WO1998003247A1

WO1998003247A1 - Process for reducing the concentration of polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons in a flue gas

Info

Publication number: WO1998003247A1
Application number: PCT/US1997/012070
Authority: WO
Inventors: Ulrich Wallbaum; Kurt Zunker
Original assignee: Dow Deutschland Inc.
Priority date: 1996-07-22
Filing date: 1997-07-11
Publication date: 1998-01-29
Also published as: AU3657997A; JP2001505478A; GB9615358D0

Abstract

The concentration of polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons in a flue gas is reduced in a process wherein the flue gas is contacted with a scrubbing liquor containing an oil and optionally water, wherein the oil and water together constitute at least 60 percent of the total weight of the scrubbing liquor and at least 50 weight percent of the oil contained in the scrubbing liquor is a vegetable oil.

Description

PROCESS FOR REDUCING THE CONCENTRATION OF POLYHALOGENATEDAROMATIC COMPOUNDS OR POLYNUCLEAR AROMATIC

HYDROCARBONS IN A FLUE GAS

It is well known that flue gases which are produced in incineration plants, such as municipal waste or hazardous waste incineration plants or incineration plants for chemical residues, by-products and/or vent gases, can contain small amounts of polyhalogenated aromatic compounds, such as polychlorinated dibenzodioxins, polychlorinated dibenzofurans, polychlorinated biphenylenes, polychlorinated phenols, polychlorinated styrenes, polychlorinated benzenes or polychlorinated naphthalenes, or polynuclear aromatic hydrocarbons, such as acenaphthene, anthracene, benzopyrenes or phenanthrene. Due to the toxicity of these compounds, much effort has been made to reduce their concentration in flue gases. Several methods are known, such as adsorption on activated carbon, activated carbon/lime mixtures or various types of catalytic reduction. Unfortunately, all these technologies are expensive and adsorption by activated carbon involves the risk of inflammation or a smouldering fire. Furthermore, the handling and disposal of the contaminated adsorbent and of open equipment in case of maintenance work requires special workplace precautions. Another method for removing dioxins and furans from flue gas is described in German Offenlegungsschrift DE-A 41 19 006. According to this method the flue gas is scrubbed to remove acidic components of the flue gas. Paraffin oil is added to the scrubbing liquor. In a two-stage process for removal of HF and HCI in the first stage and SO in the second stage, the scrubbing liquor in the second stage contains a base, such as aqueous sodium hydroxide, and paraffin oil. The paraffin oil can be separated from the aqueous phase of the scrubbing liquor and recycled to the second scrubbing stage. High amounts of paraffin oil are necessary to achieve sufficient removal of dioxins and furans. It is recommended that the paraffin oil constitutes 1/8 to 1/12 of the total amount of the scrubbing liquid. However, the reduction achieved is not always sufficient. In the Example, DE-A 41 19

3 006 discloses that the initial concentration of dioxins and furans was 0.57 ng TE/Nm and

3 that this concentration was reduced to 0.16 ng TE/Nm . This is a reduction of 70 percent, however the concentration achieved is above the limit required by the emission regulations of some countries. Another disadvantage of paraffin oil is its relative high vapor pressure which will result in enhanced hydrocarbon emissions.

German Patent DE-C 41 09 991 relates to the reduction of furans and dioxins in flue gas which is produced during the smelting of aluminum waste. According to this process finely divided metals and/or their oxides, hydroxides and/or oxyhydroxides are added to a combustion chamber in amounts which are 1 to 10 percent over the stoichiometric amounts needed to convert halogens and/or halocarbons to halides. This amount is not sufficient to fully prevent formation of dioxins and furans. The noxious compounds which remain in the flue gas are absorbed by a non-flammable oil-water emulsion which is sprayed in the flue gas. Absorption with pure oil is said to be possible, however hot gases might cause ignition of the oil. The ratio of oil to water is 0.01 to 90. After the flue gas purification the emulsion is separated into a water-rich phase and an oil-rich phase which contains the noxious compounds. The water-rich phase is recycled and the oil-rich phase is burnt. DE-C 41 09 991 does not teach which type of oil is useful in this process. Unfortunately, the process described in DE-C 41 09 991 is very complicated. It requires the addition of two different types of compounds at different stages of the incinerator system. Furthermore, the addition of finely divided metals and/or their oxides into the combustion chamber will result in the formation of dust, and consequently in the requirement to remove this dust from the flue gas by a cleaning system. This is undesirable in substantially dust free processes for incinerating gases and liquids.

