DD271277A1 - METHOD AND APPARATUS FOR REMOVING GAS CONTAMINANTS - Google Patents

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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Process and installation for removing pollutants from gas

Field of application of the invention:

The invention relates to a method for removing pollutants from preferably low-dust gases by adsorption of at least part of the pollutants on activated carbon or coke.

Characteristic of the known state of the art: In order to purify, for example, flue gases coming from thermal power plants operated with fossil fuels, it is generally sufficient to de-dust the flue gas and remove the sulfur compounds and nitrogen oxides contained in it completely or to such a large extent, that an environmental impact caused by these impurities is not to be feared. However, the known methods and measures for purifying, for example, flue gases are of little use for the removal of heavy metals from such gases. Heavy metals occur in particular in the flue gas of waste incineration plants, but also in industrial process gases. Although the amounts of heavy metal present per unit volume of gas are very low in most cases. For the large gas volumes, z. As in flue gas, however, would be the application of today's usual requirements, the burden of the environment by the heavy metals too large, than that they could be discharged with the gas in the atmosphere.

Object of the invention:

Accordingly, the object of the invention is to design the method of the kind described in the introduction in such a way that even heavy metals which are present as impurities in the gas can be easily removed therefrom.

without the function of the facilities required to remove any additional contaminants present, any deterioration and / or the materials used in or derived from the treatment of the gas may suffer significant contamination by heavy metals, and / or or discourage disposal of these materials. Further, it is desired that the material loaded with the heavy metals removed from the gas has a large heavy metal loading capacity, so that large amounts of heavy metal can be bound with small amounts of this material and thus the disposal of the loaded material is easy ,

Explanation of the essence of the invention:

To achieve this object, the invention proposes that the gas is passed through an existing of activated carbon and / or coke protective filter in which heavy metals are removed from the gas by means of adsorption by the material forming the protective filter.

It has been found that activated carbon and coke have an extraordinarily high adsorption capacity for heavy metals, which has a twofold effect. Once the loading capacity of these materials for heavy metals is very large, so that a relatively small amount of activated carbon or coke can absorb large amounts of heavy metal without the adsorption capacity suffers noticeable losses. On the other hand, the good adsorption capacity has the effect that the adsorption is very fast. Thus, it has been found that an extension of a bed of activated carbon or coke sufficient in the flow direction of the gas of 100 mm

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can to completely remove in the gas-containing mercury (Hg). Thus, little material is needed to adsorb a large amount of heavy metal. This is important because the loaded with heavy metals material must be disposed of separately in general, so for example can not be burned, as in the combustion of the heavy metals laden materials, the heavy metals get into the flue gases and would have to be deposited again by adsorption. This would lead to a constant increase in the heavy metal content in the gas.

It is also important for the use of activated carbon or coke for the adsorption of heavy metals that if, as is unavoidable in many cases, activated carbon or coke also adsorbs other substances, for example sulfur compounds contained in the gas, the adsorption capacity of activated carbon or coke for the heavy metals not impaired. Although sulfur compounds of activated carbon or coke are also adsorbed. However, it has been found that the heavy metals adsorbed by these materials possibly displace previously deposited sulfur compounds in the flow direction of the gas within the filter, so that the loading capacity of activated carbon or coke for the heavy metals is maintained despite possibly present in the gas to be cleaned sulfur compounds.

The method according to the invention is therefore also applicable with advantage if the gas to be purified contains other impurities, such as. As is the case with flue gas, which normally also contains sulfur compounds and nitrogen oxides, which must be removed before the flue gas can be released into the atmosphere. For the substantial removal of the sulfur compounds durch-

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the gas flows through a flue gas desulphurisation device, which can be arranged in the direction of flow before or after the protective filter. In addition, there is the possibility that the gas is passed after passing through the desulfurization device and the protective filter through a bed in which remaining sulfur compounds are adsorbed in the flue gas. In addition, a denoxing stage can be provided, in which nitrogen oxides contained in the gas are converted to molecular nitrogen with the addition of ammonia.

