CA2059281A1

CA2059281A1 - Process for purification of exhaust gases, in particular from waste incinerators

Info

Publication number: CA2059281A1
Application number: CA002059281A
Authority: CA
Inventors: Jurgen Ritter
Original assignee: Individual
Current assignee: Nymic Anstalt
Priority date: 1990-04-23
Filing date: 1991-04-13
Publication date: 1991-10-24
Also published as: HU210398B; EP0478744B1; WO1991016122A1; HU914082D0; AU7651291A; KR920702632A; DE59108071D1; EP0478744A1; ATE141182T1; DE4012887C2; DE4012887A1

Abstract

Abstract Process for purification of exhaust gas (1) from waste incinerators, especially refuse incinerators (2), wherein the exhaust gas (1) is dedusted, conditioned and passed through an adsorber (6) charged with activated coke (7) for separating off pollutants. The unpurified exhaust gas (1) is passed through the adsorber (6). The adsorption is controlled in such a way that the gas (8) leaving the adsorber (6) is almost free of SO2. HCl and HF for producing re-usable products, especially hydro-chloric acid and/or NaCl, is taken off from the unpuri-fied gas (8) leaving the adsorber (6). By means of multiple adsorption (6, 14, 25, 36) and regeneration (20, 29), on the one hand the exhaust gas (1) is purified and, on the other hand, the pollutants are separated from the exhaust gas (1) and further processed to give saleable products. (Figure 1).

Description

2~2~1 Description Process for purification of exhaust gases, in particular from waste incinerators The invention relates to a process for purifica-tion of oxygen-containing exhaust gases laden with SO2, HCl, NOX and at least one heav~ metal and also, in some cases, further toxic gases, especially furans and di-oxins, in particular from refuse incinerators, with the use of regenerable adsorption materials.
Exhaust gases from refuse incinerators contain HCl, HF, Sox, Hg and NOx as well as highly toxic organic PCDD/PCDF compounds, known as dioxins and furans. For the purpose of maintaining clean air, the pollutants should be separated from the exhaust gases and, as far as possible, recovered as re-usable materials of value.
Two-stage or multi-stage wet processes, operating on the basis of lime, for separating off HCl, HF and SO2 are known, wherein the reaction products occurring cure above all CaCl2 and CaSo4 [sic]. These reaction products are taken to landfill sites and deposited. A further disadvantage of these processes is that polluted effluents arise, which must either be subjected to a treatment or disposed of thermally. The effluents can again be introduced into a water course or into the sewer system only after they have been ~reated. Overall, this process is cost-intensive and pollutes the environment because of the deposition of ~he reaction products in :: ~

~ - 2 ~ 9 ~ ~ ~
landfill sites.
Concepts have been disclosed (cf.
Chem.-Ing.-Tech. 60, 1988, pages 247-255), by means of which hea~y metals removed from the exhaust gas during the purification in the wet-scrubbing process can in principle be reprocessed and recovered. Since these processes are ~ery e~pensive, underground disposal is regarded, in the case of an unfavorable cost/benefit ratio, as economically sensible and ecologically pos-sible. In the recovery of the heavy metals, 2000 tonnes of salts arise per year in a large power station, and these are to be discharged under control into the sea.
For furans and dioxins, their ready destructibility at moderate temperatures can be exploited by recycling the gases laden with these substances into the incinerator furnace.
Proposals have been disclosed to recover NaCl as an industrially marketable product, by using caustic soda solution as the neutralizing agent, and thus to reduce the landfill materials. This approach is expensive and would only be p~acticable if, in ~iew of the stringent purity requirements to be met by the NaCl employed in the chlor-alkali electrolysis, the HCl contained in the exhaust gas would first of all be largely isolated from all the other pollutants, which is not possible by the hitherto disclosed processes for the manufacture of NaC1.
Dry sorption processes are also known, which are run dry or quasi-dry and operate likewise on the basis of lime. The reaction products arising are put into landfill 3 2~2~1 as waste, together with the fly dust arising in refuse incineration. In the said processes, the further problem arises that the dioxins and furans present in ~he exhaust gas are not separated off.
A problem in the dry processes and wet processes is the separation of the mercury, present in the gaseous state, from the exhaust gas. With dry sorption processes, this separation is impossible, and with wet scrubbing it is at least not reliably possible.
DE 3,7~6,131 A1 has disclosed a process for removing pollutants from flue gas~ wherein flue gas pretreated by wet scrubbing is passed through a plurality of adsorber beds. In this case, the effect of pollutants being retained in the adsorbers with a certain selec-tivity, smaller molecules being expelled from the adsor-ber when saturation is reached, is exploited. The gas treatment provides for the removal of those gas con-stituents from the exhaust gas which might prove a hindrance to a subsequent catalytic deni~rification. In a first adsorber bed, heavy metals, especially mercury, are to be adsorbed. In one or two further beds, SO2 and HCl are adsorbed, and the laden adsorption material is then transferred into a further adsorption stage, in which excess ammonia, present in the gas, from the denitrification stage is adsorbed. The adsorbent laden with ammonia, SO2 and HCl is then burned in the boiler installation. The coke laden with heavy metal in the first adsorption stage is disposed of, i.e. put into landfill. The adsorption material used is above all .:

