AU604848B2

AU604848B2 - Process and apparatus for removing noxious substances from gas

Info

Publication number: AU604848B2
Application number: AU12117/88A
Authority: AU
Inventors: Peter Asmuth; Borchert Kassebohm; Chrysanth Marnet; Gunter Ritter; Gunter Wolfering
Original assignee: Rheinische Braunkohlenwerke AG
Current assignee: Rheinbraun AG
Priority date: 1987-02-26
Filing date: 1988-02-24
Publication date: 1991-01-03
Anticipated expiration: 2008-02-24
Also published as: JPS63232824A; ATE83166T1; DK97788D0; ES2035880T3; EP0283721B1; AU1211788A; DE3876464D1; DD271277A1; DK97788A; DE3706131A1; DK168878B1; ES2035880T5; EP0283721A3; EP0283721A2; EP0283721B2

Abstract

The noxious materials are removed from largely dedusted gases by passing the gas through a guard filter, consisting of activated carbon or coke, in order to precipitate heavy metals contained in the gas, before the latter reaches the treatment stage in which the nitrogen oxides are converted to nitrogen. <IMAGE>

Description

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FOR M 0L Si F Ref: 50939 '8 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Complete Specification Lodged: Accepted: Published: Class Int Class a 0 9 Priority: Related Art: Name and Address of Applicant: Address for Service: Rheinische Braunkohlenwerke AG.

Stuttgenweg 2 5000 Koln 41 FEDERAL REPUBLIC OF GERMANY Stadtwerke Dusseldorf AG.

Luisenstr. 105 4000 Dusseldorf 1 FEDERAL REPUBLIC OF GERMANY Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: Process and Apparatus for Removing Noxious Substances from Gas The following statement is a full description of this invention, including the best method of performing it known to me/us 5845/3

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ABSTRACT

In a process and an apparatus for removing noxious tI substances from a gas from which almost all dust has been eiairt removed, the gas is passed through a protective filter

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which employs activated carbon or coke, in order to s separate out heavy metals contained in the gas, before the gas passes into at least one treatment stage for dealing i 1 I'll. with at least one further noxious substance, as by S converting nitrogen oxides into nitrogen.

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i-i I r ~i-il~ r~ L; 0o o c o 0 C oo 0o o o ooa S 0 0 0 o o o o 0 0 0~ So o~ 0 a0 So o o 0 0 0 0 0 The present invention relates to a process for removing noxious substances from gas which preferably has a low dust content, and an apparatus for carrying out such a process.

For the purposes of cleaning for example flue gases from thermal power stations which are operated on fossile fuels, it is generally sufficient for dust to be removed from the flue gas and for the sulfur compounds and nitrogen oxides contained in the gas to be removed, if not :10 in their entirety at least to such an extent that there is no serious risk of causing environmental pollution by virtue of the presence of such substances in the gas.

However, processes and operating procedures which are used at the present time for cleaning for example flue gases 15 are not very useful for removing heavy metals from such gases. Heavy metals occur in particular in the flue gas from refuse incinerators and also in industrial process gases. Although in most cases the amounts of heavy metal which are to be found in the gas per unit of volume 20 thereof are very low, nonetheless when considering large amounts of gas, as is the case for example with flue gas, when applying the requirements and specifications which are normally applicable nowadays, the degree of environmental pollution caused by the presence of the heavy metals would be too great for them to be discharged

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2 with the gas into the atmosphere.

According to a first embodiment of the present invention there is provided a process for removing noxious substances from flue gas in which the flue gas is treated in a first treatment stage for removing sulfur compounds, then passed through a protecting filter containing activated coal or coke for removing heavy metals contained in the flue gas and then passed through at least one further treatment stage containing activated coal or coke for adsorption of remaining contents of sulfur compounds and HCl, the activated coal or coke used as adsorption means in the at least one further treatment stage being burned in a furnace the flue gas of which Is passed through the first treatment stage.

