Effects of various factors on the conversion efficiency of urea solution in a urea selective catalytic reduction system

In a urea selective catalytic reduction (SCR) system, the urea solution is injected into hot exhaust gas, after which the urea solution becomes ammonia that acts as a reducing agent for de-NO_x through evaporation, thermolysis, and hydrolysis. The formation of the reducing agent from urea decomposition is closely connected with thermofluid dynamics as well as various chemical reactions. An experimental study was performed to investigate urea decomposition in a low-temperature environment that is similar to the emission gas temperature of a large marine diesel engine. Also, this study investigated urea decomposition in conjunction with thermofluid dynamics related to the urea SCR system driving conditions. The modeled exhaust pipe was designed to control the inflow gas temperature and velocity. The urea solution injector was chosen to obtain almost identical spray performance, regardless of the urea solution flow rate, to exclude the effect of the spray on urea decomposition. A multicomponent Fourier transform infrared spectroscopy gas analyzer was used to measure the concentrations of ammonia and isocyanic acid (HNCO) in the modeled exhaust pipe. This study showed that the conversion efficiencies of ammonia and HNCO were different under the experimental conditions of this study, although there is no difference between the conversion efficiencies of ammonia and HNCO in theoretical urea thermolysis. Also, it showed that there is no need for a long residence time to improve the total conversion efficiency at a low temperature.