open-hearth coke (which cannot ~e regenerated). This process is feasible only after a prepurification of the flue gases, since otherwise all the adsorbed pollutants would be transferred back into the flue gas as a result of the combustion of the adsorbent. It is also mentioned that expensive activated coke grades might be used in the adsorption stages for the gaseous pollutants and re-generated. It is not mentioned therein what should be done with the pollutants released again during the regeneration.
For the reasons sta~ed, the known process can be used only for the final purification of prepurified flue gas.
The combustion of the open-hearth coke laden with pollutants involves the risk of the dioxins and furans not being destroyed in the case of an unduly short residence time in the boiler or, because of the presence of 2 and C12 in the boiler region, potentially even being promoted by catalytically acting copper constituents in the slag or in the fly dust, forming PCDD/PCDF recombin-ation products which, together with the unde~troyed dioxins and furans, gradually lead to these highly toxic pollutants being concentrated in the exhaust gas. For removing the dioxins and furans from the exhaust gas puriication system, this would accordingly leave only the landfill disposal of the open-hearth coke laden with pollutants, but this further increases the quantity of the waste products to be put into landfill.
A review of the features of conventional .. , ~: . - ~ .
.

~~ _ 5 _ 2~92~
processes from various contractors is published in "Energie Spectrum", July 19~9, pages 13-16. This makes it clear that, in all adsorption processes, either landfill materials arise or the laden adsorption cokes and adsorp-tion carbon are burned which, for the reasons already mentioned above, can be sensible only if the adsorption is employed only for the final puriflcation of flue gases.
In known dry sorption processes, alkaline addi-tives are added to the fuels, in order to reduce the pollutant output of acidic gases. The treatment of the flue gases of thus reduced pollutant content by means of adsorbers also causes considerable problems. For example, mercury cannot be separated off by the lime products used in the dry sorption process. Combustion of the laden open-hearth coke in the boiler would therefore lead to mercury being gradually concentrated in the exhaust gas.
For this reason, processing of the open-hearth coke is considered in this case in such a way that the residual SO2, HCl and HF and also the mercury absorbed by the open-hearth coke are thermally desorbed from the open-hearth coke, the mercury is then re-adsorbed on an open-hearth coke activated with sulfur, and the remaining pollutants as well as the open-hearth coke depleted of pollutants are recycled to the boiler. The open-hearth coke laden with mercury and activated with sulfur must be deposited in a special landfill.
In the meantLme, yet another approach is used in dry sorptions for detecting the dioxins and furans and , ,: . , , , , . -, - 6 ~ 2 ~ ~
also the mercury. A small quantity of open-hearth coke, which adsorbs the dioxlns and furans and also the mer-cury, is added to the lime product which is used in the dry sorption and which serves for separating off SO2, HCl and HF. In addition, the open-hearth coke also adsorbs certain quantities of SO2, HCl and HF.
This process again leads only to an increase in waste products, since the open-hearth coke laden with pollutants must, together with the lime reaction prod-ucts, and in most cases also mixed with fly dust, be taken to landfill sites and deposited there.
No long-term experience is yet available on the storage of open-hearth coke laden with dioxins and furans, mercury, SO2, HCl and HF. Even deposi~ion in special landfill sites appears to be no~ unobjectionable with respect to the dioxins and furans and the mercury.
Moreover, if this process were used for the purification of exhaust gases from domestic refuse incinerators, the landfill costs would considerably increase if the reac-tion products, which hitherto have been deposited less expensively in mono-landfills, would have to be deposi-tion in special landfills.
Furthermore, if this process is applied, the additional use of an activated carbon filter downstream of the dry sorption for the separation of residual SO2, HCl and HF cannot be omitted, since it is difficult with -dry sorption to reach the demanded low residual concen-trations of pollutants in the pure gas. In this case, however, the combustion of the open-hearth coke laden ., , . ~ .

