Theoretical study of the gas-phase thermal decomposition of urea

Urea is used in Selective Catalytic and Non Reduction (SCR SNCR) methods for NOx abatement post-combustion processes. The decomposition of urea-water solution exhaust systems leads to the formation NH3, which reducing agent. However, urea can also lead unwanted pyrolysis by-products, particularly region where temperature pressure conditions are not optimal or via wall-spray interactions. In this study, a gas phase kinetic model proposed explain growth by-products urea, particular, route major product: isocyanuric acid. Systematic theoretical calculations, using electronic structure calculations transition state theory, were performed explore all possible unimolecular bimolecular reactions initial species. Kinetic modeling was select relevant reaction pathways at each growing step, their rate constants refined CCSD(T)/CBS//B2PLYP-D3/cc-pvTZ calculations. Quantum showed that postulated schemes literature, based on successive + HNCO ? biuret, biuret triuret followed by cyclization into acid energetically favored. It observed most favored involves carbamimidic acid, tautomer, first yields with then involved reacting product biuret. Isocyanuric produced from between same This new mechanism tested different its phase. Exploratory simulations pseudo condensed qualitatively simulate experiments yields.