DD296617A5 - CATALYST FOR THE CLEANING OF EXHAUST GASES FROM COMBUSTION ENGINES AND GAS TURBINES - Google Patents

Abstract

The invention relates to a catalyst for purifying exhaust gases, in particular from overstoichiometrically operated internal combustion engines and gas turbines in integral honeycomb form, which has a catalyst for the selective reduction of nitrogen oxides with NH 3 in an upstream section and an oxidation catalyst in a section at the outlet side.

Description

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description

The invention relates to a catalyst for the removal of pollutants from exhaust gases, in particular of superstoichiometrically operated internal combustion engines and gas turbines.

One source of existing pollution is the emissions from combustion processes in engines and gas turbines.

The air pollutants contained in the exhaust gases nitrogen oxides, carbon monoxide and unburned hydrocarbons of various compositions and well known in terms of their polluting effect.

Since primary measures to reduce emissions of pollutants, e.g. Exhaust gas recirculation or combustion chamber modifications, so far not provide the required reduction of pollutant emissions or reduce the efficiencies of the facilities unreasonable, must also in internal combustion engines and gas turbines secondary measures such. B.

catalytic exhaust gas purification can be used.

The resulting in the combustion of liquid or gaseous fuels in internal combustion engines and gas turbine exhaust gases can not be cleaned in the superstoichiometric operation mode of operation according to the three-way principle. The removal of unburned hydrocarbons present in the exhaust gas and carbon monoxide can be carried out by catalytic oxidation of an oxidation catalyst by utilizing the oxygen contained in the exhaust gas to the environmentally friendly compounds carbon dioxide and water. The removal of nitrogen oxides succeeds because of the oxygen content only after ancestors of the selective catalytic reduction. As a selective reducing agent z. For example, ammonia, optionally from an ammonia-donating chemical, such as urea, proven that easily reacts with a suitable catalyst with the oxides of nitrogen, but only to a small extent with the oxygen.

In previous systems with internal combustion engines, the purification of the exhaust gases described above is carried out according to the following methods:

After one of the conventional processes, the exhaust gases, which are between 400 and 600 ^° C., flow directly after the engine, first of all through an oxidation catalytic converter; There, carbon monoxide and hydrocarbons are oxidized to carbon monoxide and water with the help of the oxygen contained in the exhaust gas. Subsequently, the exhaust gases are passed through a heat exchanger and cooled to the temperature necessary for carrying out the selective catalytic reduction (350 to 400 ⁰ C).

After injection of ammonia and mixing thereof with the exhaust gas, the nitrogen oxides react with the ammonia on a reduction catalyst to form nitrogen and water in the subsequent reactor. The amount of the admixed ammonia depends on the nitrogen oxide charge contained in the exhaust gas and the desired conversion rate. Due to procedural conditions it comes through stratification again and again to local overdoses of ammonia. The consequence of this is the so-called ammonia, ie. Unreacted ammonia enters the exhaust stream downstream of the reduction catalyst and can thus escape via the chimney into the atmosphere as unwanted secondary emission. When operating with sulfur-containing fuels (eg., Diesel fuel, heavy fuel oil or biogas), the ammonia slip due to reactions between ammonia and sulfur oxides contained in the exhaust gas to corrosive, sticky and efficiency-reducing deposits of ammonium hydrogen sulfate and / or ammonium sulfate in downstream equipment, eg. B. heat exchangers. The thus periodically becoming necessary washing downstream equipment also creates a sewage problem.

Weitete disadvantages of this arrangement are caused by numerous extensions and narrowing of the flow cross section additional pressure loss, and the additional high cost of the separate reactors.

The "process for the purification of oxides of nitrogen and sulfur contained exhaust gas from incineration plants" beschrierjene in the patent DE-PS 3601378 can not be readily transferred to the application "internal combustion engines". For economic reasons, due to the low sulfur dioxide concentrations in combustion engine and Gasturbinenabgbsen a sulfuric acid production must be omitted. The 250 to 550 ° C hot exhaust gas is passed there after loading with the required amount of ammonia over two in uinsm reactor successively arranged different types of catalysts. In the first stage of the catalyst, selective catalytic reduction of nitrogen oxides to nitrogen and water takes place. The subsequent oxidation catalyst is tuned in the said process to the best possible Schwefeltrioxideneugung and acid resistance and resistance to sulfur trioxide. For the low-sulfur exhaust gases from internal combustion engines and gas turbines therefore a catalyst must be used, which is optimized specifically for the catalytic oxidation of hydrocarbons and carbon monoxide.

The subject matter of the invention is a catalyst which makes it possible to avoid disadvantages of conventional exhaust-gas purification methods in internal combustion engines and gas turbines, in particular if they are operated more than stoichiometrically.

