Key reaction pathways for NO reduction by NH3 in the reduction zone during ammonia-coal co-firing: A combined experimental and DFT study

Abstract

Ammonia, as a zero-carbon fuel, is also a reducing agent for nitrogen oxides. During ammonia-coal co-firing, the reaction temperature window and atmosphere for ammonia and nitrogen oxides are usually different from those in SCR or SNCR, thus inhibiting the reduction effect of NH₃ on NO. It is of great significance to further explore the reaction mechanism and kinetic characteristics of NO reduction with a wider temperature range and different oxygen concentration for optimizing the reaction environment and reducing nitrogen oxide emissions during ammonia co-firing processes in pulverized coal boilers. To explore the characteristics of NO reduction by NH₃ under high-temperature reducing atmosphere, experiments were carried out on a one-dimensional tubular reactor at 1273–1773 K in 0–3% O₂. The effects of temperature and oxygen on the denitrification efficiency were investigated. The detailed mechanism was analyzed to obtain the skeleton reactions under the present conditions. Experimental results showed that as the O₂ concentration decreases, the temperature window for NO reduction shifts towards higher temperatures, and the maximal NO reduction efficiency is improved. The results of chemical kinetic simulation indicated that increasing O₂ concentration or temperature can promote the reaction of NH₃+OH=NH₂+H₂O and NH₂+OH=NH+H₂O, and promote the formation of NH₂ and NH radicals. NH₂ and NH radicals are essential for NO reduction under high-temperature reducing atmosphere. DFT calculations revealed that the process of NO reduction by NH₂ is that NH₂ combines with NO to form H2NNO, and then a H atom in H2NNO is transferred to O atom to form HNNOH. The decomposition of HNNOH produces different products, on the one hand, the O-N bond in HNNOH is broken to form NNH and OH radicals, on the other hand, the H atom in HNNOH is transferred from the N atom to the O atom to form N₂ and H₂O. The process of NO reduction by NH is that NH combines with NO to form HNNO, and HNNO decomposes to produce different products. On the one hand, the H-N bond in HNNO is broken to form H radical and N₂O, on the other hand, the H atom in HNNO is close to and bonded with the O atom to form N₂ and OH radical. At 1273–1773 K, the reaction rate constant of NO reduction by NH₂ is in the range of 10⁷-10¹⁰ s⁻¹, and the reaction rate constant of NH₂ radical oxidation and NH₂ conversion to NH is lower. When there is trace oxygen in the environment, the concentration of OH and NH₂ radicals can be greatly increased, and NO reduction with a large reaction rate constant can occur preferentially. Therefore, a higher NO reduction efficiency can be obtained under high-temperature reducing atmosphere.