Impact of SCR Activity on Soot Regeneration and the Converse Effects of Soot Regeneration on SCR Activity on a Vanadia-SCRF

摘要

The influence of SCR (selective catalytic reduction) activity on soot regeneration was investigated using engine test measurements with and without urea dosing on a vanadia-SCRF, also known as a vanadia SCR coated diesel particulate filter (V.SCR-DPF). The extent and rate of passive soot regeneration is significantly reduced in the presence of SCR activity especially at high temperatures (>250 °C). The reduction in soot regeneration is because some of the NO_2, which would otherwise react with the soot, is consumed by SCR reactions and consequently the rate of soot regeneration is lower when urea is dosed. The converse effects of soot oxidation on SCR activity were studied separately by analysing steady-state light-off engine measurements with different initial soot loadings on the V.SCR-DPF. The measurements show an increase in NO_X conversion with increasing soot loading. This is because the reaction of soot with NO_2 results in NO_2/NO_X ratio becoming closer to the optimal value for SCR activity of 0.5. This observation for these particular engine tests is because the V.SCR-DPF inlet NO_2/NO_X ratio was greater than 0.5. At low temperatures (<250 °C), NO_2 conversion decreases with increasing temperature, which was attributed to the decrease in the net rate of ammonium nitrate formation with increasing temperature in this temperature range, which in turn results in a decrease in NO_2 consumption. The impact of ammonium nitrate formation on NO_2 conversion was also observed to be correlated to soot loading at these low temperatures. Numerical modelling was also utilised to gain further insights into the influence of soot regeneration on SCR activity and the converse effects of SCR activity on soot regeneration on a V.SCR-DPF. The model captures qualitatively and quantitatively the decrease in soot regeneration rate as a result of NO_2 competition between passive soot oxidation and SCR reactions. The model also fairly predicts the enhanced SCR activity for the soot loaded V.SCR-DPF due to alteration of NO_2/NO_x ratio by passive soot oxidation.