Gas diffusion mechanism in multi-scale pores of coal particles and new diffusion model of dynamic diffusion coefficient

In order to solve the problem that the classical diffusion model cannot accurately fit a full-time gas diffusion process in coal particle,the experiments of gas diffusion in coal particle were carried out under various conditions using typical coals in China. The results calculated by classical diffusion model show that the experimental values of diffusion ratio are greater than the theoretical value before a certain time,however,the former is less than the latter after that time. The classical diffusion model cannot precisely fit a full-time gas diffusion process and the error is large,and then the special phenomenon for diffusion coefficient decrease with the increase of time was found. The new physical model of multi-scale pore distribution in coal was put forward. Based on new model,the authors assumed that the pore distri- bution within coal is heterogeneous and multi-scale,and has self-similar fractal structure,which causes the multistage distribution of diffusion coefficient within coal and determines the macroscopic diffusion mechanism of gas. From the surface to the center of coal,the pore distributes from big to small and the diffusion coefficient decreases accordingly. Gas diffuses rapidly from the big pore with large diffusion coefficient in the early stage and slowly from the small pore with small diffusion coefficient in the late stage until the micro pore is finally influenced. The multilevel diffusion coef- ficient distribution caused by multi-size pore results in the diffusion mechanism of diffusion coefficient decease with the increase of time. According to the assumption,two parameters,initial diffusion coefficient ( D0 ) and decay coefficient (β),were introduced to reflect the decay characteristic of dynamic diffusion coefficient. The mathematic model of dy- namic diffusion coefficient was put forward. The new model,verified by 200 data,can fit the full-time process of gas diffusion in coal under various conditions. Meanwhile,the new model covers the classical unipore diffusion model and bidisperse one and generalizes the latter ones. The new model can explain the problems caused by classical one,and in terms of accuracy,simplicity,explanatory and predictability,the new model is better than the bi-disperse model and empirical equations. The new model provides a new algorithm to determine coalbed methane content,reserves and out- burst predicting index and to explain the diffusion mechanism under various conditions.