Apparatus for mixing a gas flowing through a conduit

The invention relates to a device for mixing a gas stream flowing through a channel with the features of the preamble of claim 1 and a method using the device.

Devices for mixing a gas stream are required in the aftertreatment of flue gases resulting from the combustion of coal, waste, sewage sludge or other fuels. These flue gases contain some undesirable but unavoidable pollutants that are removed in a post-combustion flue gas cleaning plant. These pollutants include nitrogen oxides, which are reduced by the addition of a reducing agent to the flue gas.

In some known process variants, the reducing agent consists of an ammonia-water mixture which is added to the flue gas by means of pneumatic nozzles in the form of fine droplets. Due to the high temperature, these drops evaporate quickly. The reducing agent passes from the liquid phase to the gas phase. The flue gas enriched in this way with the reducing agent is fed to a catalyst in which the decomposition of the nitrogen oxides takes place. To successfully carry out the process, it is necessary to match the concentrations of both reactants in the flue gas. Too little local dosing of the reducing agent only an incomplete reduction of nitrogen oxides is achieved, which may be undesirable if low nitrogen oxide emissions are to be achieved on average over time. On the other hand, a local overdose of the reducing agent would generally lead to a fate of the reducing agent in the flue gas and thus to an unauthorized emission of this substance. For carrying out the method, intensive, uniform mixing of the flue gas with the reducing agent is thus a prerequisite for success. Furthermore, the reduction of local temperature differences is advisable, which may result from a non-uniform application of the heat exchanger or from the operation of the burner integrated in the flue gas channel. Non-uniform local profiles of the time-average temperature of the flue gas limit the achievable degree of separation of the reactor for the reduction of nitrogen oxides because of the temperature dependence of the reaction rate. By contrast, temporal fluctuations in the temperature are to some extent compensated by the thermally inert mass of the catalyst material.

The reduction of local concentration differences and local temperature differences is achieved in the prior art through the use of static mixers. Are known in the flue gas ducts installed, intersecting pipe register for introducing the reducing agent. These feeder tube registers have a large number of reductant exit points. A mixing of the flue gas with the reducing agent is achieved by vortex formed in the flow lag of the individual tubes. The achievable quality of the mixing is technically limited by the number of tubes used. In addition, an injection grille formed from crossed tubes has a considerable, undesirable pressure loss.

Good mixing results are also achieved when individual portions of a flue gas stream are set in a swirling motion, wherein the axis of the rotational movement points in the direction of the main flow direction. A known static gas mixer has a mixing element, which is formed by a curved around the main axis of the flue gas path and thus curved surface. The juxtaposition of several mixing elements of this kind leads to a good mixing. The disadvantage of this mixer can be seen on the one hand in its complicated, spatially curved structure. On the other hand, a single mixing element extends across the entire flue gas path.

In another gas mixer of the type mentioned a mixing element is used, which utilizes the wake flow of mounted on the channel wall mixer plates. These mixing elements consist of approximately trapezoidal patches attached to the wall at the trapezoidal base. Three edges of the surface element are lapped by the flue gas. The elements are inclined in the main flow direction. Retaining webs are for attachment in the throat between the mixing element and the wall, ie in the separation region of the flow. These mixing elements produce two oppositely oriented vortices with velocity components transverse to the main flow direction. This vortex pair intensifies mixing processes in the gas phase. The use of several mixing elements should ensure good mixing. A disadvantage is the relatively long, applied to the channel wall edge of the mixing element.

Other known static mixers (DE-A-4 123 161) comprise a grouping of triangular surface elements. In this case, a channel cross section is divided by a frame into a number of rectangular fields. In each field, a triangular or trapezoidal baffle is mounted, which is inclined with respect to the gas flow direction.