In view of the above-discussed disadvantages of the known processes, there is still a high demand for a new process for reducing effectively the concentration of polyhalogenated aromatic compounds, such as polychlorinated dibenzodioxins or polychlorinated dibenzofurans, or polynuclear aromatic hydrocarbons in flue gases . A preferred object of the present invention is to provide a process wherein the concentration of polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) can be

3 reduced to 0.1 ng ITE/Nm or less. Another preferred object of the present invention is to provide a process which is relatively inexpensive and uses simple equipment. A further preferred object of the present invention is the reduction of other polyhalogenated aromatic compounds and polynuclear aromatic hydrocarbons which have a similar volatility as the PCDD and PCDF in a comparable range of removal efficiency.

The present invention provides a process for reducing the concentration of polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons in a flue gas wherein the flue gas is contacted with a scrubbing liquor containing an oil and optionally water, wherein the oil and water together constitute at least 60 percent of the total weight of the scrubbing liquor and wherein at least 50 weight percent of the oil contained in the scrubbing liquor is a vegetable oil.

The term "vegetable oil" as used herein includes vegetable oils in their natural or chemically modified form, for example in their hydrogenated, partially hydrogenated or epoxidized form. Exemplary of chemically modified vegetable oils are hydrogenated or partially hydrogenated canola oil or rapeseed oil epoxidized soy bean oil.

The term "contacting the flue gas with a scrubbing liquor" as used herein includes contacting the flue gas in one or more scrubbing stages in sequence with one or more types of the above-mentioned scrubbing liquor.

It has been found that the above-mentioned process is efficient in removing polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons from flue gases. It has been surprisingly found that the above-mentioned process is particularly efficient in removing polychlorinated dibenzodioxins or polychlorinated dibenzofurans from flue gases. Surprisingly, the concentration of these compounds can be reduced to very low levels, usually to 0.1 ng .TE/Nm³ or less, in many cases even to 0.06 ng ITE/Nm³ or less, in several cases even to 0.02 ng ITE/Nm³ or less.

It has also been found that the process can be conducted continuously and on a large scale over a period of several months without exchanging the scrubbing liquid and without any measurable loss in efficiency. Furthermore, the process of the present invention can be operated in a closed system which requires very little maintenance activity. Therefore, the risk of contacting contaminated materials is significantly reduced

The use of vegetable oils, such as rapeseed oil, has been suggested for various other purposes, however not for removing polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons from flue gases.

U.S. Patent No. 4,844,721 suggests purification of polluted air which contains odorous and/or pungent pollutants, particularly styrene, acrolein or sewage odors. The polluted air is contacted with a scrubbing liquid which contains a drying or semi-drying vegetable or animal oil which has an average iodine value of at least 90. Depending on the circumstances of use, from 0.001 to 100 weight percent of the scrubbing liquid should be an oil. A preferred proportion for chemical operations are said to be from 1 to 10 percent, for organic solvents from 10 to 50 percent, for organic waste from 0.01 to 6 percent and for incineration process emissions from 0.001 to 2 percent. The absorbed styrene material is stripped from the oil by polymerizing styrene by UV light. The water portion is separated from the oil by stratification at which time much of the polymerized and unpolymerized styrenes precipitate and settle, by gravity, in the water portion. U.S. Patent No. 5,198,000 discloses a process for removing volatile organic compounds which have a boiling point of up to 170°C from a gas stream. The concentration of the volatile organic compounds in the gas stream generally is from 10 to 10.000 parts per million by volume. The gas stream is contacted with a petroleum derivative, a vegetable oil or a synthetic oil, preferably with motor oil, mineral oil or corn oil. When the oil is saturated with the volatile organic compound, the volatile organic compound is separated from the oil, the volatile organic compound is collected and the separated oil is recovered.

German Offenlegungsschrift DE-A-42 03 385 discloses the use of various vegetable oils in unchanged, emulsified or modified form as wash liquors for cleaning Another method for removing dioxins and furans from flue gas is described in German

Offenlegungsschrift DE-A 41 19 006. According to this method the flue gas is scrubbed to remove acidic components of the flue gas. Paraffin oil is added to the scrubbing liquor. In a two-stage process for removal of HF and HCI in the first stage and SO in the second stage,

Λ the scrubbing liquor in the second stage contains a base, such as aqueous sodium hydroxide, and paraffin oil. The paraffin oil can be separated from the aqueous phase of the scrubbing liquor and recycled to the second scrubbing stage. High amounts of paraffin oil are necessary to achieve sufficient removal of dioxins and furans. It is recommended that the paraffin oil constitutes 1/8 to 1/12 of the total amount of the scrubbing liquid. However, the reduction achieved is not always sufficient. In the Example, DE-A 41 19 006 discloses that

3 the initial concentration of dioxins and furans was 0.57 ng TE/Nm and that this

3 concentration was reduced to 0.16 ng TE/Nm . This is a reduction of 70 percent, however the concentration achieved is above the limit required by the emission regulations of some countries. Another disadvantage of paraffin oil is its relative high vapor pressure which will result in enhanced hydrocarbon emissions.

