AbstractThe recognition of the negative impact of NOx resulted in increasingly tighter automotive emission regulations. Companies are under pressure to develop methods, which can meet the legislative demands. After treatment solutions, and especially Selective Catalytic Reduction, became the focus of research and have shown so far promising results. However, more in depth understanding of the SCR process under different conditions is needed.
This thesis describes an investigation of the SCR performance using gas and urea injections under steady state and transient conditions undertaken on a light duty diesel engine using a 1D exhaust system designed for uniform flow across the catalyst.
Under steady state conditions, the SCR performance was examined for low and high temperature conditions. Ammonia was supplied either as 5% ammonia gas or in form of urea injection. The engine was operating at 1500 rpm and 6 and 8 bar BMEP to provide an exhaust gas temperature of 210°C and 265°C respectively. Also, the effect of SCR brick length on the NOx conversion was investigated using SCR catalysts of length 30, 45 and 75 mm. To measure the influence of NO2:NOxratio on the SCR performance, different sizes of standard DOC were used. NH3:NOxdosage levels included; α~0.5 -deficient ammonia, α~1.0 -stoichiometric ammonia, α~1.25 -excess ammonia. Gas emissions were measured before and after the SCR catalysts witha Horiba FTIR analyser during steady state and long transient tests. It was found that conditions such as temperature and NO2:NOxhad the biggest impact on the SCR performance. During the steady state engine conditions, at α~1.0 ammonia dosing and NO2:NOxratioof 0, only 17% of NO was converted in the first 30 mm of the SCR brick length. The conversion was improved at high temperature(263 °C) to31%.
A fast response CLD analyser was used during short transient testing to sample emissions with a high resolution. The short transient test withstandard0.5 and 1DOC, and fixed ammonia dosing, showed that NOx conversion was reduced during the ramp event due to deficient ammonia and a drop in the supplied NO2:NOxratio.
During urea injection experiments, urea was injected either through an oblique pipe arrangement with a mixer device placed downstream or directly into a mixing can. In this case the mixer device was replaced with a straight pipe. A 75mm SCR was fitted and to ensure that supplied NO2:NOxratio was zero, a palladium only DOC was used post a DPF. It was found that a large proportion of urea decomposition and hydrolysis was occurring on the surface of the SCR catalyst. Comparing NOx performance between urea injection and ammonia gas dosing experiment, more NO was converted for a given NH3:NOxratio when ammonia was supplied in the form of gas. That was true for low and high temperature tests.
For most studies, a long 10 degree diffuser was used in front of the SCR to provide uniform gas distribution across the catalyst. In addition SCR performance was investigated with a 180 degree sudden expansion diffuser in order to measure the influence of temperature and velocity profiles. During this study, a 45 mm SCR catalyst was used to provide a moderate amount of NO conversion and ammonia slip. The results showed that the flow and temperature distribution upstream of the SCR catalyst will have an effect on the NOx conversion, and that gas velocity has bigger impact on NOx conversion than gas temperature.
|Date of Award||2015|
|Supervisor||Stephen Benjamin (Supervisor) & Carol Roberts (Supervisor)|