WO1995018667A1 - Prevention of formation and destruction of organohalogen compounds in incineration of waste materials - Google Patents

Abstract

Organohalogen compounds (OHC), including dioxins and furans, produced in waste incinerators are destroyed by introducing inorganic bases, either alone or in combination with aliphatic hydroxy compounds, into an incineration post combustion zone that contains gaseous incineration products. The destroyer compounds may also serve to inhibit the formation of OHC by catalysis at active sites on flyash produced in the incineration and also to remove acid gases from the gaseous incineration products. In this way, the concentration of OHC on flyash precipitated from the product gas stream is decreased as dioxins and acid gas concentrations in stack emissions from the incineration process also decreases.

Description

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   TITLE OF INVENTION
PREVENTION OF FORMATION AND DESTRUCTION OF   ORGANOHALOGEN   COMPOUNDS IN INCINERATION OF
WASTE MATERIALS 
FIELD OF INVENTION
The present invention relates to the prevention of formation and destruction of organohalogen compounds including dioxins and furans in incineration of waste materials. 



   BACKGROUND TO THE INVENTION
The formation of organohalogen compounds (OHC), including polychlorinated dibenzo-p-dioxins (PCDD) and dibenzo furans (PCDF) , occurs in all combustion processes which provides the necessary favourable conditions and ingredients, such as carboneous, organohalogenated and metallic materials. Municipal, medical and industrial waste incinerators are large contributors to the release of OHC to the environment. Municipal solid waste incinerators are called "Resource Recovery" plants, promising to provide steam and electricity while simultaneously reducing trash volume by 90 percent. 



  Reduction in trash volume results in less transportation costs and land fill space. Due to these benefits, incineration is a viable alternative to landfill. 



  Hundreds of such incinerators are in operation around the world. 



   One of the significant drawbacks to the incineration procedure is that several hundred stable and toxic compounds, including polychlorinated dibenzo-p-dioxins (abbreviated herein as PCDD and collectively commonly termed "dioxins") and polychlorinated dibenzofurans (abbreviated herein as PCDF and collectively commonly termed "furans"), are formed and are present in partsper-million concentrations both in the flyash formed during combustion and in the stack emissions. The 

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 formation of OHC, including PCDD and PCDF, in the process of combustion and pyrolysis of organic compounds in presence of inorganic halogenated compounds has been known for some time.

   The major sources of release of OHC, including dioxins and furans, are various incineration processes involving combustion of municipal waste, municipal sludge, medical waste, polyvinyl chloride and other halogen-containing plastics. 



   Our research work shows that the formation of OHC, including dioxins and furans, occurs in municipal solid waste incinerators (MSWI) around the world. It has been shown in our laboratories that combustion of PVC resulted in the formation of dioxins. Increased levels of dioxins and furans have been observed in stack emissions and in flyash when PVC was incinerated. One of the mechanisms proposed for the formation of PCDD and PCDF is by reactions of molecular precursor species that are present during incineration, resulting in surface catalyzed synthesis of PCDD/PCDF on the flyash via rearrangement, free radical condensation, dechlorination, dehydrogenation, trans-chlorination, isomerization and other similar molecular reactions.

   From the fact that uncontrolled reactions occur during the incineration process, it can be presumed that all above mentioned reactions can proceed due to the presence of different kinds of metal/metal-oxides, inorganic halides, organic materials and gaseous compounds, such as   CH2=CH2,   CH=CH, H2O, CO2 and HCl in the flue gas stream. Inorganic halides and acid gases can form in the incineration process at high temperatures from halogen-containing plastics, moisture and small amounts of metals present in waste materials. These compounds constitute catalytic sites on the surface on the flyash produced during incineration to enhance the formation of PCDD/PCDF. 



   We have studied flyash samples from different incinerators around the world. The dioxins and furans 

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 have been detected in all incinerators studied. More than 600 compounds were detected in flyash samples that includes a large number of OHC. 



   Release of OHC, including dioxins and furans, by MSWI through the stack emissions and flyash is of significant public concern. MSWI flyash containing high levels of dioxins and furans when dumped in landfill sites can leach dioxins in water system. Hence, there is a need for a technology which can' provide safe incineration of waste materials. The present invention relates to a method of reducing the OHC in the stack emissions and in flyash of the MSWI. There has been previously described in our U. S. Patents Nos. 4,793,270 and 5,113,772, the disclosures of which are incorporated herein by reference, the provision-of material acting as a catalyst inhibitor in association with the flyash so as to inhibit the catalytic activity of the flyash towards the formation of chlorinated compounds, including dioxins and furans. 



   SUMMARY OF INVENTION
It has now been found that inorganic bases, either alone or in combination with aliphatic hydroxy or polyhydroxy compounds (such materials being herein termed destroyers and inhibitor/destroyers), are very effective in destroying OHC, including dioxins and furans, on MSWI flyash. It is believed that the destroyers, when heated with flyash, react with the OHC and extract halogen to form neutral stable inorganic salts and organohydroxy compounds, which then react with more destroyer and metallic constituents in flyash at higher temperatures, resulting in their decomposition. Formation of dioxins and furans by the catalytic activity of flyash and precursors in flue gases at temperature between about 250  and about 400 C has been reported in our previous U.S. Patents Nos. 4,793,270 and 5,133,772.

