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contd...
SCR Catalyst System
The basic SCR system using urea consists of three catalysts viz.,
- Hydrolysis catalyst
- SCR catalyst, and
- An oxidation catalyst to oxidize ammonia slip
NOx conversion efficiency can however, be improved at low catalyst temperatures (< 300º C) when all the NOx is converted to NO2 before entering the SCR catalyst. An additional oxidation catalyst therefore, ahead of SCR catalyst is used in the modern SCR systems. A typical SCR system for heavy-duty vehicles is shown schematically in Fig. 6.13. NOx conversions of more than 70 % have been obtained with SCR over the HD driving cycle. On road, over all reductions of close to 68 % have been obtained for heavy duty trucks. Urea consumption is about 5.5% of the fuel consumption. Urea requirements for several thousand kms of operation can be stored on board.
| Figure 6.13 |
Schematic layout of SCR catalyst system using pre-oxidation catalyst. |
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