Technology▼Solutions NASA innovation helps clean diesel exhaust Tough, new U.S. EPA standards for heavy-duty diesel engines have researchers scrambling to develop novel technologies to cut tailpipe emissions. The dilemma is that conventional catalytic converters, used on cars since 1975, are designed to work at high temperatures, making them unsuitable for the relatively cool diesel exhaust. As a result, scientists are eyeing a low-temperature catalytic converter, originally designed by NASA for CO2 lasers, as an inexpensive way to clean diesel engine exhaust while ensuring reliable engine performance. Diesel exhaust aggravates respiratory and cardiovascular disease and is likely to cause lung cancer in humans, according to the EPA. Heavy-duty trucks and buses today account for about one-third of NOx emissions and one-quarter of particulate matter emissions from cars and trucks, says the agency. To deal with the problem, EPA has required that by 2010, all heavy-duty diesel engines, such as those used in garbage trucks and buses, must emit no more than 0.20 grams per brake-horsepower-hour (g/bhp·h) NOx and 0.01 g/bhp·h particulate matter. These regulations, along with new sulfur standards for highway diesel fuel taking effect in 2006, will amount to a 90% cut in particulate matter and a 95% cut in NOx emissions from current levels. As a result, exhaust emission control devices will be, for the first time, widely needed on diesel engines, says EPA. Because diesel engines and their exhaust differ from gasoline engines, a whole new suite of technologies is required. “NASA’s Low-Temperature Oxidation Catalyst [LTOC] addresses some of the shortcomings of conventional catalysts that we referred to as the cold-start deficiency,” says Jeff Jordan, an analytical chemist with NASA. When drivers start their cars in
the morning, pollutants coming from the engine go straight from the tailpipe into the environment until the catalytic converter warms up, he says. This cold-start period lasts up to three minutes and is responsible for 70% of automobile emissions, says Jerry Summers, senior associate with Environix, Inc., a consulting firm in Wayne, Penn.
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NASA’s LTOC reduces the “lightoff” temperature—the temperature at which the catalytic converter destroys 50% of pollutants—from 300–400 oC to 150–200 oC, Jordan explains. This makes the new catalyst ideal for diesel exhaust, which typically runs around 200–300 oC, Summers says. Although the technology can be applied to conventional gasoline engines, the biggest market for the LTOC is for diesel engines, says Joe Wagner, senior project manager with the New York State Energy Research and Development Authority. In recent tests at the nonprofit Southwest Research Institute (SwRI) in San Antonio, Texas, the LTOC, in combination with two NOx absorbers and one particulate trap, cut diesel emissions by 90%, says Magdi Khair, staff engineer with SwRI, emitting 0.16 g/bhp·h NOx and less than 0.01 g/bhp·h particulate matter,
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which are low enough to meet the new standards. But getting to this point required modifying the original LTOC, developed 20 years ago as a catalytic converter for frigid, deep space lasers using CO2 as the lasing medium, Jordan says. Conventional catalytic converters are a ceramic honeycomb coated with aluminum oxide to boost the number of reaction sites, Jordan explains. A final coat of precious metals, such as platinum, provides active sites to bind pollutants and co-reactants, decreasing the energy of activation needed to oxidize smogforming hydrocarbons to relatively harmless CO2 and H2O. The LTOC is somewhat similar, but replaces aluminum oxide with tin oxide, Jordan says. Tin oxide binds and releases O2 for the needed oxidation reactions. Aluminum oxide doesn’t do this, but platinum does. Thus, the tin oxide also reduces the need for costly platinum. But tin oxide’s instability at temperatures above 700 oC was a barrier to adapting the LTOC for engine emission control, Jordan says. EPA requires catalytic converters to operate at 900 oC. At this temperature, tin oxide is reduced to tin metal, flowing over and engulfing the platinum reaction sites, he says. NASA stabilized the tin oxide with a unique combination of transition metals that also improve the surface area architecture, Jordan says. Airflow Catalyst Systems, Inc., of Rochester, N.Y., is the exclusive licensee for the LTOC for internal combustion applications, says company vice president Hugh Ogle. He expects to have a product on the market by January 2004. “We will be supplying one part of a multicomponent treatment system to meet the new standards,” Ogle says. “The new regulations are very restrictive and tight for diesel engines, and the LTOC, along with other com© 2003 American Chemical Society
ponents, will allow us to develop controls to meet the standards,” Khair adds. For diesel engines, EPA will be certifying an engine plus all of its treatment devices together as meeting the new standards, he says. As a result, Airflow Catalyst Systems has been working with engine manufacturers and running lab tests on engines equipped with the LTOC and other devices, Ogle says. If the LTOC has any role to play, it will be a minor one at the end of an emission treatment system to clean up hydrocarbons, says Joe McDonald, staff engineer with EPA’s Office of Transportation and Air Quality. The main pollutants of concern for diesel engines are particulate matter and NOx, and EPA has already demonstrated that NOx absorbers and catalyzed diesel particulate filters alone can meet the new standards, he says. Furthermore, an EPA report released in October finds that engine manufacturers are making tremendous progress in developing these technologies and are expected to have products that meet the emissions standards by 2007, he adds.
Nothing has been published on the LTOC in the peer-reviewed literature, so it is difficult to say whether it works in the real world, says Bob Sawyer, mechanical engineer at the University of California at Berkeley. “If you can assume that it works at cleaning up hydrocarbons at the end of the treatment system, that could be a positive development, but it would only treat a small percentage of the diesel exhaust problem,” he notes. “The LTOC continuously looks good in testing, and the potential is there for it to provide a breakthrough in catalytic converter technology,” Summers claims. Some of its good characteristics are that it does not convert much SO2 to sulfuric acid and much NO to NO2, which combines with water to form nitric acid and is a problem with conventional catalytic converters. The metal oxides and platinum are applied in a solution phase, which results in very low back pressure on the engine and allows the honeycomb density to be increased from 600 to 900 cells per square inch without decreasing engine efficiency, Jordan says.
The emission values demonstrated in tests at the SwRI by the system including the LTOC are very impressive and encouraging at this early stage of development, says Jerry Martin, spokesman for the California Air Resources Board (CARB). “But the work is not done yet because the manufacturer will want the engine plus its emission control system to produce emissions that are half the level of the new standard in order to provide a safety cushion,” he says. However, there is a lot of time to perfect the LTOC and its companion devices before the new standards come into play, Martin says. “If it is cost-effective and can endure the stress of heavy work for long periods and high temperatures, CARB would welcome the new technology,” he adds. The board believes that breakthroughs in diesel emission controls will soon make diesel engines just as clean as gasoline engines. Because diesel engines are more fuel-efficient than gasoline engines, clean diesel engines could claim a larger part of the American market, he says. —JANET PELLEY
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