DD227428A5 - METHOD AND DEVICE FOR PREPARING A DRY, POWDERFUL FLIGHT FLOOR DERIVATIVE AND ITS USE - Google Patents

Abstract

The invention relates to a process for the preparation of a dry pulverulent flyash derivative from calcium-containing fly ash. According to the invention, the product is prepared in such a way that excess amounts of water or aqueous suspensions are added and the excess amount of water, which is not required for quenching, is completely evaporated and removed by the heat of reaction. It describes a device for carrying out the method and the uses of the fly ash derivative for the production of products capable of deposit or as a component of building materials or raw material for the production of building materials. Ammonia and ammonium salts from coal power plant materials can also be removed and recovered.

Description

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In addition to the desulphurization of flue gases from power plants is increasingly required that the content of nitrogen oxides is as low as possible. While the sulfur is already present as an undesirable impurity in the fuels, the nitrogen oxides are formed only during combustion by reaction of nitrogen and oxygen at high temperatures. Although the amount of nitrogen oxides can be influenced by the combustion temperature and the oxygen excess, it can not be reduced below the required values of less than 200 mg of nitrogen oxides per m ^{3 of} flue gas.

In Japan, processes have been developed to remove nitrogen oxides with the aid of catalysts from flue gases by converting them catalytically with added ammonia into nitrogen and water vapor. Since this always has to be done with an excess of ammonia, this ammonia excess must then be removed from the flue gas. This produces ammonia-containing and / or ammonium salt-containing residues which can not readily be worked up, recycled or disposed of. In particular, in coal power plants, in which a desulfurization of the flue gases is carried out, the resulting dry and wet residues contain part of the ammonia used in free or chemically bound form. This content of ammonia and ammonium salts greatly disturbs the reuse or landfill of these residues.

Object of the invention

The object of the invention is to provide an improved, simple and economical process for the production of a dry, powdered fly ash derivative, in particular from power plant fly ash originating from lignite or coal firing, for environmentally friendly landfill or for use as building materials.

Explanation of the essence of the invention

The invention has for its object to make fly ash with a higher calcium oxide content safe and easy either landfillable or reusable. The invention has also set itself the task of producing a more environmentally friendly derivative of fly ash, which can then be processed easily and safely. In particular, the invention has the object to produce eirvtrockenes and powdered fly ash derivative of calcium oxide fly ash containing at least 3Gew .-% free calcium oxide, the reaction should take place with water or aqueous suspensions. In order to achieve a complete quenching of the calcium oxide to calcium hydroxide, it is necessary to always work with excess amounts of water or aqueous suspensions.

The invention has also set itself the task of removing ammonia and ammonium salts from coal power plant residues, so that they can then either recycled in the usual way or can be disposed of. The invention has also set itself the task of removing the free and chemically bound ammonia from the coal power plant residues so that it can be reused either for the process for the removal of nitrogen oxides or for other purposes.

These objects can be surprisingly achieved in that the excess, not required for the extinction amount of water is such that it is completely evaporated and removed by the heat of reaction of the quenching. This is preferably done when the amount of water is 120 to 220 mol%, preferably 180 to 200 mol% of the amount required for extinguishment. Under such conditions, the reaction time is 0.5 to 2 hours. In general, a reaction time of about 1 hour is sufficient. Variations in the reaction time result in particular from the chemical composition, the particle size and the porosity of the products used. It will therefore be possible to optimize the reaction time by simple preliminary tests, with reaction times that are too long in themselves not harmful in themselves, but lead to unnecessary costs. Too short reaction times may cause the product still contain some residual amounts of undeleted calcium oxide and optionally unreacted ammonium salts and therefore lead to undesirable post-reactions and optionally the release of ammonia in the further processing. To prevent clumping and hardening of the flyash derivative and to obtain only dry, powdery derivative, the reaction temperature must be above 100 ⁰ C.

If the fly ash used and / or the water used and / or the aqueous suspensions used contain ammonia and / or ammonium salts, the liberated ammonia is removed together with the water vapor.

Preferably, in these cases, the content of calcium oxide should be 5 to 30% by weight, based on the amount of dry coal power plant residues used. Furthermore, the pH of the reaction mixture should be above 12. For the process are suitable coal power plant residues with a content of ammonia and / or ammonium salts of more than 20 ppm. In particular, thus come coal power plant residues in question, which contain 100 to 1000 ppm ammonia in free or bound form.

