Influence of porosity on the reactivity of inertinite and vitrinite toward sodium hypochlorite: Implications for enhancing coal seam gas development

Abstract

Sodium Hypochlorite (NaClO) oxidation has been reported as a promising method for coal seam permeability enhancement and extracting valuable hydrocarbons from coal. Different coal macerals, particularly vitrinite and inertinite behave differently in NaClO with varying reaction extent exhibited. The reasons for variability are not completely understood. This study aims to reveal the controlling factors of different coal maceral oxidation by comparing their molecular structures and pore characterization. A Permian coal from Bowen Basin, Australia, was collected and separated into vitrinite-rich concentrate (bright coal) and inertinite-rich concentrate (dull coal). Their reaction phenomena with NaClO are examined and their molecular structures and pore structures are compared with multiple techniques. The vitrinite-rich and inertinite-rich concentrates share similar organic molecular structures although there is some marginal difference with the bright coal being slightly more aliphatic with higher oxygen content. By contrast, their pore structures are significantly different such that the dull coal is more porous with higher content of mesopores and better connectivity. For coal powders vitrinite is more reactive than inertinite and yet for coal particles inertinite is more reactive. Given that inertinite is more porous, these results suggest that the porosity of the inertinite is the key property making the inertinite more reactive. Therefore, it is proposed that the different extent of oxidation for different macerals from isorank coal is controlled primarily by the difference in their porosity rather than molecular structures.