*2.2.1 Conventional spouted bed reactor*

The spouted bed reactor is the core of the gasification plant. The total height of the reactor is 298 mm, with that of the conical section (angle of 30°) being 73 mm. The diameters of the cone base and cylindrical section are 12.5 and 60.3 mm, respectively. The gas inlet diameter is 7.6 mm. Despite the endothermic nature of the steam gasification process, bed isothermicity is ensured by the vigorous solid circulation of the sand in this reactor, which also promotes high heat transfer rates [23]. The CSBR is placed inside a 1250 W radiant oven. Two K-type thermocouples are located inside the reactor, one in the bed annulus and the other one close to the wall.

## *2.2.2 Fountain-enhanced spouted bed reactor*

This reactor is an improved version of that described in Section 2.2.1, which has been specifically designed for gasification process. Thus, a fountain confiner was welded to the lid in order to increase the residence time, narrow its distribution, and improve the gas-solid contact in the fountain region (**Figure 2**). Thus, several

*Main dimensions (in mm) of the spouted bed gasifier, fountain confiner, and draft tube.*

**79**

stability.

**2.3 Primary catalysts**

area of only 0.18 m2

**2.4 Product analysis**

with a surface area of 159 m2

**2.5 Experimental procedure**

order to ensure bed stability.

mass flow rate of 1.5 g min<sup>−</sup><sup>1</sup>

was 1.5 mL min<sup>−</sup><sup>1</sup>

g<sup>−</sup><sup>1</sup>

 g<sup>−</sup><sup>1</sup> .

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

modifications were introduced in order to optimize its performance. For example, the height of the reactor was increased in order to increase the residence time of the gas and promote tar cracking. This reactor may also operate in the conventional spouting regime by using a lid without confiner. It is noteworthy that its design allows using draft tubes to widen the application range of the spouting regime and improve bed stability [31, 32]. In fact, the nonporous draft tube promotes high fountains [32] by diverting most of the inlet gas stream through the draft tube, which also enhances solid cross-flow from the annulus into the spout and therefore

The main dimensions of this spouted bed reactor, the fountain confiner, and the draft tube used are depicted in **Figure 2**. According to a previous hydrodynamic study conducted under gasification conditions [33], a draft tube with 8 mm in external diameter (5.5 mm in internal diameter) and 15 mm entrainment zone height was determined as the optimum one. Thus, these geometric factors allow operating under enhanced fountain regime, with low steam flow rates ensuring great turbulence and a well-developed fountain region with a great hydrodynamic

γ-Al2O3 has been provided by Alfa Aesar and olivine by Minelco. Olivine has been calcined at 900°C for 10 h prior to use in the gasification reaction to enhance its reactivity for tar cracking. The conditions mentioned for olivine calcination have been determined as optimum by Devi et al. [14] in order to maximize tar cracking activity. The BET surface area has been measured by N2 adsorption-desorption (Micromeritics ASAP 2010). Calcined olivine has a limited porosity, with a surface

The volatile stream leaving the gasification reactor has been analyzed online by means of a GC Agilent 6890 provided with a HP-PONA column and a flame ionization detector (FID). The sample has been injected into the GC by means of a line thermostated at 280°C, once the reactor outlet stream has been diluted with an inert gas. The purpose of this system is to avoid the condensation of tars in the transfer line. The tars collected in the condensation system have been identified in a gas chromatograph/ mass spectrometer (GC/MS, Shimadzu UP-2010S provided with a HP-PONA column).

The non-condensable gases have been injected into a micro-GC (Varian 4900).

Temperature and steam/biomass ratio are the operating parameters studied in the gasification of biomass and plastics in this reactor. Additionally, biomass gasification was also performed with different primary catalysts (in situ), and the influence of using the fountain confiner was evaluated. In all runs, water flow rate

, corresponding to a steam flow rate of 1.86 L min<sup>−</sup><sup>1</sup>

The effect of temperature has been studied at 800, 850, and 900°C by feeding a

The effect of the steam/feed ratio has been studied between 0 and 2 (in mass), and the temperature has been maintained at 900°C. For a ratio of 2, the biomass or

of biomass or HDPE and using a steam/feed ratio of 1.

approximately 1.5 times that corresponding to the minimum spouting velocity in

, which is

. However, γ-Al2O3 has a much higher porous development,

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

leads to additional gas-solid contact in the fountain.

