Upper thermal maturity limit for gas generation from humic coal

• H/C atomic ratio of coal with increasing maturity presents episodic variation.
• Methyl could still be detected in the coal sample with a maturity of 4.46%Ro.
• 4.37 ml/g TOC gas is generated by the coal sample with a maturity of 5.32%Ro.
• The upper thermal maturity limit of gas generation from coal is about 5.0%Ro.

In this study, elemental compositions and chemical structure by solid-state 13C Nuclear Magnetic Resonance (13C NMR) of 19 coal samples with vitrinite reflectance (Ro) ranging from 0.35%Ro to 5.32%Ro were analyzed and measured. Pyrolysis experiments of six samples selected from the 19 coal samples were conducted to unravel the process and determine the upper thermal maturity limit for gas generation from humic coal. The variations of H/C atomic ratio of coal with increasing maturity appear to evolve through three stages which include a drastic decreasing phase before 2.0%Ro, a moderate decreasing phase from 2.0 to 6.0%Ro and a gradual decreasing phase above 6.0%Ro. The measurement results concerning the chemical structure of coal samples with increasing maturity indicate that most of the oxygen containing functional groups released was below 0.8%Ro. Aliphatic groups (especially methyl) were detected in samples with a maximum maturity of 4.46%Ro. The pyrolysis experiments for coal with different maturities using a gold tube system indicate that the main maturity intervals for CO2 and hydrocarbon gas generation from coal are below 0.8%Ro and 2.0%Ro, respectively. The pyrolysis experiment proved that only 4.37 ml/g TOC of hydrocarbon gas could be generated from the sample with a maturity of 5.32%Ro. Hence, the upper thermal maturity limit for gas generation from coal was determined to be about 5.0%Ro according to the variations of elemental composition, the chemical structure evolution and pyrolysis experiments for coal with different maturities.