Experimental study on the effect of inhibitor on coal spontaneous combustion under low temperature freezing

To investigate the impact of subzero freezing conditions on the effectiveness of inhibitors in preventing coal spontaneous combustion, coal samples containing a 15% mass fraction of MgCl2 and TPPI (Triphenyl Phosphite) were subjected to treatment at -30 degrees C. Through temperature-programmed tests, TG-DSC tests, and infrared spectrum tests, it was observed that, at 120 degrees C, the frozen coal sample subject to inhibition exhibited a notable rise in gas production, accompanied by a decrease in the rate of CO inhibition. The activation energy required for complexation was reduced in the case of frozen MgCl2 (CMF) by 5.508 kJ/mol compared to normal temperature MgCl2 (CMN). Similarly, the activation energy for frozen TPPI (CTF) was reduced by 0.216 kJ/mol compared to normal temperature TPPI (CTN). The entropy weight-RSR analysis method is used to calculate the RSR value of the combustion characteristic index. The RSR ranking is CMN > CMF > CTN > CTF > C (raw coal), with CMN having the highest grade level, and the inhibitory effect weakening after freezing. The maximum total heat release was observed in the untreated C. Furthermore, the total heat release of coal samples inhibited at normal temperatures was less than that of their frozen counterparts, indicating a higher susceptibility to spontaneous ignition in the latter. Subsequent to freezing, there was an augmentation in the content of aliphatic active groups (-CH3 and -CH2) on the coal surface. Concurrently, the micropore size of coal samples increases, gradually converting them into mesopores and macropores. Water absorption was enhanced, and the content of some hydrophilic functional groups (-OH and -COOH) also increased. Research indicates that freezing notably impacted the C=O proportion in MgCl2-treated coal samples and had a greater effect on the proportion of -OH in TPPI coal-inhibited samples. The results suggest that the inhibitory effect of utilizing an inhibitor at subzero temperatures on coal spontaneous combustion is attenuated. Moreover, the inhibitory effect was more pronounced with MgCl2 compared to TPPI.