_ 7 _ 2 ~ ~2 ~
with pollutants in the boiler is unobjectionable, since only SO2, HCl and HF are released, for which the dry sorption serves as a sink. The activated carbon filter here merely serves for the final purification of the exhaust gas, but not ror separating off dioxins, furans and Hg.
All the kno~n flue gas treatment processes lead to waste products which must be put into landfill if concentrating of toxic substances in the exhaust gases is to be avoided.
DE 3,426,059 Al discloses a process by means of which specifically organic pollutants, especially dioxins and furans, are to be removed adsorptively from the flue gas. In this case, it is assumed that, in this sep-aration, other pollutants such as SO2 and heavy metals are also separated off simultaneously. The activated carbon or activated coke used for the absorption are subjected to a conventional regeneration with inert gases in the temperature range of about 350-750C. To crack the dioxins and furans, the desorption gas taXen off from the regeneration and laden with the pollutants is heated to a temperature of from more than 1000C up to about 1400qC. By this means, the cracking temperature for the dioxins and furans is to be exceeded, so that these are reliably decomposed. The residence time at this cracking temperature should here be of the order of magnitude of a few seconds, for example 5-10 seconds. At the same time, it should be possible to carry out the cracking of the dioxins and furans during the desorption in the .. .

.
' ' ' ' '' '' 8 2 ~5 ~2 ~
regeneration stage. A concept for the further treatment of the remaining pollutants is not disclosed in this printed publication.
The invention is based on the object of indicat-ing a process for purification of flue gases of the type described above, in which no waste materials to be put into landfill arise and which can be carri~d out economically.
According to the invention, this object is achieved by a process of the type mentioned at the outset, comprising the following process steps:
- The exhaust gas is passed through a first adsor-ber and freed therein of SO2, the heavy metal andfurther toxic gases.
- HCl is removed from the gas leaving the adsorber.
- The NOX of the gas is then converted to a harm-less product.
- The Laden adsorption material of the first adsor-ber is subjected to an oxygen-free first regeneration.
- The gas released in the first regeneration is fed to a second adsorber operating oxygen-free, the heavy metal Hg being adsobed [sic] and thus being separated from the SO2 which is not adsorbed.
In the process according to the invention, the exhaust gas with its said constituents is passed into the first adsorber, where the SOz, the heavy metals, especi-ally mercury, and toxic gases such as dioxins and furans are adsorbed. In the pore structure of the adsorption ,, ;~ . :, . . .
~ . ~

:~ ; . .

2~2~
material, which is preferably activated coke (bituminous coal coke), the SO2 is catalytically converted to H2SO4.
With respect to ~he height of the adsorp~ion bed, which preferably is a moving bed, and with respect to the residence time of the gas in th~ adsorber, the adsorption is designed and controlled in such a way that the said pollutan~s are separated off down to the detectlon limit given by measurement technology. In the preferably used countercurrent process, a separation of SO2 on the one hand and of HCl likewise present in the exhaust gas and in some cases HF on the other hand can here above all be achieved. This separation becomes possible because HCl and, in some cases, HF - in contrast to SO2 - are poorly separated off by adsorption materials such as, for example, activated coke. In particular, however, the adsorbed SO2 displaces, due to its hi~her molecular weight, the little HCl and, in some cases, HF adsorbed by the adsorption material. In the countarcurrent process, HCl and HF are first adsorbed in the upper activated coke bed, since the exhaust gas flowing through this layer, before it leaves the adsorber, no longer contains any SO2, after the la~ter has already been adsorbed in the lower layers. If a moving bed is used, the upper activa-ted coke layer laden with HCl and HF gradually passes into lower layPrs, where it comes into contact with SO2 or H2SO4 from the exhaust gas. As a result, HCl and HF are desorbed again from the activated coke and SO2 is adsorbed instead. The result of this procedure is there-fore that HCl and HF only migrate through the adsorber.

' , , ' ~ . ~ ,. . ' ,; , : "
, 1' . :' ` ~, " ' ' ' '' . ' ~:, ' - . . , , .... ~ . .

