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

Abstract

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 NOx 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 NHi with oxidizing radicals such as OH/O is faster than the NO reduction by NHi/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.

Keywords

• ammonia-coal co-combustion
• micro oxygen
• nh3
• char
• no