Production law of biogenic gas made from coal and its isotope fractionation effect

To explore the mechanism of biogenic coalbed methane,the low-rank coal was selected as the parent material of biogenic gas,and the coalbed mine water carrying methanogenic bacteria was used as the source of bacteria. In a highpurity nitrogen environment,a 40 g of coal sample,20 mL of enriched bacterial solution,and 380 mL of medium were added into the 500 mL gas producing cylinder (reaction cylinder) for inoculation. After that,these gas producing cylinders were placed in a vibration incubator at 37 ℃ for a 90-day anaerobic gas production simulation experiment. Results show that the gas production mode is dominated by an acetic acid fermentation in a closed condition. Gas production efficiency is relatively low,and the process of gas production is greatly affected by the environmental pH. After entering the gas production stable period (>30 d),the pH value and the corrected gas production generally show a synchronous variation law with the gas production time. The number,abundance and activity of acid-producing bacteria and methanogens directly affect the gas environment pH and gas production. CH4 and CO2 are the main components of the produced gas,with a small amount of hydrogen. The CH4 and CO2 contents show an obvious synchronous variation law with the extension of stable gas production time. During the 90-day gas production process,the δ13C1 value is less than -55‰,and the δ13C(CO2) value is between -20. 8‰ ~ -10. 7‰,with an average of -16. 38‰. The δD1 value is between -361‰ ~ -332‰,with an average of -348‰. It can be determined that CH4 and CO2 in the gas production are organic biogenic gas. The change of carbon isotope composition of CH4 and CO2 is mainly caused by two important inherited isotope fractionation effects. The first fractionation of carbon isotope composition occurs in the process of acetic acid formation,due to the aliphatic methyl carbon isotope composition of organic parent material is relatively light,while the carboxyl carbon isotope composition of that is relatively heavy. The carbon isotope composition characteristics of that are inherited during the production of acetic acid. The second fractionation of carbon isotope composition occurs in the process of producing CH4 and CO2 by acetic acid fermentation. The methyl group in acetic acid is hydrogenated to form CH4,while the carboxyl group is dehydrogenated to form CO2. As a result,the light carbon isotopes of methyl groups are fractionated into CH4,while the heavy carbon isotopes of carboxyl groups are fractionated into CO2. Therefore,the light carbon isotope of biogenic gas from coal is fractionated into CH4,while the heavy carbon isotope of that is fractionated into CO2. The lighter the δ13C1,the heavier the δ13C(CO2). There is a negative correlation between the δ13C1 and the δ13C(CO2). During the 90 days of gas production,the longer the microbial action time is,the lighter the carbon isotopic composition of methane is. Methane has an obvious tendency to enrich the light carbon isotope.