0 - 2~2~
Downstream of the adsorber, hydrochloric acid and/or sodium chloride in marketable qualities can be produced from the HCl, if appropriate after separation from the HF
which may possibly be present.
In addition to its proper function, namely the removal of pollutants, the adsorber briefly also serves as a filter which dedusts the exhaust gas. This is especially of importance if the actual filter fails for a short time. In this case, small quantities, laden with fly dust, of activated coke, which in this case serves as a filter medium, are discharged with the aid of differen-tial pressure measurements and with the aid of a pre-cisely controllable discharge system, without having to interrupt the crude gas flow through the adsorber. In any case, however, final dedusting of the exhaust gas takes place in the adsorber by means of the activated-coke bed.
Known processes are available for the denitrifi-cation of exhaust gases from refuse incinerators. They operate with the use of SCR catalysts or of special activated cokes for the reduction of NOX by means of NH3.
For the denitrification of ~he exhaust gas purified by removal of all other pollutants, the exhaust gas is preferably fed downstream of the HCl recovery to a denitrification reactor, NH3 being admixed to the exhaust gas before and/or during the denitrification The denitrification is expediently carried out with a special activated coke. The NOX is cataly~ically reduced under the action of NH3 ~ SO tha~ the gas introduced into the atmosphere is largely free of pollutants. The use of an , 2~592~ ~

activated-coke regeneration reactor for the denitrification has the advantage that the denitrification reactor then takes on the function of a `police filter~ if, due to a fault in the up~tream reactors, exhaust gas laden with the pollutants normally already adsorbed should pass into the denitrification reactor. This even applies to traces of dioxins and furans, which are adsorbed in the activated coke of the denitrification reactor. Any dioxins and furans adsorbed are destroyed during the regeneration which is carried out from time to time. ~he remaining regeneration gas, which can then be laden with HCl and SO2 in some cases, can be introduced into the exhaust gas upstream of the first filter. If a small quantity of activated coke is continuously discharged from the d~nitrification reactor, this is of benefit to the denitrification performance of the activated-coke hed, since rheological advantages are obtained by the layer rearrangement.
For the desorption of the pollutants SO2 and heavy metal, especially Hg, which are separated off from the exhaust gas by the adsorption material in the adsorp-tion and are concentrated, the adsorption material laden with pollutan~s is fed to a thermal regeneration. In the regeneration, the H2SO~ present in the pore structure of the adsorption material, especially activated coke, is reconverted to SO2 and discharged in the rich gas, which -represents a mixture of inert fuel gas and the pollutants reconverted into the gaseous form, if, in a preferred embodiment, heating of the laden adsorption material is :
. ... .

,:
: ~ ' . !.

2 ~ :~

carried out by means of a hot inert gas. The rich gas also contains the desorb~d mercury in the gaseous form.
The inert fuel gas preferably passes, at an inlet temperature above 550~C, especially at 650C, in counter-current through the adsorption material laden with pollutants. To adjust the fuel gas to the desired tem-perature, it can be mixed with the rich gas passing out of the regeneration at about 325C. In addition, the rich gas can be adjusted in each case to the S02 content most advantageous for the utilization of materials of value by varying the rate of rich gas which is recycled and admixed to the inert gas. With the fuel gas, convective heating of the adsorption material laden with pollutants takes place and, due to the good heat transfer in countercurrent operation of the fuel gas, this leads to relatively short desorption times and to advantageous energy consumption.
With heating of the adsorption material laden with pollutants-by a fuel gas at about 650C and wîth a residence time of the adsorption material in the first regeneration stage of more than half an hour, preferably more than one hour, the dioxins and furans adhering to the adsorption material are completely destroyed. Since the regeneration is operated oxygen-free, no PCDD/PCDF
recombinates can form.
The regenerated adsorption material can be recycled for repeated use into the first adsorber. The small quantities of spent adsorption material are made up by correspondingly added fresh adsorption material.

~ .; : ., ' ' ' ~ -:
'. . .

20~2~1 The rich gas taken off from the first regenera-tion contains the desorbed SO2 and heavy metal. The rich gas is passed through a second adsorption which is operated oxygen-free and in which expediently activated coke is likewise employed as the adsorption material.
Due to the lack of oxygen, no sulfuric acid can form with the SO2, so that the SO2 is not adsorbed but migrates through the adsorber. On the other hand, the Hg is adsorbed by the activated coke. Separation of SO2 and Hg thus takes place in the second adsorption. The gas leaving the second adsorber contains SO2 in a concen-trated form, so that saleable sulfur products can be produced therefrom. This process step again serves for reducing the landfill materials, ie. for converting pollutants into re-usable materials of value.
In the process according to the invention, the processing of the pollutants removed from the flue gas by means of the adsorbers is accordingly carried out in such a way that all th~ pollutan~s can be recovered as materials of value. The activated coke preferably used is circulated between continuous adsorption and regenera-tion. With the purification concept according to the invention, concentrating due to the combustion of laden carbon or the like cannot occur, since no combustion of adsorption materials is envisaged.
The adsorption material laden with the heavy metal, especially Hg, is taken off from the second adsorber and preferably fed to a special regeneration for the recovery of the heavy metals. In this regeneration, :. .,; . " ~, , :

