基于响应曲面法的柴油机SCR性能预测

Prediction of SCR performance of diesel engine based on response surface methodology

针对柴油机选择性催化还原(Selective Catalytic Reduction,SCR)系统在不同工况下运行时性能差异较大,搭建了带SCR系统的柴油机测试台架,在对SCR系统性能测试的基础上,利用GT-POWER建立SCR系统模型,分析不同排气温度、不同排气流量、不同氨氮比对SCR性能的影响,并基于Box-Behnken设计与响应面法对柴油机SCR系统进行了研究,以排气温度、排气流量、氨氮比为变量因子,以NOX转化效率与NH3逃逸率为优化目标进行响应曲面优化。结果表明:排气温度对SCR性能影响较大,250~450℃为SCR最佳转化效率区间,NOX转化效率均在80%以上,NH3逃逸率均在5%以内;排气流量增加使NOX转化效率下降,NH3逃逸率上升,排气流量在200 kg/h以上尤为明显,排气流量每增加50 kg/h,NOX转化效率平均下降3%,NH3逃逸率平均增加4%;氨氮比增加使得NOX转化效率提升,同时NH3逃逸率增加,氨氮比在0.9以上能使NOX转化效率保持较高水平,氨氮比在0.9以下能保证较低的NH3逃逸率,氨氮比的选择尤为重要。根据响应曲面结果得出:不同的排气温度与排气流量配合不同氨氮比可提高NOX转化效率,降低NH3逃逸率,当排气温度为350℃,排气流量为200 kg/h,氨氮比为1.0时,SCR性能最佳,NOX转化效率达到96.4%,NH3逃逸率仅0.5%。该研究为SCR系统在柴油机不同工况下运行时的尿素控制提供有效的指导依据。

Selective Catalytic Reduction(SCR)is applied to a diesel engine for the promising solution of NOX emission.In this study,a diesel engine test bench with an SCR system was developed to clarify the large difference in the NOX emission of diesel engine SCR system under various exhaust conditions.A performance test of the SCR system was also used to determine the single and multi-factor interaction.An SCR model was established using GT-POWER software.A systematic analysis was made to explore the influence of exhaust temperature,exhaust mass flow,and ammonia nitrogen ratio on the SCR performance in the heavy-duty engine.Box-Behnken design and response surface method(RSM)was used to simulate the diesel engine SCR system.An RSM optimization was carried out with the NOX conversion efficiency and NH3 slip rate as optimization objectives,where the exhaust temperature,exhaust mass flow,and ammonia nitrogen ratio were variable factors.The results showed that the NOX conversion efficiency increased in the range of 150-250°C,while a high level was then observed in the range of 250-450°C,finally to decline after 450°C.There was an opposite trend for the influence of exhaust temperature on NH3 slip rate.Specifically,the NH3 slip rate remained at a low level,all within 5%above 250°C.The NOX conversion efficiency decreased,but the NH3 slip rate increased,with the increase of exhaust mass flow,especially when the exhaust mass flow was above 200 kg/h.When the exhaust mass flow increased by 50 kg/h,the NOX conversion efficiency decreased by 3%,and the NH3 slip rate increased by 4%.The high ammonia nitrogen ratio contributed to improving the NOX conversion efficiency and NH3 slip rate.Particularly,the NOX conversion efficiency maintained a high level,when the ammonia nitrogen ratio was above 0.9.Nevertheless,the NH3 slip rate maintained a low level,when the ammonia nitrogen ratio was below 0.9.It inferred that the appropriate ammonia nitrogen ratio was expected to optimize the SCR performance.In the response surface optimization,a high exhaust temperature and low exhaust mass flow with a suitable ammonia nitrogen ratio can contribute to the NOX conversion efficiency in the high level,while the NH3 slip rate in the low level.An optimal NOX conversion efficiency of SCR performance reached 96.4%,and the NH3 slip rate was only 0.5%,when the exhaust temperature was 350°C,while the exhaust mass flow rate was 200 kg/h,and the ammonia nitrogen ratio was 1.0.Consequently,an optimal combination of NOX conversion efficiency and NH3 slip rate can be achieved under the optimization of exhaust temperature,exhaust flow and ammonia nitrogen ratio.This finding can provide effective guidance for urea control in an SCR system under different operating conditions of a diesel engine.