*3.2.1 Effect of temperature and S/B ratio*

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 feed (runs with S/B = 0).

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 tar content in the gaseous product from 364.4 g Nm<sup>−</sup><sup>3</sup> at 800°C to 142.5 g Nm<sup>−</sup><sup>3</sup> at 900°C. The gas yield also increases from 0.7 m3 kg<sup>−</sup><sup>1</sup> of biomass at 800°C to 1 m3 kg<sup>−</sup><sup>1</sup> of biomass at 900°C, whereas that of char decreases from 8.9% at 800°C 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 above 800°C.

Although the content of tar is reduced to 142.5 g Nm<sup>−</sup><sup>3</sup> operating at 900°C due 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 is rather high, as observed in **Table 3**.

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 concentration has been reduced from 154 g Nm<sup>−</sup><sup>3</sup> with a S/B = 0 to 142.5 g Nm<sup>−</sup><sup>3</sup> with a 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 increases from S/B 0 to 1 (from 0.9 to 1 m3 kg<sup>−</sup><sup>1</sup> of biomass) but hardly changes as S/B is increased from 1 to 2.

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


#### **Table 3.**

*Effect of gasification temperature and S/B ratio on product fraction yields, carbon conversion efficiency, and tar concentration, at 900°C.*

need to be vaporized and the unreacted steam needs to be recovered after being condensed. Kaushal and Tyagi [48] suggest optimum S/B ratios between 0.6 and 0.85, which guarantee the thermal efficiency of the process and, at the same time, the presence of enough steam in the gasifier to promote steam reforming reactions.

The composition of the gases (on a dry basis) formed at different temperatures and different S/B ratios is displayed in **Figure 4**. As observed in **Figure 4a**, an increase in temperature enhances H2 formation due to the endothermic nature of the reactions involved (Eqs. (1)–(5)). Moreover, the inorganic species of the biomass retained in the char have a positive effect on the water-gas shift reaction (Eq. (6)) at higher temperatures [46]. Accordingly, H2 concentration increases from 28% at 800°C to 38% at 900°C, whereas that of CO decreases from 41.5 to 32.5% in the same range of temperature. Besides, concentration of methane and the other gaseous hydrocarbons (C2 to C4) decreases as temperature is raised due to the enhancement of hydrocarbon reforming reactions. As in HDPE gasification, the effect of temperature on CO2 is not of significance, as its concentration increases slightly between 800 and 900°C.

**Figure 4b** shows the composition of the gaseous stream for different S/B ratios. Given that the WGS reaction and methane and hydrocarbon reforming reactions (Eqs. (1) and (2)) are promoted at high S/B ratios, the formation of H2 and CO2 is enhanced, whereas that of CO and hydrocarbons is hindered. It is to note that this effect is more remarkable when the S/B ratio is increased from 0 to 1.

**85**

**Figure 5.**

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

The experiments with different bed materials have been carried out at a temperature of 900°C and a S/B ratio of 1. **Table 4** shows the effect of the primary catalysts used (olivine and γ-alumina) on reaction indices (gas yield, tar content, H2 production, and carbon conversion) and compares the results with those obtained using inert sand as bed material. As observed, both olivine and γ-alumina cause

improve the gasification performance, with tar reduction being slightly higher for γ-alumina (84%) than that for olivine (79%). Moreover, the carbon conversion efficiency has a drastic increase when a primary catalyst is used, attaining a value of 86.8% for olivine and 87.6% for γ-alumina. It is noteworthy that H2 production

As mentioned above, tar formation leads to operational problems in the gasification and subsequent units for syngas processing; thus, the use of a catalyst, such as olivine and γ-alumina, improves process efficiency, especially the latter, which significantly reduces tar content. Nevertheless, olivine is cheaper and more available because it is a natural material [49]. Other papers in the literature also report considerable improvements in gasification efficiency by using primary catalysts [45, 50]. The effect primary catalysts have on gas composition is displayed in **Figure 5**. As observed, γ-alumina has a greater influence on gas composition than olivine. The presence of catalysts leads to an increase in H2 and CO2 concentrations and a

) 142.5 30.2 22.4

) 1.0 1.1 1.2

) 4.5 4.3 4.3

Carbon conversion (%) 69.7 86.8 87.6

H2 production (wt%) 3.2 3.7 4.5

, respectively) compared to

). Accordingly, both catalysts

**Sand Olivine γ-Alumina**

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

a great decrease in tar content (30.1 and 22.4 g Nm<sup>−</sup><sup>3</sup>

the runs carried out with inert sand (142.5 g Nm<sup>−</sup><sup>3</sup>

peaks at 4.5 wt% when the γ-alumina is used.

