Research On The Mechanism Of NO_x Precursors Formation During The Pyrolysis Of 2,5-Diketopiperazine

Traditional waste treatment methods have numerous drawbacks,and oxygencontrolled thermal treatment technology based on pyrolysis has become an unavoidable trend.Food waste,as a major component of municipal waste,emits significant NOx emissions during the thermal treatment process due to its high nitrogen content.The formation mechanism of the NOx precursors(HCN,NH3,and HNCO)during the initial pyrolysis process is poorly understood,limiting the source control on NOx release.In this work,2,5-diketopiperazine(DKP)was selected as the Ncontaining model compound to study the formation mechanism of NOx precursors and the effects of Na+/K+on the production of NOx precursors in the pyrolysis process by combining experiments,density functional theory calculations and wave function analyses.Our current research 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.The C1-N2 bond broken via the N2-to-N5 hydrogen transfer possesses the lowest energy barrier(301.0 kJ/mol),together with the largest reaction rate constants in the range of 400-800℃.NH3 can be easily generated with low energy barriers and high rate constants at low temperatures(below 630℃).Whereas,the rate constants of the pathways for HCN formation will exceed those for NH3 generation in the range of 630-740℃.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℃,and the yield of HCN exceeded that of NH3 when the temperature was over 600℃.Based on density functional theory calculations and wave function analyses,all of the energy barriers of the initial reactions decrease with the catalysis of Na+,whereas K+can only promote part of the reactions and has weaker effects than Na+.Because of their varying effects on different reactions,Na+/K+can alter the rate-determining steps of the production pathways of NOx precursors.As for the same pathway,Na+typically results in a lower energy barrier than K+.Elevated temperatures lead to higher rate constants of K+than Na+for the favorable production pathways of NOx precursors.Based on the comparison of overall energy barriers and the rate constants,the formation of NH3 with Na+/K+ is always more competitive than HCN and HNCO.