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

modifications were introduced in order to optimize its performance. For example, the height of the reactor was increased in order to increase the residence time of the gas and promote tar cracking. This reactor may also operate in the conventional spouting regime by using a lid without confiner. It is noteworthy that its design allows using draft tubes to widen the application range of the spouting regime and improve bed stability [31, 32]. In fact, the nonporous draft tube promotes high fountains [32] by diverting most of the inlet gas stream through the draft tube, which also enhances solid cross-flow from the annulus into the spout and therefore leads to additional gas-solid contact in the fountain.

The main dimensions of this spouted bed reactor, the fountain confiner, and the draft tube used are depicted in **Figure 2**. According to a previous hydrodynamic study conducted under gasification conditions [33], a draft tube with 8 mm in external diameter (5.5 mm in internal diameter) and 15 mm entrainment zone height was determined as the optimum one. Thus, these geometric factors allow operating under enhanced fountain regime, with low steam flow rates ensuring great turbulence and a well-developed fountain region with a great hydrodynamic stability.

#### **2.3 Primary catalysts**

*Sustainable Alternative Syngas Fuel*

*2.2.1 Conventional spouted bed reactor*

*2.2.2 Fountain-enhanced spouted bed reactor*

The spouted bed reactor is the core of the gasification plant. The total height of the reactor is 298 mm, with that of the conical section (angle of 30°) being

73 mm. The diameters of the cone base and cylindrical section are 12.5 and 60.3 mm, respectively. The gas inlet diameter is 7.6 mm. Despite the endothermic nature of the steam gasification process, bed isothermicity is ensured by the vigorous solid circulation of the sand in this reactor, which also promotes high heat transfer rates [23]. The CSBR is placed inside a 1250 W radiant oven. Two K-type thermocouples are located

This reactor is an improved version of that described in Section 2.2.1, which has been specifically designed for gasification process. Thus, a fountain confiner was welded to the lid in order to increase the residence time, narrow its distribution, and improve the gas-solid contact in the fountain region (**Figure 2**). Thus, several

inside the reactor, one in the bed annulus and the other one close to the wall.

*Main dimensions (in mm) of the spouted bed gasifier, fountain confiner, and draft tube.*

**78**

**Figure 2.**

γ-Al2O3 has been provided by Alfa Aesar and olivine by Minelco. Olivine has been calcined at 900°C for 10 h prior to use in the gasification reaction to enhance its reactivity for tar cracking. The conditions mentioned for olivine calcination have been determined as optimum by Devi et al. [14] in order to maximize tar cracking activity. The BET surface area has been measured by N2 adsorption-desorption (Micromeritics ASAP 2010). Calcined olivine has a limited porosity, with a surface area of only 0.18 m2 g<sup>−</sup><sup>1</sup> . However, γ-Al2O3 has a much higher porous development, with a surface area of 159 m2 g<sup>−</sup><sup>1</sup> .

#### **2.4 Product analysis**

The volatile stream leaving the gasification reactor has been analyzed online by means of a GC Agilent 6890 provided with a HP-PONA column and a flame ionization detector (FID). The sample has been injected into the GC by means of a line thermostated at 280°C, once the reactor outlet stream has been diluted with an inert gas. The purpose of this system is to avoid the condensation of tars in the transfer line. The tars collected in the condensation system have been identified in a gas chromatograph/ mass spectrometer (GC/MS, Shimadzu UP-2010S provided with a HP-PONA column). The non-condensable gases have been injected into a micro-GC (Varian 4900).

#### **2.5 Experimental procedure**

Temperature and steam/biomass ratio are the operating parameters studied in the gasification of biomass and plastics in this reactor. Additionally, biomass gasification was also performed with different primary catalysts (in situ), and the influence of using the fountain confiner was evaluated. In all runs, water flow rate was 1.5 mL min<sup>−</sup><sup>1</sup> , corresponding to a steam flow rate of 1.86 L min<sup>−</sup><sup>1</sup> , which is approximately 1.5 times that corresponding to the minimum spouting velocity in order to ensure bed stability.

