Significance of particle size and lithotype on adsorption equilibrium in low pressure N2 and CO2 adsorption of coal: Implications for abnormal phenomenon of adsorption in N2 adsorption of semianthracite

Abstract

Low pressure N2 and CO2 adsorption was conducted on different lithotypes (vitrain and clarain) with ten different particle sizes ranging from 2.8-2.0 mm to 0.045–0.020 mm of semianthracite (Ro, max = 2.06%) having the abnormal phenomenon of adsorption in N2 adsorption. The effects of particle size and lithotype on adsorption equilibrium in N2/CO2 adsorption and associated pore structure were systematically analyzed to explore the possible reason for the abnormal phenomenon of adsorption and propose the suitable particle size for N2/CO2 adsorption. Decreasing particle size in vitrain and clarain shortens the gas flow paths and removes the constrictions, which facilitates the complete adsorption equilibrium in CO2 adsorption and improves the degree of adsorption equilibrium in N2 adsorption. For different lithotypes with the same particle size, clarain achieves complete adsorption equilibrium easier and quicker in CO2 adsorption and has higher degree of adsorption equilibrium in N2 adsorption than vitrain. The abnormal phenomenon of adsorption in N2 adsorption is resulted from the combined effect of extremely low degree of adsorption equilibrium and elastic deformation of coal. The abnormal phenomenon of adsorption is more pronounced in vitrain than in clarain and gradually disappears with decreasing particle size in both vitrain and clarain. Particle sizes of <0.075 mm (>200 mesh) and <1.0 mm (>18 mesh) are recommended as the suitable particle sizes in N2 adsorption and CO2 adsorption to accurately characterize pore structure for the coals having the abnormal phenomenon of adsorption, respectively.