A generic device, which serves to mix a plurality of gas streams or the mixing of a liquid cooling medium in a gas stream, are known from the publications DE-C-2 911 873, DE-U-8 219 268, EP-B-0 637 726. In this device planar mounting elements in the form of symmetrical surfaces are used. The edges of these built-in elements are surrounded on all sides free of the fluids to be mixed. These mounting elements are inclined at an acute angle to the flow direction introduced into the flue gas flow, that at the front edge of a Ablösewirbel arises, which is referred to in the cited documents as leading edge vortices. This leading edge vortex also has velocity components transverse to the main flow direction, thereby intensifying the mixing operations. The mounting elements of the known device are designed with circular, elliptical, oval, parabolic, diamond-shaped or triangular basic shape. They may be profiled in cross-section or provided with an angled edge or be angled V-shaped.

A disadvantage of static mixers of this known design is the nature of the introduction into the flue gas path. Due to the all-round free flushing of the edges of the mounting element, a separate supporting structure is required (DE-U-8 219 268). The shape of the mounting elements causes forces are induced by the flue gas flow, which are unsteady and make themselves felt in the component as vibration. The structures for mounting these mounting elements are designed to absorb mechanical stresses resulting from the flow-induced oscillation. This leads unfavorably regularly to heavy structures with high resistance moments. The high weight of the structures represents a serious disadvantage, because of the process, the installation position of these components in reactors for the reduction of nitrogen oxides is usually at high altitude, which in turn adversely affects the static structure of the overall reactor and assembly.

The invention has for its object to make the mounting elements of the generic device such that their weight and the weight of the supporting structures can be reduced.

This object is achieved according to the invention in a generic device by the characterizing features of claim 1. Advantageous embodiments of the invention are the subject of the dependent claims. A method using the device according to the invention is specified in claim 17.

The inventively shaped mounting elements create a wake with flow components transverse to the main flow direction, which intensify the mixing of the gas stream. The fold of the mounting element along straight lines to form the omega or W shape increases the mechanical stability of the mounting element, so that it can be made thinner and thus saving in weight. In addition, the omega or W-shape allows the installation of tension or gusset plates to further reduce the weight and / or increase the mechanical stability of the mounting element. Since these stiffening elements can be mounted on the side facing away from the flow, they do not interfere with the course of the gas flow. Furthermore, the structure for mounting the mounting element in the channel can be accommodated within the central concave curvature on the upstream side of the mounting element. The structure is thus different than in the prior art outside the vortex fields, so that they are not adversely affected. The structure can therefore be made easier.

Several embodiments of the invention are illustrated in the drawings and will be explained in more detail below. Show it:<ul id="ul0001" list-style="none" compact="compact"><li>1 is a plan view of a mounting element,</li><li>2 is a plan view of the mounting element of FIG. 1 with bending lines,</li><li>3 shows the front view of the installation element according to FIG. 2, FIG.</li><li>4 is a plan view of another installation element,</li><li>Fig. 5 is a side view of a built-in a channel mounting element</li><li>Fig. 6 shows the view perpendicular to the side view of FIG. 5 and</li><li>Fig. 7 shows the arrangement of several mounting elements.</li></ul>

The device according to the invention for mixing a gas stream uses planar mounting elements 1, whose mode of operation and arrangement within a channel 2 will be described later.

1 and 2, the geometric shape of the mounting element 1 is discussed: This form is derived from an imaginary, planar surface element in the form of a trapezoid, which is symmetrical in the illustrated case, but may also be asymmetrical. The mounting element 1 is formed by simple or multiple folding of the plane, imaginary surface element. The trapezoid has sides a, b, c, d and height h.

The sides a and c are parallel to each other, with the longer side a representing the trapezium base. The surface element is provided with an outgoing of the trapezoidal base a sweep. The sweep is caused by an angled saving of the surface element, which starts from the trapezoidal base a.

The arrows of the mounting element 1 leads to a further weight savings, on the other hand it serves in the installed state of the mounting element of the optimization of the distance between the trailing edges of the mounting element and the associated channel wall. In addition, the sweep serves to reduce unsteady motion components of the flue gas flow.

