**1. Introduction**

For the last decades, the demand for renewable energy has been increasing intensively due to the crude-oil crisis and the alert of global warming. Among the alternatives for fossil fuels to generate heat, biomass is an abundant neutral carbon source, of which its conversion to heat does not break the balance of the atmosphere's air contents [1]. Combustion of biomass has been the most direct and simple process to produce energy. However, the traditional combustion of biomass, such as wood, charcoal, straw, husks, etc., often leads to the emission of smoke, dust, fumes, volatile compounds and toxic gases due to incomplete reactions and fine particles dragged out of the system by the flue gas [2]. Although several combustion methods were invented to increase efficiency and reduce emission of pollutants, such as fixed bed rocket type, and fluidized bed technology, the direct combustion of solid fuels is still one of the major causes of the industrial air pollutant in the world [3].

In contrast, gasification of biomass can minimize the emission of pollutants. Syngas produced from gasification of biomass can be optionally purified before being combusted. Ultimately, the combustion of gaseous fuels inherently has higher efficiency than that of solid matters. That is because the oxidation of a solid object in oxygen/air is gradually happening from its outer surface into the inner layers, which can be described as a heterogeneous process, while a combustive gas like syngas can be burned at a very high mass transfer rate in a homogeneous process. A comparison is presented in **Table 1**.

The gasification phenomenon of carbonaceous materials was possibly observed in the human history as very early as the invention of fire. Gasification was found as the ignition and combustion of smoke released from smoldering coal, wood, straw,


#### **Table 1.**

*Combustion of syngas vs. combustion of solid biomass.*

#### **Figure 1.**

*Gasification of oil-extracted cashew nut shell at Laboratory of Biofuel and Biomass Research, Ho chi Minh City University of Technology (HCMUT).*

In this context, to simplify the theory, biomass can be formulated with its main general composition CaHbOc due to the much lower contents of other elements, such as N, S, P, and halogens. The involvement of inorganic compounds is not considered.

Ash

**Gasification of biomass Direct combustion of biomass**

sustainable operation.

Minimized Silica fume, dusty aerosol, and corrosive gases

Smoky and dusty with fly ash.

can shorten the lifetime of equipment.

The biomass fuel must have acceptable moisture content and relatively flammable to guarantee a

The thermal decomposition of biomass in insufficient presence of oxygen/air, known as incomplete combustion, is the most conventional gasification. Logically,

Pyrolysis: heat from the combustion zone is transferred via radiation, conduction, and convective hot streams to the surrounding biomass where oxygen/air is not sufficient or absent. Due to the heat, pyrolysis occurs to form CO2, CO, CH4, C2H4, H2O, char (C), and other organic solids and liquids as primary tar (2).

Reduction: after the above two steps, hot reactants react in situ with the biomass

Steam reforming methanation : CH4 þ H2O þ 206 kJ*=*mol ! CO þ 3H2 (5) Water gas shift reaction : CO þ H2O ! CO2 þ H2 þ 40*:*9 kJ*=*mol (6)

Reverse water gas shift reaction : CO2 þ H2 þ 41*:*2 kJ*=*mol ! H2O þ CO (8)

Water gas reaction : C þ H2O þ 118*:*5 kJ*=*mol ! CO þ H2 (3) Methanation reaction : C þ 2H2 ! CH4 þ 87*:*5 kJ*=*mol (4)

Boudouard reaction : C þ CO2 þ 159*:*9 kJ*=*mol ! 2CO (7)

CaHbOc ð Þþ biomass O2 ! CO2 þ H2O þ Q kJ ð Þ *=*mol (2)

Drying: firstly, once entering the reactor, the biomass is dried due to heat. Combustion: secondly, a part of the solid biomass, which was ignited and in contact with locally excess oxygen/air, is combusted to generate heat as the energy

the whole process can be described below as rearranged from theory [7].

**2.1 Oxygen/air as gasification agent**

Input feedstock Low-energy-density and wet

*DOI: http://dx.doi.org/10.5772/intechopen.93954*

Output flame Smokeless, free of dust and

Side product Char, ash (solids), tar, bio-

*A brief comparison between biomass gasification and combustion.*

Impact to the heat exchangers'surface

*Gasification of Biomass*

Applicability for internal combustion

Equipment design complexity

Heat receiver arrangement

engines

**Table 2.**

**549**

purified.

biomass is still feasible

toxic gases if the syngas is

Yes No

Complex Simple

oil, wood vinegar (liquids)

Mobile Fixed to the burner

source for later reactions to occur.

and with each other via a series of reactions.

grass, or other organic substances in the lack of oxygen. In 1792, the first industrial gasification system to generate electricity was reported [4]. Gasification is a thermal decomposition process of solid or liquid substances to syngas in the presence of gasification agents through a series of chemical reactions mentioned in the following sections. This technology can help converting variable low-energy-density fuels to combustive gases. It attracts significant interests in both academic and industrial fields. **Figure 1** shows a very strong flame torch produced by gasification of oilextracted cashew nut shell.

