Passive/Catalytic Regeneration
The active regeneration systems need complex control systems and are expensive. The passive regeneration systems employ catalysts to reduce soot oxidation temperatures to the levels that lie within the normal exhaust gas temperature range. The catalyst is either added to diesel fuel in the form of additives or is impregnated on the surface of the filter substrate. Another approach for passive regeneration uses a special oxidation catalyst in front of the ceramic wall flow particulate filter to promote soot oxidation. This system is known as the continuously regenerating trap (CRT).
Regeneration by Fuel Additives
Several fuel additives based on Fe, Ce, Mn, Zn, Cu and Pb lower the soot oxidation temperature. Cerium and copper based additives in 60 to 100 ppm concentration have been found very effective in lowering soot ignition temperature to about 300º C and soot regeneration has been achieved at temperatures below 400º C. The additives on oxidation produce metal oxides which promote oxidation of soot. For example, cerium fuel additive is converted to cerium oxide on combustion and on reaching the DPF it catalyses soot oxidation. The oxidation mechanism is as below;
1. Oxidation of soot:
2. Oxidation of CO
3. Ce2O3 being an unstable compound gets converted back to CeO2 in the exhaust gas as excess oxygen is available
These reactions are quite fast and are completed within 2 to 6 seconds once the temperature is sufficiently high. The fuel additive based approach for particulate trap regeneration is considered quite promising. The additive can be dosed into the fuel line when required by automatic dosing equipment on board of the vehicle.
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