Development of NOx Trap System for Commercial Vehicle - Basic Characteristics and Effects of Sulfur Poisoning - 2004-01-0580

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.