Effect of holding time on microstructure of secondary carbonized briquette and strengthening of mechanical-seepage characteristics

Physical simulation test is an effective method of occurrence mechanism and prevention and control of coal mine gas disasters. At present, the mechanical strength and permeability of briquette materials used in coal mine gas disasters physical simulation test are very different from that of raw coal. How improve the compressive strength and permeability of the briquette coal（BC） is the key problem in the briquette forming process. Based on this, several secondary carbonized briquettes with different holding time were made by a hot pressing method in this paper, and SEM, NMR, XRD, FTIR, MTS-815 rock mechanics test system and gas-containing coal thermal-fluid-solid coupling triaxial servo seepage test device were used to test and analyze secondary carbonized briquettes with different holding times. The effects of different holding times on microcrystalline structural parameters, basic structural units of aromatic hydrocarbons, alkyl side chains and various functional groups, surface morphology, T₂ spectral morphology, and porosity evolution were analyzed, the mechanical and seepage characteristics of secondary carbonized briquet under different holding times were clarified. and the optimal holding time was determined. The results show that the surface roughness and pore diameter of the secondary carbonized briquette gradually increase with the increases of holding time, and cracks appear on the surface of BC after increasing to 6.7 h. The longer holding time, the more obvious cracks are. BC cumulative porosity and mesoporous porosity increased gradually, while microporous porosity decreased gradually. The aromatic layer spacing (d₀₀₂) decreased first and then increased, while the microcrystalline diameter (L_a) and microcrystalline height (L_c) increased first and then decreased. The length of fatty chain decreased, and the degree of condensation of aromatic ring (A_ar/A_al) increased first and then decreased. The uniaxial compressive strength increases first and then decreases, and the permeability decreases first and then increases. When the holding time is 5.3 h, the optimal holding time is obtained. In this case, the uniaxial compressive strength and elastic modulus of hot pressing BC are the largest, Poisson's ratio and permeability are the smallest, and its mechanical strength, permeability and density are 9.85 MPa, 1.49 × 10⁻¹⁵ m², and 1.127 g/cm³, respectively. This study has important practical guiding significance for improving the real reducibility of the occurrence mechanism and prevention and control basic test of coal mine gas disaster and effectively preventing and controlling coal mine gas disaster accidents.