Experimental and theoretical calculations study on heterogeneous reduction of NO by char/NH3 in the reduction zone of ammonia co-firing with pulverized coal: Influence of mineral Fe

Abstract

As a carbon-free fuel, NH3 can be co-firing with pulverized coal to reduce CO2 emissions in pulverized coal-fired boilers, which has attracted increasing attention in recent years. However, the source of N in NH3 can lead to the increase in NO emissions. Studies on the NO reduction mechanism in the reduction zone by the co-firing of NH3 in a pulverized coal-fired boiler have been reported rarely. The mechanism for the heterogeneous reduction of NO with char and NH3 in the reduction zone is still not known, and the influence mechanism of mineral Fe on the char/NH3/NO heterogeneous reaction system is still not clear. In this study, high-temperature tube-furnace experiments and quantum chemical theoretical calculations are used to investigate the NO heterogeneous reduction mechanism in the reduction zone of NH3 co-firing with pulverized coal, and in-depth analysis of the influence mechanism of mineral Fe on the heterogeneous reduction of NO by char/NH3. Experimental results revealed that the heterogeneous reduction efficiency of iron-impregnated char/NH3 to NO is less than that of demineralized char/NH3 to NO. Fe inhibits the reduction of NO by char and NH3 in the reduction zone. Theoretical calculation results revealed that Fe is not conducive to the adsorption of NH and NO on the char surface. In the heterogeneous reduction of char/NH3/NO, Fe increases the activation energy of N–H bond dissociation and OH formation on the char surface, resulting in a 32.34 kJ/mol higher energy barrier value for the rate-determining step in the heterogeneous reduction of NO on the iron-impregnated char surface (IM2-Fe3 structure) than that for the heterogeneous reduction of NO on the surface of the iron-free char (IM2 structure). Fe is not conducive to the reduction of NO by char and NH3. This study provides new insights into the NO reduction mechanism in the reduction zone by the co-firing of ammonia in the pulverized coal-fired boiler and provides theoretical support for the application of this technology.