WO1999013985A1

WO1999013985A1 - Method and device for catalytic emissions cleaning

Info

Publication number: WO1999013985A1
Application number: PCT/EP1998/005800
Authority: WO
Inventors: Egon Erich; Klaus Gerhard Schmidt
Original assignee: Institut Für Umwelttechnologie Und Umweltanalytik E.V. (Iuta) - Institut An Der Gerhard-Mercator-Universität-Gesamthochschule Duisburg
Priority date: 1997-09-12
Filing date: 1998-09-11
Publication date: 1999-03-25
Also published as: EP1015109A1

Abstract

The invention relates to a method for the catalytic cleaning of streams of exhaust air. The aim of the invention is to provide a simple method and a corresponding device for carrying out such a method without the filter becoming clogged up. To this end, a solid bed reactor (3) packed with catalytically active metal chips is provided together with a temperature probe (8) located inside said solid bed reactor (3) and devices (5, 6) for measuring the concentration of pollutants in the untreated gas and in the cleaned gas. The cleaning process comprises the following steps: soaking cleaned metal chips in a dilute acid in which a catalytically active noble metal, transition metal or transition metal oxide introduced in the form of its metal salt has been dissolved; gently drying the treated metal chips; and passing the stream of exhaust air to be cleaned through a bed of the doped metal chips.

Description

Method and device for catalytic exhaust gas purification

The invention relates to a method for the catalytic cleaning of exhaust air streams and a suitable device with a new catalyst system for cleaning exhaust air streams from domestic furnaces, but which can also be used for cleaning hydrocarbon-containing exhaust air from various industries, such as the food processing and solvent processing industries.

According to the Working Group on Energy Balances of the German Environment Association, approx. 30% of the total amount of energy in Germany was used in private households in 1993, the majority of which was used for heating purposes. As a rule, heating is decentralized with natural gas and 01 as well as solid fuels (hard coal, lignite and wood).

While the combustion of natural gas and oil generally does not involve any odor pollution, the increase in the cost of solid fuels in open fireplaces can lead to a considerable environmental pollution. The same applies in particular to open fireplaces with directed exhaust gas routing, such as grill combs, which are usually a nuisance for affected residents due to burnt oils and fats. In addition, there are also a number of processes in the industrial sector that lead to odor nuisance for residents. Emissions from the food processing industries, such as restaurants, snack bars and canteen kitchens, should be mentioned. Odor nuisance also arises in the large-scale manufacture of french fries, croquettes, chips and in meat processing (e.g. in the production of smoked products and the production of sausages). Strong odor nuisance also occurs in solvent processing, such as in car paint shops.

If you first consider the combustion under optimal conditions, mainly carbon dioxide and water are emitted. Since these optimal conditions for combustion cannot be fully realized even with the help of technical measures, other, mostly disruptive reaction products (CO, N0 _X , aliphatic hydrocarbons, benzene, phenols, PAHs, formaldehyde, etc.) also arise, which are largely from the Exhaust gas should be removed.

The methods that are known from large-scale fields of application cannot be used in private households. Given the existing conditions such as pressure, temperature, volume flow, space requirements, etc., it is difficult to implement technically adapted and cost-effective processes.

The first attempts to at least dedust the exhaust gas and to retain condensable fractions by means of filter systems are not being used, because due to the steadily increasing pressure loss due to the sooting of the filter, the resulting flue gas volume flow through the natural draft of the fireplace can no longer be removed. For this reason, continuous and cost-intensive maintenance of the filters is necessary. In addition, not all polluting exhaust gas components can be removed from the flue gas by filtering separation processes.

Processes of absorption and adsorption offer the possibility of separating hydrocarbons in particular, but both measures can only be used for the domestic area with considerable technical effort.

During absorption, pollutants are separated in a washing liquid, which has to be renewed after a certain time because it is saturated with pollutants. The used washing liquid must be regenerated or disposed of.

