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Diesel Particulate Filters

Diesel Particulate Filters (DPFs) effectively capture diesel particulate emissions, preventing them from entering the atmosphere.

Diesel engines are indispensable in the transportation, construction, mining, marine and power generation industries, but they produce diesel emissions containing nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM) or soot. This is the characteristic dirty, black smoke we often see in diesel exhaust. Diesel PM can increase the risk of cardiovascular and respiratory diseases and is classified as a human carcinogen.

Wall-flow in a diesel particulate filter

Wall-flow in a diesel particulate filter

Diesel particulate filtration:

  • Channels open on inlet side are closed on outlet side
  • Exhaust is forced through walls
  • PM is trapped in the walls


Diesel PM is composed of solid carbon and ash, a soluble organic fraction (SOF) from unburned fuel or lube oil, and possibly sulfates derived from sulfur in the fuel. Diesel particulate filters are highly efficient in capturing soot and preventing it from entering the atmosphere. However, the soot must be removed from the filter to prevent excessive backpressure on the engine

 

Temperatures at which NO2 and O2 react with soot to form CO2.

Temperatures at which NO2 and O2 react with soot
to form CO2.

Soot can be removed from the filter via reaction with NO2 or reaction with O2. The reaction with NO2 occurs at temperatures typical of diesel engine exhaust and is called passive regeneration because the source of the heat is the exhaust gas itself. An upstream oxidation catalyst is installed to convert NO in the exhaust to NO2 which reacts with soot to form NO and CO2, continuously regenerating the filter.

If the temperature of the exhaust is not sufficient to achieve passive regeneration, active regeneration is required. Active filter regeneration techniques operate by raising the temperature of the filter to about 600°C. At this temperature, the soot collected in the filter will combust rapidly in oxygen. Filters can be heated electrically, with an upstream burner, with fuel injected over an upstream oxidation catalyst or via fuel-borne catalysts. Another option is to remove and clean the soot from the filters externally.