CA2095938A1

CA2095938A1 - Removal of nitrogen from flue gases at low temperatures

Info

Publication number: CA2095938A1
Application number: CA002095938A
Authority: CA
Inventors: Stephan Blumrich; Bernd Engler; Lutz Kunze; Stephan Laubenstein; Erik Peldszus; Herbert Muller
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1992-05-12
Filing date: 1993-05-11
Publication date: 1993-11-13
Also published as: BR9301807A; JPH0647282A; DE4215582C1; CN1081123A; KR940005317A; EP0569877A1

Abstract

ABSTRACT OF THE DISCLOSURE

The object of the invention is a catalyst for the removal of nitrogen from flue gases at low temperatures. The activity of a DENOX catalyst which is known per se in terms of removal of nitrogen from flue gases at low temperatures can be improved by rendering it hydrophobic, e.g.; by coating with Teflon TM.

eg (T1O2/WO3/V2O5)

Description

3 ~3 The invention rela~es to a catalyst ~or the removal of nitrogen from flue gases at low temperatures using the selective catalytic reduction (SCR) process, and additionally a process for the preparation thereof and a process for the removal of nitrogen at low temperatures using the catalyst.

If the battle against increasing environmental pollution is to be won, every possible avenue must be explored to reduce emissions. ~he principal cause of air pollution is combustive processes for generating mechanical or thermal energy from fossile fuels. Nitrogen oxides, which are one of the causes of acid rain, are among the pollutants which result from these processes. The selective catalytic reduction process (SCR process) has become widely used to remove nitrogen oxides from boiler fuel burning plant flue gases. The catalysts which are required for this process are disclosed, for example, in German Patent Specifications DE 37 40 289, DE 3~3 41 990 and DE 39 06 136, and also in VS
4,085,193. They are known as DENOX catalysts.
Vanadium pentoxide, variously doped zeolites, and also iron are utilised as catalytically active components. But it is a requirement of any catalyst that it should work within a specific operating temperature range. At high temperatures problems arise in terms of more rapid catalyst ageing, and at low temperatures deactivation processes occur as a result of masking of the active centres, by, for example, ammonium salts or interaction with water. Such deactivation is generally reversible, ie the masking substances can be driven off by heating the catalyst. The heating devices necessary for this purpose must, however, be installed and intermittently operated in the flue gas purification plant.

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Capital and operating costs for the necessary arrangements are so high that this technique is utilised only exceptionally.

Refuse incineration plant exhaust gases present conditions which are particularly unfavourable to DENOX catalysts.
Present-day refuse incinerator flue gas purification plant are characteristically required to remove all noxious substances virtually completely from the exhaust gases.
Purification stages comprising various types of carbon filters (activated carbon/open hearth coke) or fikrous filters charged with lime-activated carbon mixtures (suspended stream process) have proved advantageous for this purpose. These filters are usually operated at temperatures of between 80 and 160C, and preferably between 100 and 140C. It has also emerged that a nitrogen removal stage downstream of these filters, using the selective catalytic reduction process, is able to work very effectively because the exhaust gas contains the minimum possible level of sulphur oxide~, thus eliminating the risk of catalyst masking by ammonium salts. However, the water content of these flue gases, which is usually high (> 25 vol-%), has been found to bring about a reduction in activity.

The present invention provides a catalyst having a nitrogen removal activity which remains to the greatest possible extent intact and gives satisfactory results even when exhaust gas water contents are high. Further, the invention provides a process for the preparation of the catalyst and a process for the removal of nitrogen at low temperatures using the catalyst.

According to the invention there is provided hydrophobic impregnation of a DENOX catalyst which is known per se. The ~J~3~38 agent used to render it hydrophobic must meet specific criteria. It must not contain catalyst poisons or clog the porous structure of the catalyst. It should furthermore retain its ability to function at temperatures of up to 350C. Teflon has proved suitable as an agent which imparts a hydrophobic property and meets these requirements. A
catalyst according to the invention is obtained by impregnating a DENOX catalyst with Teflon by single or multiple immersion in a Teflon suspension, drying at 120C, and then tempering in an air current at 350C for 3 hours.
This catalyst is inserted in the flue gas current downstream of the filters usually used for flue gas purification, for the purpose of removal of nitrogen from flue gases at low temperatures. The nitrogen oxides which the flue gases still contain are, with addition of a reducing agent, preferably ammonia, reduced to nitrogen and water on the catalyst according to the invention, at temperatures of between 130 and 350C.

Catalysts which have been rendered hydrophobic are also already known from German Patent Specification DE 37 35 758.
These are catalysts on activated carbon which have been doped with noble metals, for use in oxygen reduction in aqueous media, with addition of hydrogen gas, at temperatures of up to a maximum of 60C. The experimental resuits shown in this 5pecification do not suggest that catalysts which have been rendered hydrophobic have any particular advantages in te~ms of oxygen reduction in aqueous media. It was therefore all the more surprising and unexpected to persons skilled in the art that when DENOX catalysts are rendered hydrophobic they -* ~rade Mark ~a~.~s~

exert a positive influence on the removal of nitrogen from flue gases at low temperaturss. Compared with the untreated catalysts, the catalysts which have received hydrophobic treatment exhibit appreciable advantages in terms of activity within the temperature range 130 to 350C.
,.
The invention is elucidated below in greater detail with theaid of Example and the accompanying drawings, in which:

Fig. 1: N0x conversions catalysed by a comparative catalyst and by a catalyst according to the invention, as a function of the water content of a synthetic gas Fig. 2: N0xconversions catalysed by a comparative catalyst and by a catalyst according to the invention, as a function of the water content of the flue gas in a pilot plant Fig. 3: NOX conversions catalysed by a comparative catalyst and by a catalyst according to the invention, as a function of the temperature of the flue gas in the pilot plant.

