SCR系统对公交车NO_X排放降低效果的研究

A Research on the NO_X Reduction Effects of SCR Systems in City Buses

使用便携排放测试系统测试了3辆带有SCR系统的公交车在实际运行条件下的NO_x排放,对比喷射和不喷射尿素两种情况下的NO_x排放因子,计算了SCR系统对NO_x的降低率。结果表明:国Ⅳ柴油车由于其排气温度低,催化剂体积小,SCR系统对NO_x的降低率最低,只当车速高于30 km/h以后才能逐步显现出催化效果,NO_x平均降低率只有10%;国V天然气车SCR系统对NO_x的降低率最高,它在低速时排气温度就很高,且NO_x排放中NO_2的比例较高,使其SCR系统对NO_x的降低率基本上不随车速而变化,NO_x平均降低率达85%;国V柴油车介乎两者之间,使其SCR系统对NO_x的降低率约为70%。至于NO_x+HC排放,则是国V柴油车最低,国Ⅳ柴油车最高,国V天然气车由于其甲烷的排放量很高,使其NO_x+HC排放接近而稍低于国Ⅳ柴油车。

NOX emissions from three buses with SCR systems are measured by using portable emission measurement system in real-world conditions, the NOX emission factors in both conditions （ with and without urea in-jection） are compared and their NOX reduction rates of SCR systems are calculated. The results show that the State-IV diesel bus has the lowest NOx reduction rate of SCR system due to its low exhaust temperature and less volume of catalyst, whose effects gradually become apparent only when vehicle speed is higher than 30km/h, with an average NOX reduction rate of only 10 %;the State-V CNG bus has the highest NOX reduction rate due to its high exhaust temperature even at low speed and higher proportion of NO2 in NOX emission, and its NOX reduction rate basically does not change with vehicle speed, resulting in an average NOX reduction rate of 85%; while the NOX reduction rate of State-V diesel bus is in-between with an average NOX reduction rate of around 70%. As for the emission of NOX＋ HC, the lowest is the State-V diesel bus, the highest is the State-IV diesel bus, while the NOX＋HC emission of State-V CNG bus, due to its high emission of methane, is close to but slightly lower than that of State-IV diesel bus.