3 reduced to 0.1 ng ITE/Nm or less. Another preferred object of the present invention is to provide a process which is relatively inexpensive and uses simple equipment. A further preferred object of the present invention is the reduction of other polyhalogenated aromatic compounds and polynuclear aromatic hydrocarbons which have a similar volatility as the

PCDD and PCDF in a comparable range of removal efficiency.

The term "contacting the flue gas with a scrubbing liquor" as used herein includes contacting the flue gas in one or more scrubbing stages in sequence with one or more types of the above-mentioned scrubbing liquor. It has been found that the above-mentioned process is efficient in removing polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons from flue gases. It has been surprisingly found that the above-mentioned process is particularly efficient in removing polychlorinated dibenzodioxins or polychlorinated dibenzofurans from flue gases. Surprisingly, the concentration of these compounds can be reduced to very low levels, usually to 0.1 ng ITE/Nm³ or less, in many cases even to 0.06 ng ITE/Nm³ or less, in several cases even to 0.02 ng ITE/Nm³ or less.

U.S. Patent No. 4,844,721 suggests purification of polluted air which contains odorous and/or pungent pollutants, particularly styrene, acrolein or sewage odors. The polluted air is contacted with a scrubbing liquid which contains a drying or semi-drying vegetable or animal oil which has an average iodine value of at least 90. Depending on the circumstances of use, from 0.001 to 100 weight percent of the scrubbing liquid should be an oil. A preferred proportion for chemical operations are said to be from 1 to 10 percent, for organic solvents from 10 to 50 percent, for organic waste from 0.01 to 6 percent and for incineration process emissions from 0.001 to 2 percent. The absorbed styrene material is stripped from the oil by polymerizing styrene by UV light. The water portion is separated from the oil by stratification at which time much of the polymerized and unpolymerized styrenes precipitate and settle, by gravity, in the water portion.

U.S. Patent No. 5,198,000 discloses a process for removing volatile organic compounds which have a boiling point of up to 170°C from a gas stream. The concentration of the volatile organic compounds in the gas stream generally is from 10 to 10.000 parts per million by volume. The gas stream is contacted with a petroleum derivative, a vegetable oil or a synthetic oil, preferably with motor oil, mineral oil or corn oil. When the oil is saturated with the volatile organic compound, the volatile organic compound is separated from the oil, the volatile organic compound is collected and the separated oil is recovered. German Offenlegungsschrift DE-A-42 03 385 discloses the use of various vegetable oils in unchanged, emulsified or modified form as wash liquors for cleaning gas mixtures containing solvent vapors, odorous substances or other harmful gases. The oils may be used in the form of an oil-in-water emulsion which contains at least 50 volume percent of the oil. In order to increase the absorption ability of the wash liquor, the German Offenlegungsschrift recommends the inclusion in the wash liquor of from 1 to 8 weight percent of an ester of an aliphatic dicarboxylic acid containing from 3 to 6 carbon atoms and an alcohol containing from 1 to 5 carbon atoms and/or 1 to 5 weight percent of propylene glycol and/or 2 to 10 weight percent of a glycerol ester, based on the total weight of the vegetable oil. The absorption process involves the steps of (i) leading the gas mixture into the wash liquor, (ii) causing turbulences in the rising gas stream, spraying it with the wash liquor and passing it through a droplet separator, (iii) leading the gas stream over a permanganate granulate to remove residual contaminants, and (iv) (dis)continuously withdrawing the wash liquor containing the contaminants from the container, followed by distillation to recover the contaminants. The purification process is preferably conducted at a temperature of from 15°C to 30°C. The German Offenlegungsschrift is silent about which compounds and to what extent compounds can be removed from gases by the suggested method.

European Patent Application EP-A-0 652 038 discloses a process for removing organic compounds from exhaust air, particularly for reducing the smell of the exhaust air. The process is conducted in two stages; in the first stage the exhaust air is contacted with a high boiling-oil, such as silicone oil. The high boiling-oil which is contaminated with the organic compounds is regenerated. During the regeneration step the organic compounds are condensed and collected in a solvent container. The vent stream of the first stage is contacted with a second oil, such as a vegetable oil, a vegetable oil derivative, a hydrocarbon, higher alcohols or esters. However, the European Patent Application does not disclose which compounds or the extent to which a compound can be removed from gases by the suggested method. Furthermore, this method is not very suitable for reducing the concentration of polyhalogenated aromatic compounds in flue gases.

