Apparatus for Preheating Combustion Air, Accompanied by Simultaneous Reduction of NO.sub.x Contained in the Flue Gases

1. Field of the Invention

The present invention relates to an apparatus in the form of an air preheater for preheating the air for combustion for a combustion process by acting upon the air with hot, NO.sub.x -containing flue gases from the same or an independent combustion process; the NO.sub.x contained in the flue gases is simultaneously reduced.

So-called air preheaters are used to preheat the air for combustion for a combustion process. As a result of this preheating, there is achieved, among other things, advantages which relate to combustion and which result from higher combustion chamber temperatures, such as accelerated ignition and more complete combustion. Flue gas from the combustion process itself generally serves as the material flow which gives off heat for the combustion air which is to be heated up. However, it is also possible to use flue gas from an independent combustion process.

2. Description of the Prior Art

A number of apparatuses for preheating the air for combustion are known. Such air preheaters include so-called recuperators and regenerators.

In a recuperative air preheater, the air is heated by the flue gas via heat transfer through solid walls. Preheaters of this type include, for example, plate or tube heat exchangers.

With regenerative air preheaters, the storage mass absorbs heat from the flue gas and later surrenders it to the air. For continuous operation, essentially two designs having cylindrical storage masses are encountered. One design has a rotating storage mass and stationary air and flue gas connections (Ljungstrom air preheater), and the other design has a stationary storage mass and a rotating air connection within the fixed flue gas connection which covers the entire storage mass (Rothemuhle air preheater). In both types of embodiments, the heat storage masses are designed as stationary bundles of laminations. With another type of heat storage mass, separate movable heat storage bodies are used in place of the fixed bundle of laminations; such design is disclosed, for example, in U.S. Pat. No. 4,310,046-Michalak dated Jan. 12, 1982 and corresponding German Offenlegungsschrift No. 29 51 279.6.

In large furnaces, in addition to other noxious substances, there is produced NO.sub.x which pollutes the environment to a great extent. With regard to reducing the NO.sub.x emissions, a distinction is made between primary and secondary measures. The primary measures undertake action within the combustion chamber. Thus, a reduction of the NO.sub.x formation is already achieved during the combustion process, for example by the use of burners which are developed especially for this purpose, or by feeding the air for combustion in stages.

The secondary measures undertake procedural action outside of the furnace or boiler. Such action includes flue gas treatment processes according to which a reducing agent, such as NH.sub.3, is used in the presence of a catalyst, such as vanadium compounds on a titanium oxide base, with a reduction of NO.sub.x to molecular nitrogen and water vapor being effected. This process is carried out in a separate reactor which is provided for this purpose and is equipped with the catalyst. This reactor is generally disposed between the boiler unit and the air preheater. In many units, a high temperature electrostatic precipitator is disposed ahead of the reactor.

However, this known arrangement of the NO.sub.x reactor between the air preheater and the boiler unit has a number of drawbacks. On the one hand, there results a pressure loss of the flue gas as it flows through the catalyst bed. On the other hand, an appropriate amount of space has to be provided for this reactor. Furthermore, the support construction of the system has to be reinforced due to the weight of the NO.sub.x reactor. In addition, due to the dust content of the flue gases, the catalyzers have to be periodically cleaned, for example with soot blowers, which, of course, has a negative impact on the operating cost.

An object of the present invention is therefore to design an apparatus for reduction of the NO.sub.x from the flue gas while maintaining the effectiveness of the heretofore known reactors in such a way that the loss of pressure of the flue gas as well as the space required for the unit are minimized while at the same time reducing the energy requirement for the cleaning.

