**1. Introduction**

Gasification is a thermochemical process that transforms carbonaceous materials (coal, oil and its derivatives, biomass, post-consumer and industrial solid wastes) into syngas, with CO and H2 being its major components. The gasification process takes place at high temperatures (generally in the 600–900°C range or even higher) in the presence of a gasifying agent (air, oxygen, steam, CO2, or mixtures of these components) at a lower ratio than that stoichiometrically required for combustion. Syngas production is essential due to the increasing interest in gas to liquid (GTL) processes through the synthesis of methanol, dimethyl ether, and Fischer-Tropsch. In addition, the valorization of syngas can be integrated with

#### *Sustainable Alternative Syngas Fuel*

energy recovery systems, by means of turbines, combined cycle units, or fuel cells. The gasification technology has been extensively developed for coal and oil products and is gaining increasing interest for biomass [1, 2] in which catalysts play an essential role [3]. Furthermore, the upgrading of post-consumer solid wastes by gasification is becoming a short-term promising strategy [4].

Gasification involves several steps and complex chemical reactions, which may be grouped as follows: drying, pyrolysis, cracking and reforming reactions in the gas phase, and heterogeneous char gasification. The significance of these steps on the process performance and their kinetics depends on the feedstock characteristics and gasification conditions. The pyrolysis step involves a series of complex chemical reactions of endothermic nature and leads to volatiles (gases and tars) and a solid residue or char. The homogeneous gasification reactions include a wide variety of reactions, with the balance and the extent of these reactions depending mainly on the gasifying agent used, its ratio with respect to the feed (S/feed ratio), and temperature. These reactions are as follows:

$$\begin{array}{c} \text{Steam reforming of hydrogen bonds:}\\ \text{C}\_{\text{n}}\text{H}\_{\text{m}} + \text{nH}\_{2}\text{O} \rightarrow \text{(n \text{\textdegree } 2\text{)} \text{H}\_{2} + \text{nCO } \Delta\text{H} > 0 \end{array} \tag{1}$$

$$\text{Methane reforming:}\\\text{CH}\_4 + \text{H}\_2\text{O} \Leftrightarrow \text{3H}\_2 + \text{CO} \cdot \Delta \text{H} = 206 \text{ kJ mol}^{-1} \tag{2}$$

$$\text{Char steam gaussian:}\\\text{C} + \text{H}\_2\text{O} \rightarrow \text{H}\_2 + \text{CO}\\\ \Delta \text{H} = \text{131 kJ mol}^{-1} \tag{3}$$

$$\text{Dry reforming of hydrocrbons:}$$

$$\text{C}\_{\text{n}}\overset{\cdot}{\text{H}}\_{\text{m}} + \text{nCO}\_{2} \rightarrow \text{(m/2) H}\_{2} + 2\text{nCO} \cdot \Delta \text{H} > 0\tag{4}$$

$$\text{Boundouard reaction:}\\\text{C} + \text{CO}\_2 \Leftrightarrow 2\text{CO} \cdot \Delta \text{H} = 172 \text{ kJ mol}^{-1} \tag{5}$$

$$\text{Water - gas shift reaction:}\\\text{H}\_2\text{O + CO} \Leftrightarrow \text{H}\_2 + \text{CO}\_2\\\Delta \text{H} = -41 \text{ kJ mol}^{-1} \quad \text{(6)}$$

It should be noted that gasification reactions are only those involving H2O and CO2, because O2 only promotes combustion and partial oxidation reactions that produce CO, CO2, and H2O. In addition, the exothermic nature of oxidation reactions provides the energy required for the highly endothermic steam and CO2 reforming (Eqs. (1)–(4)) and Boudouard (Eq. (5)) reactions. Steam improves H2 production by means of steam reforming reactions (Eqs. (1) and (2)) and also by enhancing the water-gas shift (Eq. (6)) equilibrium. High temperatures are required for promoting char gasification, especially CO2 gasification, whose kinetics is between 2 and 5 times slower than under steam atmosphere and does not occur below 730°C [5].

The main drawback of the syngas produced is the presence of certain impurities, such as fine particles, organic tars, NOx, and SO2, which need to be removed before its application in subsequent processes [6]. In particular, tar is the main contaminant in the gas produced, and its content ranges from 5 to 100 g Nm<sup>−</sup><sup>3</sup> , depending on the type of gasifier. However, its maximum allowable content is 5 mg Nm<sup>−</sup><sup>3</sup> in gas turbines and 100 mg Nm<sup>−</sup><sup>3</sup> in internal combustion engines [7, 8]. Tar is described as a complex mixture of condensable hydrocarbons, ranging from single-ring to

**75**

*Development of the Conical Spouted Bed Technology for Biomass and Waste Plastic Gasification*

five-ring aromatic compounds along with other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons (PAHs) [9]. These compounds may cause several operational problems, such as condensation and the subsequent plugging of downstream equipment, clogging filters, and metal corrosion, which lead to

