Passive Regeneration
The term ‘passive regeneration’ means that the accumulated diesel particulate matter
(‘DPM’) is oxidised at the prevailing temperature of the exhaust gas, rather than
due to an external heat source (which is known as ‘active regeneration’). DPM normally
oxidises at about 600 C, however since this temperature is rarely if ever reached
in a diesel exhaust system, emissions engineers have developed techniques to achieve
oxidation at lower temperatures. The two principle approaches to passive regeneration
are as follows:
- Increase the nitrogen dioxide (NO2) content of the exhaust, since NO2 oxidises DPM
at a temperature of about 300 C compared with 600 C if oxygen itself is relied upon.
The required NO2 is produced when nitrogen oxide (NO), which is always present in
diesel exhaust gas, comes into contact with a catalytic coating which is rich in
platinum. This catalytic coating can either be applied directly to the filter or
alternatively a separate catalytic converter can be installed upstream of the filter
which is commonly known as a ‘CRT’ system.
- Introduce a liquid known as a ‘fuel-borne catalyst’ to the fuel supply. This results
in microscopic particles of certain metals becoming embedded in the DPM, which help
to trigger oxidation when the exhaust gas temperature reaches about 350 C.
A comparison of the advantages and disadvantages of the two approaches can be found
in the table below.
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Nitrogen dioxide
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No extra consumable costs. No fuel-borne catalyst dosing system to install or maintain.
Simultaneously reduces CO and HC emissions.
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Process stops if exhaust temperature falls below 300 C, therefore a large proportion
of the duty cycle will need to be at or above this temperature. Requires very low
fuel sulphur levels. Increases tailpipe nitrogen dioxide emissions. Process stops
if catalytic coating becomes completely covered in DPM.
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Fuel-borne catalyst
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Once triggered by a few minutes operation at 350 C or over, the oxidation will be
self sustaining even if the exhaust gas temperature falls. Can tolerate high levels
of sulphur in fuel. No increase in nitrogen dioxide emissions. Operation can be
fine-tuned by varying the dosing.
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Additional costs of installing and maintaining a dosing system for the fuel-borne
catalyst. Consumable cost of the fuel-borne catalyst. Burning of the fuel-borne
catalyst produces ash which necessitates more frequent cleaning of the filter.
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