Chemical compound classes supporting microbial methanogenesis in coal

Microbial generation of coalbed methane (CBM) occurs in numerous basins worldwide. This study identifies specific classes of chemical compounds in two high volatile bituminous coals from the Illinois Basin in southwestern Indiana, USA that are being degraded during methanogenesis. Springfield Coal is rich in vitrinite (92.2 vol.% vitrinite in coal) whereas Lower Block Coal contains larger contributions of liptinite and inertinite and contains only 66.2 vol.% vitrinite. Anaerobic bioreactors used in the experiments contained a mineral-salts medium, operationally-defined coal extracts, and a microbial inoculum composed of an endemic microbial consortium that was obtained from co-produced water from an active CBM well. Our experiments investigated degradation of organic matter (OM) and concurrent methanogenesis using water-soluble, methanol (CH3OH)-soluble, and dichloromethane (CH2Cl2)-soluble fractions of extracted OM from coal as the sole carbon sources. Only methanol and dichloromethane were found to be effective solvents to extract sufficient OM from coals for biodegradation. The methane yield over time was small (1.7 to 8.7 μmol in total headspace) and less than 1% of carbon added to bioreactors in organic extracts was converted into methane. Extracts of vitrinite-rich Springfield Coal yielded relatively more methane than extracts of Lower Block Coal. The preference of methanogenic microbial consortia for extracts from vitrinite-rich Springfield Coal may be related to the fact that the organic matter in vitrinite is rich in functional groups containing organic nitrogen, sulfur and oxygen (NSO) that provide chemical sites with low activation energies to initiate biodegradation. A comparison of the molecular compositions of the initial organic extracts with their biodegraded residues after incubation of the bioreactors for 24 weeks indicated that biodegradation of n-alkanes and aromatic hydrocarbons occurred concurrently, whereas hopanes proved to be far more refractory. Our study documents significant biodegradation of n-alkanes (14 to 91% by GC/MS peak intensity) from coal extracts. It is particularly noteworthy that biodegradation also eliminated 6 to 58% of aromatic biomarkers, although the pool of n-alkanes was not exhausted. This significant early biodegradation of aromatic hydrocarbons in coal extracts contrasts starkly with the biodegradation pattern of petroleum where similar aromatic compounds are degraded only after n-alkanes have been severely depleted. This evidence may support the hypothesis that microbial communities engaged in coal biodegradation do not express a strong preference for either aliphatic or aromatic carbon sources, in contrast to biodegradation patterns typical of petroleum. CH3OH‐extracted organic matter from coal was generally more biodegradable than CH2Cl2-extracted organic matter from the same coal, probably because CH3OH extracts contained lower concentrations of longer straight aliphatic chains allowing faster microbial degradation.

► Methanogenesis was observed in anaerobic bioreactors using organic matter extracted from coal.
► Microbes from coalbed waters were effective in methanogenic incubations with coal extracts.
► Concurrent biodegradation of n-alkanes and aromatic hydrocarbons was observed.
► Methanogenesis was greatest in extracts of coals richer in macerals from the vitrinite group.