Nitrogen Oxide Emissions in Ammonia Combustion

Similar to hydrogen, ammonia is a zero-carbon fuel that can be synthesized from renewable energy sources such as solar and wind. Due to its better feasibility for production, preservation, and distribution, ammonia has been considered sustainable to meet the requirements of the future energy fields that are developing toward a low-carbon economy. However, the broad deployment of ammonia as fuel is limited by \({\text{NO}}_{\text{x}}\) emissions. This chapter presents the pathways of ammonia mixture reactions and the production routes of \({\text{NO}}_{\text{x}}\) emissions with different equivalence ratios. Some critical intermediate radicals are revealed for \({\text{NO}}_{\text{x}}\) formation. It is found that many factors affect the chemical reaction pathways of ammonia-based fuels, such as equivalence ratio, fuel mixture, pressure and temperature, and so forth. Ammonia combustion and \({\text{NO}}_{\text{x}}\) emissions have been investigated under different conditions on both laboratory and industrial scales. It was found that the \({\text{NO}}_{\text{x}}\) productions peaked at Φ = 0.8–0.9 for various ammonia/hydrogen blends. The NO productions from ammonia-based flames were effectively decreased with rich blends because of more generated \({\text{NH}}_{\text{i}}\) (i = 0, 1, 2) radicals. An overall equivalence ratio of 1.20 was suggested for two-stage combustion to improve combustion efficiency and emission performance. Furthermore, some practical controlling techniques, e.g., thermal \({\text{DeNO}}_{\text{x}}\) , two-stage combustion, humidification, and plasma-assisted combustion, are introduced for \({\text{NO}}_{\text{x}}\) mitigation.