The effect of temperature has been studied at 800, 850, and 900°C by feeding a mass flow rate of 1.5 g min<sup>−</sup><sup>1</sup> of biomass or HDPE and using a steam/feed ratio of 1.

The effect of the steam/feed ratio has been studied between 0 and 2 (in mass), and the temperature has been maintained at 900°C. For a ratio of 2, the biomass or plastic feed rate was reduced to 0.75 g min<sup>−</sup><sup>1</sup> in order to maintain the same steam flow rate (1.5 g min<sup>−</sup><sup>1</sup> ). The reactor contains 70 g of sand in the bed in all runs, and therefore the residence time of the products in the reactor and the hydrodynamic behavior are similar. In order to study the steam/feed ratio of 0, the steam was replaced with a N2 flow rate of 2 L min<sup>−</sup><sup>1</sup> .

In the experiments to assess the effect of the primary catalyst on product distribution, the bed contains 70 g of sand or olivine (with particle diameter in the 0.35–0.4 mm range). However, given that γ-Al2O3 has a much lower density, the bed of this material contained 25 g with a particle size greater than that of sand, in the 0.4–0.8 mm range, in order to attain a similar hydrodynamic behavior in all cases. The experiments were carried out at 900°C, with a feed rate of 1.5 g min<sup>−</sup><sup>1</sup> of HDPE or sawdust and with a steam/feed ratio of 1.

In the experiments performed with the fountain-confined spouted bed, the biomass feed rate was 0.75 g min<sup>−</sup><sup>1</sup> , with a steam/biomass ratio of 2. The bed contained 100 g of olivine, and two particles sizes have been used, i.e., 90–150 and 250–355 μm. These olivine particle size ranges are those corresponding to the optimum hydrodynamic performance of the reactor, as the minimum spouting velocity depends strongly on particle size [33]. Thus, the gas velocity in the runs with the coarse olivine fraction corresponds to approximately 1.5 times the minimum spouting velocity (so the reactor operated under conventional spouting regime), whereas in the experiments performed with the fine olivine, the gas velocity used is approximately four times higher than the minimum spouting velocity (4 ums), and the fountain-enhanced regime was therefore attained.

Furthermore, operation was carried out in two regimes in the same reactor in order to ascertain the influence the confinement system (in the standard spouting regime) has on the biomass gasification process. Thus, experiments with and without the fountain confiner were carried out at 850°C and S/B of 2, using coarse olivine (250–355 μm), with gas velocity corresponding in both cases to approximately 1.5 times ums (conventional spouting regime). The results obtained with the confiner under conventional spouting regime were compared with those obtained with this device but operating in the enhance fountain spouting regime under the same conditions and replacing the coarse olivine with the fine one in the bed. Therefore, the role of the vigorous gas-catalyst contact in the fountain-enhanced regime was assessed.

All the runs were performed in continuous mode for 20 min in order to ensure a steady-state process. The char yield was determined by weighing the mass in the reactor, as well as those retained in the cyclone and in the sintered steel filter. The char yield is given by mass unit of the whole amount of solid fed into the reactor (approximately 30 g). All the runs have been repeated several times (at least three) under the same conditions in order to guarantee reproducible results.

## **3. Results and discussion**

#### **3.1 HDPE gasification**

In this work, steam gasification of HDPE has been studied in the conventional conical spouted bed pilot plant described in Section 2.2. The effect of temperature (in the 800–900°C range) and steam/plastic (S/P) ratio (between 0 and 2) on the gas yield, tar content, carbon conversion efficiency, and H2 production is shown in **Table 2**. The reaction indices have been defined as follows: (i) gas yield as the volumetric gas production (on a dry basis) per kg of biomass in the feed (on a wet

**81**

**Table 2.**

*H2 production.*

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

(iii) carbon conversion efficiency as the ratio between the carbon units contained in the syngas and those contained in the biomass in the feed, and (iv) H2 production as

The gaseous fraction is composed of H2, CO, and CO2, together with C2−C5 hydrocarbons (mainly C3−). The tar is defined as the amount of organic compounds with a molecular weight and boiling point higher than that of benzene, a criterion that is commonly used by most authors [11, 14, 34]. The char is a carbonaceous product collected after the reaction in the reactor, sintered steel filter, and cyclone.