   Inorganic bases have been used traditionally for conversion of OHC 

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 into organohydroxy compounds under strictly controlled conditions. The use of destroyers under drastic conditions, such as high temperatures and presence of air and steam, as provided herein, results in the reaction of OHC in the flue gases and on flyash particles with the destroyer to form the organohydroxy compounds, which ultimately decompose due to further reactions with destroyers at higher temperatures. Hence, according to one aspect, the present investigation is directed towards the destruction of OHC on flyash and in flue gases produced during MSWI by the utilization of the destroyers in the manner described herein. 



   In another aspect, the present invention is directed towards the deactivation of the flyash and thus the prevention of the formation of OHC, -including dioxins and furans. Research in our laboratories and a study of the correlation of operational conditions with levels of dioxins and furans detected in the stack emissions and in the flyash show that up to about   15%   OHC compounds is formed in the combustion process and the remaining about   85%   is formed by catalytic activity of flyash and small molecules, such as   CH2=CH2'   CH=CH, H20, CO2 and HCI, present in flue gas stream. 



   Thus, based on previous U.S. Patents Nos. 4,793,270 and 5,113,772, inhibition of the formation of about 85% of the OHC compounds, including dioxins and furans, was possible by the introduction of inhibitors in the postcombustion zone of the incinerators at about 400 C. 



  Since it is possible to introduce the presentlyinvestigated destroyer compounds at a wide range of temperatures, from about 300  to about   1100 C,   the present invention enables a higher destruction efficiency to be achieved. In laboratory experiments, complete destruction of all OHC on flyash has been found when flyash was heated with inorganic bases at 400 C. 

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   Mixtures of inorganic bases and aliphatic hydroxy compounds (AHC) are highly effective when they were used in smaller amounts and even at lower temperatures (about 300  to about 500 C) and the specific utilization of such organic-inorganic complexes in decreasing PCDD/PCDF on flyash and stack emissions forms an aspect of the invention. Destroyer compounds also act as inhibitors and deactivate flyash sites responsible for the formation of the OHC. The effect of temperature on'the OHC on the flyash shows that the amount of dioxins and furans increases -with temperature up to about 400 C. When flyash containing OHC was covered with destroyer compounds and heated, then destruction of OHC started at about 300 C and complete destruction of OHC had occurred by the time the temperature reached-about 400 C. Various destroyers have shown different destruction efficiency for OHC.

   An increase in the amount of destroyer (when only inorganic bases were used) increases its efficiency for destruction of OHC compounds. However, the mixture of inorganic bases and AHC at 2.5% (by Wt) of flyash shows optimum destruction efficiency for OHC. The destruction efficiency does not improve significantly by increasing the amount of inhibitor mixture. The technique described here is useful for the destruction of OHC, including dioxins and furans, at the source, i.e. in the post combustion zone of MSWI, using destroyers in the postcombustion zone. 



   Accordingly, in one aspect of the present invention, there is provided a method for the incineration of waste materials wherein the formation of organic pollutants, especially organohalogen compounds including dioxins and furans, is minimized and the organohalogen compounds formed are removed by introduction of at least one destroyer compound in a postcombustion zone of an incinerator, which comprises incinerating municipal solid waste, medical waste, municipal sludge, industrial waste 

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 and other materials, such as wood and other cellulosic material to form flyash and gaseous products of incineration, comprising organohalogen compounds and acid gases; passing the gaseous products of incineration to a precipitation step, wherein the flyash is precipitated from the gaseous products of the incineration;

   and introducing at least one destroyer compound during the passage of the gaseous products of incineration to the precipitation step wherein at least one destroyer compound reacts with active sites on the flyash to prevent formation of organohalogen compounds from precursors in the gaseous products, to destroy organohalogen compounds on the flyash or present in the gaseous products prior to destroyer compound introduction, and to react with acid gases in the gaseous products, thereby decreasing the concentration of organohalogen compounds, including dioxins and furans, on flyash precipitated in the precipitation step and decreasing organohalogen, including PCDD and PCDF and acid gas concentrations in stack emissions vented from the precipitation step. 



   In another aspect of this invention, any material, such as flyash, soil, sediment or sludge, containing high levels of OHC can be detoxicated by mixing the material with a suitable destroyer and heating the resulting mixture at about 300  to about 900 C or, preferably, by incinerating such material in the existing incinerators along with other waste materials, and using destroyers in a postcombustion zone as described above. Accordingly, materials containing high levels of organohalogen compounds, including dioxins and furans, including soil, sediment, sludge or flyash formed in an incineration operation, may be mixed with the waste materials to be incinerated according to the procedure provided herein to effect destruction of the OHC in such materials. 

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   In another aspect of the invention, certain materials are used to effect inhibition/destruction of PCDD and PCDF which are mixtures of certain alkali metal hydroxides and certain aliphatic hydroxy or polyhydroxy compounds. Such alkali metal hydroxide compounds may comprise sodium hydroxide or potassium hydroxide, particularly potassium hydroxide, while such aliphatic hydroxy or polyhydroxy compounds may comprise ethylene glycol or polytetrahydrofuran. A particularly preferred mixture of materials is a mixture of potassium hydroxide, ethylene glycol and silica gel. 



