Biogenic methane generation from lignite coal at different temperatures

Abstract

Microbial gasification of coal resources in situ is a novel method for mining legacy and deep coal resources. The complexity of the in-situ coal environment and the singularity of strains limit the gas production efficiency. To improve the gas production efficiency in terms of strains, multiple strains with high species density were enriched from anaerobic sludge. Anaerobic fermentation experiments were conducted using lignite at 15 °C, 35 °C, and 55 °C. To obtain complete gas production curves and to reduce measurement errors, gas production volumes were collected using an online continuous micro-gas measurement device, with an error rate less than 1%. The results showed that the gas production delay periods of multiple strains were very short; additionally, there was a stagnation period in the middle of the gas production at 35 °C and 55 °C. Moreover, these strains produced the most gas at 35 °C with the lowest total organic carbon (TOC) in solution. The maximum methane ratio reached 39.94%, the maximum gas production rate was 0.1 mL/g/d, and the maximum yield was 1.22 mL/g. When gas production stopped, the potential of hydrogen in the fermentation solution decreased and the redox potential increased. A logistic model was fitted when the temperature reached 15 °C and 55 °C. A gas production curve was produced using a segmented S-shaped curve to determine the best-fit model. The results of this study provide insights into the use of multiple strains of exogenous bacteria for gasification of lignite under different coal seam temperature conditions and also provide data to support the estimation of parameters of the numerical model for the microbial gasification of lignite.