Experimental and kinetics studies on separate physicochemical effects of steam on coal char combustion

Abstract

The synthetic effect of H2O on the combustion characteristics of coal char is complicated due to its thermal (high volumetric heat capacity), chemical, and dilution (thereby low oxygen concentration) effects. To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the net effect of H2O gasification through experimental and kinetic research. The experiment research is performed at 1773 K in a high-temperature drop tube furnace. The thermal effects of H2O are stripped off the net effect by introducing Ar into the bulk gas to neutralize its high heat capacity. We assess the chemical effects of gasification (gasification endothermicity, and its additional carbon consumption) for a 100 µm coal char particle reacting in environments of varying O2, H2O, and Ar using a self-developed char burning kinetics model. The relative contributions of H2O gasification and oxidation to carbon consumption are also discussed. Results indicate that the reduced carbon consumption through the negative effect of gasification (e.g., thermal effects and the gasification endothermicity) exceeds the additional carbon consumption from gasification reaction in the early combustion stages (< 0.3 s), but it is surpassed by the latter with char consumption. Increasing both the concentrations of H2O and O2 favor the gasification endothermicity. The reduced carbon conversion ratio caused by the endothermicity effect increases as high to around 0.4 then drops down with char consumption. Besides, in the 15 vol.% O2 case, an increase in H2O concentration from 5 vol.% to 30 vol.% improves the gasification contribution from 22.9% to 46.6% in the middle stage of combustion. It also widens the particle temperature gap by endothermicity effect to over 60 K, but the gap narrows in the burnout stage. Similar results were found in the air-fired case. The relative contribution from H2O gasification reaction is less influential upon the rise of oxygen concentration, which decreases by over 15% than in 15 vol.% O2 case.