Based on the teachings of the above-discussed references it could not be expected that the concentration of polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons in a flue gas could be reduced significantly and that particularly the concentration of polychlorinated dibenzodioxins and polychlorinated dibenzofurans could be reduced to a very low level when the flue gas is contacted with a scrubbing liquor which contains an oil and optionally water, wherein the oil and water together constitute at least 60 percent of the total weight of the scrubbing liquor and wherein and at least 50 weight percent, preferably at least 75 weight percent, of the oil contained in the scrubbing liquor is a vegetable oil. The oil and the water together constitute preferably at least 80 percent, more preferably at least 90 percent of the total weight of the scrubbing liquor.

The polyhalogenated aromatic compounds in the flue gas contain at least 2 halogens, typically from 4 to 8 halogens, in a few cases even above 8 halogens. Halogens are for example bromine or chlorine. The present invention is particularly suitable for reducing the concentration of polychlorinated aromatic compounds. Exemplary of polychlorinated aromatic compounds are polychlorinated biphenylenes (PCB), polychlorinated benzenes, polychlorinated phenols, polychlorinated naphthalenes, polychlorinated styrenes, such as octachlorostyrene, and particularly the polychlorinated dibenzofurans (PCDF) and polychlorinated dibenzodioxins (PCDD), or mixtures thereof.

Exemplary of polynuclear aromatic hydrocarbons are acenaphthene, acenaphthylene, anthracene, benzoperylene, benzopyrenes, benzofluoranthenes, benzoanthracene, naphthalene, chrysene, pyrene, perylene, phenanthrene, fluorene, dibenzoanthracene, dibenzopyrene, dibenzoacridine, dibenzocarbazole and indenopyrene.

The flue gas may originate from the incineration of solid waste, for example from municipal waste incineration plants, hazardous waste incineration plants or incineration plants for chemical residues, by-products and/or vent gases. Preferably, the flue gas originates from the incineration of liquids and/or gases, for example from the incineration of liquid and/or gaseous by-products or residues of chemical production processes,

Preferably, flue gas which is emitted from an incinerator is subjected to one or more pre-purification steps before it is treated according to the process of the present invention.

Prior to the process of the present invention flue gas is preferably quenched to a temperature of from 40°C to 95°C, more preferably from 45°C to 85°C, most preferably from 50°C to 75°C. Preferred quenching agents are water or a raw acid resulting from the absorption of gaseous acidic compounds in the quenching water. Quenching methods are known in the art. Heat can be recovered in a known manner, and can for example be used for steam production. If the flue gas contains a considerable amount of a gaseous acid, such as gaseous hydrochloric acid, the flue gas is preferably passed through an acid absorber wherein it is contacted with water to reduce the acid concentration. Acid absorbers are known in the art.

To achieve optimum results in the process of the present invention, the 5 amount of dust in the flue gas which is treated according to the present invention preferably is not more than 50 mg/Nm³, more preferably not more than 10 mg/Nm³, most preferably not more than 3 mg/Nm³. If the dust level is higher, the dust concentration in the flue gas is preferably reduced prior to the process of the present invention. Processes for dust reduction are known in the art, such as electrostatic filters, bag-house filters or cyclones.

10 Prior to the process of the present invention the optionally pre-purified flue gas generally has a content of polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) of from 0.1 to 50 ng ITE/Nm³, typically from 0.1 to 10 ng ITE/Nm , in

3 3 most cases from 0.2 to 3 ng ITE/Nm . The term "ITE/Nm " means an International Tox equivalent per norm cubic meter (273K and 1013 mbar), as defined in NATO/CCMS (North

!5 Atlantic Treaty Organization, Committee on the Challenges of Modern Society) (1988),

International toxicity equivalency factor (l-TEF) method of risk assessment for complex mixtures of dioxins and related compounds; Report No. 176, Brussels; North Atlantic Treaty

Organization.

If other polyhalogenated aromatic compounds and/or polynuclear aromatic 0 hydrocarbons are present in the flue gas prior to the process of the present invention, the amount of the polyhalogenated aromatic compounds other than PCDD and PCDF generally is from 0.01 microgram/m³ to 100 micrograms/m³, typically from 0.1 microgram/m³ to 10 micrograms/m³, and in most cases from 0.5 microgram/m³ to 5 micrograms/m³ and/or the amount of the polynuclear aromatic hydrocarbons generally is from 0.01 microgram/m³ to 50 5 micrograms/m³, typically from 0.1 microgram/m to 5 micrograms/m³, and in most cases from 0.1 microgram/m³ to 1 microgram/m³.

