Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1999-08-20
2001-03-20
Beck, Shrive (Department: 1764)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C422S168000, C423S235000, C423S239100
Reexamination Certificate
active
06203770
ABSTRACT:
TECHNICAL FIELD
The invention relates to means and methods that enable the safe and reliable reduction of nitrogen oxides (NO
x
) emissions while permitting a diesel or other lean-burn engine to operate efficiently.
Diesel and lean-burn gasoline engines provide advantages in fuel economy, but produce both NO
x
and particulates during normal operation. When primary measures (actions that affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase NO
x
.
Current and proposed regulations challenge manufacturers to achieve good fuel economy and reduce particulates and NO
x
. Lean-burn engines will be necessary to achieve the fuel economy objective, but the high concentrations of oxygen in the exhaust renders typical exhaust gas catalyst systems ineffective for reducing NO
x
.
SCR (selective catalytic reduction) has been available for years in some contexts for reducing NO
x
. To date, however, SCR has depended on the use of ammonia, which has safety problems associated with its storage and transport. Urea and other reagents are safer, but have not been practical for many SCR applications—particularly mobile NO
x
sources—due to the difficulty in converting them from solid or solution form to active gaseous species, typically NH
j
and HNCO radicals.
There is a current need for a safe, economical and effective answer to the problems associated with SCR, particularly for mobile diesel and other lean-burn engines.
BACKGROUND ART
Where SCR catalysts are employed to limit NO
x
emissions from diesel engines, one has to deal with either the dangers of ammonia or a risk of fouling the catalysts under most conditions. In this regard, see R. J. Hulterman: A Selective Catalytic Reduction Of NO
x
from Diesel Engines Using Injection Of Urea: Ph.D. thesis, September 1995. Hulterman describes a number of technical challenges including clogging of atomizers, decomposition problems and system dynamics.
The limited attempts to use urea SCR for diesel engines have required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust, as disclosed in U.S. Pat. No. 5,431,893, to Hug, et al. Equipment of this type highlights the known problems with urea.
Urea takes time to break down in hot exhaust gases and may cause nozzle plugging. To protect an SCR catalyst from fouling, Hug, et al., propose bulky equipment. In addition, this disclosure highlights the necessity of maintaining the urea solution at a temperature below 100° C. to prevent hydrolysis. They propose the use of moderate urea pressures when feeding the urea and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged. The nozzles employed by Hug, et al., are apparently capable of producing moderately-fine sprays, the dispersion of which is aided by auxiliary air, but the droplets are still so large that a large pyrolization channel is required. Moreover, they employ dilute solutions that require significant heating to simply evaporate the water. See also. WO 97/01387.
In European Patent Specification 487,886 A1, urea SCR is said to be improved by obtaining quantitative hydrolysis of urea outside the supply lines. The disclosure coils for spraying fine droplets of aqueous urea onto a specially-formulated catalytic surface. The objective of the disclosure is to avoid pyrolysis and its potential for solid deposition, while achieving hydrolysis of urea to ammonia and carbon dioxide.
In European Patent Specification 615,777 A1, there is described an apparatus that feeds solid urea into a channel containing exhaust gases, which are said to be hydrolyzed in the presence of a catalyst. For successful operation the disclosure indicates that it is necessary to employ a hydrolysis catalyst, compressed air for dispersion of fine solids, means for grinding the urea into fine solids and a coating to prevent urea prills form sticking together. The disclosure notes that if the inside of the catalyzer and the nozzle tip only were coated with the catalyst, corrosion and deposition occurred. Despite achieving the goal of removing water from the process, the specification introduces solid urea into the gas stream—possibly depositing urea on the SCR catalyst.
Each of the points of caution by these references about the difficulties of using urea with SCR systems, especially for mobile sources, illustrates the trouble and lack of practical success the art has had and continues to have.
The art is awaiting the development of a process and apparatus that would permit the use of urea or other reagent in an SCR process simply, reliably, economically and safely.
DISCLOSURE OF INVENTION
It is an object of the invention to provide a safe, reliable SCR system for reducing NO
x
emissions from an internal combustion engine.
It is another object of the invention to eliminate the safety problems associated with the storage and handling of ammonia.
It is another object of the invention to eliminate wetting or fouling of SCR units while permitting the use of urea.
It is another object of the invention to eliminate the problem of line and injector plugging associated with aqueous urea for mobile applications.
It is another object of the invention to provide a simple, robust, SCR system capable of rapid response for effectively and reliably reducing NO
x
emissions from internal combustion engines, especially in mobile applications.
It is a yet further and more specific object of the invention to enable gasifiying urea by pyrolysis prior to introduction into an exhaust, thus facilitating close coupling of reagent injection means and the SCR catalyst.
It is yet another specific object of the invention to provide a simple mechanical device for accomplishing the above objects and preferably to enable close coupling of the reagent injection means and the SCR catalyst, thus permitting the placement of urea injection equipment at a greater variety of positions.
These and other objects are achieved by the present invention, which provides an improved process and apparatus for NO
x
reduction. The process, in one of its aspects, comprises: spraying an aqueous solution of urea onto heated surfaces within a pyrolysis chamber having openings therein to permit discharge of gases generated by pyrolysis of the urea and vaporization of water; maintaining the solution in contact with said heated surfaces for a time sufficient to evaporate water from the solution and pyrolyze the urea; introducing gaseous products discharged from the pyrolysis chamber into exhaust gases from the lean-burn engine; and contacting the exhaust gases containing the gaseous products discharged from the pyrolysis chamber with an SCR reactor at an exhaust gas temperature effective for selective catalytic reduction,
In one aspect, the apparatus of the invention for reducing the emissions of NO
x
from a lean-burn engine having associated therewith an exhaust system having an exhaust passage leading to an SCR reactor effective for selective catalytic NO
x
reduction, comprises: a pyrolysis chamber for pyrolizing urea fed as an aqueous solution including heated internal surfaces and having openings therein to permit discharge of gases generated by pyrolysis of urea and vaporization of water; means for passing the gases generated by pyrolysis into the exhaust gases; and means for passing the exhaust gases containing the gases generated by pyrolysis to an SCR reactor.
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Balles Eric N.
Hofmann John E.
Peter-Hoblyn Jeremy D.
Tarabulski Theodore J.
Valentine James M.
Beck Shrive
Clean Diesel Technologies, Inc.
Doroshenk Alexa A.
Ware Fressola Van der Sluys & Adolphson LLP
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