**Membrane Separation Technology in Carbon Capture Membrane Separation Technology in Carbon Capture**

Guozhao Ji and Ming Zhao Guozhao Ji and Ming Zhao

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65723

#### **Abstract**

This chapter introduces the basics of membrane technology and the application of membrane separation in carbon capture processes. A number of membranes applicable in precombustion, post-combustion or oxy-fuel combustion have been discussed. An economic comparison between conventional amine-based absorption and membrane separation demonstrates the great potential in membrane technology.

**Keywords:** membrane separation, carbon dioxide capture, pre-combustion, postcombustion, oxy-fuel combustion

#### **1. Introduction**

Gas separation by membrane is attractive in low carbon emission technologies, as it can be operated in a continuous system, which is preferred by industry, other than the conventional batch systems such as adsorption and absorption. Feeding of mixed gas and exiting of purified gas can happen at the same time. Membrane selectively permeates the desired components and retains the unwanted, resulting in separation of gas mixtures. In carbon capture and storage (CCS) processes, CO2 has to be separated from the exhaust gas streams before the subsequent transportation and storage. Membrane separation technology is one of efficient solutions for carbon capture.

There have been a number of books regarding membrane technology. However, most of them are about liquid separation and very few are found for CCS. This chapter aims at introducing and demonstrating the membrane technology in CCS. The application of membrane in carbon capture mainly includes H2 /CO2 separation for pre-combustion, CO2 /N2 separation for postcombustion and O2 /N2 separation (air separation) for oxy-fuel combustion. There is a wide variety of membrane types based on its physical and chemical property. Many of them have showed great potentials to fulfill the need of CCS.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **2. Overview of membranes**

Membrane performs as a filter. It allows certain molecules to permeate through, while blocks other specific molecules from entering the membrane as demonstrated in **Figure 1**. Membrane has already been widely used in liquid separations such as micro-filtration, ultra-filtration, reverse osmosis, forward osmosis, desalination and medical application. However, gas separation using membrane is still developing. Membrane gas separation has attracted intensive researches in CCS field during recent years.

Gas permeation flux across unit membrane area under unit pressure difference through unit membrane thickness is called permeability (mol s−1 m−2 Pa−1) and the ratio of permeabilities of different gases through the same membrane is defined as selectivity. The gas separation mechanism varies from membrane to membrane. The selectivity of different gases may result from the difference in molecular size, affinity to membrane material, molecular weight, etc., depending on the gas and membrane of interest.

In order to achieve high permeate flux, the feed gas is pressurized, while the permeate gas is connected to atmosphere or vacuum to obtain a higher driving force. However, since the thickness of a membrane is only several hundred nanometers to several microns, it is impossible to resist this force. So a membrane is normally coated onto a thick, porous substrate to achieve enough mechanical strength. The supporting substrate should offer minimum flow resistance, thus containing large pores, which allows free flow of gas that has permeated the top layer. In case of too large pores and highly rough surface on the substrate, membrane defects such as cracking and peeling may occur. An interlayer with much smaller pore size (than substrate pore size) can enable smoother transition in between. This design is referred to as asymmetric structure as shown in **Figure 2**.

In the current Research & Developments (R&Ds) of membrane, the most popular mechanism is size sieving separation. Therefore, the key parameter of a membrane is its pore size. By

**Figure 1.** The schematic of membrane separation for binary gas mixtures.

**Figure 2.** The asymmetric structure of membrane coated on a substrate.

pore size, membranes are classified into three categories that are listed in **Table 1**. In addition, one more type of membrane that is nonporous, therefore called dense membrane, is also discussed in this chapter.

#### **2.1. Advantages of membranes**

Compared to conventional CO2 removal technologies, membrane has shown great potential in CCS owing to its characteristics listed below:

#### **Low capital cost**

**2. Overview of membranes**

60 Recent Advances in Carbon Capture and Storage

researches in CCS field during recent years.

depending on the gas and membrane of interest.

to as asymmetric structure as shown in **Figure 2**.

**Figure 1.** The schematic of membrane separation for binary gas mixtures.

Membrane performs as a filter. It allows certain molecules to permeate through, while blocks other specific molecules from entering the membrane as demonstrated in **Figure 1**. Membrane has already been widely used in liquid separations such as micro-filtration, ultra-filtration, reverse osmosis, forward osmosis, desalination and medical application. However, gas separation using membrane is still developing. Membrane gas separation has attracted intensive

Gas permeation flux across unit membrane area under unit pressure difference through unit membrane thickness is called permeability (mol s−1 m−2 Pa−1) and the ratio of permeabilities of different gases through the same membrane is defined as selectivity. The gas separation mechanism varies from membrane to membrane. The selectivity of different gases may result from the difference in molecular size, affinity to membrane material, molecular weight, etc.,

In order to achieve high permeate flux, the feed gas is pressurized, while the permeate gas is connected to atmosphere or vacuum to obtain a higher driving force. However, since the thickness of a membrane is only several hundred nanometers to several microns, it is impossible to resist this force. So a membrane is normally coated onto a thick, porous substrate to achieve enough mechanical strength. The supporting substrate should offer minimum flow resistance, thus containing large pores, which allows free flow of gas that has permeated the top layer. In case of too large pores and highly rough surface on the substrate, membrane defects such as cracking and peeling may occur. An interlayer with much smaller pore size (than substrate pore size) can enable smoother transition in between. This design is referred

In the current Research & Developments (R&Ds) of membrane, the most popular mechanism is size sieving separation. Therefore, the key parameter of a membrane is its pore size. By Membrane requires little material to coat. It does not need additional facilities such as large pretreatment vessel and solvent storage.

#### **Low operating cost**

The main operating cost for membrane separation unit is only membrane replacement. Due to the smaller size and weight of membrane, the cost is much lower than the conventional techniques, which replace the large amount of solvent or sorbent.

#### **Simplicity and reliability**

Since membrane does not show fast decay in performance that most likely occurs to the traditional solvents or sorbents, it can be running unattended for long periods. Another character of membrane is that gas does not stay and reacts with membrane, so membrane has no saturation and thus avoids frequent shut down and start-up.


**Table 1.** Membrane classification by pore size.

#### **Adaptability**

Membrane system is designed and operated to remove the required percentage of CO<sup>2</sup> instead of the absolute quantity of CO2 removal. Variations in the feed CO2 concentration can be adjusted by varying the space velocity to keep constant product quality.

#### **Design efficiency**

Membrane system can integrate a number of processes into one unit, such as Hg vapor removal, H2 S removal and dehydration. Traditional CO2 removal techniques have to operate these steps separately.

#### **Easy for remote area**

Multiple membranes could be packed into one module to reduce size and weight, which not only increases membrane area in unit volume but also makes it easier to transport to remote locations. Simple installation is feasible at which spare parts are rare, labors are unskilled and additional facilities (such as solvent storage, water supply and power generation) are short in supply.

#### **2.2. Membrane fabrication**

Membrane fabrication involves how to coat the selective layer onto the porous substrate. The fabrication process has significant influence on the membrane property such as membrane uniformity and thickness. The membrane coating technique includes dip-coating, chemical vapor deposition (CVD), spinning and spraying. Among them, the most popular and mature methods are dip-coating and CVD. This section will demonstrate these two technologies.

#### *2.2.1. Dip-coating*

Dip-coating involves dipping the macro-porous substrate in a solution and in turn, the solution is coated on the substrate, which is followed by a dehydration process at a lower temperature. It is the oldest and the simplest film deposition method. The dip-coating process can be separated into five stages: immersion, start-up, deposition, drainage and evaporation **(Figure 3**).

*Immersion*: The substrate is immersed in the solution of the coating material at a constant speed to avoid jitter.

*Start-up*: The entire substrate has remained inside the solution for a while and is starting to be pulled up.

*Deposition*: The thin layer of solution deposits itself on the surface of the substrate when it is pulled up. The withdrawing speed is constant to avoid any jitters. The speed determines the thickness of the coating. Faster withdrawal speed gives thicker layer and vice versa.

*Drainage*: Excess liquid will drain from the surface back to the solution due to the gravity.

*Evaporation*: The solvent evaporates from the liquid, forming the thin layer. Evaporation normally accompanies the start-up, deposition and drainage stages.

**Figure 3.** Dip-coating stages: (a) immersion; (b) start-up; (c) deposition; (d) drainage and (e) evaporation. Reproduced from Brinker [1].

#### *2.2.2. Chemical vapor deposition (CVD)*

**Adaptability**

**Design efficiency**

these steps separately. **Easy for remote area**

**2.2. Membrane fabrication**

removal, H2

in supply.

technologies.

*2.2.1. Dip-coating*

speed to avoid jitter.

be pulled up.

instead of the absolute quantity of CO2

62 Recent Advances in Carbon Capture and Storage

Membrane system is designed and operated to remove the required percentage of CO<sup>2</sup>

Membrane system can integrate a number of processes into one unit, such as Hg vapor

Multiple membranes could be packed into one module to reduce size and weight, which not only increases membrane area in unit volume but also makes it easier to transport to remote locations. Simple installation is feasible at which spare parts are rare, labors are unskilled and additional facilities (such as solvent storage, water supply and power generation) are short

Membrane fabrication involves how to coat the selective layer onto the porous substrate. The fabrication process has significant influence on the membrane property such as membrane uniformity and thickness. The membrane coating technique includes dip-coating, chemical vapor deposition (CVD), spinning and spraying. Among them, the most popular and mature methods are dip-coating and CVD. This section will demonstrate these two

Dip-coating involves dipping the macro-porous substrate in a solution and in turn, the solution is coated on the substrate, which is followed by a dehydration process at a lower temperature. It is the oldest and the simplest film deposition method. The dip-coating process can be separated into five stages: immersion, start-up, deposition, drainage and evaporation **(Figure 3**).

*Immersion*: The substrate is immersed in the solution of the coating material at a constant

*Start-up*: The entire substrate has remained inside the solution for a while and is starting to

*Deposition*: The thin layer of solution deposits itself on the surface of the substrate when it is pulled up. The withdrawing speed is constant to avoid any jitters. The speed determines the

thickness of the coating. Faster withdrawal speed gives thicker layer and vice versa.

mally accompanies the start-up, deposition and drainage stages.

*Drainage*: Excess liquid will drain from the surface back to the solution due to the gravity.

*Evaporation*: The solvent evaporates from the liquid, forming the thin layer. Evaporation nor-

be adjusted by varying the space velocity to keep constant product quality.

S removal and dehydration. Traditional CO2

removal. Variations in the feed CO2

concentration can

removal techniques have to operate

Another common membrane coating technique is CVD. CVD modifies the properties of a substrate surface by depositing a thin layer of film via chemical reactions in a gaseous medium surrounding the substrate at elevated temperatures.

The process of CVD includes transporting the reactant gases and/or carrier gas into a reaction chamber, which is followed by a deposition process to form a film. The film coating could be performed by decomposition, oxidation, hydrolysis or compound formation. The reactions normally take place in the gaseous phase and the intermediate gases adsorb on the substrate followed by surface reactions. The detailed steps of CVD process are demonstrated in **Figure 4**.


**Figure 4.** Schematic model of a CVD process. Reproduced from Khatib et al. [2].

As illustrated above, CVD is more complicated technique than dip-coating, thus the manufacture cost of a membrane is relatively higher than that of dip-coating. The advantage of CVD is good reproducibility over dip-coating as the latter may suffer from a lack of reproducibility.

#### **2.3. Membrane separation mechanism**

A membrane can separate gas mixture because different gases have different permeability through the membrane. The permeate flux across unit membrane area under unit pressure gradient is called permeability and the ratio between permeability of gas A and that of gas B is defined as selectivity of A to B. In order to achieve separation, a greater difference between gas permeabilities is preferred. This difference comes from their physical and/or chemical properties as well as the interaction with membrane.

#### *2.3.1. Size sieving*

The most widely known separation mechanism is size sieving. The membrane pore size is just between the smaller gas molecule and larger gas molecule as depicted in **Figure 5**. The smaller gas molecule A passes the pore channel freely, while the counterpart gas B is not able to enter the pore. As a result, pure component A is obtained in the permeate stream from the gas mixture A–B. This mechanism applies to separating gas mixtures with very different molecular sizes such as H<sup>2</sup> and CO2 , H2 and hydrocarbons, etc. Some common gas kinetic diameters are given in **Table 2**. Size sieving basically performs in micro-porous membrane.

**Figure 5.** Size sieving separation mechanism.

#### *2.3.2. Surface diffusion*

As illustrated above, CVD is more complicated technique than dip-coating, thus the manufacture cost of a membrane is relatively higher than that of dip-coating. The advantage of CVD is good reproducibility over dip-coating as the latter may suffer from a lack of

A membrane can separate gas mixture because different gases have different permeability through the membrane. The permeate flux across unit membrane area under unit pressure gradient is called permeability and the ratio between permeability of gas A and that of gas B is defined as selectivity of A to B. In order to achieve separation, a greater difference between gas permeabilities is preferred. This difference comes from their physical and/or chemical

The most widely known separation mechanism is size sieving. The membrane pore size is just between the smaller gas molecule and larger gas molecule as depicted in **Figure 5**. The smaller gas molecule A passes the pore channel freely, while the counterpart gas B is not able to enter the pore. As a result, pure component A is obtained in the permeate stream from the gas mixture A–B. This mechanism applies to separating gas mixtures with very

, H2

gas kinetic diameters are given in **Table 2**. Size sieving basically performs in micro-porous

and hydrocarbons, etc. Some common

and CO2

reproducibility.

*2.3.1. Size sieving*

membrane.

**2.3. Membrane separation mechanism**

64 Recent Advances in Carbon Capture and Storage

different molecular sizes such as H<sup>2</sup>

properties as well as the interaction with membrane.

**Figure 4.** Schematic model of a CVD process. Reproduced from Khatib et al. [2].

When the membrane material has higher affinity to one particular component than the other, this affinitive component is preferentially adsorbed on the membrane surface and then the adsorbed gas molecules move along the pore surface to the permeate side until desorbing to the permeate gas. Since the membrane is occupied by the highly adsorbable component, the less adsorbed component has lower probability to access the pore, which results in a much lower permeability. In such a way, the more adsorbable gas is separated from the gas mixture (**Figure 6**). This type of mechanism is generally used to separate adsorbing gas with non-adsorbing gas such as CO2 with He, CO2 with H2 . Surface diffusion generally acts in micro- and meso-porous membranes.


**Table 2.** The kinetic diameter of different gases.

**Figure 6.** Surface diffusion separation mechanism.

#### *2.3.3. Solution diffusion*

Unlike membranes discussed above, dense membrane has no pore channel for gas transportation. However, it follows solution diffusion model. The process of gas separation using dense membranes occurs in a three-step process, which is similar to surface diffusion. The dense membrane has no pore to accommodate gas molecules, however, it can solve specific gas component. As shown in **Figure 7**, due to the difference in solubility or absorbability in the membrane material, gas A solves or absorbs in the membrane after they contact at the feed interface, while gas B still remains as gas phase at the interface. The second step is the solved A component diffusing across the membrane driven by the concentration gradient from feed interface to the permeate interface. Finally, component A desorbs from the permeate interface under a low pressure. This is a common mass transfer mechanism in polymeric membrane.

#### *2.3.4. Facilitated transport*

The solution-diffusion process is often constrained by low permeate flux rates due to a combination of low solubility and/or low diffusivity. In contrast, facilitated transport that delivers the target component by a carrier can increase the permeate flow rate. As demonstrated in **Figure 8**, the gas A and carrier C form a temporary product A–C that is from a reversible chemical reaction. The product diffuses across the membrane under the concentration gradient of this product A–C instead of the concentration gradient of A. At the permeate interface, the reverse reaction takes place and A is liberated from this reverse

**Figure 7.** Solution diffusion separation mechanism.

**Figure 8.** Facilitated transport separation mechanism.

reaction. A is released to the permeate stream and C diffuses back to the feed interface again to attach and deliver a new A. Facilitated transport mechanism normally exists in liquid membrane.

#### *2.3.5. Ion transport*

*2.3.3. Solution diffusion*

**Figure 6.** Surface diffusion separation mechanism.

66 Recent Advances in Carbon Capture and Storage

membrane.

*2.3.4. Facilitated transport*

Unlike membranes discussed above, dense membrane has no pore channel for gas transportation. However, it follows solution diffusion model. The process of gas separation using dense membranes occurs in a three-step process, which is similar to surface diffusion. The dense membrane has no pore to accommodate gas molecules, however, it can solve specific gas component. As shown in **Figure 7**, due to the difference in solubility or absorbability in the membrane material, gas A solves or absorbs in the membrane after they contact at the feed interface, while gas B still remains as gas phase at the interface. The second step is the solved A component diffusing across the membrane driven by the concentration gradient from feed interface to the permeate interface. Finally, component A desorbs from the permeate interface under a low pressure. This is a common mass transfer mechanism in polymeric

The solution-diffusion process is often constrained by low permeate flux rates due to a combination of low solubility and/or low diffusivity. In contrast, facilitated transport that delivers the target component by a carrier can increase the permeate flow rate. As demonstrated in **Figure 8**, the gas A and carrier C form a temporary product A–C that is from a reversible chemical reaction. The product diffuses across the membrane under the concentration gradient of this product A–C instead of the concentration gradient of A. At the permeate interface, the reverse reaction takes place and A is liberated from this reverse Ion transport is usually applied in air separation (O2 /N2 ). As **Figure 9** shows, only oxygen gas molecule (O2 ) can be converted into two oxygen ions (2O2−) by the surface-exchange reaction on the feed interface. Nitrogen retains in the feed side. Oxygen ions are transported across by jumping between oxygen vacancies in the membrane lattice structure. At the permeate interface, electrons liberated as the oxygen ions recombine into oxygen molecules. To maintain electrical neutrality, there is a simultaneous electrons flux going back to the feed interface neutralizing the charge caused by oxygen flux.

**Figure 9.** Ion transport separation mechanism.

#### **3. Membranes for pre-combustion capture**

Pre-combustion capture is a process that separates CO2 from the other fuel gases before the gas combustion. First, it involves the processes of converting solid, liquid or gaseous fuels into a mixture of syngas (H2 and CO) and CO2 by coal gasification or steam reforming. Afterwards water-gas shift (WGS) reaction is conducted to reduce the content of CO, thus more H2 and CO2 are generated. Membrane separation is then applied to separate H<sup>2</sup> and CO2 . Upon compression, the CO2 rich stream is transported to a storage or utilization site. Meanwhile, the nearly pure H2 stream enters the combustion chamber for power generation that emits mainly water vapor in the exhaust.

Coming from the upstream gasification, reforming and WGS, the feed gas of pre-combustion CO2 capture is hot with a temperature between 300 and 700°C. In addition, the pre-combustion separation can happen at high pressures up to 80 bar.

Pre-combustion membranes are basically classified into two categories: H<sup>2</sup> -selective membrane and CO2 -selective membrane. The former favors H2 permeation but retains CO2 in the feed side, while the latter preferentially permeates CO<sup>2</sup> .

In principle, metallic membrane is the ideal candidate for separating H2 /CO2 due to the infinite selectivity. H2 molecule dissociates as two H atoms at the membrane surface and then the atomic H diffuses to the permeate side of the membrane driven by the partial pressure drop, which is followed by the association and desorption at the permeate interface. The permeate flux is given by

$$J\_{\rm H\_z} = \frac{P\_{\rm H\_z}}{L} (\sqrt{p\_{\rm food}} - \sqrt{p\_{\rm permeate}}).\tag{1}$$

This mechanism is similar to solution diffusion and ion transport. The reason for the infinite selectivity of H2 over CO2 is that this dissociation-diffusion mechanism only applies to diatomic gases such as H2 and CO2 cannot permeate by the same mechanism. For ultrathin membrane, the rate-limiting step is the dissociation of hydrogen on the membrane surface and Pd material performs the best in hydrogen dissociation. Consequently, Pd membrane was intensively investigated in the past several decades. H2 permeability through palladium membrane varies in the range between 10−7 and 10−8mol s−1 m−1 Pa−0.5 (**Table 3**). However, the permeability was not satisfactory for the industrial requirement yet. This is due to the slow permeation of H atom in the lattice of Pd, which is one order of magnitude lower than in other metals. In order to promote the permeability, a number of palladiumbased alloys have been examined. A list of reported permeability data are summarized in **Table 4**. The alloy membranes dramatically improve the H2 permeability by 2–3 orders of magnitude.

Still, a few barriers need to be overcome for the commercialization of palladium-based membrane. First, the cost of palladium is around 18,000 US\$/Ounce (in June 2016), which is 150 times more expensive than silica membrane. Second, the H<sup>2</sup> permeation driving force is not from pressure; instead, it is from the root square of pressure (Eq. (1)). Therefore, the effect of compressing feed gas is not as significant as in other permeation mechanisms. In


**Table 3.** Hydrogen permeability through palladium membrane.

**3. Membranes for pre-combustion capture**

**Figure 9.** Ion transport separation mechanism.

68 Recent Advances in Carbon Capture and Storage

a mixture of syngas (H2

water vapor in the exhaust.

pression, the CO2

nearly pure H2

brane and CO2

nite selectivity. H2

CO2

CO2

Pre-combustion capture is a process that separates CO2

tion separation can happen at high pressures up to 80 bar.

feed side, while the latter preferentially permeates CO<sup>2</sup>

and CO) and CO2

are generated. Membrane separation is then applied to separate H<sup>2</sup>

Pre-combustion membranes are basically classified into two categories: H<sup>2</sup>


In principle, metallic membrane is the ideal candidate for separating H2

gas combustion. First, it involves the processes of converting solid, liquid or gaseous fuels into

Coming from the upstream gasification, reforming and WGS, the feed gas of pre-combustion

capture is hot with a temperature between 300 and 700°C. In addition, the pre-combus-

rich stream is transported to a storage or utilization site. Meanwhile, the

.

molecule dissociates as two H atoms at the membrane surface and then the

stream enters the combustion chamber for power generation that emits mainly

water-gas shift (WGS) reaction is conducted to reduce the content of CO, thus more H2

from the other fuel gases before the

and CO2

permeation but retains CO2

/CO2

and

. Upon com-


due to the infi-

in the

by coal gasification or steam reforming. Afterwards


**Table 4.** Hydrogen permeability through palladium-based alloy.

addition, at temperatures lower than 300°C, hydrogen embrittlement causes catastrophic failure. Furthermore, the contaminations such as CO, NH3 and sulfur compounds inhibit H2 permeation through palladium membrane. Currently, palladium membrane separation still remains in small laboratory scale.

Besides metal membrane, inorganic membrane also plays an important role in separating H<sup>2</sup> / CO2 at elevated temperatures. The separation by inorganic membrane is generally achieved by the molecular size sieving effect. Carbon molecular sieve membrane has demonstrated in pilot scale to separate H2 from refiner gas streams in the early 1990s. The disadvantage of carbon membrane is that it is only feasible in non-oxidizing condition. Another type of inorganic membrane is alumina membrane. However, the majority pore size is not in the range of micropore and cannot separate gas by the size sieving mechanism. Due to the large pore size, the selectivity of alumina membrane is fairly low.

Silica membrane shows great commercial potential for separating H2 and CO2 . It is one of the most abundant materials on the planet, thus the cost is significantly reduced. Also the good thermal and chemical stability makes it possible to work in long term without frequent replacement or maintenance. The pore diameter could be controlled around 0.3 nm by proper coating-calcining process, which is the ideal size for separating H<sup>2</sup> (σ = 0.26 nm) and CO<sup>2</sup> (σ = 0.33 nm). The performance of some reported silica membranes is summarized in **Table 5**. Due to the difficulty in measuring the membrane thickness on porous substrate, permeability of H2 divided by thickness is lumped together as permeance.

The H2 permeance of silica membrane could reach up to the order of 10−6 mol s−1 m−2 Pa−1, which strongly suggests that silica membrane is competitive in pre-combustion capture. However, exposure to high concentration water vapor leads to a decline in performance of silica membrane. Such a steady decay over long time can cause the H2 permeance decrement by an order of magnitude. This still inhibits the commercialization of silica membrane.


**Table 5.** H2 /CO2 separation performance by silica-based membrane.

addition, at temperatures lower than 300°C, hydrogen embrittlement causes catastrophic

permeation through palladium membrane. Currently, palladium membrane separation still

Besides metal membrane, inorganic membrane also plays an important role in separating H<sup>2</sup>

carbon membrane is that it is only feasible in non-oxidizing condition. Another type of inorganic membrane is alumina membrane. However, the majority pore size is not in the range of micropore and cannot separate gas by the size sieving mechanism. Due to the large pore size,

the most abundant materials on the planet, thus the cost is significantly reduced. Also the good thermal and chemical stability makes it possible to work in long term without frequent replacement or maintenance. The pore diameter could be controlled around 0.3 nm by proper

(σ = 0.33 nm). The performance of some reported silica membranes is summarized in **Table 5**. Due to the difficulty in measuring the membrane thickness on porous substrate, permeability

by an order of magnitude. This still inhibits the commercialization of silica membrane.

 permeance of silica membrane could reach up to the order of 10−6 mol s−1 m−2 Pa−1, which strongly suggests that silica membrane is competitive in pre-combustion capture. However, exposure to high concentration water vapor leads to a decline in performance of

 at elevated temperatures. The separation by inorganic membrane is generally achieved by the molecular size sieving effect. Carbon molecular sieve membrane has demonstrated

from refiner gas streams in the early 1990s. The disadvantage of

and sulfur compounds inhibit H2

**Temperature (°C) Reference**

and CO2

(σ = 0.26 nm) and CO<sup>2</sup>

permeance decrement

/

. It is one of

failure. Furthermore, the contaminations such as CO, NH3

**Table 4.** Hydrogen permeability through palladium-based alloy.

the selectivity of alumina membrane is fairly low.

