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

Abstract

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.