WO1998009716A1

WO1998009716A1 - Process for reducing dioxin and furan emissions in the stack gas from an incinerator

Info

Publication number: WO1998009716A1
Application number: PCT/US1997/015522
Authority: WO
Inventors: Lester L. Lamparski; Terry J. Nestrick; Edward E. Timm; Thomas D. Niles
Original assignee: The Dow Chemical Company
Priority date: 1996-09-06
Filing date: 1997-09-04
Publication date: 1998-03-12
Also published as: AU4179197A

Abstract

A process for reducing emissions of dioxins and furans in a stack gas from the incineration of organic materials, comprising: incinerating the organic material; adding one or more dioxine-reactive materials which are deposited on the flyash or on a supplemental particulate support optionally added upstream of an apparatus for removing flyash from the gaseous products of incineration prior to their emission in a stack gas, and which under the conditions prevailing in the apparatus will react with the dioxins in and from said gaseous products from said incineration step to produce one or more corresponding dioxin-derivative materials which are preferably substantially retained on the flyash or supplemental particulate support or both through the incinerator and which are periodically or continuously removed with said flyash and supplemental particulate support from the flyash/particulate removal apparatus, or in the alternative, adding the one or more dioxin-reactive materials via the addition of a particulate material including the one or more dioxin-reactive materials therein or thereon upstream of the flyash removal apparatus; and, maintaining the one or more dioxin-reactive materials in contact with the dioxins in sufficient concentrations and for a sufficient time to reduce the concentration of dioxins in and from said gaseous products through the formation of said dioxin-derivative materials.

Description

PROCESS FOR REDUCING DIOXIN AND FURAN EMISSIONS IN THE STACK

GAS FROM AN INCINERATOR

The present invention relates in a broad sense to processes which are capable of producing dioxins and furans and especially to those processes which additionally implicate some manner of dust control or particulate removal, and most especially is directed to reducing the level of dioxins and furans emitted to the environment from such processes. More particularly, the present invention relates to the incineration of waste materials as in a municipal solid waste incinerator, and especially to processes for reducing the levels of dioxins and furans produced on the flyash and in the stack emissions from such incineration.

Incineration, as an alternative to burying in a landfill for the disposal of urban garbage, is presently practiced throughout the world and results in a considerable decrease in the waste volume and the recovery of energy in the form of steam or electricity. Incineration, however, has come under increasing regulatory pressure due to the generation therein of a number of suspected toxic compounds, including polychlorinated dibenzo-p-dioxins (collectively commonly termed "dioxins") and polychlorinated dibenzofurans (collectively commonly termed "furans").

A large city may incinerate anywhere from 3-5 millions tons of garbage annually. For every million tons of urban waste incinerated, approximately 34 thousand tons of flyash are produced by the typical incinerator. Between 95 and 99 percent of the flyash is precipitated electrostatically and buried in landfills. The remainder is emitted from the incinerator stacks along with various gaseous byproducts, namely water vapor, HCI, S02, C02, air and volatilized organic compounds. The gaseous stack emissions introduce dioxins, furans and other highly-regulated chlorinated compounds to the atmosphere. Conventionally, these materials are present in these stack emissions in part per million concentrations, whereas under the regulations already in effect in some countries, dioxin and furan emissions are to be limited to much lower levels. There is, consequently, a growing need for a means to effectively and economically reduce the dioxin and furans content of both the solid and gaseous byproducts from municipal solid waste incinerators, with the greater emphasis at present being on the gaseous byproduct emissions.

A number, of technologies have been developed in recent years to reduce dioxin and furan emissions, particularly in the stack gases from an incinerator. United States Patent 5,449,854 is exemplary of one approach that has been taken, in providing a modified incinerator design which employs two burners. The first burner combusts halogenated organic compounds in the presence of oxygen or oxygen-enriched air to produce combustion products which potentially include unreacted oxygen and halogenated organic compounds, which can react with the excess oxygen to produce halogenated dioxins and furans. The formation of the halogenated dioxins and furans is minimized, however, by firing a second burner, burning a hydrogen-containing fuel at a fuel-rich stoichiometry into the combustion products from the first burner, so that the fuel will react with the unreacted oxygen in the combustion products from the first burner. The unreacted oxygen is consequently unavailable for combining with the halogenated organic compounds unconsumed or uncombusted from the first burner, and halogenated furans and dioxins that are formed from the first burner are consumed by the reaction of the hydrogen supplied by the fuel with the halogenated furans and dioxins.

Several post-treatment technologies have been developed which rely on a catalyst of some sort to promote the reaction or decomposition of dioxins and furans in the gases from these incinerators, recently-issued United States Patent No. 5,512,259 being exemplary of this category of processes. Another post-treatment technology which has been developed is described in German Patent Publication DE-A-4,119,006, which describes the use of paraffinic oils to scrub the off gases from an incineration facility, during a washing for the removal of acidic components in the off gases. A common disadvantage of the foregoing post-treatment technologies in the context of municipal solid waste incinerators lies in the expense of modifying the existing incinerator equipment and perhaps adding additional equipment also, not to mention the catalyst expense associated with the catalytic post-treatment routes.

