Stimulation of methane generation from nonproductive coal by addition of nutrients or a microbial consortium

Abstract

Biogenic formation of methane from coal is of great interest as an underexploited source of clean energy. The goal of some coal bed producers is to extend coal bed methane productivity and to utilize hydrocarbon wastes such as coal slurry to generate new methane. However, the process and factors controlling the process, and thus ways to stimulate it, are poorly understood. Subbituminous coal from a nonproductive well in south Texas was stimulated to produce methane in microcosms when the native population was supplemented with nutrients (biostimulation) or when nutrients and a consortium of bacteria and methanogens enriched from wetland sediment were added (bioaugmentation). The native population enriched by nutrient addition included Pseudomonas spp., Veillonellaceae, and Methanosarcina barkeri. The bioaugmented microcosm generated methane more rapidly and to a higher concentration than the biostimulated microcosm. Dissolved organics, including long-chain fatty acids, single-ring aromatics, and long-chain alkanes accumulated in the first 39 days of the bioaugmented microcosm and were then degraded, accompanied by generation of methane. The bioaugmented microcosm was dominated by Geobacter sp., and most of the methane generation was associated with growth of Methanosaeta concilii. The ability of the bioaugmentation culture to produce methane from coal intermediates was confirmed in incubations of culture with representative organic compounds. This study indicates that methane production could be stimulated at the nonproductive field site and that low microbial biomass may be limiting in situ methane generation. In addition, the microcosm study suggests that the pathway for generating methane from coal involves complex microbial partnerships.

FIG. 1.

Methane generated from coal by native populations. Data are for treatment 2 with nutrients (TX19 and TX20) and for controls with no nutrients (treatment 6) and with BES added (treatment 4). Methane was produced when TX19 was transferred into new medium with coal on day 105. The total number of methanogens (methyl coreductase A copies per ml) in TX19 was determined using qPCR. For comparison, TX20 contained 600 mcrA copies per ml on day 105. (Inset) natural log of duplicates TX19 and TX20 showing two-phase methane generation, with divergence of the two microcosms during the second phase.

FIG. 2.

Microbial populations in negative controls, source materials, and bioaugmented treatments. TRFLP OTU peak areas (in percents) relative to the sum of TRFLP peak areas are shown. (A) Treatments that did not produce methane. (B) Potential source populations WBC-2 and the native population of treatment 2 (TX19 and TX20 profiles were highly similar; TX19 is shown). (C) Treatment 3, bioaugmented with WBC-2, at two time points (39 and 78 days) during incubation.

FIG. 3.

Methane and acetate accumulation and the concentration of methanogens in treatment 3 bioaugmented microcosms. (A) Methane generation in individual microcosms prior to destructive termination after 39, 56, 78, and, 102 days of incubation and acetate concentrations after destructive termination at 8, 18, 25, 39, 56, 78, and 102 days. (B) Total methanogens as mcrA copies/ml determined by qPCR in bioaugmented microcosms after termination at 8, 18, 25, 39, 56, 78, and 102 days.

FIG. 4.

Methanogen population changes during incubation of treatment 3. DNA copies/ml using qPCR with 16S rRNA gene selective primers: nonaceticlastic Methanomicrobiales group (A), obligate aceticlast Methanosaeta (B), and facultative aceticlast Methanosarcina (C) in microcosms destructively sampled over time. Note that Methanosaeta values are plotted on log scale to depict the full range.

FIG. 5.

Changes in selected bacterial populations during incubation of treatment 3 microcosms. DNA copies/ml using qPCR with 16S rRNA gene Geobacter primers (A) and the dsrB functional gene for sulfate-reducing bacteria (SRB) (B) in microcosms destructively sampled over time. nd, analysis not done.

FIG. 6.

Proposed model of biogenic methane generation from coal based on the current study and previous work in other laboratories (9, 11, 12, 15, 19, 20, 37, 51). H₂ may be removed by methanogenesis, acetogenesis, or by a bacterial partner with a TEA. Mid-chain fatty acids were not observed during the microcosm incubation. (Reference numbers are shown in parentheses.)