When using the usual beds in particular from coke or activated carbon for desulphurisation and / or denoxing the heavy metals in the flue gas would indeed adsorbed by these beds, so that they did not get into the atmosphere with the flue gas. This also applies, at least to a certain extent, if a catalyst bed made of a different material, for example an SCR (Selective Catalytic Reduction) catalyst, is used for the denoxing. In any case, however, problems would arise elsewhere. In addition, when using SCR catalysts there is the fear that these are damaged by heavy metals, so that these very expensive catalysts no longer have the required catalytic activity after a short time.

If the beds of coke used for desulphurisation and / or denoxing, in particular hearth furnace coke, are not contaminated by heavy metals, the disposal poses no difficulties whatsoever. The coke from the bed for the desulphurisation can be returned to the furnace, whose flue gases are to be cleaned, when loaded, ie with the sulfur compounds it absorbs. Although the sulfur compounds reach the flue gases at least partially there,

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However, are removed without difficulty in a flue gas desulfurizer of conventional design, which is connected upstream of the bed for the desulphurisation. Thus, since coke is relatively cheap, combustion of the coke loaded with the sulfur compounds is the simplest and most economical way of disposal. When using a coke bed as a catalyst also for denoxification, if no other substances are adsorbed by this bed, theoretically none at all Consumption, since the coke acts only as a catalyst. If, as is inevitable in practice, it must nevertheless be replaced at certain intervals, the spent coke can also be returned to the furnace without difficulty.

On the other hand, if the cokes from both beds were also loaded with heavy metals, recirculation and incineration of this coke into the furnace would result in an accumulation of heavy metals in the flue gas, since the heavy metals introduced into the furnace with the coke return to the flue gas to a greater or lesser extent , When using other catalysts, for example the aforementioned SCR catalysts, the addition of heavy metals would result in a significant impairment of the catalytic activity. The removal of heavy metal deposits from these catalysts is very expensive.

While it would probably be possible using conventional wet desulfurization using basic sorbents, e.g. As sodium hydroxide (NaOH), limestone (CaCCU) or slaked lime (Ca (OH) ₂ ) _{in the} flue gas desulfurization at least part of the heavy metals with the sulfur from the flue gas excrete. However, this would have the consequence that in the wet desulfurization

accumulated waste products in which it is anhydrite (CaSO.) and gypsum _(CaSO4. 2H ₂ O) is, the washed Schweriretalle contained, so that the disposal or other use of these waste products, which could be used for example in the building industry, due to their content of heavy metals causes difficulties.

The application of the protective filter according to the invention for separating the heavy metals from the gas simplifies the process in any case, especially in those applications in which, as is usually the case, still other impurities must be removed from the gas, also there is a need to dispose of the carrier materials for the contaminants. The presence of heavy metals in these carrier materials loaded with other contaminants would in any case considerably complicate their disposal. This is avoided by the application of the teaching according to the invention in a simple and cheap way. This also applies to activated carbon or cokes, which are used to remove unburned hydrocarbons, eg. As polycyclic hydrocarbons and dioxins, which occur especially in flue gases from waste incineration plants, are used. The loaded with these substances activated carbons or coke can be easily burned for disposal, since the substances are at least predominantly converted to harmless compounds under the action of the high firing temperatures. With simultaneous loading of activated carbon and coke with heavy metals, this would not be possible if an accumulation of heavy metals in the flue gas should be avoided.

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The protective filter is most conveniently a bed of hearth furnace coke, which is relatively cheap and has a high adsorptivity. But it is also possible to use other cokes or activated carbon for these purposes.

