**3. Technologies of biomass gasification**

Gasification process converts biomass, a low-energy density material, into a gaseous product (LHV at 4–11 MJ/N/m3 ), which is a mixture of CO, H2 , CH4 and CO2 [10]. Gasification is a partial oxidation process and it is commonly operated at 800–900° C for biomass gasification [2]. In some cases, steam is also used as the gasification agents. The gaseous products from the gasifier can be utilized in gas engines or gas turbines for the generation of electricity. In terms of economics, it has also been proven that the performance of a biomass gasification plant with a combined cycle gas turbine (CCGT) is comparable to that of a conventional coal power plant [7], if not better.

#### **3.1. Types of gasifiers**

The gasifier, as the principle component of a gasification plant, actually provides a space for biomass and gasification agent being mixed to a certain extent, in some cases with catalysts or additives [14]. The different selection of gasifiers is actually responsible for keeping steady the production of syngas regarding the variations of biomass. Literature shows that gasifiers could be categorized into three main types: fixed bed gasifiers, fluidized gasifiers and the entrained flow gasifiers [15].

#### *3.1.1. Fixed bed gasifier*

Fixed bed gasifiers is the traditional approach applied for biomass gasification and generally operated around 1000° C. An alternative name for the fixed bed gasifier is "moving bed reactor". This is due to the movement of the biomass material in the main flow direction with very slow flowrate. The fixed bed gasifiers could be principally classified as updraft (countercurrent) and downdraft (co-current) due to the different airflow direction [14].

For the downdraft gasifier (shown in **Figure 3**), both biomass and gasification agent flow into the vessel from the top. At the "throated" area, where air or O2 is fed into system with homogeneously distribution. The temperature could rise to around 1200–1400° C, which leads to both combustion and pyrolysis of the fuel. The produced hot gases will then be reduced to H2 and CO as the main components after passing the hot char bed and will leave the gasifier unit at temperatures of about 900–1000° C. The tar content of the product gas is lower than that of the updraft gasifier, but the particulate content of the gas is higher [16]. Hence, the downdraft gasifier is suitable for downstream applications like internal combustion engines electricity generator. However, the product is withdrawn at a relatively high temperature; it needs to be

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In the fluidized gasifier, the gasification agent enters the bed at a relatively fast rate from the bottom of the vessel and exits from the top. This kind of gasification features uniform temperature distribution in the bed zone. The consistency of temperature is obtained by the application of air-fluidized bed material, which ensured the intimate mixing of fuel, hot combustion gas and bed material. Currently, three main types of fluidized gasifiers are widely used [15], bubbling fluidized bed (BFB), circulating fluidized bed (CFB) and dual

cooled to acceptable range before further usage.

*3.1.2. Fluidized gasifier*

fluidized bed (DFB).

**Figure 3.** Schematic of downdraft gasifier [16].

In an updraft gasifier (shown in **Figure 2**), the biomass material is fed from the top of the reactor, while the gasification agent enters from the bottom. The gasification agent flows through the bed of ash and biomass. The gas generated is exhausted through the top. For the reaction, the gasification agent meets the bottom char at first and achieves a complete combustion and raises temperature to c.a. 1000° C with production of H2 O and CO2 . This hot gas dries the incoming biomass near the top of the vessel and provides heat for pyrolysis of the descending biomass as well as percolates through the unreacted char bed to produce H2 and CO [15]. In this gasification system, the product gas is withdrawn from the low temperature zone; thus, the product would be contaminated with significant amount of tars. If the product is used for further downstream applications like fuel in combustion engine electricity generator, a set of cleaning processes for tar removal is essential. However, the cleaning processes require intensive operation and establishment; therefore, the application of updraft gasification is not suitable for internal combustion engines [1].