*3.2.2 Effect of primary catalyst*

Tar content (g Nm<sup>−</sup><sup>3</sup>

Char yield (g Nm<sup>−</sup><sup>3</sup>

kg<sup>−</sup><sup>1</sup>

*Effect of the primary catalysts on reaction indices.*

*Effect of primary catalysts on the composition of the gaseous fraction.*

Gas yield (m3

**Table 4.**

**Figure 4.** *Gas composition (on a dry basis) for the steam gasification at different temperatures (a) and S/B ratios (b).* *Development of the Conical Spouted Bed Technology for Biomass and Waste Plastic Gasification DOI: http://dx.doi.org/10.5772/intechopen.86761*

## *3.2.2 Effect of primary catalyst*

*Sustainable Alternative Syngas Fuel*

**S/P ratio**

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

**Temperature (°C)**

**Table 3.**

*tar concentration, at 900°C.*

slightly between 800 and 900°C.

need to be vaporized and the unreacted steam needs to be recovered after being condensed. Kaushal and Tyagi [48] suggest optimum S/B ratios between 0.6 and 0.85, which guarantee the thermal efficiency of the process and, at the same time, the presence of enough steam in the gasifier to promote steam reforming reactions. The composition of the gases (on a dry basis) formed at different temperatures

*Effect of gasification temperature and S/B ratio on product fraction yields, carbon conversion efficiency, and* 

**Carbon conversion (%)**

 1 364.2 50.4 0.7 1.9 8.9 1 243.1 59.1 0.8 2.5 6.3 1 142.5 69.8 1.0 3.2 4.5 2 142.0 70.0 1.0 3.6 3.6 0 154.0 50.4 0.9 2.3 10.7

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

**H2 production (wt%)**

**Char yield (wt%)**

and different S/B ratios is displayed in **Figure 4**. As observed in **Figure 4a**, an increase in temperature enhances H2 formation due to the endothermic nature of the reactions involved (Eqs. (1)–(5)). Moreover, the inorganic species of the biomass retained in the char have a positive effect on the water-gas shift reaction (Eq. (6)) at higher temperatures [46]. Accordingly, H2 concentration increases from 28% at 800°C to 38% at 900°C, whereas that of CO decreases from 41.5 to 32.5% in the same range of temperature. Besides, concentration of methane and the other gaseous hydrocarbons (C2 to C4) decreases as temperature is raised due to the enhancement of hydrocarbon reforming reactions. As in HDPE gasification, the effect of temperature on CO2 is not of significance, as its concentration increases

**Figure 4b** shows the composition of the gaseous stream for different S/B ratios. Given that the WGS reaction and methane and hydrocarbon reforming reactions (Eqs. (1) and (2)) are promoted at high S/B ratios, the formation of H2 and CO2 is enhanced, whereas that of CO and hydrocarbons is hindered. It is to note that this

*Gas composition (on a dry basis) for the steam gasification at different temperatures (a) and S/B ratios (b).*

effect is more remarkable when the S/B ratio is increased from 0 to 1.

**84**

**Figure 4.**

The experiments with different bed materials have been carried out at a temperature of 900°C and a S/B ratio of 1. **Table 4** shows the effect of the primary catalysts used (olivine and γ-alumina) on reaction indices (gas yield, tar content, H2 production, and carbon conversion) and compares the results with those obtained using inert sand as bed material. As observed, both olivine and γ-alumina cause a great decrease in tar content (30.1 and 22.4 g Nm<sup>−</sup><sup>3</sup> , respectively) compared to the runs carried out with inert sand (142.5 g Nm<sup>−</sup><sup>3</sup> ). Accordingly, both catalysts improve the gasification performance, with tar reduction being slightly higher for γ-alumina (84%) than that for olivine (79%). Moreover, the carbon conversion efficiency has a drastic increase when a primary catalyst is used, attaining a value of 86.8% for olivine and 87.6% for γ-alumina. It is noteworthy that H2 production peaks at 4.5 wt% when the γ-alumina is used.

As mentioned above, tar formation leads to operational problems in the gasification and subsequent units for syngas processing; thus, the use of a catalyst, such as olivine and γ-alumina, improves process efficiency, especially the latter, which significantly reduces tar content. Nevertheless, olivine is cheaper and more available because it is a natural material [49]. Other papers in the literature also report considerable improvements in gasification efficiency by using primary catalysts [45, 50].

The effect primary catalysts have on gas composition is displayed in **Figure 5**. As observed, γ-alumina has a greater influence on gas composition than olivine. The presence of catalysts leads to an increase in H2 and CO2 concentrations and a


#### **Table 4.**

*Effect of the primary catalysts on reaction indices.*

**Figure 5.** *Effect of primary catalysts on the composition of the gaseous fraction.*

reduction in that of CO due to the promotion of the water-gas shift reaction (Eq. (6)). In addition, the higher concentration of H2 by the presence of this type of catalyst is also related to the enhancement of tar cracking and reforming reactions (Eq. (1)). Moreover, γ-alumina also seems to promote methane and light hydrocarbon reforming (Eq. (3)), which can be deduced from their lower concentration in the presence of this catalyst.
