Abstract:
A reducing agent or an additive that releases an agent is often added to an exhaust gas from engines in order to chemically convert constituents, in particular nitrogen oxides, in the exhaust gas. To achieve intimate mixing of the exhaust gas with the reducing agent or additive over a short section of the gas duct, at least one mixer and at least one nozzle for introducing the additive into the exhaust gas are structurally and functionally combined with one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is a continuation of copending International Application PCT/DE00/02402, filed Jul. 21, 2000, which designated the United States.  
         BACKGROUND OF THE INVENTION  
       Field of the Invention  
         [0002]    The invention relates to a device for introducing an additive into an exhaust gas or gas stream, which can be introduced into a gas duct, in particular into an exhaust gas line from a diesel engine. The device has at least one nozzle and at least one mixer. The additive may be a reducing agent or a compound that releases such an agent.  
           [0003]    A device of this type is known, for example, from Published, Non-Prosecuted German Patent Application DE 36 42 612 A. The device introduces a reducing agent into a NOx-containing exhaust-gas duct with intensive mixing. The device is intended to achieve a homogeneous mixing of the reducing agent and the exhaust gas over a relatively short section of duct.  
           [0004]    High levels of efficiency are desired during the conversion of fossil fuel or of fuel obtained from plants into mechanical and/or thermal energy. The conversion temperatures are often so high that significant amounts of nitrogen oxides (NOx) are formed during the conversion (combustion). This applies in particular to diesel engines, which are operated, for example with mineral oil or with rapeseed oil.  
           [0005]    Since a release of the nitrogen oxide is extremely undesirable and is therefore to be avoided, the nitrogen oxides are usually catalytically reduced. However, this requires the presence of a reducing agent in the exhaust gas from an energy conversion installation, for example, in a diesel engine. It has proven expedient to admix a reducing agent of this type or an additive that releases it as a reducing-agent carrier, in particular ammonia or urea (dissolved in water), to the exhaust gas only after it has emerged from the energy conversion installation.  
           [0006]    During operation of a device of this type or of a flue-gas cleaning device, it is often difficult to achieve sufficiently intimate mixing of the exhaust gas or gas stream with the reducing agent. The reason for this is that the length of the gas or exhaust gas duct which is inherently required to achieve this, being of the order of magnitude of approximately 50 times the duct diameter, is not realistic. For relatively small installations, such as for example diesel engines with an output of up to a few 100 kW, it is in principle possible to feed a reducing agent carrier into the exhaust-gas duct via an injection device, for example via a single nozzle, and a reducing-agent generator which lies close behind the engine. However, as the rated output rises, this entails disproportionately high costs.  
           [0007]    In relatively large installations, for example, in a combined heat and power plant or a drive of a ship, with exhaust-gas duct diameters of over 200 mm, therefore, it is usual for at least two static mixers to be disposed downstream of an atomization system, which represents a complex injection device. The distances between the atomization system and the mixers are in each case to be at least equal to twice the diameter of the exhaust-gas duct. However, space for the resulting length of exhaust-gas duct is often not available.  
         SUMMARY OF THE INVENTION  
         [0008]    It is accordingly an object of the invention to provide a device for introducing an additive into an exhaust gas which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which allows the use both of an injection device or at least one nozzle for an additive and of a number of necessary mixers in an exhaust-gas duct in which, at the same time, the duct length is as short as possible. It should be possible for the device to be used in particular in a round gas duct whose diameter is between approximately 200 mm and approximately 1000 mm.  
           [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, a device for introducing an additive into an exhaust gas. The exhaust gas is introduced into an exhaust gas line from a diesel engine and has a flow direction. The device contains at least one nozzle, and at least one mixer for mixing the exhaust gas with the additive. The mixer and the nozzle combined with one another form a structural and functional unit. The mixer has a tubular body disposed approximately parallel in terms of the flow direction of the exhaust gas and houses the nozzle. The tubular body has an upstream end with a first opening formed therein and a downstream end with a second opening formed therein and communicates with the first opening. The mixer has lamellae supported by the tubular body, and the lamellae, with regard to the flow direction of the exhaust gas, face upstream at the upstream end of the tubular body and face downstream at the downstream end of the tubular body.  
           [0010]    For this purpose, the mixer and the nozzle for atomizing the additive or the reducing agent itself are combined to form a structural and functional unit. The combination can be introduced into the gas duct, i.e. can be fitted into the duct when the structural unit is operating as intended.  
