DE3605589A1 - Process for removing sulphur dioxide and nitrogen oxides from exhaust gases - Google Patents

Abstract

In a process and an apparatus for removing sulphur dioxide and nitrogen oxides from an exhaust gas, in which the exhaust gas flows through at least one first bed (14), constructed as a moving bed, of a granular carbon-containing adsorbent and then flows through a second bed (15) of such an adsorbent and in which at least upstream of the first bed ammonia (9) or an ammonia precursor is added to the exhaust gas, in which case adsorbent can be taken off from the bottom of the essentially vertically extending beds and adsorbent can be added at the top, according to the invention it is provided that the majority of sulphur dioxide is removed by addition of sulphur dioxide-binding additives (3), before the addition of ammonia, and different adsorbents are used for the beds (14, 15). <IMAGE>

Description

description

Process for removing sulfur dioxide and nitrogen oxides from exhaust gases The invention relates to a method for removing sulfur dioxide and nitrogen oxides from an exhaust gas, in which the exhaust gas by at least one as Wandering layer formed first layer of a granular, carbonaceous Adsorbent and then through a second layer of such an adsorbent flows and in which at least before the first layer ammonia or an ammonia precursor is added into the exhaust gas, the substantially perpendicularly extending Layers taken off adsorbent at the bottom and adsorbent fed in at the top can be.

From DE-OS 31 01 053, page 7, second paragraph is a two-stage Process known in which the first migrating as a migrating layer from top to bottom Adsorbent layer primarily for the adsorptive removal of sulfur oxides serves, while in a second shift, which is very similar to the first, primarily the catalytic reduction of nitrogen oxides to nitrogen in the presence of gaseous Ammonia takes place.

Furthermore, from DE-OS 31 01 053 a method is known in which the adsorbent is applied to one of the two moving layers, partially laden adsorbent is withdrawn from one moving layer and applied to the other moving layer, loaded adsorbent is withdrawn from the other moving layer and fed to a regeneration stage and finally regenerated adsorbent is given back to a moving layer. In this process, it is preferred that the moving layers are arranged one above the other, and the unloaded adsorbent is fed in at the top end and loaded adsorbent is withdrawn at the bottom end, with a connecting element between the reactors determining the two moving layers for the transfer of the partially loaded adsorbent from the upper to the lower migration layer is provided. In both known processes, the moving layers are formed by the same adsorbent, and the aim in both processes is to remove all of the sulfur oxide contained in the exhaust gas by means of the granular, carbon-containing adsorbent. The introduction of the ammonia required for the catalytic reduction of the nitrogen oxides, in the presence of sulfur dioxide in the exhaust gas, results in the following reaction equation for the formation of ammonium sulphate in the form of dust particles.

Ammonium hydrogen sulfate is also formed. The ammonium sulfates thus increase the dust content in the exhaust gas and lead to solids loading of the Adsorbent, which is preferably in the form of activated carbon or activated coke. This adversely affects the catalytic activity of the adsorbent. At the same time, there is also an undesirable increase in the pressure loss the layer because of the free flow cross-section between the adsorbent particles is reduced.

There is also the risk that the exhaust gas to the chimney will be too high Dust content consisting of coke / coal dust, ammonium sulfate / ammonium hydrogen sulfate and residual dust may be present.

Another problem is that the activity of the catalytically active adsorbent is reduced by the adsorption of sulfur dioxide as sulfuric acid according to the following reaction equation: is reduced.

It is also known for the removal of sulfur dioxide, the exhaust gas wet wash, spray adsorption wash or dry wash, sulfur dioxide-binding additives, especially calcium-containing compounds are added to the exhaust gas.

It is the object of the present invention to provide a method of the above specified type, in which the loading of the adsorbent by the The formation of ammonium hydrogen sulfate is reduced and at the same time the profitability the process is improved.

This object is achieved in that the main part of sulfur dioxide removed prior to the addition of ammonia by adding sulfur dioxide-binding additives and different adsorbents are used for the layers.

The main part of the 502 is in a pre-wash, in particular according to the dry additive process, the semi-dry process or the wet process, deposited, with lime, limestone, caustic soda, Magnesium oxide or other absorbents can be used; there are separation efficiencies achieved by over 80% to 98%.

