Gas evolution and isotopic fractionations during pyrolysis on coals of different ranks

The study was performed on 4 different coals of varied rank. Pyrolysis was conducted on coals of different ranks in gold capsules, with the yield and stable carbon isotopic values of selected gas components and liquid hydrocarbons separately measured to investigate the hydrocarbon generation chemistry, isotopic compositions, and kinetic parameters in coal. With increasing heating temperature, the methane concentrations and total gaseous hydrocarbons increase consistently with temperature. The liquid hydrocarbon concentrations first increase to maximum values and then decrease with temperature. Coals of higher ranks are more likely to primarily produce methane, while coal with low maturity is more likely to produce wet gases. The compositional differences of the four coals become similar at the late stage of the pyrolysis experiment, with a methane content >95%. All of the experiments show a similar isotope trend δ13C1 < δ13C2 < δ13C3 at the same temperature. The δ13C values of ethane and propane became less negative with increasing temperature for all four experiments. The gap between δ13C1 and δ13C2 is greater than that between δ13C2 and δ13C3. There is a linear relationship between ln(C1/C2) versus ln(C2/C3). When ln(C2/C3) increases sharply as ln(C1/C2) increases, the decomposition rates exceed the generation rates for C4 +5, C3, and C2 as secondary oil cracking occurs. At high temperatures, both parameters are larger. The activation energy distribution of 48-72 kcal/mol for Yilan coal is slightly higher than 55-66 kcal/mol for Xinjing coal, but significantly higher than the distribution of 46-61 kcal/mol for Qinan coal and 51-67 kcal/mol for Shitai coal.