In adsorptive exhaust gas cleaning, the adsorbent (e.g. activated carbon) must be regenerated or disposed of when it is loaded. Although this is economically justifiable on an industrial scale, it cannot be transferred to domestic furnaces.

An alternative method to this is catalytic exhaust gas purification. Chemical reactions convert harmful substances into harmless products. The presence of the catalyst enables the reaction to be started and maintained under mild conditions.

To convert the exhaust gas components, the use of a catalyst on a metallic carrier, similar to the pre-catalyst in an automobile, is suitable. Disadvantageous when using the in the automotive sector as a front and The main catalysts used are the high manufacturing costs, due to the complex construction of the metal packing and the ceramic honeycomb. The implementation of this technology in the field of domestic furnaces is therefore currently of no interest from an economic point of view.

To date, there are no concepts that can be implemented or used to reduce the environmental impact caused by the combustion of solid fuels in domestic furnaces. The conditions that prevail in the domestic area are too unfavorable to be able to use the known and established technologies. The time-limited operation of the furnace, the lack of a forced exhaust device and the resulting difficult pressure conditions complicate the cleaning of the exhaust gases from domestic furnaces. In addition, the temperature of the exhaust gas must be selected so that there are no condensation phenomena in the exhaust gas flow, which can lead to sooting and thus damage to the comb. In addition, there is usually no space available to use space-consuming separators.

Mature processes are available for cleaning exhaust gases from combustion plants, which are briefly described below and discussed for their suitability for exhaust gas purification in the area of domestic combustion plants.

Dry separator:

The common characteristic of dry separators is the exclusive separation of particles (dust and liquid drops) and the particles bound to them Pollutants. The dry separators can be divided according to their operating principles, whereby a distinction is made between inertial force cutters, filtering separators and electrical separators.

Mass separators include all processes in which only mass forces separate the dust particles from the carrier gas. In the filtering separators, the particles are separated from the exhaust gas by porous layers that have sufficient permeability. The particles are electrically charged in electrical separators and then separated in an electrical field.

Since no gaseous contamination can be separated from the exhaust gas with these processes, they cannot be used for exhaust gas purification in private households. In addition, due to their dimensions, the technical effort, the investment and maintenance costs, inertial force separators and electrical separators cannot be used for the intended field of application. Filtering separators (flat, bag or pocket filters) cannot be used in the case of condensate mixtures due to the sooting of the filter material and the associated pressure loss in the area of domestic exhaust gas cleaning.

Absorption process:

Absorption is understood to mean the partial or complete absorption of a gas or vapor upon contact with a washing liquid. Both gaseous components and particles from a gas stream can be retained by absorption processes. When separating particles in a washing liquid the collision of the particles is caused either by the liquid drops distributed in the exhaust gas stream, by moving liquid surfaces or by bodies flushed with liquid.

For the washing out of gaseous organic components, liquids must be selected that physically dissolve or chemically bind the components to be separated. The construction of the absorption apparatus ranges from simple gas bubble washers, continuously working trick towers, spraying devices to apparatuses with moving parts. Direct current, counter current and cross current are known as operating modes.

A technical absorption system consists of two essential parts, the actual absorber and the regenerator, in which the washing solution is freed from the component to be washed out, so that the washing agent can be reused. Only in exceptional cases is it more economical to use the exhausted absorbent only once to dispose of it after use. Due to the technical effort required to operate an absorption system, such a method is not suitable for use in a private household.

Adsorption process:

During adsorption, a substance from a gas or a liquid is bound to a solid (adsorbent), preferably in its micropores. Adsorption processes are particularly suitable for separation tasks in which an adsorbable constituent that occurs in a low concentration from a large excess of a less adsorbable matrix gas or a corresponding liquid must be separated. Such separation tasks are, for example, the separation of solvents from exhaust air streams, gas drying, the separation of odorous substances from air or water and the decolorization of solutions. The adsorption process to be used for a specific separation task depends on the physical state of the substance to be treated, on the concentration of the component to be separated, on the most suitable adsorbent and on economic considerations.

