Carbon dioxide sorption and diffusion in coals: Experimental investigation and modeling

CO2 sorption and diffusion in coal are closely related to the occurrence of coal and gas outburst, geological sequestration of CO2 in coalbeds, and enhancing coalbed methane recovery by injecting CO2. Hence, it is significant to investigate the sorption properties and diffusion models of CO2 in coal. Here we used a newly designed experimental apparatus at Peking University to investigate the sorption and diffusion properties of CO2 in natural coal samples from Dashucun Mine and Wutongzhuang Mine in Handan city, Hebei province, and Jinhuagong Mine in Datong city, Shanxi province, and obtained CO2 sorption isotherms and diffusivity models. The results indicate that, in a certain pressure range, CO2 sorption isotherms for the coal samples are consistent with the Langmuir model, which assumes that monolayer sorption occurs at the interface between coal matrix and CO2 molecules, and the sorption isotherms feature nonstandard hyperbolas in mathematics. At the same pressure and temperature, as the vitrinite content increases, coal adsorbs more CO2 molecules. The relation between the sorption capacity and the coal rank may be described as a U-type trend, and medium rank coal has the least sorption capacity. The bulk diffusivity of CO2 in coal is not constant; in the range of CO2 mass fraction greater than 1%, it increases roughly linearly with increasing mass fraction of CO2 adsorbed (or CO2 partial pressure) in coal. CO2 diffusivity in coal is approximately 10(-4) to 10(-2) mm(2)/s in magnitudes, and the diffusivity ranges in coal samples are 3x10(-4) to 8x10(-3) mm(2)/s from Dashucun Mine, 2x10(-4) to 4x10(-3) mm(2)/s from Wutongzhuang Mine, and 2x10(-4) to 4x10(-3) mm(2)/s from Jinhuagong Mine. The results of the CO2 sorption and diffusion study can be applied to help predict and prevent coal and gas outburst as well as to evaluate the feasibility in geological sequestration of CO2 and to enhance coalbed methane recovery.