BackgroundBiogenic and biogenic-thermogenic coalbed methane (CBM) are important energy reserves for unconventional natural gas. Thus, to investigate biogenic gas formation mechanisms, a series of fresh coal samples from several representative areas of China were analyzed to detect hydrogen-producing bacteria and methanogens in an in situ coal seam. Complete microbial DNA sequences were extracted from enrichment cultures grown on coal using the Miseq high-throughput sequencing technique to study the diversity of microbial communities. The species present and differences between the dominant hydrogen-producing bacteria and methanogens in the coal seam are then considered based on environmental factors.ResultsSequences in the Archaea domain were classified into four phyla and included members from Euryarchaeota, Thaumarchaeota, Woesearchaeota, and Pacearchaeota. The Bacteria domain included members of the phyla: Firmicutes, Proteobacteria, Bacteroidetes, Actinobacteria, Acidobacteria, Verrucomicrobia, Planctomycetes, Chloroflexi, and Nitrospirae. The hydrogen-producing bacteria was dominated by the genera: Clostridium, Enterobacter, Klebsiella, Citrobacter, and Bacillus; the methanogens included the genera: Methanorix, Methanosarcina, Methanoculleus, Methanobrevibacter, Methanobacterium, Methanofollis, and Methanomassiliicoccus.ConclusionTraces of hydrogen-producing bacteria and methanogens were detected in both biogenic and non-biogenic CBM areas. The diversity and abundance of bacteria in the biogenic CBM areas are relatively higher than in the areas without biogenic CBM. The community structure and distribution characteristics depend on coal rank, trace metal elements, temperature, depth and groundwater dynamic conditions. Biogenic gas was mainly composed of hydrogen and methane, the difference and diversity were caused by microbe-specific fermentation of substrates; as well as by the environmental conditions. This discovery is a significant contribution to extreme microbiology, and thus lays the foundation for research on biogenic CBM.Electronic supplementary materialThe online version of this article (10.1186/s13068-018-1237-2) contains supplementary material, which is available to authorized users.

Waste reuse energy generation is encouraged in many countries. A low H/C ratio in coal is a limiting factor that affects biomethanation. However, corn straw has a better H/C ratio, which is beneficial to methane production. A series of biological gas production experiments was carried out using lignite B, bituminous coal D, bituminous coal C, and corn straw to study the effect of synergistic biodegradation of coal combined with corn straw into biomethane. Methane production was used as an index to analyze the gas production characteristics of single coal, single corn straw, and coal and corn straw. The results show that the optimum proportion of the conversion of lignite B and corn straw to biomethane is 2:1, and the optimum proportion of the conversion of bituminous coal D, bituminous coal C, and corn straw into biomethane is 3:1, and corn straw has a significant enhancement effect on the biological gas production in this experiment. When lignite B, bituminous coal D, and bituminous coal C are, respectively, mixed with corn straw, the highest methane production is 2.05, 2.69, and 1.35 mmol/g, indicating that corn straw has different promotion effects on gas production for coals of different rank: bituminous coal D + corn straw > bituminous coal C + corn straw > lignite B + corn straw. The stage change of COD and pH was consistent with that of the total gas production under optimal proportion, and the complementary advantages or mutual inhibition of methanogens was one reason for the differences in gas production between coal combined with corn straw and single coal.

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