All the methods available for tar reduction may be classified into two groups,

should be noted that tar formation depends on the gasification conditions, particularly on temperature, so preventive treatments are recommended to operate above 1000°C. Aznar et al. [12] suggest injecting a secondary air stream into the freeboard

In situ catalytic cracking is one of the most promising techniques, as it allows reducing the need for expensive downstream operations [3, 13]. Natural minerals, such as olivine [14, 15] and dolomite [16], have been widely used in steam gasification because, apart from being active for the cracking and reforming of heavy aromatic compounds, they are inexpensive and abundant. In addition, Ni catalysts have received great attention in gasification due to their higher effectiveness for

Moreover, apart from temperature and catalysts, reactor design also plays a critical role in gasification. Different reactor configurations are commonly used for the steam gasification process, which according to their hydrodynamic behavior can be classified as follows: fixed bed, fluidized bed, entrained flow, and rotary kiln reactors, among others [4]. Fluidized beds are the most commonly used due to their advantages, such as versatility for using different types of wastes (agroforestry, post-consumer, and industrial), high heat and mass transfer rates between phases, and bed isothermicity, which allow the scaling-up of the process to the industrial level [19–21]. Nevertheless, biomass or waste particles of irregular texture require a large amount of inert solid (sand) to promote their fluidization. In addition, small particle sizes (Geldart A and B) are the best for fluidization, and therefore high amounts of energy are required to grind and sieve the feedstock. Nevertheless, there is an alternative to conventional fluidized beds, namely, the conical spouted bed reactor (CSBR), which may handle residues of different densities and sizes without significant segregation in the bed. This technology allows handling larger particles than those in fluidized beds, including those with an irregular texture, fine materials, and sticky solids, with no agglomeration or segregation problems [22]. Moreover, the highly vigorous movements of the solids lead to high heat and mass transfer rates between phases [23]. Other advantages of the CSBR over the fluidized bed are its simpler design (no distributor plate) and the lower sand/feed ratio

The main drawback of this technology for gasification is the short gas residence time, which hinders tar cracking reactions. Accordingly, certain modifications have been developed in order improve its performance in the gasification process by changing reactor hydrodynamics, which are as follows: the confinement of the fountain and the use of draft tubes. The fountain confinement device is a tube welded to the lid of the reactor that allows operating under stable conditions with fine particles and increasing the gas residence time by lengthening the path followed by the gas [24]. Therefore, gas-solid (catalyst) contact in the fountain is greatly improved, and tar cracking and reforming reactions are therefore promoted. Moreover, the draft tube also enables to widen

and a low content of PAH compounds [10, 11]. It

depending on where tar is removed: in situ (or primary) methods and postgasification (or secondary) methods. Regardless of the strategy followed, the optimum operating conditions, appropriate additives or catalysts, and a suitable reactor configuration should be established in order to obtain a gas stream with a

unacceptable levels of maintenance for engines and turbines.

*DOI: http://dx.doi.org/10.5772/intechopen.86761*

maximum tar content of 2 g Nm<sup>−</sup><sup>3</sup>

converting tar into H2-rich gas [17, 18].

required for the same capacity.

to reduce the content of tar.

#### *Development of the Conical Spouted Bed Technology for Biomass and Waste Plastic Gasification DOI: http://dx.doi.org/10.5772/intechopen.86761*

five-ring aromatic compounds along with other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons (PAHs) [9]. These compounds may cause several operational problems, such as condensation and the subsequent plugging of downstream equipment, clogging filters, and metal corrosion, which lead to unacceptable levels of maintenance for engines and turbines.

All the methods available for tar reduction may be classified into two groups, depending on where tar is removed: in situ (or primary) methods and postgasification (or secondary) methods. Regardless of the strategy followed, the optimum operating conditions, appropriate additives or catalysts, and a suitable reactor configuration should be established in order to obtain a gas stream with a maximum tar content of 2 g Nm<sup>−</sup><sup>3</sup> and a low content of PAH compounds [10, 11]. It should be noted that tar formation depends on the gasification conditions, particularly on temperature, so preventive treatments are recommended to operate above 1000°C. Aznar et al. [12] suggest injecting a secondary air stream into the freeboard to reduce the content of tar.

In situ catalytic cracking is one of the most promising techniques, as it allows reducing the need for expensive downstream operations [3, 13]. Natural minerals, such as olivine [14, 15] and dolomite [16], have been widely used in steam gasification because, apart from being active for the cracking and reforming of heavy aromatic compounds, they are inexpensive and abundant. In addition, Ni catalysts have received great attention in gasification due to their higher effectiveness for converting tar into H2-rich gas [17, 18].

