Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
1999-04-14
2001-09-11
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S177000, C422S222000
Reexamination Certificate
active
06287524
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a plate-type catalytic converter through which a flow medium flows from an inflow side to an outflow side and which includes a stack of first plates and second plates each having a catalytically active surface.
Such a plate-type catalytic converter is used, inter alia, for reducing the content of nitrogen oxides in the flue gas or waste gas of a combustion plant. The selective catalytic reduction (SCR) process is used in that case to convert nitrogen oxides together with a reducing agent introduced into the gas mixture in advance, generally ammonia NH
3
, into water and molecular nitrogen on a catalytically active surface. Instead of ammonia, the use of urea, which is decomposed into ammonia in the flue gas, is also known. For that purpose, the catalytically active material, with which the plates are coated on both sides, is generally formed of titanium dioxide TiO
2
and one or more of the additives tungsten trioxide WO
3
, molybdenum trioxide MoO
3
and vanadium pentoxide V
2
O
5
.
A plate-type catalytic converter is composed of a stack of plates provided with a catalytically active surface and a holding device for the plates. The holding device which is used is generally an element casing in which the plates are evenly spaced and oriented parallel to one another. The element casing usually has a cuboid shape which is open at the ends, allowing a flow medium, for example a flue gas, to flow through the cuboid from the inflow side to the outflow side parallel to the long sides. The plate planes are aligned parallel to the flow direction. A plate-type catalytic converter of that type is disclosed in International Application No. WO 94/26411 A1, corresponding to U.S. Pat. No. 5,820,832.
A plurality of those element casings fitted with catalytically active plates are disposed alongside one another to form a catalyst module. A complete flue-gas cleaning unit in turn includes a number of banks of such catalyst modules disposed one after the other in a flue-gas duct. For example, a DeNOx unit for reducing the content of nitrogen oxides in the flue gas from a fossil fuel-fired power station usually has three to five banks of such catalyst modules.
In order to space the individual plates in an element casing, each plate is usually provided with a bead-like structure running continuously along the plate. In that way, a plurality of separate sub-chambers through which a flow medium flows from the inflow side to the outflow side are created between two directly adjacent plates. There is essentially a laminar flow of the flow medium through those sub-chambers, producing a disadvantageous flow profile for the catalytic conversion of the reactants, for example the nitrogen oxides and the ammonia. The flow medium flowing in the center of a sub-chamber only reaches the catalytically active surface of the sub-chamber through the mechanism of diffusion and not through turbulence. That generally hinders the contact of the reactants with the catalytically active surface of the sub-chambers which is necessary for the catalytic reaction.
International Application No. WO 94/26411 A1, corresponding to U.S. Pat. No. 5,820,832, describes a plate-type catalytic converter which is fitted with first plates, that have virtually linear elevations, beads or similar structures, and in addition is fitted with second plates. The structures of the first plates are aligned virtually parallel to the flow direction, while the structures of the second plates are transverse or inclined to the flow direction. In that way, turbulence can be generated within the sub-chambers between the individual plates. However, that type of catalytic converter has a disadvantage which is that the structures of the second plates cause a high pressure loss in the flow of the flow medium, and that soot and ash particles present in the flow medium can therefore easily settle at some points in the sub-chambers due to its low flow rate. A catalytic converter of that type therefore tends to become blocked.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a plate-type catalytic converter, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has increased catalytic activity over a conventional plate-type catalytic converter of the same physical size, without the disadvantages of pressure loss or the risk of blockage due to deposits in sub-chambers.
With the foregoing and other objects in view there is provided, in accordance with the invention, a plate-type catalytic converter, comprising an inflow side and an outflow side for conducting a flow of a flow medium from the inflow side to the outflow side; and a stack of first plates and second plates each having a catalytically active surface and a leading edge facing toward the inflow side, the leading edges of the second plates set back toward the outflow side relative to the leading edges of the first plates.
The term “leading edges” is taken to mean the plate edges pointing toward the inflow side of the catalytic converter. The invention is based on the consideration that, when the flow medium enters the sub-chambers, turbulence occurs in the flow of the flow medium. Studies have shown that this turbulence is quickly calmed by the uniform cross section of the sub-chambers in the further progress of the flow and disappears after a short flow distance. The inlet zone or turbulence zone within the sub-chambers which is characterized by the turbulent flow is less than 20 cm. Upon progressing further, the flow medium experiences laminar flow through the sub-chambers. The main reason why the laminar flow has an unfavorable effect on the reaction conversion in the catalytic converter is because, due to the flow profile, the part of the flow medium most remote from the sub-chamber surface flows the fastest through the sub-chamber in question, i.e. has the shortest residence time within that sub-chamber. The mechanism of diffusion, which is controlled by time, inter alia, can therefore least take hold for that part of the flow medium. In contrast, the second plates within the catalytic converter, having respective leading edges which, in accordance with the invention, are set back in the direction of the outflow side as compared with those of the first plates, cause a further turbulence zone within the inlet zone, since they disturb the laminar flow which has begun to form between the first plates. This further turbulence zone forms precisely where the flow medium is flowing the fastest. It is therefore more pronounced than the first turbulence zone and in addition occurs where the catalytic reaction of the flow medium has taken place the least. This further turbulence zone is in addition set back in the direction of the outflow side as compared with the first turbulence zone, which begins directly behind the inflow side. The present invention therefore achieves more turbulence as compared with a conventional plate-type catalytic converter and extends the entire inlet characterized by turbulent flow. This increases the catalytic conversion.
In accordance with another feature of the invention, the first and second plates are stacked within a plate stack of the plate-type catalytic converter in such a way that the first and second plates alternate. This produces the greatest possible number of further turbulence zones within the inlet zone of the catalytic converter. In general, a single type of second plates which is shorter than the first plates is sufficient. It is, of course, possible for more than one type of second plates with a different degree of set-back to be provided.
In accordance with a further feature of the invention, the second plates are shortened by a distance by which their leading edges are set back. The plate edges thus form a plane on the outflow side. This means that, firstly, the catalytic converter is not lengthened unnecessarily and consequently its physical volume is not increased, and secondly, the amo
Hums Erich
Klatt Andreas
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Tran Hien
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