           [0011]    This immediately eliminates the length of the distance from the nozzle to the mixer of a first mixer stage, so that the duct length can preferably be shortened to only two to four times the duct diameter (two to four duct diameters). Neither the action nor the efficiency of each individual functional part, namely the injection function, on the one hand, and the static mixer function, on the other hand, is impaired.  
           [0012]    According to an expedient refinement, the mixer has a tubular body which is open at both ends, is parallel in terms of flow, is advantageously, in the installed state, coaxial with respect to the gas duct and at one end bears lamellae which face upstream and at the other end bears lamellae which face downstream. In this case, the tubular body, in a gas duct that is circular in cross section, is likewise circular in cross section. The diameter of the gas duct into which the structural unit containing the mixer and the nozzle can be fitted should expediently be no less than approximately 200 mm and no greater than approximately 1000 mm.  
           [0013]    In advantageous configurations of the tubular body, its diameter is approximately 0.5 to 0.2 times the diameter of the gas duct. The length of the tubular body approximately corresponds to 0.2 to 0.5 times the diameter of the gas duct. In this case, the tubular body is expediently supported by a tubular connection piece that projects into the gas duct transversely with respect to the gas stream and through which a tube, which serves as a feed line and as a support for the nozzle or the injection device, extends as far as the center axis of the tubular body.  
           [0014]    The nozzle is advantageously disposed in the region of the center axis of the tubular body, and therefore, in the installed state, of the center axis which is common to the gas duct and the tubular body. In this position, the nozzle or the injection device atomizes the additive, for example ammonia as the reducing agent or urea solution as the additive, in the direction of flow of the gas stream. A spray cone that emerges from the nozzle includes an angle of 20° to 60°, preferably an angle of 30°, at its cone tip.  
           [0015]    Lamellae that are supported at the ends of the tubular body preferably form an angle of from 30° to 60°, in particular an angle of 45°, with the axis of the tubular body. The lamellae are oriented in such a manner that, in the installed state of the device, they are supported on the inner side of walls of the gas duct by their free ends. In this case, the lamellae are approximately twice as wide at their free ends as at their root ends that are connected to the tubular body. Free spaces between adjacent lamellae are in this case approximately as wide as the lamellae themselves.  
           [0016]    Expediently, the lamellae which face upstream are staggered with respect to the lamellae which face downstream, so that every partial stream of the gas stream is made turbulent, in each case an identical number of the lamellae which face downstream and the lamellae which face upstream are provided. The tubular body expediently bears in each case four to eight lamellae at both end sides, the lamellae preferably are formed of planar metal sheets.  
           [0017]    The advantages which are achieved with the invention consist in particular in the fact that, on account of the spatial and functional combination of at least one mixer and the nozzle in a device which has these functional components for the introduction of an additive, such as for example ammonia or urea solution, into an exhaust gas, it is possible to achieve a particularly short configuration of an exhaust-gas duct accommodating the device, without the risk of the mixer becoming encrusted, even when using urea solution.  
           [0018]    The reason for this is that only a short trailing section in the exhaust-gas duct is required even though, at the same time, it is ensured that the temperature and velocity are made more uniform over the entire cross section of the duct.  
           [0019]    Moreover, the small number of components in the overall system leads to a reduction in manufacturing costs.  
           [0020]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0021]    Although the invention is illustrated and described herein as embodied in a device for introducing an additive into an exhaust gas, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0022]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a diagrammatic, partially, longitudinal sectional-view of a flue-gas cleaning device with a combined mixing and injection device according to the invention;  
         [0024]    [0024]FIG. 2 is a partially sectional plan view of the cleaning device;  
         [0025]    [0025]FIG. 3 is a front-elevational view of the cleaning device;  
         [0026]    [0026]FIG. 4 is a sectional view of the cleaning device taken along the line IV-IV shown in FIG. 1; and  
         [0027]    [0027]FIG. 5 is a sectional view of the cleaning device taken along the line V-V shown in FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 5 thereof, there is shown a gas or exhaust-gas duct  1  with a diameter d 1  of, for example, 200 mm to 1000 mm is connected, by a flange  2  (FIG. 1) to a preferably non-illustrated stationary diesel engine. An exhaust gas A flows through it in the direction indicated by an arrow  3 . A tubular connection piece  4  penetrates through a wall of the exhaust-gas duct  1  and projects into the exhaust-gas duct  1  transversely with respect to the direction of flow  3  of the exhaust-gas A and therefore at right angles to a duct axis  5 . If the duct cross section is circular, as in the exemplary embodiment, therefore, the tubular connection piece  4  runs in a radial direction. Outside the exhaust-gas duct  1 , the tubular connection piece  4  bears a flange  6  at its free end, on which an end plate  7  rests in a sealed manner, thus closing off an interior of the tubular connection piece  4  with respect to the outside.  
