Abstract:
A method and apparatus for mixing at least one fluid stream with a large gas stream flowing in a gas duct, especially for introducing reducing agent into flue gas containing nitrogen oxides. The gas stream is directed against at least one disk-shaped mixer element on an inlet side that is inclined at an angle counter to the direction of flow of the gas stream, wherein eddy-type whirls form at the mixer element. A swirled fluid stream is admixed with the large gas stream downstream of the mixer element.

Description:
BACKGROUND OF THE INVENTION 
     The instant application should be granted the priority dates of Jan. 28, 2006, the filing date of the corresponding German patent application 10 2006 004 068.6 as well as Jan. 26, 2007, the filing date of the International patent application PCT/EP2007/000689. 
     The present invention relates to a method of mixing at least one fluid stream with a large gas stream, especially for introducing a reducing agent into a flue gas that contains nitrogen oxides, and also relates to an apparatus for mixing at least one fluid stream with a large gas stream that flows in a gas duct, especially for introducing a reducing agent into a flue gas that contains nitrogen oxides. 
     EP 1 604 742 A1 discloses such a method in conjunction with electric filters for the dust separation of large gas streams, according to which the flow whirl, which is formed at the inclined mixer disk, is designated as a leading edge whirl. The edge of the preferably circular disk that is directed against the large gas stream is designated as the intake edge and the other edge is designated as the breaking edge. These are not linear edges but rather curved edges. 
     The vertical wall of the gas duct that guides the large gas stream has a short section of pipe for the admixture of a conditioning fluid passed perpendicularly therethrough. The short pipe section opens out, as viewed in the direction of flow of the large gas stream, behind the intake edge of the mixer disk without overlapping the mixer disk. The laminar conditioning agent stream that exits the short pipe section strikes the edge-continuous partial surface of the discharge side of the mixer disk, adjacent to the section outlet, at an angle that corresponds to the angle of inclination of the disk relative to the direction of flow of the large gas stream. 
     With this procedure, the conditioning agent is not optimally mixed into the whirl system that results at the mixer disk. 
     Column 6, lines 5-6, of EP 1 604 742 A1 indicates that methods are also expedient according to which the admixture device is mounted directly on the whirl apparatus. 
     U.S. Pat. No. 5,356,213 A, in conjunction with a gas phase oxidation process in the petrochemical field, discloses a method of mixing a small gas stream, e.g. oxygen, with a large gas stream, e.g. air, according to which the oxygen, in at least two swirled partial streams, is introduced into the air stream, which flows essentially undisturbed in a duct, via radially extending swirl vanes disposed on the end of a tubular section that opens in the direction of flow of the air, whereby the velocity of the oxygen is increased upstream of the swirl vanes and a cloth is stretched over the outlet end of the tubular section downstream of the swirl vanes. 
     With the mixing device described in EP 1 166 861 B1, in conjunction with Denox units and electric filters, the mixer disk (built-in surface) has a chamber into which leads a separate flow channel for the fluid that is to be admixed and which serves as a distribution chamber for the fluid stream. The chamber is provided on the back side (discharge side; lee side) of the mixer disk that faces away from the in-flow of the large gas stream with discharge openings from which exit in a laminar manner partial streams of the fluid stream, and is disposed in the region of the intake edge. Adjoining the distribution chamber toward the breaking edge are chambers that, however, have no distribution function nor a flow-related function, but rather serve exclusively for the reinforcement of the mixer disk. The discharge openings can be formed in the cover of the distribution chamber or in the side wall thereof. However, they can also be formed in an additional hood that is placed upon the chamber. It is furthermore possible that the chamber itself not be provided with a cover that is parallel to the mixer disk, but rather itself have a hood-shaped configuration. The flow channel for the supply of fluid can enter from the windward side of the mixer disk through the disk, or can be guided to the distribution chamber on the lee side of the mixer disk. With the method described in EP 1 166 861 B1 an additional chamber is required and the mixing-in is again effected only in the vicinity of the intake edge. 