The catalyst of the invention is characterized in that it consists of a one-piece honeycomb Exhaust gas purifying catalyst consists of a catalyst in the upstream section for the selective reduction of Nitrogen oxides by means of ammonia gas, optionally from an ammonia-donating compound, and in a downstream side Section has an oxidation catalyst. Its principle, then, is that the exhaust gases are in a single

honeycomb catalyst element immediately in succession with two different, each reaction-specific optimized catalyst formulations, which may be known in part, are brought into contact; the different catalyst zones (or reactions) for selective nitrogen oxide reduction and the subsequent one

Oxidation is hereafter referred to as zone 1 for the part of the reduction and zone 2 for the part of the oxidation. The catalyst of the invention can be completely as a supported catalyst odc but as a full catalyst with a Oxidation catalyst coating carried out in zone 2 After the former application form, zones 1 and 2 are two co-catalyst coatings on a coating

essentially inert, structurally reinforcing carrier or metal applied. Suitable carrier materials are

Ceramic body in honeycomb form of e.g. a-alumina, mullite, cordierite, zirconium mullite, batalite titanite, porcelain, thorium oxide, Steatite, boron carbide, silicon carbide, silicon nitride or Metallkörpei stainless steel or so-called. Heizleiterlegierungen (Cr / Fe / Al Alloys). After the second-mentioned, advantageous application form is based on a full catalyst in honeycomb form, consisting of a Metal oxide mixture or a metal-containing zeolite according to the claims on the downstream side Section (zone 2), an oxidation catalyst applied as a coating. The pollutant-containing exhaust gas is the exhaust gas purification system containing the catalyst in a temperature range

supplied between 250 and 55O ⁰ C. The decision criterion for the temperature levels selected in a given installation are technical or economic reasons, such as exhaust gas temperature of the engine, ammonia oxidation,

Production costs, required reduction rates for the pollutants etc. The exhaust gas is after entering a catalyst arrangement vorqeschaltote mixing device with the reducing agent

(Ammonia, optionally from an ammonia-donating chemical) mixed.

It is then passed over the catalyst zone 1, in which the selective catalytic reduction of nitrogen oxides to nitrogen

and water takes place. In Zone 2 immediately following the same catalyst element, not only are carbon monoxide and the hydrocarbons oxidized, but also the ammonia passed through Zone 1 is removed. Specific, known per se formulations for oxidation catalysts are also suitable for the conversion of sulfur dioxide in

Sulfur trioxide. The end products of the catalytic reduction and oxidation processes are carbon dioxide, water and Nitrogen or sulfur trioxide. The exhaust gas leaving the reactor after leaving the oxidation zone no longer contains any ammonia, since the ammonia is too Nitrogen and nitrogen oxides is oxidized. It can therefore no longer to the bonds, encrustations and corrosion

come through ammonia salts in the plant. The slight increase in nitrogen oxide emissions by the oxidized to nitrogen oxides

Ammonia can be kept to a minimum by simple control measures. The use of the one-piece but two-zone catalysts of the invention brings next to those already described

procedural advantages even further advantages: By eliminating multiple extensions and narrowing of the

Strömungsque. Section in bräulichen series circuits and turbulence zones between several catalyst elements

the pressure loss is significantly reduced.

Another noteworthy advantage is the cost reduction over the prior art by reducing the number

the reactors. The cost of packaging and storage of the catalyst elements in the reactor can be further reduced by the arrangement of the invention.

For the reduction and at the same time as a carrier for the designated as zone 2 oxidation region can be Vollextrudate with advantage

be used in honeycomb form according to the dependent claims 2 to 5 of this application, so-called. Full catalysts, consisting of titanium dioxide with additions of z. B. tungsten and vanadium oxide z. B. according to

DE-PS 2458888 (claim 2), German patent application P 3740289.7-41 (claim 3), German patent application P 3906136.1 (claim 4) or consisting of zeolite z. b. according to German patent application

P 3841990.4-43 (claim 5).

The oxidation zone 2 may be designed so that a suitable catalyst coating on the reduction catalyst in

is applied to a rear portion thereof, wherein, depending on the design case, the coated with the oxidizing formulation range 20 to 50% of the catalyst volume occupies (claim 6). In general, it suffices to coat 25 to 35% of each catalyst element with the oxidation catalyst. The formulation of the oxidation catalyst may (in

Based on DE-PS 2907106) correspond to those of claim 7 or from γ-alumina with additions of 3 to

35 wt .-% of cerium oxide and 1 to 5 wt .-% zirconium oxide and platinum, platinum / palladium, platinum / rhodium or palladium, wherein the noble metal is present in reduced highly dispersive form on the catalyst.