According to a second embodiment of the present invention there is provided an apparatus for removing noxious substances from flue gas including a first treatment stage for removing sulfur compounds, a 0 15 protective filter containing activated coal or coke for adsorption of heavy 0o 0 0o 000 metals contained in the flue gas, said protective filter disposed o0oo downstream of the first treatment stage in the flow direction of the flue gas, and at least one further treatment stage containing activated coal or coke for adsorption of remaining contents of sulfur compounds and HCl in 20 the flue gas, and further comprising means for conveying the activated coal 00 o 0 0 or coke used as adsorption means in the at least one further treatment ,o0 stage to a furnace the flue gas of which is passed through the first treatment stage for removing sulfur compounds.

0 00 The gas which is subjected to the treatment in accordance with the invention preferably has a low dust content, for example by virtue of removal of dust from the gas as a preliminary operation. The protective filter preferably comprises activated carbon or charcoal, or coke, more preferably hearth furnace coke produced for example from brown coal or lignite. I As will be seen in greater detail hereinafter the above-defined process of the invention, besides removing noxious substances from gas, is also capable of removing heavy metal contamination in the gas in a simple manner, and without adversely affecting the further removal of the other contaminating material that may be present in the gas. The step in the process for the removal of heavy metal is performed by means of material having a high capacity for carrying heavy metal, with the result that comparatively large amounts of heavy metal can be removed with small amounts of such material. Furthermore, in the process for removing noxious substances from gas, heavy metals contained in the gas can also be removed therefrom without adversely affecting operation of the equipment required for removing other contaminating material that 00 may be present in the gas and/or without the material 000000 o 10 which is used in such equipment or which is produced by virtue of the treatment of the gas experiencing significant contamination with heavy metals, which could r oso have an adverse effect on use and/or disposal of such material.

0o 15 In another aspect of the present invention, there is also provided apparatus for removing noxious substances O 00 from gas including a gas-treatment stage comprising a protective filter for adsorption of heavy metal contained in the gas, at least one further gas-treatment stage disposed downstream of the protective filter in the direction of flow of the gas through the apparatus, and means for passing the gas through the treatment stages.

The protective filter which is thus arranged upstream of at least one treatment stage in the direction of flow of the gas through the apparatus may comprise activated carbon and/or coke, serving for the adsorption of heavy metal contained in the gas.

It has been found that activated carbon and coke have H 5 an extremely high level of adsorption capability for heavy i metals, which produces an effect in two respects. On the i one hand, the loading capacity of such materials in respect of heavy metals is very high so that a comparatively small amount of activated carbon or charcoal 0 0 or coke can adsorb large amounts of heavy metal without 0 the adsorption capability being markedly adversely o0 0 effected. On the other hand, the high level of adsorption co o ii capability provides that adsorption takes place very Squickly. It has been found that extension of a bed 15 comprising coke or activated carbon or charcoal in the 0 G direction of flow of the gas through the apparatus of 100 Smm may be sufficient completely to remove mercury (Hg) contained in the gas. That means that only a small amount of material is required for adsorbing a large amount of heavy metal. That is important because the material which f is charged with heavy metals generally has to be disposed of separately, for example it cannot be burnt, as when the materials which are charged with the heavy metals are burnt, the heavy metals then pass into the flue gases and would have to be separated off again by means of a repeated adsorption operation. That would result in a progressive increase in the heavy metal content in the gas.

Another factor of importance in regard to the use of coke or activated carbon or charcoal for the adsorption of i heavy metals from the gas is that, if as is unavoidable in many situations coke or activated carbon or charcoal also adsorb other substances, for example sulfur compounds which are contained in the gas, that does not adversely o, O 10 affect the adsorption capability of the coke or activated 0 00 °0 0 carbon for heavy metals. Although sulfur compounds are 0 o also adsorbed by activated carbon or coke, it has been 00 0 0 0 0 0 0 found that the heavy metals which are adsorbed by those oooooo 00oo 0 materials displace any sulfur compounds which may already have been previously deposited, in the flow direction of 0 0 0 0o the gas within the filter, so that in spite of any sulfur compounds which may be contained in the gas to be cleaned, 00 0 0 0 0 0 the loading capacity of the coke or activated carbon for i adsorption of the heavy metals is still retained.

