Hydrocarbon Generation Characteristics and Kinetic Analysis of Vitrinite of Various Coal Ranks During Thermal Cracking in Two Thermal Simulation Systems

Five vitrinite samples of various coal ranks from the Shanxi Formation, North China plate, were pyrolyzed using a Rock-Eval6 pyrolyzer and an improved micro-scale sealed vessel pyrolysis instrument to investigate the characteristics and kinetic behaviors of their pyrolysates. Cumulative formation of methane and C_1–5 continuously increased as the pyrolysis temperature was increased. In contrast, the C_2–5/C_1–5 ratio is larger at a heating rate of 10 K/h heating than 30 K/h at the same temperature before the heavy gaseous components peak. Hydrocarbon generation from the vitrinite of various coal ranks that gaseous generation yield initially increases first and then decreases with increasing rank, with the maximum yield at Ro, max 2%. The results indicate that the activation energy of M1 ranges from 254.98 to 317.68 kJ/mol, with a peak of 3 kJ/mol in a diagram of activation energy distribution if its frequency factor of 7.890 × 10¹⁷/s is assumed. The activation energy of M2 ranges from 254.98 to 338.58 kJ/mol, with a peak of 313.50 kJ/mol in the diagram. The activation energy of M5 ranges from 213.18 to 246.62 kJ/mol, with a peak of 234.08 kJ/mol in the diagram. The activation energy of the total hydrocarbons of M4 in an open system is 217.36 kJ/mol. The light hydrocarbon (C_6–14) content of M1, M2, M3, M4, and M5 are, respectively, 4, 110, 111, 104, and 38 mg/g·TOC. The temperatures corresponding to peak values of heavy gaseous hydrocarbon generation vary with coal rank and increase with an increase in coal rank as hydrocarbon generation and heavy hydrocarbon cracking in high-rank coals requires more energy.