To carry out the process according to the invention, the calcium oxide-containing fly ash with water or aqueous suspensions are mixed together in a mixing apparatus and then conveyed into a reaction vessel in which the mixture 0.5 to 2h. lingers. Since the reaction temperature should be above 100 ⁰ C, it must be ensured in particular on the walls of the reaction vessel that the temperature does not drop below 100 ⁰ C. The easiest way to achieve this is through sufficient heat insulation. In principle, it would also be possible to surround the outer wall with a heating jacket, but this is less preferred because of the associated costs.

The heat of reaction of the quenching reaction is sufficient to evaporate up to 120 mol% of excess water even taking into account still existing heat losses on the walls. If the excess of water is too low, local overheating and thus incomplete extinguishment of the calcium oxide will occur. Too much excess water there is a risk that locally the temperature drops below 100 ⁰ C and it comes to unwanted clumping and irreversible hardening of the product.

To carry out the process according to the invention in the presence of ammonia and / or ammonium salts, the coal power plant residues containing ammonia and ammonium salts are mixed together with calcium-containing fly ash and the water or aqueous suspensions in a mixing device and then conveyed into a reaction vessel in which Mixture lingers for 0.5 to 2 hours. If ammonia and ammonia-containing vapors already form during this mixing process, they can be withdrawn directly and combined with the vapors from the reaction vessel.

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The dry, powdered fly ash derivative obtained according to the invention is taken off at the other end of the reaction vessel and, if desired, immediately mixed with further additives and further processed. As further additives, for example, aqueous suspensions come into question, which together with the calcium hydroxide-containing ash derivative lead to landfillable products. As aqueous suspensions it is possible to use preferably the same suspensions which have already been used for quenching the calcium oxide. In principle, however, all other suspensions of waste substances which bind with calcium hydroxide-containing fly ash derivatives can also be rendered landfillable in this form.

As aqueous suspensions both for extinguishing and for further mixing to a landfillable product come in addition to the usual sludges of wet cleaning process especially aqueous gypsum suspensions, as incurred in the hydrocyclone underflow a two-stage wet flue gas desulfurization and the example in cement-lime plaster Bauverlag 35th (1982, pp. 313-317). The underflow of the hydrocyclones used therein generally consists of an approximately 50% gypsum suspension with large compact gypsum crystals. Although these gypsum suspensions are in principle suitable to be processed into high-quality hemihydrate and used for the production of building materials. However, since they will be incurred in greater quantities in the future, as they are needed for the production of building materials, they can be made particularly good and easy to landfill together with the fly ash derivative obtained according to the invention. A particular advantage is that the lower reaches of the hydrocyclones also contain the other impurities of the flue gas scrubbing, so that these can be fed directly to the landfill in a suitable form. This is primarily chloride, which is present in flue gases of lignite in significantly lower quantities than in the flue gases of hard coal power plants. Nevertheless, this offers an elegant way, by combining dry desulfurization and wet desulfurization, to liberate the flue gases at the same time from chlorides and to deposit them safely, quickly and stably together with the residues of dry desulphurisation and wet desulphurisation. This variant of further processing is the subject of German Patent Application P 3329972.2, in which, however, the present inventive method is already described. A particular advantage of this process variant is the wastewater-free operation of the wet desulfurization.

Further aqueous suspensions which can be disposed of or further processed together with the product obtained according to the invention are sulphite-containing sulphate sludges from flue gas desulphurisation plants and other waste sludges and power plant effluents which, together with the flyash derivative prepared according to the invention, set after some time and can thus be disposed of harmlessly.

It has furthermore been found that the dry, pulverulent fly ash derivative obtainable according to the invention with a calcium hydroxide content of at least 4% by weight is also excellently suitable as a component of building materials or as a raw material for the production of building materials. For this purpose, the flyash derivative, for example, still hot immediately after discharge from the reaction vessel with the other components are mixed in dry form and transported in this form for further processing. In principle, however, it is also possible to transport and store the dry and pulverulent fly ash derivative obtainable in accordance with the invention in pure form and to mix and process it with the other components only when needed.

