Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
1999-03-26
2002-01-15
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C423S212000, C423S213200, C423S213500, C423S244010, C423S244090, C423S244100
Reexamination Certificate
active
06338831
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a storage material for sulfur oxides that contains a magnesium-aluminate spinel (MgO.Al
2
O
3
) and can be used as a so-called “sulfur trap” to remove sulfur oxides from oxygen-containing exhaust gases of industrial processes. In particular, it can be used for the catalytic exhaust gas purification of internal-combustion engines to remove the sulfur oxides from the exhaust gas in order to protect the exhaust gas catalysts from sulfur poisoning.
The principal harmful substances contained in the exhaust gas from internal-combustion engines are carbon monoxide (CO), uncombusted hydrocarbons (HC), and nitrogen oxides (NO
x
). In addition, the exhaust gas contains small amounts of hydrogen (H
2
) as well as sulfur oxides (SO
x
) that originate from the sulfur content of the fuel and the lubricating oils of the engine. Using modern exhaust gas catalysts, a high percentage of the harmful substances, with the exception of the sulfur oxides, in stoichiometric operation of an internal-combustion engine, can be reacted into the innocuous components water, carbon dioxide and nitrogen. Catalysts developed for the exhaust gas purification of stoichiometrically-operated internal-combustion engines are termed “three-way catalysts.”
Modern internal-combustion engines are increasingly operated with lean air/fuel mixtures to save fuel. While the purification of the exhaust gases of stoichiometrically-operated internal-combustion engines has reached a very high level, the purification of the exhaust gases of lean-burning internal-combustion engines still constitutes a great problem. For the major duration of their operation, these internal-combustion engines work with normalized air/fuel ratios greater than 1.3. Their exhaust gas contains about 3 to 15% by vol of oxygen. The normalized air/fuel ratio &lgr; designates the air/fuel ratio standardized to stoichiometric conditions.
Heavily oxidizing conditions are consequently present in the exhaust gas of lean-burning internal-combustion engines. Under these conditions, the nitric oxides in the exhaust gas can no longer be converted to innocuous nitrogen in a simple manner.
To solve this problem, so-called “nitric oxide storage catalysts” have inter alia been developed that oxidize the nitric oxides under lean exhaust gas conditions into nitrogen dioxide and store this in the form of nitrates. After the storage capacity of the catalyst has been reached, it is regenerated. This occurs by enriching the exhaust gas and optionally by raising the exhaust gas temperature. This decomposes the stored nitrates and releases them into the exhaust gas stream as nitrogen oxides. The released nitrogen oxides are then reduced to nitrogen at the storage catalyst with oxidation of the reductive components (hydrocarbons, carbon monoxide and hydrogen) contained in the rich exhaust gas. The storage catalyst hereby regains its original storage capacity. A storage cycle of this type lasts about 60 to 100 seconds, about 0.5 to 20 seconds being needed for the regeneration.
The mode of operation and composition of nitrogen oxides storage catalysts are known, for example, from EP 0 560 991 B1. As storage material, these catalysts contain at least one component from the group of alkali metals (potassium, sodium, lithium, cesium), the alkaline earth metals (barium, calcium) or the rare earth metals (lanthanum, yttrium). The storage catalyst contains platinum as a catalytically active element. The task of the catalytically active components is, on the one hand, to oxidize the nitrogen oxides in the exhaust gas to nitrogen dioxide under lean conditions and to reduce the released nitrogen oxides to nitrogen under rich exhaust gas conditions.
A major obstacle to the use of nitrogen oxides storage catalysts is the amount of sulfur oxides contained in the exhaust gas, since these are also oxidized at the storage catalyst under lean exhaust gas conditions and react with the storage components to form thermally very stable sulfates that cannot be destroyed during the normal regeneration of the storage catalyst. The storage capacity of the storage catalyst is thus reduced with increasing duration of operation since the storage components are blocked by sulfates.
The storage of nitrogen oxides and sulfur oxides on a storage catalyst displays pronounced temperature dependence. Storage and release of the nitrogen oxides only occur in a narrowly limited temperature interval (temperature window) that lies, for example, between about 200 and 500° C. in the case of the frequently used alkaline earth metal oxides. The lower temperature limit is kinetically determined, whereas the upper limit temperature is given by the thermal stability of the nitrates formed. The sulfates of the alkaline earth metal oxides are only decomposed at still higher temperatures under reducing exhaust gas conditions.
To prevent the storage catalyst from being poisoned by sulfates, EP 0 582 917 A1 proposes to reduce the poisoning of the storage catalyst with sulfur by means of a sulfur trap inserted in the exhaust gas stream upstream of the storage catalyst. Alkaline metals (potassium, sodium, lithium and cesium), alkaline earth metals (barium and calcium) and rare earth metals (lanthanum, yttrium) are proposed as storage materials for the sulfur trap. Here, the sulfur trap additionally comprises platinum as catalytically active component.
It is, however, a disadvantage of the proposal of EP 0 582 917 A1 that no desulfurizing of the sulfur trap is provided. In other words, after the storage capacity of the sulfur trap has been reached, the sulfur oxides contained in the exhaust gas pass through the sulfur trap unhindered and can poison the downstream nitrogen oxide storage catalyst.
EP 0 625 633 A1 provides an improvement to this concept. According to this document, a sulfur trap is also disposed in the exhaust gas stream of the internal-combustion engine upstream of the nitrogen oxides storage catalyst. This combination of sulfur trap and nitrogen oxides storage catalyst is operated in such a manner that, under lean exhaust gas conditions, sulfur oxides are stored on the sulfur trap and the nitrogen oxides on the nitrogen oxides storage catalyst. Periodic modification of the exhaust gas conditions from lean to rich decomposes the sulfates stored on the sulfur trap to sulfur dioxide and the nitrates stored on the nitrogen oxides storage catalyst to nitrogen dioxide. Herein, however, there is a danger of sulfur dioxide and nitrogen dioxide reacting together over the nitrogen oxides storage catalyst to form sulfur trioxide and nitrogen monoxide and of sulfur trioxide being stored on the nitrogen oxides storage catalyst in the form of sulfates.
As an alternative hereto, it is possible to provide for the exhaust gas temperature to be raised in order to desulfurize the sulfur trap to values that lie above the limit temperature of the storage catalyst for the storage of the nitrogen oxides. This ensures that no stored nitrogen oxides remain on the storage catalyst during the desulfurizing of the sulfur trap. In this case the above-described reaction of sulfur dioxide with the nitrogen oxides cannot occur. This does require, however, that the sulfur oxides are only released from the sulfur trap above a specific exhaust gas temperature that, taking a possible temperature difference between the sulfur trap and the storage catalyst into account, lies above the upper limit temperature of the storage catalyst.
The requirements to be fulfilled by the storage materials for the sulfur trap during application in the processes described demand a high storage capacity, a temperature T
S,DeSOx
(desulfurizing temperature) for the commencement of desulfurizing that can be adapted by specific measures to the needs of the nitrogen oxides storage catalyst and the temperature conditions in the exhaust gas installation, as well as a highest possible decomposition rate for the sulfates above the desulfurizing temperature T
S,DeSOx
.
It is an object of the present invention to provid
Domesle Rainer
Göbel Ulrich
Kreuzer Thomas
Lox Egbert
Strehlau Wolfgang
Degussa - AG
Griffin Steven P.
Ildebrando Christina
Smith , Gambrell & Russell, LLP
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