Microstructure Characteristics of Soft and Hard Coal and Its Influence on Gas Adsorption Capacity

In order to better understand the microstructure characteristics of soft and hard coal and its influence on gas adsorption capacity, three groups of soft and hard coal samples with different maturities were collected for measurements from Ordos Basin. CH4 isothermal adsorption experiments were performed to analyze the gas adsorption characteristics. Pore distribution and oxygen-containing groups of these soft and hard coals were detected by low-pressure N2 adsorption and Fourier Transform Infrared Spectroscopy (FTIR), respectively. For the collected FTIR spectrum, the peak-fitting method is used to semi-quantitatively study the distribution of oxygen-containing groups. Comparison of microstructure characteristics and methane adsorption capacity has also been made between soft and hard coal samples. The results showed that soft coals has smaller pore diameter and the micropore and mesopore surface area is 43.2–75.1 and 12.2–41.1% greater than that of corresponding hard coal, respectively, but the macropore surface area is 18.9–66.7% less than that of hard coals. The aromaticity of hard bituminous coals is smaller than that of the corresponding soft coals, while the peak areas of carbonyl and hydroxyl groups are obviously larger than that of the corresponding soft bituminous coals. However, the distribution of oxygen-containing groups in soft and hard anthracite exhibits the opposite trend. Tectonic deformation contributes to the decline of oxygen-containing groups and can enhance the maturity and pore surface area of bituminous coals, exhibiting superior methane adsorption property compared with hard bituminous coals. Though soft anthracite has developed micropore/mesopore structures, its lower maturity and higher content of oxygen-containing groups lead to the decline of gas adsorption capacity. Gas adsorption behaviors of coal are closely related to the complex microstructures, and the combined effects of physical structures and chemical groups should be taken into account.