The aforementioned hearth furnace coke has particularly favorable properties in terms of the adsorption ability for both heavy metals and the other impurities. The production of the hearth furnace coke and its properties are described in the following publications:

<H.B. King: "Coke production by lignite", Energiewirtschaftliche questions of the day, 27. Jg. 1977, Issue 8/9 P. 569 - 599;

E. Scherrer: "Production of lignite coke in Salem-Lurgi hearth furnace", brown coal, Issue 7, July 1981, pp. 242-246;

D. Boecker: "Noble grains", energy, Jg. 35, Issue 3,

The advantages of the process control according to the invention can be summarized in that the downstream of the protective filter devices for the cleaning of the gas by heavy metals are not contaminated or only to such a small extent that it is readily possible when using coke for these devices, ie For example, as an adsorbent for the removal of sulfur compounds and / or as a catalyst for the denoxification, to burn this coke in a furnace, so that a problem-free disposal is possible. When using other catalysts, such as SCR catalysts, it is not to be feared that this

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Heavy metals are poisoned and experienced in a short time a significant reduction in their catalytic effect. Furthermore, a possibly existing flue gas desulphurisation device can be operated so that no heavy metals are deposited therein and thus the sulfur-containing products of the flue gas desulphurisation device, ie in general gypsum and / or anhydrite, have no significant heavy metal impurities.

Although there is a need to dispose of the protective filter forming material, which is loaded with the separated from the flue gas heavy metals, in a special way, since it can not normally be brought into the furnace and burned there. However, the process control according to the invention has the advantage that, since the heavy metals are completely or at least predominantly adsorbed in the protective filter and the latter has a relatively long life, the amount of material loaded with heavy metal, which is to be disposed of in a special way, remains low.

Due to the large loading capacity of activated carbon or coke for heavy metals, the service life of the protective filter may be more largely determined by the dust content of the gas flowing through the filter, e.g. Flue gas than by its content of heavy metals. Although, in general, the flue gas, before it reaches the protective filter, go through a dedusting, such as an electrostatic filter. However, even if small dust residues remain in the flue gas, which are deposited in the protective filter, with the result that the protective filter is added by this dust over time. Usually this will happen at a time

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be long before the time at which the filter is saturated with heavy metals, the latter so penetrate. In order to avoid that the filter material must be replaced and disposed of only due to the caused by the accumulating dust dust increase in flow resistance, there is easily the possibility to subject the filter material to a classification process, for example by sieving, so as in the filter from the Separate flue gas deposited fine-grained dust from the filter material, so that this, as long as it is not saturated with the heavy metals, can be used in the protective filter.

The invention is particularly applicable to, but not limited to, purifying flue gas from waste incinerators and industrial gases. Rather, there is a good chance that flue gases produced by the combustion of fossil fuels also contain heavy metals, albeit in smaller quantities. Due to the large amounts of flue gases, even with a specific low content of heavy metals, a noticeable environmental pollution can arise, which can be avoided by the use of the invention. Again, the high concentration of pollutants in the material forming the protective filter is an advantage that promotes economic efficiency in particular, since little filter material is required to separate the heavy metals from the flue gas and therefore only need to dispose of correspondingly small amounts of filter material loaded with heavy metals.

Examples:

In the drawing, the flow diagram of a plant for purifying flue gases is shown.

The flue gas 2 coming from a waste incineration plant or a power plant 1 first flows through an electrostatic precipitator 4, in which it is largely dedusted. The dedusted flue gas 5 is passed into a desulfurization б in the basic sorbents, such as CaCO ₃ , 8 are entered. This plant is operated so that 5 heavy metals present in the flue gas are not or only to a limited extent deposited, so that the desulfurization leaving the leaving products 10, anhydride (CaSO ₄ ) and gypsum (CaSO ₄ .2H-O) or calcium sulfite compounds by Implementation of the sulfur compounds contained in the flue gas 5 with the basic sorbents 8 have arisen, are not contaminated by heavy metals or only to a negligible extent.