It is favorable if the noble metal content in the catalytic coating of Zone 2 is 0.25 to 2.8% by weight. Example 1-4 Based on the German patent application P 3740289.7-41, Example 17, a catalyst is in honeycomb form

(Dimensions: 150mm χ 150mm χ 440mm, cell division: 3.6mm) with a weight ratio TiO ₂ / WO ₃ of 9: 1 and one

V ₂ O ₆ content of 0.45 wt .-% prepared. As the titanium dioxide component is a flame-hydrolytically produced TiO ₂

used according to claim 3.

In order to apply the oxidation coating to 30% of the total length, an oxide mixture is produced in the first working step Y-Al ₂ O ₃ , CeO ₂ and ZrO ₂ are applied as follows: In a 25 wt .-% aqueous suspension of γ-alumina containing per 100 g of Y-Al ₂ O ₃ 60 g of CeO ₂ and 3 g of ZrO ₂ in Added form of their acetates. A coating is carried out by dipping the downstream side of the above-mentioned honeycomb body

corresponding to 10% of the total length in this suspension. Following the immersion step, the channels of the monolith with

Blown compressed air and dried at 150 ⁰ C in a stream of air. Thereafter, it is annealed at 55O ⁰ C for two hours. When Mixed oxide coating remain 80-9Og per 1 liter of catalyst volume.

The noble metal is obtained by impregnation with an aqueous solution of hexachloroplatinic acid, palladium chloride or rhodium chloride. After drying at 15O ⁰ C, the precious metals are reduced at 550 ° C in a hydrogen atmosphere. The noble metal amounts are given in Table 2.

Table 2

Precious metal amounts in the oxide catalyst coating

Example. precious metals precious metal content based on the oxidation coating Wt .-% 1 Pt 1.5 2 Pt / Pd 1.5 (2: 1) 3 Pt / Rh 1.5 (10; 1) 4 Pd 1.5

Example 5

According to the German patent 2458888, Example X-1, a honeycomb body with the same geometric dimensions, as described in Example 1, prepared. The TiO ₂ : WO ₃ weight ratio is 9: 1, the V ₂ O ₆ content 0.45 wt .-%. The TiO ₂ component used is anatase-type precipitated TiO ₂ having a specific surface area of 7OmVg. The application of the oxidation catalyst coating is carried out as described in Examples 1-4 from the downstream side to 20% of the total length. The precious metal used is 2.5% by weight of platinum, based on the coating.

Example β According to German Patent Application P 3841990, Example 32, a zeolitic honeycomb body having the same geometric dimensions as described in Example 1 is produced.

Mordenite with the modulus 19 (SiO 2 -AljOa molar ratio) was used as the zeolite. The ion exchange-incorporated active components are 1.0 wt% copper, 0.58 wt% iron, and 0.1 wt% cerium.

The application of the oxidation catalyst coating is carried out as described in Examples 1-4 to 50% of the total length. As noble metal 1.0Gow .-% platinum is applied.

example

A pilot plant, which was charged with exhaust gases from a gas engine with lean operation, was used to test the catalysts of the invention. The catalysts were inventively designed as Vollextrudate consisting of honeycomb bodies with an edge length of 150mm x 150mm and a length of 440mm. The cell division (1 bar + 1 cell opening) was 3.6 mm. The technical data of the pilot plant can be summarized as follows:

Rauchgasdjrchsatz 105m7hi.N space velocities - NO _x reduction 15000 h " ¹ - CO / HC oxidation 36000 h " ¹ Exhaust gas velocity in Reactor (empty pipe speed) 1.2 m / s Flue gas temperature 400-520 ⁰ C Molar ratio NH ₂ NO. 0.8-1.2

After 4,000 operating hours, it was possible to measure conversion rates for nitric oxide by 95% with a catalyst according to Example 1 and a molar ratio of ammonia / nitrogen oxide of 0.95. The conversion rates for carbon monoxide and hydrocarbons are shown in Table 1 and the graph Fig. 1.

Table 1 Conversion rates of the pollutants NO, CO and HC after 4000 operating hours

concentration after combination convergence Measurement Method pollutant in front of station wagon 'catalyst sion degree catalyst 160 ppm % NO _x 3,000 ppm 95 » chemiluminescence · 15 ppm Mgthode CO 2 300 ppm 1300 ppm 99 r-.DIR Ges.-HC 3 600 ppm 80 ppm 64 FID HCohneCH4 2 300 ppm 6,5Vol .-% 97 HD 0 7Vol .-% Not - Paramagnetism. NH ₃ 2,790 ppm detectable naßchem. absorption and analysis

Molar ratio 0.95, operating temperature 480 ° C

Under the applied driving conditions, ammonia could not be detected in the exhaust gas behind the catalytic converter. The plant operates in wide operating ranges ammonia-slip-free. Pollutant conversion as a function of operating time is shown graphically in Fig. 1. The conversion curve for the hydrocarbons including methane (total HC) shows that the fresh catalyst also oxidizes methane. The methane conversion, however, goes back over the course of about 1000 operating hours.

