As the Olympic Delivery Authority begins trials on diesel particulate filters, Colin Sowman finds what advantages they offer and how they are fitted.

Like it or not, there is a very high probability that over the next year or so, some plant on city centre sites in London and elsewhere will have to be fitted with a diesel particulate filter (DPF). The filters are specified in the Best Practice Guidance (BPG) and when fitted to the exhaust system they cut by more than 85% the output of small soot particles called PM10s, which can cause respiratory and cardiovascular problems.

Diesel exhaust from plant is not the only source of PM10s from a construction site and the BPG includes advice on dust suppression by paving haul roads, spraying demolition work, storing materials and so on.

However, with London and other cities around the UK striving to meet EU air quality targets or face heavy fines, it’s easy to see why local authorities are so desperate to be seen to do something about improving air quality. So long before 2011, when European legislation cuts allowable PM10 emissions from plant by 90%, some local authorities may impose the requirement to retrofit DPFs to machines on certain sites.

The technology is not new; DPFs have been mandatory in Switzerland since 2003 and even in the UK they have traditionally been fitted to machines working in confined spaces like tunnels or within buildings. More than 10,000 DPFs have been fitted to plant worldwide, but if you don’t have a machine with a filter and a customer requests one, where do you start? Well if the requirement is to conform to the BPG, then start with the Energy Saving Trust, which registers all approved manufacturers and filters.

Passive or active?

Having contacted a DPF manufacturer or installer, the first decision is whether to fit a passive or active system, says Dr Richard O’Sullivan, commercial director of DPF 
manufacturer Johnson Matthey.

Over time, all DPFs become partially blocked by the captured particulates and these must be burnt off in a process known as regeneration. Passive systems do this automatically because they get enough heat from the exhaust to start the regeneration and are, to a large extent, fit-and-forget.

However, regeneration of active systems has to be initiated by the operator on a regular basis (say at the end of each shift or during the lunch break), which can involve plugging it into the mains, activating a fuel-injected burner system, or removing the element and putting it in a special ‘oven’.

While a passive system is desirable, it is not always suitable because of the work the machine undertakes. The exhaust temperature may need to be as high as 600ºC for several minutes before a passive system spontaneously regenerates (although this can be reduced to 400ºC with fuel dosing), so if the engine is idling for extended periods, it can lead to the rapid filter blocking.

In order to decide which system is required, the best approach is to instrument the machine to measure the exhaust temperatures throughout the working day.

Johnson Matthey believes its Continuously Regenerating Trap (CRT) offers a better solution for plant as it only requires an exhaust input temperature of 250ºC for half the operating time to spontaneously regenerate. The system combines a catalyst and a particulate filter. The catalyst splits the nitrous oxides (NOx) in the exhaust into NO2 and NO because while soot needs 600º to burn in oxygen, it can ignite at just 250ºC in NO2 at a 20:1 ratio. This means it can work in the majority of plant items and applications without any operator input.

Repeated regeneration

When a DPF regenerates, a small amount of residue is left behind in the form of non-organic ash. If the engine is running on low-sulphur fuel, most of this ash comes from burning soot from engine oil that has passed into the exhaust. After repeated regeneration, the ash builds up and has to be removed – usually once or twice a year depending on the running hours, duty cycles and how much oil the engine burns.

Specialist firms usually install and clean DPFs and in Johnson Matthey’s case, all off-road aftermarket fitments are undertaken by Gloucester-based Excalibre Technologies. "When we get an enquiry about fitting a DPF to plant, the first thing we do is to check Johnson Matthey’s existing installation designs because they have fitted many machines on the Continent," says Excalibre director Alan Barnard.

Where a design exists, such as with the Volvo L70 wheel loader Excalibre modified to work in the Tyne Tunnel, the kit arrived within a week. Installation usually takes one working day and includes a readout or light and buzzer unit in the cab to warn the operator of a problem such as excessive back pressure.

"If there is not an existing installation, we make up a prototype using bits of pipe, and then produce drawings, which are sent to Germany to have proper pipes and brackets made up," says Barnard. In most cases, the DPF installation is a direct replacement for the standard silencer, which usually means it is close to the engine where the exhaust temperature is highest.

The filter housing is mounted on shock-absorbing feet and plumbed using flexible pipework to ensure the hot ceramic elements are not damaged by vibration. On the practical side, the bigger the filter element, the longer it can run between cleaning cycles. Cleaning is done by undoing a couple of clips and removing the filter element, which is then effectively placed in a vacuum cleaner that reverses the exhaust gas flow to remove the debris. This process normally takes about 30 minutes. Some users prefer to have their filters cleaned at regular intervals; 
others wait until the alerts to initiate regeneration (either manually with an active system or by working at full load for several minutes with a passive unit) become more frequent.

Most DPFs (with, perhaps, the exception of some with non-catalytic sintered elements), require the use of ultra-low-sulphur fuel. Currently, rebated (red) diesel contains up to 1,000 parts per million of sulphur and will rapidly clog the filter, and while it won’t damage the element, it can ruin a catalyst. O’Sullivan calls sulphur "catalyst poison". He says a catalyst will probably recover from a single tank full of today’s red diesel, but any more than that and there is a high risk of permanent damage.

Costs

The Environmental Industries Commission (EIC), the trade body for the likes of the DPF manufacturers, surveyed its members about the cost of installing a DPF system (see below). These prices are indicative and 
while multiple installations may be cheaper, complicated installation could be more expensive.

Warranty info

According to the Environmental Industries Commission, as long as the DPF system stays within the engine/machine manufacturer’s guidelines for back-pressure, its members do not expect there to be an issue with regards to engine warranty. However, this has yet to be put to the test in the UK. Plant manufacturers also have concerns about regenerating DPFs creating a fire hazard (they get hotter than standard exhausts), burn hazards from any exposed pipework and reduced visibility if the filter does not directly replace the standard silencer.

Indicative prices for fitting a DPF

If ultra-low sulphur diesel is not available (although given sufficient notice, most oil companies can supply it), white diesel can be used but may increase running costs. Over the next couple of years, the sulphur content of red diesel will be reduced in 
readiness for DPFs becoming something of a standard fitment beyond 2011 when Stage 3B regulations start to take effect.