A somewhat different approach has been taken in United States Patents No. 4,793,270 and 5,113,772 to Karasek et al., in which the purported catalytic effects of flyash on dioxin and furan formation in the combustion of plastics, paper and chemicals, and from several other dioxin precursors in the gaseous combustion products from a municipal solid waste incinerator, are inhibited through the injection of a catalyst inhibitor or inhibitor mixture between the furnace section of the incinerator and an electrostatic precipitator which removes flyash from the gaseous combustion products. According to these patents, the inhibitor mixtures react with catalytic sites on the flyash to inhibit the formation of dioxins and furans from the dioxin and furan precursors in the gaseous combustion products, with a reduction of dioxins in the flyash by 80 percent or more being reported, and a reduction of dioxins in the stack gas emissions by 78 percent or more being reported as well.

Still another approach has been taken in United States Patent No. 5,514,356 to Lerner, involving the addition of inert sorbents to the exhaust gas from the incineration of municipal and biomedical wastes, at temperatures which are compatible with preserving the sorbent's structural integrity (that. is, below 450 degrees Celsius) but which are above the temperatures believed to be associated with the onset of dioxin and furan formation and which are favorable for adsorption of dioxin and furan precursors thereon (that is, 290 degrees Celsius). The substantially dry, finely divided sorbents are selected to contain little or none of the chloride-reactable transition metal compounds present in the flyash from the incinerator and which have been conventionally viewed as catalyzing the formation of the dioxins and furans from their precursors, and are taught as preferentially adsorbing the dioxin and furan precursors thereon to the flyash.

The sorbents are described as being macroporous in nature, and are added in amounts which vary (as a qualitative rule) depending on the surface area of the sorbent, and to some extent on the adsorption equilibria of the precursors on the sorbent versus on the flyash at a given injection temperature. If the surface area of the sorbent is roughly the same as that of the combustion flyash, then a sorbent addition rate is suggested that is from 2 to 15 times the rate of flyash formation. For a sorbent which has 10 or more times the surface area of the combustion flyash, a sorbent rate of addition of from 1 to 10 times the flyash rate, and preferably of from 1 to 5 times the flyash rate, is suggested.

In contrast to the findings reported in United States Patents Number 4,793,270 and 5,113,772 mentioned above, it has now been discovered that injection of the alkanolamines and alkanolamine mixtures described particularly in United States Patent No. 5,113,772 (hereafter, the "772 patent) do not effect more than an insubstantial decrease in the dioxin and furan concentrations in the stack gas emitted from the electrostatic precipitator-equipped incinerators contemplated in the 772 patent. Applicants in this regard have determined that, rather than reacting with catalytic sites on the flyash and in so doing, inhibiting the flyash-catalyzed formation of dioxins and furans from dioxin and furan precursors in the combustion products from the furnace, the amines in question react with the already- formed dioxins, furans and their existing precursors.

Because of the very diffused nature of the dioxins and furans in the gas phase of a stack gas from the incinerator, however, the alkanolamines in the inhibitor mixture are prevented from reacting with the dioxins and furans in the gas phase to an extent whereby the desired reduction is seen in the dioxin and furan levels in the stack gas emitted from the incinerator.

The present invention concerns a process in which the dioxin and furan levels in the stack gas from a municipal waste incinerator can effectively be reduced through using the same aminated materials, for example, as contemplated in the '270 and 772 patents, or through using other materials (that is, other than chlorine or bromine) which will react with the dioxins and furans and the precursors thereof in the stack gas prior to their emission from the incinerator and prior to the conversion of the unconverted precursors to the undesirable dioxins and furans (these reactive materials are collectively referred to hereafter as "dioxin-reactive materials"), and which will in this fashion reduce the overall concentration of dioxins and furans which are ultimately emitted in the stack gas without having the disadvantages of the known art solutions. In order to react the dioxins and furans with an amine or other suitable dioxin-reactive material in this regard, it is necessary that the dioxins and furans which exist in very low concentrations in the stack gas be exposed to the dioxin-reactive mateπal(s) at a high effective concentration This can be achieved by adsorbing, condensing or otherwise causing the dioxin-reactive material to be deposited on a solid material and passing the stack gases through a bed of this solid pnor to their emission from the incinerator, while maintaining the dioxin-reactive mateπal(s) and the dioxins, furans and their precursors in the stack gases in contact for a sufficient time and under suitable conditions (principally, at temperatures of from 175 degrees Celsius to 350 degrees Celsius, as are normally found in the baghouse of a conventional baghouse-equipped municipal solid waste incinerator), for causing the dioxin-reactive matenal and the dioxins, furans and their precursors to react with one another

The product(s), for example, of reacting the amines and the dioxins and furans in this manner can, to the extent these are retained on the solid material, preferably then be substantially removed with the solids from the incinerator Ideally, the particle size of the solid in question is small so that when the solids are packed in a bed the diffusion path from the bulk gas to the surface of the solid is short, and the dioxins and furans also adsorb on the solid so that their effective concentration on the solid surface is significant, whereby a more complete removal can be effected of the dioxins and furans from the gas phase of the stack gas.

