Catalytic mechanism of calcium ion on the NOx precursors formation during the pyrolysis of kitchen waste

Calcium ion (Ca2+) in kitchen waste exerts a significant impact on the formation of NOx precursors to affect the release of nitrogen oxides during the incineration process. However, the catalytic mechanism of Ca2+ is still unclear. Herein, 2,4-imidazolidinedione is employed as the nitrogen-containing model to study the effect of Ca2+ on the generation of NOx precursors during pyrolysis. Through density functional theory calculations (DFT) and wave function analysis of the catalytic effect of Ca2+, the results indicate that Ca2+ has different degrees of impact on the energy barriers of the initial ring opening reaction. Ca2+ not only changes the rate determining steps of NOx precursors formation pathways, but also reduces the total energy barriers formed by NH3. At the same time, the total energy barriers of all paths formed by HCN are raised. This work theoretically explains the phenomenon of Ca2+ promoting the formation of NH3, whereas inhibiting the release of HCN in pyrolysis experiments. In addition, Ca2+ promotes partial reaction pathways for the formation of HNCO, and the total energy barriers of catalytic pathways are lower than that of the uncatalyzed pathways. Therefore, under the catalysis of Ca2+, the generation pathway of HNCO is more competitive and it is conducive to the formation of HNCO.