Effects of CO2 gasification reaction on the combustion of pulverized coal char

Abstract

Although numerous studies on CO2 gasification during coal char combustion have been documented, the effects of CO2 gasification on high-temperature carbon oxidation, which is important for evaluation of carbon conversion during pulverized coal combustion under oxy-fuel and flue gas recirculation conditions, are unclear. Thus, the char conversion characteristics at various contents of O2 and CO2 were studied in a drop-tube furnace at 1373 K. Due to the cumulative effects of CO2 gasification, which can not only suppress carbon conversion due to endothermicity but also consume part of carbon directly, there exists a minimal carbon conversion point around the CO2 content of 9–13 vol%, where the growth of promotion effect with CO2 content catches up with the suppression effect, and an inflection point around the CO2 content of 17–25 vol%, where the promotion effect of the direct gasification reaction on carbon consumption surpasses the suppression effect. Increasing O2 shifts both points towards high CO2 contents, whereas increasing combustion gas temperature decreases the values. A mathematical formula including oxidation, effect of gasification on oxidation, and CO2 gasification, is presented, which gives well prediction on high-temperature carbon conversion. With increasing CO2 content, the reduction degree of CO2 gasification on oxidation rate increases, and the synthesis oxidation rate slightly decreases, whereas there always exists a minimal point for gasification rate, gross carbon conversion rate, and gasification weightiness. With increasing O2 content, the oxidation rate, reduction degree of CO2 gasification on oxidation rate, and gross carbon conversion rate increase; the gasification rate increases below the minimal point, whereas decreases above the minimal point; the gasification weightiness decreases. Combustion gas temperature shows positive effect on all abovementioned parameters. The results derived here provide meaningful guidelines for practical application and kinetics research on pulverized coal combustion.