     Such methods, designated as mixing methods having a static mixer, are, for example, also used with SCR units, to reduce the NO x  level (Selective Catalytic Reduction) of flue gases, for example of power plant furnaces, by means of reducing agent and catalyzer. In this connection, it is customary, where the reducing agent is NH 3 , that it is stored in the form of pressure-condensed NH 3  or of ammonium hydroxide (NH 4 OH), and pre-vaporized NH 3  is sprayed into the flue gas stream via a carrier gas stream and is mixed with the flue gas stream. Where the reducing agent is urea, first an aqueous urea solution is produced that, after suitable processing, is then sprayed into the flue gas stream in gaseous form. 
     The methods are furthermore used, for example, for industrial chimneys, spray driers (See e.g. EP 0637 726 B1), heat exchangers, flue desulfurization units, and hybrid cooling towers. 
     It is an object of the present invention, for the method and the apparatus of the aforementioned general types, to improve the mixing of the fluid stream into the large gas stream. 
     SUMMARY OF THE INVENTION 
     This object is realized by a method that includes the steps of directing the large gas stream against at least one disk-shaped mixer element on an inlet or windward side thereof, wherein the mixer element is inclined at an angle counter to the direction of flow of the large gas stream, and wherein eddy-type whirls are adapted to form at the mixer element: and admixing a swirled fluid stream with the large gas stream downstream of the at least one disk-shaped mixer element. The obiect is also realized by an apparatus comprising at least one disk-shaped mixer element disposed in the gas duct and having an inlet or windward side and a discharge or lee side, wherein the mixer element forms an angle with the direction of flow of the large gas stream, and wherein eddy-type whirls are adapted to form at the mixer element: a tubular admixing device for the fluid stream: and a swirl device associated with the admixing device upstream of a fluid stream outlet of the admixing device. 
     The important thing with the present invention is that the fluid stream is add in or admixed as a swirled stream. 
     By pre swirling the fluid prior to entry into the whirl system, admixing into the large gas stream is improved. 
     To further improve the admixing, the swirled fluid stream is preferably guided essentially perpendicularly onto the mixer element, and essentially onto the center of the mixer element on the discharge side (lee side) of the mixer element, and the fluid enters the eddy-type whirls from the center along the discharge side. 
     However, it is also possible to guide the swirled fluid stream essentially perpendicularly onto the mixer element on the inlet side (windward side) and for it to exit onto the discharge side (lee side) of the mixer element through an opening provided essentially in the center of the mixer element, whereby the fluid enters into the eddy-type whirls from the center along the discharge side. 
     With the strong swirl stream or jet, the centrifugal force produces an underpressure along the jet axis and forms an outwardly pressing or extending hollow jet that with a suitable design of the opening of the supply conduit (radius, smooth transition) follows the curvature of the wall at the outlet (Coanda effect) and opens out until it forms a flat jet. 
     With both variations, due to the perpendicular orientation, there results on the lee side a rotationally symmetrically flat narrow stream or jet that extends parallel to the surface of the mixer element and expands essentially radially. With a swirled supply, the velocity of the flat jet, in addition to having a radial component, also has a circumferential component. The circumferential component is the greatest within the outlet or discharge (nozzle), and from the outlet decreases with increasing radial spacing. 
     In the case of a circular disk, the center is disposed in the center of the circle, and with a regular polygon is disposed in the center of gravity or centroid. If the mixer deviates from the regular shape, such as with an unequally sided triangle, trapezoid or the like, an adaptation of the discharge position (nozzle position) is required in order nonetheless to achieve a largely uniform distribution of the fluid across the discharge side of the disk-shaped mixer element. 
     With both methods, the fluid is distributed across the entire discharge side of the mixer element and is incorporated into the entire whirl system formed at the peripheral edge. 
     With the apparatus according to the present invention, to improve the mixing process a swirl device is provided in the admixing device upstream of the fluid outlet. 
     Swirl devices that can be used are known, for example, in the burner technology. 
     The fluid outlet of the admixing device, which has a rounded or curved configuration, can expediently be disposed on the discharge side (lee side) of the mixer element and is oriented essentially perpendicular to, and essentially onto, the center of the mixer element. 
