The effect of ammonia co-firing on NO heterogeneous reduction in the high-temperature reduction zone of coal air-staging combustion: Experimental and quantum chemistry study

Abstract

Ammonia is a carbon-free renewable energy and is considered as a potential alternative fuel. And the co-firing of coal and NH3 in boiler has attracted more and more attention. Notably, NH3 is a kind of efficient reductant. The homogeneous reduction of NO by NH3 and the heterogeneous reduction of NO by char occur in the reduction zone of high temperature and oxygen-lean during air-staged combustion. It is necessary to study the heterogeneous reaction process of NH3/char/NO in the high-temperature reduction zone. Experimental and density functional theory are employed to systematically explore the heterogeneous reduction mechanism of NO in high temperature reduction zone under NH3 co-firing in coal-fired boiler. Experimental results show that NH3 co-firing promotes NO reduction in the reduction zone, and is synergistically promoting NO heterogeneous reduction with char. In the heterogeneous reaction, two radical forms of NH and NH2, respectively, are considered in the theoretical calculation. The findings show that the energy barrier for the rate-determining step in the heterogeneous reaction of NHNO on the char surface is less than that for the rate-determining step in the homogeneous reduction of NO by NH3, which clarifies the synergistic promotion mechanism of char and NH on NO reduction. The energy barrier for the rate-determining step in the heterogeneous reaction of NH2-NO on the char surface is greater than that for the rate-determining step in the homogeneous reduction of NO by NH3. Bond order and electron localization function (ELF) analysis reveals that the NNH free radical formed by the B2 structure (intermediate structure formed by NH2 adsorbing on the char in N-down form and NO in parallel form) enhances the binding energy with the C atom and inhibits the reduction of NH2 and NO. The kinetic analysis shows that the rate constants of the rate-determining step in the heterogeneous reaction of NHNO are higher than those in the homogeneous reduction of NH3-NO and heterogeneous reaction of NH2-NO. The result not only strengthens the leading role of NH in ammonia denitrification, but also clarifies the role of char in promoting NO reduction by NH. This study enriched the mechanism of N migration and transformation under the co-firing condition of the pulverized coal and ammonia.