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

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.

Mechanical insight into the formation of H2S from thiophene pyrolysis: The influence of H2O.

The thermal utilization of waste rubber is accompanied by the release of sulfur, and the release of H2S to the gas phase is one of the crucial issues. In this work, density functional theory (DFT) calculations and wave function analysis were employed to explore the possible formation pathways of H2S and its precursor (·SH radical) during the pyrolysis of thiophene in the presence of H2O. It indicates that H2O affects the decomposition of thiophene and the formation of H2S in two patterns. First, H2O can participate in the hydrogen transfer process by acting as a catalyst or generating weak hydrogen bonds with thiophene. In this way, the hydrogen transfer reactions are promoted with lower energy barriers, and thus the formation of H2S is facilitated by H2O without changing the pyrolysis pathways. Secondly, H2O can saturate the thiophene ring by addition reactions and alter the generation pathways of H2S significantly. The energy barriers can be decreased with one or two CC bonds of thiophene being saturated. The completely saturated thiophene results in a greater decline of the overall energy barriers for H2S formation. H2O provides the H atom for H2S in the second pattern. Due to the combination of the two influence patterns, the release of H2S can be promoted greatly in the presence of H2O. The present study aims to lay a foundation for the clean thermal utilization of thiophene/rubber and to inspire the advance of desulfurization techniques.