EP0804271A1

EP0804271A1 - PROCESS FOR PURIFYING COMBUSTION GASES WITH DeNOx CATALYSTS AT TEMPERATURES BELOW 150o C

Info

Publication number: EP0804271A1
Application number: EP96902926A
Authority: EP
Inventors: Peter Dransfeld
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1995-02-11
Filing date: 1996-01-26
Publication date: 1997-11-05
Also published as: WO1996024433A1; DE19504597A1

Abstract

A process is disclosed for reducing in oxidative conditions the organic chlorinated products contained in exhaust gases that result from incomplete combustion. The process is characterised in that exhaust gases are brought into contact with a DeNOx catalyst at a temperature below 150 °C.

Description

Process for cleaning combustion gases with DeNO _x catalysts at temperatures below 150 ° C

description

The invention relates to a method for the reduction of chlorosulfonic organic products of incomplete combustion and of Queck¬ silver in combustion exhaust gases under oxidative conditions DA, by in that at a temperature below 150 ° C with a De O _x catalyst in the combustion exhaust gases Contact brings.

Incineration plants, in particular waste and waste incineration plants, still emit organic compounds as products of incomplete combustion (PIC = Products of Incomplete Combustion) and mercury even after further flue gas cleaning. Part of these PIC's are organohalogen compounds (chlorobenzenes, chlorophenols, polychlorinated biphenyls, polyhalogenated (chlorine, bromine) dibenzodioins and dibenzofurans etc.). These compounds are toxic, are extremely difficult to degrade and are therefore harmful to the environment, with late damage in particular being feared. The polyhalogenated dibenzodioxins and dibenzofurans (hereinafter collectively and briefly referred to as "dioxins *) are of particular importance with regard to the potential health risk of emissions from incineration plants.

It is already known from PCT application WO 91/04780 that a reduction in the emission of organochlorine products from incomplete combustion, in particular halogenated aromatic compounds, for example chlorobenzenes, chlorophenols, polychlorinated biphenyls, polyhalogenated (in particular polychlorinated and polybrominated) ) Dibenzodioxins and dibenzofurans, halogenated aliphatic compounds, for example tri- and tetrachloroethane, hexachlorocyclohexane etc., in particular in exhaust gases from combustion plants, in particular waste and waste incineration plants, if the exhaust gas containing the chloroorganic products is obtained at temperatures of more than Treated 260 ° C under oxidative conditions in the presence of a DeNO _x catalyst or a modified DeNO _x catalyst.

For temperatures below 150 ° C., it has hitherto not been recommended to use catalytic processes, but rather complex chemical or photochemical processes to break down polyhalogenated aromatics

(EP-A 21 294, US 4 144 152). This prejudice against the use of catalytic processes at low temperatures lies in establishes that thermal degradation is only possible above 1200 ° C (WO 88/00483). The temperature for this degradation can be reduced by using catalysts. However, it is, for example, from the overview "Separation of dioxins, furans and heavy metals * by G. Maier-Schwinning, R. Knoche and G. Schaub in VDI reports No. 1034, 1993, pp. 107 to 122, in particular Fig ¬ dung 7, known that the catalytic decomposition of dioxins decreases sharply as the temperature drops, so that the technically possible lower limit has so far been seen at 150 to 300 ° C. (WO 88/00483, DE-A 3 804 722, EP-A 387 417, WO 91/04780 and DE-P 4 327 691.1).

EP-A 559 071 describes a process with which dioxins are adsorbed on a catalyst in a first stage at temperatures from 100 to 150 ° C. and in a second stage at a temperature which is at least 50 ° C. higher, in particular at more than 300 ° C can be reduced.

In the article "Status and tendencies of flue gas cleaning after waste incineration * by A. Hackl, in" Garbage incineration and environment "vol. 4, Nov. 1990, pp. 1 to 14, edited by KJ Thome-Kozmiensky, the lower one The working temperature range of the catalyst is given as 280 ° C. (p. 11), therefore the test run I at 115 ° C in table 11 of this article is only to be understood as separation (p. 10), ie adsorption have the disadvantage in some cases that the catalyst pores clog.

High operating temperatures of the catalytic converter are often unsatisfactory.

