A thorough theoretical exploration of the effect mechanism of Fe on HCN heterogeneous formation from nitrogen-containing char

HCN is an important precursor of nitric oxide (NO) during pulverized coal combustion. A thorough theoretical exploration with density functional theory and wave functional theory was performed to elucidate the effect mechanism of Fe on HCN heterogeneous formation from nitrogen-containing char. Based on X-ray photoelectron spectroscopy analysis, an aromatic cluster model consisting of pyridinic nitrogen was chosen for simulation. Electronic structure analysis showed that abundant pi electrons were sufficiently delocalized above and below the basal model plane. Maxima of electrostatic potential existed at every hollow site at the center of a hexagon on van der Waals surface. Calculation results indicated that chemisorptions of iron atoms on seven different hollow sites were exothermic spontaneous reactions with the formation of interstitial iron carbides and iron nitrides. It was verified by reduced density gradient analysis that the Fe atom and nitrogen containing char were mainly combined by electrostatic attraction. Heterogeneous desorption of HCN from nitrogen containing char was endothermic, whether iron was in presence or not. The rate-determining step was the ring-opening process of the nitrogen heterocyclic ring when iron not bonding to the nitrogen atom, while it was the breakage of Fe-N bond when iron combined with the nitrogen atom. Thermodynamic and kinetics analyses found that the participation of Fe had inhibiting effect on heterogeneous formation of HCN except the direct binding of iron to nitrogen.