:: ', , ,`,, . ,,., ; ' ~

- - 14 - 2~ 8~
the Hg, for example, is desorbed from the activated coke by means of a fuel gas at an inlet temperature above 550C, especially at 650C. The desorption gas is con-densed, so that the heavy metals are obtained in this way in metallic form. For example, re-usable mercury is available after purification of the condensate. The regenerated activated coke can be used again for adorp-tion [sic].
By means of the novel process, the pollutants present in the exhaust gas of the waste incinerator are thus not only reliably separated off from the exhaust gas, but the pollutants are largely converted into saleable products. Landfill disposal is not necessary.
This also applies to the fly dust arising in the dedusting. This dust can be heated to a temperature of 2 1200C. At this temperature, vaporization of the heavy metals and formation of heavy metal salts takes place.
These substances can be filtered off and then be taken to a metallurgical treatment. The ballast materials are vitrified to give an inert material which can be used, for example, in road construction.
However, the Hg present in the exhaust gas from the heating of the fly dust cau~es a problem. In the known processes, this exhaust gas is mixed with the crude gas from the was~e incinerator. The Hg thus ultimately ends up in a landfill, together with the other reaction products.
To a~oid the landill disposal of reaction products with Hg, the exhaust gas emerging from the melt '' . :

2~2~

of the fly dust is fed, downstream of the filter for separating of f the heavy metal salts, to a third adsorp-tion in which the Hg is separated out of the exhaust gas.
However, residues of dioxins and furans, which are not destroyed in the thermal treatment o the fly dust because of too short a residence time, and/or recombi-nates of PCDDs and PCDFs, are also separated off in the third adsorption, so that pollution of the environment with these pollutants is reliably avoided.
Advantageously, activated coke is also used in the third adsorption. The laden activated coke is then fed to the specific regeneration for Hg recovery. The dioxins and furans are completely destroyed in the re-generation. The Hg is recovered as already described above.
In the second and/or third adsorption, the unladen activated coke from the ~g desorption is used.
Between the second and/or third adsorption and the regeneration for Hg recovery, there is a self-contained activated-coke circulation as between the first adsorber and the first regenaration. handfill disposal of acti-vated carbons laden with pollutants is thus not neces-sary.
To free the exhaust gas of aggressive S03, the exhaust gas can, after it has passed through a first filter for the separation of fly dust, be passed through a quench enriched with NH3 upstream of the first adsorber. With the NH3~ the S03 forms ammonium salts which are adsobed [sic] in the first adsorber. In an . .. .. :

:.
. , . . . . :: :

. .' ' ~ ', ' " :;
. . . .. .

-~ 2 ~ g ~

alternative embodiment, the NH3 is already added to the exhaust gas upstream of the filter provided for the separation of fly dust. The ammonium salts thus formed can then be removed from the exhaust gas togetxer with the fly dust and thermally destroyed in the processing of the fly dust.
Further features of the present invention are the sub,ects of the subclaims and will be explained in more detail with further advantages of the invention with reference to an illustrative embodiment.
In the drawing:
Fi~lre 1 shows a diagrammatic illustration of a process for purification of exhaust gases and recovery of materials of value, and Figure 2 shows a diagrammatic illustration of a plant for recovering materials of value, operated separately from a specific purification plant.
Figure l shows, in a diagrammatic form, a proces~
for purification of exhaust gases 1 from a refuse in-cinerator 2. The exhaust gas l emerges at a temperature of about 250C to 300C and is passed through an electro-static precipitator 3. In the electrostatic precipitator 3, fly dust 4 is separated off. Downstream of the elec-trostatic precipitator 3, a heat exchanger 5 and a quench enriched with NH3 (not shown) are located, which cool the exhaust gas 1 to a temperature of about 120C and free it of aggressive S03- The exhaust gas 1 cooled i~ this way is passed through an adsorber 6 which is charged with activated coke 7. From the adsorber 6, a prepurified gas .

- . , , . ~
, ' - ; , ' ~ :, " '.

