Ceramic wall filters

Filtration techniques have been widely explored to remove particulates from diesel exhaust, but, until recently, removing the collected material from, for example, a ceramic wall filter has caused problems associated with the high ignition temperature of soot (about 6(X) C). In some situations the additional temperature rise due to soot oxidation can be sufficient to actually melt the ceramic ... 

Fig. 19. Simulation of soot deposition on a filter wall, (a) Evolution of soot deposits (gray) in the wall (black is solid, white is pore space) and incipient cake formation (b) pressure drop as function of challenge soot mass demonstrating the deep-bed to cake filtration transition (c) visualization of soot deposition in an extruded ceramic (granular) filter wall and (d) development of soot deposits (black) and soot mass fraction in the wall (solid material is gray) to the onset of cake formation. Soot mass fraction scale is from 0 (violet) to the inflow value (red). In (d) the velocity on a section through the filter wall is shown, with overlay of the soot deposit shapes (see Plate 9 in Color Plate Section at the end of this book).

Figure 7 Simplified schematic diagram of a continuously regenerating trap (CRT). The platinum catalyst oxidizes hydrocarbons and carbon monoxide, and also nitric oxide to nitrogen dioxide, which is used to oxidize soot retained in the filter. In the illustration this is a ceramic-wall flow filter, which has alternate channels blocked at the front inlet and rear outlet faces.

The ceramic wall-flow filter is an innovative extension of the extruded honeycomb catalyst support described in Chapter 2. This filter concept, shown in Fig. 5. involves having the alternate cell openings on one end of the unit plugged in checkerboard fashion. The... 

This points out the critical role that mounting plays in ensuring both the mechanical and the thermal durabilities of ceramic wall-flow filters. 

Monolithic diesel particulate filters (DPF) are widely used in diesel particulate emission control. They consist of many parallel channels which are alternately plugged at either end in order to force the exhaust gases through the porous ceramic walls. The particulates are deposited on the inside wall of the inlet channel to form a thin, porous soot bed. Once a sufficient mass of particulates is collected, it is burned off to regenerate the filter. Thus, in order to achieve a successful regeneration, the DPF should operate in the thermal runaway region. 

Figure 14. Operation principle of the ceramic wall flow diesel particulate filter. Reprinted from ref.

Soot emitted from Diesel engines is hazardous for human health since it is made of inhalable particles [1] and contains gases and liquids adsorbed on its smrface, some of which (Polycyclic Aromatic Hydrocarbons) are suspected to be cancerogenic [2]. Virtually, soot-free Diesel exhaust may be obtained combining reduction of soot formation in the combustion chamber with exhaust gas treatment [3]. This latter is generally performed by a ceramic wall-flow filter that collects the carbonaceous particles while the filter regeneration is achieved by post-combustion of collected soot [3, 4]. 

PM traps are typically ceramic wall-flow filters that trap PM rather effectively. PM trap regeneration, on the other hand, has proven to be rather more challenging. Typically an additional source of heat must be added to effect the initiation of oxidative regeneration. Once regeneration is initiated, the exother-micity of the process can (and often does) overheat the filter, causing irreversible damage. Various means may be employed to reduce the required regeneration temperature. 

Extruded cordierite honeycombs also have applications in other fields because of their unique material and structural properties such as high porosity, low thermal expansion, high geometric surface area, and low gas flow restriction [2]. Utilizing their porous ceramic wall as filters, extruded honeycombs can be used as trap oxidizers to eliminate toxic particulate matter from diesel engine exhaust. 

Filtered-Particle Inspection. Solids containing extensive inteiconnected porosity, eg, sintered metallic or fired ceramic bodies formed of particles that ate typically of 0.15-mm (100-mesh) screen size, are not inspectable by normal Hquid penetrant methods. The preferred test medium consists of a suspension of dyed soHd particles, which may be contained in a Hquid vehicle dyed with a different color. Test indications can form wherever suspensions can enter cracks and other discontinuities open to the surface and be absorbed in porous material along interior crack walls. The soHd particles that form test indications ate removed by filtration along the line of the crack at the surface where they form color or fluorescent indications visible under near-ultraviolet light (1,3). 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

The typical regeneration took 10 minutes, although shorter durations also met ERA specifications for an acceptable particulate level. The key advantages of shorter regeneration are significant fuel savings and minimal thermal fatigue of the ceramic filter, both of which arc critical to the viability of a wall-flow particulate trap system. The soot filter... 

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