Multi-stage indicator gas selection and early-warning method for coal spontaneous combustion

Spontaneous combustion of residual coal in gob areas is a significant hazard that impacts the safety of mine operations. This study examines stage oxidation characteristics of coal and proposes dynamic warning methods for coal spontaneous combustion. (1) The rate of change in the yield of characteristic gases (CO, C 2 H 4 ) during heating process of coal samples with varying oxygen concentrations was analyzed. The findings suggest that oxygen concentration does not significantly impact the abrupt-change behavior of these signature gases. Four characteristic temperatures of coal spontaneous combustion were identified, and the low-temperature oxidation process of coal was categorized into five stages: self-heating, critical, pyrolysis, cracking, and fission. (2) The mechanism of coal–oxygen composite reaction aided in quantitative optimization of the coal spontaneous combustion early-warning indicators. The Graham fire coefficient R 2 and the initial appearance of C 2 H 4 and C 2 H 6 gases were identified as the main early-warning indicators. In the early-warning method, indicators such as CO, C 2 H 4 , C 2 H 6 gases, R 1 , R 3 , R 4 , and R 6 were used as auxiliary warning indicators. (3) The staged characteristics of coal spontaneous combustion were used to establish a quantitative graded early-warning system and the corresponding critical thresholds were determined. R 2 is the primary indicator across all stages; C 2 H 4 and C 2 H 6 are primary indicators for the critical stage; CO serves as an auxiliary warning indicator throughout; R 3 is applicable for early warning in the critical, pyrolysis, cracking, and fission stages; and R 1 , R 4 , and R 6 function as auxiliary indicators in the cracking and fission stages. Variation law of fire coefficient R 2 with coal temperature With increasing coal temperature, R 2 shows a monotonic increasing trend and the characteristics of phased exponential growth. The critical temperature of coal sample is the dividing point of its phased characteristics. The growth law of R 2 is basically the same under the varying oxygen concentrations. Under these conditions of different oxygen concentrations, the gas index R 2 undergoes certain changes, but does not show certain regularity with the oxygen concentration. Therefore, when using R 2 index to predict coal spontaneous combustion, the influence of oxygen concentration can be treated as an error term.