The largely freed of sulfur compounds Rauchqas 12 first flows through a further dedusting, z. Example, an electrostatic precipitator 3, and then a protective filter 14. The dust 7 deposited in the electrostatic precipitator 3 can be added to the products 10 discharged from the desulfurization device 6 since it has substantially the same chemical composition. The protective filter 14 consists essentially of a coke, preferably Herdofenkoks, filled housing. In this protective filter 14, the heavy metals contained in the flue gas 12 are so largely adsorbed by the coke that the possibly remaining in the flue gas 24 residual levels of heavy metal are negligible.

The coke 15 serving to adsorb the heavy metals, which will normally be predominantly, though not exclusively, mercury and cadmium, when its heavy metal adsorption ability is exhausted, is taken out of the protective filter 14 as laden coke 16 and is suitably used Disposed of.

Due to the large adsorptive capacity, in particular Herdofenkoks for heavy metals, the protective filter 14 has a very long life, so that due to the remaining in the flue gas 13 after the electrostatic precipitator 3 remaining dust content, the protective filter 14 is usually clogged by this dust rather than the Adsorptionsfähiqkeit therein contained coke is exhausted for heavy metals. It is therefore provided that the coke from the protective filter 14 at certain time intervals, the length of which depends on the residual dust content of the flue gas 5, a classification process using, for example, a screen 18 is subjected to the dust from the coke formed by the filter bed 14 remove. The passage 20 of the screen 18 is predominantly formed by this dust. The noticeably coarser coke grains 22, which form the screen overflow, can be reintroduced into the protective filter 14.

The coming out of the protective filter 14, purified by heavy metals flue gas 24 qeleitet by an adsorbent 26, which also consists of coke 27, preferably Herdofenkoks and to remove the after passing through the desulfurization 6 in the flue gas 12 remaining sulfur compounds used, which are adsorbed by the coke ,

The thus virtually sulfur-free flue gas 28 passes into another bed 30, which is also charged with coke 29, preferably Herdofenkoks and serves to remove the hydrochloric acid from the flue gas 28. Following this, the flue gas 32 ammonia 34 is added. In the subsequent denoxing stage 36, the catalytic reduction of the nitrogen oxides contained in the flue gas 32 to molecular nitrogen and water takes place. This conversion proceeds according to the following equation:

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4 NO + 4 NH ₃ + O ₂

In this way, with the help of activated carbon, coke or possibly SRC catalysts existing in Rauchgas32 nitrogen oxides can be reduced by 80 - 90%.

In the subsequent stage 38, the flue gas 40 coming from the denoxing stage 36 is passed through another bed 38 of coke, preferably acid-loaded hearth furnace coke, in which the unreacted quantities of the ammonia 34 added to the flue gas 32 for denoxification are removed from the flue gas 40. The thus purified flue gas 42 can then be released into the atmosphere.

The treatment stages 26, 30 and 38 are formed in the embodiment shown in the drawing as moving beds. The loaded amounts of coke 44 and 46 withdrawn from the stages 26 and 30 are first led into the stage 38, in which the unreacted ammonia 34 is removed from the flue gas 40 by adsorption. The coke 48 leaving the stage 38 is brought into the furnace 1 and burnt there. The coke from the denoxing stage 36, when it needs to be replaced, can also be burned in the furnace (not shown).

The passage 20 of the screen 18, so the dust that had been removed from the flue gas 12 in the protective filter 14 can also be returned to the furnace 1. Although it must be expected that the screen passage 20 also has a small proportion of abrasion of the coke from the protective filter 14, said abrasion is more or less heavily loaded with heavy metals. However, the proportion of abrasion of the total amount of coke from the protective filter 14 is so low that with him about

the sieve passage 20 into the furnace 1 reaching amount of heavy metals is not significant. In any case, it is so low that it is adsorbed by the latter when passing through the protective filter 14. The same applies if the dust 20 should have taken up small amounts of heavy metals.

