Hydrocarbon generation from coal, extracted coal and bitumen rich coal in confined pyrolysis experiments

摘要

Three sets of pyrolysis experiments were performed on extracted coal (R_o% 0.39), coal (initial bitumen 13.5mg/g coal) and bitumen enriched coal (total bitumen 80.9mg/g coal) at two heating rates of 2°C/h and 20°C/h in confined systems (gold capsules). For all three experiments, the yields of bitumen, σn-C_(8+), aromatic components and σC_(2-5) at first increase and then decrease with increasing EASY%Ro and reach the highest values within the EASY%R_o ranges of 0.67-1.08, 1.07-1.19, 1.46-1.79 and 1.46-1.68, respectively. In contrast, C_1/σC_(1-5) ratio at first decreases and then increases with EASY%R_o and reaches a minimum value in EASY%R_o range of 0.86-1.08, closely corresponding to the maximum values of the yields of bitumen and σn-C_(8+). Methane yields increase consistently with EASY%R_o. Nearly half of the maximum yield of methane from kerogen was generated at EASY%R_o>2.2. The differences in methane yields among the three experiments at the same thermal stress are relatively minor at EASY%R_o<2.2, but are greater with thermal stress at EASY%R_o>2.2. This demonstrates that the kerogen always retained relatively more hydrogen and hydrocarbon generative potential at the postmature stage of bitumen rich coal than the extracted coal or coal.The maximum yield of ethane is 20-25% higher in the bitumen rich coal experiment than the extracted coal or coal, while the maximum yields of C_3, C_4 and C_5 in the former are double to triple those in the latter. This result demonstrates that the added bitumen in bitumen rich coal substantially increased the generation of these wet gases. However, the averaged values of activation energies (with the same frequency factors) for both the generation and cracking of individual wet gases are similar and do not show consistent trends among the three experiments. For all three experiments, activation energies for the generation and cracking of wet gases are significantly lower than those in previously published oil pyrolysis experiments with same frequency factors (Pan et al., 2012; Organic Geochemistry 45, 29-47). Methane δ~(13)C values at the maximum temperature or EASY%R_o are close to those of initial wet gases, especially C_3, implying that the major part of methane shared a common initial precursor with wet gases, i.e., free and bound liquid alkanes.