Predicting the Distributions of Oxygen Species from Primary Devolatilization of Coals

Abstract

Starting from an extension to FLASHCHAIN, this study interprets the evolution of coal oxygen during primary devolatilization. CO 2 and H 2 O are primarily formed via bridge charring that accompanies depolymerization. At slower rates, CO plus additional CO 2 and H 2 O are expelled from side chains without disintegrating coal macromolecules. Coal-O shuttled by tar is tracked as in FLASHCHAIN. New stoichiometry specifies the partitioning of O among the precursors to CO, CO 2 , and H 2 O. CO precursors incorporate about one-third of coal-O, regardless of coal rank. In contrast, the split of coal-O between CO 2 and H 2 O precursors exhibits an erratic relation with coal quality, which has been resolved by a graphical method that connects precursor elimination during coalification to the displacements on the Van Krevelen diagram. With the roles for tar shuttling and macromolecular configuration accounted for, the model accurately interprets a database of 40 coals from all ranks with the new stoichiometry. The proportions of CO, CO 2 , and H 2 O are faithfully simulated across the entire rank spectrum, even when the observed CO 2 levels scatter by a factor of 6 among high-volatile bituminous coals very similar in carbon content. Predictions are accurate throughout devolatilization except for the latest stage of CO evolution, and within the discrepancies among independent measurements for pressures from vacuum to 7.0 MPa, pending new data to characterize the final stage of CO formation and evaluate the predicted heating rate effect.