Constraints of biomethane generation yield and carbon isotope fractionation effect in the pathway of acetotrophic with different coal-rank coals

Abstract

Biomethane is a clean, renewable, and environment-friendly resource, arousing wide attention worldwide. Eleven coals with vitrinite reflectance ranging from 0.18% to 2.50% were collected from underground coal mines; biomethane production process and carbon isotope fractionation effect in the 123 days biodegradation experiment were implemented. Results show that different coal-rank coals can be degraded by microorganisms and produce specific amounts of biomethane. However, the biomethane yield is related to the maceral contents rather than coal rank. The generation processes of methane and carbon dioxide exhibit the three-phases characteristics. The carbon isotope composition of biodegraded coal is lighter than that of raw coal. The liptinite inhibited biomethane production when the content of vitrinite is higher than 80%. The mineral promotes biomethane production when the content of vitrinite ranges from 60%~80%. The biomethane production yield is low, with vitrinite content less than 60%. The carbon isotope composition of coal organic matter is about −24.0‰, the carbon isotope compositions of CH4 (δ13C-CH4), CO2 (δ13C-CO2) and acetic acid (δ13C-ac) are −52.6‰~–22.1‰, −29.5‰~−24.5‰, and −40.7‰~−20.15‰, respectively. The CO2-CH4 isotope fractionation coefficient (αCO2-CH4) and its fractionation factors (εC) range from 1.001 to 1.025 and 0.50 to 23.80, proving that biomethane mainly generated from acetotrophic. The carbon isotope fractionation effect in the coal-methane conversion was jointly controlled by the anaerobic methane oxidation, multi-stage degradation, and substrate consumption.