Abstract:Fire is a phenomenon that covers a multiplicity of scales depending on the different processes involved. Length scales range from the nanometres when addressing material flammability to the kilometres when dealing with forest fires, while time scales cover a broad spectrum too. Heating of structural elements can be measured in hours while characteristic chemical times for reactions do not exceed the millisecond. Despite these wide ranges, a series of simple scaling laws seem to describe well a multiplicity of processes associated with fire. In this paper, flaming ignition of a solid fuel will be presented within the context of general scaling laws and forest fires. Therefore, the case of highly porous vegetable fuels will be investigated to extend the theory to the forest fires application.

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Coalfield subsurface fires can result in ecological disasters of global dimensions. These fires are difficult to control therefore can result in colossal wastage of resources (the coal itself but the resources devoted to suppression), a serious negative impact on the environment and acute health problems for large populations. However, if the heat can be effectively recycled and utilized, the combustion energy will be recovered but also heat extraction can promote suppression. Thus, leading not only to a positive energy impact but to a reduction polluting emissions and consequent health issues. This paper presents the results of a feasibility analysis of the overall recovery of underground thermal resources of a novel system of Waste Heat Recovery Units (WHRUS) that combines thermosyphon and thermoelectric technologies. Both thermal equivalent model and numerical assessment are presented. A series of realistic-scale field experiment conducted in the Xinjiang's fire zone for an extended period are discussed. Using a local geothermic assessment, the heat recovered from subsurface coal fire can be estimated as the summation of the convective and conductive components of the energy generated. The average heat generated for the fire district is estimated at approximately 495 W/m 2 and the average extraction efficiency at approximately 58%. The WHRUS shows and excellent heat transfer performance with an effective lower resistance of approximately 0.0049 W/°C and maximum thermal recovery rate greater than 90%. Finally, while the thermoelectric

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