Study on the characteristics and mechanism of DL-malic acid in inhibiting spontaneous combustion of lignite and bituminous coal

•The effects of DL-malic acid solutions with different concentrations on CSC were studied.•The inhibitory effects of malic acid on coal samples with different water-soaking times were revealed.•The inhibition mechanism of DL-malic acid was discussed both macroscopically and microscopically. Inhibitors play an important role in preventing and controlling coal spontaneous combustion (CSC). In this study, the effect of DL-malic acid, an environment-friendly inhibitor, on the CSC characteristics of lignite and bituminous coal was studied. A contrastive analysis was conducted on the inhibitory effects of malic acid solution with different concentrations on CSC. Moreover, the inhibition characteristics of malic acid on coal samples with different soaking times were analyzed. In so doing, the inhibition mechanism of malic acid on CSC was revealed. It was found that malic acid can obviously inhibit CSC of lignite and bituminous coal. After the treatment of malic acid, the concentrations of CO released by coal samples during oxidation fall, and the crossing-point temperatures increase. Compared with the raw coal, the acid-treated coal corresponds to lower concentrations of free radicals and functional groups –CH2- and –CH3 and boasts better thermal stability before 150 °C. A contrastive analysis on the inhibitory effects on coal samples with different soaking times disclose that the water-soaked coal sample is inhibited better in CSC by malic acid than the unsoaked coal sample. The malic acid solution can effectively weaken the catalytic effect of metal ions in coal and inhibit the generation of free radicals, thus inhibiting CSC well.