Effect of CO2 gasification reaction on the oxy-combustion of pulverized coal char: Measurement of high temperature gasification rate and burnout simulation

Abstract

One of the most promising strategies to reduce $CO_2$ emissions is to apply oxy-combustion of coal with flue gas recirculation (FGR) and carbon sequestration. In this technology, $CO_2$ gasification of coal char may play an important role due to the elevated $CO_2$ level with FGR, but its effect on the overall reaction rate has not been clearly understood. In this situation, many CFD studies of oxy-fuel boilers for the design and scale-up performance have paid little attention to the $CO_2$ gasification reaction with questionable rate parameters. This is due to the modelers’ lack of understanding of the gasification influence on the char reaction and, especially, the lack of available $CO_2$ gasification rate data for coal chars in the high temperature range. To give clarity to the likely impact of $CO_2$ gasification reaction on the overall char conversion in oxy coal-fired combustion environments, burnout simulations of coal char particles were carried out, adopting apparent char reactivity and a single film model that includes the Stefan flow effect on mass and energy transfer. Uncertainties that arise from related parameters and typical char combustion modeling assumptions were quantified for the model establishment. Three oxygen concentrations (21%, 30%, and 5% $O_2$), representing air, oxy, and oxygen deficient combustion environments were selected for the simulation. In contrast to previous simulations of such phenomena, the $CO_2$ gasification rate of a subbituminous coal char at high temperatures (T > 1700 K) and atmospheric pressure was derived from newly measured char particle temperatures in an optical entrained flow reactor (1 atm, 2408 K) with nearly 100 % $CO_2$. Also, these values were validated with the measured char mass loss. The experimental measurements show a significant temperature decrease from the gasifying particle (consistent with the strong endothermicity of the char-$CO_2$ gasification reaction) and show increasing mass loss with i...