The new soot filter consists of series of perforated ceramic foams. The surface of the ceramic is impregnated with a catalyst on which the incoming soot particles are burnt and released as gases. Ash particles from impurities in the diesel, which enter the filter with the soot, remain in the pores of the ceramic foam plates. The ceramic foam can absorb a large quantity of ash before it risks becoming blocked and can therefore be used for a long time. The filter is highly suitable for removing soot from so-called heavy diesel oil. Heavy diesel oil is a heavy fuel containing many minerals and metals which produce ash. Existing filter systems would become blocked if used with heavy diesel oil.
As the filter is built from separate filter plates instead of a single block, it is almost indestructible. Even if all of the plates were to break, the filter function remains intact. An inbuilt open canal prevents the filter from becoming blocked. However, a disadvantage of the design is that it requires a lot of space. This means it is not suitable for cars but it can be used for ships, which often use heavy diesel oil, as equally fixed motors or trains.
The design of the new filter is partly based on studies concerning the form and size of soot particles. Diesel particles have a structure which consists of branched lumps of soot chains. Research has shown that due to this fractal structure, diesel particles are in fact up to ten times bigger than is indicated with measuring instruments. This was established with the aid of an electron microscope. Accordingly the behaviour of the soot particles relevant to the filtering is different than was thought; they diffuse more slowly but are more likely to stick to the filter.
With effect from 2005, the statutory health standards for soot emissions from cars will be just 0.025 gram per kilometre. Due to improved insights into the performance of measuring instruments, it will in future be possible to draw up legislation based on the number and size of the particles.
The new filter distinguishes itself from current filters because the reliability was the starting point for the design rather than the filtration efficiency. In commercially available filters the opposite approach is used.
Further information can be obtained from Coen van Gulijk (Reactor and Catalysis Engineering, Delft University of Technology, now at TNO Prins Maurits Laboratory), tel. 31-15-284-3283, fax 31-15-284-3963, e-mail gulijk@pml.tno.nl. The defence of the doctoral thesis will take place on 19 September 2002. Mr Gulijk's supervisors are Prof. Jacob A. Moulijn and Dr Michiel Makkee.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).