CH4 and CO2 sorption and diffusion carried out in various temperatures on hard coal samples of various degrees of coalification

Abstract

The aim of the study was to analyse the effect of temperature on the sorption and diffusion of CO2 and CH4 on hard coal. Four coal samples, characterized by various degrees of coalification, were used in the research. The sorption measurements were carried out in three temperature values: 278 K, 313 K and 353 K and for the consecutive pressure values of 1 bar, 5 bar and 15 bar. Langmuir's sorption capacity in relation to CH4 and CO2 was the highest for the coal with the lowest degree of coalification and was 32.15 cm3/g and 76.4 cm3/g respectively. The lowest Langmuir sorption capacity in relation to CH4 and CO2 were obtained for medium-rank coal (19.23 cm3/g and 29.26 cm3/g) and high-rank coal (20.33 cm3/g and 32.89 cm3/g). The temperature growth resulted in a reduction of the sorption capacity of coal in relation to CH4 and CO2, and that decline was exponential in relation to temperature. The absolute decreases in the total sorption capacity were from 0.03 cm3/g to 0.21 cm3/g in relation to CH4, and from 0.10 cm3/g to 0.51 cm3/g in relation to CO2, per 1 K temperature increase. Within the measurement temperature range of 278–353 K, no „critical temperature point”, above which there is no reduction in CH4 and CO2 sorption, was observed. An increase in the temperature pushed up the values of the effective diffusion coefficients, in relation to CH4 by more than one order of magnitude, and in relation to CO2 by approximately four times. A decrease in the selectivity of CO2/CH4 was also observed along with the coal rank. The ratio of effective diffusion coefficient values DeCO2/DeCH4 for all the investigated coal samples declined strongly along with temperature growth.