If the catalytic stage for the removal of dioxins is operated, for example, after the stage of the aqueous flue gas scrubbing, a great deal of energy is required in order to heat the flue gases from below 100 ° C. to the working temperature of the catalyst. In addition, elemental mercury present in the flue gas is oxidized catalytically, but the mercury compounds that arise in the process, such as HgO, HgCl or HgS0, are retained less due to their vapor pressure, the higher the working temperature of the catalyst, so that an additional separation stage for the mercury compounds is needed.

The majority of incineration plants such as domestic waste and special waste incineration plants are currently equipped with cloth filters. These filters are usually operated at temperatures between 100 to 145 ° C. An adsorbing filter auxiliary layer is applied to the filters to reduce dioxin emissions. wear, which usually consists of a dusty lime / coke or lime / activated carbon mixture. This mixture has to be made expensive for this purpose and disposed of after use at high cost and with a high level of safety.

Depending on the operating temperature, the lime content in the mixture is approximately 70% to 98%. Although the lime further reduces the trace concentrations of acidic noxious gases such as HC1, HBr and S0 ₂ to a certain extent, it essentially serves to inertize the combustible or even explosive coal dust.

Using the flammable adsorbent increases the amount and cost of the material to be disposed of. In addition, handling flammable or explosive materials also requires expensive safety precautions.

Instead of carbon-based adsorbents, non-combustible mineral materials can also be used. These have a lower adsorption capacity for dioxins compared to coal, so that ultimately comparable amounts have to be disposed of. The mineral materials known so far for dioxin adsorption often have the further disadvantage that they are not suitable for the adsorption of heavy metals, in particular elemental mercury.

The disposal of the dioxin-laden adsorbent can indeed be avoided both when using carbon-based adsorbers and also from mineral materials if the material is regenerated. However, for this additional process step, it is necessary to invest in a further apparatus for destroying the dioxins at elevated temperatures (e.g. Hagenmaier drum). The operation causes additional costs and creates new problems with the evaporating mercury.

The object of the present invention was to overcome the disadvantages of

Avoid prior art while overcoming the prejudice described above.

Accordingly, the process defined at the outset was found. Further versions of the invention can be found in the subclaims.

It has been shown that the process works satisfactorily at temperatures above 70 ° C. and well above 105 ° C. Often, heating does not have to exceed 140 ° C, in some cases up to 145 ° C. The process generally works at pressures from 0.5 to 2.0, preferably 0.7 to 2.0 bar abs.

The removal of the organochlorine compounds takes place in a known manner under oxidative conditions. Oxygen is generally sufficient as an oxidizing agent. Other oxidizing agents such as H ₂ 0 are usually not required. The process is therefore carried out in such a way that sufficient oxygen is present so that the oxygen sorption outweighs the sorption 0 of possibly added reducing substances such as NH ₃ , CH or H ₂ S, for example 0 ₂ 6%, NH ₃ <30 ppm . In general, a 0 ₂ content of 0.5 to 21, preferably 3 to 21, in particular 6 to 12% by volume is used.

5 Good results are achieved if the residence time τ of the catalyst under oxidative conditions depends on the temperature of the exponential relationship (1)

0

1) τ = l, 5-10- ⁴ -e (\ -T) / [seconds] obeys, where

A = 8700 to 9100

T = temperature in Kelvin. 5

The dwell time τ is to be understood as the time in which the catalyst is exposed to oxidative conditions at the selected operating temperature. The period during which the catalyst 0 is in contact with the organochlorine products of incomplete combustion can be shorter than the residence time τ in the two-stage process described below.

It has proven to be advantageous to use the catalyst in finely divided form as so-called catalyst dust.

Good results with filter layers are achieved if the weight-average grain diameter pso, measured according to DIN 66 111, is between 1 and 200 μ, preferably 5 to 150 and in particular 10 and

40 30 μ lies. Good results in fluidized bed reactors are achieved if the weight-average grain diameter is between 10 and 5000 μm, preferably 50 and 3000 μm and in particular 100 and 2000 μm. This can hardly be achieved, for example, by conventional grinding processes.

