Experimental demonstration of NO_x reduction and ammonia slip for diesel engine SCR system

This paper investigates the characteristics of selective catalytic reduction (SCR) with a V₂O₅-WO₃/TiO₂ catalyst by studying the key parameters and determining a method for controlling ammonia injection with a sample test bench. Four parameters are defined and adopted to represent the characteristics of nitrogen oxides (NO_x) and the ammonia reaction. The effects of NH₃/NO_x ratio (NSR), catalyst temperature, and ammonia injection period on NO_x conversion efficiency and ammonia slip are investigated. The correlation between ammonia slip and ammonia saturation storage level is studied. The experimental results show that the ammonia saturation storage level has a great impact on NO_x reduction and ammonia slip. The NO_x conversion efficiency and ammonia slip strongly depend on the ammonia saturation storage level. Under such conditions, the NO_x conversion efficiency is best when the ammonia saturation storage level is 68.2~73%, and the value reaches 75% before ammonia slip. Pulse injection improves the NO_x conversion efficiency and limits ammonia slip. Comprehensive comparison shows that the injected ratio of NH₃/NO_x is first larger and then smaller than is beneficial for the rapid improvement of NO_x conversion efficiency; the appropriate NH₃/NO_x ratio and continuous injection time must be controlled, or it is easy to cause ammonia slip. Therefore, a control algorithm based on ammonia storage saturation level has been proposed. According to the difference between the actual value of ammonia storage saturation and the target value, the controller corrects the injection of urea to achieve control of ammonia storage saturation level. The period of pulse injection has little influence on the mean value of NO_x at the outlet; however, it affects the peak level of NO_x and ammonia slip. Using varied period pulse injection further improves the NO_x conversion efficiency and restrains ammonia slip. The outlet level of NO_x can be reduced by adopting a suitable ammonia pulse injection interval.