Influence of pressure on stable carbon isotope ratio and production yield of coal-derived methane

Abstract

Two immature coals have been pyrolyzed in confined gold reactors from 250 to 600 °C, but at different constant heating rates (2 and 20 K/h) and under different pressures (30 and 65 Mpa). Yields of coal-derived methane show significant differences under different pressures even with the same analytical procedure. Generally speaking, increasing pressure can result in an increasing yield of methane, even though the experiment did show a slight decrease at low temperatures (≤400 °C) with high heating rates. The range of activation energies for methane generation is obviously narrower at 65 Mpa than at 30 MPa. Under high pressure conditions of 65 MPa, methane relatively depleted in 13C would be induced at the major stage of methane generation. However, our kinetic modeling results show that isotope-specific kinetic parameters, activation energies and frequency factors of methane at high pressure are almost the same as those at low pressure. The isotope ratios for methane precursors in two coals studied were also similar to each other but with some variances under the two different pressure circumstances. The modeled δ 13C of cumulative methane by using two pressure-based series of kinetic parameters under geological conditions at 2 K/Myr exhibits less differences, which manifests that the effect of pressure can be regarded as being negligible. The modeled δ 13C values of instantaneously generated methane at two different pressures under geological conditions are, however, significantly distinctive, and this is especially the case at high maturation stage of coal.