Gas desorption-diffusion behavior from coal particles with consideration of quasi-steady and unsteady crossflow mechanisms based on dual media concept model: Experiments and numerical modelling

Abstract Gas diffusion behaviour plays a crucial role in coal and gas outburst and gas drainage. Unipore model and bidisperse model are extensively employed to describe the gas diffusion process. However, the unipore model does not characterize the entire gas diffusion process for the assumption of homogeneous spheres. Although the bidisperse model can mathematically fit the experimental data well, the assumptions of linear adsorption and independence of micropores and macropores are not always true for most of the coal. In the current study, a dual media conceptual model was proposed consisting of macropores and matrix and two diffusion models were developed with consideration of quasi-steady crossflow (model I) and unsteady crossflow (model II) mechanisms. The two models were used to model the experimental data measured in this study. Results show that the model II gave a good fitting for experimental data, which indicated that the unsteady diffusion mechanism is appropriate to describe gas transport from matrix to macropores in desorption diffusion process. Gas transport resistance in coal particle mainly comes from macropores but not the matrix, and the macropores volume distribution displays a key role in controlling gas emission. The unipore model was only equal to the gas transport in matrix of model II, therefore it gave a poor fit to experimental measured data. In addition, the diffusion models developed in this study can also be used to model other variable boundary condition experiments, such as dynamic adsorption, provided change boundary condition is in line with the experimental condition. This is a preliminary study that inspires and enlightens researchers about gas diffusion in coal.