A new insight into the role of coal adsorbed water in low-temperature oxidation: Enhanced·OH radical generation

Low temperature oxidation is the primary source of heat triggering coal spontaneous combustion. Experimental studies have found that adsorbed H₂O in coal structure can catalyze low-temperature oxidation. However, the detailed reaction pathways of adsorbed H₂O participated low-temperature oxidation process are still unclear. In this work, we for the first time elucidate the catalytic role of adsorbed H₂O in coal oxidation process at the molecular level by performing quantum chemistry calculations assisted with differential scanning calorimetry (DSC) experiment validations. Based on computations using composite theoretical method of CBS-QB3 and kinetic analyses, we found that adsorbed H₂O can increase the formation of ·OH radicals, which are critical active species for inducing coal low-temperature oxidation. On the one hand, adsorbed H₂O molecules are able to increase the formation of aliphatic hydrocarbon radicals, providing more active sites for ·OH radical generation. On the other hand, adsorbed H₂O can accelerate the formation of hydroperoxides by playing the “mediator” role in the H transfer from aliphatic structures to peroxyl radicals, providing more “precursors” for ·OH radical generation. DSC experiments using model coal samples with gradient moisture contents and different H₂O forms further demonstrate the catalytic role of adsorbed H₂O in coal low-temperature oxidation. Results from this study reveal detailed catalytic pathways of adsorbed H₂O in ·OH radical formation, providing a more comprehensive understanding on coal low-temperature oxidation theory.