Moreover, apart from temperature and catalysts, reactor design also plays a critical role in gasification. Different reactor configurations are commonly used for the steam gasification process, which according to their hydrodynamic behavior can be classified as follows: fixed bed, fluidized bed, entrained flow, and rotary kiln reactors, among others [4]. Fluidized beds are the most commonly used due to their advantages, such as versatility for using different types of wastes (agroforestry, post-consumer, and industrial), high heat and mass transfer rates between phases, and bed isothermicity, which allow the scaling-up of the process to the industrial level [19–21]. Nevertheless, biomass or waste particles of irregular texture require a large amount of inert solid (sand) to promote their fluidization. In addition, small particle sizes (Geldart A and B) are the best for fluidization, and therefore high amounts of energy are required to grind and sieve the feedstock. Nevertheless, there is an alternative to conventional fluidized beds, namely, the conical spouted bed reactor (CSBR), which may handle residues of different densities and sizes without significant segregation in the bed. This technology allows handling larger particles than those in fluidized beds, including those with an irregular texture, fine materials, and sticky solids, with no agglomeration or segregation problems [22]. Moreover, the highly vigorous movements of the solids lead to high heat and mass transfer rates between phases [23]. Other advantages of the CSBR over the fluidized bed are its simpler design (no distributor plate) and the lower sand/feed ratio required for the same capacity.

The main drawback of this technology for gasification is the short gas residence time, which hinders tar cracking reactions. Accordingly, certain modifications have been developed in order improve its performance in the gasification process by changing reactor hydrodynamics, which are as follows: the confinement of the fountain and the use of draft tubes. The fountain confinement device is a tube welded to the lid of the reactor that allows operating under stable conditions with fine particles and increasing the gas residence time by lengthening the path followed by the gas [24]. Therefore, gas-solid (catalyst) contact in the fountain is greatly improved, and tar cracking and reforming reactions are therefore promoted. Moreover, the draft tube also enables to widen

*Sustainable Alternative Syngas Fuel*

energy recovery systems, by means of turbines, combined cycle units, or fuel cells. The gasification technology has been extensively developed for coal and oil products and is gaining increasing interest for biomass [1, 2] in which catalysts play an essential role [3]. Furthermore, the upgrading of post-consumer solid wastes by

Gasification involves several steps and complex chemical reactions, which may be grouped as follows: drying, pyrolysis, cracking and reforming reactions in the gas phase, and heterogeneous char gasification. The significance of these steps on the process performance and their kinetics depends on the feedstock characteristics and gasification conditions. The pyrolysis step involves a series of complex chemical reactions of endothermic nature and leads to volatiles (gases and tars) and a solid residue or char. The homogeneous gasification reactions include a wide variety of reactions, with the balance and the extent of these reactions depending mainly on the gasifying agent used, its ratio with respect to the feed (S/feed ratio), and

Cn Hm + nH2O → (n + m/2) H2 + nCO ΔH>0 (1)

Methane reforming: CH4 + H2O⇔ 3H2 + CO ΔH = 206 kJ mol−1 (2)

Char steam gasification:C + H2O → H2 + CO ΔH = 131 kJ mol–<sup>1</sup> (3)

Boudouard reaction:C + CO2 ⇔ 2CO ΔH = 172 kJ mol−1 (5)

Water − gas shift reaction: H2O + CO ⇔ H2 + CO2 ΔH = −41 kJ mol−1 (6)

The main drawback of the syngas produced is the presence of certain impurities, such as fine particles, organic tars, NOx, and SO2, which need to be removed before its application in subsequent processes [6]. In particular, tar is the main contami-

in internal combustion engines [7, 8]. Tar is described

, depending

in gas

nant in the gas produced, and its content ranges from 5 to 100 g Nm<sup>−</sup><sup>3</sup>

on the type of gasifier. However, its maximum allowable content is 5 mg Nm<sup>−</sup><sup>3</sup>

as a complex mixture of condensable hydrocarbons, ranging from single-ring to

It should be noted that gasification reactions are only those involving H2O and CO2, because O2 only promotes combustion and partial oxidation reactions that produce CO, CO2, and H2O. In addition, the exothermic nature of oxidation reactions provides the energy required for the highly endothermic steam and CO2 reforming (Eqs. (1)–(4)) and Boudouard (Eq. (5)) reactions. Steam improves H2 production by means of steam reforming reactions (Eqs. (1) and (2)) and also by enhancing the water-gas shift (Eq. (6)) equilibrium. High temperatures are required for promoting char gasification, especially CO2 gasification, whose kinetics is between 2 and 5 times slower than under steam atmosphere and does not occur

Cn Hm + nCO2 → (m/2) H2 + 2nCO ΔH>0 (4)

gasification is becoming a short-term promising strategy [4].

temperature. These reactions are as follows:

Steam reforming of hydrocarbons:

Dry reforming of hydrocarbons:

**74**

below 730°C [5].

turbines and 100 mg Nm<sup>−</sup><sup>3</sup>

the operation range and improve the reactor's hydrodynamic behavior [24]. Thus, this chapter summarizes the main results obtained in the application of the conical spouted bed reactor in the steam gasification of biomass and waste plastics. Moreover, the influence of different primary catalysts and the incorporation of novel modifications in the reactor design, such as fountain confiner and draft tube, are also discussed.
