Dimensioning of SCR System

A clear dependence of NOx conversion on space velocity is seen in Fig. 3.10. For the lowest space velocities ( 30,000 h ) and exhaust temperatures 3(X) °C, NOx conversions 90 % were achievable while at the highest space velocity (140,000 h ) merely 40 % NOx conversion was obtained. When designing the appropriate SCR catalyst volume for an engine the required NOx conversion will limit the maximum space velocity that can be allowed. Taking the data in Fig. 3.10, if a NOx conversion of 80 % is required, the maximum space velocity needs to be around 50,000-60,000 h for this specific system. 

Although the data in Fig. 3.10 indicate that the space velocity should be as low as possible in order to maximize NOx conversion, in practical application this may not be feasible. A very low maximum space velocity will mean a large SCR catalyst volume, which will add weight to the tmck and the SCR catalyst will occupy a larger space on the tmck, meaning that the available space on the tmck 

Examples of space velocities used for heavy-duty diesel applications with vanadia SCR catalysts are in the range 20,000-70,000 h [6, 18, 34, 35]. Havenith et al. [6] used 51 dm of washcoated vanadia/alumina SCR catalyst volume, corresponding to a space velocity of 28,000 h for a 12 1 heavy-duty engine, van Helden et al. [35] used 34 dm of washcoated vanadia SCR catalyst volume (space velocity 45,000 h ) for a 12.0 and a 12.6 1 heavy-duty engine. Hofmann et al. [36] used the same SCR catalyst volume (34 dm ), but with a fully extruded vanadia SCR catalyst for a 12 1 heavy-duty diesel engine. 

Common cell densities used for heavy-duty diesel applications with vanadia SCR catalysts are 300 cpsi [19, 36] and 400 cpsi [6, 18, 35, 37, 40]. For these cell densities, the wall thicknesses for cordierite substrates range typically from 4 mil (100 pm) to 8 mil (200 pm) [18, 19, 37, 41]. In Fig. 3.12, with wall thickness is here considered the total wall thickness resulting from the substrate including the catalyst washcoat. The washcoat thickness for a coated vanadia SCR catalyst depends on the washcoat loading and could range from 20 to 100 pm [37]. For a washcoated SCR catalyst, the wall thickness could of course be reduced either by decreasing the inert substrate wall thickness or reduce the washcoat thickness as long as this does not effect the catalyst performance. 

As mentioned above, the NOx from the diesel engine exhaust consists to ca 90 % of NO, which means that the reaction will proceed to the largest extent via the NO + NH3 reaction (Rl). However, reaction with equimolar NO + NO2, reaction (R2), has a higher reaction rate compared to reaction (Rl) [42]. This means that if a part of the NO in the engine exhaust could be converted to NO2, the rate of the SCR reaction would increase. A way of doing this is to add an oxidation catalyst that oxidizes NO to NO2 upstream the SCR catalyst. The layout of this is shown in Fig. 3.13. 

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