Silica membrane shows great commercial potential for separating H2

coating-calcining process, which is the ideal size for separating H<sup>2</sup>

divided by thickness is lumped together as permeance.

silica membrane. Such a steady decay over long time can cause the H2

remains in small laboratory scale.

**Membrane Permeability** 

70 Recent Advances in Carbon Capture and Storage

**(mol s−1 m−1 Pa−0.5)**

Pd59Cu<sup>41</sup> 1.59 × 10−7 400 [6] Pd60Cu40 1.57 × 10−7 350 [5] Pd60Cu40 1.78 × 10−7 400 [5] Pd94Cu6 3.65 × 10−8 400 [14] Pd50Ni50 7.00 × 10−6 450 [15] Pd69Ag30Ru<sup>1</sup> 1.03 × 10−6 400 [13] Pd70Ag30 2.35 × 10−7 400 [13] Pd77Ag23 1.35 × 10−7 350 [16] Pd77Ag23 5.00 × 10−5 450 [17] Pd93Ag7 7.25 × 10−8 400 [14]

in pilot scale to separate H2

CO2

of H2

The H2

As a nonporous membrane, polymeric membrane permeates gases via the solution-diffusion mechanism. Permeability is a function of gas diffusivity and solubility. The hydrogen molecules diffuse faster than other gases due to the small molecular size. However, the lower solubility of hydrogen within the polymeric membrane reduces its permeability. For H2 -selective polymeric membranes, the permeability is limited by the low solubility of H2 . There is a wide range of polymeric membranes available for H2 separation from CO2 . The performance of some polymeric membranes is shown in **Table 6**. High permeabilities are observed for polyimides such as 6FDA-durene. Higher selectivities are reported for polybenzimidazole and poly(vinyl chloride), but H2 permeability is compromised.


**Table 6.** H2 /CO2 separation performance by polymeric membrane. The only shortcoming of polymeric membranes is the poor thermal stability at operating temperatures more than 100°C. Only polybenzimidazole was examined at the temperature range (300–700°C) for syngas purification. For polybenzimidazole membrane, the greatest performance in H2 permeability and H2 /CO2 selectivity is observed between 200 and 270°C. This peak performance can be related to the increasing diffusivity of the smaller H<sup>2</sup> molecule as temperature increases. More importantly, the performance of polymeric membranes depends on its stability in the environment of the real process. For example, exposure to gases such as CO2 , water vapor and H2 S may results in plasticization and mechanical fouling.

Due to the good thermal and hydrothermal stability, zeolite membranes were also viewed as another possible candidate for separation of H2 and CO2 . Zeolite has ordered pore structure. If the pore channel size is proper, efficient size sieving could be achieved. Despite the relative simple concept, only a few types of zeolite are workable since this molecular sieve mechanism requires perfect membranes. This remains a challenge for zeolite membranes. The performance of a number of reported H2 /CO2 separation using zeolite membranes is summarized in **Table 7**. In general, neither H2 permeance nor H2 /CO2 selectivity can exceed ~10<sup>6</sup> mol s−1 m−2 Pa−1 and ~50 to meet the industrial demands.

Metal organic framework (MOF) membrane has been an emerging candidate for H<sup>2</sup> /CO2 separation. In MOF materials, metal or metal oxide cluster cations are interconnected by organic anions. The coordination polymers form flexible frameworks, therefore such MOFs are called 'soft porous crystals'. **Table 8** summarizes the H<sup>2</sup> permeance and H2 /CO2 selectivity using


**Table 7.** H2 /CO2 separation performance by zeolite membranes.


**Table 8.** H2 /CO2 separation performance by MOF membranes.

The only shortcoming of polymeric membranes is the poor thermal stability at operating temperatures more than 100°C. Only polybenzimidazole was examined at the temperature range (300–700°C) for syngas purification. For polybenzimidazole membrane, the great-

/CO2

270°C. This peak performance can be related to the increasing diffusivity of the smaller H<sup>2</sup> molecule as temperature increases. More importantly, the performance of polymeric membranes depends on its stability in the environment of the real process. For example, expo-

Due to the good thermal and hydrothermal stability, zeolite membranes were also viewed

ture. If the pore channel size is proper, efficient size sieving could be achieved. Despite the relative simple concept, only a few types of zeolite are workable since this molecular sieve mechanism requires perfect membranes. This remains a challenge for zeolite membranes.

aration. In MOF materials, metal or metal oxide cluster cations are interconnected by organic anions. The coordination polymers form flexible frameworks, therefore such MOFs are called

Metal organic framework (MOF) membrane has been an emerging candidate for H<sup>2</sup>

**H2 /CO2**

MFI 2.82 × 10−7<sup>a</sup> 42.6 500 [31] MFI 1.50 × 10−7<sup>a</sup> 5 200 [32] MFI template free 1.50 × 10−8<sup>a</sup> 3 500 [33] DDR 5.00 × 10−8<sup>a</sup> 5 500 [34] DDR by CVD 2.24 × 10−8<sup>a</sup> 5.9 500 [35] Zeolite-A 9.45 × 10−10<sup>a</sup> 10 35 [36] MFI 1.76 × 10−9<sup>a</sup> 18 450 [37]


/CO2

and CO2

permeance nor H2

permeance and H2

selectivity is observed between 200 and

S may results in plasticization and mechanical

. Zeolite has ordered pore struc-

selectivity can exceed

/CO2 sep-

selectivity using

separation using zeolite membranes is

/CO2

/CO2

 **selectivity Temperature (°C) Reference**

permeability and H2

, water vapor and H2

as another possible candidate for separation of H2

The performance of a number of reported H2

summarized in **Table 7**. In general, neither H2

'soft porous crystals'. **Table 8** summarizes the H<sup>2</sup>

**Pa−1) or Permeabilityb (mol s−1 m−1 Pa−1)**

~10<sup>6</sup> mol s−1 m−2 Pa−1 and ~50 to meet the industrial demands.

 **(mol s−1 m−2**

separation performance by zeolite membranes.

est performance in H2

72 Recent Advances in Carbon Capture and Storage

sure to gases such as CO2

**Membrane Permeancea**

AIPO4

a Permeance.

b

Permeability. **Table 7.** H2

/CO2

fouling.

different MOF membranes. Despite relatively moderate permselectivity, attractively high permeances are observed. The operating temperature for MOF membranes is normally lower than the pre-combustion temperatures, owing to organic ligands. The synthesis of MOF membranes is relatively sophisticated so that the cost has to be notably reduced toward commercialization. There is still a long way for MOF membranes to fulfill the demands of industrial applications.

Unlike H2 -selective membranes, CO2 -selective membranes preferentially permeate CO2 and thus they also enable the separation of CO2 and H2 . Separating CO2 from H2 can only be realized through surface diffusion or solution diffusion driven by the difference in adsorb-ability or solubility between the gases. However, retaining the small molecules of H2 but permeating the larger CO2 is really challenging. To maximize the difference of adsorption or solution between the two gases, the temperature is required to be low, however, low temperatures are not favored by pre-combustion processes. From this point of view, CO2 -selective membranes are much less applicable than H2 -selective ones.

#### **4. Membranes for post-combustion capture**

Another situation where we need to separate CO2 is after the fuel combustion. The exhaust gas (flue gas) mainly contains CO<sup>2</sup> , H2 O and N2 . H2 O vapor is easy to be removed by condensation. More efforts are required to separate CO<sup>2</sup> and N2 prior to further treatments such as compression. Unlike pre-combustion capture, post-combustion capture separates CO2 /N2 at moderate temperatures and ambient atmosphere pressure. Such operating conditions seem less severe than those of pre-combustion processes. As a result, post-combustion capture has encountered much less difficulties and is therefore rather closer to practical application. The major challenge for post-combustion capture is the low CO2 volumetric fraction in flue gas, that is, ~15%, which results in a low driving force of CO<sup>2</sup> permeation.

The separation of CO2 /N2 mainly rely on surface diffusion and solution diffusion, which is driven by the difference in adsorb-ability and solubility between the gases. The good thing is that, compared to N2 , CO2 is more likely to be favored by majority of the membrane materials via adsorption or absorption. Furthermore, the diameter of CO2 is slightly smaller than that of N2 , which also enhances the diffusion of CO<sup>2</sup> (see **Table 2**). Therefore, for post-combustion capture, CO2 -selective membranes are generally used.

To capture CO2 from flue gas, a membrane should satisfy a few requirements such as high CO2 permeability, high CO2 /N2 selectivity, high thermal and chemical stability and acceptable costs. So far, polymer-based membranes are the only commercially viable type for CO2 removal from flue gas. The membrane materials include cellulose acetate, polymides, polysulfone and polycarbonates. **Table 9** shows the performance of several such membranes.


**Table 9.** CO2 /N2 separation performance by polymer-based membranes. Selectivity larger than 20 was observed for all the polymer-based membranes with decent permeability. The high solubility of CO2 in polymers ensures sufficient CO<sup>2</sup> /N2 selectivity. Furthermore, polymers with a high fractional free volume present excellent gas transport properties.

Mixed-matrix membrane is a new option to enhance the properties of polymeric membranes. The microstructure consists of an inorganic material in the form of micro- or nanoparticles in discrete phase incorporated into a continuous polymeric matrix. The addition of inorganic materials in a polymer matrix offers improved thermal and mechanical properties for aggressive environments and stabilizes the polymer membranes against the changes in chemical and physical environments. Carbon molecular sieves membranes also show interesting performance for CO2 separation applications. Polyimide is the most used precursor for carbon membranes. Carbon membranes improved gas transport properties for light gases (molecular size smaller than 4.0–4.5Å) with thermal and chemical stability. The major disadvantages of mixed-matrix and carbon membranes that hinder their commercialization include brittleness and the high cost that is 1–3 orders of magnitude greater than polymeric membranes.

#### **5. Membranes for oxy-fuel combustion**

**4. Membranes for post-combustion capture**

, H2

O and N2

compression. Unlike pre-combustion capture, post-combustion capture separates CO2

moderate temperatures and ambient atmosphere pressure. Such operating conditions seem less severe than those of pre-combustion processes. As a result, post-combustion capture has encountered much less difficulties and is therefore rather closer to practical application. The

driven by the difference in adsorb-ability and solubility between the gases. The good thing is

able costs. So far, polymer-based membranes are the only commercially viable type for CO2 removal from flue gas. The membrane materials include cellulose acetate, polymides, polysulfone and polycarbonates. **Table 9** shows the performance of several such membranes.

> **CO2 /N2**

3.31 × 10−10<sup>b</sup> 18.6 35 [63]

3.47 × 10−10<sup>b</sup> 18 35 [64]

2.27 × 10−10<sup>b</sup> 21.4 35 [65]

3.50 × 10−10<sup>b</sup> 15 35 [66]

Cellulose acetate 2.48 × 10−7<sup>a</sup> 40.17 Not reported [60] Polymides-TMeCat 6.30 × 10−10<sup>b</sup> 25 30 [61] Polymides-TMMPD 1.89 × 10−9<sup>b</sup> 17.1 Not reported [62] Polymides-IMDDM 6.17 × 10−10<sup>b</sup> 18.1 Not reported [62]

Polycarbonates-FBPC 4.76 × 10−11<sup>b</sup> 25.5 35 [67]

separation performance by polymer-based membranes.

. H2

and N2

permeation.

selectivity, high thermal and chemical stability and accept-

mainly rely on surface diffusion and solution diffusion, which is

is more likely to be favored by majority of the membrane materials

from flue gas, a membrane should satisfy a few requirements such as high

is after the fuel combustion. The exhaust

O vapor is easy to be removed by conden-

prior to further treatments such as

volumetric fraction in flue gas,

is slightly smaller than that

(see **Table 2**). Therefore, for post-combustion

 **selectivity Temperature (°C) Reference**

/N2 at

Another situation where we need to separate CO2

sation. More efforts are required to separate CO<sup>2</sup>

major challenge for post-combustion capture is the low CO2

via adsorption or absorption. Furthermore, the diameter of CO2


 **(mol s−1 m−2 Pa−1)** 

/N2

**or Permeabilityb (mol s−1 m−1**

that is, ~15%, which results in a low driving force of CO<sup>2</sup>

/N2

, CO2

, which also enhances the diffusion of CO<sup>2</sup>

gas (flue gas) mainly contains CO<sup>2</sup>

74 Recent Advances in Carbon Capture and Storage

The separation of CO2

that, compared to N2

of N2

CO2

capture, CO2

To capture CO2

Polysulfone-HFPSF-o-

Polysulfone-HFPSF-

PolysulfoneTMPSF-

Polycarbonates-TMHFPC

Permeability. **Table 9.** CO2

/N2

HBTMS

HBTMS

TMS

a Permeance.

b

permeability, high CO2

**Membrane Permeancea**

**Pa−1)**

In oxy-fuel combustion, oxygen is supplied for combustion instead of air. This avoids the presence of nitrogen in the exhaust gas, the major issue to be solved by post-combustion CO2 capture technologies. With the use of pure oxygen for the combustion, the major composition of the flue gases is CO<sup>2</sup> , water vapor, other impurities such as SO2 . Water vapor can be easily condensed and SO2 can be removed by conventional desulphurization methods. The remained CO2 -rich gases (80–98 vol.% CO<sup>2</sup> depending on fuel used) can be compressed, transported and stored. This process is technically feasible but consumes large amounts of oxygen coming from an energy intensive air separation (O2 /N2 ) unit.

The O2 /N2 separation follows the ion transport mechanism as depicted in **Figure 9** for air separation membrane. Oxygen molecules are converted to oxygen ions at the surface of the membrane and transported through the membrane by an applied electric voltage or oxygen partial pressure difference; these ions are reverted back to oxygen molecules after passing through the membrane. These membranes are O2 -selective in principle. Generally, fluorite-based and perovskite-based membranes are used to deliver oxygen through this mechanism.

Air separation is mostly carried out at atmosphere and meanwhile the permeate side connects to high speed sweep gas or vacuum. So, for convenience, the membrane performance is generally described as permeate flux instead of permeance. **Table 10** shows a list of oxygen permeation flux for the fluorite membranes. The oxygen permeation flux of fluorite-based membranes ranges from 10−4 to 10−6 mol s−1 m−2 between 650 and 1527°C. The highest oxygen flux was observed for Bi1.5Y0.3Sm0.2O3 compounds.


**Table 10.** Oxygen permeation flux data for fluorite membranes.


**Table 11.** Oxygen permeation flux data for perovskite membranes.

Performance of perovskite membranes are displayed in **Table 11**. Oxygen permeation flux with the magnitude of 10−2–10−5 mol s−1 m−2 between 700 and 100°C was reported. The overall oxygen flux through perovskite membrane is superior to fluorite membrane. SrCo0.8Fe0.2O3-δ exhibits the best oxygen flux.

In spite of a great number of works that attempt to efficiently separate air using membrane, the membrane technology for oxy-fuel combustion is still at its early stage of development. Compared to the conventional cryogenic air separation technique, the high temperature requirement and the resulting high costs of air separation membrane are unfavorable for commercialization. Some other issues such as high temperature sealing, chemical and mechanical stability and so on also need to be addressed prior to practical application. At present, there has not been any full scale oxy-fuel membrane project reported.

#### **6. Summary of membranes applied in CCS**

The aforementioned membranes are compared in **Table 12**. Their application situation, advantages and disadvantages are summarized accordingly.


**Table 12.** The summarization of membranes in CCS.

#### **7. Membrane mass transfer theory**

Performance of perovskite membranes are displayed in **Table 11**. Oxygen permeation flux with the magnitude of 10−2–10−5 mol s−1 m−2 between 700 and 100°C was reported. The overall oxygen flux through perovskite membrane is superior to fluorite membrane. SrCo0.8Fe0.2O3-δ

BaBi0.4Co0.2Fe0.4O3-δ 3.064 × 10−3–5.985 × 10−3 1.5 800–925 [73] BaCo0.4Fe0.5Zr0.1O3-δ 1.908 × 10−3–6.813 × 10−3 1 700–950 [74] CaTi0.8Fe0.2O3-δ 7.976 × 10−5–2.185 × 10−4 1 800–1000 [75] Gd0.6Sr0.4CoO3-δ 1.179 × 10−2 1.5 820 [76] LaCo0.8Fe0.2O3-δ 1.786 × 10−4 1.5 860 [76] La0.6Sr0.4Co0.8Cu0.2O3-δ 1.417 × 10−2 1.5 860 [76] SrCo0.8Fe0.2O3-δ 2.485 × 10−2 1 870 [77]

Bi0.75Y0.5Cu0.75O3 2.80 × 10−5–1.06 × 10−4 2 650–850 [68] Bi1.5Y0.3Sm0.2O3 4.40 × 10−3–6.36 × 10−3 1.2 825–875 [69] Ce0.8Pr0.2O2-δ 1.33 × 10−4–3.35 × 10−4 1 850–950 [70]

)0.85(CaO)0.15 1.70 × 10−4 1 870 [71]

0.9(CaO)0.1 1.36 × 10−6–9.44 × 10−5 2 1127–1527 [72]

**Thickness (mm) Temperature (°C) Reference**

 **flux (mol s−1 m−2) Thickness (mm) Temperature (°C) Reference**

In spite of a great number of works that attempt to efficiently separate air using membrane, the membrane technology for oxy-fuel combustion is still at its early stage of development. Compared to the conventional cryogenic air separation technique, the high temperature requirement and the resulting high costs of air separation membrane are unfavorable for commercialization. Some other issues such as high temperature sealing, chemical and mechanical stability and so on also need to be addressed prior to practical application. At present, there has not been any full scale oxy-fuel membrane project

The aforementioned membranes are compared in **Table 12**. Their application situation,

exhibits the best oxygen flux.

**Membrane O2**

**Membrane O2**

76 Recent Advances in Carbon Capture and Storage

(ZrO2

[(ZrO<sup>2</sup> ) 0.8(CeO2 )0.2]

> **flux (mol s−1 m−2)**

**Table 10.** Oxygen permeation flux data for fluorite membranes.

**Table 11.** Oxygen permeation flux data for perovskite membranes.

**6. Summary of membranes applied in CCS**

advantages and disadvantages are summarized accordingly.

reported.

Membrane separation technique has been intensified with the growing needs for CCS. The major two targets of membrane are chasing high permeability and selectivity. The understanding of gas transport through membrane is of great importance in providing the guidance of membrane material design and synthesis improvement.

For all mass transfer problems, a general form is always expressed as a coefficient multiplied by a driving force as

$$J = \mathbf{C} \cdot f. \tag{2}$$

where *J* is the mass transfer flux, *C* is the general transfer coefficient and *f* is the general driving force. The driving force can be the gradient of pressure, concentration, chemical potential or even electrical potential depending on the mass transfer mechanism. The coefficient can be permeability, diffusivity or other term depending on the term of driving force. For membrane mass transfer, the pressure difference and permeate flux are generally determined from experimental measurements, so the most common form in membrane industry is

$$J = \begin{pmatrix} \frac{P}{I} \end{pmatrix} \cdot \Delta p.\tag{3}$$

Membrane thickness *l* is lumped together with permeability *P* into a term called permeance (\_\_ *P <sup>l</sup>*), which is a convenient form of addressing permeation due to the difficulty in measuring the exact thickness of thin films. Generally, membrane films interpenetrate into the pores of the interlayer or substrate (interlayer-free membrane). Hence, the thickness is not homogenous.

#### **7.1. Viscous flow model**

When the pore size is large, the gas molecule-molecule collision is relatively dominant than gas molecule-wall collision. That means the mean free path is far less than the pore size

$$\frac{\lambda}{d} \ll 1,\tag{4}$$

where *λ* is the mean free path and *d* is the diameter of the pore.

In such situation, viscosity plays an important role in the mass transfer and the permeate flux across the membrane is described by viscous flow model:

 *J* = − *ε* \_\_*<sup>p</sup> τT rp* 2 \_\_\_ 8*η p*\_\_\_ *RT* d*p* \_\_\_ <sup>d</sup>*<sup>z</sup>* , (5)

where *η* is the viscosity, *R* is the gas constant, *T* is the temperature, *p* is the pressure, *ε<sup>p</sup>* is the porosity of the pore, *τT* is the tortuosity of the pore and *rp* is the pore radius. Viscosity increases with temperature for gases. From Eq. (5), it should be noted that if the transportation is in the viscous regime, the flux is a decreasing function of temperature. Although the viscosity is different from gas to gas, gas mixtures share a singular viscosity value when they are well mixed due to the intensive intermolecular collision. Therefore, there is no selectivity for all gases in the viscous regime even if they have different viscosities.

#### **7.2. Knudsen diffusion model**

When the pore size is reduced down to the scale much smaller than mean free path, the molecular-wall collision is more dominating than intermolecular collision. In this situation, the viscosity is not playing a role for the gas transportation. Instead, the pore geometry and gas molecule velocity influence more significantly in the mass transfer. This type of transport is called Knudsen diffusion. If the molecule to wall collisions is dominant over intermolecular collision, the Knudsen number must be much higher than 1.

$$\text{Ku} = \frac{\lambda}{d} \gg 1,\tag{6}$$

where *Kn* is called Knudsen number. The permeate flux is described by the Knudsen diffusion model

$$J = -\frac{2}{3} \frac{\varepsilon\_r r\_p}{\tau\_T} \sqrt{\frac{8}{\pi \text{RTM}}} \frac{\text{dp}}{\text{dz}} = -\frac{2}{3} \frac{\varepsilon\_r r\_p}{\tau\_T} \sqrt{\frac{8}{\pi \text{RTM}}} \frac{\text{dp}}{\text{l}}\tag{7}$$

where *M* is the molecular weight.

Based on Eq. (3) the permeance of Knudsen diffusion is

Membrane Separation Technology in Carbon Capture http://dx.doi.org/10.5772/65723 79

$$
\begin{pmatrix} \frac{P}{I} \end{pmatrix} = -\frac{2}{3} \frac{\varepsilon\_r r\_r}{\pi\_r I} \sqrt{\frac{8}{\pi R \Gamma \mathbf{M}}} \,\tag{8}
$$

For the same pore at a fixed temperature, the permeate flux is determined by the molar weight and in principle, the selectivity is the root square of the reciprocal of molar weights. However, due to the limited selectivity, Knudsen diffusion is rarely used in practice for separating real gas mixtures.

#### **7.3. Surface diffusion model**

measuring the exact thickness of thin films. Generally, membrane films interpenetrate into the pores of the interlayer or substrate (interlayer-free membrane). Hence, the thickness is

When the pore size is large, the gas molecule-molecule collision is relatively dominant than gas molecule-wall collision. That means the mean free path is far less than the pore size

In such situation, viscosity plays an important role in the mass transfer and the permeate flux

*ε* \_\_*<sup>p</sup> τT rp* 2 \_\_\_ 8*η p*\_\_\_ *RT* d*p* \_\_\_

where *η* is the viscosity, *R* is the gas constant, *T* is the temperature, *p* is the pressure, *ε<sup>p</sup>*

increases with temperature for gases. From Eq. (5), it should be noted that if the transportation is in the viscous regime, the flux is a decreasing function of temperature. Although the viscosity is different from gas to gas, gas mixtures share a singular viscosity value when they are well mixed due to the intensive intermolecular collision. Therefore, there is no selectivity

When the pore size is reduced down to the scale much smaller than mean free path, the molecular-wall collision is more dominating than intermolecular collision. In this situation, the viscosity is not playing a role for the gas transportation. Instead, the pore geometry and gas molecule velocity influence more significantly in the mass transfer. This type of transport is called Knudsen diffusion. If the molecule to wall collisions is dominant over intermolecular

*λ*

where *Kn* is called Knudsen number. The permeate flux is described by the Knudsen diffusion

d*p* \_\_\_ <sup>d</sup>*<sup>z</sup>* <sup>=</sup> <sup>−</sup>\_\_2 3 *<sup>ε</sup><sup>p</sup> rp* \_\_\_\_ *τ<sup>T</sup>* √

\_\_\_\_\_\_ \_\_\_\_\_\_ 8 R*T*M

*<sup>d</sup>* <sup>≪</sup> <sup>1</sup>, (4)

<sup>d</sup>*<sup>z</sup>* , (5)

*<sup>d</sup>* <sup>≫</sup> <sup>1</sup>, (6)

*<sup>l</sup>* , (7)

\_\_\_\_\_\_ \_\_\_\_\_\_ 8 R*T*M

*Δp* \_\_\_

is the pore radius. Viscosity

is

*λ*

not homogenous.

**7.1. Viscous flow model**

78 Recent Advances in Carbon Capture and Storage

\_\_

*J* = −

**7.2. Knudsen diffusion model**

*J* = −\_\_2

where *M* is the molecular weight.

model

where *λ* is the mean free path and *d* is the diameter of the pore.

the porosity of the pore, *τT* is the tortuosity of the pore and *rp*

collision, the Knudsen number must be much higher than 1.

3 *<sup>ε</sup><sup>p</sup> rp* \_\_\_\_ *τ<sup>T</sup>* √

Based on Eq. (3) the permeance of Knudsen diffusion is

*Kn* = \_\_

for all gases in the viscous regime even if they have different viscosities.

across the membrane is described by viscous flow model:

For ultra-micro-porous (dp < 5Å) material, the Lennard-Jones (L-J) potential from atoms, which forms the pore wall starts to overlap inside the pore. Consequently, there is a very deep potential well around the wall and the distance from wall to the well is around the scale of gas molecule diameter. In this situation, the gas molecule's motion is significantly affected by the potential fields. Since the intrinsic nature of gas is seeking for lower potential, thus adsorption preferentially takes place around the pore wall due to the existence of the potential well. As such, the model is called surface diffusion. A brief introduction has been given in Section 2.3.2. of this chapter, but here a more analytical and mathematical description of surface diffusion will be provided.

The original expression of mass transfer across the membrane is given by

$$J = -qD \frac{1}{RT} \frac{d\mu}{dz} \,\prime \tag{9}$$

where *q* is the molar concentration of gas in the pore, *D* is the diffusivity, *μ* is the chemical potential and *z* is the space coordinate in the membrane thickness direction.