The flyash from a conventional municipal solid waste incinerator has conveniently been found to be a suitable solid support for the alkanolamines/dioxin reaction, such that the process of the present invention most preferably comprises injecting, vaporizing and condensing or in some other fashion depositing a dioxin-reactive material such as tnethanolamine onto the flyash from a conventional municipal solid waste incinerator, collecting the tnethanolamme-treated flyash in a fabric filter or baghouse arrangement and causing the tnethanolamine to react with the dioxins, furans and their precursors present in the gas phase of the stack gas (as well as those dioxins and furans adsorbed from the gas phase onto the flyash), then removing the flyash from the fabric filter or baghouse arrangement in a conventional manner

The present invention thus broadly comprises a process for combusting organic matenal which when incinerated forms gaseous products of incineration including flyash, precursors to the formation of dioxins and furans and such dioxins and furans, wherein emissions of dioxins and furans in the stack gas from the incineration of said material are reduced by at least ten percent, and preferably by twenty percent or more, more preferably by fifty percent or more and most preferably by seventy-five percent or more, such process compπsing the steps of a) incinerating the organic matenal to form the gaseous products of incineration, b) adding one or more dioxin-reactive matenals which are adsorbed, condensed or otherwise deposited on the flyash or on a supplemental particulate support optionally and separately added upstream of a filtration apparatus for removing flyash from the gaseous products of incineration prior to their emission in a stack gas, and c) maintaining the one or more dioxin-reactive materials in contact with at least the dioxins and furans in and from said gaseous products from said incineration step for a sufficient time and at a suitable temperature for causing the one or more dioxin- reactive materials to react with the dioxins and furans to produce one or more corresponding dioxin- or furan-derivative materials. These dioxin- or furan-derivative materials are in a preferred embodiment substantially retained on the flyash or supplemental particulate support, and periodically or continuously removed with the flyash and any supplemental particulate support from the filtration apparatus.

In a related embodiment, the process involves adding the one or more dioxin-reactive materials concurrently with the addition of a particulate material including the one or more dioxin- reactive materials upstream of the filtration apparatus. The one or more dioxin-reactive materials are maintained in contact with the dioxins and furans and their precursors in sufficient concentrations and for sufficient time to reduce the concentration of dioxins and furans (and of their unconverted precursors, prior to their conversion to dioxins and furans and prior to their (referring to the dioxins and furans that would otherwise be formed from such precursors) subsequent emission in a stack gas) in and from said gaseous products through the formation of said dioxin- or furan-derivative materials and the removal of these materials by the filtration apparatus, prior to their emission in the stack gas including the gaseous incineration products.

In a further refinement, the supplemental particulate support is suited for adsorbing trace heavy metals in the gaseous incineration products stream, and in still a further refinement, the product of the reaction of the dioxins and furans with the dioxin-reactive materials on the supplemental particulate support can be separated from the support and the trace heavy metals adsorbed thereon by solvent extraction or by some other conventional means.

The process of the present invention is adaptable also to incinerators which employ an electrostatic precipitator rather than a filtration apparatus such as a baghouse filtration apparatus, by employing one or more dioxin-reactive materials which are carried on a suitable particulate support (which may be essentially inert or which may in some degree promote the reaction of the dioxin- reactive materials with the dioxins, furans and their precursors in the stack gas) in a replaceable filter arrangement located downstream from the electrostatic precipitator.

In still another, broader aspect, the dioxin and furan emissions reduction process of the present invention can be similarly employed in the context of any process which generates a gaseous stream containing dioxins or furans or the precursors of such dioxins or furans, irrespective of whether such stream normally contains particulate material that could serve to support the dioxin-reactive material(s) according to the present invention, although preferably these processes will include a filtration apparatus or some other means (such as an electrostatic precipitator) for removing particulate matter from the gaseous stream, or a device which enables or which can be adapted to provide the needed gas-solid contact at relatively small expense.

An example of a well-known, non-incinerative such process is in the pyrometallurgical processing of ores to recover metals therefrom, which in conventional operation generates dioxins and furans and employs a filtration apparatus for particulates removal and control. In this particular context, as will be reiterated and further explained below, the dioxin-reactive materials are preferably adsorbed onto activated carbon or the like or are incorporated therein prior to the activated carbon being added to the process.

Figure 1 is a schematic diagram of a municipal solid waste incinerator (including an electrostatic precipitator for removing flyash from the burner section of the incinerator) employed as described below for attempting to verify the teachings of United States Patent No. 5,113,772 and for demonstrating, with the apparatus of Figure 5, the process of the present invention. Figure 1 further includes an indication of the placement of a replaceable filter arrangement for reducing dioxin and furan emissions in the stack gas from such an incinerator, according to the present invention.

Figure 2 graphically presents the results of injecting tnethanolamine according to the 772 patent in the incinerator of Figure 1 , as described hereafter, on dioxin and furan congener group concentrations in the flyash collected from the electrostatic precipitator of the incinerator shown in Figure 1.

Figure 3 graphically presents the results of the tests giving rise to Figure 2, as related to dioxin and furan congener group concentrations in the stack gas from the incinerator of Figure 1.

Figure 4 is a schematic of a laboratory reaction vessel employed as described hereafter, to demonstrate the reactivity of triethanolamine with dioxins and furans on the flyash from the incinerator of Figure 1.

Figure 5 is a schematic illustration of a bag filtration apparatus and sampling probe employed in the Examples below, for demonstrating the process of the present invention for reducing dioxins and furans in the gas phase of a stack gas from a municipal solid waste incinerator through the use of reagent-treated flyash.