Comparative Preparation of a comparative catalyst Example 1:

A comparative catalyst of Tio2/Wo3/V2o5 was prepared in the form of a monolithic honeycomb body in accordance with the recipe given in Example 5 of DE 39 06 136 as a solid catalyst.

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Example 1: Preparation of a catalyst according to the invention A catalyst prepared in accordance with Comparative Example 1 was impregnated by a single immersion in a 5 wt-% suspension of a Teflon powder (30 ~m particle diameter; 30 W ex Du Pont). The catalyst was then dried at 120C and tempered in an air current at 350C for 3 hours.

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Example 2: Catalyst testing in a synthetic gas testing plant The NOX conver~ion activity levels of the comparative cataly~t and the catalyst according to the invention were determined in a synthetic gas testing plant, varying the water content of the synthetic gas.

Table 1: Composition of synthetic gas NO 500 ppm by vol.
2 4.5 vol-%
N2 to 100%
H2 variable Test gas temperature 1~0C
NH3/N0x molar ratio 1.2 Volumetric flow 2.59 mN3/h Space velocity SV 16S00 1/h 20 Specific catalyst loading AV 25 m/h The results of these activity measurements are listed in Table 2 and shown in graph form in Fig. 1. The activity curve of the comparative catalyst shows a marked drop once water contents exceed 15%. And by 30% water content only half of the activity of the comparati.ve catalyst remains.
A catalyst having double the volume of catalyst would have to be avai.lable in a f]ue gas purificati.on plant in order to compensate for this loss of acti.vity. The catalyst according to the inventi.on on the other hand retains its activity irrespective of the water content.

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Table 2: Activi~y mea~urements in synthetic gas Catalyst according to Comparative cataly~t S the invention H20 NOin No~t Conver- NOin NOout Co.nver-con- (ppm) (pp~) sion (%) (ppm) (ppm) sion (%) tent (vol-%) 0 500 299 40.2 487 33132.0 493 326 33.9 494 32334.6 486 331 31.9 481 31734.1 1515 486 333 31.5 511 32835.8 483 329 31.9 48S 34129.7 476 320 32.8 505 36926.9 474 331 30.2 511 4151~.8 475 332 30.1 507 43114.9 2040 474 330 30.4 502 44411.5 473 331 30.0 500 4598.2 472 330 30~1 499 4676.~ .

E~ample 3: Activity tests on a pilot plant at different flue gas water contents The catalysts were tested under field conditions on a pilot plant downstream of a natural gas furnace. The volumetric flow of the flue gas was 75 mN3/h, corresponding to a space velocity of SV = 16500 1/h. The NOX input concentration varied between 520 and 560 ppm, depending on operating 3s condition.

Starting at a water concerltrati.orl of 14 vol.-% in the exhaust gas of a natural. gas furrlace, the water content of the flue gas was increased to 62 vol-% by injecting steam.
In these experiments, which were conducted Wi.t}l a NH3/NOy molar ratio oI 0.7~ and at a temperature of ].60C the catalyst according to the inventi.on showed l~x conversion rates markedly superior to those oi the comparative catalyst. rhe results are showll as a grapll i.n ~ig. 2.

2 ~ 3 8 Example 4: Activity tests on the pilot plant at di~ferent ~lue gas temperatures Fig. 3 gives the results of the NOX conversion measurements on the pilot plant at ~lue gas temperatures of between 130 and 250C. ~he water content, at 14%, was within the region o~ the least difference, the NH3/NOX molar ratio was set at l.o for these e~periments. Marked differences in the performance of the catalysts can be observed, despite the low water content. Because of their e~cessively low activity when the flue gas temperature is below 130C, it is no longer economically viable to use the catalysts at these temperatures, since for acceptable NOX conversions still to be achieved, very great volumes of catalyst would have to be available. At above 250C, the advantage over the co~parative catalyst of the catalyst impregnated with Teflon according to the invention becomes ever less marked.
The Teflon begins to melt at temperatures above 350C, thus reducing the activity of the catalyst. Furthermore, there is little point in treatment to impart a hydrophobic property, since at temperatures such as these surface water adsorption virtually ceases.

The catalyst according to the invention, howevex, has given a performance markedly superior to that of a prior art comparative catalyst, within the claimed temperature range.

Claims

1. Catalyst for the removal of nitrogen from flue gases at low temperatures, comprising a DENOX catalyst with a hydrophobic coating.

2. Catalyst according to Claim 1, wherein the catalyst is coated with Teflon TM.

3. Process for the preparation of a catalyst according to Claim 1, wherein the DENOX catalyst is impregnated with Teflon by single or multiple immersion in a Teflon suspension, dried at 130°C, and then tempered in an air current at 350°C for 3 hours.

4. Process for the removal of nitrogen from flue gases at low temperatures, wherein the flue gas for treatment is purified by means of carbon or fibrous filters and, with addition of a reducing agent, the remaining nitrogen oxides are reduced to nitrogen and water on a catalyst according to Claim 1 at temperatures of between 130 and 350°C.

5. Process according to Claim 4, wherein the reducing agent is ammonia.