Diesel particulate filters (DPF's)

Retrofitting of older equipment with pollution control equipment such as a diesel oxidation catalyst or diesel particulate filter directly onto the engines exhaust system will reduce emissions from construction equipment.

Diesel oxidation catalysts (DOC'™s) are similar to a catalytic convertor as used in on road vehicles and can lead to a reduction on PM emissions of between 20 -“ 40%, HC by 50% and CO by 40%. Catalytic convertors use catalytic chemical conversion to transform CO and unburned HC'™s into non-toxic carbon dioxide and water. This conversion is carried out through a metallic honeycomb substrate coated with platinum, palladium and rhodium (GenCat 2008).

Flow-Through Filters (FTF'™s) or through-wall filters can be used a wide variety of construction equipment and provide greater emission benefits than a DOC. A FTF can reduce VOC and CO emissions by 50 to 89% and PM emissions by approximately 50% . A FTF includes a flow-through catalyst core and is very similar to a DOC, but it uses a different type of core material to hold the catalyst. Different manufacturers use wire mesh, wire fleece, or sintered metal cores, all coated with a precious metal catalyst and packaged into a metal container. As in the DOC the catalyst promotes the oxidation of unburned PM, VOCs and CO in the exhaust stream passing through the device. Due to the core configuration individual PM particles have greater opportunity for contact with the catalyst site than in a standard DOC. FTF'™s require a minimum exhaust gas temperature and this limits their use compared with DOC's and DPF'™s.

Diesel particulate filters (DPF'™s) are ceramic devices that collect PM in the exhaust stream by means of physical filtration; the high temperature of the exhaust heats the ceramic structure and causes the particulates to oxidise into less harmful components, once captured the accumulated deposits must be dealt with is a safe and secure manner. When DPF'™s are used with ULSD reductions in particulate matter of up to 90% can be achieved, with a reduction in both HC and CO emissions of 60-90%.

Retrofit technologies must fit the equipment application. Some technologies have exhaust temperature requirements in order to allow them to achieve the greatest emission reductions. 

Passive diesel particulate filters need to operate above a certain temperature in order to ensure regeneration, prevent the filter from becoming blocked and potentially cause engine damage due to increased backpressure. This should be a consideration when fitting to equipment that has long periods of low-load operating or idling as the required temperatures will not be achieved.

There are two types of DPF, non-regenerative where the filter is removed and replaced with a fresh one at the end of its working life and regenerative where the filter is reused. 
Non-regenerative filters are generally constructed from fibre matting in which materials such as steel wool and fibre glass are used. Housed in a steel canister the particulate matter is trapped within the fibre matting. When full the filter must be replaced with a clean one. These types of filter have a life of around 300 working hours and are therefore best suited for low usage applications or on equipment that is only used for short periods.
Regenerative filters are commonly produced from ceramic materials such as cordierite or silicon carbide. Constructed as a honeycomb monolith, channels are blocked at alternate ends forcing the exhaust stream to flow through the walls between the channels, known as ˜wall flow™. The PM cannot pass through the walls so is deposited within the channels and these deposits are then burnt away. Due to the high temperatures involved in regeneration these filters are best suited to high-use applications where the exhaust gas temperature (EGT) is high.

Diesel particulate filters can be fitted to almost any piece of machinery or vehicle, for on-road or off-road use, that uses a diesel engine.