   Such materials may be effectively employed by direct injection as an aqueous solution into a post-combustion zone of an incinerator at a temperature of about 300  to about 500 C, preferably about 350  -to about 450 C, more particularly around 400 C. 



   Accordingly, in this aspect of the invention, there is provided a method of decreasing the concentration of organohalogen compounds in flyash and vent gas stream formed in the incineration of combustible materials, which comprises effecting combustion of combustible materials in a combustion zone to form gaseous combustion products containing entrained flyash; conveying the gaseous combustion products from the combustion zone to a precipitation zone; precipitating flyash from the gaseous combustion products in the precipitation zone; venting the gaseous combustion products from the precipitation zone to provide the vent gas stream;

   and introducing to the gaseous combustion products during the conveying step an aqueous solution of at least one mixture of alkali metal hydroxide and aliphatic hydroxy or polyhydroxy compounds at a temperature of the gaseous combustion products of about 300  to about 500 C. 



   The introduction of such organic-inorganic complexes to the combustion gas stream also may be combined with introduction of an alkali metal hydroxide, in the form of 

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 an aqueous solution thereof, particularly sodium hydroxide or potassium hydroxide, directly into the combustion gas stream, at a higher temperature above about 500 C, which may include multiple introductions at different higher temperature levels of approximately 500 C, 600 C and 700 C. 



   BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a schematic diagram of the experimental set-up used for the studies of the destruction, the inhibition of formation and catalytic formation of OHC on flyash;
Figure 2 is a graphical representation of the native PCDD and PCDF present on MSWI flyash prior to (no heat) and after heat treatment of the flyash at various temperatures;
Figure 3 contains a graphical representation of the amounts of native PCDD (A) and PCDF (B) on MSWI flyash heated without and with typical destroyers at various temperatures;
Figure 4 contains chromatograms showing the response of electron captor detector for the OHC.

   Samples analyzed were 1) MSWI flyash, no treatment, 2) MSWI flyash heated to 400 C without destroyer, 3) MSWI flyash heated at 400 C with destroyer (NaOH 5% by wt.);
Figure 5 is a graphical representation of the amount of 13C PCDD produced on the flyash prior to (BG) and after destroyer treatments of the flyash at 300 C;
Figure 6 is a graphical representation of the effect of temperature on formation of dioxins from 13C pentachlorophenol precursor on flyash coated by different destroyers (2%) at various temperatures; and
Figure 7 contains graphical representation of the % decomposition of native dioxins and furans at various temperatures using various destroyers. 

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   GENERAL DESCRIPTION OF INVENTION
In the present invention, inorganic bases, either alone or in admixture with aliphatic hydroxy or polyhydroxy compounds (here termed destroyers) are employed and their effect on organohalogen compounds (OHC), including dioxins and furans, on flyash was investigated. It has been found that the destroyer compounds , when coated on the flyash, effectively destroy OHC on the flyash at temperatures above about 300 C. The destroyer compounds may be selected from inorganic bases which are hydroxides, oxides, carbonates, hydrogen carbonates and silicates of one or more alkali metals or alkaline earth metals.

   The destroyer compounds that have been found to be particularly effective are sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide   (Ca(OH)2),   calcium oxide   (CaO),   magnesium oxide   (MgO),   sodium carbonate   (Na2C03),   potassium carbonate   (KCO),   calcium carbonate   (CaC03) ,   sodium orthosilicate   (NaSiO),   sodium metasilicate   (Na2Si03)   and either used alone or in admixtures with aliphatic hydroxy or polyhydroxy compounds (AHC). Such aliphatic hydroxy or polyhydroxy compounds may include ethylene glycol, 1,2propanediol, 1,4-butanediol, polytetrahydrofuran or mixtures of these compounds.

   These destroyer compounds can be used to effectively destroy OHC on the flyash and in the flue gases of solid waste incineration by introduction to the combustion gas stream at suitable temperature. In the laboratory tests, it has been found that the flyash coated with NaOH and KOH (2 to 4%) destroys the formation of PCDD up to   99%   compared to the formation of PCDD on untreated flyash. A similar effect was observed using an even smaller percentage of mixtures of NaOH or KOH in conjunction with aliphatic hydroxy compounds. 



   Several experiments were conducted using the laboratory experimental set-up shown in Figure 1. A MSWI 

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 flyash (with native dioxins and furans produced in the incineration process on it) was heated at temperatures 200  to 500 C in air under atmospheric pressure. It was found that the amount of native dioxins and furans increased up to 350 C, probably due to presence of precursors on the flyash, which by catalytic activity of the flyash, were converted into PCDD/PCDF. Above 350 C, the amount of PCDD/PCDF on the flyash decreased, probably due to a decreasing rate of formation against the rate of decomposition of PCDD/PCDF. The flyash heated at 400 C to 500 C showed very little amount of both PCDD/PCDF. 