**Figure 2.** Schematic of updraft gasifier [16].

For the downdraft gasifier (shown in **Figure 3**), both biomass and gasification agent flow into the vessel from the top. At the "throated" area, where air or O2 is fed into system with homogeneously distribution. The temperature could rise to around 1200–1400° C, which leads to both combustion and pyrolysis of the fuel. The produced hot gases will then be reduced to H2 and CO as the main components after passing the hot char bed and will leave the gasifier unit at temperatures of about 900–1000° C. The tar content of the product gas is lower than that of the updraft gasifier, but the particulate content of the gas is higher [16]. Hence, the downdraft gasifier is suitable for downstream applications like internal combustion engines electricity generator. However, the product is withdrawn at a relatively high temperature; it needs to be cooled to acceptable range before further usage.

#### *3.1.2. Fluidized gasifier*

*3.1.1. Fixed bed gasifier*

8 Gasification for Low-grade Feedstock

Fixed bed gasifiers is the traditional approach applied for biomass gasification and generally operated around 1000° C. An alternative name for the fixed bed gasifier is "moving bed reactor". This is due to the movement of the biomass material in the main flow direction with very slow flowrate. The fixed bed gasifiers could be principally classified as updraft (countercur-

In an updraft gasifier (shown in **Figure 2**), the biomass material is fed from the top of the reactor, while the gasification agent enters from the bottom. The gasification agent flows through the bed of ash and biomass. The gas generated is exhausted through the top. For the reaction, the gasification agent meets the bottom char at first and achieves a complete combustion and

incoming biomass near the top of the vessel and provides heat for pyrolysis of the descending

this gasification system, the product gas is withdrawn from the low temperature zone; thus, the product would be contaminated with significant amount of tars. If the product is used for further downstream applications like fuel in combustion engine electricity generator, a set of cleaning processes for tar removal is essential. However, the cleaning processes require intensive operation and establishment; therefore, the application of updraft gasification is not

O and CO2

. This hot gas dries the

and CO [15]. In

rent) and downdraft (co-current) due to the different airflow direction [14].

biomass as well as percolates through the unreacted char bed to produce H2

raises temperature to c.a. 1000° C with production of H2

suitable for internal combustion engines [1].

**Figure 2.** Schematic of updraft gasifier [16].

In the fluidized gasifier, the gasification agent enters the bed at a relatively fast rate from the bottom of the vessel and exits from the top. This kind of gasification features uniform temperature distribution in the bed zone. The consistency of temperature is obtained by the application of air-fluidized bed material, which ensured the intimate mixing of fuel, hot combustion gas and bed material. Currently, three main types of fluidized gasifiers are widely used [15], bubbling fluidized bed (BFB), circulating fluidized bed (CFB) and dual fluidized bed (DFB).

**Figure 3.** Schematic of downdraft gasifier [16].

BFB gasifier applies inlet from the bottom and moves the bed of fine-grained materials. The bed temperature is maintained at 700–900° C by manipulating the ratio of fed biomass and gasification agent [16]. The flowrate of gasification agent is set to be slightly greater than the minimum velocity of fluidization of the bed material. The biomass is decomposed into char and gas products with a low tar percentage.

The CFB gasifier consists of two principle units: the gasifier unit and the circulation unit, as shown in **Figure 4**. The bed material and char in this type of gasifier is circulated between the reaction chamber and the cyclone separator, where ash and hot gas could be separated. The bed material is fully fluidized and leaves from the first unit, and then it is sent back by the second unit. The solids are moving in the solid circulation loop in greater extent of fluidization with higher residence time. Moreover, its operation pressure is also relatively higher.

Dual fluidized bed (DFB) gasifiers consist of two separated fluidized beds which are used for pyrolysis process and combustion process [14]. The first bed is operated as a pyrolysis reactor and it is heated by the second reactor with hot circulated bed material. The second reactor provides heat by burning char provided from the first reactor. The bed material plays an important role as a heat transfer medium, which prevents the dilution of the hot gas product.