As observed in **Table 2**, an increase in temperature leads to higher gas yields and lower tar and char yields, thus improving the efficiency of the whole process. The

900°C. Furthermore, the carbon conversion efficiency at 800°C is 86%, increases to 91% at 850°C, and then remains constant with further increases in temperature to

the enhancement of thermal cracking. Other authors have also observed a positive effect of temperature on the tar cracking in the gasification of waste plastics by using both steam [35] and air [36] as gasifying agents. In fact, according to certain authors, the destruction of tar aromatic hydrocarbons only occurs at temperatures

The influence of temperature on product yields has also been studied with different gasification technologies, and most of the authors agree that higher temperatures enhance syngas yield and decrease that of tar and char [37–39]. Higher char yields than those shown in **Table 2** have been reported in the literature [35, 40], which may be attributed to the characteristics of the gas-solid contact in the conical spouted bed reactor, which mitigate the limitations in the physical steps prior to gasification, which are as follows: (i) plastic melting, (ii) coating of sand particles,

**Table 2** also displays the reaction indices for different S/P values. As observed, as S/P ratio is increased from 1 to 2, the carbon conversion increases from 91.0 to 93.6%. Note that the performance is poor when operating with a S/P = 0 (pyrolysis), given that carbon conversion efficiency is as low as 68.6% due to the high tar and char yields. The lack of steam in the reactor at high temperatures promotes the formation of aromatic compounds, leading to a tar content as high as 29.5 g Nm<sup>−</sup><sup>3</sup>

The presence of steam in the reaction medium increases the gas yield and decreases that of tar. When operating only with N2 as a fluidizing agent, the tar concentration

> **Carbon conversion (%)**

 1 29.5 86.1 2.5 12.7 1.4 1 13.8 91.1 3.2 17.0 0.6 1 16.7 91.1 3.4 18.4 0.5 2 9.6 93.6 3.6 19.9 0.4 0 207.8 68.6 0.9 2.7 5.6

*Effect of gasification temperature and S/P ratio on the gas yield, tar content, carbon conversion efficiency, and* 

**Gas yield (m3 kg<sup>−</sup><sup>1</sup> )**

**H2 production (wt%)**

of HDPE at 800°C to 3.4 m3

at 800°C to 16.7 g Nm<sup>−</sup><sup>3</sup>

(on a dry basis),

kg<sup>−</sup><sup>1</sup>

of HDPE at

at 900°C due to

.

**Char yield (wt%)**

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

gas yield increases from 2.5 m3

Tar content decreases from 29.5 g Nm<sup>−</sup><sup>3</sup>

900°C.

above 850°C [13].

and (iii) pyrolysis.

**Temperature (°C)**

**S/P ratio** **Tar content (g Nm<sup>−</sup><sup>3</sup> )**

basis), (ii) tar yield expressed as the tar mass per syngas m3

the mass percentage of the H2 produced per biomass mass unit.

The mass balance closure in all the experiments was above 95%.

kg<sup>−</sup><sup>1</sup>

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

basis), (ii) tar yield expressed as the tar mass per syngas m3 (on a dry basis), (iii) carbon conversion efficiency as the ratio between the carbon units contained in the syngas and those contained in the biomass in the feed, and (iv) H2 production as the mass percentage of the H2 produced per biomass mass unit.

The gaseous fraction is composed of H2, CO, and CO2, together with C2−C5 hydrocarbons (mainly C3−). The tar is defined as the amount of organic compounds with a molecular weight and boiling point higher than that of benzene, a criterion that is commonly used by most authors [11, 14, 34]. The char is a carbonaceous product collected after the reaction in the reactor, sintered steel filter, and cyclone. The mass balance closure in all the experiments was above 95%.

