The Effect Mechanism of Fe on Coal Pyrolysis to NOx Precursors: Quantum Chemical Calculations and Mass Spectrometry Experiments

Density functional theory is adopted to thoroughly analyze the influence mechanism of Fe on the formation of NH3 and HCN. The structure of Fe adsorbed on the surface of seven-membered zigzag coal containing pyridine nitrogen is selected as the Fe-containing coal model. The effect of Fe on the nitrogen distribution during Zhundong coal pyrolysis is further studied by thermogravimetry-mass spectrometry. The theoretical calculations show that Fe increases the Mulliken charge density on the N5 surface, which increases the rate-determining step energy barrier value of NH3 generated from coal pyrolysis and inhibits the NH3 formation. On the other hand, Fe significantly enhances the bonding energy between sigma N5-C6 and pi N5-C6, increases the activation energy required for N stripping from the pyridine ring (about 69.14 kJ/mol higher than that without Fe), and inhibits HCN formation. The experimental results show that Fe catalyzes the precipitation peaks of NH3 and CH3CN about 20 K ahead of time and has no obvious catalytic effect on HCN and HNCO. In terms of the nitrogen distribution, Fe significantly promotes the CH3CN formation and shows a significant inhibitory effect on NH3, HCN, and HNCO. Kinetic results show that Fe reduces the precipitation rates of NH3 and HCN, and the inhibitory effect on HCN is more significant.