FI88363B - ROEKGASANLAEGGNING - Google Patents

Description

1 88363

Flue Gas System

The invention relates to a flue gas installation comprising a spray device for ammonia and a catalyst device 5 for reducing nitrogen oxides in the presence of ammonia and oxygen, the flue gas installation being provided downstream of the spray device with a first catalyst for reducing nitrogen oxides and a second catalyst for oxidizing ammonia.

A device for reducing nitrogen oxides from flue gases has already been proposed (DE patent application 3 627 834.3), in which a first heat exchanger, flue gas desulphurisation equipment for flue gas extraction, to recover significant heat from the flue gases 20 released from the nitrogen oxides flowing into the barrel. With this equipment, a technically sensible reduction of nitrogen oxides from flue gases is possible. The degree of reduction of nitrogen oxides achieved in this case corresponds to the type of nitrogen oxide removal equipment used, which is also called the DENOX equipment. 25 for toxins.

DE-A-2 458 888 has already disclosed a catalyst for the reduction of nitrogen oxides consisting essentially of titanium dioxide, vanadium pentoxide and other metal oxides. This catalyst: "30 is suitable for the reduction of nitrogen oxides from flue gases in the presence of ammonia and oxygen to nitrogen and water. However, these and other catalysts are characterized by the reduction of nitrogen oxides in the presence of ammonia by the amount of unchanged nitrogen oxides remaining in the flue gas." about 10% of the 2 88363 nitrogen oxides originally present. However, it is known that this emission could be further reduced when excess ammonia was available. In this case, however, unchanged ammonia would eventually remain in the flue gas. However, according to current regulations, this excess ammonia must not exceed a very low value in the flue gas of 4 mg / Nm ^. For these reasons, it has been inevitable to tolerate low emissions of unchanged nitrogen oxides in the flue gas.

The object of the invention is to show a way in which the residual concentration of nitrogen oxides in the flue gases can be further reduced without being converted to unacceptably high concentrations of other harmful substances in the flue gas.

This object is solved by the features of the characterizing part of claim 1. Other suitable embodiments are set out in claims 2-3.

The use of one catalyst in each case for the reduction of nitrogen oxides and one catalyst in each case for the oxidation of ammonia provides the conditions for injecting excess ammonia into the flue gases, without the remaining excess ammonia finally loading the flue gases released from the nitrogen oxides. At the same time, this will create the conditions for further reducing emissions of unchanged nitrogen oxides.

Catalysts for the oxidation of ammonia in the presence of oxygen to produce nitrogen are known from the Journal of Catalysis, 40, 312-317 (1975) and the Journal of Catalysis, 37, 258-266 (1975). However, these 30 studies started from a completely different task that has nothing to do with the removal of nitrogen oxides from flue gases.

A simple structure is achieved when, in a suitable embodiment of the invention, the two catalyst waves are machined together into a single unitary catalyst mass of 3,8363. This facilitates the maintenance and storage of the catalyst strip material and also eliminates confusion from maintenance work.

Advantageously, when using a reduction catalyst material consisting of titanium-5 oxide, vanadium pentoxide, molybdenum hydroxide and tungsten hydroxide, the proportion of molybdenum oxide can be increased to enhance the oxidation of ammonia. This results in a unitary catalyst body with a good conversion ratio of nitrogen oxides 10 and which is relatively insensitive to amounts of ammonia in excess of the stoichiometric amount.

Further details of the invention will be illustrated by the exemplary embodiment shown in the figures. In the drawings:

Figure 1 shows a schematic representation of a flue gas apparatus assembled in accordance with the invention leading to a chimney-15 tree, and

Figure 2 shows an enlarged schematic representation of a DENOX apparatus according to the invention.

In Fig. 1, reference numeral 1 denotes an arbitrary combustion system, the flue gas line 2 of which leads to a chimney 3. A first heat exchanger 4, a flue gas desulphurisation system 5, a second heat exchanger 6, an ammonia spraying device 7, OENOX, ·. Heat exchanger 9. The first and second heat exchangers are connected on the secondary side via pipelines 10, 11 for the heat transfer medium. The DENOX apparatus has two catalyst blocks 12, 13 connected in series in the flue gas flow direction. The first catalyst block 12 contains catalyst material for reducing nitrogen oxides in the presence of ammonia.

