Effect of temperature on the evolution and distribution for particle size of loose broken coal during the uniaxial confined compression process

Abstract

Explorations into the breakage properties of coal-rock mass in goaf is significant for stratum movement control, solid waste backfilling, and thermal-dynamic disaster prevention. In this work, a self-developed multi-field coupling experimental system for broken coal was adopted to probe both the compaction behaviors and breakage characteristics of loose coal samples as temperature increased between 30 °C and 120 °C at an increment of 10 °C, the results were compared the with those obtained under different stresses at room temperature (30 °C). Results indicate that the particle gradation tended to be reasonable gradually. Besides, for a given stress, the particle breakage rate presented a slow increasing trend initially while accelerated later under different temperatures; whereas, for the given temperature, it exhibited an opposite trend vary with the increasing stress load. For the given stress, the changing amplitude for the mass of the raw coal sample with various sizes were respectively 4.64%, 4.94%, and 7.41% under the temperature intervals of 30–60 °C, 60–90 °C, and 90–120 °C; meanwhile, for the given temperature, the changing amplitude for the mass fraction of raw coal particles were respectively 14.61%, 8.67%, and 5.9% under the stress intervals of 3–6 MPa, 6–9 MPa, and 9–12 MPa. In addition, stress played a dominant part in the breakage for coal particles, and the increase in coal temperature strengthened the stress-dominated breakage to a certain extent. Moreover, the breakage degree of coal particles under the stress-thermal synergy conditions was larger than that under pure stress conditions, and the Weibull distribution was appropriate for describing the breakage features of crushed particles in the initial stage while fractal distribution was applicable for that in the later stage. With the assistance of the scanning electron microscope (SEM), the mechanism of coal particle breakage induced by thermal stress was theoretically analyzed. Results demonstrated that with the increasing temperature, the size of cracks on coal particle surface increased while the number of cracks decreased instead. Also, the superimposition effect of the confinement stress and thermal stress primarily contributed to the stress distribution of the crushed coal particles.