Separate physicochemical effects of CO2 on the coal char combustion: An experimental and kinetic study

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The overall impact of CO2 on the coal char combustion is complicated due to the interplay between the CO2 thermal effect and chemical effect (including the gasification endothermicity, and its direct additional carbon consumption). To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the separate physicochemical effects of CO2 through experimental and kinetic research. Two oxygen concentrations (15% and 21% O2), representing industrial boiler and air combustion environments, were selected. The experiment research on its thermal effect (evaluated by carbon conversion ratios) is performed at 1773 K in a high-temperature drop tube furnace. The chemical effect of CO2 is predicted for a 100 μm coal char particle using a self-developed char burning kinetics model. Results indicate that the carbon conversion ratios show a V-type distribution with higher CO2 concentrations, as a result of its complex physicochemical effects, with minimal carbon conversion points around the CO2 concentrations of 5–10 vol.%. The thermal effect initially increases with higher CO2 concentrations from 0 to 10 vol.%, but it declines with a further increase in CO2 concentrations, while the endothermicity effect of the gasification reaction increases with higher CO2 concentrations. The relative contribution of the endothermicity effect on the char consumption is 20.4% in the 21%O2/10CO2/N2 environment, lower than the thermal effect (29.9%), but it continuously increases and becomes the most influential inhibitory factor at higher CO2 concentrations. The strongest suppression effect of CO2 corresponds to the CO2 concentrations of 10 vol.%.