When the combustion plant 1 is used, hot s. 35 gaseous gases which, in addition to dust and ash and other gaseous 4 88363 constituents, also contain an excess of oxygen. These flue gases, as they pass through the first heat exchanger 4 connected to the flue gas line 2, first flow into the chimney 3 and, after being cooled to 160 ° C, end in the flue gas desulphurisation device 5. There they are freed from dust and ash particles as well as compounds. They leave the flue gas desulphurisation plant 5 at a temperature of about 65 ° C and are heated in the next second heat exchanger 6 by means of the heat previously taken in the first heat exchanger and transferred to the heat transfer medium again at a temperature of about ° C.

Ammonia is introduced into the flue gases thus heated through the spraying device 7. The spray rate is then preferably set so that an excess of about 10% is achieved over the required stoichiometric amount of ammonia. This over-stoichiometric ammonia mixture 20 clearly improves the efficiency of the catalytic reduction of nitrogen oxides in the first catalyst block 12 with respect to the otherwise expected efficiency, i.e. the residual nitrogen oxide content is further reduced. The first catalyst block 12 can be left largely nitrogen. However, the flue gases released from the silicon oxides now contain a significant residual amount of unchanged ammonia due to the superciometric spraying of ammonia. This excess ammonia is now passed with the remaining oxygen in the flue gases to the second catalyst block 30 13. Since this catalyst block contains an oxidation catalyst known per se, it oxidizes the ammonia with oxygen to nitrogen and water according to the formula 4 NH3 + 302 -> 2 N2 + 6 H2O. 35 5 88363 Such previously known NH 4 oxidation catalysts are, for example, W 2 O 2 mixed catalysts.

The hot flue gases 5 leaving the catalyst assembly of the DENOX plant 8, free from sulfur and nitrogen oxides, are cooled in the next third heat exchanger 9 to a chimney inlet temperature of about 100 ° C and then passed through the chimney 3. With this structure, the amount of ammonia remaining in the flue gases can be reduced to less than 10 4 mg / Nm3, despite over-stoichiometric spraying, and the remaining amount of nitric oxide is directly due to over-stoichiometric ammonia injection, well below the otherwise normal 10% of the original nitrogen oxides.

It would also be possible to melt both catalyst block blocks, which may optionally consist of bulk, preferably layers of cubic catalyst stones or plates coated with catalyst mass, into a single piece. It would also be possible to process both catalyst materials into a single unitary, mixed catalyst material. The latter would have the practical advantage of being able to start with a uniform material for maintenance and renewal. Such a uniform catalyst material could also be obtained from a known catalyst for the reduction of nitrogen oxides in the presence of ammonia on the basis of titanium oxide + vanadium oxide + molybdenum oxide + tungsten oxide by increasing the proportion of molybdenum.

Claims (3)

6 68363 A flue gas installation comprising a spray device (7) for ammonia and a catalyst device 5 (8) for reducing nitrogen oxides in the presence of ammonia and oxygen, the flue gas installation (2) being provided with a first catalyst (12) downstream of the spray device (7) for reducing nitrogen oxides. and a second catalyst (13) for oxidizing ammonia in the presence of oxygen, characterized in that the first catalyst (12) is arranged in a first catalyst block upstream of the second catalyst block containing the second catalyst (13), and that the second catalyst (13) consists of a molybdenum mixed oxide catalyst. Flue gas installation according to Claim 1, characterized in that the proportion of molybdenum oxide in the mixed catalyst is increased by using a reduction catalyst consisting of oxides TiO 2, V 2 O 5 'Mo® 3 or wO 3 for nitric oxide. Flue gas installation according to Claim 1, characterized in that a maximum of 15% excess ammonia is used in the spraying device (7) in order to completely reduce nitrogen oxides. 7 88363