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Diesel Exhaust Sulfur Trap Test Development, 08-9122 Printer Friendly VersionPrincipal Investigator Inclusive Dates: 02/01/99 - 05/26/99 Background - During the last three decades, United States regulatory agencies have worked to reduce atmospheric pollution by limiting the amount of hydrocarbons (HC), particulate matter (PM), carbon monoxide (CO), and oxides of nitrogen (NOx) that can be emitted from passenger cars and trucks. As the standards become stricter, the amounts of permissible emissions are steadily being reduced. Emission levels from modern vehicles are significantly lower than they were in the 1970s, but even lower levels are being mandated. Diesel engines are particularly efficient, and are widely used in trucks, buses, and other large vehicles in which fuel economy is important. Although diesel engines produce little HC or CO emissions, they emit significant levels of PM. NOx levels are not high, but they are difficult to reduce with standard exhaust aftertreatment technologies. Diesel exhaust is net lean (has a surplus of oxygen), and standard NOx-reducing technologies do not work in a lean exhaust. New technologies to reduce NOx in diesel exhaust appear to work well when the exhaust gas contains no sulfur. However, diesel fuel contains sulfur, which enters the exhaust gas and potentially poisons the emissions control devices. To assist these devices, regulations are being proposed to reduce the amount of allowable sulfur in diesel fuel. Although 30 parts per million (ppm) was initially suggested, current data indicate some catalysts have almost zero tolerance for sulfur, so zero sulfur diesel fuel is being considered. Production of diesel fuel without any sulfur could be technologically challenging and costly. Taking an alternative solution, SwRI designed an exhaust system that uses sulfur traps (also called adsorbers or scavengers) to remove the sulfur from exhaust gases before it impinges on the aftertreatment device. This SwRI-developed system was identified as PASS, standing for Protection of Aftertreatment Systems from Sulfur. Approach - The purpose of this quick-look project was to generate proof-of-concept data that could be used to promote a full-scale consortium project to develop the PASS system. Simple lean and rich sulfur traps were formulated and prepared to show sulfur storage and release characteristics. Diesel exhaust emissions data were analyzed, and a suitable synthetic gas mixture was obtained. A basic sulfur storage and release test procedure was defined using a synthetic gas reactor, and a Fourier Transform Infra-Red spectrometer was used to measure the gas concentrations. Testing was performed to validate the test procedure and to generate the desired data. Accomplishments - Tests were performed at different trap temperatures and synthetic exhaust gas space velocities, and two different reducing agents were studied for the rich portion of the test. The tests successfully demonstrated that sulfur dioxide (SO2) can be trapped and stored on the lean trap under lean conditions and later released under rich purge conditions. They also showed that sulfides formed under rich conditions can be stored on the rich trap and released as SO2 under subsequent lean conditions. SO2 storage efficiencies of up to 93 percent were achieved, with effective storage demonstrated for a minimum of 45 minutes on stream. The project satisfactorily validated the PASS concept, and a consortium to fully develop PASS is being formed.
The PASS system removes sulfur from diesel exhaust by trapping the sulfur species. The sulfur is then bypassed around the aftertreatment device using a purge cycle and exhaust gas bypass. Engines, Fuels,
Lubricants, and Vehicle Systems Program |
