Waste heat recovery, utilization and evaluation of coalfield fire applying heat pipe combined thermoelectric generator in Xinjiang, China

Coalfield subsurface fires can result in ecological disasters of global dimensions. These fires are difficult to control therefore can cause colossal wastage of resources, negative impacts on the environment and health problems for large populations. However, if the heat can be effectively recycled and utilized, the combustion energy would be recovered to promote fire suppression and pollution reduction. This paper presents the results of feasibility analysis and overall evaluation for the harvest and utilization of underground thermal resources, assisted with an invented system of Waste Heat Recovery and Utilization System (WHRUS) that combines thermosyphon and thermoelectric technologies. Both thermal equivalent model and numerical assessment have been proposed. A series of realistic-scale field experiments initially conducted in the Xinjiang’s fire zone for an extended period are discussed. Using the local geothermic assessment, the heat recovered from subsurface coal fire can be estimated as the summation of convective and conductive type of geothermal energy. The average heat flow through the fire district is about 495 W/m2 and the average utilization efficiency is 58%. The WHRUS shows excellent performance on heat transfer with lower resistance of 0.0049 W/°C and maximum thermal recovery rate greater than 90%. Even though the thermoelectric conversion efficiency is low, the power generated remains stable and every wellbore produces more than 960 W averagely. The majority of the heat extracted is used to heat water for regional heating in a delivering rate of 38%. Through this process approximately 105 MW/a of otherwise wasted heat could be recovered and reused from the entire investigated fire zone.