45 In addition, the ratio of the specific inner surface according to BET Fi, _sp to the specific enveloping geometric surface F _u , _{sp of} the catalyst can be important in some cases. Then it is advantageous if the specified ratio of specific BET surface area to specific enveloping surface area is selected such that it obeys the exponential equation (2):

2) ^F i _'s _P ) = k _ads .3.10 ⁷ .e ^ ^τ '

F _u , _S

The following can be said about the individual sizes in (2):

B = 1600 to 7400

T = temperature in Kelvin

Fi'sp = specific inner surface of the catalyst used measured according to BET, for example DIN 66 132 in m ² / g F _u , _sp = specific enveloping geometric surface of the catalyst used in m ² / g, which is wetted by the fluid becomes. This area can be calculated directly in the case of catalyst bodies with a defined geometry. In the case of beds of spherical catalysts, for example, it is the ball surface that can be calculated from the ball diameter, divided by the weight of the catalyst bed. If the catalyst bed has particles of different sizes and irregular shapes, F _u , _S p is determined in a corresponding manner from the particle size distribution and the generally known form factor in a manner known to the person skilled in the art (see, for example, Perry's Chemical Engineer's Handbook 6th ed. Section 4, ^» Reactor Design: Basic Principles and Data "and Section 5; Davidson Clift, Harrison" Fluidization "2nd ed. (1985) Academic Press; M. Leva" Fluidization "McGraw-Hill (1959)).

kads = mass transfer coefficient gas-catalyst surface in m / s. It can be determined in a known manner according to equation (3).

3 ⁾ k _ads = ≥ • DT [m / s]

in which

Sh = Sherwood number of the respective mass transfer d = characteristic length of the respective mass transfer [m]

DT = temperature-dependent diffusion coefficient of the pollutant in the exhaust gas [m / s]

mean.

The choice of the quotient Sh / d depends on the respective mass transfer and is carried out in a manner known to the person skilled in the art. Numerical values for different technical designs (rubble layer, filter auxiliary layer, fluidized bed, honeycomb channel, etc.) can be found in the relevant specialist literature, see e.g. A. Mersmann "Substance transfer * Chapter 6, Springer (1986); H. Brauer" Substance exchange including chemical reactions "Chapters 9 and 10, Sauerländer (1971); N. Wakao and S. Kaguei" Heat and Mass

Transfer in Packed Beds "Gordon and Beach (1982); CN Satterfield" Mass Transfer in Heterogenous Catalysis * MIT Press (1970); M. Suzuki "Adsorption Engineering" Kondansha / Elsevier (1990); G. Eigenberger "Fixed Bed Reactors" in: Ullmann's Encyclopedia of Industrial Chemistry 5th ed. Vol. B4, VCH (1992); E. Tsotsas "On the heat and mass transfer in flow-through fixed beds" VDI progress reports row 3 No. 223 (1990). For the catalyst dust, which is advantageously used for filter layers or in fluidized bed reactors, the ⁽ quotient (Sh / d) is typically 10,000 -πr ¹ .

The temperature-dependent diffusion coefficient D is determined in a known manner (see, for example, VDI Warmth Atlas, 4th edition, 1988, Da 32 / Da 33) and is, for example, at 130 ° C. for tetrachlorodibenzodioxin 0.094 cm ² / s, for hexachlorodibenzodioxins 0.087 cm ² / s, for octachlorodibenzodioxins 0.082 cm ² / s and for hexachlorodibenzofurans 0.088 cm ² / s.

Since there is usually a mixture of different dioxins in the exhaust gas, it is advisable to base the calculation on the smallest diffusion coefficient in order to ensure the degradation of the corresponding dioxins in any case.

In many cases, a one-step process has proven itself.

The catalyst is usually used in arrangements such as fixed bed, moving bed, rotor adsorber, fluidized bed, circulating fluidized bed or in particular filter layer, for example in a cloth filter, if appropriate with internal or external dust recycling under oxidative conditions with the combustion. exhaust gas and thus brought into contact with the organochlorine products of incomplete combustion.

The catalyst dust is advantageously mixed into the combustion exhaust gases by means of suitable apparatuses such as compressed air injectors in the exhaust gas duct. The flow rate here is usually 10 to 30 m / s and is turbulent, so that the catalyst, like the ash contained in the exhaust gas, practically does not settle. Its concentration is usually 50 to 500, preferably 100 to 300 mg / m ³ . After a flight time of generally 0.1 to 5 seconds, the catalyst dust is filtered out of the flue gas, for example using conventional cloth filters. Good results are achieved if the filtered-out catalyst is partially, for example 50 to 95%, returned to the exhaust gas in order to increase the contact and residence time.

