Numerical study on the optimum injection of urea-water solution for SCR deNOx system of a heavy-duty diesel engine to improve deNOx performance and reduce NH3 slip

Optimization of the urea dosing system to maximize deNOx performance and to minimize NH3 slip during load changes is one of the main challenges for the mobile application of the urea-SCR (selective catalytic reduction) system. A mobile SCR system is restricted to a short distance between the engine exhaust and the catalyst entrance. Therefore, it is likely that urea residence time may not be adequate for complete thermolysis at the catalyst entrance. The key factors affecting the automobile application of SCR technology are the rapid thermolysis, good mixing of ammonia and gas, and reduction of ammonia slip. To this end, in the present study, the effects of the injection angle, urea injector location, and number of injection holes on the uniformity of the ammonia concentration distribution at the entrance of the SCR monolith as well as the maximum ammonia conversion efficiency have been examined in detailed three-dimensional numerical simulations under various engine loads of a heavy-duty diesel engine. As a result, the injection with an injection angle of 90 showed the best performance in terms of the maximum NH3 conversion efficiency. The effect of injector location more than 5D upstream was negligible in the uniformity point of view. Six- and eight-hole injectors showed no significant variation of uniformity with an injector location more than 5D, yielding over 95% of uniformity. Based on these results, the present work shows useful guidelines to achieve the optimum design of a urea-SCR injection system for improving deNOx performance and reducing ammonia slip.