20 Therefore, the process in accordance with the present 0i invention can also be used to advantage when the gas to be cleaned also contains other contaminating material, as is the case for example with flue gas which normally also contains sulfur compounds and nitrogen oxides which have to be removed before the flue gas can be discharged to the ill i; atmosphere. For the purposes of substantially removing the sulfur compounds from the gas, the gas flows through a flue gas desulfurisation device which may be arranged upstream or downstream of the protective filter, in the direction of flow of the gas. It is also advantageously possible for the gas, after passing through the desulfurisation device and the protective filter, to be passed through a bed in which any sulfur compounds that still remain in the flue gas are adsorbed. It is also possible to provide a denoxing stage in which nitrogen oxides contained in the gas are converted into molecular nitrogen, by the addition of ammonia.

2 t When using the conventional beds comprising in St 1 particular coke or activated carbon or charcoal for the post-desulfurisation step and/or the denoxing operation, the heavy metals in the flue gas would admittedly be adsorbed by those beds so that they would not pass into i i Sthe atmosphere with the discharged gas. That applies at least to a certain extent even when the denoxing operation tC is carried out using a catalyst bed of another material, for example what is known as an SCR-catalyst (Selective Catalytic Reduction catalyst). At any event however problems would occur at other points. In addition, when using SCR-catalysts there is the fear that they may suffer damage from the heavy metals so that those ca-calysts, 6 which are very expensive, would no longer enjoy the required catalytic activity after just a short period of time.

If the beds of coke and in particular hearth furnace coke which are used for the above-mentioned postdesulfurisation and/or dniioxing operations are not contaminated by heavy metals, then the disposal thereof does not give rise to any difficulties. The coke from the bed for the post-desulfurisation operation can be recycled 10 to the boiler equipment whose flue gases are to be S, cleaned, in the charged condition, that is to say with the sulfur compounds that the coke material has removed from the gas. Although in the boiler equipment mentioned above, the sulfur compounds do at least in part get back into the flue gases, they are easily removed therefrom in a flue 1 gas desulfurisation device of conventional kind which is Sdisposed upstream of the post-desulfurisation bed. As coke o is comparatively inexpensive combustion of the coke which I is charged with the sulfur compounds therefore represents

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20 the easiest and most economically viable form of disposal.

In addition, when using a coke bed as a catalyst for the denoxing operation, if no other substances are adsorbed by the bed, then theoretically there is no consumption whatsoever as the coke only acts as a catalyst. If however, as is inevitable in a practical situation, the 7 if t r I- i- i i coke bed catalyst nonetheless must be replaced at certain periods of time, then the coke which has been used can also be readily recycled to the boiler plant.

In contrast, if the cokes from both beds were also charged with heavy metals, then recycling that coke material to the boiler plant and burning it therein would result in an increase in the content of heavy metals in the flue gas as the heavy metals which are introduced into the boiler plant with the recycled coke would pass into 10 the flue gas again, to a more or less complete extent.

When using other catalysts, for example the abovementioned SCR-catalysts, accumulation of heavy metals S thereon would result in the catalytic effectiveness thereof being very seriously affected. The removal of 15 heavy metal deposits from such catalysts is a very expensive operation.

Although it would probably be possible, when carrying out a conventional wet desulfurisation operation, using basic sorbents, for example soda lye (NaOH), limestone 20 (CaCO 3 or slaked lime (Ca(OH) 2 in the flue gas desulfurisation device, for at least a part of the heavy metals to be separated out of the flue gas with the sulfur in that stage of operation, that would however have the result that the waste products which are produced in the wet desulfurisation operation and which are anhydrite (CaS04) and gypsum (CaS04.2H 2 0) would contain the washed- Out heavy metals so that because of their heavy metal content, the disposal or other utilisation of such waste products which could be used for example in the building industry gives rise to difficulties.

