Assessment of factors influencing the composition and yield of gases emitted during the self-heating of coal waste based on laboratory simulations

Coal wastes deposited in heaps may undergo self-heating and spontaneous combustion processes. Under oxygen-limited conditions in the dump, besides burning, pyrolysis processes occur. Both processes (pyrolysis and oxidation) result in emissions of volatile pollutants and have been developed in several heaps in the Upper Silesian Coal Basin (USCB), where coal exploitation has lasted for over a century.

         To examine the molecular composition and yields of volatile compounds produced during self-heating, in relation to organic matter (OM) content and its maturity, mineral matter composition, temperature, water presence and oxidation conditions, four coal waste samples were collected from the Polish part of the USCB: two from the Janina Mine (JAN-1, JAN-2) (Rr ca. 0.5%) and two from the Marcel Mine (MAC-1, MAC-2) (Rr ca. 0.9%). JAN-1 and MAC-1 are siltstones containing more quartz and less OM than JAN-2 and MAC-2 claystones. Results of the Rock-Eval analysis evidenced 1.8, 27.3, 1.9, and 21.4 wt. % TOC, respectively and the presence of the Type-III kerogen in all samples. Simulations of the self-heating were conducted in 1-L closed reactors in conditions: dry pyrolysis (DP), hydrous pyrolysis (HP) in 250, 360, and 400^oC for 72 h, and oxidation with air (OXI) in 250 and 400^oC for 72 h.

The molecular composition of generated gases (i.e., HCs (C₁-C₈), CO, CO₂, H₂, H₂S, organic S-compounds) was determined and then re-calculated as yields, taking into account the amount of gas generated (pVT) and the mass and TOC of the rock used for each experiment.

The concentration of HCs in HP and DP runs increases with temperature increase up to 58.4 mol %. The concentrations of other generated gases in these experiments strongly relate to the temperature increase of the process as well: the concentration of CO decreased, and the concentration of CO₂, H₂, H₂S and organic S-compounds increased. The presence of water and elevated TOC amounts boost the generation of S-compounds (dominated by H₂S). During all OXI experiments, only traces of HCs, H₂ and S-compounds were produced; the concentration of CO₂ increased and CO decreased with experiment temperature increase. Gases generated from TOC-rich rocks are richer in HCs, and organic S-compounds, resulting in yields of these gases, up to 4.3 kg/Mg rock and 1.4 g/Mg rock, respectively. The highest CO₂ and CO yields were recorded in OXI experiments, reaching approximately 390 and 20 kg/Mg rock, respectively; in pyrolytic experiments, these yields did not exceed 8.4 and 0.15 kg/Mg rock, respectively. After recalculating the gas yields per TOC mass in waste, it appeared that the highest HCs yields, exceeding 20 kg/Mg TOC, were recorded during HP and DP pyrolysis at 400°C of samples poor in organic carbon. CO₂ and CO yields are highest in OXI experiments of the above-mentioned samples, reaching 850-2000 and 10-50 kg/Mg TOC, respectively. The yields of these gases in pyrolysis experiments for these samples reach 24-310 kg/Mg TOC and 0.0-1.3 kg/Mg TOC, respectively. 

This study was financed from the AGH University of Krakow research subsidy (16.16.140.315) and the National Science Centre, Poland, grant No 2017/27/B/ST10/00680.