In addition to water, an oil and the optional additives as indicated below, the scrubbing liquor used in the process of the present invention may contain up to 30 percent, based on the total weight of the scrubbing liquor, of activated carbon, in order to further 0 improve the efficiency of the scrubbing liquor. Preferred amounts of the activated carbon, if present, are from 0.01 to 30 percent, more preferably from 0.1 to 10 percent, most preferably from 0.5 to 5 percent, based on the total weight of the scrubbing liquor (including the activated carbon). According to a first preferred embodiment of the present invention the optionally pre-purified flue gas is contacted with a scrubbing liquor I which contains from 60 to 99.99 weight percent, preferably from 90 to 99.98 weight percent of water and from 40 to 0.01 weight percent, preferably from 10 to 0.02 weight percent of an oil. In addition to water and an oil, based on the total weight of water and oil the scrubbing liquor may contain optional additives, such as activated carbon as previously indicated, including one or more basic compounds, inorganic salts, hypochlorite destroying compounds and/or emulsifiers. A preferred inorganic salt is sodium chloride. Preferred hypochlorite destroying compounds are hydrogen peroxide, hydrogen sulfite or sulfite salts. The presence of a base, preferably an inorganic base such as sodium or potassium hydroxide, is particularly suitable if the flue gas contains residual amounts of compounds which can be absorbed or neutralized by the base, for example, chlorine or acidic components, such as HCI, which were not completely removed in an acid absorber. If a base is present, its amount is preferably sufficient such that the pH of the scrubbing liquor I is from 7.5 to 9.5, more preferably from 8 to 9. However, the total amount of water and oil amounts to at least 60 percent, preferably at least 80 percent, more preferably at least 90 percent and most preferably at least 95 percent, based on the total weight of the scrubbing liquor I. Preferably, the scrubbing liquor I is circulated in the scrubbing system to reduce the concentration of polychlorinated dibenzodioxins or polychlorinated dibenzofurans or both in the flue gas until the total concentration of the polychlorinated dibenzodioxins and polychlorinated dibenzofurans in the oil portion of scrubbing liquor I reaches at least 0.1 microgram ITE/liter, more preferably at least 1 microgram ITE/liter and most preferably at least 10 micrograms ITE/liter.

According to a second preferred embodiment of the present invention, the optionally pre-purified flue gas is contacted with a scrubbing liquor II which contains at least 60 percent, preferably at least 80 percent, more preferably at least 90 percent, and most preferably at least 95 percent of an oil, based on the total weight of the scrubbing liquor II. The scrubbing liquor II may also contain water. The water content in the scrubbing liquor II preferably is not more than 20 percent, more preferably not more than 10 percent, most preferably not more than 5 percent, based on the total weight of the scrubbing liquor II. The scrubbing liquor II may also contain activated carbon in an amount indicated above. Other optional additives which may be comprised in the scrubbing liquor II are, for example, antioxidants, such as alkylated diphenylamines or phenolic antioxidants, or additives to deactivate metals, for example triazole derivatives. Other additives are phthalic acid dialkylesters with C,-C₁₀ - alkyl chains and/or polyethylene glycol dialkylethers with C,-C₄ - alkyl-chains. However, the total amount of water, activated carbon and other optional additives should not be more than 40 percent, preferably not more than 20 percent, more preferably not more than 10 percent, and most preferably not more than 5 percent, based on the total weight of the scrubbing liquor II. Most preferably, the scrubbing liquor II is an oil or a mixture of different oils. Preferably, the scrubbing liquor II is circulated in the scrubbing system to reduce the concentration of polychlorinated dibenzodioxins or polychlorinated dibenzofurans or both in the flue gas until the total concentration of the polychlorinated dibenzodioxins and polychlorinated dibenzofurans in the scrubbing liquor II reaches at least 1 microgram ITE/liter, more preferably at least 10 micrograms ITE/liter.

At least 50 weight percent, preferably at least 75 weight percent, more preferably at least 90 weight percent, and most preferably substantially the entire amount of the oil contained in the scrubbing liquor is a vegetable oil in its natural form or chemically modified form, such as hydrogenated or partially hydrogenated canola- or rapeseed oil or epoxidized soybean oil.

In the terms "an oil" and "a vegetable oil" as used herein, mixtures of different natural and/or chemically modified (vegetable) oils are included.