Assuming equilibrium between the membrane surface concentration and the bulk gas phase, the following relationship for the chemical potential is applicable

$$
\mu\_v = \mu + \mathbb{R}T \ln p,\tag{10}
$$

where *p* is the absolute pressure.

Using Eq. (10), Eq. (9) is converted to

$$J = -D \frac{d \ln p}{d \ln q} \frac{d q}{d z} = -D \Gamma \frac{d q}{d z} \,. \tag{11}$$

*<sup>Γ</sup>* <sup>=</sup> <sup>d</sup> ln*<sup>p</sup>* \_\_\_\_\_ d ln*q* is defined as thermodynamic factor. In micro-porous material, the adsorbed gas concentration generally follows Langmuir isotherm,

$$q = q\_{\rm sat} \frac{b\mathbf{p}}{1 + b\mathbf{p}} \, \tag{12}$$

where *b* is Langmuir equilibrium constant. Bring Eq. (12) to Eq. (11) gives

$$J = -q\_{sat}D \frac{1}{1 - \theta} \frac{d\theta}{dz} \,\prime \tag{13}$$

where *<sup>θ</sup>* <sup>=</sup> *<sup>q</sup>*\_\_\_ *q*sat is called occupancy. Thermal dynamic factor *Γ* = \_\_\_\_ <sup>1</sup> <sup>1</sup> <sup>−</sup> *<sup>θ</sup>* is derived from Langmuir isotherm. Surface diffusion is often applied in separating gas mixtures, which has very different adsorption capacity in the same material.

However, with elevated temperature, the adsorption is getting weaker and Langmuir isotherm is approaching to Henry's law.

$$q = Kp\_t \tag{14}$$

where *K* is Henry's constant. Bring Eq. (14) to Eq. (11), we get Fick's first law

$$J = -D\frac{d\boldsymbol{q}}{dz}.\tag{15}$$

Diffusivity *D* is a function of temperature. The temperature dependence usually obeys an Arrhenius relation

$$D = D\_0 \exp\left(-\frac{E\_d}{RT}\right) \tag{16}$$

where *D*<sup>0</sup> is a pre-exponential coefficient depending on the average distance, the frequency and average velocity of gas jump and *E*d is diffusion activation energy. Henry's constant is a function of temperature according to a van't Hoff relation:

$$K = K\_0 \exp\left(\frac{Q}{RT}\right) \tag{17}$$

where *K*<sup>0</sup> is a pre-exponential coefficient, *Q* is the heat of adsorption.

Eqs. (14)–(17) can be combined as

$$J = -D\_o K\_0 \exp\left(-\frac{E\_i - Q}{RT}\right) \frac{dp}{dz} = -D\_o K\_0 \exp\left(-\frac{E\_s}{RT}\right) \frac{dp}{dz}.\tag{18}$$

*E*a is called apparent activation energy, which is defined as

$$E\_a = E\_d - Q.\tag{19}$$

Apparent activation energy determines whether the permeate flux is an increasing function to temperature or not, so this type of diffusion is called activated transport.

Assuming a uniform pressure gradient, Eq. (18) is simplified to

$$J = -D\_o K\_o \exp\left(-\frac{E\_s}{RT}\right) \frac{\Delta p}{l} \,. \tag{20}$$

The permeance (\_\_ *P <sup>l</sup>*) is the coefficient between flux and pressure drop according to Eq. (3)

$$\left(\frac{P}{I}\right) = \frac{D\_o K\_b}{l} \exp\left(-\frac{E\_s}{RT}\right). \tag{21}$$

Activated transport is generally used to separate gas mixtures, which has different sign of apparent activation energy and the separation performance will be enhanced at elevated temperatures.

#### **7.4. Gas translation diffusion model**

where *<sup>θ</sup>* <sup>=</sup> *<sup>q</sup>*\_\_\_

Arrhenius relation

where *D*<sup>0</sup>

where *K*<sup>0</sup>

*E*a

The permeance (\_\_

*P*

(\_\_

*q*sat

80 Recent Advances in Carbon Capture and Storage

therm is approaching to Henry's law.

very different adsorption capacity in the same material.

*J* = −*D*

*<sup>D</sup>* <sup>=</sup> *<sup>D</sup>*<sup>0</sup> exp(<sup>−</sup> *<sup>E</sup>*\_\_\_d

function of temperature according to a van't Hoff relation:

is called apparent activation energy, which is defined as

Assuming a uniform pressure gradient, Eq. (18) is simplified to

*<sup>J</sup>* <sup>=</sup> <sup>−</sup>*D*<sup>0</sup> *<sup>K</sup>*<sup>0</sup> exp(<sup>−</sup> *<sup>E</sup>*\_\_\_a

temperature or not, so this type of diffusion is called activated transport.

*P <sup>l</sup>*) <sup>=</sup> *<sup>D</sup>*<sup>0</sup> *<sup>K</sup>* \_\_\_\_\_0

*<sup>K</sup>* <sup>=</sup> *<sup>K</sup>*<sup>0</sup> exp(\_\_\_ *<sup>Q</sup>*

Eqs. (14)–(17) can be combined as

*J* = −*D*<sup>0</sup> *K*<sup>0</sup> exp(−

is called occupancy. Thermal dynamic factor *Γ* = \_\_\_\_ <sup>1</sup>

Langmuir isotherm. Surface diffusion is often applied in separating gas mixtures, which has

However, with elevated temperature, the adsorption is getting weaker and Langmuir iso-

*q* = *Kp*, (14)

Diffusivity *D* is a function of temperature. The temperature dependence usually obeys an

and average velocity of gas jump and *E*d is diffusion activation energy. Henry's constant is a

d*q* \_\_\_

is a pre-exponential coefficient depending on the average distance, the frequency

<sup>d</sup>*<sup>z</sup>* <sup>=</sup> <sup>−</sup>*D*<sup>0</sup> *<sup>K</sup>*<sup>0</sup> exp(<sup>−</sup> *<sup>E</sup>*\_\_\_a

*RT*) *<sup>Δ</sup><sup>p</sup>* \_\_\_

*<sup>l</sup>*) is the coefficient between flux and pressure drop according to Eq. (3)

*<sup>l</sup>* exp(<sup>−</sup> *<sup>E</sup>*\_\_\_a

where *K* is Henry's constant. Bring Eq. (14) to Eq. (11), we get Fick's first law

is a pre-exponential coefficient, *Q* is the heat of adsorption.

*<sup>E</sup>*<sup>d</sup> <sup>−</sup> *<sup>Q</sup>* \_\_\_\_\_ *RT* ) <sup>d</sup>*<sup>p</sup>* \_\_\_

*E*<sup>a</sup> = *E*<sup>d</sup> − *Q*. (19)

Apparent activation energy determines whether the permeate flux is an increasing function to

<sup>1</sup> <sup>−</sup> *<sup>θ</sup>* is derived from

<sup>d</sup>*<sup>z</sup>* . (15)

*RT*), (16)

*RT*), (17)

<sup>d</sup>*<sup>z</sup>* . (18)

*<sup>l</sup>* . (20)

*RT*). (21)

*RT*) <sup>d</sup>*<sup>p</sup>* \_\_\_

If the pore size is further reduced to the molecular level, there is no potential well inside the pore. Instead, the positive potential overlaps, which forms a potential barrier. Only the gas molecules, which have kinetic energy higher than the potential barrier, are possible to make a successful jump to complete permeation. This model is called gas translation diffusion. The permeate flux of gas translation follows Fick's first law as derived in Eq. (15) with the difference in diffusion coefficient term.

$$D\_{\rm cr} = \frac{\lambda}{Z\_n} \sqrt{\frac{8RT}{\pi M}} \exp\left(\frac{\varepsilon\_n}{RT}\right) \tag{22}$$

where *λ* is the jump length, *Z*n is the number of available jump directions and *E*GT is the potential barrier. By considering ideal gas law

$$p = cRT.\tag{23}$$

Gas translation permeance should rewrite as

$$
\left(\frac{P}{I}\right) = \frac{\lambda}{Z\_n} \sqrt{\frac{8}{\pi \text{MRT}}} \exp\left(\frac{\frac{\varepsilon}{\lambda T}}{\lambda T}\right) \tag{24}
$$

#### **7.5. Oscillator model**

If we assume the pore is a cylinder, the gas molecules are hopping in the pore cylinder from entrance to the exit. The gas molecule trajectory looks like oscillating on the pore cross section. The gas travels with speed between collisions and loses all the momentum when colliding on the wall. This model is a more recent development in mass transfer theory by Bhatia et al. [78, 79].

From Newton's law,

$$
\langle \mathbf{v}\_{\mathbf{i}} \rangle = \frac{D}{k\_y T} f = \frac{f}{\overline{m}} \langle \mathbf{r} \rangle \tag{25}
$$

the gas diffusivity in the pore is derived

$$D = \frac{k\_s T}{m} \langle \mathbf{r} \rangle \tag{26}$$

where 〈*vz* 〉 is the average velocity in the permeation direction, *kB* the Boltzmann constant, *f* the force, *m* the molecule mass and 〈*τ*〉 the average hopping time. The hopping time of each molecule depends on the pore potential distribution, its radial coordinate and momentum

$$\pi(r, p\_{r'} p\_{\phi}) = \mathcal{D}m\_{r\_{\alpha}^{\mu}(r, p\_{r} p\_{\phi})}^{r\_{\alpha}(r, p\_{r} p\_{\phi})} \frac{\mathbf{d} \, r'}{p\_{r}(r', r, p\_{r'} p\_{\phi})}.\tag{27}$$

*pr* (*r*′, *r*, *pr* , *pθ*) is the radial momentum at *r*′ of a molecule, which had radial momentum *pr* at *r*. *rc*1 (*r*, *pr* , *pθ*) and *rc*<sup>0</sup> (*r*, *pr* , *pθ*) are the *r*′ solution of radial momentum *pr* (*r*′, *r*, *pr* ,*pθ*) = 0. The radial momentum is derived from the conservation of total energy or Hamiltonian

$$E\_{\rm r}(r, p\_{r'}, p\_{\vartheta}) = \phi(r) + \frac{p\_{r}^{2}}{2m} + \frac{p\_{\vartheta}^{2}}{2m} \tag{28}$$

where *φ*(*r*) is the radial L-J potential, which could be derived from pore structure and gas property. The force in radial direction is the partial derivative of total energy with respect to *r*

$$\frac{\mathrm{d}\,p\_r}{\mathrm{d}t} = -\frac{\partial E\_i}{\partial r}.\tag{29}$$

Combining Eqs. (28) and (29) gives the radial momentum

$$p\_r(r', r, p\_{r'} p\_o) = \left[2m[\phi(r) - \phi(r')] + p\_r^{-2}(r) + \frac{p\_o}{r^2} \{1 - \frac{r^2}{r'^2}\}\right]^{1/2}.\tag{30}$$

Considering a canonical distribution for *pr* and *pθ*, we have

$$\psi(r, p\_r, p\_o) = \psi\_o \exp\left[ -\frac{1}{RT} \{\phi(r) + \frac{p\_r^{\ast^2}}{2m} + \frac{p\_o^{\ast^2}}{2m} \} \right].\tag{31}$$

The diffusion coefficient expression is obtained from Eqs. (26), (30) and (31)

$$D(r\_{p'}T) = \frac{2}{\pi m f\_0^{-} r e^{\frac{\phi \eta}{4T}} \text{d}r} \int\_0^\eta e^{-\frac{\phi \eta}{4T}} \text{d}r f\_0^{\prime \alpha} e^{-\frac{p'}{2\pi RT}} \text{d}\, p\_\rho \mathbf{f}\_0^{\prime \alpha} e^{-\frac{p'}{2\pi RT}} \text{d}\, p\_\rho f\_{r\_{d(\theta'\theta,\eta)}}^{r\_{d(\theta'\theta,\eta)}} \frac{\mathbf{d}\, \mathbf{r'}}{p\_\mathbf{r}^{(\hat{r},\hat{r},\hat{p}\_\rho\mathbf{p}\_\phi)}}.\tag{32}$$

Oscillator model is a pure theoretical and analytical approach without any empirical or semiempirical factors. It takes account adsorption effect and applies to all pore sizes, pressure and temperatures.

Besides the mass transfer models introduced above, there are some other methods to study the membrane gas transport from a theoretical perspective. Monte Carlo and molecular dynamics are also major techniques to investigate the micropore mass transfer. Because this chapter focused on membrane CCS technology rather than transport phenomena, other sophisticated theories are not demonstrated here.

#### **8. Current status of membrane application**

#### **8.1. Membranes for pre-combustion**

The membrane separation for pre-combustion is not a mature technology so far. There has not been industry-scale membrane system. However, a few pilot scale pre-combustion membrane systems have demonstrated the potential of extending the system to enlarged scale.

Eltron Research & Development Inc. developed a pilot-scale pre-combustion membrane with 100 kg day−1 H2 production from 2005. They employed alloy membrane to separate H2 according to Sieverts' Law. This project successfully improved membrane-based integrated gasification combined cycle (IGCC) flow sheets, achieving carbon capture greater than 95%.

Another pilot-scale pre-combustion membrane set-up was constructed by Worcester Polytechnic Institute's (WPI) in 2010. More than 566 L H<sup>2</sup> was produced per day. Stable H2 fluxes were achieved in actual syngas atmospheres at 450°C for more than 470 h under 12 bar pressure difference. The implement MembraGuardTM (T3's technology) inhibited surface poisoning by hydrogen sulfide (H<sup>2</sup> S) and H2 permeation showed good stability for more than 250 h.

#### **8.2. Membranes for post-combustion**

*pr* (*r*′, *r*, *pr*

*rc*1 (*r*, *pr*

, *pθ*) and *rc*<sup>0</sup>

*pr*

*D*(*rp*

temperatures.

100 kg day−1 H2

(*r*, *pr*

82 Recent Advances in Carbon Capture and Storage

*E*t(*r*, *pr*

<sup>d</sup> *<sup>p</sup>* \_\_\_*<sup>r</sup>*

(*r*′ , *r*, *pr*

Considering a canonical distribution for *pr*

, *<sup>T</sup>*) <sup>=</sup> \_\_\_\_\_\_\_\_\_\_\_ <sup>2</sup> *πm*∫<sup>0</sup> <sup>∞</sup> *r e* <sup>−</sup> *φ*(*r*) \_\_\_\_ *RT* d*r* ∫0 <sup>∞</sup> *e* <sup>−</sup> *φ*(*r*) \_\_\_\_ *RT* d*r*∫<sup>0</sup>

theories are not demonstrated here.

**8.1. Membranes for pre-combustion**

**8. Current status of membrane application**

*ψ*(*r*, *pr*

Combining Eqs. (28) and (29) gives the radial momentum

, *pθ*) is the radial momentum at *r*′ of a molecule, which had radial momentum *pr*

2 \_\_\_ <sup>2</sup>*<sup>m</sup>* <sup>+</sup> *<sup>p</sup><sup>θ</sup>* 2 \_\_\_\_

, *pθ*) are the *r*′ solution of radial momentum *pr*

, *<sup>p</sup>θ*) <sup>=</sup> *<sup>φ</sup>*(*r*) <sup>+</sup> *pr*

<sup>d</sup>*<sup>t</sup>* <sup>=</sup> <sup>−</sup>

, *<sup>p</sup>θ*) <sup>=</sup> {2*m*[*φ*(*r*) <sup>−</sup> *<sup>φ</sup>*(*r*′

, *<sup>p</sup>θ*) <sup>=</sup> *<sup>ψ</sup>*<sup>0</sup> exp[−\_\_\_<sup>1</sup>

<sup>∞</sup> *e* <sup>−</sup> *pr* 2 \_\_\_\_\_ <sup>2</sup>*mRT* d *pr* ∫0 <sup>∞</sup> *e* <sup>−</sup> *<sup>p</sup><sup>θ</sup>* 2 \_\_\_\_\_\_\_ 2*mr* <sup>2</sup>

Oscillator model is a pure theoretical and analytical approach without any empirical or semiempirical factors. It takes account adsorption effect and applies to all pore sizes, pressure and

Besides the mass transfer models introduced above, there are some other methods to study the membrane gas transport from a theoretical perspective. Monte Carlo and molecular dynamics are also major techniques to investigate the micropore mass transfer. Because this chapter focused on membrane CCS technology rather than transport phenomena, other sophisticated

The membrane separation for pre-combustion is not a mature technology so far. There has not been industry-scale membrane system. However, a few pilot scale pre-combustion membrane

Eltron Research & Development Inc. developed a pilot-scale pre-combustion membrane with

production from 2005. They employed alloy membrane to separate H2

systems have demonstrated the potential of extending the system to enlarged scale.

The diffusion coefficient expression is obtained from Eqs. (26), (30) and (31)

where *φ*(*r*) is the radial L-J potential, which could be derived from pore structure and gas property. The force in radial direction is the partial derivative of total energy with respect to *r*

∂*E*\_\_\_t

)] + *pr* 2 (*r*) + *p* \_\_*<sup>θ</sup> <sup>r</sup>* <sup>2</sup> (<sup>1</sup> <sup>−</sup> *<sup>r</sup>* <sup>2</sup> \_\_ *r*′2)} 1/2

*RT*(*φ*(*r*) <sup>+</sup> *pr*

2 \_\_\_ <sup>2</sup>*<sup>m</sup>* <sup>+</sup> *<sup>p</sup><sup>θ</sup>* 2 \_\_\_\_

*RT* d *pθ*∫*rc*0(*r*,*pr*

,*pθ*) *rc*1(*r*,*pr*

and *pθ*, we have

momentum is derived from the conservation of total energy or Hamiltonian

at *r*.

,*pθ*) = 0. The radial

. (30)

, *<sup>p</sup>θ*) . (32)

accord-

<sup>2</sup>*<sup>m</sup> <sup>r</sup>* <sup>2</sup>)]. (31)

,*pθ*) d *r*′ \_\_\_\_\_\_\_\_\_\_ *pr* (*r*′ , *r*, *pr*

(*r*′, *r*, *pr*

<sup>∂</sup>*<sup>r</sup>* . (29)

<sup>2</sup>*<sup>m</sup> <sup>r</sup>* <sup>2</sup> , (28)

Membrane separation for post-combustion is a relatively mature technique. In 1995, the largest membrane-based natural gas processing plant in the world was built in Kadanwari, Pakistan. Cellulose acetate membrane was applied in this project to separate CO2 . The Kadanwari system is a two-stage unit designed to treat 25 × 10<sup>5</sup> m3 h−1 of feed gas at 90 bar. The CO2 content is reduced from 12% to less than 3%.

After Kadanwari plant, the Qadirpur plant started in the same year and the processing capacity exceeded Kadanwari plant with 31 × 10<sup>5</sup> m3 h−1 of feed gas at 59 bar. The CO2 content is reduced from 6.5 to 2%. The Qadirpur plant was upgraded to 64 × 10<sup>5</sup> m3 h−1 of feed gas in 2003.

#### **8.3. Membranes for oxy-fuel combustion**

Air separation membrane is still in its early stage. In view of the high energy requirement of ion transport mechanism, air separation membrane can hardly challenge the traditional cryogenic air separation for large scale product.

Air products, which have been developing ion transport membrane technology since 1988 and the DOE (US Department of Energy) are collecting data from a pilot plant near Baltimore in Maryland, with the capacity of 5 tons of oxygen per day. This facility will lead to the next step of designing and building a larger membrane air separation unit (150 tons oxygen per day).

#### **9. Techno-economic of membrane**

The conventional CO2 capture process is absorption (with ammines). Amine-based absorption is the most common technology. However, the corrosion, degradation and high regeneration energy of amine significantly increase the electricity cost. Substantial technological improvements and alternative technologies are highly needed to lower the CO2 capture cost.

The economic indicator CO2 avoided (\$/ton) is an established term for measuring and comparing different CO<sup>2</sup> capture strategies such as absorption, adsorption, cryogenic separation and membrane separation. It is the additional cost of establishing and running a CO2 capture facility for an industrial plant or power plant compared to the respective plant without CO2 capture. The CO2 avoided is expressed as:

\$\text{C.}\$\newline\text{or }\text{\$\text{\$\alpha\$}\$ : \$\text{позоста}\$ ранс с \$\text{рет }\text{рин}\$ спорансте он не переште рани члнос \$\text{°C}\_2\$ (сартне. The CO\${}\_{2}\$\text{avoided}\$ is expressed as: 
$$\text{CO}\_{2}\text{ }\text{avoided} = \frac{\text{LOC}(\text{cap}\text{ге}) - \text{LOC}(\text{ref.})}{\text{CO}\_{2}\text{ }\text{emission} (\text{ref.}) - \text{CO}\_{2}\text{ }\text{emission} (\text{cap}\text{ге})}\text{.}\tag{33}$$

where ref. and capture mean the reference plant without capture and the respective plant with CO2 capture facility. LCOE is the levelized cost of electricity which is expressed as: LCOE <sup>=</sup> sum of cost over lifetime \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ sum of electrical energy produced over lifetime . (34)

$$\text{LCOE} = \frac{\text{sum of cost over lifetime}}{\text{sum of electrical energy produced over lifetime}}.\tag{34}$$

A brief techno-economic comparison was made between two power plants using conventional amine scrubbers in and a power plant using polymer membrane (**Table 13**). The estimates are subject to uncertainty because we cannot accurately predict all input parameters such as fuel price, operational and maintenance cost. The aim of the comparison is not to give absolute costs, but to illustrate indicatively that the costs per ton CO2 avoided. The overall comparison indicates that the case employing membrane separation results in slightly lower LCOE and CO<sup>2</sup> avoided than traditional amine-based solvent scrubbing. Although this cannot judge the membrane economical advantage, the comparison at least indicates that membrane separation is competitive to the amine-based solvent scrubbing. However, significant efforts are still required to improve the membrane properties so as to achieve higher stability, permeate purity and recovery.


**Table 13.** Techno-economic comparisons between amine-based CO2 removal and membrane separation.

#### **Author details**

where ref. and capture mean the reference plant without capture and the respective plant

A brief techno-economic comparison was made between two power plants using conventional amine scrubbers in and a power plant using polymer membrane (**Table 13**). The estimates are subject to uncertainty because we cannot accurately predict all input parameters such as fuel price, operational and maintenance cost. The aim of the comparison is not to give

comparison indicates that the case employing membrane separation results in slightly lower

not judge the membrane economical advantage, the comparison at least indicates that membrane separation is competitive to the amine-based solvent scrubbing. However, significant efforts are still required to improve the membrane properties so as to achieve higher stability,

**University**

Location USA USA USA Coal type Bitcoal Bitcoal Illinois#6 Plant size (MW) 575 600 580

emission (kg/MWh) Reference 811 836 760

Net power output (MW) Reference 528 600 550

Efficiency penalty (%) 9.9 10.9 7 Capital costs (\$/kW) Reference 1696 2104 1727

LCOE (\$/MWh) Reference 62 77 62

**Table 13.** Techno-economic comparisons between amine-based CO2

avoided (\$/ton) 58 71 46

CCS technology Amine-based Amine-based Membrane-based

avoided than traditional amine-based solvent scrubbing. Although this can-

**Electric Power Research Institute**

90 85 90

Capture 107 126 87

Capture 493 550 461

Reference 41.4 40 41.4 Capture 31.5 29.1 34.4

Capture 2759 3516 2627

Capture 104 127 93

removal and membrane separation.

avoided. The overall

**Membrane Technology and Research, Inc**

absolute costs, but to illustrate indicatively that the costs per ton CO2

 capture facility. LCOE is the levelized cost of electricity which is expressed as: LCOE <sup>=</sup> sum of cost over lifetime \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ sum of electrical energy produced over lifetime . (34)

with CO2

LCOE and CO<sup>2</sup>

Designed CO2

Net plant efficiency

(LHV, %)

CO2

rate (%)

CO2

permeate purity and recovery.

84 Recent Advances in Carbon Capture and Storage

capture

**Organization Carnegie Mellon** 

Guozhao Ji and Ming Zhao\*

\*Address all correspondence to: ming.zhao@tsinghua.edu.cn

School of Environment, Tsinghua University, Beijing, China

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#### **Emerging New Types of Absorbents for Postcombustion Carbon Capture Emerging New Types of Absorbents for Postcombustion Carbon Capture**

Quan Zhuang, Bruce Clements and Bingyun Li Quan Zhuang, Bruce Clements and Bingyun Li

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65739

#### **Abstract**

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Carbon capture is the most probable technology in combating anthropogenic increase of CO2 in the atmosphere. Works on developing emerging absorbents for improving carbon capture performance and reducing process energy consumption are actively going on. The most worked‐on emerging absorbents, including liquid‐liquid biphasic, liquid‐solid biphasic, enzymatic, and encapsulated absorbents, already show encouraging results in improved energy efficiency, enhanced CO2 absorption kinetics, increased cyclic CO2 loading, or reduced regeneration temperature. In this chapter, the latest research and development progress of these emerging absorbents are reviewed along with the future directions in moving these technologies to higher‐technology readiness levels.