Referring now to the drawings, and more particularly to Figure 1, a municipal solid waste incinerator 10 is schematically illustrated, which conventionally comprises a trash chute 12, a grate and a furnace 14, a boiler 16, an economizer 18, an electrostatic precipitator 20, a fan 22 and a stack 24. For purposes of verifying the teachings of the 772 patent in relation to a claimed reduction in dioxin and furan levels in the gas phase of a stack gas from the stack 24, injection lances 26 were employed for injecting a triethanolamine (TEA) solution (10 wt. percent in deionized water, at a rate equivalent to 5 weight percent on a pure TEA basis of the electrostatic precipitator ash collection rate) into the flue gas duct of the incinerator 10 as indicated in Figure 1 at "A", in the form of a spray of fine droplets that completely covered the area of the duct with good mixing, and that evaporated within a few feet of the injection point at 350 degrees to 370 degrees Celsius.

Samples were collected of the gaseous products of incineration between the economizer 18 and the electrostatic precipitator 20 (at "B" in Figure 1) as well as of the stack gas at the stack 24, by employing an unheated probe assembly as taught in Marklund et al., "A New Method for Sampling Halogenated Dioxins and Furans and Related Compounds in Flue Gases", Second Annual Conference on Municipal Waste Combustion, Conference Proceedings of the Air Waste Management Association, Pittsburgh, Pennsylvania, 1991, pp. 814-818.

The method employed for gathering these samples and for analyzing the same for the dioxin and furan congener groups generally paralleled the procedure taught in U.S. Environmental Protection Agency Method No. 23, as outlined in the United States Code of Federal Regulations, Title 40, Part 60, except that because of space limitations it was not possible for the sample taken between the economizer 18 and the electrostatic precipitator 20 to traverse the sampling probe across the flue gas duct as outlined in Method 23. A pitot tube traversal of the duct was instead conducted to determine an appropriate representative position within the duct from which to take the process sample, and this location was then used. Ash samples were simultaneously collected also from the electrostatic precipitator 20's ash handling system (not shown), and analyzed by the same method for the dioxin and furan congener groups.

The verification trial was conducted over a period of several days, as shown in Figures 2 and 3. As can be seen from Figure 2, the injection of TEA was observed to cause a substantial reduction in ash-borne dioxin and furan congener group concentrations, in keeping with the teachings of the 772 patent. In contrast, as shown in Figure 3, the injection of TEA was not seen to effect more than an insubstantial reduction in the dioxin and furan concentrations in the stack gas, if in fact any true reduction could be observed or substantiated as having occurred at all, given the limits of precision at these concentrations. With reference to the first sample point, at two days following the start of TEA injection, the measured dioxin congener group concentration was 118.6 nanograms per dry standard cubic meter (or DSCM, corrected to 7 percent oxygen at 20 degrees Celsius and 760 mm Hg), as compared with an initial concentration (at normal operating conditions and before the start of TEA injection) of 123.2 nanograms per DSCM - or, an apparent percentage reduction of only 3.7 percent, versus the 78 percent or more reduction reported in col. 4, lines 32-35 of the 772 patent. Furan group concentrations were virtually unchanged, from an initial value of 244.7 nanograms per DSCM to a value after two days of TEA injection of 244.5 nanograms per DSCM, so that the total measured dioxin and furan concentrations declined (or not) over this same period from an initial value of 367.9 nanograms per DSCM to 363.1 nanograms per DSCM - an apparent 1.3 percent overall reduction.

In order to gain a better understanding of the disparities in the present data for the flyash, solid phase of the flue gas and in the gaseous phase in regards to the proposed mechanism of dioxin and furan inhibition per the 772 patent, and in the percentage reduction in stack gas dioxin levels reported by the 772 patent and found herein, an apparatus 28 as depicted in Figure 4 was initially constructed and used to determine the nature of the interaction between the flyash from the electrostatic precipitator 20 and the dioxins and furans thereon on the one hand, and triethanolamine on the other.

The apparatus 28 was constructed starting with a cleaned glass tube 30 having an inside diameter of 4 mm and a length of 150 mm. One end of the tube 30, referred to hereafter as the cold zone 32, was loosely plugged with clean quartz wool 34 to provide a means for containing the dry powdered materials to be added to the tube 30 while permitting gas flow therethrough. Approximately 0.5 grams of cleaned and activated silica gel (designated as "36" in Figure 4) were packed loosely in the tube 30 adjacent the quartz wool plug 34, followed by a second quartz wool plug 38. Then, 0.50 grams of flyash ("40") collected from the electrostatic precipitator 20 during normal (non-TEA) operation of the incinerator 10 were packed loosely in the tube 30 adjacent the second quartz wool plug 38, the ash having previously been sieved through a screen with 149 micrometer openings. A third quartz wool plug 42 was then added. Triethanolamine (0.005 grams) was dissolved in an equal weight of water, and the solution deposited on a thin strip of Whatman GF/C glass fiber filter media (Whatman, Inc., Clifton, NJ), thereby yielding triethanolamine in the equivalent of .0 weight percent of the ash 40 in the tube 30. The triethanolamine-doped filter media 44 was then inserted in the tube in the heated zone 46 as shown in Figure 4.

At time zero, this apparatus 28 was inserted into a furnace comprised of the preheated (to 300 degrees Celsius) injection port assembly of a gas chromatograph, in such a manner that only the heated zone 46 was exposed to the 300 degree Celsius temperatures in the furnace. Immediately after inserting the apparatus 28 into the furnace, a flow of purified air was provided to the apparatus at a flow rate of 4.1 standard cubic centimeters per minute in the direction suggested in Figure 4. After allowing 30 minutes for the triethanolamine to evaporate from the filter media 44 and to react with the dioxins and furans on the ash 40, the apparatus 28 was withdrawn from the furnace. The apparatus 28 was then broken up or disassembled, treated with an aliquot of 7 wt. percent aqueous HCI solution in a beaker to liberate organic materials from the ash, and the entire residual solids content exhaustively extracted with benzene in a Soxhlet-Dean-Stark extractor to extract the dioxins and furans therefrom.