Useful vegetable oils, are for example, rapeseed oil, olive, olive pit, castor oil, almond, peanut, hazel nut, beet, sesame, beechnut, corn, sunflower, soya, cottonseed, cereal, poppy, hemp, walnut, linseed or wood oil or mixtures thereof. The most preferred oil is rapeseed oil or canola oil. Other oils which may be mixed with one or more vegetable oils are for example pure or used mineral oils, such as paraffin oil, or animal oils, such as sardine oil, shark oil, whale oil or herring oil. The most preferred oils are rapeseed oil, canola oil, sunflower oil or epoxidized soy bean oil. In the practice of the process of the present invention the flue gas is preferably contacted with the scrubbing liquor in a packed gas wash tower, spray tower or bubble separator. More preferably, a packed wash tower is used with a total amount of scrubbing liquor of preferably from 0.1 to 5 liters, more preferably from 0.3. to 2 liters, per norm-cubicmeter (Nm³)of maximum possible flue gas flow. Most preferably a packed wash tower is used wherein the flue gas and the scrubbing liquor flow countercurrently and with a total amount of scrubbing liquor of 0.3 to 2 liters per Nm³of maximum possible flue gas flow.

The process of the present invention can be conducted in one or more scrubbing stages in sequence. Applying two or more, preferably two, scrubbing stages in sequence is particularly useful if the concentrations of polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons cannot be reduced to the desired level in one stage. Different types of scrubbing liquors, such as scrubbing liquors of type I or II, can be used in the different stages and preferably a scrubbing liquor II is used in the last stage.

The flue gas temperature preferably is from 40°C to 95°C, more preferably from 45°C to 85°C, most preferably from 50°C to 75°C. The temperature of the scrubbing liquor is generally at least as high as and preferably slightly higher than the temperature of the flue gas. The preferred temperature of the scrubbing liquor I is up to 30°C, more preferably up to 15°C and most preferably up to 5°C above the temperature of the flue gas. The preferred temperature of the scrubbing liquor II is up to 30°C, more preferably from 5°C to 20°C and most preferably from 10°C to 20°C above the temperature of the flue gas. The contact time of the flue gas with the scrubbing liquor is generally not less than 1 second, preferably from 3 to 20 seconds, more preferably from 5 to 10 seconds. In a preferred embodiment of the present invention wherein the flue gas is contacted with the scrubbing liquor in a packed wash tower, the scrubbing liquor is preferably recirculated between the wash tower sump and the packing section.

The scrubbing liquor is generally disposed after its physical conditions, such as viscosity or particle/dust content or loading with polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons, have reached a level which is undesirable for sufficient removal performance. Preferably the scrubbing liquor is disposed by incineration, more preferably in the same incinerator from which the flue gas is emitted which is treated according to the process of the present invention. Disposal can be achieved in batches or by continuous removal of a small side stream of loaded scrubbing liquor. The disposal of this side stream is preferably done by continuous removal of an amount of scrubbing liquor I at a rate of from 0.05 to 1 percent per hour of the total amount of scrubbing liquor I or by continuous removal of an amount of scrubbing liquor II at a rate of from 0.01 to 0.1 percent per hour of the total amount of scrubbing liquor II present in the process. When the scrubbing liquor is exchanged in batches, preferably from 30 to 90 percent, more preferably from 50 to 70 percent of the total amount of scrubbing liquor is exchanged by fresh material.

The concentration of polynuclear aromatic hydrocarbons or polyhalogenated aromatic compounds other than PCDD and PDCF in the flue gas is generally reduced by at least 20 percent, in many cases by at least 40 percent in the process of the present invention.

It has surprisingly been found that the concentration of polychlorinated dibenzodioxins and/or polychlorinated dibenzofurans in the flue gas is generally reduced by 90 percent or more, often even by 95 percent or more and in many cases even by 98 percent or more due to the contact of the flue gas with the scrubbing liquor according to the process of the present invention. Similar reduction rates can be expected for other polynuclear aromatic hydrocarbons or polyhalogenated aromatic compounds with similar volatility as polychlorinated dibenzodioxins and/or polychlorinated dibenzofurans It has even more surprisingly been found that the indicated reduction can be achieved in a single stage

The concentration of the polynuclear aromatic hydrocarbons and the polyhalogenated aromatic compounds in the flue gas can be further reduced by carrying out the process of the present invention in two or more stages and/or by frequently exchanging at least a portion of the used scrubbing liquor such that the scrubbing liquor will not be highly loaded with contaminants