This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in conjunction with the accompanying drawings, in which:

FIG. 1 is a view that schematically illustrates an air preheater in the form of a tube heat exchanger which operates according to the cross flow principle and is inventively provided with a catalyst coating on the heat exchange elements;

FIG. 2 is a detailed view of a portion of a heat exchange element in the form of a finned tube which is provided with a catalyst coating;

FIG. 3 is a detailed view of a portion of a heat exchange element in the form of a catalyst-coated finned plate for a plate air preheater;

FIG. 4 is a cross-sectional view of a Ljungstrom-type air preheater which has a catalyst-coated heat storage mass;

FIG. 5 is a view that shows a rotary air preheater having radial gas feed and a catalyst-coated heat storage mass in the form of individual elements; and

FIG. 6 is a view that shows a combination of the tube air preheater having catalyst-coated heat exchange elements, and a subsequently disposed air preheater having a self-cleaning heat storage mass.

The apparatus of the present invention is characterized primarily by the use of a regenerative or recuperative type air preheater in which those surfaces of the heat exchange elements which are exposed to the flue gas are provided with a coating which acts as a catalyst for the NO.sub.x reduction.

To avoid the formation of deposits on the catalyst contact surfaces as a result of the flue gas, which is cooled off as it flows through the heat exchanger, falling below the condensation temperature, it is furthermore proposed pursuant to the present invention that the catalyst coating on the heat exchange elements only be provided in the region which has a flue gas temperature greater than the condensation temperature of the reaction products of the materials contained in the flue gas and of the added reducing agent.

Pursuant to another specific embodiment of the present invention, the air preheater which is provided with a catalyst coating on the heat exchange elements is designed for a flue gas exit temperature greater than the condensation temperature of the reaction products of the materials contained in the flue gas and of the added reducing agent, while connected downstream from this preheater is an air preheater having a self-cleaning heat storage mass which during operation is kept in a fluidized bed, and which comprises a plurality of separately movable individual elements.

Pursuant to yet another proposal of the present invention, individual heat exchange elements are provided one after the other in the direction of flow of the flue gas with a catalyst coating which is respectively optimally adapted to the reaction conditions for the flue gas temperature which is present in that region.

The advantages achieved with the present invention consist in that the space required for the system is considerably reduced due to the integration of the NO.sub.x reactor in the air preheater; likewise, the pressure losses which otherwise occur in an NO.sub.x reactor are avoided. By eliminating a separate housing for the catalyzers, simpler and hence more economical support constructions can be utilized. Since not only the catalyzers but also the heat exchange surfaces must be cleaned at certain time intervals, for example by means of soot blowers, only one such device is required for the apparatus of the present invention. A significant advantage of the present invention also is provided by coating the heat exchange elements in the downstream direction with different catalyst materials, the activity of which is respectively adapted to the optimum reaction conditions for the flue gas temperature which exists in this region. As a result, a high efficiency is achieved for the NO.sub.x reduction. Not only the catalyst but also the heat exchange elements are elements of the system which can wear out due to dust abrasion and corrosion, and must be periodically replaced. Since the heat exchange elements are inventively provided with a catalyst coating, with the apparatus of the present invention only one operation is required in order to effect such replacement.

Referring now to the drawings in detail, the air preheater 1 illustrated in FIG. 1 is a tube heat exchanger; the flue gas 2 and the air 3 which is to be heated up are guided in a cross flow. The air preheater 1 is divided into four zones 4, 5, 6, and 7 which are connected with one another via guide channels 8 for the air. Only the heat exchange elements in the zones 4, 5, and 6 are provided with a catalyst coating. Since, in contrast to the situation which exists in a separate reduction reactor, the flue gas is cooled off as it flows through a heat exchanger, it is inventively proposed to select the catalyst material in the individual zones in such a way that the activity of this material is adapted to the flue gas temperature which exists in a given zone. In this way, a high rate of reduction is achieved. The heat exchange element for the zones 4 to 6 can, for example, be the finned tube 9a shown in FIG. 2. With this embodiment, a particularly large contact surface of the catalyst is provided. Due to the fact that relative to the material of the heat exchange element 9a, for example steel tubes 11 (FIG. 2), the catalyst coating or layer 10 has a considerably lower heat conductivity, there is possible with this embodiment to provide a compact air preheater due to the greater heat exchange surface for the flue gas.

