**2.1.1 Chemical absorption**

Chemical absorption is a widely used process with a series of pilot plants distributed around the world. The oldest commercial CCGS plant is located in Sleipner, Norway and has used this process since 1996 (Solomon, 2007).

The process entails using a solvent, normally an amine, which chemically reacts with CO2, forming a compound. As shown in Figure 6, this reaction occurs in an absorption tower, whose size basically depends on the flow of the flue gas from the industrial process. The compound thus formed is transferred to the regeneration unit where its temperature is raised to release the CO2. The solvent free of CO2 then returns to the absorption tower to repeat the cycle.

Fig. 6. Absorption and regeneration processes

One example of commercial chemical absorption processes is the chilled ammonia process (CAP), which was developed by Alstom Power and is utilized in pilot plants to capture carbon developed by that company in partnership with American electric utilities. The first pilot plant, with generating capacity of 1.7 Mwatts, was the Pleasant Prairie thermoelectric plant of WE Energies in Wisconsin. The second was the Mountaineer thermoelectric plant, with capacity of 20 Mwatts, owned by American Electric Power in West Virginia (Sherrick et al., 2009). This plant operated from October 2009 to May 2011, for a total of over 6,500 hours,

Carbon Capture and Storage – Technologies and Risk Management 245

CH4 + H2O CO + 3H2 (7)

Equation 7 is highly endothermic, consuming a high amount of heat, while equation 8 is slightly endothermic, producing only a small amount of heat. After the reformation process, which is carried out in the steam methane reformer (SMR) unit, the synthetic gas (syngas) generated is composed basically of hydrogen and carbon dioxide associated with some impurities, depending on the composition of the natural gas reformed. The syngas is then sent to the adsorption unit, which works by the principle of pressure swing adsorption

The project, which received funding of U\$ 284 million from the US DOE, will include a CO2 separation unit and a drying and compression unit in the process (Figure 8), besides interconnection with an existing pipeline to send the CO2 to the site for geological sequestration. The units are slated to start operating at the end of 2012 and start of 2013 and

The vacuum swing adsorption (VSA) process is a variation of the PSA process, whereby the adsorption is carried out at a pressure near atmospheric pressure and the desorption occurs

In theory the oxy-combustion process involves burning a fuel using O2 instead of air as the oxidant. In this process, the N2 is separated in advance, eliminating the presence of nitrous oxide (N2O) in the exhaust gas. Since the sulfur removal units are already obligatorily included in industrial processes that burn fossil fuels, except for particulates and other impurities the exhaust gas contains a high concentration of CO2. However, all oxycombustion systems in practice work with a mixture of O2 with recirculated exhaust gas. Therefore, the oxy-combustion only increases the CO2 concentration in the exhaust gas,

Fig. 8. Port Arthur 2 with CO2 separation and compressor/drier units

CO + H2O CO2 + H2 (8)

Equations 7 and 8 show the chemical reactions that produce hydrogen from methane.

(PSA) to separate the hydrogen to be exported.

will capture 1 MtCO2 per year.

by producing a vacuum in the chambers.

**2.1.3 Oxy-combustion** 

and reached the goal of validating the technology, capturing over 50 KtCO2 in this period and permanently storing over 37 KtCO2 in a saline aquifer located at a depth of 2,400 meters.

The overall chemical reactions associated with the CAP are defined by equations 1 to 4:

$$\text{CO}\_2\text{(g)} \Leftrightarrow \text{CO}\_2\text{(aq)}\tag{1}$$

$$\text{(NH}\_4\text{)}\_2\text{CO}\_3\text{(aq)} + \text{CO}\_2\text{(aq)} + \text{H}\_2\text{O} \text{ (l)} \Leftrightarrow \text{2(NH}\_4\text{)}\text{HCO}\_3\text{ (aq)}\tag{2}$$

$$\text{(NH}\_4\text{)HCO}\_3\text{(aq)} \Leftrightarrow \text{(NH}\_4\text{)HCO}\_3\text{(s)}\tag{3}$$

$$\text{(NH4)}\\\text{2CO3 (aq)} \Leftrightarrow \text{(NH4)}\\\text{NH4CO2 (aq)} + \text{H}\_2\text{O (l)}\tag{4}$$

These reactions are all reversible and their directions depend on the pressure, temperature and concentration in the system. The equations are exothermic from left to right and endothermic from right to left, requiring the removal or addition of heat.