Of course, the protective filter 14 will receive not only heavy metal but also other contaminants as far as they are adsorbable, for example sulfur compounds. However, this does not impair the adsorptive capacity of the carbonaceous material present in the protection filter 14. At some point, the protective filter 14 will be saturated with the sulfur compounds and thus can not adsorb any further sulfur compounds. However, this is irrelevant since the bed 26 is provided anyway for the removal of the residual sulfur compounds still contained in the flue gas 24. The same applies to the loading of the coke 15 in the protective filter 14 with HCl, although it has been found that in the adsorption processes SO on the one hand and HCl on the other hand influence each other such that the concentration of SO ₂ and the H ₂ SO formed therefrom. decreases with the bed depth, whereas the concentration of HCl increases in the direction of flow in the bed. This will be due to the fact that SO ₂ or H ₂ SO ₄ already removed from the coke adsorbed HCl again separates from the coke surface and displaced in the flow direction. For this reason, it may be appropriate to provide for the adsorption of SO ₂ on the one hand and HCl on the other hand separate beds, although it is also possible deviating from the in the drawing darqestellten embodiment, several beds, possibly also under the influence of the denoxation serving Bed 36, to summarize.

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There are still other modifications of the embodiment shown in the drawing possible. Thus, a further de-dusting device can be provided between desulphurisation device 6 and protective filter 14. It is also possible to provide only a dedusting device arranged between desulphurization device b and protective filter 14. This does not change the basic procedure. It is crucial that a Schutzfilteri 4 is provided which adsorbs the heavy metals from the flue gas 12, so that the substances used in the subsequent purification stages as adsorbents and / or catalysts by the heavy metals are not or at least not significantly contaminated.

Claims (14)

claims: 1. A method for removing pollutants from preferably low-dust gas by adsorption of at least part of the pollutants of activated carbon or coke, wherein the gas flows through at least two successively located in the flow direction treatment stages, characterized in that the gas for the deposition of contained in it Heavy metals is passed through a protective filter before the gas enters at least one further treatment stage, in which at least one further pollutant is separated from the gas. 2. The method according to claim 1, characterized in that the gas also flows through a gas desulfurization. 3. The method according to claim 2, characterized in that the gas flows through the gas desulphurisation device after it has passed the protective filter. 4. The method according to claim 2, characterized in that the gas flows through the gas desulphurisation device before it passes through the protective filter. 5. The method according to any one of claims 1 to 4, characterized in that the gas is passed after passing through gas desulphurisation and protective filter through a bed in which sulfur compounds still contained in the gas are adsorbed. 27t 277 6. The method according to claim 1, characterized in that the gas flows through a stage for the conversion of nitrogen oxides to nitrogen after passing through the protective filter. 7. The method according to claim 1, characterized in that the gas flows through a device for dedusting the gas in front of the protective filter. 8. The method according to claim 5, characterized in that for the protective filter Herdofenkoks is used. 9. The method according to claim 1, characterized in that the settling in the protective filter residual dust from the gas is separated by a Klassiervorganq of the protective filter forming substances. An apparatus for removing pollutants from gases having at least two treatment stages, at least one of which is provided with means for receiving an activated carbon or coke adsorbent, and means for passing the gas through the treatment stages and for renewal therein located adsorbent, characterized in that in the flow direction before at least one of the treatment stages (6, 26, 30, 36, 38) provided with an activated carbon or coke protective filter (14) is arranged, which serves to adsorb the heavy metals contained in the gas. 11. Plant according to claim 10, characterized in that the protective filter (14) in the flow direction of the gas in front of a gas desulfurization device (6) formed treatment stage is arranged. 2 7 ί 277 12. Plant according to claim 10, characterized in that the protective filter (14) in the flow direction of the gas behind a gas desulfurization device (6) formed treatment stage is arranged. 13. Plant according to claim 10, characterized in that between the protective filter (14) and as a means (36) for denoxifying the gas formed treatment stage an activated carbon or coke containing treatment stage (26) for adsorbing the sulfur compounds still contained in the gas is arranged. 14. Plant according to claim 10, characterized in that the protective filter (14) at least predominantly consists of hearth furnace coke, which has been produced from lignite. - For this 1 sheet Zeichnuna -