DOC is used to reduce the typical diesel engine exhaust gases, CO and THC while DPF is used to remove PM. SCR with urea catalyst is used to reduce NOx. But there is N2O formation as the side reaction in the process of NOx reduction. N2O is one of the greenhouse gases and causes 310 times the greenhouse effect than CO2. Therefore, in this study, NOx emission characteristics and N2O emission from DOC, DPF and SCR were investigated.
The engine used in this study was a 57 kW 4-cylinder, 3,400 cc mechanical fuel injection off-road diesel turbo engine. Catalysts used were iron(Fe), copper(Cu) Zeolite and Vanadium titania (V/TiO2) for SCR and Adblue certified urea was used as the reducing agent.
The operating conditions for this study were 1,250 rpm, 1,600 rpm, 2,200 rpm and 2,500 rpm for 25%, 50%, 75% and 100% loads. Catalyst for the after-treatment systems were in the order of DOC, DPF and SCR. The exhaust gas and temperature were measured before and after the catalyst in each condition. The urea injection was computed based on SCR inlet NO and NO2 concentration, and air at the SCR inlet and the urea was increased until the NOx reduction was more than 80%.
As the result, about 90% of CO and THC were oxidized in DOC and DPF while NO was oxidized into NO2 from NOx with the NO2/NOx ratio of more than 80% at 250℃ ~ 300℃. The N2O formation at DOC and DPF was relatively low at 3 ppm.
As the result of Fe-Zeolite testing, NO conversion at SCR was more than 80% at 250℃ ~ 450℃ and NO2 conversion was 65% ~ 90% at 200℃ ~ 550℃. The N2O formation with Fe-Zeolite was the largest at 52 ppm at 1,250 rpm 50% load at 300℃. The N2O formation with SCR was relatively high at more than 10 ppm at 300℃±50℃.
As the result of Cu-Zeolite testing, NO conversion was more than 80% at 250℃ ~ 550℃ which was better than Fe-Zeolite at higher temperature. NO2 conversion was more than 80% at temperature higher than 300℃. N2O formation at 250℃ ~ 550℃ was in the range of 5 ppm and 15 ppm.
As the result of V/TiO2 testing, NO conversion was excellent at 75% at 200℃ ~ 550℃. However, NO2 was rapidly decreased to 40% at 250℃ ~ 340℃. The NO2/NOx was more than 80% at the same temperature range which showed the V/TiO2 catalyst was not suitable for the NO2 conversion. However, NO2 conversion was more than 80% as the NO2/NOx ratio was decreased when the temperature was higher than 350℃. N2O formation was largely influenced by NO2 reduction with low efficiency of less than 5 ppm. When the temperature reached higher than 500℃, the formation was about 10 ppm.