A deep insight into catalytic effects of alkali and alkaline earth metals (AAEMs) in coal ash on NH3 oxidation during ammonia and coal co-firing process

Abstract

Ammonia as an eco-friendly and carbon-free energy has been greatly concerned at home and abroad. It has been utilized in the ammonia and coal co-firing process, demonstrating robust market viability. Nonetheless, due to the element N within ammonia causing the production of NO_x, the catalytic effect and reaction mechanics of AAEMs in coal ash on the NH₃ oxidation remain elusive. In this study, the catalytic effects of AAEMs (K, Ca, Na and Mg) on NH₃ oxidation were investigated through experiments, and the reaction mechanism was elucidated from a microscopic perspective by density functional theory (DFT). The experimental results show that the addition of AAEMs does enhance the oxidation potential of NH₃ during coal and ammonia co-firing process, and promote the transformation of NH₃ to NO, N₂O and N₂. However, the promotion effect of different AAEMs is different, consequently influencing the selectivity of NH₃ oxidation pathway. At 600–800 °C, alkaline earth metals (Ca and Mg) facilitate the oxidation of NH₃ while alkali metals (K and Na) have the opposite effect. Mg has the highest promoting effect on the conversion of NH₃ to NO at 600–800 °C, but the catalytic effect of K and Na is more dominant from 800 °C to 900 °C. The transformation from NH₃ to N₂O remains the same, but the catalytic impact of Ca is mostly pronounced. AAEMs can reduce thermal decomposition activation energy of N₂O, and the decomposition of N₂O can be promoted at 800 °C. The DFT results indicate that AAEMs can enhance the adsorption capacity of NH₃ on the active site of the coal ash surface, and reduce the activation energy of NH₃ oxidation, thereby accelerating the transformation of NH₃ to other nitrogen-containing species. This study provides new insight into the evidence for the enhanced catalytic effect of AAEMs on NH₃ oxidation during ammonia and coal co-firing process.