Reduction of NOx emissions from diesel engines by solid SCR technology
LIU Yingshuai1,2,3,, HU Guangdi1,,, QI Baohua2 1.School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China 2.Ningbo Free Trade Zone Workstation, Ningbo 315899, China 3.Institute of Intelligent Manufacturing, Weifang University of Science & Technology, Weifang 261000, China
Abstract:The urea-SCR technology is prone to low NOx conversion efficiency and urea crystallization in the low exhaust temperature actual operation process of the vehicle. To address this, engine test bench studies are conducted to investigate the effectiveness of solid SCR technology on NOx emission reduction. Results show that the volume of the solid SCR system is only 1/3 of the urea-SCR when carrying the same amount of reductant. The NOx conversion efficiency of the solid SCR system reaches 40% when the exhaust temperature is 160 ℃. Based on the same ammonia nitrogen ratio setting, the NOx conversion efficiency improves by 3.3% and 4.5% by applying the WHSC and WHTC cycles respectively. The NOx conversion efficiency improves significantly when the starting temperature of the solid SCR is reduced to 160 ℃, which is 9.7% and 15.5% higher than that of the solid SCR system and urea-SCR systems at 200 ℃, respectively. The emission of NOx pollutants from diesel vehicles with the urea-SCR system is significantly higher, which is 2.38 and 1.73 times that of solid SCR system with injection temperature at 160 ℃ and 200 ℃ according to the work based windows analysis ((0, 20]). Key words:selective catalytic reduction/ exhaust emission of diesel engine/ leakage of ammonia/ conversion efficiency/ metal ammine chloride salts.
图1固态SCR系统工作示意图 Figure1.Schematic diagram of solid SCR system
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1.School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China 2.Ningbo Free Trade Zone Workstation, Ningbo 315899, China 3.Institute of Intelligent Manufacturing, Weifang University of Science & Technology, Weifang 261000, China Received Date: 2020-05-13 Accepted Date: 2020-10-10 Available Online: 2021-02-22 Keywords:selective catalytic reduction/ exhaust emission of diesel engine/ leakage of ammonia/ conversion efficiency/ metal ammine chloride salts Abstract:The urea-SCR technology is prone to low NOx conversion efficiency and urea crystallization in the low exhaust temperature actual operation process of the vehicle. To address this, engine test bench studies are conducted to investigate the effectiveness of solid SCR technology on NOx emission reduction. Results show that the volume of the solid SCR system is only 1/3 of the urea-SCR when carrying the same amount of reductant. The NOx conversion efficiency of the solid SCR system reaches 40% when the exhaust temperature is 160 ℃. Based on the same ammonia nitrogen ratio setting, the NOx conversion efficiency improves by 3.3% and 4.5% by applying the WHSC and WHTC cycles respectively. The NOx conversion efficiency improves significantly when the starting temperature of the solid SCR is reduced to 160 ℃, which is 9.7% and 15.5% higher than that of the solid SCR system and urea-SCR systems at 200 ℃, respectively. The emission of NOx pollutants from diesel vehicles with the urea-SCR system is significantly higher, which is 2.38 and 1.73 times that of solid SCR system with injection temperature at 160 ℃ and 200 ℃ according to the work based windows analysis ((0, 20]).
固态SCR系统分金属氨盐存储罐、内部加热器、减压阀、不锈钢罐,以及后处理控制单元(aftertreatment control unit,ACU)、喷射控制阀、喷射装置、压力调节阀和氨气输送管等组成,其结构[22-23]如图1所示。固体SCR技术原理分3步:1)将金属氨盐 (Sr(NH3)8Cl2)存于封闭不锈钢罐体内,当其受热至一定温度后,NH3以氨气形式释放出来。NH3的喷射压力通过压力调节阀闭环调节,稳定在400 kPa;2)ACU接收发动机控制器(electronic control unit,ECU)的氨氮比通信信号(controller area network,CAN)后,实时喷射NH3到发动机排气管根据柴油机不同工况实时调整NH3喷射量;3)NH3在SCR催化剂的作用下与NOx发生化学反应,从而减少柴油机尾气中NOx的排放。为使NH3剂量阀保持精确计量,对系统监测密闭容器压力值及温控单元进行闭环控制。固态SCR技术同尿素SCR技术相比,不受尿素热解水解温度的限制,不存在尿素结晶、尿素结石堵塞排气管路的风险。而金属氨盐(Sr(NH3)8Cl2)做为NH3的存储介质[24],具有氨存储效率高、低温活性高等优点。另外,催化剂氨存储量对NOx转化效率有明显的影响,所以选择催化剂氨存储性能好、低温活性好的催化剂,可以有效提升SCR系统在低温下的NOx转化效率。