As observed in **Table 2**, an increase in temperature leads to higher gas yields and lower tar and char yields, thus improving the efficiency of the whole process. The gas yield increases from 2.5 m3 kg<sup>−</sup><sup>1</sup> of HDPE at 800°C to 3.4 m3 kg<sup>−</sup><sup>1</sup> of HDPE at 900°C. Furthermore, the carbon conversion efficiency at 800°C is 86%, increases to 91% at 850°C, and then remains constant with further increases in temperature to 900°C.

Tar content decreases from 29.5 g Nm<sup>−</sup><sup>3</sup> at 800°C to 16.7 g Nm<sup>−</sup><sup>3</sup> at 900°C due to the enhancement of thermal cracking. Other authors have also observed a positive effect of temperature on the tar cracking in the gasification of waste plastics by using both steam [35] and air [36] as gasifying agents. In fact, according to certain authors, the destruction of tar aromatic hydrocarbons only occurs at temperatures above 850°C [13].

The influence of temperature on product yields has also been studied with different gasification technologies, and most of the authors agree that higher temperatures enhance syngas yield and decrease that of tar and char [37–39]. Higher char yields than those shown in **Table 2** have been reported in the literature [35, 40], which may be attributed to the characteristics of the gas-solid contact in the conical spouted bed reactor, which mitigate the limitations in the physical steps prior to gasification, which are as follows: (i) plastic melting, (ii) coating of sand particles, and (iii) pyrolysis.

**Table 2** also displays the reaction indices for different S/P values. As observed, as S/P ratio is increased from 1 to 2, the carbon conversion increases from 91.0 to 93.6%. Note that the performance is poor when operating with a S/P = 0 (pyrolysis), given that carbon conversion efficiency is as low as 68.6% due to the high tar and char yields. The lack of steam in the reactor at high temperatures promotes the formation of aromatic compounds, leading to a tar content as high as 29.5 g Nm<sup>−</sup><sup>3</sup> . The presence of steam in the reaction medium increases the gas yield and decreases that of tar. When operating only with N2 as a fluidizing agent, the tar concentration


#### **Table 2.**

*Sustainable Alternative Syngas Fuel*

flow rate (1.5 g min<sup>−</sup><sup>1</sup>

plastic feed rate was reduced to 0.75 g min<sup>−</sup><sup>1</sup>

replaced with a N2 flow rate of 2 L min<sup>−</sup><sup>1</sup>

or sawdust and with a steam/feed ratio of 1.

the fountain-enhanced regime was therefore attained.

biomass feed rate was 0.75 g min<sup>−</sup><sup>1</sup>

regime was assessed.

**3. Results and discussion**

**3.1 HDPE gasification**

in order to maintain the same steam

of HDPE

). The reactor contains 70 g of sand in the bed in all runs, and

, with a steam/biomass ratio of 2. The bed

therefore the residence time of the products in the reactor and the hydrodynamic behavior are similar. In order to study the steam/feed ratio of 0, the steam was

. In the experiments to assess the effect of the primary catalyst on product distribution, the bed contains 70 g of sand or olivine (with particle diameter in the 0.35–0.4 mm range). However, given that γ-Al2O3 has a much lower density, the bed of this material contained 25 g with a particle size greater than that of sand, in the 0.4–0.8 mm range, in order to attain a similar hydrodynamic behavior in all cases.

In the experiments performed with the fountain-confined spouted bed, the

contained 100 g of olivine, and two particles sizes have been used, i.e., 90–150 and 250–355 μm. These olivine particle size ranges are those corresponding to the optimum hydrodynamic performance of the reactor, as the minimum spouting velocity depends strongly on particle size [33]. Thus, the gas velocity in the runs with the coarse olivine fraction corresponds to approximately 1.5 times the minimum spouting velocity (so the reactor operated under conventional spouting regime), whereas in the experiments performed with the fine olivine, the gas velocity used is approximately four times higher than the minimum spouting velocity (4 ums), and

