Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2003-05-02
2004-08-24
Tran, Binh Q. (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, C060S287000, C060S291000, C060S292000, C060S295000, C060S303000
Reexamination Certificate
active
06779339
ABSTRACT:
FIELD OF THE INVENTION
This method provides a technique for on-vehicle NO
X
adsorber desulfation especially for use in NO
X
adsorber catalyst equipped diesel vehicles employing a multi-leg exhaust flow path.
THE PRIOR ART
New emission reduction standards for lean burn heavy-duty diesel engines are to be implemented starting in model year 2007. The new standards will require catalysts and systems that can suppress the emission of oxides of nitrogen (NO
X
) from these engines into the atmosphere. Current NO
X
adsorber catalyst formulations typically contain a combination of one or more of the following substances: alkali metals such as potassium (K), sodium (Na), lithium (Li) and cesium (Cs); alkali earth metals such as barium (Ba) and calcium (Ca); rare earth metals such as lanthanum (La) and yttrium (Y); and precious metals such as platinum (Pt) and rhodium (Rh). The precious metals in the catalyst wash coat oxidize NO and NO
2
to nitrate ion (NO
3
−
) and the nitrate ion is subsequently absorbed by the NO
X
adsorbent (alkali metals, alkali earth metals and rare earth metals) to form stable nitrates. These nitrate ions are then desorbed in a rich exhaust environment (lambda<<1) at normal engine operating temperatures and reduced over precious metal sites to diatomic nitrogen.
It has been found in development testing that the NO
X
storage and reduction capacity of the adsorber decreases over time. The main mechanisms responsible for the decrease in adsorber NO
X
storage and reduction capacity are thermal degradation of the adsorber wash coat and poisoning due to the presence of sulfur in diesel fuel. The sulfur is first oxidized during combustion, forming SO
2
. The SO
2
is then further oxidized to SO
3
and sulfate ion (SO
4
2−
) via the reaction with O
2
−
or O
2−
on the surface of the platinum in the adsorber wash coat. The sulfate ion is then adsorbed by the NO
X
adsorbent (alkali metals, alkali earth metals, and rare earth metals) to form stable sulfates (for example BaSO
4
), reducing the number of sites available for NO
X
adsorption. These sulfates have a higher binding affinity for alkali/alkali-earth/rare earth metals than nitrates, thus requiring temperatures that are much higher than those present in typical diesel exhaust to be desorbed. Higher temperatures, in conjunction with a rich exhaust environment (lambda<<1) are required to remove the sulfate ion from the NO
X
adsorbent. The process of removal of sulfates from NO
X
adsorbers will be referred to herein as desulfation.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for desulfating NO
X
adsorber catalysts in a multi exhaust path flow system utilizing in-exhaust fuel injection and exhaust flow bypass. This method minimizes temperature extremes on the surface of the NO
X
adsorber, while achieving the precise temperatures required for desulfation. Thus, overall thermal degradation of the adsorber catalyst due to high temperatures is kept to a minimum or ultimately eliminated. This method allows the sulfates to desorb from the adsorber catalyst, as H
2
S and SO
2
, at exhaust lambda values <1.
The present invention generates an exotherm on particulates accumulated in a trap upstream of a NO
X
adsorber, convectively transfers heat to the NO
X
adsorber, and minimizes local temperature extremes on the surface of the NO
X
adsorber. This, in turn, reduces the chances for thermal damage, e.g., deactivation of the adsorber NO
X
storage and reduction function due to sintering and migration of the wash coat into the catalyst substrate (i.e., migration of alkali, alkali earth, and rare earth metals). The present invention also allows better control over the desulfation and does not affect the drivability of the vehicle, as has been reported in connection with single leg systems.
More specifically, the present invention splits an exhaust stream into two or more paths or legs, each leg of the system containing of a NO
X
adsorber and a particulate trap, preferably a catalyzed diesel particulate filter (hereinafter “CDPF”), upstream of the NO
X
adsorber. While desulfating one of the multiple flow paths, the desulfating path is bypassed so that only a very small fraction of the exhaust flows through the desulfating NO
X
adsorber. This flow is due to incomplete sealing of the exhaust brake used to shut flow off to the bypassed leg. The incorporation of the perpendicular exhaust bypass loop allows controlled addition of exhaust from the adsorbing leg to the desulfation leg. This addition of exhaust allows for control over the mass of oxygen in the desulfating leg. Reductant is added via secondary fuel injection directly into the desulfating leg, upstream of the CDPF. The oxygen causes an exothermic oxidation of the reductant across the CDPF. The extent of the exotherm is determined by the lambda value in the desulfating leg, which is a product of the amount of reductant and oxygen present in the leg. Lambda can also be defined as the ratio of actual oxygen concentration to the oxygen concentration required for stoichiometry. The exotherm causes a rise in the CDPF temperature which is monitored. In pilot plant experimentation the CDPF temperature was measured by six thermocouples inserted along the CDPF horizontally. The heat is convectively transferred from the CDPF to the NO
X
adsorber catalysts by manipulation of the exhaust bypass flow rate from the adsorbing leg to the desulfating leg. Heat transfer can also occur without using the perpendicular exhaust bypass loop by momentarily opening the desulfating leg to full exhaust flow and then closing off the flow once the heat has transferred. Once the adsorber is heated to the desired temperature, reductant is added to facilitate sulfur release. The perpendicular exhaust bypass loop flow is controlled to maintain an exotherm across the CDPF and to allow heat transfer from the CDPF to the NO
X
adsorber to maintain the desired desulfation temperature.
Accordingly, the present invention provides a method for treating an exhaust gas stream which is in a lean state, fuel-lean of stoichiometric, and which contains NO
X
and SO
2
. The method includes splitting the exhaust gas stream into major and minor exhaust gas portions for flow through at least first and second separate flow paths, each of the flow paths containing a particulate trap and, downstream of the particulate trap, a NO
X
adsorber containing a NO
X
oxidation catalyst and a nitrate adsorbent. The major portion of the exhaust gas is passed in the lean state, for a first period of operation, along at least one of the flow paths and through, in succession, the particulate trap and the NO
X
adsorber to convert NO
X
to nitrate, to convert the SO
2
to sulfate and to adsorb the nitrate and the sulfate on the nitrate adsorbent. Temperatures in the particulate trap and the NO
X
adsorber are monitored. After the first period of operation, the flows of the exhaust gas portions are switched so that the one flow path receives the minor exhaust gas portion for a second period of time and, during at least a part of the second period of time, fuel is introduced into the one flow path, upstream of the particulate trap, for combustion in the particulate trap to produce a fuel-rich, reducing exhaust flow. During that same second period of operation, a bypass portion of the exhaust gas is diverted from another flow path at a point upstream of the particulate trap and is introduced into the one flow path also upstream of its particulate trap. When the temperature of the particulate trap in the one flow path reaches a first predetermined temperature, heat of fuel thereto is discontinued and exhaust gas flow is increased to transfer heat from the particulate trap to the NO
X
adsorber, to raise the temperature of the NO
X
adsorber to a second predetermined temperature for desulfation. The major and minor exhaust gas portions are periodically switched between flow paths so that a second period of operation is effected in one flow path while a first
Bunker Byron J.
Laroo Christopher A.
McDonald Joseph F.
Moss Robert E.
Olson Brian A.
Lorusso, Loud & Kelly
Loud George A.
The United States of America as represented by the Environmental
Tran Binh Q.
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