     However, it is also possible to associate the rounded or curved fluid outlet of the admixing device with an essentially central opening of the mixer element out of which the swirled fluid stream, which is supplied essentially perpendicularly to the mixer element, exits toward the discharge side. 
     The mixer disk preferably has a circular, elliptical, oval, parabola, diamond, or triangular configuration, as is known from DE 37 23 618 C1, Column 2, lines 40-45. A polygonal configuration, for example 8-cornered, is also possible. Particularly preferred is the shape of a symmetrically structured 8-polygon, and further preferably a regular 8-polygon. A polygon in the form of a trapezoid is also particularly suitable. 
     The mixer disk is preferably inclined relative to the direction of flow of the gas stream at an angle in the range of between 30° to 90°. 
     To limit the intake of dust into the supply device caused by a return flow, a dust-protecting plate is expediently disposed upstream or in the opening in the mixer element. 
     The swirl device is preferably disposed in the supply conduit of the admixing device or is disposed upstream of the supply conduit. 
     The swirl device is-preferably selected from the group: swirl producer with radial grating, swirl producer with axial grating, swirl producer with tangential in-flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in greater detail subsequently by way of example and with the aid of the figures, in which: 
         FIG. 1 : shows a 3-dimensional representation of a horseshoe eddy with eddy wake obtained at a circular disk against which a gas stream flows and that is inclined relative to the stream at an angle a, 
         FIG. 2 : is a side view transverse to the line A-A in  FIG. 1 , 
         FIG. 3 : is a front view onto the lee side of the disk transverse to the line B-B in the illustration of  FIG. 1 , 
         FIG. 4 : is a partial cross-sectional view/side view comparable to  FIG. 2  of a first embodiment of the inventive apparatus with an 8-cornered mixer disk, according to which the swirled fluid stream is guided onto the discharge side (lee side) of the mixer disk and exits to the discharge side through an opening in the mixer disk, 
         FIG. 5 : is a perspective illustration of the apparatus of  FIG. 4  with a view onto the discharge side of the mixer disk, 
         FIG. 6 : is a partial cross-sectional view/side view comparable to  FIG. 4  of a second embodiment of the inventive apparatus, according to which the swirled fluid stream is guided centrally onto the discharge side of the mixer disk. 
         FIG. 7 : is a partial cross-sectional view/side view according to  FIG. 4  with a dust-protecting plate as a built-in return flow blocking device, 
         FIG. 8 : is a vertical-cross sectional view through a swirl device embodied as a radial grating, 
         FIG. 9 : is a horizontal cross-sectional view through the radial grating of  FIG. 8 , 
         FIG. 10 : is a vertical cross-sectional view through a swirled device embodied as an axial grating, 
         FIG. 11 : is a horizontal cross-sectional view through the axial grating of  FIG. 10 , 
         FIG. 12 : is a vertical cross-sectional view through a swirl device having a spiral housing, 
         FIG. 13 : is a horizontal cross-sectional view through the swirl device of  FIG. 12 , and 
         FIG. 14 : is a swirl device subassembly having a plurality of radial gratings. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The formation of eddy trails or wakes involves a natural phenomenon in three-dimensional flows at a body (See Prandtl, Oswabtsch, Wieghardt; GUIDE THROUGH FLUID DYAMICS, 9 TH  Edition 1990; ISBN 3-528-28209-6, page 229, FIG. 4.41 and pertaining text). 
     The formation, the shape and position of such eddy wakes in the downflow of mixer disks are first schematically illustrated in  FIGS. 1-3  and will be described in conjunction therewith. 
