Effect of assist-air of twin fluid atomizer on urea thermal decomposition

Recently, because of International Maritime Organization (IMO) regulations concerning NOx emissions, there has been growing interest in the use of selective catalytic reduction (SCR) systems to use urea to reduce NOx emissions of large marine ships. The conversion efficiency of the reducing agent is an important design factor because the system efficiency and the dimensions of the SCR catalyst are closely related to how much of the injected urea solution is decomposed into a reducing agent and to the optimal distance between the urea solution injector and the SCR catalyst. An experimental study was performed using a laboratory scaled exhaust system and a twin fluid atomizer type urea solution injector to investigate the conversion efficiency of the reducing agent under low temperature conditions similar to the emission gas temperatures of large marine diesel engines. In addition, a numerical simulation was carried out to evaluate the effect of the assist-air of a twin fluid atomizer on the inflow gas temperature drop. The conversion efficiency of the reducing agent increased as the inflow gas temperature rises. In addition, the average conversion efficiency measured at a given residence time was found to increase slightly with an increase in the inflow gas velocity. The inflow gas temperature drop caused by the assist-air of a twin fluid atomizer used as the urea solution injector has reduced the conversion efficiency of the reducing agent even though the ratio of the inflow gas flow rate to the air flow rate is relatively small.