Study on the Adsorption Characteristics of Tectonic Coal under Infrared Radiation Heating

Abstract The fundamental mechanistic question addressed in this study is how infrared radiation heating (IRH) modulates the methane adsorption capacity of tectonic coal – a critical yet poorly understood issue for thermal-enhanced coalbed methane (CBM) recovery. Unlike conventional conductive or convective heating, IRH delivers noncontact electromagnetic energy that directly excites molecular vibrations and rotations within the coal matrix, generating volumetric heating with higher thermal efficiency and no secondary contamination. To elucidate this mechanism, we adopted an integrated methodology combining high-pressure isothermal adsorption experiments under controlled IRH power levels, theoretical derivation of the Langmuir adsorption equilibrium constant b based on heat transfer and adsorption kinetics, thermodynamic calculation of key parameters (ΔG0, ΔH0, ΔS0, and equivalent heat of adsorption qst), and quantitative microstructural analysis via SEM coupled with ImageJ pore statistics. Our key findings are fourfold: (1) Both the saturation adsorption capacity a and the adsorption equilibrium constant b decrease with increasing IRH power, with a following a sharp initial decline that plateaus at higher power – a trend quantitatively captured by our theoretically derived b(T) model; (2) Thermodynamic analysis confirms that methane adsorption on coal is spontaneous physical adsorption (ΔG0 < 0, ΔH0 ≈ −30 kJ/mol), and IRH weakens the adsorption affinity by raising the system energy state; (3) The equivalent heat of adsorption qst is positively correlated with uptake, demonstrating that IRH-driven desorption is an endothermic process that requires energy input to overcome adsorption barriers; (4) Microstructurally, IRH reduces micropore abundance from 45–51% to 34–43% while increasing small- and mesopore fractions, indicating that pore enlargement and interconnectivity – rather than mere temperature rise – underlie the reduced specific surface area and adsorption site availability. Collectively, these results establish that IRH enhances CBM recovery through a dual mechanism: direct weakening of adsorption thermodynamic affinity and concurrent improvement of pore transport pathways. This work provides both theoretical insights and experimental validation for the rational design of IRH-based stimulation strategies in low-permeability coal seams.