#### *3.1.3. Entrained flow gasifier*

Entrained flow gasifiers are generally classified into two types: top-fed gasifier and side-fed gasifier (shown in **Figure 5**), which is according to how and where the fuel and gasification agent is fed. This type of gasifier is suitable for integrated gasification combined cycle (IGCC) plants. It is extensively applied in large-scale gasification and is widely employed for coal, biomass and refinery residues. The gasification temperature of this kind of gasifier could reach 1400° C with a pressure range of 20–70 bar [14]. This high temperature could accelerate tar cracking and mitigate severe tar issue of biomass gasification. However, this kind of high temperature gasification requires a finely fed biomass material (<0.1–0.4 mm), which makes this process unsuitable for most biomass materials (such as wood). Therefore, this process is

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Tar is a major inherent problem in biomass gasification; it can cause a lot of issues such as equipment blockages, lower system efficiency, poor quality gas output and increased maintenance. Tar consists of a group of very complicated mixtures with more than 200 components. Several key components include benzene, toluene, single-ring aromatic hydrocarbon, naphthalene and so on. The formation of tar was due to lower temperature of gasification. It was confirmed that increased temperature of gasification could reduce the content of tar in the outflow and it was believed that higher temperature can promote the cracking of tar [18]. Currently, there are a lot of methods that could be employed for tar minimization, and they can be divided into two categories depending on where the removal technology is applied. Firstly, tar could be removed inside the gasifier by choosing an appropriate operation parameter or using a catalyst. Previous research indicates that both particle size and surface areavolume ratio of loading feedstock have a significant effect on tar yields [19, 20]. It showed that the gasification of pine saw dust only produced 0.4 wt% of tar at 700° C when the particle size was smaller than 75 micron. While if particle size increased to the range of 600–1000 micron, the tar yield would be higher than 10 wt% even at 900° C. From the view of thermal kinetics, the gasification of larger size of particles needs to overcome greater resistance of thermal conductivity; in other words, it needs more time to complete heat transfer and the devolatilization of biomass materials. On the other hand, small particle size also can contribute to a fast diffusion of the gasification agent and shorten time duration of the whole process. However,

not considered in detail.

**Figure 5.** Schematic diagram of an entrained flow gasifier (side-fed) [17].

**3.2. Tar removal**

**Figure 4.** Schematic diagram of circulating fluidized bed gasifier (CFB) [17].

Biomass Gasification: An Overview of Technological Barriers and Socio-Environmental Impact http://dx.doi.org/10.5772/intechopen.74191 11

**Figure 5.** Schematic diagram of an entrained flow gasifier (side-fed) [17].

temperature gasification requires a finely fed biomass material (<0.1–0.4 mm), which makes this process unsuitable for most biomass materials (such as wood). Therefore, this process is not considered in detail.

### **3.2. Tar removal**

BFB gasifier applies inlet from the bottom and moves the bed of fine-grained materials. The bed temperature is maintained at 700–900° C by manipulating the ratio of fed biomass and gasification agent [16]. The flowrate of gasification agent is set to be slightly greater than the minimum velocity of fluidization of the bed material. The biomass is decomposed into char

The CFB gasifier consists of two principle units: the gasifier unit and the circulation unit, as shown in **Figure 4**. The bed material and char in this type of gasifier is circulated between the reaction chamber and the cyclone separator, where ash and hot gas could be separated. The bed material is fully fluidized and leaves from the first unit, and then it is sent back by the second unit. The solids are moving in the solid circulation loop in greater extent of fluidization with

Dual fluidized bed (DFB) gasifiers consist of two separated fluidized beds which are used for pyrolysis process and combustion process [14]. The first bed is operated as a pyrolysis reactor and it is heated by the second reactor with hot circulated bed material. The second reactor provides heat by burning char provided from the first reactor. The bed material plays an important role as a heat transfer medium, which prevents the dilution of the hot

Entrained flow gasifiers are generally classified into two types: top-fed gasifier and side-fed gasifier (shown in **Figure 5**), which is according to how and where the fuel and gasification agent is fed. This type of gasifier is suitable for integrated gasification combined cycle (IGCC) plants. It is extensively applied in large-scale gasification and is widely employed for coal, biomass and refinery residues. The gasification temperature of this kind of gasifier could reach 1400° C with a pressure range of 20–70 bar [14]. This high temperature could accelerate tar cracking and mitigate severe tar issue of biomass gasification. However, this kind of high

higher residence time. Moreover, its operation pressure is also relatively higher.