-- 2~2~

8 issues, from which th~ HCl is used in a process step 9 for the manufacture of hydrochloric acid or NaCl. Subse-quently, the gas 10, which contains almost only NOX and has a temperature below 70C, is heated to about 120C by means of a heat exchanger 11. The heat exchanger 11 is connected to the heat exchanger 5 via a line 12. After the gas 10 has been heated, admixing 13 of NH3 in the form of aqueous ammonia takes place. The gas 10 mixed with ~H3 is fed to a denitrification reactor 14, which is charged wi~h activated coke 15. Downstream of the denitr-ification reactor 14, a blower 16 is arranged which draws in the gas 1, 8, 10 and passes pure gas 17 to a stack which is not shown here.
The line 12, which mutually connects the two heat exchangers 5 and 11, has a branch 18, through which a part heat can be passed to the process step 9 for the manufacture of hydrochloric acid or NaCl.
~ he activated coke 7 of the adsorber 6 is, as diagrammatically indicated by an arrow 19, fed to the regenerator 20. The activated coke 7 regenerated in the regenerator 20 is re-used in the adsorber 6, which is indicated by the arrows 21 and 22. The arrow 23 symbo-lizes the addition of fresh activated coke, since small quantities of activated coke are consumed in the process sequence.
Rich gas 24, which cont~ins SO2 and Hg, issues from the regenerator 20. ~he rich gas 24 is passed through a second adsorption 25, whereby Hg is adsorbed from the rich gas 24. The second adsorption 25 is carried ,; . ,~, ,. ~ ; . j :

- ' ' ! ' ~ ~ ~ ; . ` ' ' .

. ' ! ' ' ' . : . . . .
. ,' ' ' ' ' . . ..

2~2~:~

out with activated coke 26 on which the Hg deposits. The gas 27 leaving the second adsorber 25 contains SO2 in a concentrated form, so tha~ sulfur products can advan-~a~eously be manufactured from this.
The activated coke 26 laden with Hg i~ fed along an arrow 28 to a second regeneration 29. The gas 30 leaving the second regenerator 29 is condensed and the condensate is subsequently purified, after which utiliz-able mercury is available.
At least a part of the regenerated activated coke 26 is fed from the second regenerator 29 to the second adsorber 25, as indicated by the arrow 31.
The fly dust 4 separated off by the electrostatic precipitator 3 from the exhaust gas 1 is fed to a melting furnace 32. Downstream of the melting furnace 32, heavy metals 33 suitable for smelting and, for example, glassy granules 34 which can be used for road construction are obtained.
The exhaust gas 35 leaving the melting furnace 32 is passed into a third adsorber 36, in which the exhaust gas 35 is purified by means of activated coke 26. The purified gas 37 lea~ing the third adsorber 36 is dis-charged to a stack which is not shown.
The activated coke 26 laden in the third adsorp-tion 36 is, as symbolized by an arrow 38, trans~erred into the second regenerator 29 and regenerated. A part of the regenerated activated coke 26 is, as indicated by an arrow 39, recycled to the third adsorbar 36 in exchange for laden activated coke.

:

' ,; : ' ~, , ;;:

2~5~2~:~

The function of the first embodiment is now described in more detail below.
The exhaust gas 1 issues from ~he refuse inciner~
ator 2 at a temperature of about 250-300C. The exhaust gas 1 essentially contains the pollutants HCl, HF, SOx, Hg and NOX and also PCDD compounds and PCDF compounds.
Together with the exhaust gas 1, ~he fly dust 4 is also transported, which is separated off from the exhaust gas 1 by the electrostatic precipitator 3 and fed to the smelting furnace 32. Downstream of the electrostatic precipitator 3, the exhaust gas 1 is cooled by means of the heat exchanger 5 and the quench to a temperature of about 120C and freed of S03, and then passed through the adsorber 6 charged with activated coke 7. In the adsorber 6, SO2, Hg, dioxins and furans are separated off from the exhaust gas l. By contrast, HCl and HF pass through the adsorber 6, 50 that the prepurified gas 8 contains only HCl and HF in addition to NOX. The exclusion of HCl and HF is achieved here due to the fact that, as a result of the countercurrent flow of the exhaust gas 1 through the activated-coke bed migrating slowly downward3 through the adsorber, the SO2 again and again displaces the HCl and HF, so that ultLmately no HCl and HF can be permanently adsorbed by the activated coke.
In addition to the main task of the adsorber 6, namely the separation of pollutants, the adsorber 6 also serves for final dedusting of the exhau~t gas 1. ~his ensures that the prepurified gas 8 is largely free of fly dust 4 even in the event of short-term failure of the ,:. ', . : , ' .,..... ; .,. ':. ,:', ' ., . . ~ - .

