Formation mechanism of NOx precursors during the pyrolysis of 2,5-diketopiperazine based on experimental and theoretical study.

Abstract Incineration of food waste leads to the release of NOx pollutants, whereas the formation mechanism of the NOx precursors (HCN, NH3, and HNCO) during the initial pyrolysis process is far from well-studied, limiting the source control on NOx release. In this work, 2,5-diketopiperazine (DKP) was selected as the N-containing model compound to study the formation mechanism of NOx precursors in food waste pyrolysis, by combining experiments and density functional theory (DFT) calculations. The C1-N2 bond broken via the N2-to-N5 H-transfer possesses the lowest energy barrier, together with the largest reaction rate constants in the range of 400–800 °C. NH3 can be easily generated with low energy barriers and high rate constants at low temperatures (below 630 °C). Whereas, the rate constants of the pathways for HCN formation will exceed those for NH3 generation in the range of 630–740 °C. In addition, the DKP pyrolysis can also lead to the formation of HNCO with a very low energy barrier, and it can convert into HCN and NH3 through further hydrogenation and decomposition. These calculation results are exactly consistent with the experimental results that NH3 was the main precursor in the range of 400–600 °C, and the yield of HCN exceeded that of NH3 when the temperature was over 600 °C. Our current work on the formation mechanism of NOx precursors during the pyrolysis of DKP can provide theoretical guidance for the development of NOx control technology in the pyrolysis/combustion process of organic waste.