**Figure 4.** Schematic diagram of circulating fluidized bed gasifier (CFB) [17].

and gas products with a low tar percentage.

10 Gasification for Low-grade Feedstock

gas product.

*3.1.3. Entrained flow gasifier*

Tar is a major inherent problem in biomass gasification; it can cause a lot of issues such as equipment blockages, lower system efficiency, poor quality gas output and increased maintenance. Tar consists of a group of very complicated mixtures with more than 200 components. Several key components include benzene, toluene, single-ring aromatic hydrocarbon, naphthalene and so on. The formation of tar was due to lower temperature of gasification. It was confirmed that increased temperature of gasification could reduce the content of tar in the outflow and it was believed that higher temperature can promote the cracking of tar [18]. Currently, there are a lot of methods that could be employed for tar minimization, and they can be divided into two categories depending on where the removal technology is applied.

Firstly, tar could be removed inside the gasifier by choosing an appropriate operation parameter or using a catalyst. Previous research indicates that both particle size and surface areavolume ratio of loading feedstock have a significant effect on tar yields [19, 20]. It showed that the gasification of pine saw dust only produced 0.4 wt% of tar at 700° C when the particle size was smaller than 75 micron. While if particle size increased to the range of 600–1000 micron, the tar yield would be higher than 10 wt% even at 900° C. From the view of thermal kinetics, the gasification of larger size of particles needs to overcome greater resistance of thermal conductivity; in other words, it needs more time to complete heat transfer and the devolatilization of biomass materials. On the other hand, small particle size also can contribute to a fast diffusion of the gasification agent and shorten time duration of the whole process. However, the small size of feedstock particle required much more energy input during the biomass pre-preparation process. In addition, it is also effective by applying an optimal design of gasification reactor. A collaborative project between Switzerland and India demonstrated that an open-top fixed bed would produce much less tar and particulates than a closed-top fixed bed [15]. The reason behind this is that the open-top fixed bed could introduce dual air from the top and nozzles actually increase the residence time for degrading tar.

reduce the concentration of tar to a certain level and then the second stage employed transition metal-based catalysts bed for near-completed removal of tar. But this kind of two-stage reforming process would increase operational cost clearly. In the research scale, some people applied noble metal catalysts and achieved highly catalytic activity as well as better carbonresistant ability. However, high cost and low accessibility still restrain the wide utilization of noble metal-based catalysts before the technical breakthrough of catalyst regeneration. Alkali metal catalyst is an alternative with good catalytic performance and also exhibits outstanding coke resistance. It is due to this that alkali metal could suppress directly decomposition of hydrocarbon by avoiding quick adsorption of tar components. But alkali metal evaporates under high temperature gasification condition. In many practical process, biomass ash has been reused as an alkali catalyst because most biomass contains abundant alkali metal elements and it is believed that this type of natural catalyst with properties of low cost and

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In the future, the development of novel and economic catalysts is still a promising option for tar elimination. At this stage, the biggest barrier for the catalyst development is the unclear mechanism of complex tar reformation. Therefore, employing model tar components for the study of coke formation mechanism is still very important and will be an effective way out. For the catalyst synthesis, composite catalysts with different components should be considered. It is also favored that if the developed catalyst could be applied under a low temperature condition (400–600° **C**), it will minimize cost effectively in a practical operation by using waste heat. In addition, the practical application of the catalyst also requires solving many scale-up issues, such as variation of temperature and pressure, impurities, fly ash and catalyst collapse

Biomass gasification could exploit an abundant variety of waste materials as feedstock such as agricultural residues and food waste. It actually achieves resource recovery and mitigates CO2 emission as an environmental benefit. However, power generation from biomass gasification

