MICROBIAL METHANOGENESIS OF COAL: THE IMPORTANCE OF ORGANIC INTERMEDIATES

Coalbed methane is a significant energy resource, currently accounting for about 10% of natural gas production in the USA, and anticipated to increase during coming decades. An important and potentially renewable component of coalbed methane is produced by anaerobic microbial activity (microbial methanogenesis), but little is known about microbial degradation of the complex organic geopolymers in coal to produce methane, the nature of the microbial community involved, and important intermediates in the degradation pathway. We are conducting laboratory experiments to examine in detail the microbial degradation of coal to methane.

Subbituminous coal samples from cores drilled in south Texas and the Powder River Basin, WY were used in initial experiments. Experiments were conducted in sealed anaerobic bioreactors with coal and a buffered bicarbonate aqueous solution with nutrients or the coal-associated water with nutrients. A microbial consortium enriched from a natural wetland environment (WBC2) was added to the bioreactors for the experiments. Methane production from the endemic microbial populations was also monitored. WBC2 was observed to produce methane from all the coal samples, and methane production from coal was observed from two of the endemic microbial populations. Typical methane production curves over time involved an initial lag period (substrate and microbial population buildup), an exponential growth phase (active methane production from substrate), and a plateauing of methane production (equilibrium between substrate production and utilization). Free fatty acids (principally acetate) were most abundant in the bioreactor water during the exponential growth phase, suggesting that these compounds are important intermediates in the production of methane from coal. Long-chain fatty acids (hexadecanoic acid) and aliphatic hydrocarbons were also observed to accumulate in the bioreactor solution during the growth phase. These substances may be intermediates in the biodegradation pathway. The dominance of aliphatic hydrocarbons and long-chain fatty acids in the bioreactor aqueous phase also suggests that aliphatic functionalities rather than the aromatic backbone of coal are primarily attacked by microbial enzymes during biodegradation of coal.