Effect of oxygen concentration on the heterogeneous reduction of NH₃/Char/NO in the high temperature reduction zone during ammonia-coal co-combustion

Mixing carbon-free fuel ammonia into pulverized coal boiler can effectively reduce CO₂ emissions from power plants, which has attracted more and more attention in recent years. However, ammonia and coal co-combustion increases the path of NO_x formation. The study of NO reduction mechanism in high temperature reduction zone of ammonia and coal co-combustion process is particularly critical to achieve a low-nitrogen emission. Considering that the high-temperature reduction zone is not an oxygen free environment, it is necessary to explore the influence mechanism of micro-oxygen in reduction zone on NO reduction by unburned ammonia coupled coal char. In this paper, the influence mechanism of micro-oxygen concentration (0, 0.5%, 1.0%, 1.5% and 2.0%) on NH₃/NO homogeneous and NH₃/char/NO heterogeneous reduction is studied in a high temperature horizontal tubular furnace. The temperature range is 1 373−1 773 K. The results show that the efficiency of homogeneous reduction of NO by NH₃ increases with the temperature increase under high temperature and oxygen-free environment. When the temperature is higher than 1573 K, the homogeneous reduction of NO by NH₃ decreases slightly with the increase of temperature. The optimal reduction temperature of unburned ammonia for NO is 1 573 K. Char significantly improves the efficiency of NO reduction by NH₃ at high temperature, and broadens the optimal reduction temperature of NO to 1 673 K. The reduction of NO by char and NH₃ is synergistic. Compared with the absence of oxygen, when the temperature is not higher than 1 500 K and the oxygen concentration is within 2.0%, the micro-oxygen promotes the formation of the reduced free radical NH₂/NH, which promotes the homogeneous reduction of NO by NH₃. Further increasing the temperature is conducive to the oxidation of NH₂/NH, so that the homogeneous reduction efficiency of NO by NH₃ in the presence of oxygen is lower than that in the absence of oxygen. In the heterogeneous reduction system of NH₃/char/NO, when the temperature is not higher than 1 473 K and the oxygen concentration is not higher than 0.5%, the heterogeneous reduction efficiency of NO under micro-oxygen is higher than that without oxygen. With the further increase of oxygen concentration and temperature, the oxidation reaction of NH_i with oxidizing radicals such as OH/O is faster than the NO reduction by NH_i/char, the reduction efficiency of NO in the NH₃/char/NO heterogeneous system decreases. The results indicate that there is an optimal oxygen concentration to promote the reduction of NO by NH₃/char in the high temperature reduction zone.