Besides capturing CO2, the CAP also removes other residual gases in its cleaning and cooling stages, such as SO2, SO3, HCl and HF. Equations 5 and 6 show the overall chemical reactions associated with the removal of SO2.

$$\text{SOz}(\text{g}) + 2\text{NH}\_3(\text{g}) + \text{H}\_2\text{O}(\text{aq}) \Leftrightarrow \text{(NH}\_4)\_2\text{SO}\_3(\text{aq})\tag{5}$$

$$\text{(NHl)}\text{2SO}\text{(aq)} + 1/2\text{O}\_2\text{(g)} \Leftrightarrow \text{(NHl)}\text{2SO}\text{(aq)}\tag{6}$$

#### **2.1.2 Physical adsorption**

Physical adsorption consists of capturing CO2 by the surface of a solid material, such as activated charcoal or a zeolite, placed in the path of the flow of the gas targeted for removal of CO2. The CO2 adsorbs to the surface of the solid particles by surface forces (non-chemical forces). The adsorption process is facilitated by keeping the process at low temperature or high pressure. Once the adsorbent material reaches a determined CO2 saturation level, the exhaust gas flow is diverted to another path and the chamber containing the adsorbent material is heated or its pressure is reduced to release the CO2, in a process called desorption.

An example of the physical adsorption is a project for hydrogen production units in Port Arthur, Texas run by the company Air Products. This was one of the three projects chosen in Phase II of the Industrial Carbon Capture and Sequestration Program (ICCS) of the US Department of Energy (US DOE). The Port Arthur Units 1 and 2, whose block diagrams are shown in Figure 7, work based on the traditional process of reform of natural gas by the action of steam.

Fig. 7. Port Arthur 1 and 2 –Hydrogen production units - Source: Air Products (2011)

and reached the goal of validating the technology, capturing over 50 KtCO2 in this period and permanently storing over 37 KtCO2 in a saline aquifer located at a depth of 2,400 meters.

CO2 (g) CO2 (aq) (1)

(NH4)2CO3 (aq) + CO2 (aq) + H2O (l) 2(NH4)HCO3 (aq) (2)

(NH4)HCO3 (aq) (NH4)HCO3 (s) (3)

 (NH4)2CO3 (aq) (NH4)NH2CO2 (aq) + H2O (l) (4) These reactions are all reversible and their directions depend on the pressure, temperature and concentration in the system. The equations are exothermic from left to right and

Besides capturing CO2, the CAP also removes other residual gases in its cleaning and cooling stages, such as SO2, SO3, HCl and HF. Equations 5 and 6 show the overall chemical

SO2(g) + 2NH3(g) + H2O(aq) (NH4)2SO3(aq) (5)

(NH4)2SO3(aq)+ 1/2O2(g) (NH4)2SO4(aq) (6)

Physical adsorption consists of capturing CO2 by the surface of a solid material, such as activated charcoal or a zeolite, placed in the path of the flow of the gas targeted for removal of CO2. The CO2 adsorbs to the surface of the solid particles by surface forces (non-chemical forces). The adsorption process is facilitated by keeping the process at low temperature or high pressure. Once the adsorbent material reaches a determined CO2 saturation level, the exhaust gas flow is diverted to another path and the chamber containing the adsorbent material is

An example of the physical adsorption is a project for hydrogen production units in Port Arthur, Texas run by the company Air Products. This was one of the three projects chosen in Phase II of the Industrial Carbon Capture and Sequestration Program (ICCS) of the US Department of Energy (US DOE). The Port Arthur Units 1 and 2, whose block diagrams are shown in Figure 7, work based on the traditional process of reform of natural gas by

heated or its pressure is reduced to release the CO2, in a process called desorption.

Fig. 7. Port Arthur 1 and 2 –Hydrogen production units - Source: Air Products (2011)

endothermic from right to left, requiring the removal or addition of heat.

reactions associated with the removal of SO2.

**2.1.2 Physical adsorption** 

the action of steam.

The overall chemical reactions associated with the CAP are defined by equations 1 to 4:

Equations 7 and 8 show the chemical reactions that produce hydrogen from methane.

$$\text{CH}\_4 + \text{H}\_2\text{O} \Leftrightarrow \text{CO} + 3\text{H}\_2\tag{7}$$

$$\text{CO} + \text{H}\_2\text{O} \Leftrightarrow \text{CO}\_2 + \text{H}\_2\tag{8}$$

Equation 7 is highly endothermic, consuming a high amount of heat, while equation 8 is slightly endothermic, producing only a small amount of heat. After the reformation process, which is carried out in the steam methane reformer (SMR) unit, the synthetic gas (syngas) generated is composed basically of hydrogen and carbon dioxide associated with some impurities, depending on the composition of the natural gas reformed. The syngas is then sent to the adsorption unit, which works by the principle of pressure swing adsorption (PSA) to separate the hydrogen to be exported.

The project, which received funding of U\$ 284 million from the US DOE, will include a CO2 separation unit and a drying and compression unit in the process (Figure 8), besides interconnection with an existing pipeline to send the CO2 to the site for geological sequestration. The units are slated to start operating at the end of 2012 and start of 2013 and will capture 1 MtCO2 per year.

Fig. 8. Port Arthur 2 with CO2 separation and compressor/drier units

The vacuum swing adsorption (VSA) process is a variation of the PSA process, whereby the adsorption is carried out at a pressure near atmospheric pressure and the desorption occurs by producing a vacuum in the chambers.
