Simulation study on nitrogen transformation characteristics of NH3/Coal Co-firing under deeply air-staged condition

Abstract

Carbon emissions have become a prominent global concern, with the utilization of fossil fuels serving as the primary contributor. Ammonia, being a hydrogen carrier without carbon, has the potential to be utilized through combustion, resulting in the production of water and nitrogen as ideal byproducts, which are environmentally benign. However, individual ammonia is difficult to combust due to its unfavorable combustion characteristics. Hence, ammonia needs to be blended with other fuels that possess superior combustion characteristics. Additionally, NH3 inherently contains a significant amount of nitrogen, a fuel element that can contribute to the increased release of NOx emissions. In this work, a simulation was conducted using CHEMKIN to investigate the nitrogen transformation characteristics during ammonia/coal co-firing under deeply air-staged combustion. Experimental studies primarily focus on macroscopic factors, such as temperature and pressure, while often neglecting the analysis of reaction mechanisms. The utilization of the chemical reaction kinetics simulation method can provide valuable insights into the reactions and substances that exert the most significant impact on the formation of NOx, as well as elucidate the pathway of fuel-nitrogen conversion during combustion. Simulation results demonstrate that as the NH3 co-firing ratio (CR-NH3) increases, the volume content of NOx initially rises, reaching its peak at approximately 10% CR-NH3, and subsequently declines. The conversion rate of NOx increases as the combustion temperature in the primary combustion area rises. The primary reactions have a greater influence on the formation of NO at low-temperature conditions. However, it is important to note that low temperature also decreases the combustion efficiency. The incorporation of overfire air (OFA) in advance has the potential to mitigate nitrogen reduction in the primary combustion area, while simultaneously improving NOx emissions. With the oxygen concentration of primary air increased, the conversion rate of NOx gradually decreases. It undergoes a sharp change before reaching 21% oxygen concentration, after which it levels off. In addition, high oxygen concentration increases the pathway of NO consumption. When the excess air coefficient of OFA falls within the range of 0.35–0.45, the NOx conversion rate can be minimized. The present study aims to offer technical assistance and guidance for effectively controlling NOx emissions in ammonia/coal co-firing boilers.