## 3 Results and Discussion
### Gasification Performance of Fuels and Their Mixture with Steam Reagent  
During steam gasification, several reactions occur simultaneously within the reactor, the most important of which are the following [11]:  
\[\text{C}+\text{H}_{2}\text{O}\rightarrow\text{CO}+\text{H}_{2}\qquad\Delta\text{H }=\text{131 kJ/mol} \tag{4}\]  
\[\text{C}+\text{2H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{2H}_{2}\quad \Delta\text{H}=\text{90 kJ/mol} \tag{5}\]  
\[\text{C}+\text{CO}_{2}\rightarrow\text{2 CO}\qquad\Delta\text{H}=\text{172 kJ/mol} \tag{6}\]  
\[\text{C}+\text{2H}_{2}\rightarrow\text{CH}_{4}\qquad\Delta\text{H}=\text{-75 kJ/mol} \tag{7}\]  
Figure 5: (**a**) Evolution of gases during pyrolysis of barley straw fuel. (**b**) Higher heating value of liquid and gaseous pyrolysis products of fuels. \[\mathrm{CO}+\mathrm{H_{2}O}\rightarrow\mathrm{CO_{2}}+\mathrm{H_{2}} \Delta\mathrm{H}=-41\mathrm{\ kJ/mol} \tag{8}\] \[\mathrm{CH_{4}}+\mathrm{H_{2}O}\rightarrow\mathrm{CO}+3\mathrm{H_{2}} \Delta\mathrm{H}=206\mathrm{\ kJ/mol} \tag{9}\]  
The water-gas shift reaction (8) is in equilibrium and the formation of \(\mathrm{H_{2}}\) is favored at high temperatures, while reactions (7) and (9) are favored at high pressures. Figure 6 illustrates the effect of gasification temperature on fuel conversion, and Figure 7 illustrates the composition of the product gas as a function of temperature during the process for the lignite, the biomass and their mixture. As the reaction temperature was increased from 600 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C, gasification reactions were promoted and conversion increased. At the final temperature, the lignite was converted to gas by 70%, barley straw by 80.7% and the mixture by 73.5%. The higher conversion of barley straw biochar is attributed to its enhanced specific surface area (Table 1) in comparison to the lignite, as well as its enrichment in potassium minerals such as arcanite, sylvie and fairchilide, previously presented in Figure 3, which are known to act as catalysts during the gasification process, increasing the reaction rate and thereby the conversion [4; 6; 17; 19]. From Figure 7, it can be seen that the product gas mixture of both fuels consisted mainly of \(\mathrm{H_{2}}\), \(\mathrm{CO_{2}}\) and \(\mathrm{CO}\) and lower amounts of \(\mathrm{CH_{4}}\). Hydrocarbons were produced in minor quantities (0.03-0.1% mol), and thus, were excluded from the graph. As the temperature was raised from 700 \({}^{\circ}\)C to 900 \({}^{\circ}\)C, the concentrations of \(\mathrm{H_{2}}\) and \(\mathrm{CO}\) in the product gas increased, whereas that of \(\mathrm{CO_{2}}\) decreased. Thus, it can be speculated that reactions (4) and (6) and the reverse water-gas shift reaction (8) prevailed during the process. Above 900 \({}^{\circ}\)C, reactions (4), (5) and (8) were promoted, further increasing the percentage of \(\mathrm{H_{2}}\) in the product gas at the expense of \(\mathrm{CO}\). Also, the drop in the concentration of \(\mathrm{CH_{4}}\) with increasing temperature shows that \(\mathrm{CH_{4}}\) produced by reaction (7) was further consumed by reaction (9). Figure 7 shows that the amount of syngas (\(\mathrm{CO}+\mathrm{H_{2}}\)) produced from these materials varied between 70% and 72% mol. A comparison of the present results with those of previous investigations is difficult, because of the variety of fuels and experimental conditions used. Figure 6: Effect of gasification temperature on fuel conversion. Figure 7: Composition of product gas as a function of gasification temperature of (**a**) Kardia lignite, (**b**) barley straw and (**c**) Kardia lignite/barley straw 50:50 chars. For coals of various rank, the concentration of syngas has been reported to range between 58% and 83% [6; 17; 26], while for some agricultural residues, such as cotton straw, corn stalk and rice husk, it is reported to vary between 61% and 74% at temperatures of 800-900 \({}^{\circ}\)C [4; 5; 31].