Gas-Phase Reaction of NOX Formation in Oxyfuel Coal Combustion at Low Temperature

Oxyfuel coal combustion with flue-gas recirculation is known to be one of the most promising methods for reducing CO 2 emissions from pulverized coal combustion power plants. In the oxyfuel system, the combustion atmosphere consists of only O 2 separated from air and flue gas recirculated from a stack. Therefore, the flue gas is dominated by CO 2 without any nitrogen. The most attractive potential aspect of oxyfuel coal combustion would be the ability to achieve a carbon capture and storage (CCS) system, which would be a “quick-impact” approach to reducing global CO 2 emissions. In the oxyfuel system, several minor components in the flue gases are also recirculated to the combustion zone, together with the CO 2 . The effects of these additional impurities on the flue gas composition are one of our concerns. In this study, NO X emissions were experimentally monitored in an oxyfuel coal combustion atmosphere. The effect of NO X recirculation on NO X formation in the combustion zone was investigated in particular. In addition, elementary reaction kinetics for NO X formation was numerically analyzed under oxyfuel coal combustion conditions identical to the experiments. All experiments and numerical analyses were conducted at temperatures between 1073 K and 1223 K; these temperatures are relatively low, compared with various coal combustion conditions. In particular, this temperature range corresponds to fluidized-bed combustions. As a result, NO emissions under CO 2 –O 2 conditions were confirmed to be lower than those under “air” conditions. In addition, the concentration of NO under oxyfuel conditions was unchanged by flue-gas recirculation. In contrast, the concentration of N 2 O under CO 2 –O 2 conditions was higher than that under air conditions, and, furthermore, it was increased by flue-gas recirculation.