Effect of chemical structure on mechanical-seepage characteristics of briquette under different heating temperatures

Abstract

Based on the low strength and high permeability of briquette coal (BC) used in physical simulations of coal mine disasters, we propose a new briquette production method that can improve the mechanical strength and reduce the permeability of BC. The reduction of physical simulation experiments of coal and gas outburst was improved, which is of significance for the prevention and control of coal mine gas disasters. Based on this method, we developed a set of briquette hot-pressing systems and optimized the proportion of coal powder used in the briquette formation process. Mechanical property and seepage tests of the briquettes were carried out under uniaxial and triaxial compression. The effects of different temperatures on the physical (size, density, ignition loss rate) and mechanical (Poisson's ratio, elastic modulus and compressive strength) coal parameters and the effect of the permeability were analyzed using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy analyses. The morphology and pore, crystallite, and molecular structures were used to determine the cause of the increase in the mechanical strength of BC. Based on the packing density theory, the optimal ratio of the 40–60, 100–120, and 180–200 mesh particle sizes is 91.77:5.87:2.35. With the increase in the temperature from 150 to 450 °C, the microstructure changes from loose to compact, the number of pores and cracks increase, the degree of order of the microcrystalline structure gradually increases, the distance between the microcrystalline layers d002 continuously decreases, aliphatic chains shorten, and the aromaticity increases. The rate of the change of the loss rate and density of the hot-pressed briquette first increase, then decrease, and then increase again. The mechanical parameters (strength and elastic modulus) first increase and then decrease, whereas the Poisson's ratio first decreases and then increases, and the permeability increases. Considering the density, strength, and permeability of hot-pressed briquettes, the optimal heating temperature at which the similarity between the hot-pressed BC and raw coal is the highest at 300 °C.