  After the heat treatment at various temperatures, flyash samples were dried by heating at 150 C to remove toluene used for extraction. A catalytic activity test of the flyash samples next was conducted at 300 C. The procedures for catalytic activity tests were similar to that reported in our previous U.S. Patents Nos. 4,793,270 and   5,113,772.   It was found that flyash heated up to 500 C retained its catalytic activity to produce PCDD from precursors, such as PCP, at lower temperatures such as 250  to 400 C. 



   In another set of laboratory experiments, heat treatment of flyash coated by 2 to 5% destroyer compounds shows that destruction of PCDD/PCDF started at 300 C and was completed by the time the temperature reached 400 C. 



  Hence, the addition of destroyers makes the flyash complex destructive for organic compounds, even at 300 C, otherwise flyash promotes the formation of dioxins and furans (see Figures 2 and 3). The effectiveness of destruction depends upon the amount of destroyer used and the temperature employed for the destruction. About 6   wt%   destroyer at 300 C has the same effect as about 1 wt% destroyer at 400 C. This result indicates that very little destroyer is required to achieve complete destruction of the OHC, if destroyer can be introduced at temperatures between 400  and 900 C, or up to   1100 C,   in 

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 the postcombustion zone of the incinerator, depending upon the type of the destroyer used. Destruction efficiency also depends upon the melting point of the destroyer used.

   NaOH and KOH are highly effective when used above about 390 C,   Na2C03   above about 800 C, and sodium meta- and ortho-silicates above about 900 C. 



   In general, destroyer compounds are used in any amount within a range of from about 0.5 to about 6 wt% of the flyash produced in the incineration step. This quantity generally corresponds to about 0.025 to about 0.3 wt% of-the waste material incinerated. 



   The destroyer may be introduced to the postcombustion zone of the incinerator in any convenient form, for example, granular form, powder form or, preferably, as an aqueous solution. 



   Depending on the destroyer compound or compounds used, the destroyer compound may be introduced between the combustion chamber and precipitation zone of the incinerator at a suitable temperature of from about 300  to about 1100 C, preferably from about 300  to about 800 C and particularly from about 300  to about 600 C. 



   The destroyer effect achieved herein also may be combined with the inhibition of formation of OHCs by flyash catalysis, as described in USPs 4,793,270 and 5,113,772, by introduction of a suitable inhibitor material as described therein, to the incinerator gas stream between the combustion zone and the precipitation zone, generally at a temperature of from about 300  to about 500 C. The present invention employs, in one embodiment, mixtures of materials as inhibitor/destroyers not contemplated in our earlier patents. 



   As will be apparent from the disclosure of USP 5,113,772, some of the compounds which were used as inhibitors of OHC formation in that patent are used herein as destroyers of OHC. It is preferred herein to employ the same compounds for destruction and inhibition. 

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  In general, higher temperatures are required to effect destruction of OHC with such compounds than inhibition of OHC formation. The destroyer and inhibitor compounds, therefore, may be added at different locations, and accordingly, different temperatures, between the incinerator and the precipitator. The present invention employs, in one embodiment, mixtures of materials as inhibitor/destroyers not contemplated in our earlier patents. 



   Several mixtures of the above described inorganic bases and-their combination with AHCs were tested in laboratory experiments. A coating of 5 wt% NaOH on flyash completely destroyed all the OHC, including PCDD/PCDF, on the flyash at 400 C. In addition, the flyash was permanently deactivated-for the formation of PCDD/PCDF. Deactivation of flyash is very important because it provides the means to introduce destroyers at any temperatures above 400 C for higher efficiency with a lower amount of destroyer. In this way, destroyers effectively destroy an estimated 10 to 15% OHC compounds produced in the furnace when introduced at about 400  to   1100 C   in the post combustion zone of the incinerator. 



  The procedure also deactivates the flyash to prevent further formation of PCDD/PCDF (about 85 to 90%) by catalytic activity of the flyash in the cooler parts of the incinerator. 



   Acid gases which may be present in the incinerator gas stream, such as HCI, SO2 and NO2, also may react with the alkaline destroyer compounds, so that such acid gases are removed from the gas stream, whereby their concentration in the stack emissions decreases. 



   The dioxins and the furans exhibit varying degrees of toxicity, depending on the number of chlorine atoms present and the position of the chlorine substitution. 



  Usually toxicity of a particular sample is estimated based on amount of tetra- to octa- chlorinated dioxins 

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 and furans present. The formation of PCDD and PCDF for precursor compounds, such as chlorobenzenes, chlorophenols and chlorodiphenyl ethers on flyash is illustrated by the following equation: 
 EMI13.1 
 r I, 9----o -Q -4 GD jf ' rt " 4 Cl. 



  30 1 (M. CL - ci,, Cl. P'FLY ASH --4 PCDD m=1-6 0..- , O , m = 1 - 6 n = 1 - Jkk PCDD CHLOROBENZENES CHLOROPHENOLS io = CELOROBENZENES CELDROPEENOLS (:4 PCDD 1-8 
EXAMPLES Example 1:
This Example shows the effect of heat on the amount of native dioxins and furans present on MSWI flyash. 



   An experimental apparatus shown in the Figure 1 was used to test the effect of heat treatment and catalytic activity of flyash samples. The apparatus comprised a glass tube (25 X 1 cm I. D.) passing through the oven. 



