Spontaneous coal fires in heaps and beyond – a forgotten factor of the dynamics of climate change

BIO Web of Conferences 196, 02001 (2025) SMILS III https://doi.org/10.1051/bioconf/202519602001 Spontaneous coal fires in heaps and beyond – a forgotten factor of the dynamics of climate change Łukasz Kruszewski1,* 1Institute Poland of Geological Sciences, Polish Academy of Sciences, Twarda 51/55, 00-818 Warszawa, Abstract. Spontaneous coal fires are known from both post-coal mining waste-rock heaps and natural geoenvironments. They represent a still not fully scientifically encompassed phenomenon. They are very difficult to be fully contained, thus sustaining long-time emissions in the heaps. Some of them may burn for 70 years and more. As a worldwide issue, both from environmental and human safety point of view, these fires pose a forgotten factor of the dynamics of coal fires, as they add to the worldwide CO2 emissions’ budget. However, CO2 is just a tip of an iceberg in these emissions, as shown here by the example of the Upper Silesian heaps of Poland. 1 Introduction 1.1 Spontaneous coal fires – general information Spontaneous coal fires (SCF) pose a global issue that concerns both natural (e.g., coal mines, unexploited coalfields (e.g., [1])) and industrial (post-coal-mining waste-rock heaps, PCWH) [2-4]. They are rarely mentioned as a factor of climate change. Meanwhile, coal (and also bitumen) burning induces formation of fumarole that are analogous to volcanic hydrothermal vents. Examples of such fumaroles from Polish PCWH are shown in Figure 1. These events are typically linked to self-heating, spontaneous ignition, and combustion of the fuel. Their causes are complex, with key factors including oxidation of organic matter, formation of oxocarbons, the role of coal petrology (maceral composition), the physicochemical behavior and spatial interactions of coal macerals, and the catalytic or fluxing effects of iron minerals [5-7]. Importantly, in heaps not only coal but also associated barren sedimentary rocks—shales, sandstones, carbonate concretions, and others—may burn and degas [7, 8]. Some well-known cases have, however, been human-induced. One of the most famous examples is the Centralia fire in Pennsylvania, USA, which has been active for over 57 years [4]. *Correspondingauthor: © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). BIO Web of Conferences 196, 02001 (2025) SMILS III https://doi.org/10.1051/bioconf/202519602001 Fig. 1. Examples of coal-fire fumaroles in the Upper Silesian post-mining waste-rock heaps. A – hot (>200 oC surface temperature) gas vent richly encrusted with salammoniac (NH 4Cl), sulfur (S8) and NH4-Bi-Pb-Sn-Cd-Ag-Cl-I-Br mineralization; B – another, highly hydrated fumarole in the same heap, winter time; C – hot fumaroles of another heap, richly encrusted by salammoniac covered by orange-red ammonium iodobismuthate – a new mineral; this site strongly resembles La Fossa volcano, Italy; D – shiny coal tar and minor salammoniac freshly desublimed from reduced gases; E – very hot organics-rich fumarole in a fire front; F – in situ complex gas analysis of sulfur-encrusted gypsum (CaSO4∙2H2O)-rich fumaroles using portable GASMET DX4000 system. 2 BIO Web of Conferences 196, 02001 (2025) SMILS III https://doi.org/10.1051/bioconf/202519602001 In China and India, both anthropogenic and natural coal fires affect areas spanning many square kilometers. In China, they occur mainly in the northern provinces—Xinjiang, Inner Mongolia, and Ningxia. In Tajikistan, famous lignite fires of the Kukhi-Malik – Ravat – Jijikrut are (Fan-Yagnob Coal Basin) were reportedly observed by Alexander the Great [8], suggesting continuous activity for more than 2000 years. The PCWH are widespread wherever coal is exploited [4, 6-8, 9-10]. Such heaps typically contain both residual coal and barren rock and undergo intense physicochemical transformations driven by three main processes [8, 11–13]: (1) pyrometamorphism – hightemperature contact metamorphism during the burnout stage, (2) exhalative processes – condensation of hot coal-fire gases (CFG) and their interaction with waste material, analogous to hydrothermal processes; and finally (3) low-temperature supergene alteration – formation of hydrated sulfates of NH4+, Mg, Al, Fe, Ca, K, and Na. 1.2 Chemistry of coal burning Coal burning induces decomposition and simplification of the complex structure of the coal macromolecule that comprises carbon, hydrogen, oxygen, nitrogen, sulfur and some minor elements. Examples of chemical reactions related to this process are given in Figure 2. 1.3 Coal fires versus climate change The SCF are rarely mentioned as a factor of the climate change and its dynamics. Meanwhile, the north China SCF area is potentially responsible for as much as 2-3% of the yearly, worldwide CO2 production from fossil fuels burning [1]. Numerous studies of coal-fire gas (CFG) rely on the Gas Chromatography (GC) method and simple analytical tools, thus reporting, e.g., SO 2 as the sole form of sulfur. We have proven that the GC method – especially as not necessarily an in situ method – may not be ideal and may not represent the completely true and whole spectrum of the gaseous compounds in the heap emissions. With the sophisticated portable FTIR spectrometer (GASMET DX-4000), used, i.a., in situ, we have shown that this is a large oversimplification, with dimethyl sulfide (DMS), (CH3)2S, thiophene, C4H4S, and occasionally dimethyl disulfide(DMDS), (CH3)2S2, being volumetrically more important [14]. One of the major discoveries is also highly probable admixture of SF6 in concentrations much higher (1.n ppmv) than ones typically measured in natural environment [15]. 3 BIO Web of Conferences 196, 02001 (2025) SMILS III https://doi.org/10.1051/bioconf/202519602001 Fig. 2. Examples gas-generation reactions due coal burning (from [7], also given in [13]). 4 BIO Web of Conferences 196, 02001 (2025) SMILS III 2 https://doi.org/10.1051/bioconf/202519602001 Examples from Poland A few-year-long spatiotemporal study of the gaseous emissions in the Polish PCWH mainly of the Upper Silesian Coal Basin area, that encompassed ~100 fumarolic vents in various objects allowed to deeply dive into both the chemical composition of the local CFG and its variations. We have used a sophisticated in situ analysis by the means of the FTIR spectrometry (GASMET DX4000 system) that allows to study hot, chemically aggressive and ash-carrying fumes. An important feature of this system is capability of immediate reporting of both organic and inorganic compounds. The inorganic part not only includes separated NOx (N2O, NO, NO2) but also water and simple gases that do not interact with due to a special heating procedure applied. The method is based on a multi-step approach: (1) precise, certified, fully quantitative analysis of 30+ preselected gases with co (...truncated)