Mechanism of Na on NO reduction by ammonia and char during ammonia-coal co-firing process
Coal-fired boilers can realize significant carbon emission reduction by using co-combustion with ammonia which is one of the low-carbon fuels.NO generation and emission in ammonia-coal co-firing is very critical.Ca,Fe,K,Na,and other typical mineral elements in coal have different effects on NO generation and emission during coal combustion and ammonia-coal co-firing.The effects of Ca,Fe and K on NO generation and reduction in coal-ammonia co-firing have been investigated,but the role of Na is still not clear.To investigate the mechanism of NO reduction in the high-temperature oxygen-poor region during ammonia-coal co-firing and the role of the mineral element Na,quantum chemical calculations were carried out.Firstly,the char model for adsorption of Na atom at various positions was constructed using a typical Zigzag model.The calculation results showed that Na atom were more likely to be adsorbed on unsaturated carbon at the edge of the char rather than adsorbed parallel to the char.Next,adsorption energy calculations were carried out for different adsorption orders of NO and NH.The results showed that NO and NH were more stable to adsorbed on both sides of Na atom.Compared to configuration without the participation of Na,the maximum increase in adsorption energy of the configuration with Na participation is-49.31 kJ/mol.Then,the detailed steps of the reaction were explored for six reactant configurations.The migration of H atom to the char surface to form the N2 molecules showed a significantly lower rate-limiting step energy barrier.Meanwhile,the results showed that the involvement of Na significantly reduced the energy barrier of the rate-limiting step of the reaction by 77.12 kJ/mol(Path 5 compared to Path 2).Finally,kinetic calculations of the rate-limiting steps of the six paths were performed.The results showed that the presence of Na promoted the reaction and was more pronounced at low temperatures(4 259-fold for Path 5 compared to Path 2),as well as reduced the temperature dependence of the reaction.
ammonia-coal co-firingNaNOheterogeneous reductionDensity functional theory
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