Of course, the other uses depend to a considerable extent on the particular chemical composition of the dry and powder flyash derivative. The chemical composition, in turn, depends to a considerable extent on the coal or lignite used in each case, so that the widest variety of uses arise. Crucial is according to the invention only that the fly ash used has at least 3% free calcium oxide, so that the fly ash derivative contains about 4Gew .-% calcium hydroxide. If there are various types of fly ash in sufficient spatial proximity, some of which have a relatively high calcium oxide content and some others a relatively low calcium oxide content, they may also be mixed together before the quenching reaction and subjected together to the process of the invention. If the low-calcium fly ash has only a very low content of calcium oxide, it is in principle also possible to subsequently mix it with the calcium hydroxide-containing derivative according to the invention and to process it further. Products produced according to the invention with a relatively high calcium oxide content can be further processed with appropriate additives to form mortars, cleaning materials, but also preformed building materials such as calcareous sandstones. It is also possible to use this material for backfilling in mining, for the construction of dams or as an asset for landscaping.

Even when using coal power plant residues with a content of ammonia and / or ammonium salts, the products according to the invention can be used, further processed or disposed of.

The only decisive factor is that the content of ammonia and ammonium salts according to the invention is lower than 10 ppm and therefore does not lead to disturbing post-reactions or odor nuisance even after prolonged storage. The free and the bound ammonia of the coal power plant residues used is discharged under the conditions of the process of the invention almost completely with the vapors from the reaction vessel. It is readily possible to separate ammonia separately from these vapors and to reuse the resulting ammonia. In the simplest case, this ammonia can be used again to react with the nitrogen oxides in the system to remove the nitrogen oxides from the flue gases. If desired, this ammonia can also be used for other purposes in the processing and processing of power plant residues.

In the accompanying drawings, a device according to the invention for carrying out the method is explained in more detail. In this figure mean:

1 - the reaction vessel;

2 - thermal insulation of walls;

3 - the filling opening for the mixture;

4 - the mixing device for fly ash with water or aqueous suspensions and optionally also with ammonia

and ammonium salt-containing coal power plant residues;

5 - the discharge device for the dry, powdered fly ash derivative at the bottom of the reaction vessel;

6 - the vapor withdrawal for evaporated, excess water, possibly also together with ammonia;

7 - a conveyor system for fly ash;

8 - a belt scale for fly ash;

9 - a fly ash silo;

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10 - a pneumatic conveying device for fly ash,

11 - the supply line for water or aqueous suspension to the mixer,

12 - a mixer for the finished fly ash with other components,

13 - the supply line for further components,

14 - conveyor belt from the mixing plant,

15 - Belt scale for the final product,

16 - Loading silo

In the inventive device fly ash from the fly ash silo 9 on the pneumatic Fordereinrichtung 20 and Fordersystem 7 is required on the belt weigher 8 and there is added to the mixing device 4. At the same time via the supply line 11 Wassei ¹ or aqueous suspension is metered in. The mixture is fed through the Einfülloffnung in the reaction vessel 1, in which it gradually decreases from top to bottom The discharge device 5 must be regulated so that the reaction time is 0.5 to 2h, preferably 1 h The reaction time can be regulated by the Fordergeschwindigkeit the discharge device 5 The supplied amount It is possible to automatically regulate the reaction time and thus the metering and removal by measuring the reaction temperature on the outer wall The excess amounts of water are optionally together with the ammonia as steam by the Brudenabz taken 6 ug and further recycled in a suitable manner. For example, it is possible to condense the vapors and recycle the condensate into the process. It is also possible to condense the vapors so that the ammonia can be recycled to the nitrogen oxides removal process solid or liquid other components to be mixed, this is done in the further mixer 12, which transports the final product thus obtained on the conveyor 14 in the loading silo 16

Essential to the invention, however, are only the combinations of the mixing device 4, the infill 3, the trigger 6, the discharge device 5 and the heat-insulated walls 2

Ausfuhrungsbeispiei

The inventive method is explained in more detail in the following examples

example 1

In a continuous mixer fly ash with a content of free calcium oxide of about 20%, which comes from a fired in the dry additive process lignite boiler, mixed with water in the ratio 1 0.15 The mixture product is heated by the Abloschreaktion and becomes the Nachloschen transported in a warm isoherten reaction vessel, which has a Brudenabzug and a Austragvorncht After a reaction time of about 1 hour, the Abloschprodukt is removed from the container

In order to arrive at a product capable of being landfilled, the quenching product is mixed directly with further water in the ratio of 1 to 25. This results in a moisture-moist product which is maximally compressed in the landfill with the usual compaction equipment. Samples of the material thus obtained had the following values