Use of the protective filter in accordance with the present invention for separating the heavy metals out of the gas does at any event simplify the operating procedure i involved, in particular therefore including in those 10 situations of use in which, as is generally the case, V there are other contaminating materials and impurities which have to be removed from the gas, while also

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involving the necessity to dispose of the carrier toI 00 materials for the contaminating substances. The presence 15 of heavy metals in those materials which are charged with 19 II other contaminating materials would considerably 04 complicate the disposal thereof but that is avoided in a simple and inexpensive fashion by means of the procedure of the process of the invention. That also applies moreover in regard to activated carbon or cokes which are used for the removal of unburnt hydrocarbons, for example polycyclic hydrocarbons and dioxins which occur in particular in flue gases from refuse incinerators. The cokes or activated carbons or charcoals which are charged with those substances can be readily burnt in order to dispose thereof as the substances are at any event converted predominantly into harmless compounds, under the effect of the high firing temperatures involved. If activated carbon and coke were simultaneously charged with heavy metals, that would not be a possible mode of operation if an increase in the heavy metal content in the flue gas is to be avoided.

j In a preferred embodiment the protective filter comprises a bed of hearth furnace coke which is a S' ,.10 comparatively inexpensive material with a high adsorption I capability. It is also possible however to use other cokes or activated carbon for this purpose.

The above-mentioned hearth furnace coke has quite I t particularly advantageous properties in regard to its adsorption capability both in respect of heavy metals and t 0 in respect of other contaminating materials. The V production of hearth furnace coke and the properties iji a thereof will be generally familiar to the man skilled in the art, being described for example in the following publications: I H.B. Kbnigs: 'Feinkokserzeugung durch Braunkohle', Energiewirtschaftliche Tagesfragen, 27th volume 1977, issue 8/9, pages 569-599; E. Scherrer: 'Herstellung von Braunkohlenkoks im Salem- Lurgi-Herdofen', Braunkohle, issue 7, July 1981, pages 242 246; and 2T1 D. Boecker: 'Edle Korner', Energie, volume 35, issue 3, 1983.

The advantages of the operating procedure of the invention may be summarised along the lines that the items of equipment for cleaning the gas, which are disposed j downstream of the protective filter, are not contaminated by heavy metals or, if they are so contaminated, such contamination is only to such a low degree that, when using coke in those items of equipment, that is to say for o 10 example as an adsorption agent for the removal of sulfur 0...0 compounds and/or as a catalyst for a denoxing operation, o0 it is readily possible for that coke to be burnt for 00 0 0 0 00 example in a boiler plant so that disposal of that ooooo000000 0 0 material does not give rise to difficulties. When using So 15 other catalysts, for example SCR-catalysts, there is no o fear that they may be poisoned by heavy metals and thus 0 00 suffer a serious reduction in their level of catalytic effectiveness, in a short period of time. In addition, any flue gas desulfurisation device which may be included in 20 the installation can be operated in such a way that no Sheavy metals are separated out therein, so that the sulfur-bearing products of the flue gas desulfurisation device, being therefore generally gypsum and/or anhydrite, do not suffer from any serious contamination due to heavy metals.

11 L- Although it is generally necessary for the material which forms the protective filter and which is charged with the heavy metals removed from the flue gas to be disposed of in a particular fashion as it normally cannot be introduced into and burnt in the boiler plant for example, the operating procedure of the present invention does however give the advantage that, as the heavy metals are entirely or at least predominantly adsorbed in the protective filter and the latter has a comparatively long service life, the amount of material which is charged with heavy metal and which thus has to be disposed of in a particular fashion is small.

Due to the high loading capacity of activated charcoal or carbon, or coke, in respect of heavy metals, depending on the respective factors involved, the service life of the filter can be determined to a greater degree by the dust content of the gas such as flue gas which flows through the filter, than by the heavy metal content thereof. Although generally before the flue gas reaches the protective filter it is passed through a device for removing dust therefrom, for example an electrostatic filter, nonetheless the flue gas still contains residual amounts of dust, even if only small, which are separated out in the protective filter. That means that the protective filter becomes clogged by the dust, in the 7 .i I I 0 00000 0 0 0 0 0 00O 00 0 D 00 do o o eo o o o~ passage of time. Normally, that will happen at a time which is long after the time at which the filter is saturated with heavy metals so that the latter pass through. In order to ensure tht the filter material does not have to be replaced and disposed of simply because of the increase in flow resistance caused by dust accumulating therein, it is readily possible for the filter material to be subjected to a grading or screening operation, for example by sieving, in order in that way to 10 separate from the filter material the very fine-grain dust which is separated out of the flue gas in the filter, whereby, as long as the filter material is not yet saturated with heavy metals, it can continue to be used in the protective filter.