Gasification is an advanced technology to convert biomass to syngas fuel under different atmospheres (oxygen/air, steam, H2, CO2). The product syngas can also be used as precursors to synthesize valuable chemicals via Fischer-Tropsch (F-T) reactions [5]. **Table 2** highlights some key differences between gasification and direct combustion of biomass.

#### **2. Biomass gasification reactions**

The combustion of a solid fuel is a thermal and oxidation decomposition with the involvement of oxygen in air. Generally, for biomass, it can be simply expressed as:

$$\rm C\_{a}H\_{b}O\_{c}N\_{d}S\_{e} + O\_{2}/\text{air} \rightarrow CO\_{2} + H\_{2}O + N\_{x}O\_{y} + SO\_{z} + Heat \tag{1}$$

This process can be observed with two visual phenomena: first, thermal decomposition on the outer surface of the solid phase to release volatile and combustive components, which join thermal reactions in the gas phase secondly, as the formation of flames [6]. Differing from direct combustion, gasification limits the process at the first step to produce syngas. Conventionally, oxygen/air is used as gasification agent in this case. However, other gasification agents also can be employed to enhance the conversion efficiency as presented followings.


#### **Table 2.**

grass, or other organic substances in the lack of oxygen. In 1792, the first industrial gasification system to generate electricity was reported [4]. Gasification is a thermal decomposition process of solid or liquid substances to syngas in the presence of gasification agents through a series of chemical reactions mentioned in the following sections. This technology can help converting variable low-energy-density fuels to combustive gases. It attracts significant interests in both academic and industrial fields. **Figure 1** shows a very strong flame torch produced by gasification of oil-

*Gasification of oil-extracted cashew nut shell at Laboratory of Biofuel and Biomass Research, Ho chi Minh City*

**Combustion of syngas from gasification of biomass**

Type of reactions Homogeneous Heterogeneous

Mass transfer rate Almost instant Slow, depending on the solid

Uniformity Very high None Process nature Simple Complex

*Combustion of syngas vs. combustion of solid biomass.*

*Biotechnological Applications of Biomass*

**Direct combustion of solid**

surface – oxygen/air contact

**biomass**

Gasification is an advanced technology to convert biomass to syngas fuel under different atmospheres (oxygen/air, steam, H2, CO2). The product syngas can also be used as precursors to synthesize valuable chemicals via Fischer-Tropsch (F-T) reactions [5]. **Table 2** highlights some key differences between gasification and

The combustion of a solid fuel is a thermal and oxidation decomposition with the involvement of oxygen in air. Generally, for biomass, it can be simply expressed as:

This process can be observed with two visual phenomena: first, thermal decomposition on the outer surface of the solid phase to release volatile and combustive components, which join thermal reactions in the gas phase secondly, as the formation of flames [6]. Differing from direct combustion, gasification limits the process at the first step to produce syngas. Conventionally, oxygen/air is used as gasification agent in this case. However, other gasification agents also can be employed to

enhance the conversion efficiency as presented followings.

CaHbOcNdSe þ O2*=*air ➔ CO2 þ H2O þ NxOy þ SOz þ Heat (1)

extracted cashew nut shell.

*University of Technology (HCMUT).*

**Table 1.**

**Figure 1.**

**548**

direct combustion of biomass.

**2. Biomass gasification reactions**

*A brief comparison between biomass gasification and combustion.*

In this context, to simplify the theory, biomass can be formulated with its main general composition CaHbOc due to the much lower contents of other elements, such as N, S, P, and halogens. The involvement of inorganic compounds is not considered.

### **2.1 Oxygen/air as gasification agent**

The thermal decomposition of biomass in insufficient presence of oxygen/air, known as incomplete combustion, is the most conventional gasification. Logically, the whole process can be described below as rearranged from theory [7].

Drying: firstly, once entering the reactor, the biomass is dried due to heat.

Combustion: secondly, a part of the solid biomass, which was ignited and in contact with locally excess oxygen/air, is combusted to generate heat as the energy source for later reactions to occur.

$$\rm C\_aH\_bO\_c \ (biomass) + O\_2 \to CO\_2 + H\_2O + Q \ (kJ/mol) \tag{2}$$

Pyrolysis: heat from the combustion zone is transferred via radiation, conduction, and convective hot streams to the surrounding biomass where oxygen/air is not sufficient or absent. Due to the heat, pyrolysis occurs to form CO2, CO, CH4, C2H4, H2O, char (C), and other organic solids and liquids as primary tar (2).

