# Allothermal Gasification of Peat and Lignite by a Focused Light Flow
## 3 Results and Discussion
### Fuel Gasification at Various Light Flow Intensities
The second difference from the previous observations was the fact that an increase in the light intensity did not directly increase the slope of the curves (i.e., the gasification acceleration was different from that of bituminous coals). It was clear that some curves coincided although the intensity of heat pumping was increasing. This suggested that the oxidation heat even at the previous light intensity value exceeded the corresponding increase in the external heat flow. Thus, the heat effect of the fuel smoldering had a significant influence on the peat and lignite conversion dynamics. This fact also meant that the conversion was not purely allothermal. However, switching off the external light flow  
Figure 5: Weight of syngas vs. conversion time of gasification of peat (**a**) and lignite (**b**) at different light flow intensities. led to a fast decay of the expansion of fuel smoldering. The main part of the hot zone cooled down for some seconds, and the residual islands of the fuel smoldering disappeared within 10-15 s.  
The light spot with a diameter of approximately 2-2.5 mm allowed the gasification of about 3 mm\({}^{3}\) of the fuel (\(\sim\)1% of the sample weight) in a hundred seconds. A rather low conversion rate was conditioned by a relatively low density of the heat flow, as well as by the high ash content of the fuel. The ash deposited in the oxidized area reduced the heat penetration into the depth due to lower thermal conductivity. The process dynamics showed that the proposed technique could be applied when a thin layer of the fuel was fed into the reactor. This layer should continuously move under a spot of focused light at a certain velocity that depends on the thickness of the layer and the actual content of the fuel. In this case, an optimal conversion rate with good usability could be estimated. The obtained findings indicated that the allothermal gasification processes of peat and lignite developed in a similar way. Identical intensity of light energy pumping and similar conversion rates showed the similarity of the undergoing processes. However, the essential differences in the chemical composition of syngas together with the surface temperatures meant that these fuels differed greatly. One of the important differences was the dependence of the surface temperature on the light flow intensity. As can be seen, the heating of lignite was less efficient due to the difference in the light absorption even in a wet state. A very different dependence of the concentrations of syngas components on the initial concentration of water showed that the contribution of steam-carbon reactions to gasification was quite different for these two fuel types. The excess of water led to a dramatic decrease in the syngas production, but this fall was much sharper for lignite. Varying both the initial moisture content of the fuel and the light flow intensity, gasification could be optimized in accordance with the established goals (maximum gas production, minimum amount of non-combustible gases, etc.). Such flexibility of manipulation of the process performance is almost impossible with autothermal processes. It can serve to investigate the fundamental properties of the fuel gasification or to optimize the industrial applications of the proposed approach. In contrast to the well-known autothermal techniques, the light-induced gasification had some specific aspects:  
* Very low inertia of the undergoing processes that started and stopped within seconds. * Easy adjustment of operating parameters for a chosen type of fuel. * High efficiency of using solar light (the absorption coefficient of water-fuel mixtures was above 60% for a wide range of fossil fuels). * Convenient scaling that allowed the creation of table-sized scientific setups, as well as big solar concentrators. Of course, the upscaling of the gasifier performance required a proportional increase in the heat source power, when using the allothermal approach. In the case of solar light, it meant an increase in the active area of light concentrator mirrors.