Since a NOx trap catalyst cyclically releases and reduces NOx with rich exhaust gas, generating of a rich spike becomes important for application to diesel engines, which always operate with overall lean combustion. In addition, a NOx trap catalyst is poisoned and degraded in performance by the presence of SO2 in the exhaust gas.
When the NOx absorbing efficiency thus decreases, it is necessary to regenerate the catalyst by a sulfur purge (desulfation) process in order to remove SO2. It is apparent that there are many factors and effects to understand before one can apply this catalyst system to a diesel engine, therefore we have carried out an inquiry into a performance of the NOx catalyst used the model gas equipment with known gas mixtures. The rich spike was generated with diesel fuel (light oil), resulting in a transient equivalence ratio spike > 1, to simulate diesel exhaust gas. Results indicated a NOx conversion efficiency exceeding 90% after optimizing the diesel fuel addition method. Moreover, the transient mode engine tests showed an average conversion efficiency exceeding 70%.
However, during the engine tests the performance began to decrease due to degradation assumed to result from SO2 poisoning. It was then important to establish the optimum sulfur purge method during model gas testing. The sulfur desorption rate was improved by alternating between rich and lean gas at a 600°C temperature. Following regeneration of the degraded catalyst (after SO2 poisoning), sulfur desorption and performance recovery were observed.
However, since the sulfur purge requires a high catalyst temperature, fuel consumption and thermal catalyst stability are items to be mindful of. We note that low-sulfur fuel is preferred for the NOx trap catalyst.