Reduction: after the above two steps, hot reactants react in situ with the biomass and with each other via a series of reactions.

$$\text{Water gas reaction}: \text{C} + \text{H}\_2\text{O} + \text{118.5 kJ/mol} \rightarrow \text{CO} + \text{H}\_2\tag{3}$$

Methanation reaction : C þ 2H2 ! CH4 þ 87*:*5 kJ*=*mol (4)

	- Water gas shift reaction : CO þ H2O ! CO2 þ H2 þ 40*:*9 kJ*=*mol (6)
		- Boudouard reaction : C þ CO2 þ 159*:*9 kJ*=*mol ! 2CO (7)

The main weakness of gasification by oxygen/air is due to a large portion of inert nitrogen in the agent (79–80%), which makes the resulted syngas diluted. It can be roughly estimated that syngas from this type of gasification mainly contains around 30–60% of nitrogen and 10–15% of CO2 since its heating value is typically between 4 and 6 MJ/m<sup>3</sup> (for comparison, HHV of H2 = 12.76 MJ/m<sup>3</sup> , CO = 12.63 MJ/m<sup>3</sup> , CH4 39.76 MJ/m<sup>3</sup> and CH4 is commonly much less than CO and H2) [7–9]. Low quality syngas is the main disadvantage of this technique for applications which require high temperature and steady operation, such as internal combustion engine, metallurgy, and melting glass industries.

Air-based gasification processes are sensitive and complex, which are influenced by a number of factors, such as biomass composition and particle geometry, gasification agent composition and flow rate, equipment design, etc. Among these, the ratio of actual air-fuel ratio to the stoichiometric air-fuel ratio (ER) is used as a parameter to calculate and to simulate the process [10].

$$ER = \frac{\text{Stoichiometric Air (Nm}^3)}{\text{Actual Air Supplied } \left(\text{N}m^3\right)} \text{ and ER < 1.0} \tag{9}$$

**2.3 Saturated and superheated steam as gasification agent**

*SCR* ¼

**2.4 Other gasification agents: H2 and CO2**

pressure, which rises several safety concerns.

purposes of CO2-gasification [27].

750–800°C [19].

*Gasification of Biomass*

*DOI: http://dx.doi.org/10.5772/intechopen.93954*

process.

reactions.

**551**

Water gas (3) and water gas shift (6) reactions are the reasons steam can be introduced to oxygen/air gasification or wet biomass is accepted, of which moisture is more tolerated than that in direct combustion. Higher generation yields of H2 and CO are obtained so the final syngas mixture gets higher heating value. However, these two reactions are endothermic while the vaporization enthalpy of water has a large value (at atmospheric pressure that is 40.65 kJ/mol) so

saturated steam or water can make the pyrolysis zone lose heat, drop temperature, leading to lower conversion yield. Lower quantity becomes a contrast to higher quality of syngas formation in this case. Subsequently, the process even gets faded if sufficient heat is not guaranteed. To achieve both quantity and quality of syngas, heat should be redeemed by using superheated steam instead of saturated steam or water in wet biomass so that the gasification temperature is maintained above

The ratio of steam to carbon content of the biomass fuel (SCR) is used as a crucial operating parameter in biomass gasification with steam feeding [20]:

*Steam mass flow rate kg*

*Carbon feed rate kg*

Steam flow rate (kg/s) to biomass (kg/s) ratio (S/B) is also used like SCR [21].

Not many studies on gasification by hydrogen and carbon dioxide were found although these two agents are reactants in methanation (4) and Boudouard (7)

Methanation reaction can be increased when more H2 exists in the reaction zone of a gasifier. Since methanation is exothermic, hydrogen can be mixed with air in air-based gasification or can be used as the only gasification agents without slagging problems in the gasifiers like conventional oxygen/air gasification. Pure hydrogen gasification is expected to be able to run at lower temperature and milder conditions because less heat is generated from methanation reaction (ΔH = �87.5 kJ/mol) than from combustion step in air-based gasification [23], which may lead to the absence of oils and tars [24]. However, catalysts are needed because the reaction rates are very low [25]. Otherwise, hydrogen gasification should be carried out in high H2

CO2 is a Boudouard reactant, as well as it can react with H2 in the mixture via reverse water gas shift reaction. Hot flue gas is a popular product in industry, which includes steam, CO2, and heat from direct combustion of fuel, thus can be considered as a gasification agent [26]. This technique is available if a combustion process

is combined with gasification because air-based gasification already has its combustion zone. CO2 utilization and enhancement of CO formation can be the

