Theoretical study of the gas-phase thermal decomposition of urea

Abstract

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