**Keywords:** postcombustion capture, biphasic absorbent, lipophilic amine, ionic liq‐ uids, amino acids, enzymes, encapsulated

#### **1. Introduction**

Postcombustion carbon capture is considered one of the most promising and feasible technol‐ ogies for reducing carbon dioxide (CO2) emissions from energy‐intensive industries such as coal‐fired power plants. This is because postcombustion carbon capture has relatively higher level of technology readiness, lower energy penalty, and favorable cost compared to other carbon capture technologies (e.g., oxy‐fuel, integrated gasification combined cycle or IGCC) [1]. Conventional aqueous alkanolamine‐based carbon capture adsorbents were developed over half a century ago for natural gas/CO2 separation as well as syngas/CO2 separation, both work

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

at high absorption pressures. Research work has been conducted to extend the conventional absorbents for coal‐fired power plant CO2 capture. These are classified as first‐generation absorbents [2]. However, for the application of coal‐fired power plant CO2 capture, the flue gas is at about ambient atmosphere. This difference of the CO2 absorption operation pressure makes the first‐generation absorbents not satisfactory. Among the traditional alkanolamines, 30 wt% monoethanolamine (MEA) with a cyclic CO2 loading of 4–5 wt% and a regeneration temperature of about 120°C is regarded as a benchmark absorbent [3]. In a continuous operation, a huge volume of the liquid absorbent has to be pumped back and forth between the absorber and the stripper during absorption and regeneration. For regeneration, a significant amount of water in the absorbent (an aqueous solution of MEA) is vaporized in the regenerator to flow upward acting as both a heat transfer agent and a stripping gas. The energy penalty of the regeneration could be as high as 4.2 GJ/tCO2 [4]. The power generation efficiency would be reduced by about eight percentage points from a range of 28–34% to 20–26%. Therefore, research efforts are continuing in the hope to improve carbon capture performance and to reduce energy penalties. As the third‐generation absorbents [2] (second generation: demonstration in 2020–2025 time frame; third generation: at early development stage), biphasic absorbents (liquid‐liquid as well as liquid‐solid phase change), enzymatic‐enhanced, and encapsulated absorbents are attracting ever increasing research interest [5–8].

This chapter presents a review on these emerging absorbents and identifies directions for further research at pilot scale and beyond. We will examine the achievement on the CO2 absorption energy efficiency, enhanced CO2 absorption kinetics, and increased cyclic CO2 loading or lower regeneration temperature.

#### **2. Liquid‐liquid biphasic absorbent systems**

Liquid‐liquid biphasic absorbent systems generally have one liquid phase fed into an absorber while upon CO2 absorption or increase of temperature, the absorbent turns into two immiscible liquid phases (one CO2‐rich and the other CO2‐lean phases) [9, 10]. Because of the separation of the two liquid phases, during regeneration, only the CO2‐rich phase, a smaller flow than in a conventional alkanolamine case, is sent to the stripper/regenerator. The CO2‐lean phase is mixed with the regenerated stream (now in lean state) and sent back to the absorber to perform another round of CO2 absorption. By doing so, the regeneration heat consumption can be drastically reduced and therefore, compared to the conventional postcombustion carbon capture absorbents, biphasic absorbent systems may reduce energy consumption and capital cost (requiring relatively smaller strippers).

It is found that, up to 2016, the active developers of the liquid‐liquid biphasic absorbents are 3H Company, IFP Energies nouvelles, Korea Institute of Energy Research, Norwegian Uni‐ versity of Technology, Tsinghua University, and University of Dortmund. In August 2015, DOE approved and funded 16 transformative carbon capture projects, two of which were on biphasic absorbents [11]. It is likely that more biphasic absorbent work will be published in the next few years.

#### **2.1. Mechanism of liquid‐liquid phase separation**

at high absorption pressures. Research work has been conducted to extend the conventional absorbents for coal‐fired power plant CO2 capture. These are classified as first‐generation absorbents [2]. However, for the application of coal‐fired power plant CO2 capture, the flue gas is at about ambient atmosphere. This difference of the CO2 absorption operation pressure makes the first‐generation absorbents not satisfactory. Among the traditional alkanolamines, 30 wt% monoethanolamine (MEA) with a cyclic CO2 loading of 4–5 wt% and a regeneration temperature of about 120°C is regarded as a benchmark absorbent [3]. In a continuous operation, a huge volume of the liquid absorbent has to be pumped back and forth between the absorber and the stripper during absorption and regeneration. For regeneration, a significant amount of water in the absorbent (an aqueous solution of MEA) is vaporized in the regenerator to flow upward acting as both a heat transfer agent and a stripping gas. The energy penalty of the regeneration could be as high as 4.2 GJ/tCO2 [4]. The power generation efficiency would be reduced by about eight percentage points from a range of 28–34% to 20–26%. Therefore, research efforts are continuing in the hope to improve carbon capture performance and to reduce energy penalties. As the third‐generation absorbents [2] (second generation: demonstration in 2020–2025 time frame; third generation: at early development stage), biphasic absorbents (liquid‐liquid as well as liquid‐solid phase change), enzymatic‐enhanced, and encapsulated absorbents are attracting

This chapter presents a review on these emerging absorbents and identifies directions for further research at pilot scale and beyond. We will examine the achievement on the CO2 absorption energy efficiency, enhanced CO2 absorption kinetics, and increased cyclic CO2

Liquid‐liquid biphasic absorbent systems generally have one liquid phase fed into an absorber while upon CO2 absorption or increase of temperature, the absorbent turns into two immiscible liquid phases (one CO2‐rich and the other CO2‐lean phases) [9, 10]. Because of the separation of the two liquid phases, during regeneration, only the CO2‐rich phase, a smaller flow than in a conventional alkanolamine case, is sent to the stripper/regenerator. The CO2‐lean phase is mixed with the regenerated stream (now in lean state) and sent back to the absorber to perform another round of CO2 absorption. By doing so, the regeneration heat consumption can be drastically reduced and therefore, compared to the conventional postcombustion carbon capture absorbents, biphasic absorbent systems may reduce energy consumption and capital

It is found that, up to 2016, the active developers of the liquid‐liquid biphasic absorbents are 3H Company, IFP Energies nouvelles, Korea Institute of Energy Research, Norwegian Uni‐ versity of Technology, Tsinghua University, and University of Dortmund. In August 2015, DOE approved and funded 16 transformative carbon capture projects, two of which were on biphasic absorbents [11]. It is likely that more biphasic absorbent work will be published in

ever increasing research interest [5–8].

92 Recent Advances in Carbon Capture and Storage

loading or lower regeneration temperature.

cost (requiring relatively smaller strippers).

the next few years.

**2. Liquid‐liquid biphasic absorbent systems**

The solubility or liquid‐liquid phase separation in aqueous amine systems is determined by the relative strength of the molecular interactions among the amine molecules, among the water molecules, and between the amine and the water molecules [12, 13]; nonaqueous liquid‐ liquid biphasic absorbent would follow the same principle. For instance, in a system of pentane and water, the interaction of pentane with water is weaker than the interaction among water molecules, and therefore pentane does not dissolve well in water. By contrast, in a system of ethanol and water, the interaction of ethanol and water is stronger than the interaction among ethanol molecules and as a result ethanol dissolves well in water. The relative strength of the molecular interactions is known to be influenced by temperature, and change in temperature could turn on a liquid‐liquid phase separation from a homogeneous solution, or vice versa, i.e., two liquid phases merging into one homogeneous liquid phase [9]. The possibilities of solubility or phase separation are summarized in **Figure 1** [12, 14]. The real situation of an amine and water could be complicated. Only systems with a lower critical solution temperature (LCST, case C) and both upper critical solution temperature (UCST) and LCST (case B), are potentially suitable for phase separation absorbents. When temperature is increased to above LCST, by breakdown of strong cohesive interactions between the solute and solvent [15], a homogeneous solution changes into two immiscible liquid phases (cases B and C in **Figure 1**). Since postcombustion carbon capture is operated at about 40°C, the LCST of a phase separation absorbent should be higher than the absorption temperature. When CO2 is absorbed, new chemical species (e.g., carbamate, protonated amine, and carbonate and/or bicarbonate ions) are formed. These new species may lower the LCST of the system and result in phase separation at CO2 absorption temperature.

**Figure 1.** Partial miscibility curves of binary liquid‐liquid mixtures. (A) UCST; (B) UCST, and LCST; (C) LCST [15].

#### **2.2. Nonaqueous liquid‐liquid biphasic absorbents**

3H Company filed seven patents on nonaqueous solution of amine dissolved in an alcohol as self‐concentrating absorbents [16]. The amines described in their patents include alamine 336, dibutylamine, diethanolamine (DEA), diisopropylamine, MEA, and piperazine, and the alcohols used as solvents include decylalcohol and isooctanol. Yeo Il Yoon's group at the Korea Institute of Energy Research reported a study on absorbent systems of MEA, DEA (diethanol‐ amine) in 1‐heptanol, 1‐octanol, and isooctanol [17]. They found that, using a bubbling tube at 40°C with 30% CO2 in N2, the absorbent changed into two immiscible liquid phases when CO2 was absorbed. The CO2‐rich phase was found to be dominant with amine‐bonded CO2 and unreacted amine, while the CO2‐lean phase was mainly alcohol with a small amount of free amine. Proton nuclear magnetic resonance (1 H NMR) characterization further showed that MEA or DEA carbamate and protonated amines existed in the rich phase, possibly in ion pairs such as MEACOO‐ MEAH+ .

In one of Hu's studies, the CO2‐rich phase had a CO2 loading of about 27 wt% with a volume only about 30% of the aqueous MEA case [18]. A batch mode of the rich phase stripping of a nonaqueous biphasic absorbent at 115–125°C indicated deeper regenerability down to about 90% of the absorbed CO2 compared with that of the aqueous MEA absorbent, a regeneration of about 50%. In Hu's study, the regenerated stream, now mainly MEA, was combined with the lean stream (mainly alcohol) and sent back to the absorber. Therefore, the potential net cyclic CO2 loading (the ratio of the weight of CO2 released in regeneration to the weight of the absorbent) would double that of the aqueous amine absorbent, and by regenerating the CO2‐ rich phase only (a much smaller volume compared to the whole absorbent), less thermal energy consumption and a smaller stripper are expected.

In Hu's study of biphasic absorbents [18], the boiling points of the alcohols were within 176– 195°C, which was higher than the regeneration temperature and the alcohols did not evaporate. Improvements could be made by applying, e.g., a stream of CO2 from the regenerator as a stripping gas, using a much smaller reboiler to raise the temperature of this CO2 stream 20– 30°C higher than the regeneration temperature, and feeding the gas directly into the regener‐ ator. Meanwhile, the energy efficiency of the self‐concentrating systems should be examined and compared with the conventional MEA technology.

In addition, in these biphasic absorbent systems, the absorbents are nonaqueous and likely will absorb the moistures from flue gases and may cause potential problems including mutual dissolution amongst amine‐alcohol‐water and may need for water separation. The relatively high viscosity of the absorbents is another concern; the viscosity of alcohol amine absorbents was found to increase upon CO2 absorption [10].

#### **2.3. Aqueous liquid‐liquid biphasic absorbents**

There are more works reported on aqueous liquid‐liquid biphasic absorbents. A group from the University of Dortmund theoretically analyzed the solubility of liquid lipophilic amines in water. Different from the hydroxyl group‐bearing conventional alkanolamines, lipophilic amines have relatively lower solubility in water. The amines screened are listed in **Table 1** [12, 13], tested either alone or mixed.


dibutylamine, diethanolamine (DEA), diisopropylamine, MEA, and piperazine, and the alcohols used as solvents include decylalcohol and isooctanol. Yeo Il Yoon's group at the Korea Institute of Energy Research reported a study on absorbent systems of MEA, DEA (diethanol‐ amine) in 1‐heptanol, 1‐octanol, and isooctanol [17]. They found that, using a bubbling tube at 40°C with 30% CO2 in N2, the absorbent changed into two immiscible liquid phases when CO2 was absorbed. The CO2‐rich phase was found to be dominant with amine‐bonded CO2 and unreacted amine, while the CO2‐lean phase was mainly alcohol with a small amount of

MEA or DEA carbamate and protonated amines existed in the rich phase, possibly in ion pairs

In one of Hu's studies, the CO2‐rich phase had a CO2 loading of about 27 wt% with a volume only about 30% of the aqueous MEA case [18]. A batch mode of the rich phase stripping of a nonaqueous biphasic absorbent at 115–125°C indicated deeper regenerability down to about 90% of the absorbed CO2 compared with that of the aqueous MEA absorbent, a regeneration of about 50%. In Hu's study, the regenerated stream, now mainly MEA, was combined with the lean stream (mainly alcohol) and sent back to the absorber. Therefore, the potential net cyclic CO2 loading (the ratio of the weight of CO2 released in regeneration to the weight of the absorbent) would double that of the aqueous amine absorbent, and by regenerating the CO2‐ rich phase only (a much smaller volume compared to the whole absorbent), less thermal energy

In Hu's study of biphasic absorbents [18], the boiling points of the alcohols were within 176– 195°C, which was higher than the regeneration temperature and the alcohols did not evaporate. Improvements could be made by applying, e.g., a stream of CO2 from the regenerator as a stripping gas, using a much smaller reboiler to raise the temperature of this CO2 stream 20– 30°C higher than the regeneration temperature, and feeding the gas directly into the regener‐ ator. Meanwhile, the energy efficiency of the self‐concentrating systems should be examined

In addition, in these biphasic absorbent systems, the absorbents are nonaqueous and likely will absorb the moistures from flue gases and may cause potential problems including mutual dissolution amongst amine‐alcohol‐water and may need for water separation. The relatively high viscosity of the absorbents is another concern; the viscosity of alcohol amine absorbents

There are more works reported on aqueous liquid‐liquid biphasic absorbents. A group from the University of Dortmund theoretically analyzed the solubility of liquid lipophilic amines in water. Different from the hydroxyl group‐bearing conventional alkanolamines, lipophilic amines have relatively lower solubility in water. The amines screened are listed in **Table 1** [12,

H NMR) characterization further showed that

free amine. Proton nuclear magnetic resonance (1

94 Recent Advances in Carbon Capture and Storage

MEAH+ .

consumption and a smaller stripper are expected.

and compared with the conventional MEA technology.

was found to increase upon CO2 absorption [10].

**2.3. Aqueous liquid‐liquid biphasic absorbents**

13], tested either alone or mixed.

such as MEACOO‐


**Table 1.** Lipophilic amines screened in the University of Dortmund's study [9, 12, 13].

Among the lipophilic amines tested, no candidates were found to be suitable as liquid‐liquid phase separation absorbents because of their low CO2 loading, lack of phase separation, or, complicated phase change behaviors or solid precipitation upon CO2 absorption. By mixing two different amines (the so‐called bi‐amine systems), however, would enable a CO2 capture performance that neither of the two components would show alone, such as phase separation, because the physical and chemical properties of the two components may supplement each other. In many cases, one (the so‐called activator) of the two amine components has a relatively higher CO2 absorption capacity and the other one (the so‐called promoter) functions to improve phase separation and/or to enhance reaction rates of absorption or regeneration. Two mixtures (i.e., MCA‐DSBA and DPA‐DMCA) with a ratio of 3:1 have been identified at an optimum total concentration of 3‐4 M (**Table 1**). The CO2 absorption isotherms of these two mixtures were evaluated and compared with those of aqueous MEA [12]. At 40°C (the typical postcombustion capture temperature), both MCA‐DSBA and DPA‐DMCA were found to have higher CO2 loadings compared to MEA (**Figure 2**). Due to its favorable CO2 absorption isotherm, MCA‐DSBA performed a little more superior to DPA‐DMCA. By contrast, at 65– 70°C (could be used as regeneration temperature), the two mixture systems had much lower CO2 loading than MEA (even at 120°C). This indicated that the cyclic loading of these mixture systems may reach about 10 wt% at a lower regeneration temperature of 65–70°C and could double the cyclic loading of MEA at 120°C. Therefore, the bi‐amine systems may have the potential to achieve better regeneration (up to 90%).

Besides the two‐component bi‐amine mixtures, three‐component mixtures can also be developed. Since some aqueous lipophilic amines have LCST lower than 40°C, the absorbent could be in two liquid phases before CO2 absorption takes place. A solubilizer could be added to increase the LCST. One of the examples is DMCA‐MCA‐AMP in a ratio of 3:1:1 (see **Table 1**), where AMP was used as a solubilizer to increase the LCST (to > 40°C) of DMCA‐ MCA [13]. It was found that the CO2 loading of DMCA‐MCA‐AMP was 3 mol/L (ca. 13.2 wt %) at 40°C and at a CO2 partial pressure of 0.15 bar and, at 75°C, over 90% of CO2 was regen‐

erated. Therefore, it seems that the addition of AMP led to the increase of LCST about 15–20°C but without impairing the CO2 absorption and desorption performance.

**Type Chemical Abbreviation Molar mass**

Among the lipophilic amines tested, no candidates were found to be suitable as liquid‐liquid phase separation absorbents because of their low CO2 loading, lack of phase separation, or, complicated phase change behaviors or solid precipitation upon CO2 absorption. By mixing two different amines (the so‐called bi‐amine systems), however, would enable a CO2 capture performance that neither of the two components would show alone, such as phase separation, because the physical and chemical properties of the two components may supplement each other. In many cases, one (the so‐called activator) of the two amine components has a relatively higher CO2 absorption capacity and the other one (the so‐called promoter) functions to improve phase separation and/or to enhance reaction rates of absorption or regeneration. Two mixtures (i.e., MCA‐DSBA and DPA‐DMCA) with a ratio of 3:1 have been identified at an optimum total concentration of 3‐4 M (**Table 1**). The CO2 absorption isotherms of these two mixtures were evaluated and compared with those of aqueous MEA [12]. At 40°C (the typical postcombustion capture temperature), both MCA‐DSBA and DPA‐DMCA were found to have higher CO2 loadings compared to MEA (**Figure 2**). Due to its favorable CO2 absorption isotherm, MCA‐DSBA performed a little more superior to DPA‐DMCA. By contrast, at 65– 70°C (could be used as regeneration temperature), the two mixture systems had much lower CO2 loading than MEA (even at 120°C). This indicated that the cyclic loading of these mixture systems may reach about 10 wt% at a lower regeneration temperature of 65–70°C and could double the cyclic loading of MEA at 120°C. Therefore, the bi‐amine systems may have the

Besides the two‐component bi‐amine mixtures, three‐component mixtures can also be developed. Since some aqueous lipophilic amines have LCST lower than 40°C, the absorbent could be in two liquid phases before CO2 absorption takes place. A solubilizer could be added to increase the LCST. One of the examples is DMCA‐MCA‐AMP in a ratio of 3:1:1 (see **Table 1**), where AMP was used as a solubilizer to increase the LCST (to > 40°C) of DMCA‐ MCA [13]. It was found that the CO2 loading of DMCA‐MCA‐AMP was 3 mol/L (ca. 13.2 wt %) at 40°C and at a CO2 partial pressure of 0.15 bar and, at 75°C, over 90% of CO2 was regen‐

Other amines Monoethanolamine MEA 61.08

**Table 1.** Lipophilic amines screened in the University of Dortmund's study [9, 12, 13].

96 Recent Advances in Carbon Capture and Storage

potential to achieve better regeneration (up to 90%).

N‐Methylpiperidine MPD 99.17 N‐Ethylpiperidine EPD 113.2

N‐Methyldiethanolamine MDEA 119.16 2‐Amino‐2‐methyl‐1‐propanol AMP 89.14 2‐Amino‐2‐methyl‐1,3‐propanediol AMPD 105.14 N,N,N',N'‐Tetramethyl‐1,6‐hexanediami TMHDA 172.31 N‐Methylmorpholine MMP 101.15 N,N‐dimethyl‐1,3‐propanediamine DMPDA 102.18 Piperazine pZ 86.14

**Figure 2.** Loading curves (CO2 absorption isotherms) of 3M 1:3 MCA:DSBA and DPA:DMCA at absorption and regen‐ eration temperatures [12].

The Dortmund group also disclosed some coded proprietary biphasic absorbents [19, 20].

IFP Energies nouvelles has screened a large number of amines and identified DMX‐1 [21]. DMX (implying de‐mixing, i.e., phase separation) is an aqueous amine solution [22] (US 8,361,424 B2, US 8,500,865 B2, US 8,562,927 B2, US 2011/0185901 A1, WO2007/104856 A1, and US 2007/0286783 A1). After absorbing CO2 at 40°C, the absorbent is heated to achieve phase separation and form a CO2‐rich phase and a CO2‐lean phase. In the subsequent regeneration process, a decanter is installed between the cross heat exchanger of lean (outlet stream from the stripper) and rich phase (CO2‐loaded stream from the absorber), and the regeneration is operated at 90°C under which two phases are formed. The CO2‐rich phase, up to 75% of the absorbed CO2, obtained in the decanter is sent to the stripper and the remaining CO2 is stripped in the stripper. This process could reduce the stripping burden thereby enhancing the regen‐ eration efficiency [23].

In another report, two amine solutions (Amine B and Amine D) of a single tertiary alkanola‐ mine with high dielectric constant have been studied [24]. The high dielectric constant of the alkanolamine is believed to trigger phase separation and to prevent solid precipitation. It has been reported that their CO2 absorption loadings at 40°C and 0.1 bar are comparable to that of MEA (**Table 2**), although the CO2 loading in the CO2‐rich phase is not as concentrated as that with the 3H absorbents [18, 25]. Their CO2 absorption isotherm is shown to be different from that of MEA (**Figure 3**) with a sharp decrease of the CO2 loading in the low‐pressure region, which is similar to the absorbents developed by the Dortmund's group (**Figure 2**). This feature makes it possible to achieve high cyclic loadings (e.g., 10.56–14.08 wt%). Compared to MEA technology, the use of DMX‐1 could lead to 3.8% increase in power plant efficiency and 15.4% reduction in cost [26].


**Table 2.** IFP DMX absorbent performance (data from [24]) [9].

**Figure 3.** Partial pressure of CO2 versus loading of a 30% wt MEA (□), a 30% wt molecule B (▲), 30% (◊) and 50% wt (●) molecule D aqueous solutions, at 40°C [24].

In another two studies, approximately 30 aqueous amines (lipophilic amines and alkanola‐ mines) and the combination of them were screened and shown in **Table 3** (The amines appeared in **Table 1** are not included in **Table 3**) [27, 28]. Two promising examples were identified as mixtures of 2M BDA/4M DEEA (2B4D) and 2M DMBA/4M DEEA (2D4D); BDA and DMBA are lipophilic amines, and DEEA is a tertiary alkanolamine. These two mixtures were found to have about 97% of the absorbed CO2 in the lower phase along with a total loading of 0.51 mol CO2/mol amine, and had a cyclic loading of 46% higher than MEA (30 wt%). Their CO2 absorption isotherms were similar to those of the DMX absorbents [24] and the Dort‐ mund's biphasic absorbents [12, 13], and their overall performance was also similar to the DMX absorbents. 1 H NMR phase composition analysis and CO2 absorption kinetics studies showed that the biphasic solvent separation was due to the fast reaction rate of BDA with CO2 and the limited solubility of DEEA in the reaction products. It was concluded that the phase separation was determined by thermal dynamics of all of the species existing in the CO2‐loaded system, the temperature, and pressure of the CO2 (**Table 4**).


**Table 3.** Amines screened (data from [27, 28]) [9].

MEA (**Table 2**), although the CO2 loading in the CO2‐rich phase is not as concentrated as that with the 3H absorbents [18, 25]. Their CO2 absorption isotherm is shown to be different from that of MEA (**Figure 3**) with a sharp decrease of the CO2 loading in the low‐pressure region, which is similar to the absorbents developed by the Dortmund's group (**Figure 2**). This feature makes it possible to achieve high cyclic loadings (e.g., 10.56–14.08 wt%). Compared to MEA technology, the use of DMX‐1 could lead to 3.8% increase in power plant efficiency and 15.4%

**Figure 3.** Partial pressure of CO2 versus loading of a 30% wt MEA (□), a 30% wt molecule B (▲), 30% (◊) and 50% wt

In another two studies, approximately 30 aqueous amines (lipophilic amines and alkanola‐ mines) and the combination of them were screened and shown in **Table 3** (The amines appeared in **Table 1** are not included in **Table 3**) [27, 28]. Two promising examples were identified as mixtures of 2M BDA/4M DEEA (2B4D) and 2M DMBA/4M DEEA (2D4D); BDA

Amine wt% 30 30 50 30 mol CO2 per kg before flash (40°C; 0,1bar CO2) 2.6 2.4 3.2 2.8 CO2 wt% in absorbent 11.44 10.56 14.08 12.32 CO2 % flashed 15 50 65 75 CO2‐rich phase % 89 63 73 Flow reduction in stripper % 11 37 27

**MEA Molecule D Molecule B**

reduction in cost [26].

98 Recent Advances in Carbon Capture and Storage

**Table 2.** IFP DMX absorbent performance (data from [24]) [9].

(●) molecule D aqueous solutions, at 40°C [24].


**Table 4.** Phase composition of CO2‐loaded 2B4D (data from [27, 28]).

An absorbent with a composition of BDA and DEEA the same as those found in the CO2‐rich phase was further studied. Phase separation was observed in this absorbent and the single liquid phase started to become two liquid phases at CO2 loadings at 0.099 mol CO2/mol amine, and from loadings of 0.187 to 0.313 mol CO2/mol amine, BDA further reacted with CO2 while DEEA transferred to the upper phase. Between loadings of 0.313 and 0.345 mol CO2/mol, DEEA reacted with CO2 with the products transferred to lower phase until the equilibrium loading of 0.505 mol CO2/mol amine was achieved [29] (**Table 5**).


Data in the table were read from the graphs in [30]. This operation may introduce an uncertainty of ±5–10%.

**Table 5.** CO2 Absorption and regeneration of aqueous amine or binary amine systems.

To identify absorbents with low regeneration energy, researchers from the Norwegian University of Science and Technology screened multiple aqueous amines or mixture of amines [30]. They found that DMMEA/PZ (3M/1M, 5M/2M), especially 3M/1M, and DMMEA/MEA (5M/2M), had high CO2 absorption rate and characteristics of deep regeneration at low temperature, almost doubling the cyclic loading of the MEA. Further, these researchers developed biphasic absorbents of DEEA/MAPA, tertiary alkanolamine and lipophilic amine [31, 32]. They examined the vapor liquid equilibrium (VLE) of CO2‐DEEA‐MAPA‐H2O system at 40, 60, and 80°C, from which the CO2 absorption loading, absorption phase split ratio, and phase compositions were derived as presented in **Table 6**. The absorbed CO2 was found to be highly concentrated in the CO2‐rich phase, which alone was shown to have a much deeper regeneration hence a higher cyclic loading compared to aqueous MEA (**Figure 4** (Figure 17 in [31]). However, the CO2‐DEEA‐MAPA‐H2O system had a higher viscosity compared to MEA and its CO2 absorption kinetics was faster but dropped down slower with increasing CO2 loading. In their pilot plant trials at a scale of 80–90 m3 /h flue gas with CO2‐rich/CO2‐lean phase separation and CO2‐rich phase regeneration, their system was shown to be superior to aqueous MEA, which was tested in the pilot plant as well [33, 34]. This finding was supported by modeling the biphasic absorbent with an energy consumption of 2.2‐2.4GJ/tCO2 and compared to 3.7GL/tCO2 using 30wt% aqueous MEA [32, 35].