This extract was in turn conventionally processed for subsequent analysis by a High Resolution Gas Chromatograph - Low Resolution Mass Spectrometer (HRGC - LRMS) method, and the dioxin and furan group concentrations found to be 290 nanograms per gram of ash and 158 nanograms per gram of ash, respectively.

The same procedure was then used with an apparatus constructed in the manner of the apparatus 28, but which omitted the triethanolamine-doped filter media 44. The dioxin and furan congener group totals in this case were 1204 nanograms per gram of ash and 441 nanograms per gram of ash, respectively. Finally, a sample of the sieved ash was extracted and analyzed in the same manner, and was determined to contain 329 nanograms of dioxins per gram of ash and 170 nanograms of furans per gram of ash.

The results of these three experiments are summarized below in Table 1 :

Table 1

Temp. (°C) Reagent Total Dioxins Total Furans

(Wt. Pet.) (ng/g) (ng/g)

25 0 329 170

300 0 1204 441

300 1 290 158

These results demonstrate, as expected, that the concentration of dioxins and furans from the untreated flyash is increased substantially through heating, and that the treated ash produces fewer dioxins and furans after heating than untreated ash. Unexpected, however, is the finding that the dioxin and furan concentrations are reduced for the treated ash after heating, as compared to before heating. The conclusion drawn from this finding and from the foregoing verification study is that, rather than inhibiting dioxin and furan formation at the 350 to 370 degrees Celsius, pre-electrostatic precipitator injection conditions suggested in the 772 patent and experienced above through reaction with active catalytic sites on the flyash (as taught in the 772 patent), the triethanolamine is instead reacting with the dioxins and furans on the ash from the electrostatic precipitator 20.

A literature survey in fact reveals that the reaction of an aryl chloride with an amine under these conditions is well-known, whereby 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD) will react with monoethanolamine (MEA), for example, to produce an aminated chlorodioxin and HCI, and whereby hexachlorobenzene and pentachlorophenol (as precursors) will react with MEA to form aminated chlorobenzenes and aminated chlorophenols. Additionally, through a period of approximately 4 hours of operation of the incinerator 10 under normal operating conditions (without TEA injection), gas samples were taken from the incinerator 10 as per EPA Method 23. Four-hour samples were in this manner collected on each of three consecutive days, and these were conventionally analyzed for chlorinated dioxin and furan congener groups. Ash samples were taken on an hourly basis during this same time frame, and the ash samples collected for a given 4-hour run were combined and these three combined daily samples also conventionally analyzed for chlorinated dioxin and furan congener groups. The daily gas-phase and ash-phase analyses were then averaged, and the averages conventionally employed to construct a mass balance for dioxins and furans around the electrostatic precipitator 20, under the assumptions a) that the dioxin and furan congeners are present only either in the solid phase (adsorbed onto the flyash particle surfaces) or in the gas phase, b) that there are no dioxins or furans formed after the process sample is collected (that is, in the electrostatic precipitator 20), c) that no gas/solid transfer occurs of the dioxins and furans in the electrostatic precipitator 20, d) there are no gas or ash leaks from the electrostatic precipitator 20 and e) a uniform distribution of the dioxins and furans on the ash particles entering and leaving the electrostatic precipitator 20.

By way of background, the stack gas exiting the stack of an incineration unit consists of two physical phases, that is, a gas phase comprising typical flue gases (for example, nitrogen, oxygen, C02, CO, H20, HCI, NOx and S02) and a solid phase comprised of unprecipitated/unremoved flyash with any adsorbed species thereon. Conventional wisdom has held that the dioxins and furans exiting the stack of an incinerator in the stack gas are predominantly in the solid phase, as adsorbed onto the flyash. Contrary to the conventional wisdom, however, it has been found through the mass balance conducted herein that most of the dioxin and furan species in the stack gas from a conventional municipal solid waste incinerator 10 as depicted in Figure 1 are present in the gas phase.

Taken with the results of the earlier-described experiments, and without wishing to be bound by any particular theory of operation which may be expressed herein, it is inferred that fundamentally the alkanolamine "inhibitor materials" of the 772 patent are reacting with the chlorinated dioxins and furans and their respective precursors at a relatively slow rate compared to the gas transit time through the electrostatic precipitator 20 of the incinerator 10, with the gas phase dioxin and furan species concentrations perhaps being essentially established in equilibrium with those in the solid phase before the flyash is separated by the electrostatic precipitator 20. The ash, which has been coated with triethanolamine for example through a condensation mechanism as the temperature falls in the gas train of the incinerator 10, experiences a displacement reaction as indicated previously between the amine and the chlorinated species in the ash hoppers of the electrostatic precipitator 20, and the ash- borne dioxin and furan concentrations are accordingly reduced without affecting the stack gas concentrations. A process which exposes the gas-phase dioxins, furans and their precursors to a sufficient concentration of one or more materials which will react therewith at the temperatures prevailing in the gas train of an incinerator (that is, at temperatures of 175 degrees Celsius and greater, more preferably at temperatures of from 175 degrees Celsius to 350 degrees Celsius, still more preferably at temperatures of from 200 degrees Celsius to 300 degrees Celsius and especially at temperatures of 225 degrees Celsius up to 275 degrees Celsius) and for a sufficient contact time for the reaction to occur, so that dioxin-derivative species are formed therefrom which preferably are substantially retained on the flyash (or a supplemental particulate support on which the one or more dioxin-reactive materials are condensed or which contains one or more dioxin-reactive materials) and which can be removed therewith, was however expected to be able to successfully reduce the dioxin and furan emissions in the stack gas from an incinerator 10.

