The influence of methane and CO2 adsorption on the functional groups of coals: Insights from a Fourier transform infrared investigation

Understanding the mechanism of CO2 and CH4 storage in coals is critical for CO2 sequestration in coal seams with enhanced coalbed methane recovery (CO2-ECBM) and the prevention of mine gas disasters. The primary objective of this study is to investigate the potential chemical interactions of CO2 and CH4 with coal due to high-pressure adsorption. The functional groups of two different-rank coals before and after high-pressure CH4 and CO2 adsorption treatments were studied using Fourier transform infrared spectroscopy (FTIR) spectrometry and curve fitting. The changes in the chemical structure of coal were evaluated using structural parameters derived from FTIR. The coal adsorption treatments were conducted at 40 °C and 6 MPa. The results indicate that both coal rank samples underwent functional group and structural parameter changes after high-pressure CO2 and supercritical CH4 adsorption and thus that chemical interactions occur at high pressure. Moreover, the variation law of functional groups is complex due to its dependence on the coal rank and gas type. FTIR spectra of long-flame coal and anthracite coal have clear functional group distribution differences. The amount of oxygen-containing functional groups decreases with the increase in coal rank. This study demonstrates the utility of FTIR spectrometry and curve fitting to quantitatively evaluate the chemical structure of coal samples. This preliminary study is intended to enrich the theories of gas adsorption in coals, guide applications of CO2-ECBM, and assist in the prevention of mine gas disasters.