The adsorbent can e.g. in granular form in a fixed bed, in a moving bed or fluidized bed or, for liquids only, in powder form. In general, periodic regeneration or reactivation of the adsorbent is required, but in some circumstances, single use may prove more economical.

The technical effort with which adsorption processes are operated is disproportionately large for private households, so that these processes cannot be used.

Thermal oxidation:

Oxidizable gaseous, vaporous or particulate pollutants can be eliminated by burning them from exhaust gases. During the complete oxidation, the chemically bound carbon and hydrogen are transferred to the substances carbon dioxide (C0 ₂ ) and water (H ₂ 0). During the thermal oxidation, sulfur, nitrogen and halogen compounds are converted into their oxides, which must be removed from the exhaust gas flow by other purification processes if emission limit values are exceeded. Due to the different combustion temperatures, two methods of thermal oxidation are distinguished, flame combustion and thermal combustion.

Flame combustion is used when the energy content of the exhaust gas is so large that stable combustion is possible in a burner without the need for additional energy. The oxidation temperature is above 1200 ° C. This process is often used in the petrochemical industry and in the refinery sector in order to be able to dispose of the excess gaseous hydrocarbon compounds that occur in the event of malfunctions.

Thermal combustion or thermal post-combustion (TNV) requires temperatures of 600 to 1100 ° C, a closed reaction space and usually requires additional fuel to convert the pollutants. In contrast to flame combustion, when using thermal combustion, the pollutant concentration is lower than the lower ignition limit. This method is preferably used to remove organic compounds from exhaust gases (e.g. solvent vapors) with a high degree of conversion.

Catalytic oxidation:

The principle of catalytic combustion is based on the conversion of pollutants on a suitable catalyst at the lowest possible temperatures. A complete removal of organic components from an exhaust gas stream is already possible at temperatures from 150 to 500 ° C. Catalysts have the ability to To reduce the activation energy of chemical reactions, which, for example, accelerates the oxidation of hydrocarbons without the catalyst being subject to any change. Organic components are converted to carbon dioxide and water according to the following reaction equation.

C _m H "+ {m +) 0 ₂ <-> m C0 ₂ + ^ ₂ 0

The catalyst itself consists of a catalytically active component, which in most cases is applied to a support material. Precious metals such as platinum and palladium or the oxides of the transition metals copper and chromium (but also Ti, V, Mo, Fe, Ni) are available as catalytically active components. The advantage of precious metal catalysts is their high reactivity and their particular suitability for the conversion of hydrocarbons and carbon monoxide.

Transition metal oxide catalysts are less sensitive to sulfur and halogens (catalyst poisons) and are therefore also used for exhaust gases containing chlorine and sulfur.

In the known catalysts, the support material used is generally metals as shaped sheets, wires, fabrics, nets, metal oxides such as Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , MgO as shaped bodies, or natural and synthetic minerals such as pumice, mullite , Cordierite, stealite, zeolites as moldings, for use.

Catalytic combustion has so far been used primarily in the automotive industry, paint shops, printing plants and the chemical industry. The preferred conditions of use for catalytic combustion are:

small and medium quantities of gas, low oxygen concentrations, changing loads and quantities of gas, frequent start-up and shutdown processes.

The invention is therefore based on the object of providing a method for the catalytic cleaning of exhaust air flows and a device suitable therefor, in which reliable filtering of the exhaust air flow is ensured without great design effort, without the filter becoming soiled. Furthermore, the catalyst system should also be able to be used retrospectively in existing exhaust-air-generating systems without major modifications to the system being necessary. The new catalytic converter should be used particularly in the exhaust gas flow of domestic combustion plants in order to be able to implement the odorous pollutants.