Volume weight 1.7kp / dm ³ ,

7-day compressive strength 3.5MN / m ² ,

28-day compressive strength 11.5MN / m ² ,

28-day transmittance coefficient 2 χ 10 " ⁹ m / s

Example 2

In a continuous mixer, fly ash with a content of free calcium oxide of about 20%, which originates from a lignite boiler fired in the dry additive process, is mixed with a plaster suspension containing 50% solids in a ratio of 0.3. The gypsum suspension originates from the hydrocyclone underflow a 2-stage flue gas desulfurization The suspension is heated by the Abloschreaktion and transported to the extinguishing in a warisoherten on the walls Behalter, which has a Brudenabzug After a reaction time of about 1 hour, the Abloschprodukt is discharged from the reaction vessel and placed in another continuous mixer, in which it is mixed in a ratio of 1 0.35 with another 50% gypsum suspension The amount of gypsum suspension in the first stage is chosen so that no clumping occurs, but already almost completely extinguished The amount of gypsum suspension in the second stage is so by preliminary experiments wt hlt that the product in the landfill with conventional compression Advised can be maximally compressed in the landfill site, the product is applied in layer thicknesses of 25 to 50cm, and then in each case with the suitable compression furfemteilige Schuttungen Advised compacted samples of the material so obtained had the following values

Volume weight 1.7kp / dm ³ ,

7-day pressure resistance 2.5MN / m ² ,

28-day compressive strength 9MN / m ² ,

28-day transmittance coefficient 6 χ 10 ~ ⁹ m / s

Example 3

Hard coal fly ash with an ammonia content of about 400 ppm, as obtained by an ammonia-operated process for the removal of nitrogen oxides (DeNOx process) in the electrostatic precipitator is in a Dun hlaufmischer with about 10% by weight of calcium oxide, based on the fly ash, and with water in proportion 1 0.07 blurred The blended product heated by the Abloschreaktion to about 120 ° C and is transported to the extinguishing in a warm isoherten reaction vessel, which has a discharge and a Brudenabzug Kondensationsvornchtung for the liberated ammonia After a reaction time of about 1 hour, the Abloschprodukt taken from the container with an ammonia content of less than 10 ppm It can be used in a known manner as a concrete additive

Example 4

One part of hard coal fly ash with an ammonia content of about 400 ppm, as obtained by an ammonia-operated process of removal of nitrogen oxides (DeNOx process) in the electrostatic precipitator, and 1 part calcium sulfite / calcium sulfate mixture, as it arises in a dry desulfurization are in a continuous mixer with about 10Gew -%

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Calcium oxide, based on the fly ash, mixed with water in the ratio 1 0.07 The mixture product is heated by the Abloschreaktion to about 110 ⁰ C and is transported to the extinguishing in a warm insulated reaction vessel containing a discharge and a Brudenabzug Kondensationsvomchtung for the liberated ammonia After a reaction time of about 1 hour, the flaking product is removed from the container with an ammonia content of less than 10 ppm and can now be further processed in a known manner to a deponerable product

Example 5

1 part brown coal fly ash with a content of free calcium oxide of about 20% and an ammonia content of about 200 ppm, as obtained by an ammonia-operated process for the removal of nitrogen oxides (DeNOx process) in the electrostatic precipitator, containing 1 part 50% solids Gypsum suspension with an ammonia content of about 400 ppm, as obtained in a wet desulfurization as Hydrozyklonenunterlauf, mixed in the ratio 1 0.3 The mixture product expected by the Abioschreaktion to about 130 ⁰ C and is transported to the extinguishing in a warm insulated reaction vessel This has a discharge device After a reaction time of about 1 hour, the flaking product with an ammonia content of less than 10 ppm is removed from the container and can then be further processed in a known manner into a product capable of being landfilled

Claims (11)