**Table 6.** Phase compositions of DEEA/MAPA (5M/2M) [31, 32].

reacted with CO2 with the products transferred to lower phase until the equilibrium loading

**CO2 absorption capacity (wt%)**

**Cyclic capacity (wt%)**

/h flue gas with CO2‐rich/CO2‐lean phase

**Regeneration depth (%)**

of 0.505 mol CO2/mol amine was achieved [29] (**Table 5**).

DMMEA/MAPA 5 M/1 M 0.47 14.08

**Concentration Initial CO2**

**absorption rate (NL/min)**

AMP/PZ 3 M/1 M 0.47 11.44 0 0 AMP/MAPA 3 M/1 M 0.44 11.44 8.36 73.1 DMMEA/PZ 3 M/1 M 0.44 10.12 9.24 91.3 DMMEA/MAPA 3 M/2 M 0.49 13.2 7.48 56.7 DMMEA/PZ 5 M/2 M 0.48 12.76 11.44 89.7 DMMEA/MEA 5 M/2 M 0.44 10.56 9.68 91.7 DMMEA/MAPA 5 M/2 M 0.49 12.76 9.24 72.4

DEEA/PZ 3 M/1 M 0.5 12.32 0 0.0 DEEA/MEA 5 M/2 M 0.44 6.6 0 0.0 TRIZMA/PZ 3 M/1 M 0.44 7.92 7.04 88.9 TRIZMA/MEA 3 M/1 M 0.33 4.4 3.96 90.0 TRIZMA/MAPA 3 M/1 M 0.43 8.36 5.28 63.2 MEA 5 M 0.48 11.44 4.84 42.3 MAPA 5 M 0.5 19.36 4.84 25.0 Data in the table were read from the graphs in [30]. This operation may introduce an uncertainty of ±5–10%.

**Table 5.** CO2 Absorption and regeneration of aqueous amine or binary amine systems.

loading. In their pilot plant trials at a scale of 80–90 m3

To identify absorbents with low regeneration energy, researchers from the Norwegian University of Science and Technology screened multiple aqueous amines or mixture of amines [30]. They found that DMMEA/PZ (3M/1M, 5M/2M), especially 3M/1M, and DMMEA/MEA (5M/2M), had high CO2 absorption rate and characteristics of deep regeneration at low temperature, almost doubling the cyclic loading of the MEA. Further, these researchers developed biphasic absorbents of DEEA/MAPA, tertiary alkanolamine and lipophilic amine [31, 32]. They examined the vapor liquid equilibrium (VLE) of CO2‐DEEA‐MAPA‐H2O system at 40, 60, and 80°C, from which the CO2 absorption loading, absorption phase split ratio, and phase compositions were derived as presented in **Table 6**. The absorbed CO2 was found to be highly concentrated in the CO2‐rich phase, which alone was shown to have a much deeper regeneration hence a higher cyclic loading compared to aqueous MEA (**Figure 4** (Figure 17 in [31]). However, the CO2‐DEEA‐MAPA‐H2O system had a higher viscosity compared to MEA and its CO2 absorption kinetics was faster but dropped down slower with increasing CO2

**Amine or mixture of amines**

100 Recent Advances in Carbon Capture and Storage

**Figure 4.** The total pressure from lower phase samples with absorption taken at 40°C from the screening apparatus. PCO2: (Δ) 6 kPa, (Ο) 8 kPa, (◊) 10 kPa, and (□) 13 kPa; (green line) MEA at loading 0.5 mol CO2/mol MEA (model from [36]) [31].

In summary, up to now, researchers and developers have achieved encouraging results in the area of liquid‐liquid biphasic CO2 absorbents, and some biphasic absorbents can be regener‐ ated at lower temperatures with deeper regenerability than the bench mark aqueous MEA (**Table 7**).


**Table 7.** Summary of the developed biphasic absorbents for CO2 capture.

## **3. Liquid‐solid biphasic absorbent systems**

There is a category of liquid absorbents forming solid precipitates after CO2 absorption such as carbamate, bicarbonate, or carbonate in solid states. According to Le Chatelier's Principle [37], formation of a solid product during CO2 absorption and its removal from the solution phase shifts the reaction equilibrium toward the production of more products. This phenom‐ enon could be engineered and developed to potentially more efficient carbon capture tech‐ nology.

#### **3.1. Emulsion of alkanolamine and ionic liquid (IL)**

Research has been going on to use ILs as absorbents for CO2 capture because ILs have negligible volatility, nonflammability, high thermal stability, and virtually unlimited chemical tunability. However, stand‐alone, ILs are not competitive enough when compared to CO2 capture efficiency of aqueous alkanolamine systems. An idea is to try hybrid system coupling advan‐ tages of alkanolamines with those of room‐temperature ILs (RTILs) and to achieve potential synergies arising from each of the individual components [38].

A mixture of diethanolamine (DEA) and 1‐alkyl‐3‐methylimidazolium bis(trifluoromethyl‐ sulfonyl)imide, which is hydrophobic, was tested for CO2 absorption (**Figure 5**). This emulsion could capture CO2 up to the stoichiometric maximum through crystalizing CO2‐ bonding product (DEA‐carbamate) while avoiding equilibrium limitations and thus mak‐ ing efficient utilization of the absorbent molecules [39]. Similar precipitation of carbamate Emerging New Types of Absorbents for Postcombustion Carbon Capture http://dx.doi.org/10.5772/65739 103

**Figure 5.** Immiscible alkanolamine/RTIL system for efficient CO2 captures [39].

**Developer Biphasic absorbent Phase separation**

Non‐aqueous, MEA in 1‐heptanol, isooctanol,

MCA/DSBA; DPA/DMCA; DMCA/MCA/AMP

**Table 7.** Summary of the developed biphasic absorbents for CO2 capture.

**3. Liquid‐solid biphasic absorbent systems**

**3.1. Emulsion of alkanolamine and ionic liquid (IL)**

synergies arising from each of the individual components [38].

3H Company Nonaqueous, alkanolamine/ alcohol

102 Recent Advances in Carbon Capture and Storage

1‐octanol

Korean Institute of Energy Research

Dortmund University

IFP Energies nouvelles

Tsinghua University

Norwegian University of S&T

nology.

**temperature**

Absorption temperature, 40°C

Absorption temperature

DMX‐1; Amine B; Amine D 90°C >90°C 90+% [24]

There is a category of liquid absorbents forming solid precipitates after CO2 absorption such as carbamate, bicarbonate, or carbonate in solid states. According to Le Chatelier's Principle [37], formation of a solid product during CO2 absorption and its removal from the solution phase shifts the reaction equilibrium toward the production of more products. This phenom‐ enon could be engineered and developed to potentially more efficient carbon capture tech‐

Research has been going on to use ILs as absorbents for CO2 capture because ILs have negligible volatility, nonflammability, high thermal stability, and virtually unlimited chemical tunability. However, stand‐alone, ILs are not competitive enough when compared to CO2 capture efficiency of aqueous alkanolamine systems. An idea is to try hybrid system coupling advan‐ tages of alkanolamines with those of room‐temperature ILs (RTILs) and to achieve potential

A mixture of diethanolamine (DEA) and 1‐alkyl‐3‐methylimidazolium bis(trifluoromethyl‐ sulfonyl)imide, which is hydrophobic, was tested for CO2 absorption (**Figure 5**). This emulsion could capture CO2 up to the stoichiometric maximum through crystalizing CO2‐ bonding product (DEA‐carbamate) while avoiding equilibrium limitations and thus mak‐ ing efficient utilization of the absorbent molecules [39]. Similar precipitation of carbamate

DEEA/BDA 40°C 90°C 90% [27, 28]

DEEM/MAPA 80°C >80°C ∼90% [31, 34, 35]

**Regeneration temperature**

**Regeneration depth**

Up to 125°C 90% [18, 25]

N/A N/A [17]

60–70°C Up to 75°C 90+% [12, 13]

**Source**

upon CO2 absorption was also observed with ILs such as 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]), 1‐butyl‐3‐methylimidazolium bis(tri‐ fluoromethylsulfonyl)imide ([BMIM][Tf2N]), and 1‐hexyl‐3‐methylimidazolium bis(trifluor‐ omethylsulfonyl)imide ([HMIM][Tf2N]). The density of the solid phase precipitates was lighter thereby quickly rising to the surface and easing the separation for regeneration (**Figure 6a**–**c**). Hydrophobicity of ILs plays a role in the separation of solid products from

**Figure 6.** DEA/RTIL system for CO2 capture: (a–c) (without surfactant) after CO2 capture; (d) (with surfactant) before and after CO2 capture; (e) CO2 capture capacity profiles of the DEA/RTIL system at atmospheric pressure and 25°C; and (f) basic structural unit in DEA‐carbamate (C9H22N2O6) crystal [39].

the liquid phase. When a surfactant, Triton® X‐100, was added to the [HMIM][Tf2N]‐ based system, the carbamate product remained dispersed in the suspension (**Figure 6d**). CO2 loading capacity up to the stoichiometric maximum (0.5 mole of CO2 per mole of DEA) can be achieved. The three absorbents showed similar CO2 uptake rates (**Figure 6e**). The crystallization of the carbamate product, which was composed of protonated‐DEA cation and DEA‐carbamate anion (**Figure 6f**), enabled higher CO2 uptake, and solid pre‐ cipitation may have facilitated the separation thereby offering advantages in regenerating a smaller (only solid carbamate) volume with less energy consumption.

Other systems of aqueous solutions of N‐methyldiethanolamine (MDEA) and guanidinium tris(pentafluoroethyl) trifluorophosphate [gua]+ [FAP]‐ IL showed similar solid formation after the absorption of CO2 at high pressures. The formed CO2‐bonding solid products could be easily regenerated (**Figure 7**) [40].

**Figure 7.** A photo of double layer CO2‐rich mixtures [40].

#### **3.2. Chilled ammonia**

Aqueous ammonia can absorb CO2 to produce solid ammonium carbamate/bicarbonate, which could be separated from the solution thereby allowing an efficient recycling of the unreacted scrubbing solution [31, 41, 42]. Chilled ammonia process could be developed using aqueous ammonia to absorb CO2 at lower temperature (2–10°C), in which the ammonia slip from the absorber could be reduced and the flue gas volume could be smaller [43].

Precipitation of pheromone in ethanol‐water chilled ammonia solution was also observed after CO2 absorption [44]. In this CO2 absorption process, solid mixtures of ammonium bicarbonate and ammonium carbamate, or of ammonium carbamate alone were formed. Selective formation or precipitation of solid ammonium carbamate could be obtained by reacting gaseous CO2 and NH3 in anhydrous ethanol, 1‐propanol, and N,N‐dimethylforma‐ mide (DMF) in a flow reactor that can operate continuously. After filtering the solid precipi‐ tates, the unreacted ammonia solution could be reclaimed into the absorber. Such a chilled ammonia process may be applied to capture CO2 from flue gas of coal‐fired boilers, natural gas combined cycle systems, and other energy heavy industrial applications [45].

#### **3.3. Triethylenetetramine (TETA)/ethanol solution as absorbent**

the liquid phase. When a surfactant, Triton® X‐100, was added to the [HMIM][Tf2N]‐ based system, the carbamate product remained dispersed in the suspension (**Figure 6d**). CO2 loading capacity up to the stoichiometric maximum (0.5 mole of CO2 per mole of DEA) can be achieved. The three absorbents showed similar CO2 uptake rates (**Figure 6e**). The crystallization of the carbamate product, which was composed of protonated‐DEA cation and DEA‐carbamate anion (**Figure 6f**), enabled higher CO2 uptake, and solid pre‐ cipitation may have facilitated the separation thereby offering advantages in regenerating

Other systems of aqueous solutions of N‐methyldiethanolamine (MDEA) and guanidinium

the absorption of CO2 at high pressures. The formed CO2‐bonding solid products could be

Aqueous ammonia can absorb CO2 to produce solid ammonium carbamate/bicarbonate, which could be separated from the solution thereby allowing an efficient recycling of the unreacted scrubbing solution [31, 41, 42]. Chilled ammonia process could be developed using aqueous ammonia to absorb CO2 at lower temperature (2–10°C), in which the ammonia slip from the

Precipitation of pheromone in ethanol‐water chilled ammonia solution was also observed after CO2 absorption [44]. In this CO2 absorption process, solid mixtures of ammonium bicarbonate and ammonium carbamate, or of ammonium carbamate alone were formed. Selective formation or precipitation of solid ammonium carbamate could be obtained by reacting gaseous CO2 and NH3 in anhydrous ethanol, 1‐propanol, and N,N‐dimethylforma‐ mide (DMF) in a flow reactor that can operate continuously. After filtering the solid precipi‐

absorber could be reduced and the flue gas volume could be smaller [43].

[FAP]‐

IL showed similar solid formation after

a smaller (only solid carbamate) volume with less energy consumption.

tris(pentafluoroethyl) trifluorophosphate [gua]+

**Figure 7.** A photo of double layer CO2‐rich mixtures [40].

**3.2. Chilled ammonia**

easily regenerated (**Figure 7**) [40].

104 Recent Advances in Carbon Capture and Storage

When CO2 is absorbed into a solution of triethylenetetramine (TETA) dissolved in ethanol, solid precipitates formed (**Figure 8a** and **b**) in contrast to TETA/water solution [46]. Moreover, TETA/ethanol solution showed improvement in CO2 absorption rate, absorption capacity, and absorbent regenerability. Ethanol not only promoted the solubility of CO2 in the liquid phase but also facilitated the chemical reaction between TETA and CO2. The CO2 capacity of the solid phase as TETA‐carbamate accounted for about 81.8% of the total CO2 absorbed (**Figure 8c**). The TETA/ethanol solution was found to be relatively stable throughout multiabsorption‐ desorption cycles (**Figure 8d**). One hurdle of applying the TETA/ethanol solution for CO2 removal is that ethanol has a high vapor pressure and this must be taken into consideration for further development of this absorbent system and later designing for possible commercial applications.

**Figure 8.** TETA/ethanol solution (a) before CO2 absorption and (b) after CO2 absorption. (c) Partition of carbon dioxide in the solid phase and liquid phase. (d) Cycling absorption/regeneration runs of TETA/ethanol solution for CO2 ab‐ sorption [46].

#### **3.4. Amino acid salt as liquid‐solid phase change absorbent**

Being environmental friendly, ionic nature and low volatile, amino acid salts are of great interest as potential solvents for CO2 capture [47, 48, 49]. Moreover, amino acid salt solutions have good resistance to an oxygen‐rich flue gas stream. The reactivity of amino acid salts with CO2 is similar to those of alkanolamines due to the presence of identical amino functional groups in their molecules. Some of them such as the potassium salts of glycine, sarcosine, and proline, react faster with CO2 than MEA thereby kinetically favorable [50, 51].

Multiple amino‐acid salts were found to precipitate after reacting with CO2 to a certain degree [52]. During the absorption of CO2 in aqueous potassium salts of N‐methylalanine, DL‐alanine, and α‐aminoisobutyric acid, solid precipitates were observed [53]. Various types of solid precipitates could be achieved by varying the amino acid structure and solubility. Amino acids with a primary amino group may form only zwitterion species precipitates [54], while amino acids with a hindered amino group and with relatively high zwitterion solubility (e.g., proline) may form potassium bicarbonate precipitates [55].

By Le Chatelier's Principle, the driving force for CO2 absorption can be maintained at a high level even at high loadings. Thus the absorber performance could be significantly improved. This effect was indicated in **Figure 9** (enhanced absorption) where the possible precipitates were highlighted. In **Figure 9**, besides the heat input necessary to regenerate the solvent, in the case of precipitating amino acids two more effects are possible: Enhanced absorption (purple) due to the precipitation of reaction products during absorption and enhanced

**Figure 9.** Conventional amine‐based process for CO2 capture where the reactions specific to amino acid salts have been added at the bottom of the absorber and the stripper [56].

desorption (red) due to a lower pH that results from increasing the amino acid to K+ ratio in solution [56]. Because of the high loadings, the regeneration energy consumption was reduced [57]. As can be seen from **Figure 10**, at a given CO2 partial pressure, a precipitating‐based process would have higher loading than a conventional absorption process without precipi‐ tation, and a combined process (simultaneous absorption and precipitating process) is expected to result in increased capacity.

groups in their molecules. Some of them such as the potassium salts of glycine, sarcosine, and

Multiple amino‐acid salts were found to precipitate after reacting with CO2 to a certain degree [52]. During the absorption of CO2 in aqueous potassium salts of N‐methylalanine, DL‐alanine, and α‐aminoisobutyric acid, solid precipitates were observed [53]. Various types of solid precipitates could be achieved by varying the amino acid structure and solubility. Amino acids with a primary amino group may form only zwitterion species precipitates [54], while amino acids with a hindered amino group and with relatively high zwitterion solubility (e.g., proline)

By Le Chatelier's Principle, the driving force for CO2 absorption can be maintained at a high level even at high loadings. Thus the absorber performance could be significantly improved. This effect was indicated in **Figure 9** (enhanced absorption) where the possible precipitates were highlighted. In **Figure 9**, besides the heat input necessary to regenerate the solvent, in the case of precipitating amino acids two more effects are possible: Enhanced absorption (purple) due to the precipitation of reaction products during absorption and enhanced

**Figure 9.** Conventional amine‐based process for CO2 capture where the reactions specific to amino acid salts have been

proline, react faster with CO2 than MEA thereby kinetically favorable [50, 51].

may form potassium bicarbonate precipitates [55].

106 Recent Advances in Carbon Capture and Storage

added at the bottom of the absorber and the stripper [56].

**Figure 10.** Schematic picture to depict the difference between a precipitating and a nonprecipitating system in terms of CO2 pressure as a function of loading [58].

**Figure 11** shows a schematic representation of the DECAB process for liquid‐solid phase change amino acid salt absorbent [56]. The flue gas (at 40°C) is contacted with CO2 pre‐ loaded absorbent in a spray‐tower, resulting in that the CO2 undergoes a chemical reaction with the absorbent that leads to the formation of carbamate and carbonate ions, as shown in **Figure 11**. As absorption goes on, the pH of the absorbent solution as well as solubility of the amino acid decreases. Finally, the CO2‐bonding amino‐acid precipitates as an amino acid zwitterion. In the process, the solid precipitates are collected at the bottom of the tow‐ er. The remaining CO2 in the flue gas is captured in the absorption column, where the de‐ pleted flue gas is contacted with lean absorbent. The absorption column is a conventional packed absorption column filled with structured packing. There, the CO2 partial pressure is reduced to the desired value for 90% CO2 removal. The rich stream containing the solids, is further processed in the stripper, via the lean‐rich heat exchanger, to release the CO2. The lean‐rich heat exchanger also needs to be able to handle solids (e.g., spiral heat exchanger). The CO2 absorption depth needs to be controlled so that only in the spray‐tower the solid products are formed [56].

**Figure 11.** DECAB process concept for CO2 capture. Enhanced absorption due to the precipitation of reaction products during absorption is highlighted in purple [56].

#### **4. Enzymatically catalyzed absorbent systems**

A special enzyme, carbonic anhydrase (CA), works in vertebrates' lungs to facilitate oxygen and CO2 exchange through respiration in a very fast and effective way. Attempts to incorporate this type of enzyme to carbon capture absorbent systems have shown encouraging results [59]. When a small quantity of the enzyme is used as catalyst in a CO2 capture absorbent system (usually an aqueous amine absorbent), it enhances the reaction rate and enables rapid approach to equilibrium between dissolved CO2 and HCO3 ‐ in aqueous solutions. The idea is a natural extension of the experience that some enzymes have been successfully deployed to increase the efficiency of other industrial processes [60, 61]. The enhancement effect of the enzyme is so significant that the size of an absorber could be reduced up to 90% smaller than the conventional amine case [61]. However, enzymes are bio active compounds. How to maintain its long period activity is an issue. The Canadian company, *CO2 Solutions*, reported that their developed enzymatic catalytic amine system could work for 15 days for CO2 capture in temperature ranges of 40–70°C [61].

The way to implement enzyme more effectively in CO2 capture process is to support or immobilize it onto some sort of carrier to provide sustained stability under working conditions. It may need to be able to stand for the high regeneration temperature if the enzyme is not confined in the absorber only. Report shows that some developed enzyme systems could stand for moderate regeneration temperatures (e.g., around 70–80°C) [59]. The immobilized enzyme should also be strong toward contaminants encountered in the common flue gas.

A porous organosilica coating containing CA to separate CO2 from a flowing gas stream has been tested [62]. This coating was applied to ceramic random packing and placed in a counter‐ current absorber column, where it demonstrated a high rate enhancement for 400 days at 45°C and a total turnover number of ~48 million moles CO2/mole enzyme. In another development, the same coating formulation was deposited onto stainless steel structured packing. This coating technique was used to produce 275 liters of packing for pilot testing at the National Carbon Capture Center in Wilsonville, AL, on coal‐fired flue gas. This unit operated for nearly 5 months at 40°C in the same carbonate solution and exhibited a steady 80% CO2 capture.

A type of magnetic polymer microspheres functionalized with epoxy group was prepared, and CA enzyme was immobilized on the carriers by selectively covalent binding [63]. The parameters affecting CA immobilization, such as pH, temperature, and enzyme dose were investigated. The kinetic parameters of the immobilized and the free CA were also evaluated. The value of the Michaelis–Menten constant (*K*m) and the maximum velocity (*V*max) of the immobilized CA were 8.077 mmol/L (mM) and 0.027 μmol/(min mL), respectively, while those of the free CA were 6.091 mM and 0.091 μmol/(min mL), respectively. Moreover, the perform‐ ance of the thermal stability, storage stability, and reusability of the immobilized CA confirmed that CA immobilized on the epoxy‐functionalized magnetic polymer microspheres possessed a stable and efficient catalytic ability on CO2 hydration, which seemed to be a suitable candidate for CO2 capture [63].

In another development, new materials of Fe3O4 magnetic microspheres were functionalized with carboxyl groups and prepared for CA immobilization to capture CO2. The optimum conditions for immobilization, such as carrier dose, enzyme dose, pH, shaking speed, tem‐ perature, and contact time, were examined. The pH and thermal stability of the free and the immobilized CA were compared. The results showed that the immobilized CA had a better enzyme activity, a higher pH and thermal stability than those of free CA. Meanwhile, CO2 capture was significantly enhanced by the free and immobilized CA in tris(hydroxymethyl) aminomethane (Tris) buffer solution. Moreover, the immobilized CA maintained 58.5% of its initial catalytic activity after 10 recovery cycles due to the protection effect of the magnetic microspheres. All the results confirmed the potential merits of using the carboxyl‐functional‐ ized Fe3O4 magnetic microspheres immobilized CA to remove CO2 from air or flue gas [64].

#### **5. Encapsulated absorbents**

The CO2 absorption depth needs to be controlled so that only in the spray‐tower the solid

**Figure 11.** DECAB process concept for CO2 capture. Enhanced absorption due to the precipitation of reaction products

A special enzyme, carbonic anhydrase (CA), works in vertebrates' lungs to facilitate oxygen and CO2 exchange through respiration in a very fast and effective way. Attempts to incorporate this type of enzyme to carbon capture absorbent systems have shown encouraging results [59]. When a small quantity of the enzyme is used as catalyst in a CO2 capture absorbent system (usually an aqueous amine absorbent), it enhances the reaction rate and enables rapid approach

extension of the experience that some enzymes have been successfully deployed to increase the efficiency of other industrial processes [60, 61]. The enhancement effect of the enzyme is so significant that the size of an absorber could be reduced up to 90% smaller than the conventional amine case [61]. However, enzymes are bio active compounds. How to maintain its long period activity is an issue. The Canadian company, *CO2 Solutions*, reported that their developed enzymatic catalytic amine system could work for 15 days for CO2 capture in

The way to implement enzyme more effectively in CO2 capture process is to support or immobilize it onto some sort of carrier to provide sustained stability under working conditions. It may need to be able to stand for the high regeneration temperature if the enzyme is not confined in the absorber only. Report shows that some developed enzyme systems could stand for moderate regeneration temperatures (e.g., around 70–80°C) [59]. The immobilized enzyme

should also be strong toward contaminants encountered in the common flue gas.

‐ in aqueous solutions. The idea is a natural

products are formed [56].

108 Recent Advances in Carbon Capture and Storage

during absorption is highlighted in purple [56].

**4. Enzymatically catalyzed absorbent systems**

to equilibrium between dissolved CO2 and HCO3

temperature ranges of 40–70°C [61].