The previous removal of the main part of sulfur dioxide is the first layer of an exhaust gas with only a residual content of SO2 is applied, so that after the ammonia has been added, there is only a reduced formation of ammonium sulphates comes, as soon as a separation of residual SO2 is achieved.

In the first shift, the and deposited in particulate or aerosol form impurities, such as on Alkali and / or alkaline earth compounds to be returned to the laundry, in particular Calcium compounds, residual airborne dust, heavy metal compounds and the resulting Ammonium hydrogen sulfate. Furthermore, there is an adsorption of not yet bound Residual SO2.

The adsorbents can be catalytic in their grain size Differentiate between activity and / or its basic material. While for the second shift high-quality activated carbon / activated coke is used, coke is used for the first layer or coal used in relatively coarse-grained to fragmentary form, which is essential is cheaper than the high quality activated carbon / activated coke. That for the first shift The adsorbent used then has a smaller pore volume, but still a very good retention capacity for dust-like particles and still a sufficient one catalytic activity for the removal of the residual SO2.

The adsorbents can differ in their basic material, if, for example, hard coal or lignite is used as the base material. For the first Layer is preferred lignite coke, especially broken coke, Lignite activated coke or lignite activated carbon. The preferred upper grain size of the grain is 10 mm, whereby in the case of the use of adsorbents based on lignite an upper grain size of 4 mm is again preferred.

The lower grain limit can be freely selected; however, it must be ensured be that the adsorbent is not through the exhaust gas flowing across the layer is carried out with. It will therefore preferably be in the range from 1 to 3 mm. For the high-quality activated coke / activated charcoal for the second layer is the top one Grain boundary usually at 5 to 10 mm.

As long as the adsorbent of the first layer does its job the integration of the residual SO2 and the dust separation, it can after corresponding fine particle separation is fed back to the upper end of the moving layer will. If this is no longer the case, it can be deducted from the first layer Adsorbents are fed to a furnace and burned there, in particular the furnace from which the flue gases originate. However, it is also possible no Material returned from the bottom of the first layer, but everything loaded and feed contaminated material to the furnace. This firing is preferred a smelting furnace or a dust furnace with dry ash extraction to avoid the with to bind the adsorbent entrained constituents in the slag or ash. The adsorbent can be ground before it is introduced into the furnace, together with the main fuel of the combustion system or a separate one Mill. Furthermore, the adsorbent can be used together with recirculated fly ash and / or mixed with the main fuel. The adsorbent can also be fed to the furnace as additional fuel, like this in DE-OS 33 17 507 is described.

As a higher quality adsorbent for the second adsorbent layer is used in the form of activated coke / activated carbon, it is useful if the withdrawn Adsorbent regenerated in a known manner and the second layer is fed back.

Since there is an SO # pre-wash, the desorption gas is preferred the regeneration of the exhaust gas flow before the addition of the sulfur-binding additives.

It is of course also possible to use the adsorbent of the second layer to be fed to the combustion system. In principle, it is also possible to use the second layer to be trained as a moving layer. Since in the process according to the invention however, the second layer is only very slowly loaded and thus deactivated instead of a slowly moving shifting layer, it was also replaced in batches and shifts can therefore be used for predetermined times. It is appropriate the ammonia not only before the first shift, but also before the second shift to add. Before the first shift, the exhaust gas is related to the SO2 and NOx content a lean stoichiometric amount of ammonia is added to the exhaust gas, while before the second Layer of ammonia based on the NOx content in sub-stoichiometric to essentially stoichiometric ratio is added. To a minimum content of SO in the exhaust gas before the second layer is obtained, the rate of migration of the adsorbent in the first layer, preferably depending on a minimum SO2 content in the exhaust gas after the first layer and / or the pressure loss in the first layer regulated.

The exhaust gas can flow through the layers transversely to the direction of extent. But it is also possible, in particular, that in the formation of the layers as a migrating layer, the exhaust gas flows through the layer in countercurrent, ie in particular the first layer is flowed through in countercurrent.

