Interactions of HCN with NO in pressurized oxy-combustion

The process of recycling flue gas to the boiler of a pressurized oxy-combustion system could affect the final concentration of NOx, which is very important for an appropriate design of the carbon capture process. During the recycling process, HCN, as the main intermediate of the reburn reaction, may interact with NO significantly to determine the final NO emissions. The current study aims to assess the gas-phase interactions of HCN and NO in a CO₂ atmosphere, which is customary to pressurized oxy-combustion conditions. A kinetic modeling study of the oxidation of HCN in the presence of NO has been performed under POC conditions and in the 973–1473 K temperature range. The influences of the stoichiometry, pressure, H₂O concentration, and NO inlet concentration are analyzed. The results show that the HCN–NO interaction reduces both HCN and NO and that the interaction increases as pressure increases when the temperature is above 1073 K, whereas the opposite is found in low-temperature regions. The effect of stoichiometric parameters also strongly depends on temperature but follows the opposite trend compared with the effect of pressure. These differences contribute to the suppressive effect on O₂ decomposition and the positive effect on H₂O decomposition at elevated pressures. An optimal temperature for NO reduction is shifted to lower temperature with increasing pressure and H₂O concentration. The analysis of the total fixed nitrogen (TFN) indicates that for an actual boiler (5% O₂), the temperature should be controlled at 1373, 1223, 1173, and 1173 K, respectively, as the pressure increases from 1 to 15 atm. Finally, a 21-species and 69-step reduced chemistry is developed and validated to accurately predict the interaction of HCN with NO at elevated pressures.