Many attractive options for carbon capture solvents suffer from high viscosity, making it difficult to generate large surface areas for fast absorption, and amine‐based aqueous liquids suffer from potential environmental impacts from ammonia (product of amine decomposi‐ tion), and amine vapor release. Microencapsulated carbon sorbents (MECSs) are a new class of carbon capture materials consisting of a CO2‐absorbing liquid absorbent contained or confined within solid, CO2‐permeable polymer shells. As part of a US‐DOE ARPA‐E program, a team from the University of Illinois Urbana‐Champaign, Babcock and Wilcox, and the Lawrence Livermore National Laboratory has created this new type of encapsulated form of carbon capture absorbent in which the operating fluid, amines, or carbonates in the tests are enclosed in a thin polymer shell forming 200–400 μm beads [65]. While mass transport across the polymer shell is reduced compared to the bulk liquid, the large surface area of the beads improves overall mass transfer more than off‐setting this disadvantage. The liquid, as well as any degradation products or precipitates, remain encapsulated within the beads, which can be thermally regenerated repeatedly. Encapsulated absorbents have the capacity of the liquid absorbents as well as the physical behavior of solid sorbents. It could be imagined for them to be useful in fairly conventional‐style capture applications, as well as unprecedentedly new approaches facilitated by their high surface area. The developed beads appear to be both chemically and mechanically stable under typical industrial conditions. There are engineering constraints that the beads must satisfy for several application strategies, including their use in fluidized beds and these should be further studied. The US group has encapsulated MEA, piperazine, and a variety of other carbonate solutions, which appear to be optimal for this application, demonstrating rapid CO2 uptake and desorption using colorimetric methods, which permit rapid spectroscopic determination of the extent of CO2 uptake and release. Carbonate capsules are created using a silicone polymer shell which is both rugged and permeable to CO2. Results of mechanical/thermal cycling tests demonstrate long‐term stability of silicone‐encapsulated carbonate [65].

Especially, MECS enhances the rate of CO2 absorption for solvents with slow kinetics and prevent solid precipitates from scaling and fouling equipment, two factors that have previ‐ ously limited the use of sodium carbonate solution for carbon capture. Researchers have examined the thermodynamics of sodium carbonate slurries for carbon capture [66]. Modeling work has been carried out on the vapor‐liquid‐solid equilibria of sodium carbonate and several features that can contribute to an energy‐efficient capture process have been derived: very high CO2 pressures in stripping conditions, relatively low water vapor pressures in stripping conditions, and good swing capacity. These would make a more effective and efficient CO2 absorption and desorption cycle. The high potential energy savings have been indicated compared with an MEA system [66].

#### **6. Direction for further development**

#### **6.1. Liquid‐liquid biphasic absorbent**

The following aspects are recommended for future studies:

**•** As to liquid‐liquid biphasic absorbent systems that have already been identified, detailed, and comprehensive CO2 absorption and kinetics studies are much needed. Continuous mass transfer from CO2‐lean phase to CO2‐rich phase, diffusion of multispecies within the absorbent, and viscosity should be considered as CO2 absorption continues. Based on knowledge obtained from these current amines studied, screening, designing, and synthe‐ sizing of new amines with improved properties are possible.


#### **6.2. Other emerging absorbents**

carbon capture absorbent in which the operating fluid, amines, or carbonates in the tests are enclosed in a thin polymer shell forming 200–400 μm beads [65]. While mass transport across the polymer shell is reduced compared to the bulk liquid, the large surface area of the beads improves overall mass transfer more than off‐setting this disadvantage. The liquid, as well as any degradation products or precipitates, remain encapsulated within the beads, which can be thermally regenerated repeatedly. Encapsulated absorbents have the capacity of the liquid absorbents as well as the physical behavior of solid sorbents. It could be imagined for them to be useful in fairly conventional‐style capture applications, as well as unprecedentedly new approaches facilitated by their high surface area. The developed beads appear to be both chemically and mechanically stable under typical industrial conditions. There are engineering constraints that the beads must satisfy for several application strategies, including their use in fluidized beds and these should be further studied. The US group has encapsulated MEA, piperazine, and a variety of other carbonate solutions, which appear to be optimal for this application, demonstrating rapid CO2 uptake and desorption using colorimetric methods, which permit rapid spectroscopic determination of the extent of CO2 uptake and release. Carbonate capsules are created using a silicone polymer shell which is both rugged and permeable to CO2. Results of mechanical/thermal cycling tests demonstrate long‐term stability

Especially, MECS enhances the rate of CO2 absorption for solvents with slow kinetics and prevent solid precipitates from scaling and fouling equipment, two factors that have previ‐ ously limited the use of sodium carbonate solution for carbon capture. Researchers have examined the thermodynamics of sodium carbonate slurries for carbon capture [66]. Modeling work has been carried out on the vapor‐liquid‐solid equilibria of sodium carbonate and several features that can contribute to an energy‐efficient capture process have been derived: very high CO2 pressures in stripping conditions, relatively low water vapor pressures in stripping conditions, and good swing capacity. These would make a more effective and efficient CO2 absorption and desorption cycle. The high potential energy savings have been indicated

**•** As to liquid‐liquid biphasic absorbent systems that have already been identified, detailed, and comprehensive CO2 absorption and kinetics studies are much needed. Continuous mass transfer from CO2‐lean phase to CO2‐rich phase, diffusion of multispecies within the absorbent, and viscosity should be considered as CO2 absorption continues. Based on knowledge obtained from these current amines studied, screening, designing, and synthe‐

of silicone‐encapsulated carbonate [65].

110 Recent Advances in Carbon Capture and Storage

compared with an MEA system [66].

**6.1. Liquid‐liquid biphasic absorbent**

**6. Direction for further development**

The following aspects are recommended for future studies:

sizing of new amines with improved properties are possible.

For liquid‐solid phase change absorbents, one of the biggest challenges related to their applications is to have a process design that could handle solids/precipitates transportation, heat exchanging, and regeneration. Process designs like the ones (i.e., DECAB process, DECAB Plus process) proposed for liquid‐solid phase change amino acid salt absorbent could be adapted for other liquid‐solid phase change systems as well [58].

Enzymes are macromolecular proteins and their current research has been focusing on their effects on CO2 capture performance. The aspects of their active life span, the process of mass production or extraction from natural sources, and the related costs should be further explored.

For the encapsulated absorbents, within capsule kinetics and overall mass transfer studies should be pursued. The technology of mass production and the costs are also key factors in determining its commercial viability for CO2 capture application. Techno‐economic feasibility studies are keenly welcome.

In summary, most of these technologies are still at laboratory research stage, and there remain challenges associated with the scale‐up of these technologies to meet the needs of CO2 capture from power generation as well as other energy heavy industries. Future efforts should be focused on developing basic theoretical and mechanistic understandings of phase change, mass transfer, and CO2 absorption and desorption phenomena, to perform pilot plant testing to generate design parameters and process requirements, and to create in parallel techno‐ economic plant design fundamentals and packages for proving the feasibilities of these emerging carbon capture absorbents.

## **Acknowledgements**

Financial supports from EcoEII program, OERD (Office of Energy Research & Development), NRCan (Natural Resources Canada), and WV NASA EPSCoR are greatly acknowledged.

## **Author details**

Quan Zhuang1\*, Bruce Clements1 and Bingyun Li2,3

\*Address all correspondence to: quan.zhuang@canada.ca

1 Natural Resources Canada, CanmetENERGY, Ottawa, Canada

2 National Energy Technology Laboratory‐Regional University Alliance, Morgantown, West Virginia, USA

3 Department of Orthopaedics, School of Medicine, West Virginia University, Morgantown, West Virginia, USA

#### **References**


[10] Zhuang, Q., Clements, B. "CO2 Capture by Biphasic Absorbent—Absorption Perform‐ ance and VLE Characteristics," in preparation, 2016.

**Author details**

Virginia, USA

**References**

West Virginia, USA

Quan Zhuang1\*, Bruce Clements1

112 Recent Advances in Carbon Capture and Storage

Forum; 16 December 2015.

November 2010.

2015.

\*Address all correspondence to: quan.zhuang@canada.ca

1 Natural Resources Canada, CanmetENERGY, Ottawa, Canada

and Bingyun Li2,3

2 National Energy Technology Laboratory‐Regional University Alliance, Morgantown, West

3 Department of Orthopaedics, School of Medicine, West Virginia University, Morgantown,

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#### **Bio-inspired Systems for Carbon Dioxide Capture, Sequestration and Utilization Bio-inspired Systems for Carbon Dioxide Capture, Sequestration and Utilization**

Gonçalo V. S. M. Carrera, Luís C. Branco and Manuel Nunes da Ponte Gonçalo V. S. M. Carrera, Luís C. Branco and Manuel Nunes da Ponte

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65861

#### **Abstract**

[54] Kumar, P. S., Hogendoorn, J. A., Timmer, S. J., Feron, P. H. M., Versteeg, G. F. Equili‐ brium solubility of CO2 in aqueous potassium taurate solutions: Part 2. Experimental

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[56] Sanchez‐Fernandez, E., Mercader, F. D. M., Misiak, K., Van D. H, L., Linders, M., Goetheer, E. New process concepts for CO2 capture based on precipitating amino acids.

[57] Brouwer, J., Feron, P., Ten A., N. Amino‐acid salts for CO2 capture from flue gases. Fourth Annual Conference on Carbon Capture and Sequestration, Alexandria, Virgin‐

[58] Fernandez, E. S., Goetheer, E. L. DECAB: Process development of a phase change

[59] Sonja, S., House, L. "Chapter 2–Enzyme‐Catalyzed Solvents for CO2 Separation, Novel Materials for Carbon Dioxide Mitigation Technology," Elsevier, Amsterdam, 2015. [60] Nguyen, L. Low‐Cost Enzyme‐Based Technology for Carbon Capture, 2012 NETL

[61] Carley, J. A. "Enzyme‐Enabled Carbon Capture, Lowering the CCS Cost Barrier,"

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[63] Jing, G., et al., Immobilization of carbonic anhydrase on epoxy‐functionalized magnetic polymer microspheres for CO2 capture. Process Biochem. 50(12) (December 2015) 2234–

[64] Bihong, L., et al., Immobilization of carbonic anhydrase on carboxyl‐functionalized ferroferric oxide for CO2 capture, Int. J. Biol. Macromol. 79 (August 2015) 719–725. [65] Aines, R. D., et al. Encapsulated solvents for carbon dioxide capture. Energy Procedia.

[66] Stolaroff, J. K., Thermodynamic assessment of microencapsulated sodium carbonate

slurry for carbon capture, Energy Procedia. 63 (2014) 2331–2335.

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116 Recent Advances in Carbon Capture and Storage

Energy Procedia. 37 (2013) 1160–1171.

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absorption process. Energy Procedia. 4 (2011) 868–875.

CO2 Capture Technology Meeting, Pittsburgh, PA, July 11, 2012.

Dioxide Mitigation Technology, pp. 117–147. Elsevier 2015.

This chapter reviews the study and development of biological, enzymatic and biomolecular systems for carbon dioxide capture and further sequestration or even utilization. Regardless of the interest on the use of the captured CO2 as C1 synthon on the manufacture of added-value compounds, there is a tremendous unbalance between the requirements of the contemporary society (leading to a massive production of carbon dioxide) and the framework of commercialization of the products from CO2 utilization. In this context, viable options are storage as a solid in the form of calcium or magnesium carbonate and conversion into other energetic frameworks. In addition, it is important to highlight that the conventional energy resources are progressively being replaced by renewable resources. While the change in energetic paradigm is not accomplished, systems that capture and convert carbon dioxide are highly sought. To this end, bio-inspired systems will be presented, starting from the use of compounds from the chiral pool, such as amino acids, saccharides and related bio-polymers, involved in the physical and chemical capture, sequestration and/or utilization of CO2. Additionally, enzymatic systems are presented in the context of sequestration of CO2 in the form of solid carbonates or even utilization of this C1 synthon in the preparation of fuels and commodity chemicals. Carbonic anhydrase is by far the most studied enzyme, as it catalyses the inter-conversion between CO2 and hydrogencarbonate in an effective mode. The biological option comprises the utilization of methanogens, acetogens and other organisms leading to the formation of added-value compounds. Most of the described systems are based on microbial electro-synthesis model and microbial carboncapture cell prototypes.

**Keywords:** carbon dioxide, amino acids, saccharides, bio-polymers, enzymes, carbonic anhydrase

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

In the present context, our civilization's standards of life are grounded on enormous emissions of Green House Gases (GHGs) to the atmosphere, in concrete carbon dioxide. Simultaneously, biological systems available in nature have restricted capacity on the fixation of CO2, and accumulation of this GHG is creating impact on our environment. It is essential to develop and implement technologies that simultaneously avoid further accumulation and increase the rate of CO2 incorporation in added value products. The use of renewable energies, such as solar, hydroelectric, wind, geothermic, hydrogen, tides and biofuels are progressively being implemented depending on the specific resources of each country and commercial adjustment of the energetic paradigm that can be valid and affordable for the near 7.5 billion human habitants in our planet (2016) [1]. Regardless of the progresses on the implementation of renewable energy resources, conventional fuels continue to be the main source of energy worldwide, leading to accumulation of CO2 in the atmosphere.

The most recent data from IPCC is clear [2]. The total cumulative anthropogenic emissions linked to CO2 (1750–2011) are 2040 ± 310 GtCO2. Nearly 50% of the cumulative emissions took place in the last 40 years (1970–2011), consistent with a steady rise in CO2 emissions during that period. It is important to highlight that 40% of the anthropogenic emissions (1750–2011) persisted in the atmosphere (880 ± 35 GtCO2). While the anticipated change of the energetic paradigm is not assimilated, systems that capture CO2 in an effective mode, and incorporate it in safe and useful products, are highly desirable.

An additional point, that should be presented, is illustrated in **Figure 1**, corresponding to the pattern of GHG emissions by economic sector, being useful in the definition of target sectors more able to be optimized in respect to the CO2 footprint and development of innovative strategies adjusted to a specific challenge. The most representative sector is electricity and heat production applied to the other segments as an indirect source of CO2 emissions (except other energy). The first sector is followed by agriculture, forestry and other land use (AFOLU). Industry, transport, other energy and buildings are the other sectors, with lower percentage on direct GHG emissions. The data here presented (**Figure 1**) is associated with the year 2010 (the most recent data available from IPCC).

From the more representative GHGs emitted to the atmosphere, CO2 presents by far, the highest percentage of associated emissions (**Figure 2**, 2010) which demonstrates the importance of the commercially available systems for CO2 capture and fixation, urgency in the development and implementation of straightforward and sustainable alternative systems complementary to the change in the energetic paradigm already on course.

In order to accomplish an effective CO2 uptake exist diverse prototypes and mature technology: (A) absorption; (B) adsorption; (C) cryogenic; (D) membrane [3]. All the pointed instances, except (C), incorporate bio-inspired systems, as represented in the literature, and will be described briefly.

**1. Introduction**

118 Recent Advances in Carbon Capture and Storage

accumulation of CO2 in the atmosphere.

it in safe and useful products, are highly desirable.

(the most recent data available from IPCC).

described briefly.

In the present context, our civilization's standards of life are grounded on enormous emissions of Green House Gases (GHGs) to the atmosphere, in concrete carbon dioxide. Simultaneously, biological systems available in nature have restricted capacity on the fixation of CO2, and accumulation of this GHG is creating impact on our environment. It is essential to develop and implement technologies that simultaneously avoid further accumulation and increase the rate of CO2 incorporation in added value products. The use of renewable energies, such as solar, hydroelectric, wind, geothermic, hydrogen, tides and biofuels are progressively being implemented depending on the specific resources of each country and commercial adjustment of the energetic paradigm that can be valid and affordable for the near 7.5 billion human habitants in our planet (2016) [1]. Regardless of the progresses on the implementation of renewable energy resources, conventional fuels continue to be the main source of energy worldwide, leading to

The most recent data from IPCC is clear [2]. The total cumulative anthropogenic emissions linked to CO2 (1750–2011) are 2040 ± 310 GtCO2. Nearly 50% of the cumulative emissions took place in the last 40 years (1970–2011), consistent with a steady rise in CO2 emissions during that period. It is important to highlight that 40% of the anthropogenic emissions (1750–2011) persisted in the atmosphere (880 ± 35 GtCO2). While the anticipated change of the energetic paradigm is not assimilated, systems that capture CO2 in an effective mode, and incorporate

An additional point, that should be presented, is illustrated in **Figure 1**, corresponding to the pattern of GHG emissions by economic sector, being useful in the definition of target sectors more able to be optimized in respect to the CO2 footprint and development of innovative strategies adjusted to a specific challenge. The most representative sector is electricity and heat production applied to the other segments as an indirect source of CO2 emissions (except other energy). The first sector is followed by agriculture, forestry and other land use (AFOLU). Industry, transport, other energy and buildings are the other sectors, with lower percentage on direct GHG emissions. The data here presented (**Figure 1**) is associated with the year 2010

From the more representative GHGs emitted to the atmosphere, CO2 presents by far, the highest percentage of associated emissions (**Figure 2**, 2010) which demonstrates the importance of the commercially available systems for CO2 capture and fixation, urgency in the development and implementation of straightforward and sustainable alternative systems

In order to accomplish an effective CO2 uptake exist diverse prototypes and mature technology: (A) absorption; (B) adsorption; (C) cryogenic; (D) membrane [3]. All the pointed instances, except (C), incorporate bio-inspired systems, as represented in the literature, and will be

complementary to the change in the energetic paradigm already on course.

**Figure 1.** Percentages of direct GHG emissions by economic sector from a total of 49 Gt CO2-equivalent during 2010. AFOLU: Agriculture, Forestry and Other Land Use [2].

**Figure 2.** Percentages of GHGs from the 49 Gt CO2-equivalent emissions during 2010. FOLU: Forestry and Other Land Use, F Gases: fluorinated gases covered under the Kyoto Protocol [2].

(A) Absorption: this topic includes physical and chemical absorption. In both situations, CO2 is captured in the volume of a solution. The first framework comprises the physical interaction between high-pressure CO2 and a solution by intermolecular interactions. In this context, molecular solvents and ionic liquids as well carry out this action. Chemical absorption is conventionally performed with solutions of alkanolamines (**Figure 3**):

**Figure 3.** Conventional amines used in chemical absorption of CO2. MEA: Monoethanolamine; DEA: Diethanolamine; MDEA: Methyldiethanolamine.

This established technology presents various drawbacks, such as the compulsory dilution of the alkanolamine in the aqueous environment to avoid deterioration of materials and excessive release of heat when reaction is performed. The utilization of these systems leads to mitigated CO2 uptake (7 wt% using a 30 wt% aqueous solution of MEA). Moreover, there is a highenergy penalty incorporated into the system due to a high-heat capacity of the aqueous environment acting as a sink in the process of CO2 release [4, 5]. Finally, the solvent is volatilized during the operations precluding, an effective regeneration of the CO2 capture system. These strategies are used when the concentration of CO2 is low.

The mechanism associated with these systems encompasses the formation of carbamates, in a 0.5:1 stoichiometry (half of the converted alkanolamine is in the form of carbamate and the other is presented as ammonium). Two different paths lead to this same end: The two-step Zwitterion mechanism is started by a nucleophilic attack of the amine group on CO2 leading to the presence in the same moiety of positive and negative charges. In a further step, a proton is transferred to another alkanolamine, and a salt is formed. In the single-step termolecular mechanism, the nucleophilic attack and proton transfer occurs simultaneously. The other product of reaction is hydrogencarbonate corresponding to the product of reaction between CO2 and water. This reaction is slower; nevertheless, the stoichiometry of CO2 incorporation is one, which is a factor of two superior to the formation of carbamate by the same quantity of aminoalcohol. Giving the product, the conventional capture agents can act as generic bases or nucleophiles in the specific instance of CO2 as well. Another property associated with these systems is that, at given pH, hydrogencarbonate might coexist with carbonic acid and carbonate. The hydroxide anion has a lower expression, though the reaction with CO2 is faster than with water. Another mechanism associated with these systems is the hydrolysis of carbamate to generate hydrogencarbonate. Given the degree of substitution, the amine functionality can be more CO2-philic or more alkaline, with a concomitant contribution on the definition of the reaction profile.

(B) Adsorption is obtained by the capture of CO2 when interacting with a solid surface. Contrarily to chemical absorption, the interaction between CO2 and the surface is moderated (intermolecular forces). Activated carbons and molecular sieves are conventionally used. Three different modes of action characterize this type of framework, related with the 'switch' used between adsorption/desorption: pressure, temperature and electric power control the behaviour of these systems.

(D) Membrane is a partially permeable structure that separates CO2 from gas mixtures. With this recent technology, CO2 is separated from diverse sources, such as post-combustion flue gas, syngas and natural gas. Two operational methods are available: (a) Gas separation membrane: based on the preferential permeation of a specific component of a mixture. (b) Gas absorption membrane: centred on the specific affinity of the previously referred chemical absorber solutions towards CO2.

The bio-inspired systems described in the following topics represent an effective alternative to the available frameworks, with potential to be integrated in the previously mentioned capture systems as well as in sequestration and utilization frameworks.

Aqueous solutions of amino acids are straightforward alternatives to alkanolamine-based systems. The amino acid frameworks are presented as stand-alone salts, as zwitterionic structures activated by bases, or as salts doped with superbases. Different mechanisms of reaction/association are presented according the structure and composition of the system. A different case study is based on the use of highly abundant saccharides and related biopolymers, which may constitute invaluable systems for CO2 capture, sequestration and/or utilization, presented as liquid solutions, gels, confined hydrated foams or solid adsorbents.

Enzymes, especially carbonic anhydrase (CA), are useful catalysts for CO2 sequestration and utilization. In the case of CA, the high rates of reaction and the mild reaction conditions constitute clear advantages of these systems in the laboratory environment. But on a pilot or even industrial scale, harsh conditions of temperature and the presence of contaminants in CO2 streams hinder the utilization of this enzyme. Possible solutions already on praxis are expression of the genetic code associated with this enzyme from thermophiles on readily available organisms, immobilization on diverse supports or generation of catalysts inspired on the mode of action of carbonic anhydrase.

Finally, the use of microbes is addressed in this chapter on the production of added value products from CO2, with special focus on the use of methanogens and acetogens in microbial electro-synthesis and microbial capture cell frameworks.

## **2. Bio-inspired systems**

**Figure 3.** Conventional amines used in chemical absorption of CO2. MEA: Monoethanolamine; DEA: Diethanolamine;

This established technology presents various drawbacks, such as the compulsory dilution of the alkanolamine in the aqueous environment to avoid deterioration of materials and excessive release of heat when reaction is performed. The utilization of these systems leads to mitigated CO2 uptake (7 wt% using a 30 wt% aqueous solution of MEA). Moreover, there is a highenergy penalty incorporated into the system due to a high-heat capacity of the aqueous environment acting as a sink in the process of CO2 release [4, 5]. Finally, the solvent is volatilized during the operations precluding, an effective regeneration of the CO2 capture system. These

The mechanism associated with these systems encompasses the formation of carbamates, in a 0.5:1 stoichiometry (half of the converted alkanolamine is in the form of carbamate and the other is presented as ammonium). Two different paths lead to this same end: The two-step Zwitterion mechanism is started by a nucleophilic attack of the amine group on CO2 leading to the presence in the same moiety of positive and negative charges. In a further step, a proton is transferred to another alkanolamine, and a salt is formed. In the single-step termolecular mechanism, the nucleophilic attack and proton transfer occurs simultaneously. The other product of reaction is hydrogencarbonate corresponding to the product of reaction between CO2 and water. This reaction is slower; nevertheless, the stoichiometry of CO2 incorporation is one, which is a factor of two superior to the formation of carbamate by the same quantity of aminoalcohol. Giving the product, the conventional capture agents can act as generic bases or nucleophiles in the specific instance of CO2 as well. Another property associated with these systems is that, at given pH, hydrogencarbonate might coexist with carbonic acid and carbonate. The hydroxide anion has a lower expression, though the reaction with CO2 is faster than with water. Another mechanism associated with these systems is the hydrolysis of carbamate to generate hydrogencarbonate. Given the degree of substitution, the amine functionality can be more CO2-philic or more alkaline, with a concomitant contribution on the

(B) Adsorption is obtained by the capture of CO2 when interacting with a solid surface. Contrarily to chemical absorption, the interaction between CO2 and the surface is moderated (intermolecular forces). Activated carbons and molecular sieves are conventionally used. Three different modes of action characterize this type of framework, related with the 'switch' used

MDEA: Methyldiethanolamine.

120 Recent Advances in Carbon Capture and Storage

definition of the reaction profile.

strategies are used when the concentration of CO2 is low.

The systems proposed here (biological, enzymatic and bio-molecular) constitute solid alternatives to the conventional platforms available in the market. These bio-inspired frameworks present diverse advantages such as low corrosion, easy disposal and biodegradability, naturally produced and possibility to tune capacity of CO2-incorporation according to the configuration of the system. Nevertheless, there are practical issues that should be addressed in order to make the current technology available thrive in the various challenges assigned.

#### **2.1. Bio-molecular**

#### *2.1.1. Amino acids*

The amino acid systems highlighted include the use of these frameworks as anions, in the presence of an inorganic cation (sodium or potassium), as aprotic ionic liquids or as protic mixtures, using either organic or inorganic bases. The main advantage of amino acid-based systems over conventional alkanolamines relies on high stability to oxidative degradation, high chemical reactivity with CO2, low vapour pressures (compatible with temperature of flue gases), and high surface tension (fundamental on the design of membranes). Various studies exist when an inorganic cation is used [6–18]. Here a selection will be emphasized to establish the structure of amino acid/CO2 absorption-desorption properties relationship (**Figure 4**, **Table 1**). In this comparative study, the conventional MEA aqueous solution is used as a reference. It presents considerably high initial rates of CO2 absorption and desorption, and high CO2 uptake is obtained as well (**Table 1**).

**Figure 4.** Amino acids used in CO2-capture studies, and presented along this chapter.


**Table 1.** CO2 absorption and desorption properties of amino acid-based systems.

**2.1. Bio-molecular**

122 Recent Advances in Carbon Capture and Storage

*2.1.1. Amino acids*

The amino acid systems highlighted include the use of these frameworks as anions, in the presence of an inorganic cation (sodium or potassium), as aprotic ionic liquids or as protic mixtures, using either organic or inorganic bases. The main advantage of amino acid-based systems over conventional alkanolamines relies on high stability to oxidative degradation, high chemical reactivity with CO2, low vapour pressures (compatible with temperature of flue gases), and high surface tension (fundamental on the design of membranes). Various studies exist when an inorganic cation is used [6–18]. Here a selection will be emphasized to establish the structure of amino acid/CO2 absorption-desorption properties relationship (**Figure 4**, **Table 1**). In this comparative study, the conventional MEA aqueous solution is used as a reference. It presents considerably high initial rates of CO2 absorption and desorp-

tion, and high CO2 uptake is obtained as well (**Table 1**).