-1- 663 Invention claims: 1. A process for the preparation of a dry powdered fly ash derivative of calcium oxide fly ash containing at least 3 wt .-% free calcium oxide by reaction with excess amounts of water or aqueous suspensions, characterized in that the excess, not required for the extinction amount of water is so dimensioned in that it is completely evaporated and removed by the heat of reaction of the quenching. 2. The method according to item 1, characterized in that the amount of water is 120 to 220Mol% of the amount required for cancellation. 3. The method according to item 1, characterized in that the amount of water is 180 to 200 mol% of the amount required for the quenching. 4. The method according to any one of items 1 to 3, characterized in that the reaction time 0.5 to 2h. is. 5. The method according to any one of items 1 to 4, characterized in that the reaction temperature is above 100 ⁰ C. 6. The method according to any one of items 1 to 5, characterized in that the fly ash used and / or the water and / or the aqueous suspensions contain ammonia and / or ammonium salts and the liberated ammonia is removed together with the water vapor. 7. The method according to item 6, characterized in that the amount of calcium oxide 5 to 30Gew .-%, based on the amount of dry coal power plant residues used is. 8. The method according to item 6 or 7, characterized in that the pH of the reaction mixture is greater than 12. 9. Apparatus for carrying out the method according to points 1 to 5, characterized in that it consists of a) a dry fly ash mixing device (4) with water or aqueous suspensions, b) a reaction vessel (1) with a filling opening (3) for the mixture of the mixing device (4), a vapor withdrawal (6) for the evaporated water, a discharge device (5) for the dry, powdered fly ash derivative at the bottom of the reaction vessel (1 ) and thermally insulated walls (2). 10. Apparatus for carrying out the method according to items 6 to 8, characterized in that it consists of a) a mixing device (4) for solid coal power plant residues with water or aqueous suspensions b) a reaction vessel (1) with a filling opening (3) for the mixture of the mixing device (4), a discharge device (5), for the dry, powdery derivative at the bottom of the reaction vessel (1) with heat-insulated walls (2), c) a vapor withdrawal (6) for the evaporated water and the liberated ammonia, d) a condensation device for ammonia and possibly a recirculation line for condensed ammonia for flue gas cleaning system. 11. The device according to item 9 or 10, characterized in that the reaction vessel (1) is designed cylindroconical. For this 1 page drawings Field of application of the invention The present invention is a process for the preparation of a dry, powdered fly ash derivative from calcium oxide-containing fly ash containing at least 3Gew .-% free calcium oxide by reaction with excess amounts of water or aqueous suspensions. Furthermore, the invention relates to a preferred apparatus for carrying out the method and uses of the present invention fly ash derivative. Characteristic of the known technical solutions Fly ash is a by-product of all incinerators of coal dust and accumulates in such significant amounts that so far only small parts of the resulting fly ash could be meaningfully reused. The largest amounts of fly ash are still dumped. If the fly ash contains high proportions of water-soluble salts, in particular water-soluble sulfates, the fly ash must be dumped in a form that contaminates neither the groundwater nor caused by wind air pollutants. The fly ash accumulating in brown coal incineration plants presents particular difficulties in landfilling because it contains water-soluble salts; see. Brown coal, Issue 2, February 1981, pp. 7-11. To reduce the dust load, the fly ash was moistened before shipment. DTe here partially chemical reactions led during transport to slightly solidified agglomerated but retained a loose structure even after the subsequent tilting by a spreader. These ash beds were permeable to water and led to contamination in later contact with groundwater, in particular by water-soluble sulfates. An improvement in the situation has occurred in that the ash is added immediately before tilting with 20 to 40% water to a mushy consistency and this mixture compacted by shaking with a vibrator. This results in firmer and less permeable landfills. However, the method described in brown coal, Issue 2, is useful only until the incinerators have been switched to a desulfurization with limestone chippings. Here, the brown coal is added before combustion a certain amount of limestone, which binds the formed SO ₂ to a considerable extent. This dry additive process is not only for new construction facilities on lignite-based, but due to the simple principle of "lime to coal" with relatively little technical effort in existing plants can be used, see VGB Kraftwerkstechnik 63 (1983) pp 327-331. Such fly ash with a relatively high proportion of calcium oxide results in the addition of water to such a violent Ablöschreaktion that the previous simple humidification of the ash with water is no longer possible Landfilling such ash behaves differently than the mixed with water fly ash of conventional incinerators of brown coal. In hard coal dust incinerators, it has hitherto been customary to purify and desulphurise the flue gases by wet scrubbing. The resulting wash liquors or sludge have occasionally been mixed and used with the simultaneously incurred fly ash; see. DE-OS 2400350, 3109879, 3113714, 3124003, 3117436, 3119422, 2357407 and EP-OS 22318. In the future, it is to be expected that instead of wet cleaning desulfurization takes place in hard coal incineration plants by adding limestone to the fluidized bed. Again, fly ash will be incurred with an increased content of calcium oxide. Such fly ash behave completely different in the implementation with water, so that the current uses hardly come into question.