DK174013B1 - Process for reducing nitrogen oxides in a flue gas, and a suitable flue gas conduit - Google Patents

Description

in DK 174013 B1

The invention relates to a process for reducing the nitrogen oxides in a flue gas by catalytic reduction of the nitrogen oxides by means of ammonia supplied to the flue gas.

The invention further relates to a flue gas conduit with an injection nozzle for supplying ammonia to the flue gas and with a catalyst device arranged in the flow direction after the injection nozzle and, firstly, contains a first catalyst material for reducing the nitrogen oxides in the presence and catalyst material for oxidation in the presence of oxygen, or Methods and flue gas lines of the kind mentioned above are known from DE 2 919 812 A1 and from DE 3 601 378 C2.

The flue gas pipelines known from these documents include a catalyst material intended for the oxidation of sulfur dioxide contained in the waste gas flowing through the flue gas to sulfur trioxide. ice A device for reducing nitrogen oxides in flue gases is already known (DE publication no. 36 27 834 A1), in which a first heat exchanger, a flue gas desulphuriser, a second heat exchanger connected to the first heat exchanger is arranged one after the other. for reheating the desulfurized flue gases, a plant for reducing the nitrogen oxides in the flue gases, and an additional heat exchanger for reheating with the sensitive heat of the chimney flue-gaseous flue gases. This device enables an energetically rational reduction of nitrogen oxides in the flue gases. The degree of reduction of nitrogen oxides obtained by this corresponds to the nature of the de-titration plant, also called 25 DENOX plants.

DE Patent Specification 24 38 888 C3 already discloses a catalyst for reducing nitrogen oxides consisting essentially of titanium dioxide, vanadium pentoxide and additional metal oxides. This catalyst is suitable for reducing nitrogen oxides in flue gases in the presence of ammonia and oxygen to 30 nitrogen and water. However, it is a peculiarity of this and other catalysts for reducing nitrogen oxides in the presence of ammonia that the amount of the non-reacted nitrogen oxides remaining in the flue gas - the so-called "emission" (Schlupf) - amounts to approx. 10% of the nitrogen oxides originally present. It is well known that this emission could be further reduced by 35 if ammonia excess was worked. In this case, however, ammonia would not remain in the flue gas eventually. This excess ammonia, according to the applicable emission laws, must not exceed a very low value in the flue gas at 4 mg / m3 nitrogen. For these reasons, it has so far been inevitable to have to tolerate a small release of unreacted nitrogen oxides into the flue gas.

It is an object of the invention to avoid the disadvantages of the known plants.

5 The object of the invention is to find a way to reduce the residual content of nitrogen oxides in the flue gases, without, in turn, introducing prohibitively large additions of other harmful substances into the flue gas.

For the purpose of this task, a method for reducing the oxides of nitrogen in a flue gas by catalytic reduction of the nitrogen oxides by means of ammonia supplied to the flue gas is disclosed, in which to reduce the nitrogen oxides an excess stoichiometric amount of ammonia is added to the flue gas. ammonia is oxidized in a catalyst which, in the presence of oxygen, is selectively effective for the reaction 4NH3 + 30 * - * 2N * + 6H * 0. For the purpose of this task, a flue gas pipe is also provided with an injection nozzle for supplying ammonia to the flue gas and with a catalyst device which is arranged in the flow direction after the injection nozzle and, in order, first contains a catalyst material for reducing the nitrogen oxide under the nitrogen oxide and another catalyst material for oxidation in the presence of oxygen, the second catalyst material selectively catalyzing the reaction 4NH3 + 30 * - »2N2 + 6H * 0.

Further suitable embodiments are set out in the dependent claims, with reference to preferred embodiments of the method, respectively. flue duct.

25 The incorporation of a catalyst material for reducing nitrogen oxide there and a catalyst material for oxidation of ammonia provides a prerequisite for injecting an excess of ammonia into the flue gases without depleting the latter excess ammonia gases. Thus, at the same time, a precondition for the release of non-reacted nitrogen oxides is also provided.

