A lumped model for thermal decomposition of urea. Uncertainties analysis and selective non-catalytic reduction of NO

Abstract

A two-step mechanism for non-catalytic thermal decomposition of urea was investigated. It involves a first order reaction for the thermolysis of urea and a pseudo-first order reaction for hydrolysis of isocyanic acid. Rate constants of these reactions were tuned on experimental measurements of NH 3 and HNCO concentrations at 423–723 K and 1083–1383 K in isothermal flow reactors made of aluminum and ceramic, respectively. A regression analysis by taking the uncertainties in the kinetic parameters into account shown that the resultant pairs of Arrhenius expressions for thermo-hydrolyzing urea were different with a 90% confidence when obtained from the different sets of experimental data. Additional results of ammonia concentration from experiments of urea solution decomposition reported in the literature revealed the reliability of the Arrhenius expressions tuned on the reaction rates obtained in the highest range of temperature in the ceramic reactor. The consistency of these equations was also verified by a comparison between calculated and experimental rates of selective non-catalytic reduction of nitric oxide with urea as reducing agent. A statistical analysis based on the identification of possible tendencies in these residuals again evidences the weakness of the Arrhenius expressions based on the experiments performed at lower temperatures, which was attributed to a non-expected catalytic effect of aluminum reactor walls on reactions rates of urea thermal decomposition.