contd...
Selective Catalytic Reduction (SCR)
The selective catalytic reduction of NOx by ammonia has been applied now for several decades in the stationary systems like gas turbines, utility boilers, diesel engine power plants, and incinerators. Now, SCR is being used in heavy duty diesel vehicles quite widely in Europe to meet Euro 4 and later emission standards. On the road vehicles, urea is used as the carrier of ammonia. Hydrolysis of urea is carried out at first on a catalyst on board of the vehicle to produce ammonia and carbon dioxide. Ammonia then, reacts on the SCR catalyst with the NOx and converts it to nitrogen. Vanadium and titanium oxide mixture (V2O5 + TiO2 + WO3) coated on a ceramic honeycomb substrate of 200- 400 cpsi is used as SCR catalyst. The basic chemical reactions in the urea-SCR process are as follows:
Hydrolysis of Urea:
| (NH2)2 CO + H2O → CO2 + 2 NH3 |
(6.5) |
NOx Reduction:
| 4NO + 4 NH3 +O2→ 4 N2 + 6 H2O |
(6.6) |
| 6NO2 + 8 NH3 →7N2 + 12 H2O |
(6.7) |
Urea concentrations of 30 to 40 % in water solution are stored on board as the temperature of crystallization is the lowest (-11º C) for 33% solution.
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