In the embodiment illustrated in FIG. 1, the zone 7 in the heat exchanger is provided with smooth tubes which have no catalyst coating. The reason for this is that the flue gas 2, which cools off as it flows through the heat exchanger, passes in the zone 7 through a temperature range in which deposits could form on the heat exchange elements due to the condensation of reaction products from the materials contained in the flue gas and from the added reducing agent. Thus, for example, the SO.sub.3 from flue gas reacts with the NH.sub.3 reducing agent to form ammonium sulfite, the condensation temperature of which is below approximately 250.degree. C. In order from time to time to rid the heat exchange elements of these deposits, a soot blower 12 is provided. Soot blowers for cleaning the catalyst contact surfaces, for example, can be disposed at the inlet for the flue gas. In place of tubes as heat exchange elements, plates or so-called plate-fins can be used in the air preheater. Such a use does not make a difference in principle in the manner of functioning thereof relative to an air preheater which is provided with tubes. A particularly suitable plate-fin element 9b is illustrated in FIG. 3. As in the case of the heat exchange element of FIG. 2, here also an increased surface, a greater catalyst contact surface and a greater heat exchange surface is produced for the flue gas.

FIG. 4 is a cross section through an air preheater of the so-called Ljungstrom type, which has a cylindrical storage mass which rotates during operation. In this case also, the storage mass is divided into several zones 4, 5, 6, and 7, with the storage mass in the lowermost zone 7 not being coated with catalyst material. As in the previous embodiment, the activity of the catalyst materials in the individual zones is adapted to the respective flue gas temperature in that zone, so that each layer or coating has optimum reaction conditions for the flue gas temperature which exists in that zone. In the embodiment of FIG. 4, a gap 13 is provided in the storage mass between the lowermost zone 7 and that zone 6 disposed thereabove; a cleaning device 12 is provided in the gap 13. With this cleaning device 12, cleaning can selectively be undertaken by means of compressed air 14 or water 15.

The regenerative air preheater illustrated in FIG. 5 has a rotating drum 16 and radial gas feed; the heat storage mass comprises a plurality of movable individual elements 17 in the form of, for example, spheres, hollow spheres, and/or saddle-shaped bodies, which are similarly provided with a catalyst coating or layer. Pursuant to this embodiment, the drum 16 is divided in the radial direction into individual chambers 18, with the heat conductive elements 17 in the individual chambers 18 being provided with catalyst coatings which are respectively adapted to the flue gas temperature. With this type of air preheater, which has separate, movable individual elements 17 as the heat storage mass, it is not necessary to provide cleaning means, since these elements rid themselves of deposits due to the fact that they rub against one another, and due to their whirling movements.

Another possibility for continuously keeping the heat exchange surfaces free of deposits is shown in FIG. 6. In this embodiment, disposed one after the other are a tube or plate air preheater 1 of the type illustrated in FIG. 1, and an air preheater in the form of a column heat exchanger 19 having a storage mass comprising a plurality of separate movable elements 20 which are circulated by means of a transporting mechanism 21. Only the heat exchange elements 9 of the plate or tube heat exchanger 1 are provided with a catalyst coating. In order to avoid deposits on the catalyst contact surfaces, this air preheater 1 is designed in such a way that the temperature of the exiting flue gases 22 is still greater than the condensation temperature of the reaction products of the materials contained in the flue gas and of the added reducing agent. In the subsequent column heat exchanger 19, deposits are avoided by the fact that the heat storage elements 20 are continuously kept in a fluidized bed due to the fact that the flue gases 22 and the combustion air 3 which is to be heated up flow in an upward direction. As a result, a self-cleaning effect of the subsequently arranged air preheater 19 is achieved.

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.