**Figure 4.** Amino acids used in CO2-capture studies, and presented along this chapter.

Diverse potassium salts of amino acids from chiral pool dissolved in aqueous media were tested [6] (**Table 1**). Glycine (GLY) salt presents similar performances as conventional MEA solutions. Differently, alanine (ALA) is identified by a different reaction profile with CO2, the level of CO2 incorporation and the associated initial rate of absorption are lower. Nevertheless, the rates of CO2 release at moderate temperatures are higher. The reason for such behaviour are the bulkiness of the associated substituent group and the moderate pKa of the amine functionality, contributing to the mitigated propensity of CO2 to be chemically captured, either in the form of carbamate (amino acid as nucleophile) or of bicarbonate (alanine as base).β-Alanine (BALA) is an extended amino acid with concomitant bulkiness and high pKa of its amine functionality, contributing to lower CO2 absorption and desorption performances with respect to GLY, nevertheless, BALA presents improved CO2 absorption and poorer desorption performances when compared with Alanine. With α-aminobutyric acid (AABA), the amine functionality should act preferably as base, due to probable 5-element-ring-hydrogen bond interaction between carboxyl and amine functionalities owing to the size of the linear substituent group, leading to a more favourable incorporation of CO2 in the form of hydrogencarbonate after reaction with water. Improved incorporation with a factor of two, with respect to carbamate leads to improved absorption performances when compared with Alanine.

The performance of γ-aminobutyric acid (GABA) in terms of CO2 uptake is higher than the previously mentioned amino acids, the high pKa leads to the formation of hydrogencarbonate, which is a slower reaction than the formation of carbamate. This information is in line with the lower initial ratio of CO2 absorption obtained when compared with GLY. α-Methyl alanine (AMALA) presents two methyl groups in the α-position and a high pKa of the amine functionality which, similar to AABA, acts as enhanced base by hydrogen-bond stabilization of the protonated base by the carboxyl group of the amino acid. AMALA is characterized by the highest value of CO2 uptake among the presented amino acids; nevertheless, it presents the lowest initial rate of CO2 absorption. These results are compatible with the preferential formation of hydrogencarbonate instead of carbamate. Serine (SER) and cysteine (CYS) are hydroxyl and thio-functionalized amino acids, respectively, that may create destabilization as hydrogen donors when the protonated amine functionality is interacting with the carboxyl group of the amino acid. Considering this aspect, the formation of carbamate should be predominant with respect to the hydrogencarbonate counterpart, and the CO2 uptake is low. The initial rate of desorption associated with these two amino acids is high due to the steric repulsion between the formed carbamate and the bulky hydroxyl or thiol functionalities.

Proline (PRO), 4-hydroxy proline (HYPRO) and pyroglutamic acid (PGA) are cyclic, secondary amino acids, with PRO presenting high pKa and, possibly due to stabilization of protonated amine by the carboxyl group, may lead to enhanced CO2 uptake, the highest pKa among the presented amino acids, leading to a higher hydroxide/water ratio and an enhanced kinetics for CO2 uptake. The amine functionality of HYPRO presents lower pKa than PRO-equivalent, leading, possibly, to a less favourable hydrogencarbonate/carbamate ratio and low CO2 uptake. PGA presents an electro-tractor carbonyl group conjugated with the amine functionality hampering its performance either as a nucleophile or base, leading to a low level of CO2 incorporation.

Asparagine (ASN) and Glutamine (GLN) are linear amide-functionalized amino acids, presenting low pKa associated with the corresponding amine functionalities, leading to low levels of CO2 uptake. Due to steric repulsion, the ratio of the kinetics of CO2 uptake/desorption is mitigated. Diglycine (DIGLY) leads to a low level of carbon dioxide incorporation due to inherent bulkiness associated to the chemical structure. Arginine (ARG) presents highly basic guanidine functionality and a not so basic amine group. The combination of both functionalities leads to the highest level of CO2 incorporation, despite the presence of both functionalities. Taurine (TAU) amine group presents low pKa, considering this; the ratio carbamate/hydrogencarbonate should be high.

Diverse potassium salts of amino acids from chiral pool dissolved in aqueous media were tested [6] (**Table 1**). Glycine (GLY) salt presents similar performances as conventional MEA solutions. Differently, alanine (ALA) is identified by a different reaction profile with CO2, the level of CO2 incorporation and the associated initial rate of absorption are lower. Nevertheless, the rates of CO2 release at moderate temperatures are higher. The reason for such behaviour are the bulkiness of the associated substituent group and the moderate pKa of the amine functionality, contributing to the mitigated propensity of CO2 to be chemically captured, either in the form of carbamate (amino acid as nucleophile) or of bicarbonate (alanine as base).β-Alanine (BALA) is an extended amino acid with concomitant bulkiness and high pKa of its amine functionality, contributing to lower CO2 absorption and desorption performances with respect to GLY, nevertheless, BALA presents improved CO2 absorption and poorer desorption performances when compared with Alanine. With α-aminobutyric acid (AABA), the amine functionality should act preferably as base, due to probable 5-element-ring-hydrogen bond interaction between carboxyl and amine functionalities owing to the size of the linear substituent group, leading to a more favourable incorporation of CO2 in the form of hydrogencarbonate after reaction with water. Improved incorporation with a factor of two, with respect to carbamate leads to improved absorption performances when compared with Alanine.

124 Recent Advances in Carbon Capture and Storage

The performance of γ-aminobutyric acid (GABA) in terms of CO2 uptake is higher than the previously mentioned amino acids, the high pKa leads to the formation of hydrogencarbonate, which is a slower reaction than the formation of carbamate. This information is in line with the lower initial ratio of CO2 absorption obtained when compared with GLY. α-Methyl alanine (AMALA) presents two methyl groups in the α-position and a high pKa of the amine functionality which, similar to AABA, acts as enhanced base by hydrogen-bond stabilization of the protonated base by the carboxyl group of the amino acid. AMALA is characterized by the highest value of CO2 uptake among the presented amino acids; nevertheless, it presents the lowest initial rate of CO2 absorption. These results are compatible with the preferential formation of hydrogencarbonate instead of carbamate. Serine (SER) and cysteine (CYS) are hydroxyl and thio-functionalized amino acids, respectively, that may create destabilization as hydrogen donors when the protonated amine functionality is interacting with the carboxyl group of the amino acid. Considering this aspect, the formation of carbamate should be predominant with respect to the hydrogencarbonate counterpart, and the CO2 uptake is low. The initial rate of desorption associated with these two amino acids is high due to the steric repulsion between the formed carbamate and the bulky hydroxyl or thiol functionalities.

Proline (PRO), 4-hydroxy proline (HYPRO) and pyroglutamic acid (PGA) are cyclic, secondary amino acids, with PRO presenting high pKa and, possibly due to stabilization of protonated amine by the carboxyl group, may lead to enhanced CO2 uptake, the highest pKa among the presented amino acids, leading to a higher hydroxide/water ratio and an enhanced kinetics for CO2 uptake. The amine functionality of HYPRO presents lower pKa than PRO-equivalent, leading, possibly, to a less favourable hydrogencarbonate/carbamate ratio and low CO2 uptake. PGA presents an electro-tractor carbonyl group conjugated with the amine functionality hampering its performance either as a nucleophile or base, leading to a low level of CO2

incorporation.

In another study, [7] mostly secondary but also tertiary bulky amines in PEG150 solvent are tested. With [Na] *i-*PrNHGLY (**Figure 4**, **Table 1**), the level of CO2 incorporation was near 100% with respect to the amine unit. The main reason for such result is the formation of carbamic acid instead of carbamate leading to a nearly full use of the amine functionality to capture CO2 instead of half (in the carbamate/ammonium salt). To this result contributes the stabilization of the carbamic acid functionality by the carboxyl group, the steric repulsion associated with the amine substituents, avoiding deprotonation of the carbamic functionality, and the type of chemical environment [7]. The carbamic acid reaction profile and steric hindrance lead to low energy requirements for CO2 desorption (313 K). An interesting work was carried out by Wang et al. [17], where, from a diverse set of amino acid sodium salts (aqueous solutions) was possible to observe with Alanine, a phase-split between a CO2-rich and a CO2-lean phase. The CO2-rich phase was composed of carbamate and hydrogencarbonate. The decrease on the volume of solution leads to a significant decrease in energy input for the CO2 strip.

In a different study [19], (**Figure 4**, **Table 2**) a diverse set of amino acid ionic liquids (neat) for CO2 capture was tested. The best performance was obtained with [N66614] LYS, leading to nearly two equivalents of CO2 reacting with the two amine functionalities existing in lysine. The reason behind this outstanding result relies on the formation of carbamic acid (as in Ref. [7]). One additional point is addressed to the catalytic effect of the carboxyl group, promoting the proton transfer, starting from zwitterionic structure (after nucleophilic attack to CO2) to the carbamic acid end product. In this work the effect of the cation in the CO2 uptake was checked. [N66614] and [P66614] were tested in combination with the LYS anion. It was observed that the initial rate of absorption was higher with [P66614], nevertheless [N66614] lead to high CO2 uptake (**Table 2**). [N66614]LYS is associated with stronger hydrogen-bond interactions after chemisorption of CO2 leading to an increment of viscosity and poorer kinetics. Due to the different stabilized-arrangement of this ionic liquid, after incorporation of CO2, nearly all the amine groups are converted to carbamic acid functionalities. [P66614]LYS presents another configuration after CO2 uptake with concomitant half of the amine groups presented as carbamic acid functionalities, ¼ as carbamate and the remaining ¼ as ammonium.

A conceptual study was carried out in our laboratories [20] concerning the preparation of reversible ionic liquids using GLY, ALA, valine (VAL), leucine (LEU), phenylalanine (PHE) and tryptophan (TRP) that activated by an organic superbase, 1,8-diazabicyclo[5.4.0]undec-7 ene (DBU) or 1,1,3,3-tetramethylguanidine (TMG), react with CO2 to obtain carbamate-based ionic liquids. The system can revert back to the early configuration upon heating at an appropriate temperature. It was possible to observe for the DBU series a decrease in the temperature associated with CO2 release when the bulkiness of the substituent group of the amino acid increases. A similar order was observed with the TMG set, however not completely defined.


**Table 2.** CO2 absorption and desorption characteristics of amino acid-based systems.

Other studies concerning the application of amino acid-based ionic liquids in CO2 capture are presented here. Lv et al. [21] tested 1-aminopropyl-3-methylimidazolium glycinate aqueous solution and could achieve 1.23 mol of CO2 loading per mol of ionic liquid. The reaction was followed by 13C-NMR and it comprised two steps: (1) initial formation of carbamate, followed by (2) hydrolysis of this functionality to obtain hydrogencarbonate. The same author [22], reported the use of [C2OHMIM] GLY aqueous solution to 0.575 mol of CO2 uptake/mol ionic liquid. In that study the effect of O2 on the absorption performances was determined, which were better than in the case of MEA solution. In another study, Li et al. [23], tested [P4444] salts of GLY, ALA and PRO, in combination with PEG solvents, on the design of membranes for CO2/H2 separation. In a different framework, amino acids are combined with inorganic bases (mainly carbonate salts) in aqueous solutions to promote kinetics and capacity of CO2 uptake. GLY, sarcosine (SAR), PRO [24] and ARG, as well, were used in that context [25].

Finally, as an example for the promoting effect of adding the conventional alkanolamines to amino acid systems, Gao et al. [26] combined MDEA aqueous solution with [N1111] GLY. The reason behind this option relies on the fact that MDEA, a tertiary amine, acts as base in the considerable slow reaction between water and CO2 to obtain hydrogencarbonate, leading to high CO2 uptake. The GLY-based amino acid presents good kinetic performances, however low CO2 incorporation in the form of carbamate. Considering the reaction of carbamate hydrolysis and the complementary reactive profiles of MDEA and [N1111] GLY, such systems are presented here as an alternative to conventional frameworks.

#### *2.1.2. Saccharides and related bio-polymers*

appropriate temperature. It was possible to observe for the DBU series a decrease in the temperature associated with CO2 release when the bulkiness of the substituent group of the amino acid increases. A similar order was observed with the TMG set, however not completely

> **Initial rate absorpt (mol CO2 mol amine−1 min−1)**

**Initial rate desorpt (mol CO2 mol amine−1 min−1)**

**CO2 uptake (mol CO2 mol amine−1)**

(24 h)

(48 h)

(48 h)

(48 h)

(48 h)

(24 h)

**pKa, parent amine /amino − acid**

2.18, 8.95, 10.53

2.02, 8.80

2.2, 9.1

1.80, 9.33, 6.04

1.82, 8.99, 12.48

2.28, 9.21

10.53

**Absorpt method**

neat 100, r.t, 353 - - - 2.1

neat 100, r.t, 353 - - - 2.0

neat 100, r.t, 353 - - - 1.9

neat 100, r.t, 353 - - - 1.9

neat 100, r.t, 353 - - - 1.3

neat 100, r.t, 353 - - - 1.2

GLY, sarcosine (SAR), PRO [24] and ARG, as well, were used in that context [25].

Finally, as an example for the promoting effect of adding the conventional alkanolamines to amino acid systems, Gao et al. [26] combined MDEA aqueous solution with [N1111] GLY. The

**Table 2.** CO2 absorption and desorption characteristics of amino acid-based systems.

[P66614]LYS [19] neat 100, r.t, 353 - - - 1.6 (48 h) 2.18, 8.95,

Other studies concerning the application of amino acid-based ionic liquids in CO2 capture are presented here. Lv et al. [21] tested 1-aminopropyl-3-methylimidazolium glycinate aqueous solution and could achieve 1.23 mol of CO2 loading per mol of ionic liquid. The reaction was followed by 13C-NMR and it comprised two steps: (1) initial formation of carbamate, followed by (2) hydrolysis of this functionality to obtain hydrogencarbonate. The same author [22], reported the use of [C2OHMIM] GLY aqueous solution to 0.575 mol of CO2 uptake/mol ionic liquid. In that study the effect of O2 on the absorption performances was determined, which were better than in the case of MEA solution. In another study, Li et al. [23], tested [P4444] salts of GLY, ALA and PRO, in combination with PEG solvents, on the design of membranes for CO2/H2 separation. In a different framework, amino acids are combined with inorganic bases (mainly carbonate salts) in aqueous solutions to promote kinetics and capacity of CO2 uptake.

defined.

**Amino acid system**

[N66614]LYS [19]

[N66614]ASN [19]

[N66614]GLN [19]

[N66614]HIS [19]

[N66614]ARG [19]

[N66614]MET [19]

**Solvent / Concentration of the capture agent** 

126 Recent Advances in Carbon Capture and Storage

*P, T***abs,** *T***desorp (kPa, K, K)**

> The use of saccharides and related bio-polymers (**Figure 5**) constitute natural and abundant alternatives for CO2 fixation, chemisorption and adsorption. Concerning fixation, Sun et al. [27] developed a superbase/cellulose catalytic system to obtain cyclic carbonates from epoxides and CO2. Cellulose acts as a hydrogen bond donor and the superbase as the nucleophile in the activation of the epoxide. High conversions and selectivities are associated with DBU/cellulose. In a different study, Tamboli et al. [28], reported the use of chitosan/DBU dissolved in 1 mesyl-3-methylimidazolium (mesylMIM)-based ionic liquids for preparation of dimethyl carbonate (DMC) from methanol and CO2. From the chemisorption perspective, it was tested in our laboratories [29] the use of monosaccharides, oligosaccharides or a polysaccharideactivated, by combination with adjustable proportion of liquid DBU or TMG as organic superbases, for CO2 capture. Of course, it is necessary to consider that low ratios of superbase lead to highly viscous solutions with hampered capacity for CO2 mass transfer and concomitant poor performances in the capture of this GHG. In a different perspective, an excess of the superbase would lead to high dilution of the capture agent with associated limitation on the wt% of CO2 uptake. It was necessary to carry out an optimization for maximal performances (D-Mannose:DBU —0.625/1 in equivalents leading to 13.9 wt% of CO2 uptake and 3.3/5 alcohol functionalities converted to carbonates). It is also important to consider an effective stirring to overcome the increase of viscosity with the progress of reaction, which limits CO2 uptake.

**Figure 5.** Examples of mono-saccharide and related biopolymers.

In a different study, Eftaiha et al. [30] used chitin acetate in DMSO for CO2 capture. The mechanism involves activation of the alcohol groups by DMSO followed by conversion into carbonates which are stabilized by the ammonium groups available in chitin acetate. A very interesting concept was reported by Sehaqui et al. [31] and used in the preparation of cellulosepolyethyleneimine foams and study of its properties on CO2 capture from air. To this end, it is essential a high relative humidity for optimal CO2 uptake. The confined/activated water might have a crucial role in the mode of action of this system. There are other studies that deal with carbon dioxide chemisorption by the use of chitin or chitosan dissolved in ionic liquids such as the one reported by Xie et al. [32]. The use of saccharide-based polymers is also associated with adsorption systems. In this context, carbon spheres were prepared from alginate and chitosan, after thermal treatment between 673 and 1073 K, leading to an excellent capacity for CO2 adsorption. The high conductivity presented by alginate-based spheres was crucial for the development of an adsorption/desorption system based on the use of electric power as a "switch" with low energetics associated with CO2 capture and release.

#### **2.2. Enzymatic**

CO2 fixation and conversion catalysed by enzymatic-cellular systems is essential in the recycling processes inherent to life. Six major routes [33] are available (**Figure 6**):


**6.** Finally the dicarboxylate/4-hydroxybutyrate cycle is found in *Thermoproteales* and *Desulfurococcales* and comprises two carbon dioxide conversion steps, (2a) and (2b), existing in the reductive citric acid cycle (**Figure 6**).

carbonates which are stabilized by the ammonium groups available in chitin acetate. A very interesting concept was reported by Sehaqui et al. [31] and used in the preparation of cellulosepolyethyleneimine foams and study of its properties on CO2 capture from air. To this end, it is essential a high relative humidity for optimal CO2 uptake. The confined/activated water might have a crucial role in the mode of action of this system. There are other studies that deal with carbon dioxide chemisorption by the use of chitin or chitosan dissolved in ionic liquids such as the one reported by Xie et al. [32]. The use of saccharide-based polymers is also associated with adsorption systems. In this context, carbon spheres were prepared from alginate and chitosan, after thermal treatment between 673 and 1073 K, leading to an excellent capacity for CO2 adsorption. The high conductivity presented by alginate-based spheres was crucial for the development of an adsorption/desorption system based on the use of electric power as a

CO2 fixation and conversion catalysed by enzymatic-cellular systems is essential in the

**1.** Calvin cycle: it is one of the most important processes in nature, being the major route for carbon dioxide conversion. It is present in a great majority of photosynthetic organisms and besides the CO2 fixation (thermodynamically favourable), catalysed by 1,5-ribulose bisphosphate carboxylase (RuBisCO), in each cycle are produced saccharides, fatty acids

**2.** Reductive citric acid cycle: this cycle, comprising four carboxylation steps exists, basically, in the conversion of CO2 and water into carbon compounds. It is found mainly in some thermophilic bacteria that grow on H2, and bacteria that reduce sulphate. The first carboxylation step, the conversion of succinyl CoA into 2-oxoglutarate is thermodynamically unfavourable (Δ*G*°′ = 19 kJ/mol), such as the conversion of 2-oxoglycerate into isocitrate (Δ*G*°′ = 8 kJ/mol) and the production of pyruvate from acetyl CoA (Δ*G*°′ = 19 kJ/mol). The fourth carboxylation step, the conversion of phosphoenolpyruvate into oxaloacetate is thermodynamically favourable (Δ*G*°′ = −24 kJ/mol) and due to its incorporation into this

**3.** Reductive acetyl CoA route: it is a non-cyclic route, existing in some acetogenic and methanogenic micro-organisms. This process includes the conversion of CO2 into formic acid (Δ*G*°′ = 22 kJ/mol) catalysed by formate dehydrogenase. The other fixation step

**4.** The 3-hydroxypropionate cycle was found in *Chloroflexaceae*, a phototrophic bacterium and comprises two favourable CO2 conversion steps, from the thermodynamic point of view. Malonyl CoA from acetyl CoA (Δ*G*°′ = −14 kJ/mol) and conversion of propionyl CoA

**5.** A recently found cycle is the 3-hydroxypropionate/4-hydroxybutyrate cycle existing in *Metallosphaera* and includes three CO2 conversion steps, existing in the (2) reductive citric

into methylmalonyl CoA (Δ*G*°' = −11 kJ/mol) are the two CO2 conversion steps.

recycling processes inherent to life. Six major routes [33] are available (**Figure 6**):

cycle the promotion of the three previous carboxylation steps is assured.

comprises the conversion of carbon dioxide into CO (Δ*G*°′ = 0 kJ/mol).

acid cycle and (4) 3-hydroxypropionate cycle (**Figure 6**).

"switch" with low energetics associated with CO2 capture and release.

**2.2. Enzymatic**

and amino acids.

128 Recent Advances in Carbon Capture and Storage

**Figure 6.** Enzymatic-catalysed reactions of CO2 incorporation involved in the six main cellular metabolic routes [33].

In all the conversion steps (**Figure 6**) the source of the C1 synthon is either CO2 itself or HCO3 as the intermediate. The preparation of HCO3 is catalysed by the ubiquitous enzyme carbonic anhydrase (CA):

This enzyme, one of the fastest enzymes known, has a diversity of structures and is catalogued in five different families (α, β, γ, δ and ζ). As an illustrative example, a typical mechanism of a α-CA, which contains in the active site, a Zn(II) centre coordinated with three HIS and one water molecule forming hydrogen bond with the hydroxyl group of THR (activated by GLU) will be described. This concerted interaction, convert water/hydroxide to an enhanced nucleophile towards CO2, leading to the formation of hydrogencarbonate. Afterwards a new water molecule replaces HCO3 in the Zn(II) coordination site and a new cycle is initiated [34]. Considering the diverse enzymatic reactions available (**Figure 6**) and the specific instance of CA (**Figure 7**), some concerns should be addressed. From the presented enzymatic-catalysed reactions (**Figure 6**) only a parcel can be used *in vitro* for conversion of CO2, which is not the case of the reactions involving coenzyme A (CoA), that lead to uncommon/useless products. Other reactions are thermodynamically unfavourable under standard conditions, and pH, ionic strength or temperature tuning should be carried out [35]. It is important to highlight that the possibility of tuning reaction parameters is restrained due to possible inactivation/ denaturation of enzyme under specific reaction conditions.

$$\begin{array}{c} \begin{array}{c} \text{H}\_{2}\text{O} \end{array} + \begin{array}{c} \text{CO}\_{2} \end{array} \xrightarrow{\begin{array}{c} \text{C} \end{array}} \begin{array}{c} \text{HCO}\_{3}^{\text{-}} \end{array} + \begin{array}{c} \text{H}^{\text{-}} \end{array} \end{array} \end{array}$$

**Figure 7.** CO2 conversion to HCO3 catalysed by carbonic anhydrase.

Other option is the coupling of reactions leading to a net thermodynamic feasible transformation [36]; however, as the reaction system becomes more complex, acceptable operational conditions become usually narrower. Due to the efficiency of CA, the enzymatic reaction associated is, by far, the most represented reaction of CO2 incorporation in the literature, usually associated with the direct sequestration of CO2 in the form of solid calcium or magnesium carbonates [34, 37], separation operations [38] and even coupled to other enzymatic CO2 incorporation reaction in order to increase drastically the concentration of the C1 synthon in the form of hydrogencarbonate in water and improve the reaction outcome [39]. The high temperatures and inhibitors produced during combustion processes and high commercial value associated with this enzyme, limit the efficiency of CA, nevertheless diverse frameworks were developed in order to overcome operational constraints such as: direct use of carbonic anhydrase mimics [40], support immobilization of the enzyme [41], (to avoid denaturation under harsh conditions), and combination with motion [42] generated by a chemical-engine (to overcome the diffusion constraints associated with immobilization) appear as useful frameworks. Testing carbonic anhydrase from thermophiles [43] and its genetic edition with overexpression in *Escherichia coli* are other alternatives [44].

#### **2.3. Biological**

The concluding topic comprises the use of micro-organisms as productive units of addedvalue products from CO2 (sequestration and/or utilization). Diverse concepts were explored

in the literature such as: (1) microbial electro-synthesis [45, 46], where electrons are supplied from a cathode to micro-organisms, which converts CO2 into added-value products, usually methane and/or acetate. The electrons are provided by electric current, preferentially from renewable resources. Another framework is (2) microbial carbon capture cell [47], where, different from microbial electro-synthesis, the source of electrons comes from the microbialassisted degradation of organic compounds from wastewater in the anode with formation of protons, electrons and CO2. Usually the anode and the cathode are separated by an ion exchange membrane. The cathode receives the generated electrons and, assisted by micro-organisms, converts CO2 into useful chemicals. Another option available is, (3) microbial electrolytic carbon capture, which similar to microbial electro-synthesis, is used as an external electric power source to increase the potential generated by degradation of organic compounds in the anode assisted by micro-organisms. H2 and OH− are generated in the cathode with the anion reacting with CO2 to obtain hydrogencarbonate [48]. Usually, depending on the microbial cultures on the cathode, it is possible to obtain methane, acetate and other compounds. When acetate is required acetogens are used, nevertheless there is competition associated with the formation of methane, which is inhibited by the addition of compounds, such as 2-bromoethanesulfonate. Other systems available are based solely on the application of light in photo-reactors in order to photosynthetic micro-organisms produce added value compounds. All these options should be optimized in order to be effectively used in real situations.

