Sustainable utilisation and transformation of the thermal energy from coalfield fires: A comprehensive review

•Global distribution and thermal characteristics for coalfield fires were found.•Heat transfer properties and influencing factors for thermosyphon were analysed.•Material performance and cooling system of thermoelectric device were optimised.•An enhanced thermoelectric conversion and utilisation system was proposed. Coalfield fires provide high-temperature heat storage at relatively shallow depths, their overburden and upper strata contain immense amounts of thermal energy, which is identical to the reservoir condition of shallow geothermal energy. Therefore, heat waste resources produced by coalfield fires can be developed and utilised as low-medium grade geothermal resources. By comparing the available methods for thermal energy extraction and conversion, it was found that the heat transfer technology using two-phase closed thermosyphon and thermoelectric conversion using thermoelectric conversion device were the most suitable for use in coalfield fire areas. As a highly effective heat conduction scheme, the two-phase closed thermosyphon was proposed as a feasible means of thermal energy extraction. Meanwhile, thermoelectric conversion device was utilised to transform thermal energy directly into electrical energy to realise the reuse of the waste heat. Some influencing factors on the heat transfer properties of two-phase closed thermosyphon were analysed, and efficiency of the thermoelectric conversion device was investigated. The thermoelectric parameters for these thermoelectric materials of interest were evaluated, and several effective methods for the improvement of thermoelectric conversion efficiency were exposed. In addition, an enhanced thermoelectric conversion and utilisation system with increased conversion efficiency was proposed. The results presented in the current study can provide a theoretical basis and technical support information for the feasible management and adequate control of coalfield fires and utilisation of the resulting thermal energy.