To demonstrate this concept, an apparatus 48 was constructed as shown in Figure 5, comprising a slightly-modified commercially-available bag filter apparatus 48 (Model D1022 Stainless Steel In-Stack Thimble Filter Holder, Graseby Nutech, North Augusta, South Carolina) including a 12 mm outside diameter sampling tube 50 equipped with 0.64 cm. diameter sampling orifice 52, and leading to a sealed bag housing 54 containing a glass fiber filter bag 56 from Whatman International, Maidstone, England, Item No. 2814300 (85 mm in length, 33 mm in outside diameter, 1.5 mm wall thickness). A conduit 58 at the opposite end 60 from the sampling tube 50 was provided for conveying the filtered gas stream from the apparatus 48 to a sample collection train (not shown) suited for dioxin and furan collection.

The apparatus 48 was inserted into the ductwork of the incinerator 10 at the pre-electrostatic precipitator sampling location ("B") employed in the verification trials described previously, with the probe tip aligned with the flue gas flow in the incinerator duct to receive an ash-laden process gas stream therethrough at a velocity which was matched to the free stream velocity of the flue gas in the duct. The sampling orifice 52 was in this regard sized so that an isokinetic sample was taken of the gas in the incinerator duct, at a flow rate corresponding to a filter bag being operated at a typical air to cloth ratio of 5 SCFM (standard cubic feet per minute) of gas flow per square foot of exposed filter bag area. The temperature of the filter bag housing 54 was measured at 240 degrees Celsius, or the same as the free stream flue gas temperature at the same location.

The filtered gas passing through the filter bag 56 was directed into the above-mentioned sample collection train, which consisted of a water-cooled XAD-2 styrene-divinylbenzene copolymer resin (Rohm & Haas, Philadelphia, PA) trap followed by impingers and a sampling pump. The dioxin and furan congener group concentrations in the filtered gas were then determined as before. To determine the ability of TEA-coated flyash collected upon a filter bag such as filter bag 56 in the apparatus 48 to react with and thereby reduce gas phase dioxin and furan concentrations in the filtered stack gas downstream of a conventional baghouse filtration apparatus, the apparatus 48 was inserted into the ductwork of the incinerator 10 as just described, and gas flow through the apparatus 48 continued for approximately 100 minutes to establish an ash cake on the bag 56 of TEA-coated flyash After this period of time, the gas flow through the apparatus 48 was interrupted, the sample collection train detached from the apparatus 48 and the apparatus 48 was withdrawn without disturbing the ash cake on the bag 56. A clean sample train was attached to the apparatus 48, the apparatus 48 was re-inserted and gas flow re-established for approximately 50 minutes The dioxin and furan congener group concentrations were then determined, for comparison to those found without any TEA injection and those found with TEA injection in the manner of the 772 patent (after two days of such injection).

The results were as reported in Table 2, with the dioxin and furan congener group concentrations being reported in nanograms per dry standard cubic meter.

Table 2

Sample Dioxin Furan Total D/F Pet

Cone Cone Cone Reduction

No Injection 123.2 244.7 367.9 —

772 Patent 1186 244.5 363 1 1.3

Present Invention 13.2 79 7 92 9 74 7

Taken with the results described in Table 1 above, these results clearly show that the process of the present invention enables a reduction in not only the dioxin and furan content of flyash removed from a municipal solid waste incinerator by a baghouse filtration apparatus or by an electrostatic precipitator, but also in the dioxin and furan emissions in the stack gas from such an incinerator; in the case of an ESP-equipped incinerator, this reduction can be accomplished as stated previously, by contacting the dioxins and furans and the precursors thereof remaining in the gaseous incineration products stream following the electrostatic precipitator with one or more dioxin-reactive matenals carried on a suitable particulate support in a replaceable filter arrangement downstream of the electrostatic precipitator. Such an arrangement is shown schematically for example in Figure 1 , in the placement of a filter 21 between the electrostatic precipitator 20 and the fan 22

It will be appreciated from the foregoing that in the baghouse-equipped incinerators for which the present invention is most ideally suited, an alternate or supplemental particulate support for the dioxin-reactive materιal(s) could be used as well, which could be added separately from the dioxin- reactive material(s) or which could incorporate in the pores (where applicable) or on the surface thereof the dioxin reactive material(s) and, further, which may desirably have the added capacity to remove trace heavy metals (mercury, for example) or other undesirable components from the combustion products. An example that is contemplated would be the use of activated carbon for such trace heavy metals removal. An additional example would be the use of an alumina support which is augmented with an alternate (to the preferred alkanolamines), dioxin-reactive material in the form of oxalic acid.