         [0029]    That end of the tubular connection piece  4  which lies in the exhaust-gas duct  1  bears a tubular body  8  which is open at both end sides and is disposed coaxially with respect to the exhaust-gas duct  1 . The duct axis  5  therefore forms the center axis of both the exhaust-gas duct  1  and the tubular body  8 . The tubular body  8  has a diameter d 2  (FIG. 5) that corresponds to 0.2 to 0.5 times the diameter d 1  of the exhaust-gas duct  1 .  
         [0030]    The tubular body  8  has a length that is approximately equal to 0.2 to 0.5 times the diameter d 1  of the exhaust-gas duct  1 .  
         [0031]    A number of first lamellae  9 , which face upstream with respect to the direction of flow  3  of the exhaust gas A, are secured to an inflow-side end of the tubular body  8 . A preferably identical number of second lamellae  10 , which correspondingly extend downstream, are secured to an outflow-side end of the tubular body  4 . The lamellae  9  and the lamellae  10  are disposed staggered with respect to one another, i.e. each of the lamellae  9  lies between in each case two lamellae  10 , and vice versa, as seen in the direction of the duct axis  5 . This can be seen relatively clearly from FIGS.  3  to  5 .  
         [0032]    The lamellae  9  and  10  in each case form an angle α from 30° to 60°, preferably of α=45° (see FIG. 1), with the duct axis  5  and are supported, by their free ends  9   a  and  10   a,  respectively, on the inner side or inner wall  1 ′ of the exhaust-gas duct  1 . The free ends  9   a,    10   a  of the lamellae  9  and  10  are approximately twice as wide as their root ends  9   b  and  10   b,  respectively, which are secured to the tubular body  8 . By way of example, four to eight lamellae  9 ,  10  are secured to each end of the tubular body  8 , the lamellae  9 ,  10  contain planar metal sheets. Consequently, they cause particularly powerful turbulence in the flowing exhaust gas A.  
         [0033]    The tubular connection piece  4  contains a tube  11  that is supported by the end plate  6  and on which, at a free end side in the tubular body  8 , a nozzle  12  is held. The tube  11  is connected via a line, in a manner which is not illustrated in more detail, to a reservoir for an additive which releases a reducing agent or for a reducing-agent carrier R, for example for an aqueous urea solution, and expediently, in addition, to an air connection for cooling and atomizing the urea solution which is, for example, aqueous. A non-illustrated pump forces the reducing-agent carrier R, e.g. the aqueous urea solution, or a corresponding reducing agent, e.g. ammonia, through the tube  11  and through the nozzle  12 .  
         [0034]    The geometric dimensions of the nozzle  12  are such that, at the delivery pressure of the reducing-agent carrier R which is reached, a spray cone  13  is formed by the carrier R when it emerges from the nozzle  12 , the tip of which cone includes an angle β of from 20° to 60°, preferably of β=30° to 45°. The nozzle  12  and the tubular body  8  with the lamellae  9  and  10  therefore form a structural and functional unit, the tubular body  8  and the lamellae  9 ,  10  attached thereto representing, in functional terms, a static mixer or a mixing stage. The nozzle  12  in this case lies on the common center axis of the exhaust-gas duct  1  and of the mixer  8 ,  9 ,  10 , which corresponds to the duct axis  5 .  
         [0035]    In operation, the hot exhaust gas A from the diesel engine flows through the exhaust-gas duct  1  in the direction of flow  3 . In the process, a small proportion of the exhaust gas A, and therefore at least one exhaust-gas partial stream, flows through the tubular body  8 . When a predetermined operating state is reached, the reducing-agent carrier R is sprayed or injected through the nozzle  12  into the flowing exhaust gas A.  
         [0036]    In the region of the tubular body  8 , considerable turbulence caused by the lamellae  9  and  10  occurs in the flowing exhaust gas A, which turbulence also acts on the spray cone  13 , thus ensuring very intimate mixing of the exhaust gas A with the reducing-agent carrier R.  
         [0037]    In the state which prevails in the exhaust-gas duct  1 , the, for example, aqueous urea solution is broken down by hydrolysis into gaseous ammonia and water, so that in the gas mixture which is formed in this way, when a catalytic converter which is suitable and provided for this purpose, but is not shown, is reached, nitrogen oxides contained in the exhaust gas A are virtually completely reduced to form nitrogen.