**1. Introduction**

Evidence suggests that conventional energy production has limited capacity to meet growing demand and that additional demands will have to be met by unorthodox sources. Since the world is now drifting toward sustainable development, renewable energy technologies are gaining traction. One of such renewable energy technologies that has received great attention in recent times include biomass gasification, which is one of three main (combustion and pyrolysis) thermochemical conversion pathways used to recover energy from biomass materials. Gasification

produces energy from biomass and involves heating the biomass at elevated temperatures (above 1000°C) under a limited supply of oxygen to produce a mixture of gases (H2, CO, CO2) collectively referred to as syngas. However, the combustible constituents of the syngas are CO and H2 and can be used as fuel in gas engines for heat and electricity generation as well as for the production of chemicals (such as alcohols, organic acids, ammonia, and methanol) via the Fischer-Tropsch process [1]. The significance of gasification technology is such that it helps waste management, at the same time, produces energy and other valuable products needed for economic growth. Systems designed to gasify coal is assumed to be able to use biomass as well, however, differences in the characteristics of coal and biomass can have a significant impact on the sizing and design of the combustion chamber of the gasification system, as well as on the location of the gasifying agent [2]. A graphical representation of a gasification process, which depicts feedstock flexibility and the production of a wide range of products, is presented in **Figure 1**.

The gasification technology has existed for several decades and has, as of today, been commercialized in very few countries of the world like Sweden, Germany, Canada, the United States, India, and China. The use of this technology offers a number of ecological and economic advantages such as low emission of pollutants, reduction in the environmental effects of waste disposal, generation of non-hazardous by-products when biomass is used as the feedstock, and lower operating cost [4].

Gasification occurs in a gasifier under a series of chemical reactions that are mostly endothermic in nature; however, to provide the heat required for the reactions to proceed successfully, and the heat needed for drying and pyrolysis to occur, a certain amount of exothermic combustion is allowed in the gasifier [4, 5]. The gasification reactions are described in greater detail in subsequent sections. The gasifier and its configuration are key factors that affect the entire gasification process, including the reactions occurring and their products [6]. This is true because gasifiers are generally classified into three broad groups, namely: the fixed bed gasifiers, the fluidized bed

#### **Figure 1.**

*A schematic representation of a gasification process depicting feedstock flexibility and the wide range of products that can be obtained from the process [3].*

*The Technical Challenges of the Gasification Technologies Currently in Use and Ways… DOI: http://dx.doi.org/10.5772/intechopen.102593*


#### **Table 1.**

*The main characteristics of the three types of gasifiers commonly used for the recovery of heat and electricity from biomass [7].*

gasifiers, and the entrained flow gasifiers. **Table 1** shows the main characteristics of these three gasifiers.

Although the gasification technology may be considered as a useful technology for the recovery of energy from biomass materials, the technological choices with regards to the type of gasification system (fixed bed, fluidized beds, or entrained flow reactors) for the conversion of biomass are still faced with a host of technical barriers that have hindered the significant exploitation of the gasification technology and biomass energy as a whole. The quality of the syngas produced from the gasification process, the lack of feedstock flexibility and its mechanism of conversion are the main obstacles. This chapter, therefore, presents an overview of the gasification technology and discusses its main technical barriers with reference to the gasification systems commonly used today. The status of current research in gasification and future research focus are also presented.

### **2. Types of gasification systems and their configurations**

There are different types of gasification systems but the most commonly used are the fixed-bed, fluidized-bed, and entrained-flow gasification systems. The main differences between these gasifiers are connected to their mechanism of heating and the way feedstock and gasifying agents are introduced in the gasification process, as well as by the location of syngas output [8–10]. However, the technological choices toward these gasifiers are guided by the nature and availability of biomass feedstocks. While

#### **Figure 2.**

*Schematic representations of the gasification systems in use today: (a) fixed bed; (b) fluidized bed; (c) entrained flow. Reproduced with permission from [13, 15].*

the characteristics of biomass feedstocks intended for gasification are detailed in [11], the principles of operation of the types of gasifiers mentioned above and their merits and demerits are equally well described in [12, 13] and in [14]. These gasification systems may appear as simple devices but their successful operations are not so simple. The gasifiers are still faced with a host of technical issues that have hindered their broader market penetration. These technological barriers are described in Section 5. Nonetheless, in order to fully comprehend the technical barriers of each of these gasifiers, it is important to understand the differences between the gasifiers in terms of configuration, which also affects the thermodynamics of their operation. Therefore, a schematic diagram of each gasifier type is presented in **Figure 2**.

