17.7: Diesel Particulate Filters
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An important and demanding filtration requirement is that for automotive Diesel engine exhaust, from which very fine (10-100 nm) carbon particles must be removed. Such particulate is present in many combustion exhausts - candle flames are particularly rich in it - and the level in Diesel exhaust is relatively high. The gas flow rate for a Diesel car is ~250 kg h-1, the exhaust pipe diameter is ~150 mm and a back-pressure above ~100 mbar across the filter would impair operation of the engine. The filter surface area is raised via a honeycomb geometry of the type illustrated below (doi:10.1595/147106709x390977), so that the high flow rate creates only a fairly low flux through the filter wall, Q, of ~0.1 m3 m-2 s-1 (ie a gas velocity of ~0.1 m s-1), and the wall thickness, Δx, is kept low (~1 mm). The Carman-Kozeny equation was used to create the figure below giving the pressure drop as a function of pore size, based on the filter material having the structure of a set of parallel cylinders, with a porosity level of 50%. The plot suggests that pore dimensions finer that a few microns will lead to an unacceptably high pressure drop and in fact DPFs are normally made by lightly sintering together relatively coarse particulate, creating pores with dimensions of tens of microns.
This appears problematic for filtration of substantially sub-micron particles. Fortunately, the carbon particles in Diesel exhaust adhere well to each other, so that a network of them builds up fairly quickly in the pores. When “clean”, however, the filtration efficiency is relatively low, and filters are repeatedly returned to this state, since they need to be “regenerated” every few hundred km (by injecting fuel into the exhaust, so that the accumulated carbon particles are burnt off). This is illustrated in the figure below, which shows experimental data for the pressure drop across a DPF, as a function of mass gain during operation, and the initial filtration efficiency after regeneration.
In practice, there are several factors that influence the choice of material and production route for a DPF, including a need to be resistant to high temperatures and thermal shock. Some details are supplied here.