The invention is also directed to an apparatus for implementation of the process consisting of at least two with a carbonaceous adsorbent filled and one behind the other arranged layer reactors, of which at least the first is a moving bed reactor, made up of feeders at the top the reactors and discharge devices at the lower end of the reactors and exhaust gas supply and discharge lines and from at least one device for the supply of ammonia in the exhaust.

According to the invention the device is characterized in that a Washer for dry or wet washing of the exhaust gas with binding sulfur dioxide Additives are provided and the layer reactors with different adsorbents are filled.

It is useful if the two layer reactors by arrangement a gas-permeable partition are formed in a container, if only the Supply of ammonia is provided in the supply line.

However, if ammonia is also to be added after the first shift, it is advantageous if the two layer reactors are from two separate vessels are formed, which are connected to one another by a connecting line another device for the supply of ammonia is provided.

The second layer reactor can also be a moving bed reactor or be a batch loadable reactor.

It is also preferred that the first one is loaded Moving bed reactor is a deep bed reactor which is designed as a cross-flow or counter-flow reactor.

Finally, it is preferred that the two reactors have moving bed reactors with different walking speeds are.

The method according to the invention and the device are intended below on the basis of two exemplary embodiments in conjunction with the accompanying figures explained. They show: FIG. 1 a process scheme of a first process procedure, in which the exhaust gas is subjected to, for example, spray absorption scrubbing, and two migrating layers are arranged in a container, and the adsorbent the first layer is at least partially fed back to the migrating layer and 2 shows a process scheme in which the adsorption material withdrawn from the first layer is fed in total to the combustion system.

The sulfur dioxide coming from a furnace (1) of a steam generator and exhaust gas containing nitrogen oxides is, as shown in FIG. 1, a spray adsorption scrubbing in a Sprühsorptionsreaktor (2) subjected, in which the exhaust gas with a through An atomizer device (3) treats the finely sprayed additive-water suspension and is freed from the majority of sulfur dioxide in the process.

During this binding of sulfur dioxide, the sprayed one dries Suspension in the reactor (2) to a powder, which at an exit point (4) a funnel-shaped part of the reactor (2) is removed from the process. That partially desulphurized and coarsely dedusted exhaust gas contains airborne dust in the form of dust, additive, Sulfites and sulfates of the additive and heavy metal compounds.

The exhaust gas reaches a filter system (6) via a line (5), an electric or a fabric filter, a cyclone or other known device can be for dry dust separation. With correspondingly small amounts of dust in the partially desulphurized exhaust gas may

this filter system can be dispensed with. The one through the filter (6) Dust separated from the exhaust gas is at a deepest point (7) via its own filter Discharge organs discharged. The filter system is only used with the dry process Use, but not with wet processes.

The now pre-cleaned exhaust gas reaches a moving layer adsorber (10), in which blinds (11) and (12) and a gas-permeable Partition wall (13) two moving layers (14) and (15) are determined through which the exhaust gas flows transversely. By adding the ammonia, the nitrogen oxides contained in the exhaust gas are essentially according to the following reaction equations: reduced, the gas temperature preferably being in the range from 80 to 180 ° C., again preferably 80 to 150 ° C.

The first moving layer (14) of the adsorber (10) is made from a bunker (16) loaded with fresh coke with a grain size of up to 10 mm, in the case of the Use of coke or activated charcoal from brown coal with a grain size of up to 4 mm. In this moving layer designed as a deep bed filter (14) will the remaining dust in the flue gas due to deposits on the surface of the coke pieces filtered out.

At the same time, the exhaust gas is of the comparatively small part freed of the still present sulfur dioxide. Also the ammonium hydrogen sulfate that forms is filtered out by deposits on the surface of the coke pieces.

The loaded broken coke is at an exit point (17) from the Adsorber (10) withdrawn and fed to a vibrating screen (18), where deposited dust particles, as well as coke debris are excreted. The broken coke is, if it is still usable is returned to the bunker (16) via a transport line (19). Otherwise it will it is fed to the furnace of the steam generator via a transport line (20), i.e. disposed of in a particularly simple and economically justifiable manner, since a relatively complex and therefore expensive regeneration of the broken coke from economic sources Considerations ruled out. When incinerated in the furnace, this is due to the broken coke adsorbed sulfur dioxide and released again from the exhaust gas in the spray absorption reactor (2) deposited.