One of the major risks is the potential emission of toxic producer gas and particulates. The

dation of trace elements in feedstock [24]. As one of the most dangerous constituent, CO can permeate into human blood system and combine with hemoglobin to stop oxygen adsorption and distribution. Long-term exposure to CO causes asthma, lung inflammation, schizophre-

, NOx

inhalation, ingestion and dermal system of human [25]. Hence, the entire gasification process should prevent leakage and an efficient gas clean-up system is essential. In recent years, the hazard of particles emission (PM2.5) attracts public attention increasingly, due to its carcinogenicity. PM2.5 particles can adsorb many soluble organic compounds including alkanes,

and volatile organics involves incomplete combustion and oxi-

and volatile organics could also destruct

disposability should attract special attention

**4. Socio-environmental impact**

**4.1. Health and safety hazard**

production of CO, SOx

poses several key hazards and socio-environmental impacts.

, NOx

nia and cardiac defects. Toxic gases like SOx

Secondly, in many processes, tar is removed as a downstream step after gasification, including mechanical method, thermal cracking and catalysis. The details of some common technologies have been listed in **Table 1**. Wet gas cleaning method has been accepted at an early stage. Its equipment investment is relatively low and the operation is also easy to handle. But this technology would also create a lot of waste water and bring serious environmental issues. Therefore, dry gas cleaning method becomes more widespread via various types of filters, rotating particle separators and dry cyclones. Although the dry method avoids waste water issues, its efficiency of tar removal is not good enough if compared with wet method. On the other hand, the replacement, renewal or disposal of filter materials reduces the financial effectiveness of the entire gasification system. This similar situation could also be applied to thermal cracking method and higher operation temperature requires much more energy input.

In the recent two decades, catalytic cracking has attracted more and more attention and has already become the central branch of research. Catalytic cracking is more like a downstream catalytic reforming unit and could easily degrade comparative stable tar to a significant extent. The previous research indicated that the catalytic cracking unit could promote gas yield by 10: 20 vol% and increase the heating value by c.a. 15% [23]. Ni-based catalyst is applied most widely and especially preferred for hydrogen or syngas production. Nickel has a very good catalytic activity and a preferable price advantage. While the application of Ni catalysts needs to avoid extremely high heavy-tar content flue gas, which will form a serious carbon deposition over the catalyst surface and lead to a quick deactivation. The other transition metal-based catalysts, such as co, Fe and cu, also have similar issues. Thus, some applications used the two-stage catalytic reforming process: the first stage used dolomite to


**Table 1.** Post-gasification tar removal methods [15].

reduce the concentration of tar to a certain level and then the second stage employed transition metal-based catalysts bed for near-completed removal of tar. But this kind of two-stage reforming process would increase operational cost clearly. In the research scale, some people applied noble metal catalysts and achieved highly catalytic activity as well as better carbonresistant ability. However, high cost and low accessibility still restrain the wide utilization of noble metal-based catalysts before the technical breakthrough of catalyst regeneration. Alkali metal catalyst is an alternative with good catalytic performance and also exhibits outstanding coke resistance. It is due to this that alkali metal could suppress directly decomposition of hydrocarbon by avoiding quick adsorption of tar components. But alkali metal evaporates under high temperature gasification condition. In many practical process, biomass ash has been reused as an alkali catalyst because most biomass contains abundant alkali metal elements and it is believed that this type of natural catalyst with properties of low cost and disposability should attract special attention

In the future, the development of novel and economic catalysts is still a promising option for tar elimination. At this stage, the biggest barrier for the catalyst development is the unclear mechanism of complex tar reformation. Therefore, employing model tar components for the study of coke formation mechanism is still very important and will be an effective way out. For the catalyst synthesis, composite catalysts with different components should be considered. It is also favored that if the developed catalyst could be applied under a low temperature condition (400–600° **C**), it will minimize cost effectively in a practical operation by using waste heat. In addition, the practical application of the catalyst also requires solving many scale-up issues, such as variation of temperature and pressure, impurities, fly ash and catalyst collapse
