Unveiling the transition phase in coal low-temperature oxidation via surface potential monitoring

Traditional temperature and gas monitoring techniques struggle to pinpoint the transition phase during coal low-temperature oxidation due to dynamic condition disturbance. Coal's low-temperature oxidation process is tied to the evolution of functional groups, whose different electronegativities and charge transfer properties alter the coal's surface potential. Hence, tracing surface potential changes can reveal the progression of coal low-temperature oxidation. This study investigates surface potential behavior during low-temperature oxidation experiments on lignite, bituminous, and anthracite, alongside gas product and microstructure analyses using gas chromatography and in situ FTIR. Multifractal analysis of surface potential signals shows that broader Δα anomaly regions and earlier Δfm extreme points indicate stronger coal spontaneous combustion tendencies. Surface potential oscillations and abrupt multifractal spectrum width changes act as early precursors to the low-temperature oxidation's transition stage. Additionally, the transition phase obtained through surface potential occurs earlier by 1.9 °C, 2.7 °C, and 2.9 °C than that detected by index gases. Mechanistic investigations confirm a strong correlation between total functional group electronegativity and surface potential, though the relationship varies among different functional groups. Grey correlation analysis indicates that C=O, the primary CO source, minimally affects surface potential, whereas C=C and free hydroxyl groups significantly influence electronegativity and surface potential at lower temperatures. SEM analysis identifies micro-crack propagation and electron escape as drivers of surface potential oscillations. These findings establish surface potential as a feasible early-warning indicator for detecting the transition phase of coal low-temperature oxidation.