Shape up to soot

Construction plant could in future emit cleaner exhaust fumes, thanks to the development of new diesel particulate filter technology.

Diesel particulate filters — first introduced in cars in Europe in 2000 — which are traditionally made from ceramic materials such as silicon carbide, trap and remove soot and dangerous particulate matter from exhausts.

Inside each one is a honeycomb structure with alternate channels plugged at opposite ends. The exhaust gases pass through the open end of a channel, and at the opposite end a plug forces the gases through a porous channel wall. This allows fumes to escape through the neighbouring channel, while trapping as much as 90 per cent of particulates.

Most diggers and construction machines discharge unfiltered fumes into the air because they are custom-made in small batches, and producing filters for their special geometric requirements is not cost efficient.

the Fraunhofer Institute for Ceramic Technologies and Systems, in collaboration with German filter manufacturer CleanDieselCeramics (CDC) has developed a new honeycomb structure which promises to make it cheaper to manufacture diesel soot filters in a variety of shapes.

The filters contain irregular four-cornered shaped honeycomb structures bonded together in a large block. When the filter starts to work and heat up, the bonds expand like joints. A solid ceramic block would break apart under this kind of pressure.

Conventional diesel filters are constructed using rectangular honeycomb structures bonded together in a large block. The drawback of these is that the block has to be ground into a cylindrical shape at the end of production — wasting time and valuable material. The smooth finishing also requires expensive machinery.

'By machining you lose about 20 per cent of expensive material,' said project manager Jörg Adler. 'We wanted to develop a system that would be produced without machining.'

The new honeycomb shape changes the geometry of the filter's channels. With previous designs, fumes flow into the filter through hundreds of square channels criss-crossing the filter. Now fumes can travel through hundreds of smaller triangle-shaped channels — which Adler says enlarges the filter surface.

'It is also a more stable shape than conventional designs,' said Adler, adding that the filter is less sensitive to lateral pressure because the channels lack harsh 90º angles.

The researchers have also tested a silicon carbide ceramic material for the filter. This, said Adler, will allow the size of the pores to be easily and precisely adjusted for optimum filtration.

'As far as the performance and quality of our filter is concerned, we can hold our own against anything on the market,' claimed Adler.

The researchers have tested and optimised the production method on a pilot production line at Fraunhofer, and CDC is building its first plant in Germany this spring. It will annually produce 40,000 filters for installation in machinery as an upgrade kit.

Adler said different systems and materials can be used as filters, but silicon carbide seems to have the advantage at the moment. However, he added that things could change as manufacturers demand more cost-effective and reliable materials as governments increase legal emission standards.

Diesel particulate matter is emitted by all kinds of incomplete combustion (where there is inadequate ventilation) of diesel fuel, and comprises carbon, inorganic oxides and hydrocarbons, some of which are highly toxic.

'Nitrogen oxide and particulate matter in diesel exhaust are considered to be crucial pollutants concerning health,' said Adler. 'To meet future emission standards worldwide, emission control technologies such as diesel particulate trap systems will be necessary.'