     A circular disk  1  is inclined at an angle a relative to the flowing gas stream  2 , which in  FIG. 1  comes from below. On the windward or inlet side  1   a  of the disk, the gas stream is deflected from its main direction of flow, and there results a high-pressure zone. The partial stream  2   a  of the gas stream  2  flows at a prescribed angle along and below the disk. On the lee or discharge side  1   b  of the disk, a low-pressure zone forms that is filled beyond the edge of the disk by the partial stream  2   a  of the gas stream  2 . Due to the flow deflection at the edge of the disk, there is formed a horseshoe eddy  3  having the eddy axis  3   a,  which is shown by a dashed line and that continues downstream of the disk in the form of an eddy wake having two symmetrically rotating eddies or whirls. The lateral eddies of the horseshoe eddy continue as wakes, overlap or superpose with the gas stream (principal stream), and expand with the principal stream. The flow condition within the eddy wake is very turbulent. The schematically illustrated boundary  3   b  of the horseshoe eddy and wake should not be seen as a sharply defined demarcation. The position and the structure of the two eddies as well as the opposite directions of rotation thereof, can be experimentally determined with suitable measuring techniques. 
     Comparable eddies having wakes are formed with other disk shapes, such as elliptical, oval, parabola, diamond, polygonal or triangular shapes. 
     The turbulent intermixing of eddy wakes and gas stream is utilized to distribute a gas stream that is metered-in in a nearly point-type manner over a very large cross-sectional area. 
     With the embodiment of the inventive apparatus shown in  FIGS. 4 and 5 , a supply conduit  4 , which serves for the supply of the fluid F that is to be mixed in, extends via a linear section  4   a  through the wall  5  of a Is duct K that guides the large gas stream  2  and in which an 8-conered mixer disk  1  is disposed at an angle. Adjoining the conduit section  4   a  is an angled-off conduit section  4   b  that is oriented perpendicular to the center M of the windward or inlet side la of the mixer disk  1 . An opening  6  is provided in the middle M of the disk. 
     As schematically indicated in  FIG. 4 , a swirl device DE, in the form of an axial grid or grating AG, is disposed in the conduit section  4   a  and is thus arranged ahead or upstream of the outlet  4   c  of the conduit. 
     The axial grating AG and further examples for possible swirl devices are described starting on page 16, 2 nd  paragraph. 
     The outlet end of the angled-off conduit section  4   b  is provided with a curved outlet  4   c  that is rounded with the prescribed radius R, i.e. is constantly flared or expanded, and is oriented toward the opening  6  and is connected to the disk. The radius of curvature or rounding-off R is preferably R=D/2, with D being the conduit diameter. 
     The swirled fluid stream  7  exiting the outlet lc is distributed over the discharge side under the influence of the swirl, as is schematically illustrated in  FIG. 5  by the arrows  7   a.  Mixing in or admixture into the flow whirls or eddy-type whirls  3  is effected in the region of the peripheral edge of the mixer disk. 
     With the embodiment of the inventive apparatus shown in  FIG. 6 , with the 8-cornered mixer disk  8 , a supply conduit  9  extends via a linear section  9   a  through the wall  5  of a duct K that guides the large gas stream  2  and in which the mixer disk  8  is disposed at an angle. Adjoining the conduit section  9   a  is an angled-off conduit section  9   b  that is oriented perpendicular to the center M of the lee or discharge side  8   b  of the mixer disk  8 , and that is provided on its outlet end with a rounded-off or curved outlet  9   c . The outlet  9   c  is spaced from the discharge side  8   b  (lee side) of the mixer disk  8 . A swirl device DE is provided in the conduit section  9   a.    
     However, the disposition of the swirl device DE in, or an association of the swirl device to, the angled-off section, as with the embodiment of  FIGS. 4 and 5 , is preferred, since then the swirled stream can exit from the outlet without further deflection in the conduit. 
     The swirled fluid stream  10  exiting the outlet  9   c  is distributed over the discharge side  8   b,  as is correspondingly schematically illustrated for the fluid stream  7  by the arrows  7   a  in  FIG. 5 . The mixing-in or admixing into the flow whirls or eddy-type whirls  3  is effective in the region of the peripheral edge of the mixer disk  8 . 
     The conduits  4  and  9  need not necessarily be angled-off in the duct K; the fluid need only be supplied to the mixer disk in a perpendicular manner, i.e. an inclined guiding of the conduit through the duct wall  5  would also be conceivable. It is furthermore conceivable for at least one further conduit section to extend parallel to the wall of the duct. 
