Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation
Frequently large volumes of nitrogen gas (N2) are reported in desorbed gas from coal using the standard gas content testing method. This study was undertaken to investigate the hypothesis of N2 generation from microbial activities in coal. This is critical for both estimating greenhouse gas emissions from coal mining, and developing efficient strategies for gas drainage and ventilation in underground mining.
The composition of gas desorbed from coal is generally corrected for possible air contamination. This is done by removing the oxygen (O2) and a portion of the N2 from the gas mixture. This portion of N2 is estimated by multiplying the O2 content by the ratio of N2 to O2 in standard air. The corrected N2 in the gas mixture after removing the air N2 is called 'excess N2'. In reporting the gas composition, commercial gas laboratories assume that the 'excess N2' is released from the coal. However, the magnitudes of excess N2 in coal seam gas (CSG) reported by these laboratories are sometimes much higher than expected. Previous studies of coal pyrolysis show that N2 can be produced during coalification from bituminous to anthracitic ranks, and that the volume of N2 generated thermogenically is normally less than ~10% of the total gas volume. Hence, a few per cent of the amount of N2 in the CSG can be considered to be of thermogenic origin (usually <10%). However, commercial laboratories often report values in excess of 30%, particularly at the later stage of gas desorption testing or from gas testing for shallow samples. Some of the excess N2 can be attributed to the depletion of O2 for very low-gas-content coals, particularly when air is not evacuated from the desorption device.
Another source of N2 in CSG could be biogenic activities: particularly for coal seams at shallow depth. The occurrence of biogenic methane (CH4) has commonly been reported in Australian coalfields. It can thus be hypothesised that N2 can also be generated by similar processes.
To investigate the possibility of microbial N2 production, we developed and designed a method of culturing N2 in coal. The method was applied to two sets of coal core samples sourced from two exploration boreholes, drilled 50 km apart, which intersect two coal measure sequences in the Hunter Coalfield. Samples were collected from five consecutive coal seams at each location, at shallow depths of 50-250 m. The formation water was collected from the same boreholes, or from other boreholes in the vicinity of the coal-sampled holes.
Cultures were established in triplicate in gas-tight, He-filled, pre-sterilised vials containing coal and formation water to maintain anaerobic and N2-free conditions. Cultures were incubated in the dark under in situ temperature conditions. Gas molecular and isotopic composition analyses were undertaken on three occasions spaced in three to four-month intervals. The generation of biogenic CH4 was monitored concurrently to confirm the presence of microbial activity in the cultures.
Gas samples collected from incubated vials were measured for molecular composition, and a new method was trialled for identifying biogenic N2 based on the relative abundance of nitrogen 15 (δ15N) stable isotopes in the collected gas.
Overall, the data show that biogenic activities have taken place and that both CH4 and N2 were produced from these cultures. CH4 was continuously produced; N2 was produced and also consumed, most likely during microbial processes. However, based on the limited number of measurements and the relatively short time span of experimentation, we are unsure of the exact production and consumption patterns of biogenic N2 in coal and the factors that affect these patterns.
Comparison of treatment samples (in which microbial activities could occur) and control samples (in which microbial activities were prevented by the use of biocide) show that the δ15N values shifted positively from the δ15N values of standard air for one borehole, and negatively for the other borehole. Since the control samples show δ15N values similar to that of air, the shift is meaningful, and could open a way to distinguish biogenic N2 from atmospheric N2 in desorbed CSG.