Claims (8)

1. A catalyst for purifying exhaust gases, in particular from superstoichiometrically operated internal combustion engines and gas turbines, characterized in that it consists of a one-piece Wabon-shaped Abgasreinigiingskatalysator, in a upstream section a catalyst for the selective reduction of nitrogen oxides by means of ammonia gas, optionally from a Ammoniakspendenden compound, and in an outflow section has an oxidation catalyst. 2. A catalyst according to claim 1, characterized in that the reduction catalyst from a solid catalyst of an intimate mixture of the components (A) titanium in the form of oxides, (B) at least one metal from the group B.1 iron and vanadium in the form of oxides and / or sulfates, and or the group Θ.2 molybdenum, tungsten, niobium, copper, chromium in the form of oxides, and / or (C) tin in the form of oxides,
and or (D) metals from the group beryllium, magnesium, zinc, boron, aluminum, yttrium, rare earth elements, silicon, antimony, bismuth and manganese in the form of oxides, where the components are in the atomic ratios A: B: C: D = 1: 0.01 to 10: 0 to 0.2: 0 to 0.15
available. 3. A catalyst according to claim 1, characterized in that the reduction catalyst of a solid catalyst of an intimate mixture of titanium dioxide as component A), tungsten oxide as component Bi and vanadium, iron, niobium, copper, chromium and / or molybdenum as component B ₂ with an atomic ratio between the metals of components A) and B) of 1: 001 to 1, wherein component A) is a finely divided oxide obtainable by flame hydrolysis of TiCl ₄ having an X-ray structure predominantly anatase, a BET surface area of 50 ± 15 m ² / g, a density of 3.8 g / cm ³ , a mean primary particle size of 30 nm, a pH of 3-4 measured in 4% aqueous dispersion, and one at pH 6.6 isoelectric point that after two hours of annealing at 1000 ⁰ C, a TiO ₂ content of 99.5Gew .-.%, an Al ₂ O ₃ content of 0.3Gew .-%, an SiO ₂ content of 0.2Gew. -%, a Fe ₂ O ₃ content of 0.01 wt .-% and an HCl Content of 0.3% by weight and which shows a weight loss after drying for ² hours at 105 ° C. of 1.5% by weight and after ² hours' annealing at 1000 ° C. of 2% by weight. 4. Catalyst according to claim 1, characterized in that the reduction catalyst contains the components A) titanium oxide B ₁ ) at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, Yttrium, Lanthanum, Cerund
B ₂ ) at least one oxide of vanadium, niobium, molybdenum, iron, copper, chromium with an atomic ratio between the elements of components A) and B) of 1: 0.001 to 1, preferably 1: 0.002 to 0.4, and component A) in the form of a reactive high surface area titanium oxide having a BET surface area of 40-500, preferably 50-300, in particular 60-150 m ² / g, which is present completely or for the most part in the anatase modification is present. 5. A catalyst according to claim 1, characterized in that the reduction catalyst of a copper and / or iron and optionally also cerium or Mo-containing acid-resistant zeolite full catalyst, optionally of the mordenite type. 6. Catalyst according to claims 1 to 5, characterized in that the Oxidaton. catalyst is applied as a coating on a downstream section of a reduction extrudate catalyst according to any one of claims 2 to 5, wherein said coating) accounts for 20 to 50%, in particular 25 to 35% of the total catalyst volume. 7. Catalyst according to claims 1 to 6, characterized in that the oxidation catalyst from 2 to 70 wt .-% CeO ₂ and 0 to 20Gew .-% ZrO ₂ and optionally iron oxide, alkaline earth oxides and / or rare earth metal oxides containing alumina of the transition series as a carrier and an active phase applied to the carrier consists of 0.01 to 3% by weight of platinum, palladium and / or rhodium with a weight ratio between platinum and / or palladium and the optionally present rhodium of 2: 1 to 30: 1. 8. Use of the catalyst for the catalytic purification of exhaust gases in particular from superstoichiometrically operated internal combustion engines and gas turbines from the pollutants nitrogen oxides, hydrocarbons, carbon monoxide, ammonia slip from the denitration and optionally ochwefoldioxid.