With regard to the cleaning method according to the invention, this task is solved by the following steps:

Potion of cleaned metal shavings in a dilute acid, in which a catalytically active noble metal, transition metal or transition metal oxide was introduced, in the form of its metal salts,

gentle drying of the prepared metal chips and

Passing the exhaust gas stream to be cleaned through a fill of the doped metal chips. In terms of the device, the object is achieved by a fixed bed reactor with a bed of catalytically active metal chips contained therein, a temperature sensor inside the fixed bed reactor and devices for measuring the raw and clean gas concentration of pollutants.

According to the invention, an inexpensive catalyst is presented, with which flue gas cleaning can also be carried out for small combustion plants and the cleaning of the odor-intensive organic compounds from various hydrocarbon-containing exhaust air streams. The cleaned metal chips used according to the invention as the carrier material of the catalyst, which are doped with a catalytically active transition metal or transition metal oxides, represent a particularly inexpensive carrier material. This applies in particular to those metal chips which are obtained as a waste product during metal cutting and which are impregnated with impregnation how especially cooling lubricants must be carefully cleaned.

A bed of cleaned metal chips is used for cleaning. Metal chip beds offer the advantages that, on the one hand, no great pressure loss builds up, particles can be separated out by the filtering action of the bed, and the sooting of filtering separators, which has so far been typical, does not occur. On the other hand, the good thermal capacity of the metals allows the temperature required for the catalytic process to be reached quickly. The bed is lightly compacted and preferably placed in the - already existing - exhaust pipe (stovepipe). The following criteria are important when selecting the chips. The chips should have their own catalytic activity as large as possible. For this, Cr-Ni chips are available, possibly with proportions of Mo, Ti, V. The pressure loss that builds up over the chippings must be as low as possible in order to be able to take advantage of the natural draft of the chimney. This means that in most cases there is no need for an additional blower in the comb. The conversion of the pollutants takes place at the catalytically active centers on the surface of the catalyst. For this reason, the inner surface should be as large as possible in order to obtain sufficient activity with a small catalyst volume. The height of the catalyst is decisive for the pressure loss and must therefore be as small as possible.

Since the own catalytic activity of the metal chips is not sufficient, the chips are doped according to the invention with a further catalytic component. The noble metals Pt and Pd, but also other catalytically active transition metals and metal oxides, optionally with doping, are particularly suitable for this purpose.

First of all, the metal chips used for the production of the catalyst must be carefully freed from cooling lubricants. Then, for example, palladium (II) acetylacetonate is dissolved in dilute nitric acid. The chips are drenched with this solution. After the nitric acid mixture has evaporated to such an extent that only a viscous liquid remains, the chips can be dried in the drying cabinet in a manner that is gentle on the temperature (for example, 2 hours at 80 ° C.). After this drying, the chips have taken on a reddish to brown color and can be left in an N ₂ atmosphere at a temperature of 400 ° C. for a further 3 hours. The surface of the chips is approx. 9-10 m ² / g (BET analysis). The catalyst can be activated reductively with carbon monoxide and / or hydrogen. Due to the fact that the chips are self-activated, activation can also be dispensed with in the catalyst system according to the invention.

The invention is explained in more detail below on the basis of a drawing documenting a model test. The drawing shows:

1 shows the schematic structure of the trainer with the catalyst system according to the invention,

Fig. 2 shows the measured raw and clean gas concentration as a function of the catalyst temperature and

Fig. 3 shows the degree of separation depending on the catalyst temperature.

The invention is explained by way of example in an experiment with a model gas (n-hexane and air). Hexane and air are mixed homogeneously in a mixing section 1 and then passed with the aid of a pump 2 over the catalyst 3 containing the metal chip bed described above. The concentration of the hexane in the mixture is adjusted by tempering the template. The catalyst 3 is brought from the outside by an electric heater 4 to the desired temperature (catalyst light-off temperature), which according to the invention can be below 200 ° C. Before and after the catalyst 3, the hexane concentration (as methane equivalent concentration) is measured by two flame ionization detectors 5, 6. The direct comparison of the raw and clean gas concentration in a process computer 7 gives the conversion of hexane and thus the activity of the catalyst 3.