  Part of the flow tube was a reservoir where flyash to be tested was packed. A flyash sample from MSWI with all compounds produced in the process of the incineration on it was heated at various temperatures to examine the effect of heat. In each experiment, 1.5 g of the flyash was placed in the glass tube. The section of the tube containing flyash was heated at various temperatures for 60 minutes using 3 ml/minute flow of air through the tube. After completing the experiment, the flyash was spiked with an internal standard for recovery estimates. 



  Organic compounds present on the flyash were extracted by eluting with 220 ml toluene. Extracts were concentrated by rotary evaporation to a few ml and final concentration in a sample vial to 500  l under a gentle stream of N2. 

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   Figure 2 shows a graph of the amount of native dioxins and furans present on MSWI flyash prior to (no heat) and after heat treatment of the flyash at various temperatures. The results of these experiments reveal that the flyash produced in the incineration process contains high levels of dioxins/furans (no heat plot) and precursors. The MSWI flyash is highly catalytically active, which produces more dioxins/furans, probably from native precursors present on.the flyash, when heated from 200  to 400 C. 



   After the heat treatment at various temperatures, flyash samples were dried by heating at 150 C to remove toluene used for extraction. Then a catalytic activity test of these flyash samples was conducted at 300 C. In the catalytic activity test, a volume of 50  l of 5  g/ l 13C-PCP solution in methanol was deposited on the glass beads on top of the flyash and the solvent was allowed to evaporate. The section of the tube containing flyash and 13C-PCP was heated at 300 C for 60 minutes using 3 ml/minute flow of air. After completing the experiment, the flyash was spiked with an internal standard for recovery estimates and extracted as mentioned above. 



  Analysis of extracts shows the formation of 13C PCDD. 



  This result indicates that merely heating the flyash does not affect its catalytic activity. 



  Example 2:
This Example shows the effect of 5% destroyers on native PCDD/PCDF on the flyash. 



   In these experiments, fresh flyash (10 g) containing native PCDD/PCDF was coated with a 5% destroyer solution in water. A 1.5 g portion of destroyer coated and dried flyash was used in the heat treatment described in Example 1. The amount of PCDD/PCDF detected in flyash coated by typical destroyers, namely NaOH and PSEGC (a polysilicate-ethylene glycol complex formed from KOH:ethylene glycol:silicagel in respective weight ratios 

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 17: 60:23) and heated at various temperatures is shown in Figure 3.

   Comparing the amounts of PCDD/PCDF on flyash heated at various temperatures without (Figure 2) and with NaOH, PSEGC and SMSEG (a sodium metasilicateethylene glycol complex formed from sodium metasilicate: ethylene glycol in a weight ratio of 15:85) destroyers (Figure 3), it can be seen that no PCDD/PCDF was produced between 300 C and 500 C on destroyer-coated flyash. In fact, the native PCDD/PCDF content of destroyer-coated flyash was reduced at 300 C and completely eliminated at 400 C. The experiment at 500 C also did not reveal any residual native PCDD/PCDF. 



  Similar results were obtained for KPTHF, a mixture of KOH: PTHF (KOH: polytetrahydrofuran:: 25: 75) and other destroyers with vary little variation in the destruction efficiency. 



   Figure 4 shows the plot of gas chromatography/ electron capture detector response for various chlorinated compounds on flyash from MSWI, the flyash heated at 400 C, and the flyash coated by 5 wt% NaOH and heated at 400 C. It is clear from Figure 4 that a large number of organohalogen compounds, including PCDD/PCDF, are present on flyash from MSWI (bottom tracing) . Merely heating the flyash at 400 C does not destroy the OHC on the flyash (middle tracing) . However, 5 wt% destroyer on the flyash effectively destroys all the OHC, including PCDD/PCDF, on the flyash (top tracing). 



   After the heat treatment at various temperatures of the flyash samples coated by various destroyers, they were extracted using toluene, after that they were dried by heating at 150 C to remove toluene used for extraction. A catalytic activity test of these flyash samples then was conducted using 13C PCP at 300 C, as described in the Example 1. Only traces of 13C labelled PCDD were produced on these flyash samples from 13C-PCP. 

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  This result indicates that the catalytically active sites on the flyash were removed by the destroyer. 



  Example 3:
This Example shows the effect of amount and type of destroyer used for coating the flyash in the formation of 13C PCDD from 13C PCP. 



   In these experiments, nine flyash samples (1.5 g each) were coated by three different destroyers, namely   Na2C03,   KOH and NaOH (1 wt%, 2 wt% and 6 wt%) separately. 



  Catalytic activity tests were conducted on each coated sample at 300 C, as described in Example 1. The analyses of the flyash extracts are shown in Figure 5. 



  Background(BG) represents the amount of 13C PCDD produced from 13C PCP on the flyash with no destroyer at 300 C. 



  From the plots, it can be seen that, as the amount of destroyer (wt%) increases, the formation of PCDD decreases. The effect is consistent for all three destroyers used. There was a slight difference in activity of the destroyers to prevent the formation of PCDD. 



  Example 4:
This Example shows the effect of temperature on the formation of 13C PCDD from 13C PCP on the flyash coated by various destroyers. 



