The competitive behavior for O2 and CO2 reaction during char oxy-fuel combustion: effects of temperature and inherent minerals

Abstract

Coal O2/CO2 combustion is a promising carbon capture and storage technology for coal-fired power plant. Char consumption rate prediction is essential for the boiler design, but challenging for O2/CO2 combustion since char reaction behavior within dual reactive gas media is complex. There is also a doubt of whether or not the O2 reaction and CO2 reaction occur independently, whereby the overall rate would be simply additive, or the two reactions compete for the same char actives, in which case the rate prediction could be more complicated. Here, the competitive behavior for O2 and CO2 reactions of char active sites was investigated via thermogravimetric approach, with a focus on the effect of temperature and inherent mineral composition. For char combustion within O2/CO2 environment with molar fractions of 30/70, the contribution of O2 to char consumption is the same as the O2 reaction alone. However, as being out-competed by O2, only 37% CO2 could occupy char active sites and react. Consequently, the overall char reaction rate within O2/CO2 is only 83% of the sum of that for individual gases at 900 °C. With increasing temperature, this ratio becomes even smaller, as the contribution of CO2 that being out-competed (to the sum of individual gases) is larger. With increasing O2 fraction, it occupied more active sites, thus further inhibiting CO2 reaction. The inherent alkali and alkaline earth metal provides the char with more active sites, thus muting the competition and inhibition for CO2 gasification.