### **3. The gasification reaction chemistry**

The key mechanism of the gasification technology involves the conversion of solid carbonaceous materials like biomass into flammable gas by partial oxidation. However, the chemistry involved in the process is quite complex and can be achieved via a series of physical and chemical transformation reactions that occur inside the gasification system [4, 16]. The major chemical reactions occurring are those that involve the degradation of large organic molecules into carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), water in the form of steam (H2O), and methane (CH4). These reactions take place in accordance with the chemical bonding theory and can be represented thus [3]:

The combustion reactions include:

$$\text{C} + \forall \text{O}\_2 \rightarrow \text{CO} \ (-111 \text{ MJ/kmol}) \tag{1}$$

$$\text{CO} + \text{V}\text{O}\_2 \rightarrow \text{CO}\_2 \,(-283 \text{ MJ/kmol}) \tag{2}$$

H2 þ ½O2 ! H2O ð Þ �242 MJ*=*kmol (3)

*The Technical Challenges of the Gasification Technologies Currently in Use and Ways… DOI: http://dx.doi.org/10.5772/intechopen.102593*

Other key gasification reactions are:

$$\text{C} + \text{H}\_2\text{O} \leftrightarrow \text{CO} + \text{H}\_2 \text{ (} + \text{131 MJ/kmol)}\tag{4}$$

$$\text{C} + \text{CO}\_2 \leftrightarrow 2\text{CO} \ (+172 \text{ MJ/kmol}) \tag{5}$$

$$\text{C} + 2\text{H}\_2 \leftrightarrow \text{CH}\_4 \text{ (}-75 \text{ MJ/kmol)}\tag{6}$$

The above reactions occur under standard operating conditions of gasification and are considered important reactions that form the major part of the syngas produced in the gasification process [4, 16]. While reaction (4) may be referred to as the "Water-Gas Reaction", reactions (5) and (6) are termed the "Boudouard Reaction" and the "Methanation Reaction" respectively. Reactions (4) and (5) are the main reduction reactions. However, under high carbon conversion conditions, reactions (4)–(6), being heterogeneous in nature, are reduced to the following homogeneous gas-phase reactions [16]:

$$\text{CO} + \text{H}\_2\text{O} \leftrightarrow \text{CO}\_2 + \text{H}\_2 \text{ (} - \text{41 MJ/kmol)}\tag{7}$$

$$\text{CH}\_4 + \text{H}\_2\text{O} \leftrightarrow \text{CO}\_2 + \text{3H}\_2 \text{ (+206 MJ/kmol)}\tag{8}$$

Reactions (7) and (8) are known respectively as the "Water-Gas-Shift Reaction" and the "Steam-Methane-Reforming Reaction". These two reactions (7) and (8) play a key role in determining the final equilibrium of the composition of the syngas produced in the gasification process [3, 16]. Under a limited supply of oxygen to the gasifier, the sulfur composition of the feedstock is converted to hydrogen sulfide (H2S), with a minute amount forming carbonyl sulfide (COS). The nitrogen (N) chemically bound in the feedstock is converted to gaseous nitrogen (N2), ammonia (NH3), and traces of hydrogen cyanide (HCN). The chlorine in the feedstock is mainly converted to hydrogen chloride (HCl). It is important to however state that the concentrations of sulfur, nitrogen and chloride in the feedstock for gasification are sufficiently low that their effects in the gasification process are quite insignificant; trace elements (such as arsenic, mercury, and other heavy metals) that are associated with both the organic and inorganic components of the feedstock are mostly contained in the fractions of ash and slag formed during gasification, as well as in the gases emitted, and must be expunged from the syngas prior to further use [16].

#### **3.1 The kinetics of gasification reactions**

Temperature increases in a gasification process lead to dehydration, volatilization, and degradation of the biomass feedstock. The gasification process reactions are mostly reversible reactions. The order of the reactions and their conversion rates are often subject to the limitations of the reaction kinetics and thermodynamic equilibrium of the gasification process. For instance, reactions (1)–(3) presented in a previous section are combustion reactions that actually go to completion when equilibrium positions of the reactions shift to the right. However, not all reactants in a gasification process can be completely converted into products; as such, stoichiometric calculations may be required to determine the products of a completed reaction [5].

While the kinetics of a reaction can determine how fast products are formed and whether the reactions in the gasifier go to completion, the equilibrium state of the reaction determines to what extent the reaction can progress. The thermal efficiency of the gasification process and the composition of the syngas produced are strongly

influenced by the thermodynamic equilibrium of the water-gas-shift reaction and the steam-methane-reforming reaction (reactions (7) and (8), Section 3) [3, 4]. A useful tool for evaluating important design parameters of a gasification technology is thermodynamic modeling. With this tool, process efficiency can be optimized at different operating conditions; the relative quantities of gasifying agents such as oxygen and steam can also be calculated including the composition of the product syngas.