After the process of the present invention the flue gas generally has a content of polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) of up 0 1 ng ITE/Nm³, typically even only up to 0.06 ng ITE/Nm³ and under optimal conditions even only up to 0 02 ng ITE/Nm³

The invention is further illustrated by the following examples which should not be construed to limit the scope of the present invention. Unless stated otherwise all parts and percentages are given by weight

Example 1

Pre-cleaned flue gas of a hazardous waste rotary kiln incinerator was treated with a scrubbing liquor II as described below This incinerator typically emitted 30,000 to 40,000 Nm³/h flue gas which was in compliance with the regulations of the German directive for waste incineration (17 federal directive for emission protection, December 1990) except for the emission of polychlorinated dibenzodioxins and dibenzofurans The pre-cleaned flue gas typically contained less than 5 mg/Nm³ dust and less than 5 mg/Nm³ HCI, but from 0 1 to 0 5 ng ITE/Nm³ of PCDD and PCDF. The flue gas was saturated with water vapor and had a temperature of 55 to 60°C A side stream of 100 to 150 Nm³/h was fed into a packed wash tower of 2 meters height and 250 mm diameter which was filled with stainless steel rings The height of the packing was about 1.3 meters. An oil reservoir underneath the wash tower contained 750 to 800 liters of pure unmodified rapeseed oil. It was pumped by a membrane pump at a rate of 100 to 150 ters/h on top of the packing and recirculated to the reservoir The oil and gas flows were set countercurrently. An electrical heating device attached to the reservoir kept the oil at a temperature of from 70°C to 75°C The gas pipes were insulated and electrically heated to prevent condensation. The oil pipe was insulated to prevent temperature losses of the oil before it entered the wash tower. The gas flow speed in the pipe to the wash tower was adjusted to be equal to the gas flow speed in the chimney of the incinerator in order to provide the same amount of dust to the oil wash tower compared to the typical dust load in the gas of the chimney. Sampling for PCDD and PCDF was always conducted by using two identical apparatus according to the most commonly used German method for PCDD and PCDF sample collection (Verein Deutscher Ingenieure.VDI 3499/2; March 1993 draft). Flue gas samples were taken simultaneously before and after the wash tower. Sampling times were chosen to provide a limit of detection of at least 0.001 ng ITE/Nm³.

The wash tower was operated continuously for 3 months with the operating parameters as described above and the scrubbing liquor II was circulated in a totally closed loop mode. After 4 days of operation the first PCDD and PCDF sample collection was done for 2 hours. Within the following 70 days 4 additional sample collections were conducted. The PCDD and PCDF ITE values before and after the wash tower and the relative reduction rates are given in the following table.

No. of Experiment 1 2 3 4 5

Days of operation 4 25 46 73 74

ITE [ng/Nm³] before 0.10 0.41 0.41 0.15 0.27 wash tower

ITE [ng/Nm³] after wash 003 003 0.01 0.005 tower

% Reduction 94.0% 99.3% 99.3% 93.4% 98.1 %

Experiment No. 4 was a sample collection during an interruption of oil circulation. The flue gas was passing through the wash tower with an oil wetted packing, but without any oil flow for 3 hours. Experiment No. 5 was a sample collection during a time period with a 5 to 10 times higher dust load in the flue gas than normal. Dust concentrations were in the range of 20 to 30 mg/Nm³. The wash tower was taken out of operation after 93 days. The viscosity of the rapeseed oil changed during the entire time period from 35 mPas to 52 mPas (at 40°C) and the iodine value decreased from 98 to 88.

Example 2

A reservoir arranged to the wash tower as described in Example 1 was filled with an emulsion of rapeseed oil in water. The oil concentration in water was about 1000 parts per million. All other operating conditions and parameters including the procedure of sample collection remained as described in Example 1. The following table shows the achieved results:

No. of Experiment

Days of operation 16 17

ITE [ng/Nm³] before wash tower 0.13 0.17 0.18

ITE [ng/Nm³] after wash tower 0.01 1 0.006 0.006

% Reduction 91.5% 96.5% 96.7%

Experiment No. 3 was a sample collection during a time period with 5 to 10 times higher dust load in the flue gas than normal. Dust concentrations were in the range of 20 to 30 mg/Nm³.

