Particle removal Particulate material emissions

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 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 mixture of flue gas and dry particles (including fly ash) flows out of the top of the reactor into a cyclone separator. Most (about 90%) of the entrained particles are removed in the cyclone. A major portion of the collected material is recycled to the reactor, while the balance is discharged as byproduct. Gas from the cyclone next passes through a high efficiency particulate collector system then to the stack. Material from diis collector is added to the cyclone discharge to form the total byproduct stream. The process has been used in several waste incineration units in Europe, and a 10 MW demonstration plant was started up at TVA s National Center for Emissions Research in November 1992 (Aiipol, 1993). 

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