In similar fashion, the replaceable filter arrangement in an electrostatic precipitator-equipped incinerator will preferably employ activated carbon or a similar particulate support which can serve to remove trace heavy metals from the stack gas, in addition to providing a surface for the interaction of the dioxin-reactive material(s) and the dioxins, furans and precursors thereof in accord with the present invention.

Preferably, where activated carbon or the like is employed in this trace heavy metals removal capacity, the dioxin-reactive material(s) will be such that, when reacted with the dioxins, furans and precursors thereof, the dioxin-derivative materials produced thereby can be removed (for example, by solvent extraction) from the activated carbon support and the trace heavy metals adsorbed thereon. The alkanolamines are considered to be preferred in this context as well.

Those skilled in the art will further recognize that the process of the present invention may be equally beneficially employed in the context of other processes and apparatus besides incineration and incinerators, that produce (in at least some modes of operation) and emit undesired dioxins and furans in a gaseous stream, and which preferably, though not necessarily, conveniently further include some sort of particulate control or other gas-solid contacting means that could be adapted with relatively little expense to employ one or more dioxin-reactive materials in a dioxin-reducing capacity. One non- limiting example of such a non-incinerative process would be in the pyrometallurgical processing of ores in the primary metals industry, although other such processes will undoubtedly be readily apparent to those skilled in the art who are familiar with the teachings contained herein.

Claims

ClaimsWhat is claimed is:

1. A process for combusting organic material which when incinerated forms gaseous products of incineration including flyash, precursors to the formation of dioxins and furans and such dioxins and furans, wherein emissions of dioxins in a stack gas from the incineration of said material are reduced, comprising:

incinerating said organic material to form said gaseous products of incineration;

adding one or more dioxin-reactive materials to said gaseous products of incineration, which materials are deposited on the flyash or on a supplemental particulate support optionally and separately added upstream of an apparatus for removing flyash from the gaseous products prior to their emission in a stack gas, or in the alternative, adding the one or more dioxin-reactive materials with the addition of a particulate material including the one or more dioxin-reactive materials therein or thereon upstream of the flyash removal apparatus; and

maintaining the one or more dioxin-reactive materials in contact with the dioxins in the gaseous incineration products in sufficient concentrations and for a sufficient time to reduce the concentration of the dioxins in and from said gaseous products by at least ten percent, through the formation of dioxin- derivative materials by reaction with said one or more dioxin-reactive materials.

2. A process as defined in Claim 1 , wherein the dioxin-derivative materials are substantially retained on the flyash or supplemental particulate support, or in the alternative, on the particulate material including the one or more dioxin-reactive materials therein or thereon which had been added upstream of the flyash removal apparatus, and further wherein the dioxin-derivative materials are periodically or continuously removed with said flyash and, where present, with said particulate material or materials from the flyash removal apparatus.

3. A process as defined in Claim 1 , wherein the one or more dioxin-reactive materials include one or more amines.

4. A process as defined in Claim 3, wherein the one or more amines comprise one or more alkanolamines.

5. A process as defined in Claim 4, wherein the one or more dioxin-reactive materials include triethanolamine.

6. A process as defined in Claim 1 , wherein the one or more dioxin-reactive materials include oxalic acid.

7. A process as defined in Claim 6, wherein the oxalic acid is added via the addition of an alumina support which has been augmented with oxalic acid.

8. A process as defined in Claim 1 , wherein the one or more dioxin-reactive materials are added via a particulate support which incorporates the one or more dioxin-reactive materials therein or are added separately therefrom but are retained on the particulate support, with the particulate support being suited for adsorbing trace heavy metals in the gaseous incineration products stream.

9. A process as defined in Claim 8, wherein the one or more dioxin-reactive materials are selected to react with the dioxins among the gaseous products from the incineration step to produce one or more corresponding dioxin-derivative materials which can be physically or chemically separated from the support and the trace heavy metals adsorbed thereon.

10. A process as defined in Claim 9, wherein triethanolamine is added as a dioxin-reactive material and further wherein the particulate support is an activated carbon.

11. A process as defined in Claim 1 , wherein the process is conducted in an incinerator which employs a baghouse filtration apparatus, and wherein the one or more dioxin-reactive materials are added upstream of the baghouse filtration apparatus, vaporized and condensed onto the flyash and any supplemental particulate material added and collected on the fabric filters of the baghouse filtration apparatus.

12. A process for combusting organic material which when incinerated forms gaseous products of incineration including flyash, precursors to the formation of dioxins and furans and such dioxins and furans, wherein emissions of dioxins in a stack gas from the incineration of said material are reduced, comprising:

precipitating flyash from the gaseous products of incineration by means of an electrostatic precipitator; and

contacting the dioxins and furans and the precursors thereof remaining in the gaseous incineration products stream following the electrostatic precipitator with one or more dioxin-reactive materials carried on a particulate support which is either inert to or which promotes the reaction of the dioxin- reactive materials with one or more of the dioxins and furans and precursors thereof in the gaseous products from the incineration step, in a replaceable filter arrangement, the dioxin-reactive materials being present in the filter arrangement in sufficient concentrations being and maintained in contact with the dioxins, furans and precursors thereof for a sufficient time at the temperatures prevailing in the filter arrangement to reduce the concentration of the dioxins in and from said gaseous products from said incineration step through reaction therewith to form one or more corresponding dioxin-derivative materials therefrom.