   In these experiments, three different destroyers, namely   NaCO,   KOH and NaOH were coated (2 wt%) separately on flyash samples. Catalytic activity tests were conducted on each coated sample at 300 , 350  and 400 C, as described in Example 1. The analyses of the flyash extracts are shown in Figure 6. Background(BG) represents the amount of 13C PCDD produced from 13C PCP on the flyash with no destroyer at 300 C. From the plots, it can be seen that the effectiveness of the destroyer increased with the temperature increases. 



   Figure 7 shows the percentage reduction or destruction of native dioxins and furans using various 

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 inhibitor/destroyers. The inhibitor/destroyers used were in these experiments, namely: i) KPTHF, a mixture of KOH:polytetrahydrofuran (25:75), 5% (by wt) ii) SMSEG, sodium metasilicate: ethylene glycol (15: 85,   5%   (by wt) iii) NaOH, 5% (by wt) iv) PSEGC, potassium hydroxide:ethylene glycol: silica gel:::17:60:23, 5% (by wt)
The results seen in Figure 7 are compared with those found earl-ier. 



   From the results of Examples 3 and 4, it can be seen that the catalytic activity of flyash for the formation of PCDD from precursors was effectively reduced, either by using smaller amounts of destroyers at higher temperatures or by using larger amounts of destroyers at lower temperatures. 



  Example 5:
This Example illustrates a commercial scale plant test. 



   A waste incinerator plant had three independent incineration lines comprising an incineration grate for municipal waste, a waste to heat boiler (100 ton per day) and a multi-stage flue gas purification plant. From a refuse bunker, the waste is supplied to the incineration grate where it is burned at 1000 C. Flue gases flow through the boiler at a velocity of 6 to 12 M/second. 



   The inhibitor/destroyer mixtures were injected into the waste flue gas stream. KOH or NaOH as the destroyers were injected in a temperature window of 590  + 50 C and PSEGC or SMSGC (see Example 4) as the inhibitor/destroyers were injected in a temperature window of 375  + 50 C. The amount of each inhibitor and destroyer injection was 7 to 10% by weight of the flyash produced. An injection nozzle unit was employed containing a pressure atomizer with a mixing chamber in 

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 which the destroyer or inhibitor was mixed with 13 times the amount of water by volume. The nozzles had a outside diameter of 12 mm and an opening of 1.6 mm and were operated with water at an injection pressure of 6 to 8 bar to distribute the destroyer/inhibitor uniformly throughout the flue gas. 



   Plant test I was conducted using combination I consisting of KOH as destroyer and PSEGC as inhibitor/destroyer, plant test II was conducted using combination II consisting of NaOH as destroyer and SMSGC as inhibitor/destroyer and plant test III was conducted using combination I and activated carbon. On the fourth day of each test, the flue gas samples were collected for PCDD/PCDF measurements. Several flyash samples were collected from the electrostatic precipitator on the third and fourth day of each test. Sampling and analysis of the flue gas for PCDD/PCDF concentration were done with standard techniques. 



   The amount of PCDD/PCDF detected on the flyash samples during the three tests is shown in the following Table I: 

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   TABLE I: AMOUNT OF PCDD/PCDF (ng/g) ON THE FLYASH SAMPLES COLLECTED PRIOR TO AND DURING TESTS I-III IN THE SWISS INCINERATOR.   
 EMI19.1 
 
<tb> 



  Test <SEP> No. <SEP> Sample <SEP> No. <SEP> PCDD <SEP> PCDF
<tb> ng/g <SEP> % <SEP> Reduction <SEP> ng/g <SEP> % <SEP> Reduction <SEP> 
<tb> Background <SEP> 1 <SEP> 150- <SEP> 85sample
<tb> Test <SEP> 1 <SEP> 1 <SEP> 27 <SEP> 82 <SEP> 17 <SEP> 80
<tb> 2 <SEP> 25 <SEP> 84 <SEP> 12 <SEP> 86
<tb> Test <SEP> 2 <SEP> 1 <SEP> 27 <SEP> 82 <SEP> 15 <SEP> 82
<tb> t <SEP> 2 <SEP> 13 <SEP> 91 <SEP> 5 <SEP> 94
<tb> Test <SEP> 3 <SEP> 1 <SEP> n. <SEP> d. <SEP> > 99 <SEP> 15 <SEP> 82
<tb> 2 <SEP> 6 <SEP> 96 <SEP> 12 <SEP> 86
<tb> 
   n.d.

   = not detected   

 <Desc/Clms Page number 20> 

 As may be seen from these results, with all other conditions remaining the same, injecting the destroyers and inhibitors into the post combustion zone at temperatures of 550 C and 350 C respectively, caused a marked reduction in the amount of PCDD/PCDF on the flyash samples from the electrostatic precipitator. A comparison between the individual tests shows that the best reduction of PCDD/PCDF of 99% was achieved in Test III. An overall reduction in PCDF of about 80 to 94% was achieved in Tests I to III. 