Admittedly, catalysts for oxidation of ammonia in the presence of oxygen are known from the Journal of Catalysis, 40, 312-317 (1975) and from the Journal of Catalysis, 37, 258-266 (1975). However, these studies were based on completely different issues without referring to 35 flue gas nitrification.

The efficiency of the flue gas pipe according to the invention is maximized if the first catalyst material is suitably contained in a catalyst block which is built in the flow direction in front of the second catalyst material contained in the flow direction. In this case, further, the unabated excess of ammonia is available by reducing nitrogen oxides, and this residual amount is oxidized only when the nitrogen oxides in the flue gas are degraded.

A simplified construction is obtained if both catalyst materials in one appropriate embodiment of the invention are assembled together into a single catalyst mass. This facilitates the maintenance and spare part storage of the catalyst material and eliminates any confusion on maintenance work. Conveniently, the amount of molybdenum oxide to enhance the ammonia oxidation can be increased by using a titanium dioxide, vanadium pentoxide, molybdenum oxide and tungsten oxide reduction catalyst material. This leads to a single catalyst body which has good nitrogen oxide conversion rates and is relatively insensitive to over stoichiometric ammonium oxide amounts.

Preferably, both catalyst materials are processed into a single catalyst mass which oxidizes ammonia to nitrogen, with the catalyst arrangement embedded in a hot section of the flue gas conduit (2).

Conveniently, a molybdenum blend catalyst is used as an oxidation catalyst, but Cu (II) Na-Y-Zeolite can also be used as an oxidation catalyst.

A maximum of 15% excess ammonia is induced to completely reduce the nitrogen oxides.

Further details of the invention are explained by means of an exemplary embodiment shown in the figures. In the drawing: FIG. In a schematic representation of a flue pipe equipped according to the invention according to the invention, and FIG. 2 is an enlarged view of a DENOX plant according to the invention.

In FIG. 1, reference numeral 1 indicates any combustion plant whose flue gas line 2 leads to a chimney 3. In the flue gas line 2 35, a first heat exchanger 4, a flue gas desulfurization system S, a second heat exchanger 6, an ammonia injector 7 are injected successively. , a DENOX system 8 and a third heat exchanger 9. The first and DK 174013 B1 4 the second heat exchanger are interconnected on the secondary side via pipelines 10, 11 to the heat transport medium. In the DENOX plant, two catalyst blocks 12, 13 are successively coupled in the direction of flow of the flue gases. The first catalyst block 12 contains a catalyst material for reducing nitrogen oxides in the presence of ammonia. The second catalyst block 13 contains a catalyst material for oxidation of ammonia in the presence of oxygen.

During the operation of the combustion plant 1, hot flue gases are generated which, in addition to dust and ash and other gaseous components, also contain excess amounts of oxygen. These flue gases flow on their way to the chimney 3 through the first first heat exchanger 4 connected to the flue pipe 2 and come after they have been cooled to approx. 160 ° C, into the flue gas desulphurisation plant 5. There they are liberated in a manner known per se for dust and ash particles and for sulfur containing compounds. They leave the flue gas-desulfurization plant 5 by approx. 65 ° C and reheated in the following, the second was heat exchanger 6 by means of the heat extracted previously extracted in the first heat exchanger and transferred to the catalysts 12, 13 adapted to the optimum gas temperature adapted to the catalysts used in the DENOX plant - in the present case. ca. 300 ° C.

In the flue gases thus heated, 7 ammonia is supplied via the injection nozzle. The injected amount is preferably adjusted below such that, relative to the required stoichiometric amounts of ammonia, approx.

10% profit available. In this over-stoichiometric ammonia mixture, the efficiency of the catalytic reduction of nitrogen oxides in the first catalyst block 12 is clearly improved over the otherwise expected efficiency, i.e. the remaining nitrogen content is further reduced. However, as a result of the over-stoichiometric injection of ammonia extracted from the first catalyst block 12, which is as far as possible de-titrated, there is now a noticeable residual amount of unreacted ammonia.

This excess ammonia is now fed with the oxy residual amount in the flue gas to the second catalyst block 13. Since this catalyst block contains a per se known oxidation catalyst, the ammonia in it is oxidized with the oxygen to nitrogen and water according to the formula:

4NH3 + 302 - * 2Nj + 6HjO

Such prior art NH 3 oxidation catalysts are e.g. W03-mixing catalysts.