The associated swept height is shown in FIG. 1 denoted by + p. The trapezoid base a is therefore only an imaginary line. The sweep height may also assume negative values ​​and be formed by a projection extending from the trapezoidal base. In this case, the shape of the swept trapezium merges into the mathematical form of a kite with a capped tip. Such a negative sweep height is shown in FIG. 4 shown. The height of the arrow to be chosen depends on the height h of the imaginary trapezoid. The absolute amount of the ratio between the height h of the imaginary trapezoid and the sweep p is within the limits between 0.1 and 0.75, so the relation is:<maths id="math0001" num=""><math display="block"><mrow><mtext>0.1 &lt;|</mtext><mfrac><mrow><mtext mathvariant="italic">p</mtext></mrow><mrow><mtext mathvariant="italic">H</mtext></mrow></mfrac><mtext>| &lt;0.75</mtext></mrow></math><img file="EP0956897A2_D0001.tif" /></maths>

The built-in element 1 is installed in a channel 2 through which a gas flow flows in such a way that the short side c faces the main flow direction in the installed position. The short side of the imaginary trapezoid becomes the head edge. The sides of the imaginary trapezium form the side edges of the mounting element 1, and the resulting by the sweep, facing in the flow direction edge becomes the trailing edge.

The gravity axis of the mounting element 1 is set at an angle relative to the main flow direction of the gas flow. As a result of employment creates a main flow facing side (bottom) and a side facing away from the main flow side (top). In addition, the gravity axis relative to the main flow direction can be rotated by an angle, resulting in symmetrical mounting elements 1 to an asymmetric flow of the mounting element through the gas flow.

For mechanical stabilization, the built-in component described above is folded along three straight lines 3. The designated as the main axis, mean of these lines 3 coincides before folding with the gravity axis of the surface element. As shown in FIG. 2 3, the lines 3 may run parallel to one another, starting from the top edge and terminating in the trailing edge. According to FIG. 4 the two outer of the lines 3 can also form an angle in the direction of the trailing edge, the main axis forming the bisecting line. In this embodiment, the Abkantlinien also end at the trailing edge, but go out from the side edges of the mounting element 1.

As shown in Fig. 3, starting from a flat plate, the mounting element 1 along the lines 3 is bent so that in the cross section of the mounting element 1, the shape of the Greek letter ω and the Latin letter W is formed. The folded installation element 1 is inserted into the gas flow in such a way that a concave curvature 4 and on both sides of this concave curvature 4 two convex curvatures 5 are formed towards the flow.

By folding four surfaces have arisen, the angled abut the Abkantlinien 3 together. In this case, the two inner surfaces form the concave curvature 4 with respect to the flow of the gas stream. In each case an inner surface and an outer surface form the convex curvatures 5. The outer surfaces widen in the flow direction and are wider at least at their widest point than the inner surfaces. The two outer surfaces enclose an angle of about 120 °, while the angle formed by the inner surfaces is about 90 °. The angle subtended by the two outer surfaces may be between 90 ° and 180 °, and the angle subtended by the two inner surfaces may vary between 0 ° and 120 °.

In the Figs. 5 and Figure 6 shows how a single mounting element 1 is installed in a channel 2 through which the flue gas from a combustion process flows. It can be seen that the main axis of the mounting element 1 is aligned at an angle to the flow direction of the flue gas. The flow direction is indicated by the arrow 6. In this installation position, the head edge and the side edges of the mounting element 1 facing the flow direction. The pointing in the flow direction trailing edge is shown in FIG. 5 aligned parallel to one of the walls of the channel 2. The trailing edge may also be inclined at a slight angle against the wall of the channel 2.

