Characteristics and multiscale study of deep coal spontaneous combustion under thermal-fluid-solid coupling

This study elucidates the evolution mechanisms of spontaneous combustion in deep coal bodies under thermal-fluid-solid coupling through macroscopic programmed temperature-rise experiments using a coal spontaneous combustion temperature-programmed heating system, mesoscale triaxial compression-seepage tests using a triaxial seepage test apparatus, and microscopic electron spin resonance (ESR) spectroscopy. As temperature rose from 60 °C to 210 °C, oxygen consumption and CO/CO 2 /C 2 H 4 production increased continuously. Dry coal oxygen consumption first decreased, then increased with stress; 8 % moisture coal continued to rise with stress; 15 % moisture coal exhibited different response characteristics around 140 °C. Triaxial tests showed that confining pressure from 5 MPa to 15 MPa increased elastic modulus from 1980 MPa to 2493 MPa and peak strength from 32.8 MPa to 41.7 MPa; moisture from 0 % to 15 % reduced peak strength to 21.3 – 26.7 MPa. Permeability decreased sharply at 0.3 – 0.9 MPa CO 2 and stabilized beyond. ESR revealed that 8 % moisture coal reached 8.23 × 10 16 g −1 at 210 °C, exceeding 6.27 × 10 16 g −1 in dry coal. The results confirm that deep coal spontaneous combustion arises from synergistic effects of “stress-enhanced reactions, delayed moisture release, and oxygen supply constrained by seepage,” refining the mechanism and providing new insights for prevention and control.