CFD modeling on char surface reaction behavior of pulverized coal MILD-oxy combustion: Effects of oxygen and steam

Gasification reaction is not supposed to be omitted for pulverized coal MILD-oxy combustion regime, especially for the wet recycle (steam up to 40 vol%). This paper numerically investigates the individual effects of steam and oxygen on pulverized coal MILD-oxy combustion to have better understanding of char reaction behavior under this combustion mode. Results show that, the flame temperature affected by steam is because of thermal radiation, whereas that of O-2 is because of chemical reactivity. The maximum value of the turbulent Damkohler number (Da,(t)) is within 0.25-1.9 for all cases, implying that the rate of chemical reaction is comparable with that of turbulent flow. For 30% O-2 and 40% steam cases, gasification reaction accounts for up to similar to 62%. With a given O-2 level, the reaction of char-H2O competes the active sites with char-O-2 and char-CO2, whereas for a certain level of steam, a positive synergy exists between char-H2O and char-CO2. The effect of CO + H2O -> CO2+ H-2 on generating H-2 becomes increasingly significant when O-2 content is > 21%, and easy to occur under low steam concentration. Increasing the O-2 and steam is conducive for the decrease in overall char burnout rate. As the steam content increases, the reduction on char burnout rate is insignificant for 18% and 21% O-2 cases, but it is serious for 24%-30% O-2 cases.