Compositional analysis and isotope sourcing of gases generated from self-heating coal waste dump: the case study from France

The accumulation of organic matter in coal waste dumps can result in self-heating or spontaneous ignition, which lead to the release of various gaseous products into the atmosphere. GHGs and trace compounds emitted from self-heating coal waste dump located in the Nord-Pas-de-Calais region of Northern France were investigated under this study in September 2024. Tracking hotspot locations across coal waste dump confirmed various patterns of temperature and gaseous emissions from the investigated area. The temperature measured in boreholes drilled to the depth up to 0.6 meters on the top and slopes of the dump ranged between +51.0 and +83.1 °C. The non-uniform subsurface temperatures can be explained by the varied content of coal and carbon-containing rocks deposited at the dump, along with the diverse air inflow to the thermally active sites. The composition and source of the gaseous compounds emitted during self-heating were directly influenced by the various thermal activity stages and properties of the organic matter present in the dump.

Different generation patterns of released gases are related to the self-heating stage, including exothermic oxidation and pyrolysis. At thermally active sites (but below +68 °C) on the top of the dump (well-ventilated with free access of oxygen) the emission included CO₂ 12.8 vol%. Otherwise, at the sites on the wet dump slope (preventing oxygen entering), where prominent thermal activity was noted (temperature rise of about 80 ºC), the switch to pyrolysis was confirmed by showing a peak of CO₂ (18.3 vol%), with a significant drop in O₂ content (1.48 vol%). Apart from CO₂, much higher was the concentration of CH₄ reaching 4260 ppmv, and CO averaging 54 ppmv, above background H₂, high levels of pyrolytic ethane 327 ppmv and propane 69 ppmv as well as C₄ – C₆ hydrocarbons.

The generation processes of the gases on both types of sites were confirmed by C and H isotopic analyses (CO₂, CH₄, C₂H₆, H₂) and will be discussed in detail during the presentation of the paper. Recapitulating, the stable isotope tracing of the emitted gases was useful and can also be indicative for future monitoring of the thermal stage of self-heating coal waste dumps. Additionally, sulfur and nitrogen heterocyclic compounds such as furane, thiophene, and pyridine were detected in trace quantities. Although substantial amounts of gasses (mainly CO₂ and CH₄) escaped from the emission hotspot on the dump, their concentrations measured above the surface at sites without thermal activity were not significantly higher than local background levels. The surface flux mapping of entire dump, depth profiling of temperature and gas concentrations, their generative and degradation processes will be the main areas of future investigations.

 

This work was funded by the Polish Ministry of Science and Higher Education under Grant No. 2022/44/C/ST10/00112. The isotopic analysis has been supported by the ATMO-ACCESS (grant agreement No. ATMO-TNA-4—0000000041 and No. C1-ISOLAB/CESAR-9).

Acknowledgment for organization of and assistance during the onsite measurements and samples acquisition on the dump to Fabrice Quirin, Vincent Adam, Gaetan Bentivegna from Bureau de Recherches Géologiques et Minières, Unité Territoriale Après-Mine Nord, Billy-Montigny, France.