Spray characteristics of a urea solution injector and optimal mixer location to improve droplet uniformity and NOx conversion efficiency for selective catalytic reduction

The urea–SCR catalyst system has the advantages of high NOx conversion efficiency and a wide range of operating conditions. The key factors for successful implementation of urea–SCR technology are good mixing of urea (ammonia) and gas to reduce ammonia slip. Urea mixer components are required to facilitate evaporation and mixing, because it is difficult to evaporate urea in the liquid state; the distance to the mixer is the most critical factor affecting mixer performance. In this study, to determine the optimal distance from the injector to the mixer for urea injection, we used laser diagnostics and a high speed camera to analyze the spray characteristics of a urea injector and to determine the distribution of urea solution droplets in a transparent manifold. The spray angle with a certain value (α) was measured, the injection quantity was about 9.4208 g/s, the SMD was approximately 65 μm, and then the position Bd was the optimal location for installation of mixer. In addition, we calculated the droplet uniformity index (DUI) from the acquired images using digital image processing to clarify the distribution of the sprayed droplets. Analysis of the quantitative characteristics and spatial distribution of the droplets revealed that addition of mixers resulted in more uniform distribution of the urea solution in the exhaust pipe and improved the performance of the urea–SCR catalyst system. A grid-channel plate-type mixer was optimal for maintaining a high the DUI which increased 32% than the case without a mixer, and the NOx conversion efficiency which was 85–90%.