Catalytic Diesel Filters

By Norman G. Clark

Norman G. Clark - Nett Technologies is a manufacturer of catalytic emission control products specializing in exhaust pollution control for construction and mining machinery, industrial forklifts, generator sets, urban buses and trucks.

A catalytic converter is a device that uses a catalyst to convert three harmful compounds in exhausts into harmless compounds, such as water and carbon dioxide. A catalyst increases a natural reaction.

The three exhaust compounds are:

  • Hydrocarbons – in the form of unburned gasoline
  • Carbon Monoxide – formed by the combustion of gasoline
  • Nitrogen Oxides – created when the heat in the engine forces nitrogen in the air to combine with oxygen

Carbon Monoxide is a fatal poison for any air-breathing animal. (CO)

Nitrogen Oxides leads to smog and acid rain. (NOx)

Hydro Carbons produce environmentally unsafe smog. (HC)

The catalyst also converts Diesel Particulate Matter (DPM) which consists of soot, ash, dirt, sand and dust.

So A Catalytic Converter heats up and then increases the reactions of CO, HC, NOx and DPM. The higher the temperature, the better the catalyst works. A minimum exhaust temperature of 200°C is needed for “light off” of the catalyst. The higher the temperature of the exhaust is, the more efficient the catalyst will be.

They can be used and are efficient, on all CNG, LPG and Diesel Petrol engines.

How the Filter Works

Nett® diesel filters utilize cordierite or silicon carbide wall-flow monoliths to trap the soot produced by heavy-duty diesel engines. The cylindrical filter element consists of many square parallel channels running in the axial direction, separated by thin porous walls.

The channels are open at one end, but plugged at the other. This way the particle laden exhaust gases are forced to flow through the walls. Gas is able to escape through the pores in the wall material. Particulates, however, are too large to escape and are trapped in the filter walls.

A proprietary catalyst is coated onto the inside surface of the filter monolith. The catalyst lowers the soot combustion temperature allowing the filter to regenerate. The accumulated soot is oxidized in the filter during regular operation of the engine.

Periods with exhaust temperatures of 325-400°C (615-750°F), which are necessary for proper filter regeneration, occur on most heavy-duty diesel engine applications.

The catalyzed filter monolith is wrapped in a fiber mat and packaged into a stainless steel housing, which is installed in the vehicle’s exhaust system.

Efficiency and Pressure Drop

The soot filtration efficiency of the Nett® catalytic diesel filter increases with the soot loading in the unit. Even at low soot loads the filter efficiency exceeds 90% (blue line in the graph).

The visible smoke is completely eliminated by the filter. Due to the presence of the catalyst, reductions in carbon monoxide and hydrocarbon emissions are also observed.

Typical exhaust gas pressure drop on a properly regenerating filter (red line) is between 5 and 10 kPa (20-40" H2O). Applications with higher exhaust temperatures regenerate better, accumulate less soot in the filter, and experience lower pressure drop.

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