# Allothermal Gasification of Peat and Lignite by a Focused Light Flow
## 3 Results and Discussion
### Fuel Gasification at Various Light Flow Intensities  
As was shown above, the intensity of the light flow that pumped heat into the reaction volume determined the temperature of the fuel surface. This had a considerable effect on the heat dissipation inside the fuel layer, which was also conditioned by the fuel properties. Using the findings from the previous section, the fuel moisture content could be determined that provided the highest ratio of CO and CO\({}_{2}\) concentrations, that is the fuel composition with the highest calorific value of syngas. These fuels were peat with a moisture content of about 25 wt.% and lignite with a water content of about 30 wt.%. The gasification of such compositions using different light flow intensities had some specific aspects. Figure 4 shows the maximum concentrations of different syngas components vs. the incoming light intensity. The concentration values were normalized according to the procedure described above, using the normalization factors presented in Table 3.  
As one can see, the concentrations of most gases were growing steadily with a higher light flow intensity. However, there were some obvious jumps in concentrations for peat conversion when the light intensity was about 100 W/cm\({}^{2}\). The sharpest jumps were for methane, CO\({}_{2}\), CO, and NO. This was very similar to what was observed during the light-induced gasification of coal processing waste [22; 23; 24] when efficient gasification started just after the light intensity exceeded a certain threshold value. The same was true for the current case: low-intensity heating initiated the fuel  
Figure 4: Chemical composition of syngas vs. light flow intensity. The normalization factors of concentrations of each gas are shown in Table 3. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Fuel** & **CH\({}_{4}\) ppm** & **H\({}_{2}\) ppm** & **CO\({}_{p}\) ppm** & **CO\({}_{2}\) ppm** & **SO\({}_{2}\) ppm** & **NO\({}_{p}\) ppm** \\ \hline Peat & 95 & 1314 & 4411 & 5600 & 25 & 39 \\ Lignite & 105 & 761 & 4240 & 5100 & 10 & 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Normalization factors for the concentrations of syngas components presented in Figure 4.  
pyrolysis with some smoldering centers, and gasification started when heating was strong enough. It is clear from Figure 2 that such intensity corresponded to the surface temperature of approximately 900 K. Thus, we could calculate this surface temperature as an effective temperature threshold for peat conversion. The curves for lignite showed that there was a certain acceleration of CO, CO\({}_{2}\), and SO\({}_{2}\) production when the light flow intensity was above 80 W/cm\({}^{2}\). However, this increase was much more gradual than in the case of peat conversion. The corresponding surface temperature was about 800 K. Taking into account that the HPD for this fuel mixture was 15-20% higher than for peat, it could be concluded that the fuel surface became colder because heat penetrated into the sample depth. This clearly explained the faster increase in the production of some gases as compared with peat samples. This meant that lignite conversion had a greater effective depth than peat conversion. A further increase in the light flow intensity accelerated the combustion of both peat and lignite. It was clear that such an increase made no sense in terms of allothermal fuel conversion. Changes in the weight of the fuel sample, when heated by the light flow, are shown in Figure 5. These changes were equal to the weight of the gasified matter. It was clear that the curves corresponding to higher intensities looked essentially non-linear at the beginning for both peat and lignite. This meant that the weight of the gasified fuel was not proportional to the absorbed energy of the light flow. The duration of this non-linear stage was 5-10 times as long as it was in the gasification of coal processing waste [24]. The contribution of carbon oxidation heat here was comparable with the heat income from an external source.