# Allothermal Gasification of Peat and Lignite by a Focused Light Flow
## 3 Results and Discussion
### Influence of Fuel Moisture on the Chemical Composition of Syngas  
Heating the fuel by a high-intensity light flow accelerated the temperature increase, which was much faster than it was during the heating by partial combustion of the fuel (like in autothermal gasification). This meant that most of the water from the fuel evaporated through a layer of hot carbon, facilitating steam-carbon reactions. They slightly decreased the fuel temperature growth, suppressing the ignition. However, the fuel layers that were close to the surface could still ignite, producing CO\({}_{2}\). Their combustion was weak and unstable, occurring mostly as smoldering and ending quickly when the incoming light flow was switched off. Nevertheless, the presence of smoldering supplied considerable heat to the fuel. Therefore, the gasification of peat and lignite was not purely allothermal. The contribution of combustion heat to the process dynamics was not decisive, but it noticeably affected the rate of syngas production. The changes in the chemical composition of syngas, when varying the fuel moisture, are shown in Figure 3. The light flow intensity was about 100 W/cm\({}^{2}\) in this set of experiments. The gas concentrations were normalized here for a better joint representation of the curves with the values, differing by an order of magnitude or even more. The normalization factor meant the maximum value of the concentration of certain gas that corresponded to \(C=1\) on the curve. The values of the normalization factors are shown in Table 2.  
Figure 2: Fuel surface temperature vs. light flow intensity during gasification. As seen from Figure 2(a), the concentration of combustible gases significantly increased with the water concentration growth from 10 wt.% to 20 wt.%. This fact reflected the acceleration of steam-carbon reactions, as well as an almost twofold decrease in the HPD in the bulk of the fuel. A decrease in HPD meant that the temperature was redistributed, and the near-surface layer was heated more rapidly with a lower temperature in the deep layers. It accelerated combustion in near-surface layers with the corresponding growth of the CO\({}_{2}\), SO\({}_{2}\), and NO production. A further increase in the water content up to 40 wt.% did not significantly affect the production of combustible gases and CO\({}_{2}\) with NO. However, the production of sulfur dioxide gradually went down. This indicated that the evaporation of water facilitated the fuel cooling. An even further increase in the water concentration led to a considerable decrease in the fuel temperature inside the illuminated area with a corresponding reduction in the production of all the gases. However, the ratio of the dominating combustible component (CO) to the dominating passive gas (CO\({}_{2}\)) changed here, showing a way to increase the calorific value of syngas. The next step of the water concentration increase was followed by a decrease in the production of gases and made no sense in terms of the fuel conversion. Figure 2(b) shows the curves of gasification results vs. lignite moisture. An increase in the water concentration from 15 wt.% to 35 wt.% intensified the production of most gases (excluding SO\({}_{2}\)). However, the production of carbon oxides did not grow so sharply as in the case of the peat-based mixture. A further increase in the water content by 10 wt.% lead to a substantial decrease in the production of all gases. The fuel composition with such a big amount of water could not be gasified using the proposed method within a reasonable time because of the long duration of drying of extra \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Fuel** & **CH4, ppm** & **H2, ppm** & **CO, ppm** & **CO2, ppm** & **SO2, ppm** & **NO, ppm** \\ \hline Peat & 142 & 1410 & 6090 & 9530 & 43 & 53 \\ Lignite & 356 & 2613 & 15,170 & 22,610 & 33 & 26 \\ \hline \hline \end{tabular}
\end{table}