In addition, the method according to the invention can also be carried out in two stages. In a first step, the catalyst is loaded with the incomplete combustion of the organochlorine products and regenerated in a second step under oxidative conditions.

If catalyst dust is used, the loaded catalyst can be filtered out of the flue gas by, for example, cyclones or cloth filters and discharged in a silo where the second step (regeneration) takes place under oxidative conditions.

The residual oxygen of the combustion gases or additionally introduced atmospheric oxygen can be used for regeneration, for example. The loading step can be carried out under oxidative conditions. However, this is not absolutely necessary.

For the sake of simplicity, both steps can be operated at the same temperature. Sometimes, however, it is favorable for plant-specific reasons to carry out the regeneration at a different temperature than the loading. The duration of the regeneration τ (seconds) in the second step is often chosen in accordance with the above relation (1).

DeNOχ catalysts have been known for a long time and are used for the catalytic reduction of nitrogen oxides NO _x with ammonia in nitric acid plants and large combustion plants. The DeNO _x catalysts and their manufacture are described, for example, in VDI Report No. 730, 1989, pages 121 to 156, and in

DE-AS 26 58 539, DE-OSen 34 33 197, 34 38 367 and 35 31 810 and US-PS 4 085 193 and 4 378 338. To these publications and the literature citations contained therein are hereby fully referred to.

In particular, DeNOχ catalysts of the titanium oxide type, iron oxide or zeolite type, which may contain conventional donors, come into consideration. Titanium oxide catalysts are preferred, in particular those with a titanium oxide content of> 70% by weight, based on the total weight of the catalyst.

The modified DeNOx catalysts are those which additionally contain nickel, chromium, copper and / or cobalt oxides as donors. They are contained in an amount of 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the total weight of the catalyst.

Catalysts containing donor tungsten oxide (WO ₃ ), molybdenum oxide (MOO ₃ ) and / or in particular vanadium oxide (V ₂ θ ₅ > are particularly preferred. The type and amount of the additives are chosen so that the conversion rate mentioned and the Degradation rate can be increased.

The amount of donors is generally at least 0.5% by weight, preferably 1 to 20% by weight.

Preferred compositions (% by weight, based on the total weight) for catalysts used according to the invention are

Ti0 ₂ 70 to 95% W0 ₃ 2 to 10%

V ₂ 0 ₅ 0.5 to 5%,

M0O3 0 to 5%, preferably 0 to 4%

Rest: usual fillers, e.g. glass fiber, clay, possibly SO ₄ .

The catalysts generally have the following data:

Pore volume: 100 to 400 mm ³ / g BET surface area: 20 to 100 m ² / g according to the pore radius: 50 to 200 A.

Surprisingly, it was also found that used DeNO _x catalysts, ie exhausted for denitrification, are still well suited for the oxidative destruction of halogenated, for example chlorinated, compounds such as aromatics. These catalysts are particularly preferred. Depending on the strategy followed of the regular replacement of the DeNOχ catalyst, these are generally DeNO _x catalysts and their activity to reduce nitrogen oxides to less than 70% of the initial activity.

Approximately 33,000 m ³ of DeNO _x catalyst volume are currently installed in power plants in the Federal Republic. In the course of their use, their ability to denitrify is generally reduced by known catalyst poisons such as oxides or salts of phosphorus, arsenic or also by alkali and alkaline earth metals. The replacement requirement is estimated at approximately 4000 m ³ annually. A corresponding amount of deactivated old catalysts has to be disposed of, because material recycling is currently not possible if one refrains from smelting processes for producing a slag, which can possibly be used as path building material. However, these are only individual measures that are very expensive in terms of energy and / or apparatus. Heavy metals evaporate, creating new problems. On the basis of the present invention, it is now possible, inter alia, to recycle the deactivated old catalysts previously considered to be waste materials for material recycling.