Example 3

A reservoir arranged to the wash tower as described in Example 1 was filled with an emulsion of partially hydrogenated rapeseed oil with a iodine value of 82 to 90 and a linolenic acid content of less than 1 percent in water. The oil concentration in water was about 3500 parts per million. All other operating conditions and parameters including the procedure of sample collection remained as described in Example 1. The following table shows the achieved results: No. of Experiment 1 2

Days of operation 1 24

ITE [ng/Nm³] before wash tower 0.433 0.371

ITE [ng/Nm³] after wash tower 0.053 0.044

% Reduction 87.8% 88.2%

Example 4

A reservoir arranged to the wash tower as described in Example 1 was filled with an emulsion of epoxidized soy bean oil in water. The oil concentration in water was about 3000 parts per million. All other operating conditions and parameters including the procedure of sample collection remained as described in Example 1. The following table shows the achieved results:

No. of Experiment 1

Days of operation

ITE [ng/Nm³] before wash tower 0.486 0.383

ITE [ng/Nm³] after wash tower 0.021 0.014

% Reduction 95.8%

96.3%

Example 5

Flue gas emitted from an incinerator for chlorinated liquid by-products and process vent gases was first sent through an acid absorption tower and was then treated in a scrubbing tower. The flue gas flow varied in the range of from 5,000 to 10,000 Nm³/h. The scrubbing The scrubbing tower was 12.3 meters high and had a diameter of 2.5 meters and a packing height of 4.5 meters. The volume of the packing was about 22 cubic meters and packing rings with a diameter of 2 inches were used. The wash liquor contained 2.3 percent of NaOH, 2.3 percent of NaCI, 94 to 95 percent of water and several other inorganic components, such as sodium carbonate or sodium bicarbonate. The wash liquor was circulated at a rate of 40 to 70 m³/h. Between 6 and 8 m³/h of wash liquor were continuously discharged and also continuously replaced by fresh material to keep the total volume of the liquor constant. For the experiments according to the present invention 400 to 600 parts per million of rapeseed oil were constantly added to the wash liquor. PCDD and PCDF emission prior to the addition of the rapeseed oil was 3.93 ng ITE/Nm³. Oil addition to the wash liquor was started. Thereafter, two samples, one at 20 hours and another at 24 hours, were collected over a 2 hour period PCDD and PCDF emissions were 0.070 and 0.052 ng ITE/Nm³.

Claims

Claims:

1. A process for reducing the concentration of polyhalogenated aromatic compounds or polynuclear aromatic hydrocarbons in a flue gas wherein the flue gas is contacted with a scrubbing liquor containing an oil and optionally water, wherein the oil and water together constitute at least 60 percent of the total weight of the scrubbing liquor and wherein at least 50 weight percent of the oil contained in the scrubbing liquor is a vegetable oil.

2. The process of Claim 1 wherein the flue gas is contacted with a scrubbing liquor I containing a mixture of from 90 to 99.99 weight percent of water and from 10 to 0.01 weight percent of an oil based on the total weight of water and oil.

3. The process of Claim 1 wherein the flue gas is contacted with a scrubbing liquor II containing at least 60 percent of an oil, based on the total weight of the scrubbing liquor.

4. The process of any one of Claims 1 to 3 wherein the scrubbing liquor contains up to 30 percent of activated carbon, based on the total weight of the scrubbing liquor including the activated carbon.

5. The process of Claim 2 wherein the scrubbing liquor I contains a mixture of from 90 to 99.98 weight percent of water and from 10 to 0.02 weight percent of an oil, and wherein the water and oil together constitute at least 80 percent of the total weight of the scrubbing liquor.

6. The process of any one of Claims 1 to 5 wherein at least 75 weight percent of the oil contained in the scrubbing liquor is a vegetable oil.

7. The process of any one of Claims 1 to 6 wherein the vegetable oil is rapeseed oil, canola oil, sunflower oil or epoxidized soy bean oil.

8. The process of any one of Claims 1 to 7 wherein the dust concentration in the flue gas is not more than 50 mg/Nm³ when the flue gas is contacted with the scrubbing liquor.

9. The process of any one of Claims 1 to 8 wherein the temperature of the flue gas is from 40°C to 95°C and the temperature of the scrubbing liquor is up to 30°C above the temperature of the flue gas.

10. The process of any one of Claims 1 to 9 wherein the polyhalogenated aromatic compounds in the flue gas are polychlorinated dibenzodioxins, polychlorinated dibenzofurans, polychlorinated biphenylenes, polychlorinated phenols, polychlorinated benzenes, polychlorinated naphthalenes, polychlorinated styrenes or a mixture of two or more thereof.

1 1. The process of any one of Claims 1 to 10 wherein the concentration of polychlorinated dibenzodioxins or polychlorinated dibenzofurans in the flue gas is reduced by at least 90 percent.

12. The process of any one of Claims 1 to 11 wherein at least a portion of the scrubbing liquor is incinerated after the flue gas has contacted the scrubbing liquor.