An embodiment of the process and apparatus according to the invention will now be described by way of example with reference to the accompanying drawing in which the single Figure shows a flow chart diagrammatically illustrating an appa.atus for cleaning flue gas.

Referring now to the drawing, reference numeral 1 indicates a plant such as a thermal power station or a 460* refuse incinerator producing flue gas as indicated at 2.

The flue gas firstly flows through a dust separator device illustrated in the form of an electrostatic filter 4 in which the dust contained in the flue gas 2 is substantially removed therefrom. The flue gas with its reduced dust content, as indicated at 5, is then passed into a desulfurisation unit 6 into which are introduced basic sorbents, for example Ca0 3 as indicated at 8. The 31 unit 6 is operated in such a way that any heavy metals 0 oo 10 present in the flue gas 5 are not separated off therein or oo are separated off only to such a slight extent that the o o0a products leaving the desulfurisation unit 6 at 10, being o0 0 o 0 °o anhydrite (CaSO 4 and gypsum (CaS0 4 .2H 2 0) or calcium o00000 sulfite compounds which are produced by reaction of the sulfur compounds contained in the flue gas 5 with the O 00 basic sorbents 8, are not contaminated by heavy metals or are contaminated only to a negligible degree.

The flue gas 12 which is thus substantially freed of sulfur compounds then firstly flows through a further dust 20 removal unit, illustrated for example in the form of an electrostatic filter 3, and then a protective filter 14.

The dust which is removed in the filter 3 can be added to the products 10 which are discharged from the desulfurisation unit 6 as that dust is of substantially the same chemical composition. The protective filter 14 substantially comprises a housing filled with for example activated carbon or charcoal, or coke, preferably hearth furnace coke. In the protective filter 14 the heavy metals contained in the flue gas 12 are adsorbed by the coke to such an extent that the residual heavy metal content that may possibly remain in the flue gas leaving the protective filter 14 at 24 is negligibly small.

When the heavy metal adsorption capability of the coke indicated at 15 which provides for the adsorption of the heavy metals which are normally predominantly if not S.exclusively mercury and cadmium is exhausted, the coke is removed from the protective filter 14 as charged coke, as indicated at 16, and disposed of in a suitable fashion. By virtue of the high adsorption capability in particular of hearth furnace coke in respect of heavy metals, the protective filter 14 generally has a very long service life so that the protective filter 14 normally becomes clogged by the residual dust content which still remains in the flue gas 13 even after having passed through the electrostatic filter 3, before the adsorption capability of the coke in the protective filter 14 for adsorbing heavy metals is exhausted. To take account of that i clogging phenomenon, the illustrated system provides the option that the coke from the protective filter 14 can be subjected at certain periods of time, the duration of which depends on the residual dust content in the flue gas to a grading or screening operation using for example a sieve as indicated at 18 in the drawing, to remove the dust from the filter bed of the protective filter 14, which is formed by the coke. The material which passes through the sieve 18, as indicated at 20, is quite predominantly formed by the dust which has accumulated in the protective filter. The noticeably larger grains of coke which do not therefore pass through the sieve, as indicated at 22, can be re-introduced into the protective filter 14.

The flue gas which issues at 24 from the protective filter 14 and from which heavy metals have been removed is then passed through an adsorption bed 26 which also comprises coke 27, for example and preferably hearth furnace coke, and which serves for removal of any sulfur compounds which may still remain in the flue gas after passing through the desulfurisation unit 6. Those residual sulfur compounds are thus adsorbed by the coke in the adsorption bed 26.