Steam feeding makes the ratio of hydrogen to carbon in the whole reaction mixture increase, which was found to yield more H2, and increase the heating value of the syngas, while tar content decreases significantly [22]. This technique is positively meaningful in biomass gasification because it does not only increase the quality of the syngas but also reduce tar-clogging problems to sustain the

*s* 

(10)

*s*

Gasification ER is theoretically usually from 0.19 to 0.43, and a range of 0.25–0.29 was studied to be considered as the optimum ER in gasification of some popular biomass [11].

### **2.2 Oxygen-enriched air**

To obtain more concentrated syngas, nitrogen must be limited from the gasification agent in air-based systems while sufficient oxygen is still guaranteed for combustion to generate heat [12]. This method does not change the nature of the gasification process since nitrogen is an inert gas not involved in the reactions. Several techniques were introduced to remove nitrogen, thus increase oxygen content in the input air stream, such as pressure swing adsorption (PSA) [13], temperature swing adsorption [14], carbon membranes [15], etc. Oxygen concentration in studies on gasification with oxygen- enriched air is found limited by less than 50%, and no study on 100% oxygen gasification, possibly because of a high risk of explosion [16–18].

**Figure 2** shows the visual change in an air-based syngas flame (wood pellet as feedstock) when oxygen concentration in the gasifying agent increased from that of normal air to 30%. With normal air, the syngas flame is thinner with smoke, while oxygen-enriched air makes the flame stronger, thicker, and less smoke. The flame temperature was measured as 874 and 933°C, respectively.

**Figure 2.**

*Experimental gasification of wood pellet (a) showing the flame of syngas when using (b) normal air (21% vol. as O2) and (c) oxygen-enriched air (30% vol. as O2)*

The main weakness of gasification by oxygen/air is due to a large portion of inert nitrogen in the agent (79–80%), which makes the resulted syngas diluted. It can be roughly estimated that syngas from this type of gasification mainly contains around 30–60% of nitrogen and 10–15% of CO2 since its heating value is typically between

39.76 MJ/m<sup>3</sup> and CH4 is commonly much less than CO and H2) [7–9]. Low quality syngas is the main disadvantage of this technique for applications which require high temperature and steady operation, such as internal combustion engine, metal-

Air-based gasification processes are sensitive and complex, which are influenced by a number of factors, such as biomass composition and particle geometry, gasification agent composition and flow rate, equipment design, etc. Among these, the ratio of actual air-fuel ratio to the stoichiometric air-fuel ratio (ER) is used as a

*Actual Air Supplied Nm*<sup>3</sup> and ER<1*:*<sup>0</sup> (9)

, CO = 12.63 MJ/m<sup>3</sup>

, CH4

4 and 6 MJ/m<sup>3</sup> (for comparison, HHV of H2 = 12.76 MJ/m<sup>3</sup>

parameter to calculate and to simulate the process [10].

*ER* <sup>¼</sup> *Stoichiometric Air Nm*<sup>3</sup>

Gasification ER is theoretically usually from 0.19 to 0.43, and a range of 0.25–0.29 was studied to be considered as the optimum ER in gasification of some

To obtain more concentrated syngas, nitrogen must be limited from the gasification agent in air-based systems while sufficient oxygen is still guaranteed for combustion to generate heat [12]. This method does not change the nature of the gasification process since nitrogen is an inert gas not involved in the reactions. Several techniques were introduced to remove nitrogen, thus increase oxygen content in the input air stream, such as pressure swing adsorption (PSA) [13], temperature swing adsorption [14], carbon membranes [15], etc. Oxygen concentration in studies on gasification with oxygen- enriched air is found limited by less than 50%, and no study on 100% oxygen gasification, possibly because of a high risk of

**Figure 2** shows the visual change in an air-based syngas flame (wood pellet as feedstock) when oxygen concentration in the gasifying agent increased from that of normal air to 30%. With normal air, the syngas flame is thinner with smoke, while oxygen-enriched air makes the flame stronger, thicker, and less smoke. The flame

*Experimental gasification of wood pellet (a) showing the flame of syngas when using (b) normal air*

temperature was measured as 874 and 933°C, respectively.

*(21% vol. as O2) and (c) oxygen-enriched air (30% vol. as O2)*

lurgy, and melting glass industries.

*Biotechnological Applications of Biomass*

popular biomass [11].

explosion [16–18].

**Figure 2.**

**550**

**2.2 Oxygen-enriched air**
