Analysis of the Risks, and the Mitigation of Heating and Combustion in Coal Slag Heaps

Abstract The coal extraction since 1830 has produced considerable amounts of mining waste. When stored in slag heaps, this waste that can sometimes heat up and then combust. This then results in potentially dangerous phenomena likely to impact humans and the environment: burns, pollution, fire, explosion, instability. This heating and its progression towards combustion can arise from internal causes (self-heating linked to exothermic processes of oxidation and hydration of coal and/or sulphide minerals) or external causes (natural or accidental fires, lightning, short circuits, etc.). In order to quantify the types of risks associated to the heating and combustion a litterature review was carried out. We began by assessing the total number of coal slag heaps built worldwide, and then the proportion that are (or have been) heating or burning. This analysis identified 180,000 coal slag heaps built since the start of coal extraction, of which, according to an international state-of-the-art review and the results of a questionnaire sent to over a hundred people, about a third are (or have been) heating or burning. Here, the main initiating mechanisms of these processes are presented, the feared events they trigger, and the resulting potentially dangerous phenomena for humans and the environment. These include safety issues (burns, explosions, instabilities), health issues (air or water pollution, olfactory pollution) and/or environmental issues (release of ecotoxic compounds, acid mists or greenhouse gas emissions). Then, a risk analysis based on this data is produced, and an inventory of the main mitigation methods (covering, excavation, monitoring, etc.) that make it possible to effectively treat these slag heaps. This data is illustrated by a few examples from two coal-producing European countries (Poland and the Czech Republic) that we visited recently.