Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2000-02-22
2001-11-06
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S274000, C060S286000, C060S297000, C060S303000
Reexamination Certificate
active
06311484
ABSTRACT:
This invention relates generally to NOx emissions and more particularly to systems for reducing NOx emissions in mobile or vehicular applications.
The invention is particularly applicable to and will be described with specific reference to a system for reducing NOx transient emissions in vehicles powered diesel engines. However, those skilled in the art, will recognize that the invention has broader application and could be used in mobile applications powered by gasoline engines operated at “lean burn” conditions.
BACKGROUND OF THE INVENTION
This invention is directed to the removal of nitrogen oxides (NOx) from the exhaust gases of internal combustion engines, particularly diesel engines, which operate at combustion conditions with air in large excess of that required for stoichiometric combustion, i.e., lean. It is well known that fuel efficiency improvements in excess of 10% can be achieved in gasoline engines operated at “lean burn” conditions when compared to today's engines which cycle the air to fuel ratio about stoichiometric. Diesel engines, by their nature, operate at lean conditions and achieve 20-30% better fuel economy than stoichiometric gasoline engines.
Unfortunately, the presence of excess air makes the catalytic reduction of nitrogen oxides difficult. Emission regulations impose a limit on the quantity of specific emissions, including NOx, that a vehicle can emit during a specified drive cycle such as an FTP (“federal test procedure”) in the United States or an MVEG (“mobile vehicle emission group”) in Europe. The regulations are increasingly limiting the amount of nitrogen oxides that can be emitted during the regulated drive cycle.
There are numerous ways known in the art to remove NOx from a waste gas. This invention is directed to a catalytic reduction method for removing NOx. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Two types of catalytic reduction are practiced. The first type is non-selective catalyst reduction (NSCR) and the second type is selective catalyst reduction (SCR). This invention relates to hydrocarbon (HC) lean-NOx reaction which can be either NSCR or SCR.
In the selective catalyst reduction method, a reducing agent or reductant is supplied to the exhaust stream and the mixture is then contacted with a catalyst. A common nitrogen oxide reducing agent typically used in industrial processes is urea or ammonia, which despite the number of prior art automotive patents, is not favored for vehicular applications because of the infrastructure required for reductant sale to the public. Additionally, any SCR method using a separate reducing agent requiring separate on-board holding tanks and environmental provisions (such as provisions to keep the tank from freezing) present difficult engineering problems to overcome.
Perhaps one of the more sophisticated approaches to using urea/ammonia system in a mobile application is disclosed in a series of patents which include U.S. Pat. No. 5,833,932 issued Nov. 10, 1998; U.S. Pat. No. 5,785,937, issued Jul. 28, 1998; U.S. Pat. No. 5,643,536, issued Jul. 1, 1997; and U.S. Pat. No. 5,628,186, issued May 13, 1997. While these patents discuss reducing reagents in a general sense, they are clearly limited to urea/ammonia reductants. According to this system, a catalytic converter having composition defined in the '932 patent, has a reducing agent storage capacity per unit length that increases in the direction of gas flow. This allows for positioning of instrumentation along the length of the catalyst as disclosed in the '536 patent to determine the quantity of ammonia stored in the catalyst. The catalyst is thus charged with the reducing agent such that transient emissions can be converted by the reducing agent stored in the catalytic converter. As explained in the '186 patent, should the vehicle experience a sudden increase in load or acceleration, and without having to wait for an increase in the temperature of the catalytic converter, the stored reducing agent is utilized to reduce NOx, thereby preventing overloading of the catalytic converter, i.e., ammonia breakthrough. The ammonia is metered on/off by the length sensors to “charge” the catalyst with stored ammonia. On acceleration, the metering is stopped to prevent ammonia slip (col.
7
, '536 patent).
Urea/ammonia SCR systems are characterized in that the catalysts have the ability to store ammonia at temperatures of the exhaust gases, at least at the relatively low operating temperature ranges of exhaust gases produced by diesel engines. Ammonia SCR systems can therefore be developed by catalyst sizing and ammonia slip control techniques (such as described above) to assure a sufficient quantity of ammonia is present to reduce the NOx emissions generated by the engine and particularly the increased NOx emissions produced, transiently, during engine acceleration or engine load increase periods.
Because of the infrastructure limitations of a urea/ammonia SCR system in mobile applications, there is prior art for the use of hydrocarbons (HC) as a selective reducing reagent for NOx emissions. While reducing catalytic converters (principally base metal zeolites, copper or cobalt ZSM-5 for high temperatures) are able to reduce NOx emissions in the presence of HC at relatively high temperatures (300 to 450° C.), they are not able to store and release the HC at the higher temperatures. It is well known that HC can be adsorbed in zeolite based catalysts at temperatures below 200° C. which are then desorbed at temperatures of about 200° C. or higher (see any number of HC trap patents, for example, U.S. Pat. No. 5,804,155 incorporated by reference herein and SAE paper No. 950747 incorporated by reference herein). There is also prior art for use of HC as a non-selective reducing agent or reductant for NOx emissions. The reducing catalytic converters in this case are precious metal based catalysts and more usually platinum zeolites typically based on Pt ZSM-5 which are active for NOx reduction at lower temperatures (180 to 250° C.).
Because normal operating temperatures of diesel engines produce exhaust gas temperatures above 200° C., it is not usually possible to store and release HC as in ammonia systems. This is a fundamental difference between ammonia based SCR systems and HC based reaction systems.
Despite this fundamental distinction, there is a segment of the prior art that teaches HC can be adsorbed and desorbed (stored and released) at temperatures which include a portion of the normal operating range of the diesel engine. This conclusion appears to be based on the observation that zeolite containing catalysts show better NOx reduction conversion percentages than non-zeolite containing catalysts.
In Mercedes-Benz U.S. Pat. No. 5,935,530, issued Aug. 10, 1999, a three stage catalytic converter is disclosed having an intermediate section which is said to store HC when the engine runs at reduced load and release the stored HC when the engine is at load so that the secondary injection of HC would not have to change in synchronization with the changing engine load (col.
6
). The known adsorber catalyst is defined to include a precious metal catalyst. The data disclosed in the '530 patent is based on an artificial gas composition heated at various temperatures and to which a fixed propane/propene ratio is added. The data shows, (as noted in the SAE references), that for a given temperature range, propene-propane will achieve a high NOx reduction. However, there is no evidence that transient NOx emissions can be controlled by this catalyst design.
Johnson Matthey U.S. Pat. No. 5,943,857 issued Aug. 31, 1999, is also directed to a storage of HC, but storage occurring below a temperature range of 190° C. and a desorbtion of the stored HC at temperature ranges stated to be at 198° C. to 200° C. The '857 patent shows NOx reduction levels achieved between catalyst with and without zeolites and shows that zeolite co
Dettling Joseph C.
Roth Stanley A.
Yassine Mahmoud
Denion Thomas
Engelhard Corporation
Negin Richard A.
Tran Binh
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