The flue gas at 28 which is thus practically free of sulfur then passes into a further bed 30 which is also charged with coke 29, preferably hearth furnace coke, and which serves to remove hydrochloric acid from the flue gas 28. Ammonia is then added to the flue gas 32, as indicated at 34. The gas then passes into a denoxing stage 36 which provides for catalytic reduction of the nitrogen oxides contained in the flue gas 32, to provide molecular nitrogen and water. That reaction takes place in accordance with the following equation: 4 NH 0 '4 N 6H0.

3 2 2 2 In that way, the nitrogen oxides contained in the flue gas 32 can be reduced by 80 to 90% by means of activated carbon, coke or possibly an SCR-catalyst.

10 In the subsequent stage 38, the flue gas coming from the denoxing stage 36, at 40, is passed through a further bed 38 of coke, preferably acid-charged hearth furnace coke, in which the unreacted amount of ammonia 3/ which was added to the flue gas 32 for the denoxing operation is removed from the flue gas 40. The gas 42 which has been cleaned in that manner can then be discharged to atmosphere.

In the embodiment illustrated in the drawing, the treatment stages 26, 30 and 38 are in the form of travelling beds. The amounts of charged coke which are removed from the stages 26 and 30, as indicated at 44 and 46, are firstly passed into the stage 38 in which the unreacted ammonia 34 is removed from the gas 40 by adsorption. The coke leaving the stage 38, at 48, is introduced into the plant 1 where it is burnt. If the coke from the denoxing stage 36 has to be replaced, it can also be burnt in the plant 1 (that option is not shown).

The material 20 which passes through the sieve 18, being therefore the dust which had been removed from the flue gas 12 in the protective filter 14, can also be recycled to the plant 1. Although it must be borne in mind that the material 20 which passes through the sieve 18 also has a small amount of coke material which has come from the protective filter 14 as a result of abrasion from S 10 the coke therein, pith that abrasion material being 0 000a 0:00 charged with heavy metals to a greater or lesser degree, o the amount of such material in relation to the total 000 amount of coke coming from the protective filter 14 is so 0 00 0 0 small that the quantity of heavy metals which passes therewith into the plant 1 by way of the sieving operation at 18 is not significant. At any event it is so slight that it is adsorbed by the protective filter 14 when it a, passes therethrough. The same situation arises if the dust which passes through the filter 18 has also picked up small amounts of heavy metals.

It will be appreciated that the protective filter 144 will remove not only heavy metals but also other contamination materials, insofar as they are capable of being adsorbed thereat, for example s;ulfur compounds.

However that does not adversely affect the adsorption capability of the carbonaceous material present in the protective filter 14. At some time the protective filter 14 will become saturated with the sulfur compounds and will thus not be able to adsorb any further such compounds. That is immaterial however as the bed 26 is provided in any case for removal of the residual sulfur compounds which are still contained in the flue gas 24 leaving the protective filter 14. A similar point also applies in regard to the coke 15 in the protective filter 1 0 14 becoming charged with HCl, although it has been found o that in the adsorption operations SO on the one hand and o 02 oo HCl on the other hand influence each other in such a way 00 o o 0 0 that the concentration of S02 or the H2s4 formed therefrom decreases with the depth of bed whereas the concentration of HCl increases in the bed in the direction 0 o of flow of the gas. That is to be attributed to the fact a that S0 2 or H 2 So 4 detaches HCl which has already been 0 0 0 0 adsorbed by the coke, from the surface of the coke again, and displaces it in the direction of flow. For that reason ooo 0 20 it may be desirable to provide separate beds for the i adsorption of S02 on the one hand and HCl on the octher i. hand, although, departing from the embodiment illustrated in the drawing, it is also readily possible to combine together a plurality of beds, possibly even under the influence of the bed 36 which serves for denoxing purposes.

19 j- i Various modifications may be made in the embodiment of the invention illustrated in the drawing. Thus a j further dust removal device may be disposed between the desulfurisation unit 6 and the protective filter 14. It is also possible to provide only one dust removal device which is disposed between the desulfurisation unit 6 and the protective filter 14. That does not alter the fundamental operating procedure involved, the crucial consideration being the provision of the protective filter 14 for adsorbing the heavy metals from the flue gas 12 so that the substances used in the following cleaning stages as adsorption agents and/or catalysts are not contaminated or at any event are not contaminated to a substantial degree by heavy metals in the flue gas.

