Effects of nitrogen content in fuel on NOX emissions under oxy-fuel coal combustion

Oxy-fuel coal combustion with flue gas re-circulation is known to be one of the most promising methods for reducing CO2 emissions from coal combustion power plants. In the oxy-fuel system, the combustion atmosphere consists of only O2 separated from air and flue gas re-circulated from a stack. Therefore, the flue gas is dominated by CO2 without any nitrogen gas. The most attractive potential aspect of oxy-fuel coal combustion would be the ability to achieve a CCS (Carbon Capture & Storage) system, which would be a ‘quick-impact’ approach to reducing global CO2 emissions in near future. In the oxy-fuel system, several minor components in the flue gases are also re-circulated to the combustion zone together with the CO2. The effects of these additional impurities on the flue gas composition are drawing concerns. We have reported [1] that carbon conversion under CO2-O2 combustion is lower than that under air combustion. The concentration of NO from coal combustion in a CO2-O2 atmosphere is lower than that in air, while the concentration of N2O from coal combustion in a CO2-O2 atmosphere is higher than that in air. N2O accumulates a little as a result of flue gas recirculation under oxy-fuel conditions. Objectives in this study are to estimate the combustion behavior and NOX formations under oxy-fuel combustion conditions for various bituminous coal having different nitrogen contents. Both NO and N2O are regarded as NOX compounds here. The coal combustion experiments to observe NOX emissions were conducted under air, CO2-O2 and oxy-fuel combustion conditions. NOX emissions were estimated under flue-gas re-circulation conditions, in which a small amount of NOX was added to the inlet gas to simulate the oxy-fuel condition. All experiments were operated at temperatures of 1073K, which is relatively low compared with general coal combustion conditions. In particular, this temperature range corresponds to fluidized bed combustions.