   The amount of total PCDD/PCDF detected in flue gases in background samples and samples collected during Tests I to III are shown in the following Table II: 

 <Desc/Clms Page number 21> 

   TABLE II: AMOUNT OF PCDD/PCDF (ng/m3) USEPA FOR RAW AND PURE GAS PRIOR TO AND AFTER INHIBITOR INJECTIONS DURING TESTS I-III AT SWISS INCINERATOR.   
 EMI21.1 
 
<tb> 



  Sample <SEP> No. <SEP> PCDD <SEP> PCDF <SEP> TOTAL
<tb> (PCDD+PCDF)
<tb> Flue <SEP> Gas <SEP> Samples <SEP> prior <SEP> to <SEP> (Background) <SEP> ng/m3 <SEP> ng/m3 <SEP> (ng/m3)
<tb> and <SEP> During <SEP> inhibition <SEP> Tests <SEP> I, <SEP> II <SEP> and <SEP> III
<tb> Background <SEP> for <SEP> Test <SEP> I <SEP> Raw <SEP> Gas <SEP> 23.62 <SEP> 26.07 <SEP> 49.67
<tb> Background <SEP> for <SEP> Test <SEP> II <SEP> Raw <SEP> Gas <SEP> 52.42 <SEP> 48.36 <SEP> 100.78
<tb> Background <SEP> for <SEP> Test <SEP> III <SEP> Raw <SEP> Gas <SEP> 26.26 <SEP> 34.17 <SEP> 60.42
<tb> Background <SEP> for <SEP> Test <SEP> I <SEP> Pure <SEP> Gas <SEP> 29.60 <SEP> 24.27 <SEP> 53.87
<tb> Background <SEP> for <SEP> Test <SEP> II <SEP> Pure <SEP> Gas <SEP> 42.29 <SEP> 40.59 <SEP> 82.88
<tb> Background <SEP> for <SEP> Test <SEP> III <SEP> Pure <SEP> Gas <SEP> 38.10 <SEP> 40.86 <SEP> 78.95
<tb> 

  Inhibition <SEP> Test <SEP> I <SEP> Raw <SEP> Gas <SEP> 46.70 <SEP> 33.46 <SEP> 80.60
<tb> Inhibition <SEP> Test <SEP> II <SEP> Raw <SEP> Gas <SEP> 88.05 <SEP> 159.76 <SEP> 247.81
<tb> Inhibition <SEP> Test <SEP> III <SEP> Raw <SEP> Gas <SEP> 6.80 <SEP> 7.15 <SEP> 23.95
<tb> Inhibition <SEP> Test <SEP> I <SEP> Pure <SEP> Gas <SEP> 19.87 <SEP> 19.43 <SEP> 39.30
<tb> Inhibition <SEP> Test <SEP> II <SEP> Pure <SEP> Gas <SEP> 26.38 <SEP> 6.49 <SEP> 32.87
<tb> Inhibition <SEP> Test <SEP> III <SEP> Pure <SEP> Gas <SEP> 5.67 <SEP> 5.54 <SEP> 11.21
<tb> 
 

 <Desc/Clms Page number 22> 

 As may be seen from the results of Table II, the PCDD/PCDF concentration related to the toxic equivalent (TE) of USEPA was reduced in Tests I to III up to 91% in raw gas and up to 95% in pure gas. 



   The results contained in this Example demonstrate that the formation of PCDD/PCDF during the incineration of waste can be essentially suppressed both the flyash from the electrostatic precipitator and in the flue gas by injection of destroyers and inhibitors into the flue gas from the incinerators. 



     SUMMARY   OF DISCLOSURE
In the summary of this disclosure, the present invention provides a method for destruction of organohalogen compounds (OHC), including dioxins and furans, and for the prevention of -formation of the OHC and the acid gases in the combustion gas stream from waste incinerators by employing certain inorganic bases, alone or their combination with aliphatic hydroxy compounds (collectively called herein destroyers).

   The destroyers, such as alkali metal/alkaline earth metal oxides, hydroxides, carbonates, bicarbonates and silicates and mixtures of alkali metal/alkaline earth metals oxides, hydroxides, carbonates, bicarbonates, silicates and their mixtures with aliphatic hydroxy and/or polyhydroxy compounds, proved to be highly effective in the destruction and the prevention of formation of the OHC, including toxic dioxins and furans, in the post combustion zone of incinerators. 



  Modifications are possible within the scope of this invention.

Claims

CLAIMS What we claim is: 1. A method for incinerating of waste materials wherein organic pollutants formation, especially organohalogen compounds including dioxins and furans, is prevented and organohalogen compounds formed are destroyed by introduction of at least one destroyer compound in a postcombustion zone of an incinerator, which comprises: incinerating waste materials selected from municipal solid waste, medical waste, municipal sludge, industrial waste and-other materials, to form flyash and gaseous products of incineration, comprising organohalogen compounds and acid gases; passing said gaseous products of incineration to a precipitation step, wherein said flyash in precipitated from the gaseous products of the incineration;

and introducing at least one destroyer compound to said gaseous products of incineration during said passage of said gaseous products of incineration to said precipitation step wherein at least one destroyer compound reacts with active sites on the flyash to prevent formation of organohalogen compounds from precursors in said gaseous products, to destroy organohalogen compounds on the flyash or present in the gaseous products prior to destroyer compound introduction, and to react with acid gases in said gaseous products, thereby decreasing the concentration of organohalogen compounds, including dioxins and furans, on flyash precipitated in said precipitation step and decreasing organohalogen and acid gas concentrations in stack emissions from the precipitation step.