The migration speed of the layer (14) is so great that the exhaust gas with a minimal SO-2 content through the partition (13) into the second migration layer (15) occurs.

If necessary, further layers can follow this layer will. The second migration layer is made of high quality adsorbent in the form charged by activated carbon and / or activated coke. Because of the extremely low content the second layer is deactivated only very slowly in the presence of sulfur dioxide, whereby a small migration speed of the adsorbent in the migration layer (15) becomes possible and at the same time a smaller number of regeneration steps is required is. Because of the low load, it is also possible to apply layer (15) in batches to replace. That too regenerating adsorbent is attached to a Exit point (21) of the adsorber (10) discharged and via a transport line (22) conveyed to a regeneration device (23), where they can be reused in a known manner a desorption process, for example a thermal treatment or subjected to microwave irradiation. After that, the regenerated Adsorbent via a transport line (24) to the second migration layer (15) returned. The desorption gas occurring in the regeneration device (23) is drawn off via a line (25) and the exhaust gas flow upstream of the spray sorption reactor (2) fed. It could also become liquid depending on the sulfur dioxide concentration Sulfur dioxide, elemental sulfur, sulfuric acid or other sulfur-containing Products are processed. The completely cleaned exhaust gas leaves the entire system via a chimney (26) connected downstream of the adsorber (10).

In the embodiment according to FIG. 2, the same are as far as possible Reference numerals have been used.

The embodiment according to FIG. 2 differs from the embodiment according to FIG. 1 essentially in that instead of a moving layer adsorber moving bed reactors (27) and (28) separated from one another by a container (2) are provided in each of which the first layer (14) and the second layer (15) are performed. The two moving layer reactors (27) and (28) are through a Line (29) connected to each other, in which a measuring point (30) for the detection the sulfur dioxide and nitrogen oxide content is arranged. It can also be separate Measuring points be provided. Furthermore, the line (29) is an admixing device (31) assigned for the admixture of ammonia. This admixing device (31) has a schematically shown valve (32) for controlling the Contribution of ammonia in the line (29). Line (9) is another NO, S02 measuring point (33) which controls a valve (34) which is assigned to the admixing device (9) (such an arrangement can also be used in FIG. 1).

The admixing device (9) is used to relate to NOx and ammonia S02 is supplied over stoichiometrically, while ammonia via the admixing device (31) is supplied essentially stoichiometrically based on NOx.

The rotary valve assigned to the extraction point (17) is controlled by the measuring point (30) controlled so that the flow rate in the moving layer (14) is set so that the residual S02 content in line (29) is just against 0 tends to.

In the embodiment according to FIG. 2, there is a sieving and recirculation of the adsorbent is not provided, but rather the entire withdrawn adsorbent is fed to the combustion system (1) via line (20).

The ratio of that fed to the furnace (1) via line (20) combustible material in the embodiment according to FIG. 1 can be varied by possibly more or less material via the line (19) via the migrating layer is driven in a circle.

Of course, the withdrawn part must be replaced in FIG. 1 while in the embodiment according to FIG.

2 the entire withdrawn part must be continuously replaced. That cheap adsorbents used in the first layer, preferably lignite coke, Lignite activated coke, or lignite activated carbon, can be used economically in the furnace to be burned; regeneration is not economical. This could be only worthwhile for the higher quality adsorbent in the second layer.

Although the method in connection with FIGS. 1 and 2 only in connection with a spray adsorption step to remove most of the sulfur oxides has been described, other wet or dry washing methods can also be used will. In a wet washing process, droplet separation would be after the Scrubber required.

It would also be conceivable in the embodiment according to FIG both migrating layers are arranged in a container by suitable measures carry out an introduction of ammonia in the area of the partition. However, it is easier to put the ammonia in a connecting line between two separate containers bring in.

