Organic geochemical characteristics of coal biogasification

Abstract

To investigate the gas production mechanism and geochemical characteristics of biogenic coalbed methane, two pieces of lignite were selected as the degradation substrate of coal biogasification, and the coal seam native bacteria were used as the bacteria source to carry out the anaerobic gas production simulation experiment for 90 days. The results show that the gas generation mode is mostly acetic acid fermentation under anaerobic closed circumstances, and the gas production efficiency is low. The major components of biogas are CH4 and CO2, and the gas composition is dry. The entire gas production process can be divided into three stages: rapid growth, fluctuating growth, and decline. After entering the steady stage of gas production, the contents of CH4 and CO2 exhibit a clear synchronous change law. In this process, two significant inherited isotope fractionation effects caused the changes in the carbon isotopic composition of CH4 and CO2, which eventually led to the fractionation of light carbon isotopes of methyl into CH4 and heavy carbon isotopes of carboxyl into CO2, The lighter the δ13CCH4, The heavier the δ13CCO2, and the biochemical methane tends to enrich light carbon isotopes. In addition, as the biodegradation time increased, the Pr/nC17 and Ph/nC18 ratios of the two coal samples generally increased, while the OEP value and ΣnC21-/ΣnC22+ ratios generally decreased, indicating that the native bacteria of coal seam are better at degrading odd carbon of short-chain n-alkanes in soluble organic matter. At the same time, these microorganisms can also make use of aromatic compounds in coal, so it can be seen that the way of coal seam origin bacteria using coal is diversified.