Reaction Mechanism Study On The Sncr De-nox During A Moderate Temperature Range Using Melamine And Urea-spiked Hydrazine Hydrate As Reductant

In China,the nitrogen oxides mainly come from coal-fired power plant boilers and the emissions are increasing year by year,which has caused increasingly serious environmental problems.Selective non-catalytic reduction?SNCR?denitrification process is attracting more and more attention due to its simple equipment,low operating cost and simultaneous application with other NO_X control technologies.But traditional SNCR de-NOX process generally has the problems such as high temperature range,low denitration rate and so on.In recent years,the SNCR de-NO_X technology mainly focus on developing new SNCR denitration reductant to improve the denitration efficiency during a moderate temperature range.The advantages of melamine and hydrazine as SNCR reductants during a moderate temperature range were confirmed in different experiments,but their denitrification mechanism is still unclear.In this work,the SNCR denitrification mechanism of melamine and urea-mixed hydrazine was investigated via density functional theory simulation at B3LYP/6-311++G?d,p?level.The results show that melamine can react with NO directly,and electrons transfer from the HOMO orbital of melamine to the LUMO orbital of NO.The initial reactions have three possible paths,of which path2 is more competitive and R?IM1 has the highest barrier of 56.41 kcal/mol.H and OH play important roles in the subsequent reactions.The initial products of P₁ and P₃ can both convert to P₂.P₂ can be combined with H,then the C-N bond is broken,and final products of C₃N₅H₅ and N₂ are obtained.In the traditional NH₃-SNCR de-NO_X process,the energy required for the reaction of H+O2?2OH is 79.56 kcal/mol,which is higher than the reaction of melamine and NO.In N₂H₄-SNCR denitrification process,except the direct decomposition of hydrazine,the reaction of N2H4+NO and N2H4+O2 can also produce NH2 under appropriate conditions.N2H4 and NO can transfer to N2H3+HNO or H2NNO+NH2,and the required energy barriers are 27.55 and 32.76 kcal/mol,respectively.N2H4 and O₂ react to generate N₂H₃,N₂H₂,OH and HO₂ through the reaction of N₂H₄+O₂?N2H3+HO2?N2H2+2OH.The energy barrier is 19.26 kcal/mol.In urea-mixed hydrazine's SNCR de-NO_X process,urea first decomposes into NH₃ and HNCO,then HNCO and NH3 react with H and HO2 in the system to produce NH2,respectively.The reaction energy barrier of NH₃+HO₂ and H+HNCO are only 0.38 and 0.69kcal/mol lower than that of the reation N₂H₄+NO and N₂H₄+O_2.The results can provide theoretical guidance for the further experimental research and industrialization of the moderate-temperature SNCR denitrification technology.