-;

2 ~
_ 20 -electrostatic precipitator 3.
Accordingly, due to the adsorption in the adsorber 6, only HCl and HF are still present in the prepurified gas 8, in addition to NO~. There~ore, the prepurified gas 8 is suitable for use in the process step 9 for the manufacture of hydrochloric acid or NaCl. These products obtained in the process step 9 are saleable and/or re-usable, so that the process is satisfactory in a special way from both environmental and economic aspects.
After the process step 9, the gas 10 is at a temperature below 70C. By means of the second heat exchanger 11, which is connected via the line 12 to the first heat exchanger 5, the gas 10 is heated to about 100C. Subsequently thereto, NH3 iS admixed with the gas 10, which is passed through the denitrification reactor 14 which is filled with a special activated coke 15. ~his achieves a ca~alytic decomposition of NOX. At the exit from the denitrification reactor 14, the pure gas 17 issues, which has been freed of pollutants and which i5 passed by the blower 16 via the stack into the atmos-phere. If a moving bed is used in the denitrification reactor 14, the latter can serve as a final dedusting stage.
The activated coke 7 from the adsorber 6, which is laden essentially with Hg, H2SO4, dioxins and furans, is fed to the first regenerator 20. In the regenerator 20, H2S04 and Hg are thermally desorbed from the activated coke 7, namely by direct heating of the activated coke 7 ' 2 ~ ~

by an inert fuel gas which is passed at a gas inlet temperature of about 650C in countercurrent through the activated coke 7. The rich gas 24 leaves the regeneration 20 at about 325C.
Apart from the temperature of about 650C, it is essential that the first regenerator 20 is operated oxygen-free and the duration of the regeneration is about 1 hour. This ensures that the dioxins and furans are completely destroyed and a formation of recombinates of PCDDs and PCDFs is precluded.
The regenerated activated coke 7, which is free of pollutants, is recycled to the adsorber 6, in exchange for laden activated coke 7.
The rich gas 24 leaving the regenerator 20 contains essentially SO2 and Hg. This rich gas 24 i5 fed to the second adsorption 25 which is operated oxygen-free. Due to the oxygen-free operation, no sulfuric acid can form from the S02, so that only the Hg is adsor~ed in the activated coke 26 of the second adsorption 25, but on the other hand the SO2 migrates through the second ad-sorber 25. In this way, the S02 is separated from the Hg in the second adsorption 25, which is carxied out at approximately 120C. In the gas 27 leaving the second adsorber 25, the S02 is present in a concentrated form and serves for the manufacture of saleable sulfur pro-ducts.
The Hg-containing activated coke 26 from the second adsorption 25 is regenerated in the second re-generation 29 which is likewise operated oxygen-free. At :, . .
:: :
I .
: :":' ' ' ., , - 22 ~ 2 ~ ~2~ ~
the temperature of about 600-650C prevailing during the second regeneration 29, the Hg is desorbed. The desorp-tion gas is then condensed, in order to obtain Hg in metallic form in this way. After purification of the condensate, utilizable mercury is available.
The activated coke 26 regenerated in the second regeneration 2g is at least partially recycled to the second adsorption 25 in exchange for laden activated coke 26.
As already stated above, the fly dust 4 is separated off from the exhaust gas 1 downstream of the refuse incinerator 2 by means of the electrostatic pre-cipitator 3. This fly dust is heated in the melting furnace 32 to a temperature of about 1200C or higher.
The heavy metals contained in the fly dust 4 vaporize and form heavy metal salts which can be taken to smelting.
The ballast materials are vitrified to give glassy granules 34. Due to its volatility, the mercury adhering to the fly dust does not form any heavy metal salts in the melting furnace 32 and is discharged with the exhaust gas 3~ from the melting furnace 32. The exhaust gas 35 also still contains dioxins and furans, which were not destroyed in the melting furnace 32 because of too short a residence time, and possibly also PCDD and PCDF recom-binates. These pollutants, ie. ~g, dioxins and furans, are adsorbed from the exhau~t gas 35 by the activated coke 26 in the third adsorp~ion 36 which is operated at about 120C. The pollutant-free ga~ 37, whose temperature is about 120C, is fed to a stack which is not shown .. . .
~ ' ''' " ' , .

':

- 23 _ 2~
here.
The activated coke 26, laden with Hg, dioxins and furans, from the third adsorption 36 is regenerated in the second regeneration 29. The dioxins and furans are destroyed here, as already described in connection with the first reg~neration 20, and the mercury is reco~ered.
As compared with the regeneration 20 and the adsorber 6, the second regeneration 29 and the adsorbers 25 and 36 are relatively small and compact, since the quantities of activated coke used therein and the volu-metric gas flows passing through the adsorbers 25 and 36 are small as compared with the volumetric flow of the exhaust gas 1.
The overall result of this process, apart from the purification of the exhaust gas l, is that the pollutants present in the exhaust gas 1 are separated out by repeated adsorption and regeneration, and saleable products can be manufactured from them.
The illustration in Figure 2 is based on the same process steps as those described with reference to Pigure 1. An essential difference is, however, that the actual refuse incinerator 2 with the downstream purification plant for the exhaust gas 1 is not directly connected to a plant 40 for recovering materials of value. The laden activated coke 7 from one or more refuse incinerators 2 and from other exhaust gas purification plants 41 and, for example, from a small power station 4~, a soil decontamination 43, an industrial boiler 44 or from industrial process gas 45, are fed to the plant 40 for .

