Diffusion distance variations in coal pulverization based on equivalent matrix size: Implications for coal and gas outburst indicators

Abstract

Coal matrix is rich in complex pores, constituting channels for gas diffusion. Diffusion distance plays a crucial role in diffusion resistance, directly influencing the intermediate processes of “desorption-diffusion-seepage” in coal. However, current research often quantifies diffusion distance using average particle size, failing to represent the true diffusion distance. In this study, we introduce a novel concept of equivalent matrix size based on gas desorption experiments. Diffusion coefficients were obtained by fitting the time-varying diffusion model, and the mass exchange of interporosity flow was estimated. Additionally, temporal matrix shape factors were analyzed to determine the matrix size with the aid of numerical simulation. Results demonstrate a temporal evolution in matrix shape factors, namely a decrease in the initial stage, stability in the medium stage, and inconsistency in the later stage. During pulverization, fracture and matrix damage occur simultaneously, with fractures undergoing more severe fragmentation. Lost time significantly impacts the accuracy of accessing lost gas amount ML and outburst indicator K1. Upon incorporating the matrix size, ML for XT and QD samples were overestimated by 0.83 cm3/g and 0.14 cm3/g, and K1 was overestimated by 21.2 % and 2.7 %, which could help establish a theoretical foundation for predicting coal and gas outbursts.