According to a further teaching of the invention, a thermocouple 8 is provided in the interior of the catalyst 3. The temperature of the metal chips is measured in order to determine the influence on the catalytic activity of the chips as precisely as possible. The pressure loss over the chip bed was determined using a

Differential pressure measuring device 9 determines and can be tolerated with a few Pascals.

The measurement results were determined under the following test conditions:

Volume flow: 300 1 / h

Hexane concentration (raw gas): 100 mg hexane / m ³

Ambient temperature: 25 ° C

Mass of the chip bed: 500 g Volume of the catalyst bed: 0.43 10 ^{~ 3} m ³

Pd content: 5% by weight

Pressure: normal pressure

Based on experience from individual tests, it can be assumed that in real use the temperature of the exhaust gas from domestic combustion systems is sufficient to keep the temperature in the catalytic converter at a constantly high value, so that a degree of separation of clearly> 90% can be achieved. It may be cheap to reduce odors in private Area to install an electrical preheater.

2 shows the raw and clean gas concentration as a function of the catalyst temperature. The first conversion of hexane can be found at 233 ° C (catalyst start temperature). As can be seen from FIG. 3, the degree of separation increases markedly with increasing temperature, so that a conversion of 86% is achieved at a temperature of 346 ° C. The pressure loss over the bed could be neglected.

Claims

Claims:

1. A process for the catalytic purification of exhaust air flows, characterized by the following steps:

Impregnation of cleaned metal chips in a dilute acid in which a catalytically active noble metal, transition metal or transition metal oxide, introduced in the form of its metal salts, has been dissolved,

gentle drying of the prepared metal chips and

Passing the exhaust gas stream to be cleaned through a bed of the doped metal chips.

2. The method according to claim 1, characterized in that the metal salt used is palladium (II) acetylacetonate and nitric acid is used as the acid.

3. The method according to claim 1 or 2, dadur ch ge kenn z e i chnet that Cr-Ni chips are used as metal chips.

4. The method according to claim 3, characterized in that the Cr-Ni chips contain proportions of Mo, Ti, V.

5. The method according to any one of claims 1 to 4, characterized in that the gentle drying of the chips in the drying cabinet takes place at temperatures of 70 to 90 ° C.

6. The method according to any one of claims 1 to 5, characterized in that the duration of gentle drying is 2 to 3 hours.

7. The method according to claim 5 or 6, characterized in that the metal chips are left for several hours in an N ₂ atmosphere after drying in the drying cabinet.

8. The method according to claim 7, characterized in that the temperature of the N ₂ atmosphere is 400 ° C.

9. The method according to any one of claims 1 to 8, characterized in that the doped metal chips are heated to a starting temperature before the first use.

10. The method according to claim 9, characterized in that the catalyst light-off temperature is below 200 ° C.

11. The method according to any one of claims 1 to 10, characterized in that the temperature in the reaction area can be above 350 ° C.

12. Device for the catalytic cleaning of exhaust air streams, characterized by a fixed bed reactor

(3) with a fill contained therein - I o-

catalytically active metal chips, a temperature sensor (8) inside the fixed bed reactor (3) and devices (5, - 6) for measuring the raw and clean gas concentration of pollutants.

13. The apparatus according to claim 12, characterized in that as a fixed bed reactor (3) a section of an existing in the exhaust air generating exhaust pipe is used, in which the bed is introduced.

14. The apparatus according to claim 12 or 13, characterized in that the evaluation of the measured raw and clean gas values takes place in a process computer (7).

15. The device according to one of claims 12 to 14, characterized by a differential pressure measuring device (9) for tolerating the pressure loss in the fixed bed reactor (3).

16. Device according to one of claims 12 or 13, characterized in that the fixed bed reactor (3) has a heater (4).

17. Use of metal chip waste from the metalworking industry for the process for the catalytic cleaning of exhaust air streams according to claims 1 to 11, characterized in that the metal chips are cleaned of impurities such as cooling lubricants in particular before impregnation.