Analysis of the mechanism for H2S generation and the main factors controlling gas production in coal seams

Abstract

H2S is a poisonous and reactive gas found in coal mines, and it brings serious risks to coal mining. However, research on the mechanism for H2S generation in coal seams is rare, and information on the causes of the concentration anomalies is relatively limited. Simulations of coal thermal evolution and the mechanism for coal thermochemical sulfate reduction (TSR) are also scarce. Herein, the processes for H2S generation from coal under different conditions were simulated by combining high-temperature and high-pressure sealed simulations of TSR under different conditions. Gas formation in the TSR reactions occurring during pyrolysis, different sulfate types, salinities and coal particle sizes were discussed, H2S and other gas products were compared, the assumptions regarding TSR reactants and the product generation from coal were illustrated, and the H2S generation mechanism occurring during coal pyrolysis and the TSR reaction were revealed. H2S, CH4, C2-C5, and nonhydrocarbon gases were detected after the pyrolyses. The TSR reaction was the main source of thermogenic H2S and can be divided into the TSR starting stage, the TSR and coal pyrolysis symbiotic H2S stage, and the TSR autocatalytic reaction stage. TSR reactions exhibit different reactivities under different sulfate conditions and starting temperatures. The concentrations and yields of the H2S generated by the TSR reactions of MgSO4, CaSO4 and Na2SO4 decreased successively. The stronger the electronic induction effect of a cation was, the lower the energy barrier for the TSR reaction. High salinity led to contact ion pairs (CIP) involving sulfates and then accelerated the TSR reaction and H2S generation. Smaller coal particle sizes promoted heat and mass transfer during the TSR reaction, which enhanced H2S production.