Towards Quantitative Prediction of Urea Thermo-Hydrolysis and Deposits Formation in Exhaust Selective Catalytic Reduction (SCR) Systems 2019-01-0992
In order to assist in fast design cycle of Diesel engines selective catalytic reduction (SCR) exhaust systems, significant endeavour is currently afford to improve numerical simulation accuracy of urea thermo-hydrolysis. In this article, the achievements of a recently developed urea semi-detailed decomposition chemical scheme are assessed using several available databases from the literature.
First, evaporation and thermo-hydrolysis of an urea-water solution (UWS) single-droplet, hanged on a thermocouple with 127 µm diameter, have been simulated at ambient temperature conditions ranging from 473K to 773K. It has been shown that the numerical results, in terms of rates of evaporation and gasification of urea, as well as droplet temperature history are very close to the experiments if the heat flux coming from the thermocouple wire is properly considered.
Next verification of the models has been carried out using UWS sprays injected in 6-m long pipe under typical of Diesel engine exhaust manifold conditions. In this case, good agreement with experiments in terms of urea to ammonia (NH3) conversion efficiencies have been obtained under different temperatures and residence times. In addition, it proved to that by-products (like biuret, Cyanuric acid and even ammelide) can be formed in the spray particles upon water evaporation is completed during their travel to the exhaust catalyst inlet, especially for high gas temperature conditions.
Chaouki Habchi, Shaoping Quan, Scott Drennan, Julien Bohbot