Effects of flue gas re-circulation and nitrogen contents in coal on NOX emissions under oxy-fuel coal combustion

• NOX emissions were estimated in simulated oxy-fuel combustion conditions.
• NOX concentrations profiles in the reactor were analyzed by drop tube furnace experiments.
• Small amounts of NO and N2O were added to inlet gas as recirculated impurities.
• Contributions of recirculated NOX to NOX emissions were small in oxy-fuel coal combustion.
• Conversion ratios to NO and N2O were governed by the ratio of char-N to fuel-N.

Oxy-fuel coal combustion has drawn attention as a useful technique to achieve carbon dioxide capture and storage (CCS). It is the technique that enriches CO2 in exhaust gas by flue gas recirculation with additional pure O2, which makes it easy to capture and liquidize CO2 downstream. However, in the oxy-fuel combustion system, various impurities in the flue gas are brought back to the combustion zone with recirculated flue gas, and more have accumulated in a boiler compared with the air combustion system. NOX formations, in particular, can be affected by nitrogen contents in the raw coal and recirculated multiple NOX compounds, such as NO and N2O. Then, in this study, we experimentally estimated the effects of NO and N2O recirculation by a drop tube furnace equipment simulating oxy-fuel atmosphere for three coal samples with different nitrogen contents. As a result, contributions of recirculated NOX to NOX emissions were found to be very small in oxy-fuel coal combustion. Instead, nitrogen conversion ratios to NO and N2O were governed by the ratio of char-N to fuel-N.