# Allothermal Gasification of Peat and Lignite by a Focused Light Flow
## 1 Introduction  
Over recent years, human civilization has gone through a chain of energetic crises. Some of them were caused by the conflicts of interests between the world's centers of power, when the availability of oil and gas decreased significantly for many countries. However, there were some objective reasons for that, as well: an increase in the global population and the depletion of numerous readily available deposits of fossil fuels that were in use for a long time [1]. The deficit of high-quality industrial fuels moves people to use widespread and low-demand fossil fuels (oil shale, peat, lignite) and other raw materials earlier classified as waste [2; 3; 4]. Using different types of domestic waste and biomass as fuel represents yet another way of replacing traditional fuels [5; 6]. However, using such quasi-renewable fuels through direct burning calls for a complicated modification of heaters and involves additional processing stages (like esterification) of the fuel. A permanent increase in the atmospheric pollution causes climate changes. Therefore, the new fuels and corresponding techniques of their application have to be environmentally friendly. However, replacing coals, oil, and natural gas by low-quality fuels on a permanent basis is quite expensive and only makes sense in a few industrial sites. A much more attractive option is to preprocess such fuels to a state when a sub-product is suitable for burners with the current design. This is implemented by the thermal conversion of a fuel (pyrolysis and/or gasification) to a combustible gas [5; 6; 7]. It typically produces synthesis gas (syngas)--a mixture of combustible and non-combustible gases (CO, CO\({}_{2}\), H\({}_{2}\), CH\({}_{4}\), etc.) with the water steam. Syngas can serve as an inexpensive replacement of natural gas for the power industry or as a good crude material for the chemical industry [8; 9; 10]. Therefore, it is a relevant task currently to developefficient and inexpensive methods of converting fossil fuels into synthesis gas. The most well-known gasification techniques suggest the conversion of fuel through its partial combustion [11; 12; 13]. This method is good, but it requires special efforts to keep the efficiency at a high enough level when the fuel has low reactivity and high ash content [14; 15]. In this case, the production of heat has low intensity, and an essential part of it is spent on heating the concomitant passive matter. The second argument for the development of a new gasification method is that a certain heat flow is required for the optimal conversion efficiency. Combustion gives us very few possibilities for heat flow manipulation (the maximum heating intensity is limited by the fuel reactivity, and even reducing the temperatures is not so easy for autothermal processes). Applying focused sunlight as a heat source of fuel gasification promises a relatively inexpensive industrial conversion technique. Taking into account that the light absorption coefficients of fossil fuels are much higher than those of high-quality photovoltaic devices, this approach looks quite promising. There are already some known attempts to use solar light for fuel processing [16; 17; 18]. However, these approaches are quite different from the ones proposed in this paper in terms of the types of fuels used, their moisture content, and the fuel particle sizes. An important factor of solid fuel gasification is the reaction of carbon with water [11; 12]. On the one hand, it rapidly produces CO and H\({}_{2}\), and on the other hand, it suppresses the direct oxidation process, thus preventing the fuel ignition. Consequently, most of the gasification approaches deal with water-fuel compositions [13; 19]. The existing allothermal (allothermal processes here are thermochemical processes where the heat needed for the transformation of precursors comes from the external heat sources) approaches are suitable for relatively pure carbon.