DK 174013 B1 5

The heat emitted from the catalyst device in the DENOX system 8 is desulphurised and de-sulfurized flue gases in the subsequent third heat exchanger 9 to the chimney entry temperature of approx. 100 ° C and then escapes through the chimney 2. In this construction, the amount of ammonia remaining in the flue gas, despite over-stoichiometric injection, can be reduced below 4 mg / m3 nitrogen, and the residual nitrogen amount is precisely due to the over-stoichiometric ammonia injection. usual 10% for the original nitrogen oxides.

It would also be possible to merge the two catalyst blocks, which may optionally consist of rice bricks, layers of preferably cubic catalyst bricks or of catalyst mass coated sheets, into a single block. It would also be possible to process the two catalyst materials into a single, cohesive, mixed catalyst material. The latter would have the practical advantage that a single material can be used in maintenance and extension. Such an ice aggregate catalyst material could also be provided by increasing the molybdenum amount on the basis of a known catalyst for reducing nitrogen oxides in the presence of ammonia based on titanium oxide + vanadium oxide + molybdenum oxide + tungsten oxide.

Claims (13)

A process (for reducing the nitrogen oxides in a flue gas by catalytic reduction of the nitrogen oxides by means of 5 ammonia added to the flue gas, characterized in that an excess stoichiometric amount of ammonia in the flue gas is added to the flue gas and the unreacted ammonia is not reacted. a catalyst which, in the presence of oxygen, is selectively effective for the reaction 4NH, + 30, - 2N, + 6H, 010 Process according to claim 1, characterized in that the excess ammonia excess is not more than 15% of the stoichiometric reduction needed for the reduction of nitrogen oxides. Process according to claim 1 or 2, characterized in that the catalytic reduction of the nitrogen oxides and the catalytic oxidation of the excess ammonia takes place in two different, successively coupled catalysts, wherein the nitrogen oxides are reduced in a catalyst as in at least titanium oxide and that for the oxidation of the excess ammonia, zeolite is used at most in the second catalyst. Process according to claim 1 or 2, characterized in that the catalytic reduction of the nitrogen oxides and the catalytic oxidation of the excess ammonia are carried out with catalysts which have been processed into a single catalyst block. Process according to claims 1 - 4, characterized in that the flue gas can be passed through a desulphurisation plant without it. 6. Flue gas conduit with an injection nozzle for supplying ammonia to the flue gas and with a catalyst device disposed in the flow direction after the injection nozzle and sequentially containing a first catalyst material for reducing the oxides of nitrogen in the presence of 30 ammonia ammonia and a second catalyst for oxygen, characterized in that the second catalyst material selectively catalyzes the reaction 4NH3 + 30, -> 2N, + 6H, 0 35 Flue gas conduit according to claim 6, characterized in that the second catalyst material contains a molybdenum mixture oxide catalyst. DK 174013 B1 Flue gas pipe according to claim 6, characterized in that the second catalyst material contains Cu (II) Na-Y-Zeolite. 9. A flue gas line having an injection nozzle for ammonia and a catalyst device disposed in the flow direction after the injection nozzle containing a first catalyst material for reducing the nitrogen oxides in the presence of the ammonia and a second catalyst material for the oxidation in the presence of oxygen both catalyst materials are processed into a single catalyst mass which oxidizes ammonia to nitrogen. in Island Flue gas pipe according to claim 9, characterized in that the first catalyst mass is a TiO 2 / V 2 O 2 / MoO 3 / WO 3 reduction catalyst and that the second catalyst mass for amplifying the ammonia oxidation has an increased MoO 3 amount. Flue gas pipe according to any one of claims 6 to 10, characterized in that the ice catalyst arrangement is arranged in one and the same catalyst housing (8). Flue gas pipe according to any one of claims 6-11, characterized in that the catalyst arrangement is built into a hot section of the flue gas pipe (2 ° 0 to 500 ° C). Flue gas pipe according to any of claims 6 · 12, characterized in that an additional desulphurisation system is used for the flue gas.