Numerical study on the effects of reaction conditions on single-particle char combustion in O2/H2O atmospheres

Abstract

The distributions of temperature, reaction rate, and component concentration in char are significant for the stable char-O2/H2O combustion. Based on the single-particle char combustion numerical model in O2/H2O atmospheres, the present work quantitatively studied the effects of reaction conditions on distributions of temperature, reaction rate, and component concentration in char under O2/H2O conditions. The simulation results show that the char surface temperature is always higher than the center temperature in the heating stage. The H2O and O2 concentrations have little effect on the char heating time and heating rate. The char center temperature gradually exceeds the surface temperature after the heating stage, and this gap narrows with the rise in H2O concentration and the decrease in O2 concentration. The C-O2 and C-H2O reaction rates first increase and then decrease along the char radius, and the ratios of the C-H2O reaction rate gradually reduce along the radius. At 1373 K with 5 % O2/20 % H2O, the ratios are below 20 %. The ratios increase significantly with the rise of temperature to 1673 K, especially in the char center, more than 70 %. The H2O and O2 concentrations in char are higher on the surface than in the center. However, the CO and H2 concentrations show the opposite characteristics, and the distribution of CO2 is highly uniform in char. The char-O2/H2O reaction gradually shifts to the diffusion controlled regime with the rise in temperature and O2 concentration, while it slowly turns to the chemical kinetic controlled regime with the increase in H2O concentration.