     As shown in  FIG. 7 , a dust-protecting plate  11  can be associated with the opening  6 . If the strong swirl stream or jet is used in a dust-containing environment, dust can pass back into the supply conduit  4  via the return flow RS that forms along the jet axis. Due to the centrifugal force, the dust possibly migrates partially against the conduit wall, where it could lead to deposits. The dust-protecting plate  11  can prevent this. 
     It is to be understood that with large duct cross sections a plurality of mixer disks having fluid supply associated therewith can be distributed over the cross-section of the duct in a plane or can be staggered in one or more planes. 
     Examples of possible swirl devices are described in the following: With burners, a peripheral component is frequently superimposed over a combustion air stream in order to radially expand the flame. This expansion can be carried out until the flame breaks open and lies against the combustion chamber wall as a so-called flat flame. Typical for swirled burner flows is an underpressure zone that is intentionally utilized to stabilize the flame. 
     The following techniques are, for example, known: With the swirl generation using a radial grid or grating RG pursuant to  FIGS. 8 and 9 , a plurality of fins or deflectors LF are disposed tangentially to an inner circle IK in a housing G. The swirled fluid stream FD is withdrawn centrally. 
     With the swirl generation using an axial grid or grating AG pursuant to  FIGS. 10 and 11 , the fluid stream F enters the axial grating AG, which is built up of a plurality of inclined guide vanes FL that are radially disposed in a conduit R; the fluid stream is swirled in the axial grating to form the stream FD. 
     With the swirl generation using the tangential supply pursuant to  FIGS. 12 and 13 , the fluid stream F is introduced into a spiral housing S and leaves it centrally as the swirled stream FD. The spiral housing serves as fins or deflectors, so that no guide vanes are required. 
       FIG. 14  shows that a plurality of swirl devices, such as radial gratings, can be combined into a subassembly in the event that a swirled fluid stream is to be supplied to a plurality of mixer disks. A plurality of radial gratings RG are disposed in a common housing GM, and a plurality of swirled partial streams FD 1 , FD 2 , FD 3  are withdrawn. 
     Other swirl devices are also possible; reference is made, for example, to DE 40 21 817 A1, where the guide devices do not have a vane-like configuration as with the embodiment of  FIGS. 10 and 11 , but rather are groove-like in the conduit wall. Similar grooves are found in the fuel injection device of EP 1 605 204 A2. 
     The specification incorporates by reference the disclosure of German 10 2006 004 068.6 filed Jan. 28. 2006, as well as International application PCT/EP2007/000689, filed Jan. 26, 2007. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings. but also encompasses any modifications within the scope of the appended claims. 
     Reference Numeral List 
     
         
           1  round mixer disk 
           1  a windward side of the disk 
           1   b  lee side of the disk 
           2  gas stream 
           2   a  gas stream, partial stream on the windward side of  1   a  of the disk 
           2   b  gas stream, partial stream toward the lee side  1   b  of the disk 
           3  horseshoe eddy and wake 
           3   a  eddy axis 
           3   b  outer boundary of the eddy 
           4  supply conduit 
           4   a  linear conduit section 
           4   b  angled-off conduit section 
           4   c  curved outlet 
           5  wall of the flue gas duct K 
           6  opening 
           7  swirled fluid stream 
           7   a  arrows of the fluid stream 
           8  8-cornered mixer disk 
           8   a  windward side 
           8   b  lee side 
           9  supply conduit 
           9   a  linear conduit section 
           9   b  angled-off conduit section 
           9   c  curved outlet 
           10  swirled fluid stream 
           11  dust-protecting plate 
         AG axial grid or grating 
         D diameter of conduit  4 ,  9   
         DE swirl device 
         F fluid stream 
         FD swirled fluid stream 
         G housing 
         GM common housing 
         IK inner circle 
         K flue gas duct 
         L guide vanes 
         LF fin or deflector 
         M center of the disks  1 ,  8   
         R radius of curvature of the expansion 
         RG radial grid or grating 
         RS return flow 
         S spiral housing