Methane adsorption thermodynamics of coal sample subjected to liquid nitrogen freezing–thawing process

Abstract In order to explore the characteristics and thermodynamic parameters of methane (CH4) adsorption in coal sample subjected to liquid nitrogen (LN2) freezing–thawing. The adsorption experiments of coal sample unfrozen, 5 cycles frozen, and 10 cycles frozen were conducted with the temperature range of 313–353 K at 10 K intervals. The influence of freezing–thawing on the adsorption amounts and thermodynamic parameters of CH4 adsorbed by coal sample were quantitatively compared. Electron microscope scanning (SEM) were carried out to analyse the pores structure distribution on the coal sample surface. The changes of pore structure on the coal sample surface under different freezing–thawing cycles were counted. The results showed that, the adsorption amounts of CH4 were positively correlated with the cycles of freezing–thawing. The standard adsorption enthalpy Hθ, the standard adsorption Gibbs free energy Gθ, and the standard adsorption entropy Sθ, isosteric heat of adsorption, and mean isosteric heat of adsorption all increased with the cycles of freezing–thawing. The freezing–thawing of LN2 was observed to saliently enhance the spontaneity and heat-transfer effect of adsorption process. The freezing–thawing of LN2 can effectively change the pores structure of the coal sample surface, and pores size presented a normal distribution. The study of adsorption characteristics and pore changes can provide reference for the technology development with LN2 cracked coal sample for enhancing coal bed methane (CBM) drainage.