Gas generation from coal: taking Jurassic coal in the Minhe Basin as an example

The gas generation features of coals at different maturities were studied by the anhydrous pyrolysis of Jurassic coal from the Minhe Basin in sealed gold tubes at 50 MPa. The gas component yields (C1, C2, C3, i-C4, n-C4, i-C5, n-C5, and CO2); the δ13C of C1, C2, C3, and CO2; and the mass of the liquid hydrocarbons (C6+) were measured. On the basis of these data, the stage changes of δ13C1, δ13C2, δ13C3, and δ13CO2 were calculated. The diagrams of δ13C1–δ13C2 vs ln (C1/C2) and δ13C2–δ13C1 vs δ13C3–δ13C2 were used to evaluate the gas generation features of the coal maturity stages. At the high maturity evolution stage (T > 527.6 °C at 2 °C/h), the stage change of δ13C1 and the CH4 yield are much higher than that of CO2, suggesting that high maturity coal could still generate methane. When T < 455 °C, CO2 is generated by breaking bonds between carbons and heteroatoms. The reaction between different sources of coke and water may be the reason for the complicated stage change in $$\delta^{{{13}}} {\text{C}}_{{{\text{CO}}_{{2}} }}$$ δ 13 C CO 2 when the temperature was higher than 455 °C. With increasing pyrolysis temperature, δ13C1–δ13C2 vs ln (C1/C2) has four evolution stages corresponding to the early stage of breaking bonds between carbon and hetero atoms, the later stage of breaking bonds between carbon and hetero atoms, the cracking of C6+ and coal demethylation, and the cracking of C2–5. The δ13C2–δ13C1 vs δ13C3–δ13C2 has three evolution stages corresponding to the breaking bonds between carbon and hetero atoms, demethylation and cracking of C6+, and cracking of C2–5.