Interactions Between Soot and de-NOx Activity

SCRF systems may appear as an attractive solution due to their higher de-NOj performance at clean state compared to the SCR catalysts, as presented in 

However, the general case of soot -loaded SCRF has to be considered in order to account for the competitive interactions between SCR activity and soot oxidation which will be discussed in this section. 

Watling et al. [50] also investigated the effect of the presence of soot on conversion by performing a series of simulations for a Cu-SCRF at various soot loadings and temperatures between 150-550 °C. The gas feed to the SCRF contained 500 ppm and 15 % O2 with a space velocity of 55,000 h . A realistic NO2/NOJC ratio was used at the inlet, as presented in Fig. 13.15a. 

The results, presented in Fig. 13.15b, show no significant effect of soot on NO conversion. At low temperatures, the NO2 consumption by soot is slow and has little impact on the N02/NO c ratio. At mid temperatures, the presence of soot actually enhances the de-NO performance. As mentioned before, optimum SCR activity is achieved when inlet N02/NO c = 0.5 due to the fast SCR reaction. NO2 /NOx ratio is higher than 0.5 between 300 °C and 400 °C. Soot oxidation with NO2 results in a decrease in N02/NO c to a value closer to 0.5 and thus enhancing the NO conversion. 

Colombo et al. [48] performed a simulation study of the de-NOj activity effect on soot regeneration. Simulations were carried out at a fixed temperature of 500 °C and with a feed mixture containing equal amounts of ammonia and NOj. The initial soot load was set to 8 g/1 and simulations were repeated with two different NO2/NO , ratios, namely 0 and 1. For both reacting systems, simulations were first performed considering the SCR reactivity and then repeated by deactivating the de-NO reactions. The computed evolution of the total soot mass and of wall soot was followed as a function of time, as shown in Fig. 13.16. 

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