**3.2 Biomass gasification**

*Sustainable Alternative Syngas Fuel*

, but this concentration is drastically reduced to 16.7 g Nm<sup>−</sup><sup>3</sup>

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

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

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

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

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].

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

HDPE when the S/P ratio is raised from 1 to 2. The following aspects can

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.

value of 2 is used. Similarly, gas yield increases slightly from 3.4 m3

the optimum one does not increase the gas yield [42, 43].

negligible (the concentration is almost steady).

when operating with S/P ratios of 1 and 2, respectively. These results

and

HDPE to

kg<sup>−</sup><sup>1</sup>

is 207.8 g Nm<sup>−</sup><sup>3</sup>

9.6 g Nm<sup>−</sup><sup>3</sup>

3.6 m3

kg<sup>−</sup><sup>1</sup>

temperatures.

**82**

**Figure 3.**
