A kinetic evaluation on NO₂ formation in the post-flame region of pressurized oxy-combustion process

Pressurized oxy-combustion (POC) is a promising technology that can significantly reduce the energy penalty associated with first generation oxy-combustion for CO2 capture in coal-fired power plants. However, higher pressure enhances the production of strong acid gases, including NO2 and SO3, which could aggravate the corrosion threat during flue gas recirculation. In this study, the conversion of NO to NO2 has been kinetically evaluated in a plug flow reactor under representative post-flame conditions of pressurized oxy-combustion. The effects of residence time, temperature, pressure, O2, H2O, CO, and SO2 on NO2 formation are studied. The calculations show that when pressure is increased from 1 to 15 bar, NO2 is increased from 1 to 60 ppm, and the acid dew point increases by over 80^oC. Higher pressure and temperature greatly reduce the time required to reach equilibrium, e.g., at 15 bar and 1300^oC, equilibrium is reached in 1 millisecond and the NO2/NO is about 0.8%. The formation and destruction of NO2 is generally through the reversible reactions: NO+O+M=NO2+M, HO2+NO=NO2+OH, and NO+O2=NO2+O. With increasing pressure and decreasing temperature, O plays a much more important role than HO2 in the oxidation of NO. A higher water vapor content accelerates NO2 formation in all cases by providing more O and HO2 radicals. The addition of CO or SO2 also promotes the formation of NO2. Finally, NO2 formation in a POC furnace is compared with that in a practical atmospheric air-combustion (AAC) furnace and the comparison shows that NO2 formation in a POC furnace can be over 10 times that of an AAC furnace.