Furthermore, operation was carried out in two regimes in the same reactor in order to ascertain the influence the confinement system (in the standard spouting regime) has on the biomass gasification process. Thus, experiments with and without the fountain confiner were carried out at 850°C and S/B of 2, using coarse olivine (250–355 μm), with gas velocity corresponding in both cases to approximately 1.5 times ums (conventional spouting regime). The results obtained with the confiner under conventional spouting regime were compared with those obtained with this device but operating in the enhance fountain spouting regime under the same conditions and replacing the coarse olivine with the fine one in the bed. Therefore, the role of the vigorous gas-catalyst contact in the fountain-enhanced

All the runs were performed in continuous mode for 20 min in order to ensure a steady-state process. The char yield was determined by weighing the mass in the reactor, as well as those retained in the cyclone and in the sintered steel filter. The char yield is given by mass unit of the whole amount of solid fed into the reactor (approximately 30 g). All the runs have been repeated several times (at least three)

In this work, steam gasification of HDPE has been studied in the conventional conical spouted bed pilot plant described in Section 2.2. The effect of temperature (in the 800–900°C range) and steam/plastic (S/P) ratio (between 0 and 2) on the gas yield, tar content, carbon conversion efficiency, and H2 production is shown in **Table 2**. The reaction indices have been defined as follows: (i) gas yield as the volumetric gas production (on a dry basis) per kg of biomass in the feed (on a wet

under the same conditions in order to guarantee reproducible results.

The experiments were carried out at 900°C, with a feed rate of 1.5 g min<sup>−</sup><sup>1</sup>

**80**

*Effect of gasification temperature and S/P ratio on the gas yield, tar content, carbon conversion efficiency, and H2 production.*

is 207.8 g Nm<sup>−</sup><sup>3</sup> , but this concentration is drastically reduced to 16.7 g Nm<sup>−</sup><sup>3</sup> and 9.6 g Nm<sup>−</sup><sup>3</sup> when operating with S/P ratios of 1 and 2, respectively. These results suggest that an increase in S/P ratio enhances the cracking of tar compounds, as reported by Herguido et al. [41] in the steam gasification of biomass.

The presence of steam in the reaction environment also improves H2 production, increasing significantly from 2.7 to 18.4 wt% when the S/P ratio is increased from 0 to 1. However, the increase in H2 production (19.9 wt%) is moderate when a S/P value of 2 is used. Similarly, gas yield increases slightly from 3.4 m3 kg<sup>−</sup><sup>1</sup> HDPE to 3.6 m3 kg<sup>−</sup><sup>1</sup> HDPE when the S/P ratio is raised from 1 to 2. The following aspects can explain these results: (i) promotion of hydrocarbon reforming reactions (Eq. (1)) as steam concentration is higher and (ii) low tar and char formation rate, although this effect is of lower significance. A similar trend has been reported in the literature, although some authors attain a saturating trend, i.e., a higher steam/tire ratio than the optimum one does not increase the gas yield [42, 43].

Moreover, **Figure 3** displays the composition of the gases formed at different temperatures (**Figure 3a**) and S/P ratios (**Figure 3b**). As observed in **Figure 3a**, an increase in temperature leads to an increase in the concentrations of H2, CO, and CH4 in the gaseous stream, which are 60.3, 28.2, and 7.2% vol., respectively, at 900°C. Temperature has an opposite effect on C2–C5 hydrocarbons (made up mainly of olefins, with ethylene being the major one), whereas that on CO2 was almost negligible (the concentration is almost steady).

The higher concentration of H2 and CO can be explained by the endothermic nature of steam and dry reforming reactions (Eqs. (1) and (4)), which are promoted at higher temperatures, whereas that of CH4 is due to the endothermicity of HDPE cracking reactions. On the contrary, the C2–C5 hydrocarbons formed are probably reformed, and therefore their yield decreases as temperature is higher. It should be noted that the water-gas shift reaction (Eq. (6)) is exothermic, and therefore thermodynamic equilibrium shifts toward the formation of CO at high temperatures.

Regarding the gas composition (**Figure 3b**), an increase in S/P ratio from 1 to 2 does not lead to a significant change, but the composition of the gas when only pyrolysis is performed (S/P = 0) is very different. As observed, the presence of steam favors H2 and CO2 formation but reduces that of CO and CH4 because the higher concentration of steam in the reactor enhances both water-gas shift and methane reforming reactions. Other authors have observed a similar effect of S/P ratio on the gas composition in the gasification of different polymeric materials [42, 43].

