A chelated calcium-procyanidine-attapulgite composite inhibitor for the suppression of coal oxidation

•Pioneering study on slowing down coal oxidation by chelated calcium-procyanidine-attapulgite composite inhibitor.•The radical scavenging of the inhibitor was investigated.•The mechanism of chelated composite inhibitor in slowing down coal oxidation was analyzed. In this study, the impact of chelated calcium-procyanidine-attapulgite composite inhibitor on coal oxidation process is studied. The hydroxyl and superoxide anion radical scavenging activity for procyanidine-calcium chloride complexes was measured, and the result show that the complex with ligand-to-calcium ion ratio 1:5 has the highest radical clearing activity. Due to the water adsorption quality of CaCl2, the thermal analysis of samples with/without procyanidine-calcium chloride complexes indicated that the complex can slow down the coal oxidation well and the best ratio of procyanidine and CaCl2 should be determined at 1:7. Then the water retension capacity of procyanidine, CaCl2, ATP and composite inhibitors were compared, and the result indicated that ATP can improve the water retention of the composite inhibitor, so ATP as introduced into the composite inhibitor, and the optimal ratio of chelated inhibitor and ATP was determined as 1:5 by TG-DSC curves. Therefore, a chelated calcium-procyanidine-attapulgite composite inhibitor was chosen as an additive to suppress coal oxidation. The measurements of CPT, oxygen consumption during low temperature oxidation and CO emissions all demonstrated that the addition of the composite inhibitor to coal samples has an obvious suppression effect on the oxidation process. The in situ FTIR spectra of coal samples during the oxidation process show that the coal oxidation slows down due to the addition of chelated calcium-procyanidine-attapulgite composite inhibitor by eliminating free radicals and keeping the coal hydrated.