The mounting element 1 is mounted on a support 7 which is supported on two opposite walls of the channel 2. This support 7 is arranged on the underside of the installation element 1, which is flown by the flue gas, within the concave curvature 4. In the arrangement at this point, the carrier 7 exerts no adverse effect on the flow field of the flue gases flowing at the edge.

On the upper side of the mounting element 1 5 clamping 8 or gusset plates are arranged within the two outer, convex to the flow convexity. These spans 8 each connect two legs of the omega-shaped mounting element 1 with each other and thus increase the mechanical stability of the mounting element 1. Since the tensioners 8 are mounted on the side facing away from the flow of the mounting element 1, they do not interfere with the course of the gas flow out.

The top edge and the side edges of the lying in the flue gas installation element 1 are lapped by the flue gas on all sides. As a result, detachment vortices arise at the top edge and at the side edges, they spread out in the shape of a circle in the shape of a circle and form a vortex field which, by virtue of its rotation, generates a flow component transverse to the main flow direction. Due to the pulse exchange associated with it, this crossflow component leads transversely to the flow direction to a good mixing of the flue gas flow.

The favorable effect of the mounting element 1 on the mixing of the flue gas stream can be applied in an advantageous manner to the admixture of a reducing agent to the flue gas for the purpose of reducing the nitrogen oxides present in the flue gas. As a reducing agent, an ammonia-water mixture is used, which is injected by means of air in atomized form in the flue gas. The injection takes place via a lance 10 provided with an exit head 9. This lance 10 is inserted into the channel 2, that the outlet head 9 is located in the slipstream generated by the mounting element 1. The gases within the windshield mix with the flue gas of the mainstream. As a result, a very uniform mixing of the reducing agent is achieved in the flue gas. In this way, local under or over concentrations of the reducing agent in the flue gas and local temperature differences can be avoided.

To support the mixing effect 1 holes 11 or holes are arranged in the mounting element. Through these openings 11 reaches flue gas to a small extent from the upstream side to the downstream side of the mounting element. 1 These openings 11 can be made by simply cutting out the mounting element 1 forming sheet. Advantageous in terms of generating additional turbulence is the preparation of apertures 11 by attaching slots in the sheet metal of the mounting element 1 and by bending the aperture 11 corresponding surface element of the sheet metal plane. With the attachment of two slots, which enclose an angle and intersect at a point, a triangle can be bent out of the metal sheet. This triangle acts for the passing through the opening 11 flue gas partial flow as a detachment edge. This partial flow is thereby excited to turbulent mixing. Flue gas and reducing agent, which are present in the lee of the area, experience a turbulent mixing with the passing through the opening 11 flue gas partial stream. The size of the resulting vortex corresponds approximately to the diameter of the opening 11 in the mounting element. 1 Accordingly, these vortices are always smaller in size than the largest vertebral elements that arise through the mounting element 1 itself. The advantage of the arrangement consists in the fact that first a mixture of the reducing agent is carried out in fluidized structures of average dimensions. Only then are these medium-sized vortex structures turbulently mixed by vortex structures of the largest dimensions. As a result, shorter mixing lengths are achieved overall. It should be noted that in FIG. 6 different forms of openings 11 are shown. In practice, only one of these forms will be used at a time.

In the Figs. 5 and 6, only a single mounting element 1 is shown in the channel 2. However, it may be advantageous to arrange a plurality of these mounting elements 1 approximately in one plane in a channel 2 (FIG. 7). Various arrangements are possible. Thus, the mounting elements 1 can be arranged approximately in a plane which is oriented perpendicular to the flow direction. The mounting elements 1 can also be arranged by juxtaposition of a plurality of elements in a plane which is oriented in the flow direction. Furthermore, the mounting elements 1 can also be lined up in one or more planes, which runs obliquely to the flow direction or run. This results in a staggered, formed arrangement of built-in elements. 1 Especially this arrangement can help to reduce the flow resistance or to reduce the flue gas side pressure loss to overcome the flow resistance of the overall arrangement on.