- , . ~:, .

.

2 ~

recovering materials of value. The plant 40 for recover-ing materials of value is thus utilized in common by a plurality of smaller plants, in which the construction of a special plant 40 for recovering materials of value is not worthwhile from economic aspects.

... :., . :. .: . . :;
,,, :: : ~ . ,

Claims

1. Process for purification of oxygen-containing exhaust gases (1) laden with SO2, HCl, NOx and at least one heavy metal and also, in some cases, further toxic gases, especially furans and dioxins, in particular from refuse incinerators, with the use of regenerable adsorp-tion materials, comprising the following process steps:
- The exhaust gas (1) is passed through a first adsorber (6) and freed therein of SO2, the heavy metal and further toxic gases.
- HCl is removed from the gas leaving the adsorber (6).
- The NOx of the gas is then converted to a harm-less product.
- The laden adsorption material (7) of the first adsorber (6) is subjected to an oxygen-free first regeneration (20).
- The gas released in the first regeneration (20) is fed to a second adsorber (25) operating oxygen-free, the heavy metal (Hg) being adsobed [sic] and thus being separated from the SO2 which is not adsorbed.

2. Process according to claim 1, wherein the SO2 gas passing through the second adsorber (25) is processed to give sulfuric acid.

3. Process according to claim 1 or 2, wherein the heavy metal (Hg) adsorbed in the second adsorber (25) is separated from the adsorption material in a second regeneration stage (29) and recovered in metallic form.

4. Process according to one of claims 1 to 3, wherein the first regeneration is carried out at a temperature which decomposes the adsorbed toxic gases, especially dioxins and furans.

5. Process according to claim 4, wherein the tem-perature is above 550°C.

6. Process according to claim 5, wherein the tem-perature is 650°C.

7. Process according to one of claims 1 to 6, wherein, for carrying out the first regeneration, hot inert gas is passed in countercurrent through the laden adsorption material (7) located in a reactor.

8. Process according to one of claims 1 to 7, wherein the residence time of the adsorption material (7) in the reactor for the first regeneration is half an hour or more.

9. Process according to one of claims 1 to 8, wherein the NOx, after it has passed through the first adsorber (6) and has been separated from HCl gas, is catalytically decomposed in a reactor (14) by addition of ammonia.

10. Process according to claim 9, wherein the gas is heated by means of a heat exchanger (11) to a temperature of 100°C before the catalytic decomposition.

11. Process according to claim 9 or 10, wherein the exhaust gas is cooled upstream of the first adsorber (6) by means of a heat exchanger (5).

12. Process according to claims 10 and 11, wherein the two heat exchangers are mutually connected.

13. Process according to one of claims 1 to 12, wherein a dust filter (3) is provided upstream of the first adsorber (6).

14. Process according to claim 13, wherein an elec-trostatic precipitator is used as the dust filter (3).

15. Process according to claim 13 or 14, wherein the fly dust (4) filtered off in the dust filter (3) is intensively heated.

16. Process according to claim 15, wherein the exhaust gas (35) formed in the intensive heating of the fly dust is passed to a third adsorber (36).

17. Process according to claim 16, wherein the adsorption material (26) of the third adsorber is sub-jected to an oxygen-free regeneration at a temperature which decomposes adsorbed toxic gases.

18. Process according to claim 17, wherein the adsorption material (26) of the third adsorber (36) is fed to the second regeneration stage (29).

19. Process according to claim 17 or 18, wherein the residence time of the adsorption material (26) of the third adsorber (36) during regeneration is half an hour or more.

20. Process according to claim 8 or 19, wherein the residence time is one hour or more.

21. Process according to one of claims 1 to 20, wherein activated coke is used as the adsorption material (7, 26).

22. Process according to one of claims 1 to 21, wherein the exhaust gas is passed through a quench enriched with NH3 upstream of the first adsorber (6).

23. Process according to one of claims 1 to 21, wherein NH3 is fed into the exhaust gas (1) upstream of the filter (3).

24. Process according to one of claims 1 to 23, wherein hydrochloric acid or NaCl is produced in a process step from the gas leaving the first adsorber (6).

Patent attorneys Gramm + Lins Li/ho/ne