Thermal effect on bearing behavior and instability of coal-backfill composites

Abstract

The coal pillars’ solidity and continuous operation of gasifiers can be pointedly enhanced by employing backfill technology for underground coal gasification. To investigate the load-coupling features and instability mechanisms of coal pillars and backfill materials exposed to thermal environments, the strength features of coal-backfill composites were tested across 11 temperature groups, and their thermal damage mechanisms were analyzed using computed tomography (CT) and scan electron microscopy (SEM). A constitutive model for coal-backfill composites was also developed. The findings reveal that the mechanical behavior of coal-backfill composites under uniaxial compression presents a double-peak occurrence at different temperatures. The first peak strength is associated with the backfill body, while the secondary is linked to the coal body. Both strengths decrease progressively as temperatures rise. After 1000 ℃ treatments, the coal-backfill composite can still maintain at 5.49 MPa. Besides, high temperatures significantly lessen composite's energy storage capacity, resulting in lower energy during failure. It decreased by 93.48 % at 1000 ℃. At extreme temperatures, the backfill body, which has superior temperature resistance, primarily bears the load, and destruction occurs first. This occurs because the porosity growth in backfill is too much lesser than in coal under high-temperature conditions. Beyond 500 °C, pyrolysis reactions within the coal lead to extensive structural damage and growing porosity, the maximum increase was 661.07 %. The thermal damage constitutive model demonstrates that these stresses are proportional to the elastic modulus ratio between the backfill and coal body.