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Catalytic Diesel Filters How the Filter Works Efficiency and Pressure Drop Catalyst Types Substrate Types Standard Models How the Filter Works Nett® diesel filters utilize cordierite or silicon carbide wall-flow monoliths to trap the soot produced by heavy-duty diesel engines. The cylindrical filter element consists of many square parallel channels running in the axial direction, separated by thin porous walls. The channels are open at one end, but plugged at the other. This way the particle laden exhaust gases are forced to flow through the walls. Gas is able to escape through the pores in the wall material. Particulates, however, are too large to escape and are trapped in the filter walls. A proprietary catalyst is coated onto the inside surface of the filter monolith. The catalyst lowers the soot combustion temperature allowing the filter to regenerate. The accumulated soot is oxidized in the filter during regular operation of the engine. Periods with exhaust temperatures of 325-400°C (615-750°F), which are necessary for proper filter regeneration, occur on most heavy-duty diesel engine applications. The catalyzed filter monolith is wrapped in a fiber mat and packaged into a stainless steel housing, which is installed in the vehicle’s exhaust system. Efficiency and Pressure Drop The soot filtration efficiency of the Nett® catalytic diesel filter increases with the soot loading in the unit. Even at low soot loads the filter efficiency exceeds 90% (blue line in the graph). The visible smoke is completely eliminated by the filter. Due to the presence of the catalyst, reductions in carbon monoxide and hydrocarbon emissions are also observed. Typical exhaust gas pressure drop on a properly regenerating filter (red line) is between 5 and 10 kPa (20-40" H2O). Applications with higher exhaust temperatures regenerate better, accumulate less soot in the filter, and experience lower pressure drop. Catalyst Types Depending on the application, Nett® particulate filters can be ordered with one of the following catalyst types: Standard catalyst Sulfate-suppressed catalyst The standard catalyst provides the best activity and the lowest regeneration temperature requirements. However, if high sulfur fuels are used, the catalyst may generate sulfate particulates at elevated temperatures. As a general guideline, the standard catalyst should be used in the following applications: All applications with ultra low sulfur fuel (S < 50 ppm). All applications, regardless of fuel sulfur level, where sulfates are not considered a particulate emission and are not regulated. Examples of such applications include underground non-coal mining in the U.S. (particulates measured as total carbon) or occupational health environments in Germany (particulates measured as elemental carbon). The sulfate-suppressed catalyst eliminates the sulfate make at the expense of somewhat increased regeneration temperature requirement, typically by 10-15 °C. This catalyst is used for applications with high sulfur fuel (S > 50 ppm) which are sensitive to sulfate particulate emissions, such as most urban buses operated on high sulfur fuels. Substrate Types Nett® diesel particulate filters are available with two types of substrates: Cordierite substrates Silicon carbide (SiC) substrates Cordierite substrates perform satisfactory in most heavy-duty applications with high exhaust temperature. However, in low-temperature applications which may experience “uncontrolled regenerations”, cordierite is more susceptible for thermal stress related damage. Silicon carbide has higher maximum operating temperature limits and better durability in high thermal stress applications. Disadvantages of SiC include higher weight and higher cost. Standard Models Standard models of cordierite and SiC diesel particulate filters are listed in Table 1 and Table 2, respectively. Filter sizing guidelines in the table are approximate. Sizing for particular engines and applications should be consulted with our office before ordering. Many diesel fleets around the world run older, "high-emitter" engines that aren't suited for retrofits with catalyzed diesel particulate filters (DPFs), or they may have nothing but high-sulfur fuel and thus can't use catalyzed filters. Still other fleets might have relatively newer, cleaner engines and even low-sulfur fuel, but the economic incentives might not be big enough to cover the costs of installing today's DPF systems. More: The cost of replacing an old engine with a new "clean" engine might be cost-prohibitive for some fleets. Still other fleets might consider installing a relatively cheap, uncatalyzed DPF, then use an off-board regeneration system (such as electric heating). But this might require a high-voltage line, or a special oven, or might seem a bit too complicated or complex for some fleets. But a new, simple invention -- selected by United Kingdom's National Endowment for Science, Technology & the Arts (NESTA) for a competitive R&D award -- could help many such fleets take a huge bite out of diesel PM emissions, at an estimated one-quarter the cost of catalyzed DPFs. The so-called Diesel Emission Reduction Technology (DERT) employs ordinary cloth fibers such as cotton, linen or jute, in a series of five to seven corrugated "cassettes" stuffed inside a steel sleeve that would be attached to the end of the exhaust tailpipe. Rather than oxidizing soot as with typical DPFs, this system merely collects the soot for periodic disposal at a central recycling site. The filters could be easily removed, washed and re-used several times, with the water/soot waste product perhaps mixed with boiler fuel for burning, or with asphalt for road-paving. As compared to the rather complex ash-removal/servicing intervals for conventional catalyzed DPFs, these "DERT" cassettes could be easily removed and swapped-out with fresh filters on a weekly or monthly basis, depending upon engine PM emission rate and vehicle mileage. Tests on an engine dynamometer show that the cloth-based DPF does a good job of PM capture (65% with just one cassette, higher with multiple cassettes) and can withstand the relatively low end-of-pipe exhaust temperatures. A remaining engineering challenge is to develop a sealing material that's more resistant to oxidation than the cotton fiber, explains DERT inventor Julian Hasinski of UK-based Aerotech Engineering. The company has paid for patent extension to the U.S "and the patent is centered on the maintenance of a useable temperature range," Hasinski told us. The device has an estimated installation cost of 500 pounds-sterling (about U.S. $820) and the filter itself has a manufacturing cost of about (U.S.)$8-13. Relatively newer vehicles with low engine-out PM emission probably would require changing the leading filter cassette monthly, whereas older, "dirtier" engines could require lead-filter swap-out weekly, Hasinski estimates. Washed filters are expected to be recycled about 10 times. So, filter replacement cost might be as little as $l00-$150/year -- or perhaps double that when including a retail mark-up. At the high-end (more frequent cassette replacements), annual maintenance cost might be around $600/year. While all DPFs have an impact on backpressure to the engine, the key is to avoid excessive backpressure. The DERT device is designed as an "expansion chamber" to avoid such backpressure, and daily or weekly filter inspections would check soot-build-up impact on backpressure. The U.K. already offers tax breaks for clean-diesel retrofit technologies including DPFs, so the DERT system cost would be more than offset after one year, Hasinski says. Fleets in other parts of the world -- where clean-diesel retrofit tax breaks might not exist -- could find the DERT technology a far cheaper alternative than, for example, government-mandated conversions to "clean" technologies such as compressed natural gas (CNG) or LP Gas. It's also a lot cheaper than the capital cost of catalyzed DPFs -- especially important to fleet operators in "developing countries" where money is especially hard to come by. Aerotech now hopes to take the DERT device to field trials with a diesel taxi, to prove durability, efficiency and serviceability. City-vehicle fleets -- both light and heavy-duty -- are the logical target markets initially, because of the government tax incentives, he says. Concurrently, Aerotech is starting to initiate talks with potential licensees, and "I'm quite open to investment suggestions," Hasinski told us. Bio-Diesel, Green/Black-Diesel Filters & Supplies What happens to the millions of gallons of used oil and lubricants that are drained from our nation's cars and trucks each year? The question demands an urgent answer when you realize that one gallon of used oil can contaminate millions of gallons of ground water. Waste oil is free for the taking and should be dealt with by proper burning techniques that save the environment as well as vital energy reserves. Do your part to save the environment while keeping a few more dollars in your pocket |