Adsorption rate characteristics of methane and CO2 in coal samples from Raniganj and Jharia coalfields of India

A set of six sub-bituminous to high-volatile bituminous coal samples was chosen to analyze the adsorption rate during the isotherm tests. The isotherm tests were conducted at 30 °C on dry, powdered coal samples up to a maximum experimental pressure of ~ 8 MPa and ~ 6.5 MPa for methane and CO2, respectively. It was observed that the rate of adsorption for CO2 is higher than that for methane for the same experimental pressure–temperature condition and, for a particular pressure step, the equilibrium was reached earlier for CO2 compared to methane. It was also observed that at an increased pressure range, the rates of adsorption and the diffusion were generally decreased. The experimental data were fitted to unipore diffusion model but this model failed to predict the experimental adsorption kinetics data for the entire time range. However, the modification of unipore model, as suggested by Mianowski and Marecka (2009), was able to satisfactorily predict the experimental kinetics data at various pressure stages for the entire time range with the introduction of a kinetic parameter, C, accounting for the effect of gas diffusivity. The adsorption kinetics data was “unusable” at the initial time range due to variation in temperature–pressure in the sample cell after gas was injected. It was also observed that this “unusable” data is small at lower pressure range (~ 15 s), but could be significant (~ 30–50 s) at higher-pressure range.

► Rate of adsorption for CO2 is higher than methane at the same temperature and pressure.
► Pressure has a negative impact on the rate of adsorption and gas diffusivity.
► Linear approximation of the unipore model can fit the tested data for total time.