A particular advantage of the process according to the invention is that it can also be used to reduce the mercury content in combustion exhaust gases at the same time to reduce chlorine-organic products from incomplete combustion. Mercury is reduced particularly well if there is a comparatively large amount of sulfur dioxide (10 to 100 mg / Nm ³ ) in the flue gas. Particularly good mercury reduction properties are achieved if 5 to 50 parts by weight of VOs and / or W0 _{3 are} added to 100 parts by weight of the above-mentioned catalyst compositions. In the case of powdered catalyst compositions, it has proven to be advantageous to use the metal oxides with the same average grain size.

Embodiment

A used catalyst based on Ti0 with vanadium and tungsten doping with the

composition

82.9% by weight of TiO ₂ ,

1.5 wt% V ₂ θ ₅ li d 7, 8 wt% W0 ₃ .

Rest: fillers

were ground from a coal-fired power plant.

The mean grain diameter was dpso = 50 μm. This material was mixed into the flue gas upstream of the cloth filter of a domestic waste incineration plant, so that a concentration of fresh catalyst dust of about 20 mg / m ^{3 was} established. The residence time in the flue gas until separation in the cloth filter was approx. 1 second, the pressure 950 bar. Furthermore, in order to save catalyst dust and to increase the dwell time of the dust, 90% of the dust separated in the cloth filter was returned, so that the total amount of fresh and used catalyst dust in the flue gas duct was approximately 200 mg / m ³ . The cleaning cycle of the cloth filter was regulated by the pressure difference across the cloth filter. Typically, cleaning was carried out about 1 to 2 times a day. The mean contact time was about 5 to 10 days. The operating temperature of the flue gas, like that of the cloth filter, was 130 ° C. The discharge cone of the cloth filter and the return line for the pneumatically conveyed dust were heated to 130 ° C.

Measurement result 1

The S0 concentration in the flue gas before the cloth filter was 4 mg / m ³ . The dioxin concentration was determined according to known methods in the flue gas before and after the cloth filter and in the removed filter dust.

The ratio of the dioxin concentration in the inlet to the dioxin concentration in the outlet was on average [D in] / [D out] = 48.

The concentration after the cloth filter was always below 0.1 ng / m ³ . A dioxin balance around the cloth filter with the catalytically active filter auxiliary layer was created from the measured flue gas and filter dust mass flows. The one in the filter dust discharge The dioxin concentration measured only made up 7% of the concentration to be expected on the basis of the balance.

The majority (93%) was broken down oxidatively. This results in a degradation rate of over 90%. The dioxin concentration in the filter dust discharge remained below the threshold value specified in the Hazardous Substances Ordinance.

Mercury was determined using the method described by Braun, Metzger, Vogg in Rubbish and Waste 18, pages 62 to 71 and pages 89 to 95 (1986) (see also Braun, Metzger Chemosphere 16, page 821 ff (1987)). The method allows a separate determination of elemental mercury (H _g ) and mercury compounds (HgVerb)

Concentration before cloth filter Concentration after cloth filter [μg / m ³ ] [μg / m ³ ] Hg HgVerb. Hg HgVerb.

48 32> 2 19

Measurement result 2

The sulfur dioxide concentration in the flue gas upstream of the cloth filter was 45 mg / m ³ in this case.

Concentration before cloth filter Concentration after cloth filter [μg / m ³ ] [μg / m ³ ] Hg HgVerb. Hg HgVer.

59 28> 2 8

Claims

claims

1. A process for reducing chlorine-organic products incomplete combustion in combustion gases under oxidative conditions, characterized in that the combustion gases are brought into contact with a DeNO _x catalyst at a temperature below 150 ° C.

2. The method according to claim 1, characterized in that the catalyst is loaded with the products in a first step and regenerated in a second step.

3. The method according to claim 1 or 2, characterized in that a DeNO _x catalyst used with regard to the NOχ reduction is used as the DeNOx catalyst.

4. The method according to claims 1 to 3, characterized gekennzeich¬ net that a used power plant DeNO _x catalyst is used.

5. The method according to claims 1 to 4, characterized gekennzeich¬ net that a DeNO _x catalyst of the titanium oxide type, iron oxide type or zeolite type, optionally with conventional donors, is used.

6. Process according to Claims 1 to 5, characterized in that a DeNOx catalyst is used which contains a tungsten oxide, molybdenum oxide and / or vanadium oxide as the donor.