Diesel Particulate Filter consists of a filter monolith manufactured as an extruded honeycomb structure which contains hundreds of parallel channels. All the channels are plugged at alternate ends, the adjacent channels are open & closed. This gives the open face a ‘chessboard’ appearance. When the exhaust gas and soot pass down the length of each open channel the gases only can pass through the porous wall whereas the carbon soot remains trapped within the structure and therefore filtered.
The next stage in the process is to remove the soot by regeneration of the filter. As the soot progressively builds up in the filter honeycomb it creates addition back pressure which must be reduced so that the filter can work continuously and efficiently while the engine is in operation. There are a number of different methods to initiate the regeneration process. The most widely used and effective for on-road vehicles is the use of a catalyst, A precious metal based oxidation catalyst is coated onto the filter. The catalyst initiates an oxidation reaction with the carbon soot at a temperature of around 350°C.
The benefits of the wall flow filter in a stationary engine application are that they are by far the most effective method of high performance particulate reduction, they are not consumed and they can achieve good sound attenuation eliminating the need for additional silencing.
The advantages of precious metal catalyst regeneration are that there is no additional energy requirements, no external heater or dosing control system and no consumables. In addition to dealing with the particulate the catalyst will also reduce the Carbon Monoxide, Hydrocarbon and the unwanted diesel odours in the exhaust gases. The consequences of this are that vast lengths of exhaust pipework can be reduced or eliminated as vents from gensets can be located in basements or discharge at low levels into the atmosphere. These advantages offer capital cost reductions to the engineer and better aesthetics to the architect.
Specific care has to be taken in the design of a stationary diesel generator when incorporating a particulate wall flow filter system.
The back pressure generated by the filter is considerable and it will account for 50-75% of the total allowable back pressure on a typical stationary diesel engine exhaust installation.
The catalyst & filter option requires that the engine must run at above (350°C) say 60-75% load for at least 20% of its cycle. This will create uncertainty because it relies on the site maintenance engineer when starting the engine on a weekly check to bring the system onto load, if the engine is idling the temperature regime can not be achieved. If the temperature regime is not reached the filter will not regenerate and this could lead to filter blockage and eventual engine stalling. This can be overcome by including loadbanks as an integral part of the original system, therefore allowing the engineer to freely test the engine on load without the unwanted concern of bring the system online.
The type of fuel should be considered, as the system is susceptible to different levels of sulphur in the diesel fuel. Ultra Low Sulphur Diesel (0.005% NFE50 or better) is preferred.