The above-described process and apparatus can be .applied in particular to the cleaning of flue gas from refuse incinerators and industrial gases without however being restricted thereto. On the contrary it is entirely !i possible for flue gases which are produced by the I j 20 combustion of fossile fuels also to contain heavy metals, even if in smaller quantities. By virtue of the large amounts of flue gases involved, even if the gas has a low specific content of heavy metals, that can still give rise to serious environmental pollution which can thus be at least reduced by means of the process and apparatus described above. In that context also the high level of Mrw- 4 r concentration of noxious substances in the material forming the protective filter represents an advantage which is to the benefit in particular of the economy of the process as a small amount of filter material is required for removing the heavy metals from the flue gas and consequently only correspondingly small amounts of filter material charged with heavy metals need to be disposed of. The material of the protective filter is also generally readily available, a preferred material being 10 hearth furnace coke which has been produced for example from brown ccal.

It will be appreciated that the above-described process and apparatus have been set forth solely by way of example and illustration of the present inventionand other modifications may be made therein.

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Claims

1. A process for removing noxious substances from flue gas in which the flue gas is treated in a first treatment stage for removing sulfur compounds, then passed through a protecting filter containing activated coal or coke for removing heavy metals contained in the flue gas and then passed through at least one further treatment stage containing activated coal or coke for adsorption of remaining contents of sulfur compounds and HC1, the activated coal or coke used as adsorption means in the at least one further treatment stage being burned in a furnace the flue gas of which is passed through the first treatment stage.

2. A process according to claim 1 wherein the flue gas after passing through the protective filter and the at least one further treatment stage flows through a stage for conversion of nitrogen oxides Into nitrogen.

3. A process according to claim 1 or claim 2 wherein upstream of the protective filter the gas flows through a means for removing dust from the gas.

4. A process according to any one of claims 1 to 3 wherein hearth furnace coke is used for the protective filter.

A process according to any one of the preceding claims wherein residual dust which is deposited from the gas in the protective filter is separated from the filter material by a classifying operation.

6. A process according to any one of the preceding claims wherein the activated coal or coke in the further treatment stage is hearth furnace coke.

7. A process according to any one of the preceding claims wherein hearth furnace coke is used in the stage for conversion of nitrogen oxides into nitrogen.

8. A process for removing noxious substances from flue gas, substantially as hereinbefore described with reference to the accompanying drawing.

9. An apparatus for removing noxious substances from flue gas including a first treatment stage for removing sulfur compounds, a protective filter containing activated coal or coke for adsorption of heavy metals contained in the flue gas, said protective filter disposed KNK: 9Y ly '0 4 23 downstream of the first treatment stage in the flow direction of the flue gas, and at least one further treatment stage containing activated coal or coke for adsorption of remaining contents of sulfur compounds and HC1 in the flue gas, and further comprising means for conveying the activated coal or coke used .as adsorption means in the at least one further treatment stage to a furnace the flue gas of which is passed through the first treatment stage for removing sulfur compounds.

An apparatus according to claim 9 further including a treatment stage in the form of a means for denoxing of the gas and wherein said further treatment stage containing activated coal or coke is disposed between said protective filter and said denoxing stage.

11. An apparatus according to claim 10, wherein hearth furnace coke is used in the treatment stage for denoxing the gas.

12. An apparatus according to any one of claims 9 to 11, wherein said protective filter at least predominantly comprises hearth furnace coke which has been produced from brown coal.

13. An apparatus according to any one of claims 9 to 12, wherein the activated coal or coke in the further treatment stage is hearth furnace coke.

14. An apparatus for removing noxious substances from flue gas, substantially as hereinbefore described with reference to the accompanying drawing. DATED this FIRST day of AUGUST 1990 Rheinische Braunkohlenwerke AG Stadtwerke Dusseldorf AG I Patent Attorneys for the Applicants SPRUSON FERGUSON KWK:910y i u^,;A