Claims (19)

Claims 1. A method for removing sulfur dioxide and Nitrogen oxides from an exhaust gas, in which the exhaust gas passes through at least one as a moving layer formed first layer of a granular, carbonaceous adsorbent and thereafter flowing through a second layer of such adsorbent and in the case of at least before the first layer ammonia or an ammonia precursor is added into the exhaust gas, the substantially perpendicularly extending Layers taken off adsorbent at the bottom and adsorbent fed in at the top It can be stated that the main part is sulfur dioxide removed prior to the addition of ammonia by adding sulfur dioxide-binding additives and different adsorbents are used for the layers. 2. The method according to claim 1, d a d u r c h g e k e n n z e i c h n e t that for the first layer coke or coal in relatively coarse-grained to fragmentary Form and high-quality activated carbon / activated coke is used for the second layer. 3. The method according to claim 1 or 2, d a d u r c h g e k e n n notices that for the first layer lignite coke, lignite activated coke or lignite activated carbon is used. 4. The method according to any one of claims 1 to 3, d a d u r c h g e k It is noted that at least a portion of the # n Adsorbent is fed to a furnace. 5. The method according to claim 4, d a d u r c h g e k e n n z e i c h n e t that the adsorbent is ground before being introduced into the furnace will. 6. The method according to any one of claims 1 to 5, d a d u r c h g e k It is noted that the adsorbent withdrawn from the second layer regenerated in a manner known per se and fed back to the second layer will. 7. The method according to any one of claims 1 to 6, d a d u r c h g e k It is noted that the desorption gas of the regeneration precedes the exhaust gas flow Addition of the sulfur-binding additives is supplied. 8. The method according to any one of claims 1 to 7, d a d u r c h g e k It is noted that the second layer is used as a wandering layer or is replaced in batches. 9. The method according to any one of claims 1 to 8, d a d u r c h g e k It is noted that additional ammonia was added before the second shift will. 10. The method according to any one of claims 1 to 9, d a d u r c h g e it is not indicated that before the first shift, the exhaust gas in relation to the S02 and a superstoichiometric amount of ammonia is added to the NOx content in the exhaust gas. 11. The method according to any one of claims 1 to 10, d a d u r c h g e It is not noted that ammonia related to the NOx # content before the second shift admixed in a substoichiometric to essentially stoichiometric ratio will. 12. The method according to any one of claims 1 to 11, d a d u r c h g e it is not indicated that the migration speed of the adsorbent in the first layer depending on a minimum SO2 content in the exhaust gas after first layer and / or the pressure drop in the first layer is regulated. 13. Device for performing the method according to one of the claims 1 to 12, consisting of at least two with a carbonaceous adsorbent filled and one behind the other arranged layer reactors, of which at least the first is a moving bed reactor, made up of feeders at the top the reactors and discharge devices at the lower end of the reactors and exhaust gas supply and discharge lines and from at least one device for the supply of ammonia in the exhaust gas, that is, that a scrubber (2) for the dry or wet scrubbing of the exhaust gas with additives that bind sulfur dioxide (3) is provided and the layer reactors (14, 15) with different adsorbents are filled. 14. The apparatus of claim 13, d a d u r c h g e k e n n z e i c h n e t that the two layer reactors (14, 15) are permeable by the arrangement of a gas Partition wall (13) are formed in a container (10). 15. The device according to claim 13, d a d u r c h g e k e n n z e i c h n e t that the two Layer reactors by two separate ones Containers (27, 28) are formed, which are connected by a connecting line (29) are on which a further device (31) is provided for the supply of ammonia is. 16. Device according to one of claims 13 to 15, d a d u r c h it is noted that the second layer reactor (15; 28) is also a Moving bed reactor or a batch loadable reactor. 17. Device according to one of claims 13 to 16, d a d u r c h It is not noted that the first reactor is loaded with an adsorbent Lignite base and the second reactor filled with higher quality activated carbon / activated coke is. 18. Device according to one of claims 13 to 17, d a d u r c h it is noted that the first moving bed reactor is a deep bed reactor which is designed as a cross-flow or counter-flow reactor. 19. Device according to one of claims 13 to 18, d a d u r c h it is not indicated that the two reactors are moving bed reactors (14, 15) are at different travel speeds.