13. A process as defined in Claim 12, wherein the dioxin-derivative materials are substantially retained on the filter arrangement and are removable therewith as a filter is replaced.

14. A process as defined in Claim 12, wherein the one or more dioxin-reactive materials include one or more amines.

15. A process as defined in Claim 14, wherein the one or more amines comprise one or more alkanolamines.

16. A process as defined in Claim 15, wherein the one or more dioxin-reactive materials include triethanolamine.

17. A process as defined in Claim 12, wherein the one or more dioxin-reactive materials include oxalic acid.

18. A process as defined in Claim 17, wherein the oxalic acid is added via the addition of an alumina support which has been augmented with oxalic acid.

19. A process as defined in Claim 12, wherein the particulate support in the replaceable filter arrangement is suited for additionally adsorbing trace heavy metals in the gaseous incineration products stream.

20. A process as defined in Claim 19, wherein the one or more dioxin-reactive materials are selected to react with the dioxins to produce one or more corresponding dioxin-derivative materials which can be separated from the support and the trace heavy metals adsorbed thereon, and to thereby regenerate the filter assembly for further use, by physical or chemical processing.

21. A process as defined in Claim 20, wherein triethanolamine is employed as a dioxin- reactive material and further wherein the support is an activated carbon.

22. In a process for the combustion of organic material which when incinerated forms gaseous products of incineration including flyash, precursors to the formation of dioxins and furans and such dioxins and furans, as conducted in an incinerator which employs a baghouse filtration apparatus for removing flyash from the gaseous products of incineration prior to their emission in a stack gas, the improvement which comprises:

adding one or more dioxin-reactive materials to the gaseous products of incineration upstream of the baghouse filtration apparatus which dioxin-reactive materials are deposited on the flyash or on a supplemental particulate support optionally and separately added upstream of the baghouse filtration apparatus, and which under the conditions prevailing in the baghouse filtration apparatus will react with at least the dioxins in and from said gaseous products to produce one or more corresponding dioxin- derivative materials therefrom which are substantially retained on the flyash or supplemental particulate support and which can be periodically removed with said flyash and supplemental particulate support from the baghouse filtration apparatus, or in the alternative, adding the one or more dioxin-reactive materials via the addition of a particulate material including the one or more dioxin- reactive materials therein upstream of the baghouse filtration apparatus; and

maintaining the one or more dioxin-reactive materials in contact with the dioxins in sufficient concentrations and for a sufficient time under the conditions prevailing in the baghouse filtration apparatus to reduce the concentration of dioxins in and from said gaseous products from said incineration step through the formation of said dioxin-derivative materials and the periodic removal of the flyash and supplemental particulate support including such dioxin-derivative materials.

23. In a process which generates a gaseous stream containing dioxins, and particulate matter, and which incorporates an apparatus for removing particulate matter from the gaseous stream, the improvement which comprises:

adding one or more dioxin-reactive materials to the dioxin-and particulate- containing gaseous stream upstream of the particulate removal apparatus which are deposited on the particulate matter collected in the apparatus, and under the conditions prevailing in the apparatus will react with the dioxins from said gaseous stream to produce one or more corresponding dioxin-derivative materials; and

maintaining the one or more dioxin-reactive materials in contact with the dioxins in sufficient concentrations and for a sufficient time under the conditions prevailing in the apparatus to reduce the concentration of the dioxins emitted through said gaseous stream by at least ten percent through the formation of said dioxin-derivative materials therefrom.

24. A process as defined in Claim 23, which is employed in the pyrometallurgical processing of ores to produce metals therefrom.

25. A process for the removal or reduction of dioxins from a gas stream containing such dioxins, which consists essentially of passing the gas stream through a gas-solid contacting device including one or more dioxin-reactive materials on a support which is inert to or which promotes the reaction of the dioxins in the gas stream with the one or more dioxin-reactive materials, and maintaining the dioxins in the gas stream in contact with the one or more supported, dioxin-reactive materials for a sufficient time and under sufficient conditions for causing the dioxins in the gas stream to react with the dioxin-reactive materials and to form dioxin-derivative materials therefrom.

26. A process as defined in Claim 1 , wherein the one or more dioxin-reactive materials are employed in sufficient amounts to accomplish at least a twenty percent reduction in dioxins in and from said gaseous products of incineration.

27. A process as defined in claim 26, wherein at least a fifty percent reduction in dioxin concentration is accomplished.

28. A process as defined in claim 27, wherein at least a seventy-five percent reduction in dioxin concentration is accomplished.

29. A process as defined in Claim 12, wherein the one or more dioxin-reactive materials are employed in sufficient amounts to accomplish at least a ten percent reduction in dioxins in and from said gaseous products of incineration.

30. A process as defined in Claim 30, wherein at least a twenty percent reduction in dioxin concentration is accomplished.

31. A process as defined in Claim 30, wherein at least a fifty percent reduction in dioxin concentration is accomplished.

32. A process as defined in Claim 31, wherein at least a seventy-five percent reduction in dioxin concentration is accomplished.

33. A process as defined in Claim 23, wherein the one or more dioxin-reactive materials are employed in sufficient amounts to accomplish at least a twenty percent reduction in dioxins emitted by means of said gaseous stream.

34. A process as defined in Claim 33, wherein at least fifty percent reduction in dioxin emissions is accomplished.

35. A process as defined in Claim 34, wherein at least a seventy-five percent reduction in dioxin emissions is accomplished.