51 / 2023-05-15 20:36:14

Theoretical model and experimental verification of free gas diffusion in coal particle micropores

Coal particles,Gas diffusion theory,Free gas density gradient,Modeling,Experimental verification

Mastering the law of gas diffusion and transport in coal pores is the key theoretical basis for coal seam gas exploitation and dynamic mine gas disaster prevention. At present, a large number of coal particle gas adsorption/desorption experiments have proved that the classical Fick diffusion theory model has a large deviation from the experimental data on the whole time scale. Although the Fick model of dynamic diffusion coefficient emerged, Darcy model and anomalous diffusion model later can correct the above deviation to some extent, they all have their own defects and cannot explain the essential reasons of gas diffusion in coal particle well. The Fick model maintains that the gas diffusion process in the coal matrix is driven by the gas concentration/content gradient, while the Darcy model holds that the gas pressure gradient dominates the gas flow in the coal matrix. Whether it is gas seepage or diffusion in coal seam, its essence is mass transfer, and density, as a universal physical quantity, can be associated with concentration and pressure. Based on the above situation, this work analyzes the difference between free gas and adsorbed gas in coal particles on gas diffusion, puts forward the hypothesis that the mass gas flux is proportional to the density gradient of free gas, and establishes the corresponding new theoretical model of free gas diffusion in coal particles. Then, the finite difference numerical method is used to independently compile the program software under various conditions, and the accuracy and universality of the new diffusion model were investigated through the coal particle gas adsorption/desorption experiments under various environmental factors (different coal types, different pressure change processes, different gases, different particle sizes, and different temperatures). The simulation results and experimental data match well throughout the entire desorption time period, and the diffusion model proposed in this work can be used to describe the diffusion and migration behavior of gas in coal particles. It is interesting that the key flow coefficient (microchannel diffusion coefficient) in the proposed new diffusion model does not change with time, and the fluctuation of gas pressure is also small. The microchannel diffusion coefficient shows a trend of first decreasing and then increasing with the increase of coal rank/coal metamorphism degree. In summary, the microchannel diffusion coefficient is only related to the pore structure and gas properties of coal, and is not related to state parameters such as gas pressure and time. Finally, this work comprehensively discusses the scientificity and rationality of the new free gas diffusion model in coal particle compared to traditional Fick and Darcy models from the perspectives of theoretical basis, mathematical form, prediction accuracy, and sensitivity to key flow coefficients. The research content can enrich and improve the theory of gas diffusion and flow in coal, providing a theoretical basis for gas disaster prevention and gas resource utilization. The new model of free gas diffusion is to some extent an improved version of Fick model and Darcy model, which is more suitable to reveal the mechanism of gas diffusion and migration in coal matrix.