**83**

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

The same reaction indices in plastic gasification, i.e., the gas yield, tar content, carbon conversion efficiency, and H2 production, have been analyzed in this section (**Table 3**). Temperature is one of the more influential variables in steam gasification, and its effect has been studied in the 800–900°C range for a steam/biomass ratio of 1. Moreover, the effect of steam/biomass ratio has been studied in the 0–2 range at 900°C. Note that no steam was fed into the reactor in the runs carried out with a S/B ratio of 0, but the sawdust contained a moisture content of approximately 10%, and water is formed during the thermal degradation of biomass. Therefore, some steam reforming will occur even in the runs without water in the

As observed in **Table 3**, the temperature plays a crucial role in the efficiency of the gasification process. An increase in the gasification temperature reduces the

to 4.5% at 900°C. In the same line, the carbon conversion efficiency in the process is considerably higher as temperature is increased, and char yield is therefore lower. In fact, char gasification kinetics is enhanced by temperature due to the highly endothermic nature of char steam gasification (Eq. (3)) and Boudouard (Eq. (4)) reactions. The increase in char conversion with temperature is related to the shift in equilibrium in both reactions [44]. However, this result depends on the char residence time in the reactor. Thus, char gasification reaction kinetics is slow, even

to the positive effect of temperature on tar cracking and reforming reactions, this value is still high for syngas applications. It should be noted that no defluidization problems are observed in the steam gasification, which is due to the vigorous solid cyclic movement in the conical spouted bed. However, the conventional spouted bed regime leads to short residence times (below 0.5 s), which are beneficial to increase the yield of bio-oil in pyrolysis processes, but in gasification they are responsible for the limited tar cracking, whose concentration in the gaseous stream

Given that the tar yield is highly dependent on several parameters, such as residence time, temperature, and S/B ratio, the results showed in the literature vary greatly depending on the technology used, but all of them evidence a significant decrease in tar content in the gaseous product stream with temperature [45–47]. With respect to the experiments carried out with different S/B ratios (**Table 3**), an increase in this parameter improves the gasification performance by increasing the gas yield and carbon efficiency and lowering that of tar. For example, tar con-

S/B = 1, given that an increase in the S/B ratio promotes tar cracking and reforming reactions (Eq. (1)). However, a further increase in the S/B ratio from 1 to 2 only reduces slightly the tar content of the gaseous product. Likewise, the gas yield

kg<sup>−</sup><sup>1</sup>

The reduction in the tar and char content leads to an increase in the carbon conversion efficiency, attaining the maximum value of 70% with a S/B = 2. Although gasification efficiency is improved in terms of biomass conversion, the energy efficiency of the process is lower when high S/B ratios are used, given that more water

of biomass at 900°C, whereas that of char decreases from 8.9% at 800°C

kg<sup>−</sup><sup>1</sup>

at 800°C to 142.5 g Nm<sup>−</sup><sup>3</sup>

of biomass at 800°C to

operating at 900°C due

with a S/B = 0 to 142.5 g Nm<sup>−</sup><sup>3</sup>

of biomass) but hardly changes as

with a

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

*3.2.1 Effect of temperature and S/B ratio*

tar content in the gaseous product from 364.4 g Nm<sup>−</sup><sup>3</sup>

Although the content of tar is reduced to 142.5 g Nm<sup>−</sup><sup>3</sup>

at 900°C. The gas yield also increases from 0.7 m3

is rather high, as observed in **Table 3**.

centration has been reduced from 154 g Nm<sup>−</sup><sup>3</sup>

increases from S/B 0 to 1 (from 0.9 to 1 m3

S/B is increased from 1 to 2.

**3.2 Biomass gasification**

feed (runs with S/B = 0).

1 m3 kg<sup>−</sup><sup>1</sup>

above 800°C.

**Figure 3.** *Effect of gasification temperature (a) and S/P ratio (b) on the gaseous fraction composition.*
