Determination of diffusion coefficient and convective heat transfer coefficient for non-isothermal desorption-diffusion of gas from coal particles

Abstract

Experiments investigating gas adsorption/desorption-diffusion in coal particles typically involve putting coal particles into a sample cell in a water bath to ensure a constant-temperature environment. Numerical simulations of such experiments have also been performed under isothermal condition. However, the coal particle temperature is not constant, and the influence of temperature variation on the diffusion coefficient back-calculation is often neglected. In this study, CO2 desorption-diffusion experiments are conducted with coal particles first, using the abovementioned water bath technique, and the coal particle temperature is monitored. Then, a coupled thermal-gas model for CO2 adsorption/desorption-diffusion in coal particle is proposed, in which the interaction between gas adsorption/desorption and coal temperature variation is considered. Finally, numerical simulations of the CO2 desorption-diffusion experiments are described, and the applied thermal boundary conditions are validated. The impacts of temperature variation on the desorbed-diffused gas amount and the diffusion coefficient back-calculation are also evaluated. The experimental results show that the coal particle temperature first decreases and then increases. Thus, setting an isothermal boundary condition for the particle surface will not lead to an accurate simulation of the temperature variation, which will ultimately also affect the accuracy of the diffusion coefficient back-calculation. Instead, adopting a convective heat boundary condition for the particle surface can better reproduce the evolution of desorbed-diffused gas and coal temperature changes, thereby improving the accuracy of the diffusion coefficient back-calculation. The results also indicate that the magnitude of coal diffusion coefficient and the size of coal particle both have significant effects on the temperature variation and the subsequent desorption-diffusion of adsorbed gas.