#### **3. Conclusions and challenges**

This enzyme, one of the fastest enzymes known, has a diversity of structures and is catalogued in five different families (α, β, γ, δ and ζ). As an illustrative example, a typical mechanism of a α-CA, which contains in the active site, a Zn(II) centre coordinated with three HIS and one water molecule forming hydrogen bond with the hydroxyl group of THR (activated by GLU) will be described. This concerted interaction, convert water/hydroxide to an enhanced nucleophile towards CO2, leading to the formation of hydrogencarbonate. Afterwards a new water molecule replaces HCO3 in the Zn(II) coordination site and a new cycle is initiated [34]. Considering the diverse enzymatic reactions available (**Figure 6**) and the specific instance of CA (**Figure 7**), some concerns should be addressed. From the presented enzymatic-catalysed reactions (**Figure 6**) only a parcel can be used *in vitro* for conversion of CO2, which is not the case of the reactions involving coenzyme A (CoA), that lead to uncommon/useless products. Other reactions are thermodynamically unfavourable under standard conditions, and pH, ionic strength or temperature tuning should be carried out [35]. It is important to highlight that the possibility of tuning reaction parameters is restrained due to possible inactivation/

Other option is the coupling of reactions leading to a net thermodynamic feasible transformation [36]; however, as the reaction system becomes more complex, acceptable operational conditions become usually narrower. Due to the efficiency of CA, the enzymatic reaction associated is, by far, the most represented reaction of CO2 incorporation in the literature, usually associated with the direct sequestration of CO2 in the form of solid calcium or magnesium carbonates [34, 37], separation operations [38] and even coupled to other enzymatic CO2 incorporation reaction in order to increase drastically the concentration of the C1 synthon in the form of hydrogencarbonate in water and improve the reaction outcome [39]. The high temperatures and inhibitors produced during combustion processes and high commercial value associated with this enzyme, limit the efficiency of CA, nevertheless diverse frameworks were developed in order to overcome operational constraints such as: direct use of carbonic anhydrase mimics [40], support immobilization of the enzyme [41], (to avoid denaturation under harsh conditions), and combination with motion [42] generated by a chemical-engine (to overcome the diffusion constraints associated with immobilization) appear as useful frameworks. Testing carbonic anhydrase from thermophiles [43] and its genetic edition with

The concluding topic comprises the use of micro-organisms as productive units of addedvalue products from CO2 (sequestration and/or utilization). Diverse concepts were explored

denaturation of enzyme under specific reaction conditions.

130 Recent Advances in Carbon Capture and Storage

**Figure 7.** CO2 conversion to HCO3 catalysed by carbonic anhydrase.

overexpression in *Escherichia coli* are other alternatives [44].

**2.3. Biological**

While renewable sources of energy do not definitely replace conventional fuels, the use of bio-inspired systems for CO2 capture, sequestration and utilization, constitutes an already open window of opportunity in the context of mitigation of environmental effects associated with excessive anthropogenic GHG emissions. Characteristics such as abundance, low corrosion, biodegradability and possibility to tune interaction with CO2, constitute clear advantages of the described bio-inspired systems. With bio-molecular frameworks, it is important to overcome the kinetic constraints associated with increase of viscosity when CO2 is captured. Simultaneously the conception of robust systems requiring low energetics for CO2 release is important. Additionally, the straightforward use of the specific functionalities of amino acids, saccharides and related bio-polymers on the enhancement of the levels of fixation and utilization of CO2 is essential. Concerning enzymatic systems is of extreme importance, for stand-alone and especially multi-enzymatic systems, the design/ optimization of the system configuration to obtain the target product in high yields or simply sequester CO2 as a solid. Additional attention should be devoted to the design of robust systems compatible with the real conditions of combustion/processing of gases, addressed to immobilization, design of enzymatic mimics and genetic engineering. Finally, the biological systems available should be improved with application studies in order to overcome robustness and selectivity constraints associated with electro/photochemical systems.

#### **Abbreviations**



#### **Author details**

**Abbreviations**

AABA: α-Aminobutyric acid ADP: Adenosine Diphosphate

132 Recent Advances in Carbon Capture and Storage

AMALA: α-Methyl Alanine ARG: Arginine ASN: Asparagine

ATP: Adenosine Triphosphate

CA: Carbonic Anhydrase CoA: Coenzyme A CYS: Cysteine

DEA: Diethanolamine DIGLY: Diglycine

LEU: Leucine LYS: Lysine

MET: Methionine

MDEA: Methyldiethanolamine MEA: Monoetanolamine

[N1111]: Tetramethylammonium [N66614]: trihexyltetradecylammonium

[P4444]: Tetrabutylphosphonium [P66614]: trihexyltetradecylphosphonium

PEG: Polyethyleneglycol PGA: Pyroglutamic acid PHE: Phenylalanine *i*-PrNHAla: N-*i*-propylalanine *i*-PrNHBALA: N-*i*-propyl-β-Alanine *i*-PrNHGLY: N-*i*-propylglycine

[MesylMIM]: 1-mesyl-3-methylimidazolium

*n*-DiPrNHGLY: N-*n*-dipropylglycine DMC: dimethyl carbonate GABA: γ-Aminobutyric acid GHGs: Green House Gases GLN: Glutamine GLU: Glutamate GLY: Glycine HIS: Histidine HYPRO: 4-hydroxy proline

ALA: Alanine

BALA: β-Alanine *t*-BuNHGLY: N-*t*-butylglycine

AFOLU: Agriculture, Forestry and Other Land Use

[C2OHMIM]: 1-(2-hydroxyethyl)-3-methylimidazolium

IPCC: Intergovernment Panel on Climate Change

NADP: Nicotinamide Adenine Dinucleotide Phosphate

DBU: 1,8-Diazabicyclo[5.4.0]undec-7-ene

Gonçalo V. S. M. Carrera, Luís C. Branco and Manuel Nunes da Ponte\*

\*Address all correspondence to: mnponte@fct.unl.pt

LAQV – REQUIMTE – Faculty of Science and Technology – NOVA University of Lisbon, Caparica, Portugal

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#### **Synergistic Effect on CO2 Capture by Binary Solvent System Synergistic Effect on CO2 Capture by Binary Solvent System**

Quan Zhuang and Bruce Clements Quan Zhuang and Bruce Clements

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65763

#### **Abstract**

CO2 absorption into a binary solvent system was studied using a batch‐mode gas/liquid absorption apparatus. The binary system composed of potassium carbonate (K2CO3) and piperazine (PZ) showed a strong synergistic effect, whereby the binary solvent performed better than either of the individual solvents for CO2 absorption. The other pairs of solvents tested (K2CO3/monoethanolamine (MEA) and K2CO3/NaOH) showed no synergistic effects. The results indicate that this synergistic effect only occurs with specific pairs of solvents. The mechanism for the synergistic effect is postulated that the activated CO2 on PZ migrates to K2CO3, or a more reactive intermediate complex between K2CO3 and PZ is formed.

**Keywords:** post‐combustion, carbon capture, binary solvent, synergy effect, pipera‐ zine, potassium carbonate, CO2 absorption

#### **1. Introduction**

There has been a growing concern over greenhouse gas emissions as they are considered to be the direct cause of global warming [1, 2]. Postcombustion capture technology is widely being studied for capturing CO2 produced in power generation plants [3–5]. Compared with other CO2 capture technologies such as oxy‐fuel combustion and integrated gasification combined cycle (IGCC), postcombustion capture is regarded as the most probable technology to be first employed when carbon capture becomes a reality in the near future in terms of technology

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

readiness level, flexibility, and economics [6]. Postcombustion capture technology uses liquid solvents to make efficient contact with CO2‐containing flue gas, during which CO2 interacts and reacts with the solvent and is removed from the flue gas stream. After absorption, the CO2‐laden solvent undergoes a regeneration operation, releasing pure CO2 which is then compressed, transported, and sequestered. The regenerated solvent, now at lean state, is returned to start the next cycle of CO2 capture. The whole operation is a continuous process. The same or similar technologies have been applied for decades for natural gas purification and syngas CO2 separation [7–9]. For greenhouse gas CO2 mitigation applications, commercial solvents such as amine, potassium carbonate, and methanol are currently being tested, however, improved solvents are required to reduce the cost and increase the efficiency of postcombustion capture systems. At the moment, solvents that are being developed for CO2 capture include noncon‐ ventional amines, aqueous ammonia, amino acids, ionic liquids, and mixtures of two or more solvents, i.e., hybrid systems [10, 11].

Potassium carbonate is known to be used in industrial CO2 separation processes, such as Benfield and Catcarb [12], as the main solvent with or without proprietary additives. It has advantages over amines: lower cost, lower heat of absorption, thermal stability, nonvolatile, less corrosiveness, low toxicity, and environmentally friendly. A major downside for using K2CO3, however, is its slow absorption rate and low CO2 absorption capacity, resulting in poor CO2 mass transfer rate relative to amines. The way to overcome the aforementioned short‐ comings of K2CO3 is to add promoter, i.e., a hybrid solvent. Hybrid solvent systems have the potential to perform better than the individual components alone. Physicochemical properties of different solvents can supplement each other. Synergistic effect or cooperative effect of hybrid solvents has been found in applications in other areas such as extraction and coal swelling [13, 14]. The mechanisms of the synergistic effects are suggested to be engendered by thermodynamics and hydrogen bonding.

We have been studying CO2 absorption using an aqueous potassium carbonate solvent solution with the addition of other solvents in an attempt to improve CO2 absorption performance. In this chapter, we report results of a synergistic effect that became apparent during these studies. When small amount of piperazine (PZ) is added to the potassium carbonate solution, both CO2 absorption rate and capacity are significantly enhanced, exceeding the mathematical sum of the CO2 absorption rate and the capacity of the individual solvents.

Piperazine itself is an active absorbent for CO2 [15]. For some engineering reasons, it has only been used as an additive or a promoter to other common CO2 capture amines [16]. With amine solvents, piperazine has shown promotional effect. For instance, the CESAR‐1 solvent is an aqueous blend of AMP (2‐amino‐2‐methyl‐1‐propanol) and PZ which showed a reduction of about 20% in the regeneration energy and 45% in the solvent circulation rate compared to those of MEA‐based CO2 capture process under similar process condition [17].

There have been some reports on the promotional/synergistic effect on CO2 capture by K2CO3 and PZ [18]. This study builds upon previous achievements and provides convincing experi‐ mental evidence of the synergistic effect.

#### **2. Experimental**

readiness level, flexibility, and economics [6]. Postcombustion capture technology uses liquid solvents to make efficient contact with CO2‐containing flue gas, during which CO2 interacts and reacts with the solvent and is removed from the flue gas stream. After absorption, the CO2‐laden solvent undergoes a regeneration operation, releasing pure CO2 which is then compressed, transported, and sequestered. The regenerated solvent, now at lean state, is returned to start the next cycle of CO2 capture. The whole operation is a continuous process. The same or similar technologies have been applied for decades for natural gas purification and syngas CO2 separation [7–9]. For greenhouse gas CO2 mitigation applications, commercial solvents such as amine, potassium carbonate, and methanol are currently being tested, however, improved solvents are required to reduce the cost and increase the efficiency of postcombustion capture systems. At the moment, solvents that are being developed for CO2 capture include noncon‐ ventional amines, aqueous ammonia, amino acids, ionic liquids, and mixtures of two or more

Potassium carbonate is known to be used in industrial CO2 separation processes, such as Benfield and Catcarb [12], as the main solvent with or without proprietary additives. It has advantages over amines: lower cost, lower heat of absorption, thermal stability, nonvolatile, less corrosiveness, low toxicity, and environmentally friendly. A major downside for using K2CO3, however, is its slow absorption rate and low CO2 absorption capacity, resulting in poor CO2 mass transfer rate relative to amines. The way to overcome the aforementioned short‐ comings of K2CO3 is to add promoter, i.e., a hybrid solvent. Hybrid solvent systems have the potential to perform better than the individual components alone. Physicochemical properties of different solvents can supplement each other. Synergistic effect or cooperative effect of hybrid solvents has been found in applications in other areas such as extraction and coal swelling [13, 14]. The mechanisms of the synergistic effects are suggested to be engendered by

We have been studying CO2 absorption using an aqueous potassium carbonate solvent solution with the addition of other solvents in an attempt to improve CO2 absorption performance. In this chapter, we report results of a synergistic effect that became apparent during these studies. When small amount of piperazine (PZ) is added to the potassium carbonate solution, both CO2 absorption rate and capacity are significantly enhanced, exceeding the mathematical sum

Piperazine itself is an active absorbent for CO2 [15]. For some engineering reasons, it has only been used as an additive or a promoter to other common CO2 capture amines [16]. With amine solvents, piperazine has shown promotional effect. For instance, the CESAR‐1 solvent is an aqueous blend of AMP (2‐amino‐2‐methyl‐1‐propanol) and PZ which showed a reduction of about 20% in the regeneration energy and 45% in the solvent circulation rate compared to

There have been some reports on the promotional/synergistic effect on CO2 capture by K2CO3 and PZ [18]. This study builds upon previous achievements and provides convincing experi‐

of the CO2 absorption rate and the capacity of the individual solvents.

those of MEA‐based CO2 capture process under similar process condition [17].

solvents, i.e., hybrid systems [10, 11].

140 Recent Advances in Carbon Capture and Storage

thermodynamics and hydrogen bonding.

mental evidence of the synergistic effect.

A batch‐mode liquid‐gas absorption apparatus was constructed in CanmetENERGY, Ottawa. A schematic and a photo of the apparatus are shown in **Figure 1**. All of the connections within the system are vacuum‐proof. The volume of the four‐neck flask is 690 ml. The solute gas used in the experiment is a mixture of CO2 and air (49 v% of CO2). CO2 absorption tests were carried out at 21℃ (room temperature). The flask was placed in a water bath to maintain a constant temperature (CO2 absorption is exothermic). First, the flask was purged by the solute gas for 10 min. Then all of the valves of the flask were closed, leaving the gas in the flask at ambient pressure. After this, 10 ml of solvent was introduced into the flask by opening the two valves of the funnel, and then closing them quickly so that the flask becomes a closed system with gaseous solute in contact with liquid solvent. The liquid was agitated by a magnetic stirrer at 350 rpm (there was no difference on the CO2 absorption results with rpm in the range of 300– 400). When the CO2 was absorbed, the pressure in the flask decreased. This pressure was monitored with a solid state pressure sensor/transducer (PX209‐30V15GI) from Omega. A monotonous pressure declining curve was obtained, revealing the CO2 absorption kinetics (rate of decline) as well as capacity (the final level‐off of the decline).

**Figure 1.** Batch mode gas‐liquid absorption apparatus.

The solvents used and their concentrations in aqueous solution are shown in **Table 1**. In the test, the primary solvent was aqueous potassium carbonate, K2CO3. Other solvents were used as secondary promoters to see if there was a synergistic effect between the primary and secondary solvents. The hybrid solvents were obtained by mixing the individual solvents (shown in **Table 1**) with certain ratio (quantity in ml). Water was added to adjust the effective concentration and the final volume in a test.

Three test series were completed, one for each of the secondary solvents. These included:

Test Series 1—K2CO3 (primary solvent) with PZ (secondary solvent)


Test Series 2—K2CO3 (primary solvent) with MEA (secondary solvent)


Test Series 3—K2CO3 (primary solvent) with NaOH (secondary solvent)



**Table 1.** Properties of chemicals and solvents used in the experiment.

#### **3. Results and discussion**

The CO2 absorption results for test series 1 are shown in **Figure 2**. After the solvent was introduced into the flask filled with CO2/air, the chemisorption occurred as demonstrated by the pressure decrease. From the results in **Figure 2**, it can be seen that K2CO3 showed a slow absorption rate and low absorption capacity. Piperazine's CO2 absorption rate was faster and had higher capacity. When the two solvents were mixed, the binary solvent system absorbed more CO2 at an even faster rate. The mathematical sum of the individual CO2 absorption curves of the K2CO3 and piperazine (the sum of the green curve and the light blue curve) is shown in **Figure 2** as well (dark blue line). It is clear that the binary solvent system performed much better for CO2 absorption than the mathematical sum of the individual solvents. The two curves (orange and purple in **Figure 2**) showing the CO2 absorption results of the binary solvent system from two different tests under the same conditions indicate that the apparatus worked very well with a high degree of repeatability.

Test Series 1—K2CO3 (primary solvent) with PZ (secondary solvent)

Test Series 2—K2CO3 (primary solvent) with MEA (secondary solvent)

Test Series 3—K2CO3 (primary solvent) with NaOH (secondary solvent)

**Density (g/cm3 )**

C2H7NO 1.01 61.08 15

The CO2 absorption results for test series 1 are shown in **Figure 2**. After the solvent was introduced into the flask filled with CO2/air, the chemisorption occurred as demonstrated by the pressure decrease. From the results in **Figure 2**, it can be seen that K2CO3 showed a slow absorption rate and low absorption capacity. Piperazine's CO2 absorption rate was faster and

**Molar mass (g/mol)**

**Concentration used**

**Structure**

**(% wt)**

**•** 3 ml NaOH/7 ml H2O (NaOH represents its solution in **Table 1**)

**•** 3 ml MEA/7 ml H2O (MEA represents its solution in **Table 1**)

**•** 7 ml K2CO3/3 ml H2O (K2CO3 represents its solution in **Table 1**)

**•** 3 ml PZ/7 ml H2O (PZ represents its solution in **Table 1**)

**•** 7 ml K2CO3/3 ml PZ

142 Recent Advances in Carbon Capture and Storage

**•** 7 ml K2CO3/3ml H2O

**•** 7 ml K2CO3/3 ml MEA

**•** 7 ml K2CO3/3 ml H2O

**•** 7 ml K2CO3/3 ml NaOH

Ethanolamine (MEA)

**Solvent Molecular.** 

**3. Results and discussion**

**formula**

Potassium carbonate K2CO3 2.43 138.21 33

Piperazine (PZ) C4H10N2 1.98 86.14 16

Sodium hydroxide NaOH 2.13 40.00 15

**Table 1.** Properties of chemicals and solvents used in the experiment.

**Figure 2.** Test series 1—CO2 absorption with binary solvent system of K2CO3 and piperazine.

The test results of the binary solvent system of K2CO3 and MEA are shown in **Figure 3**. The component solvents of K2CO3 and MEA were of similar effectiveness for CO2 absorption. The binary solvent system showed only a slight synergistic effect.

**Figure 3.** Test series 2—CO2 absorption with binary solvent system of K2CO3 and MEA.

In order to investigate the necessary and/or sufficient conditions for the synergistic effect of a stronger CO2 solvent with a milder solvent (e.g., PZ with K2CO3), the binary solvent system of K2CO3 with NaOH was tested (**Figure 4**). It can be seen from **Figure 4** that, although NaOH is a much stronger CO2 solvent than K2CO3, the binary solvent system of K2CO3 and NaOH does not show any synergistic effect.

**Figure 4.** Test series 3—CO2 absorption with binary solvent system of K2CO3 and NaOH.

**Figure 5.** The CO2 absorption by binary solvent versus the ratio of K2CO3:PZ.

Therefore, it is a necessary but not a sufficient condition for a binary solvent system with different CO2 absorption capacities and kinetics to generate synergistic effect. Among the three pairs, only the binary solvent of K2CO3 and PZ showed a positive synergistic effect on CO2 absorption.

In order to investigate the necessary and/or sufficient conditions for the synergistic effect of a stronger CO2 solvent with a milder solvent (e.g., PZ with K2CO3), the binary solvent system of K2CO3 with NaOH was tested (**Figure 4**). It can be seen from **Figure 4** that, although NaOH is a much stronger CO2 solvent than K2CO3, the binary solvent system of K2CO3 and NaOH does

**Figure 4.** Test series 3—CO2 absorption with binary solvent system of K2CO3 and NaOH.

**Figure 5.** The CO2 absorption by binary solvent versus the ratio of K2CO3:PZ.

not show any synergistic effect.

144 Recent Advances in Carbon Capture and Storage

As shown by our experiment (**Figure 2**) and others [19], PZ is a stronger and faster CO2 solvent than K2CO3. When the ratio of K2CO3 and PZ was varied, the CO2 absorption curves shifted from the curve of K2CO3 to the curve of PZ, as shown in **Figure 5**. The binary solvent systems between the two pure solvents exhibit synergistic effect. Illustrated in **Figure 6** is the synergistic performance of the binary solvent as well as the relationships with the two pure solvents (this is only a general illustration).

**Figure 6.** Illustration of synergistic effect by a binary solvent system, e.g., K2CO3/PZ.

PZ is an expensive solvent. Whether or not it is suitable, alone, as a CO2 capture solvent is still being explored in terms of thermal stability, corrosiveness and cost, etc. [19]. As shown by this study, it is promising to apply a binary solvent of K2CO3 and PZ at a ratio that maximizes the synergistic effect on CO2 capture. Savings from operating at this condition could be realized in terms of solvent cost, reduction of the absorber and regenerator sizes due to the improved CO2 absorption rate and capacity. More effective solvents would require smaller absorbers and regenerators, leading to lower capital costs.

J. Tim Cullinane and Gary T. Rochelle have reported the promotional effect of K2CO3 and PZ by kinetics [18]. They concluded that the promotional effect comes from the kinetics of the two individual solvents and that the two solvents absorb CO2 independently. These cannot explain the observations of this study. The promotional or synergistic effect of PZ to K2CO3 has been suggested to occur through an intermediate formed between CO2 (aq) and PZ [20–22]. This hypothesis, however, still needs to be verified experimentally. Our results indicate that there may be a more interactive mechanism affecting the hybrid solvent performance. Having a binary solvent system with one solvent more effective than the other is a necessary condition for the synergistic effect (the pairs of K2CO3 and PZ, K2CO3, and NaOH), but not a sufficient condition (K2CO3 and NaOH). There must be other reasons behind the synergistic effect. Here we postulate two mechanisms:


The factors of electron donner strength, dielectric constants, solubility parameters of the individual absorbent, and hydrogen‐bonding/nonhydrogen‐bonding may influence the degree of synergistic effects. There needs more research work to capture and characterize the reactive intermediate complex or transition state, to prove or disprove these postulated mechanisms.

#### **4. Conclusion**

The idea of combining solvents to improve absorption is effective for piperazine and K2CO3. These two solvents interact together and generate a greater absorption than each of the individual solvents. The other solvents, i.e., MEA and NaOH, when mixed with K2CO3 did not improve CO2 absorption, implying that the synergistic effect only occurs selectively between specific pairs of solvents. The solution of 3 ml piperazine with 7 ml potassium carbonate is the optimal ratio that increases CO2 absorption using the least amount of piperazine. The results of these tests show the possibility of using piperazine and K2CO3 solution at an industrial scale. If correctly implemented, it would result in savings in capital by reducing the absorber size compared to use K2CO3 alone. The next step for this project is to apply these results within a larger system. The major conclusions from the tests conducted are summarized below:


## **Acknowledgements**

binary solvent system with one solvent more effective than the other is a necessary condition for the synergistic effect (the pairs of K2CO3 and PZ, K2CO3, and NaOH), but not a sufficient condition (K2CO3 and NaOH). There must be other reasons behind the synergistic effect. Here

**•** CO2 transition (or spill over or migration): CO2 is reactivated by solvent B forming a labile state [[B] · [CO2](aq)], then transfers or migrates to solvent A to finish CO2 absorption

**•** Reactive complex intermediate structure between the two solvents: in the CO2 absorption system, there occur some kind of interactions between the two solvents by hydrogen bonding or local ionic attraction, forming a more reactive intermediate complex [A·B] with

The factors of electron donner strength, dielectric constants, solubility parameters of the individual absorbent, and hydrogen‐bonding/nonhydrogen‐bonding may influence the degree of synergistic effects. There needs more research work to capture and characterize the reactive intermediate complex or transition state, to prove or disprove these postulated

The idea of combining solvents to improve absorption is effective for piperazine and K2CO3. These two solvents interact together and generate a greater absorption than each of the individual solvents. The other solvents, i.e., MEA and NaOH, when mixed with K2CO3 did not improve CO2 absorption, implying that the synergistic effect only occurs selectively between specific pairs of solvents. The solution of 3 ml piperazine with 7 ml potassium carbonate is the optimal ratio that increases CO2 absorption using the least amount of piperazine. The results of these tests show the possibility of using piperazine and K2CO3 solution at an industrial scale. If correctly implemented, it would result in savings in capital by reducing the absorber size compared to use K2CO3 alone. The next step for this project is to apply these results within a larger system. The major conclusions from the tests conducted are summarized below:

**•** This synergistic effect only happens between this specific pair of solvents and is not universal. Other than the thermodynamic reasons behind the effect, there seems to be some additional mechanism that enhances the reaction (potentially a labile [CO2] formation followed by migration or some more reactive intermediate complex structure formed

**•** 3 ml piperazine/7 ml K2CO3 ratio is the most effective (faster absorption rate and higher

**•** A synergistic effect between K2CO3 and piperazine was observed.

between the two solvent molecules).

absorption capacity).

we postulate two mechanisms:

146 Recent Advances in Carbon Capture and Storage

improved CO2 absorption ability.

mechanisms.

**4. Conclusion**

(**Figure 6**). Likely hydrogen bonding is involved.

The project was financially supported by the Canadian Federal Government ecoEII Program. Thanks goes to Mr. Zlatko Lovrenovic, coop student from University of Ottawa, for his contribution to the project. An extra financial support from AirScience Technologies Inc., Montreal, Canada, is gratefully acknowledged.

#### **Author details**

Quan Zhuang\* and Bruce Clements

\*Address all correspondence to: quan.zhuang@canada.ca

Natural Resources Canada, CanmetENERGY, Ottawa, Ontario, Canada

#### **References**