2. The method of claim 1 wherein said at least one destroyer compound is introduced to said gaseous products of incineration by introducing destroyer compounds directly into a postcombustion zone of the waste incinerator. <Desc/Clms Page number 24>

3. The method of claim 1 wherein acid gases and gaseous organohalogen compounds including dioxins and furans, react with the destroyer compounds, whereby their concentration in the stack emissions decreases.

4. The method of claim 3 wherein the organic compounds, including dioxins, are destroyed on at least one of flyash produced during incineration or flyash introduced from another incinerator.

5. The method of any one of claims 1 to 4 wherein waste materials containing high levels of organohalogen compounds,-including dioxins and furans, including soil, sediments, sludge or flyash from other incinerations are added to and mixed with the waste materials prior to incineration thereof.

6. The method of any one of claims 1 to 5 wherein the at least one destroyer compound is at least one hydroxide, oxide, carbonate, hydrogen carbonate or silicates of an alkali metal or alkaline earth metal.

7. The method of claim 6 wherein said at least one destroyer compound is selected from NaOH, KOH, Ca(OH), MgO, CaO, Na2CO3,K2CO3, CaC03, NaHC03, KHC03 , Ca (HCO3)2, NaSIO and Na4Si04.

8. The method of any one of claims 1 to 7 wherein the destroyer compound is a mixture of one or more of the destroyer compounds identified in claim 6 or 7 with at least one aliphatic hydroxy or polyhydroxy compound.

9. The method of claim 8 wherein said aliphatic hydroxy or polyhydroxy compound is selected from ethylene glycol, 1,2-propanediol, 1,4-butanediol, polytetrahydrofuran and mixtures of these compounds.

10. The method of claim 9 wherein the destroyer compound is a mixture of potassium hydroxide, ethylene glycol and silica gel.

11. The method of any one of claims 1 to 10 wherein destroyer compounds are introduced directly into the <Desc/Clms Page number 25> gaseous products of incineration at a temperature of about 1100 to about 300 C.

12. The method of claim 11 wherein said temperature is about 800 to about 300 C, especially about 600 to about 300 C.

13. The method of any one of claims 1 to 12 wherein said at least one destroyer compound is introduced in an amount of from about 0.5 to about 6 wt% of the flyash produced.

14. The method of claim 13 wherein said at least one destroyer -compound is introduced in an amount which corresponds to about 0.025 to about 0.3% of the waste materials incinerated.

15. The method of any one of claims 1 to 14 wherein said at least one destroyer compound is introduced in granular form, in powder form, as an aqueous solution that is sprayed into the postcombustion zone or combinations thereof.

16. The method of any one of claims 1 to 15, wherein said at least one destroyer compound is introduced directly into a post-combustion zone of the waste incinerator (a) in the form of an organic-inorganic complex at a temperature of about 300 to about 500 C, and (b) in the form of an alkali metal hydroxide at a temperature of above about 500 C.

17. The method of claim 16 wherein said organicinorganic complex is a mixture of an alkali metal hydroxide and an aliphatic hydroxy or polyhydroxy compound.

18. The method of claim 17 wherein said alkali metal hydroxide is potassium hydroxide or sodium hydroxide.

19. The method of claim 18 wherein said organicinorganic complex is a mixture of potassium hydroxide with ethylene glycol and silica gel. <Desc/Clms Page number 26> 20. The method of any one of claim 16 to 19 wherein said organic-inorganic complex is introduced at a temperature of about 350 to about 450 C.

21. A method of decreasing the concentration of organohalogen compounds in flyash and vent gas stream formed in the incineration of combustible materials, which comprises: effecting combustion of combustible materials in a combustion zone to form gaseous combustion products containing entrained flyash, conveying said gaseous combustion products from said combustion zone to a precipitation zone, precipitating flyash from said gaseous combustion products in said precipitation zone, venting said gaseous combustion products from said precipitation zone to provide said vent gas stream, and introducing to said gaseous combustion products during said conveying step an aqueous solution of at least one mixture of alkali metal hydroxide and aliphatic hydroxy or polyhydroxy compounds at a temperature of said gaseous combustion products of about 300 to about 500 C.

22. The method of claim 21 wherein said aqueous solution is introduced at a gaseous combustion products temperature of said gaseous combustion products is from about 350 to about 450 C.

23. The method of claim 22 wherein said alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

24. The method of claim 23 wherein said aliphatic hydroxy or polyhydroxy compound is ethylene glycol or polytetrahydrofuran.

25. The method of claim 21 wherein said aqueous solution is an aqueous solution of a mixture of potassium hydroxide, ethylene glycol and silica gel.

26. The method of claim 22 including introducing to said gaseous combustion products during said conveying step an aqueous solution of an alkali metal hydroxide at a <Desc/Clms Page number 27> temperature of said gaseous combustion product of at least about 500 C.

27. The method of claim 26 wherein said introduction of said gaseous solution of alkali metal hydroxide is effected at multiple different temperatures of said gaseous combustion products above about 500 C.

28. The method of claim 26 wherein said alkali metal hydroxide is sodium hydroxide or potassium hydroxide.