Numerical assessment of NOx evolution in ammonia oxidation and its control by reburning in pressurized oxy-combustion

Abstract

Pressurized oxy-combustion (POC) is an emerging technology and has a higher efficiency compared to atmospheric combustion for carbon capture, utilization, and sequestration (CCUS). NOx is one of the significant conventional pollutants produced in POC, which has not only a disastrous effect on the environment but also aggravate the corrosion in the CCUS system. Ammonia is one of the primary gaseous precursors for NOx generation in solid fuel combustion. Nonetheless, the evolution of NOx from ammonia oxidation in pressurized combustion is still rarely studied, especially in the oxy-environment. Therefore, it is imperative to study the NOx formation from ammonia oxidation and its control in POC. In this study, first, the formation of NOx from ammonia oxidation in POC is kinetically evaluated. Different parameters are investigated at elevated pressure i.e., the effects of oxy and the air-environment, temperature, H2O, and SO2 concentration, on NOx formation. After that, reburning process is analyzed at high pressure to control the NOx using the actual POC conditions. The results illustrate that the ammonia oxidation temperature continues to decrease as the pressure rises from 1 atm to 10 atm. Pressures larger than 10 atm have no effect on the oxidation of NH3. The NO formation from NH3 oxidation continuously decreases with increasing the pressure, which means that higher pressure inhibits the NOx formation. The NO formation in air is significantly higher than the oxy-environment, due to the significant thermal NOx formation in air environment. The water vapor enhances the NO formation in oxy-environment significantly at 1 atm, however this effect is suppressed by elevated pressure. There is no effect of SO2 on NH3 oxidation and NO formation at both atmospheric and elevated pressure. The NO reduction by reburning process is enhanced from 43% to 76% with a pressure increase from 1 atm to 15 atm. In addition, the conversion of NO to N2 is boosted by elevating the pressure.