EP3660398B1 - Combustion method in a power plant - Google Patents

Description

The present invention relates to a firing method in a power plant, which has at least one mill, at least one steam generator connected to the at least one mill with a combustion chamber and a flue gas cleaning system connected downstream of the steam generator, which has at least one filter device and at least one flue gas desulphurisation system connected downstream of the filter device, wherein in the at least one mill raw coal is ground to coal dust, the coal dust is burned in the combustion chamber, through the resulting heat steam, i.e. water vapor or saturated steam or superheated steam, is generated in the steam generator, the flue gas arising in the combustion chamber is passed into the flue gas cleaning system , where the flue gas is filtered in the filter device, with filter ash accumulating as a waste product, and sulfur and / or at least one sulfur compound is separated from the filtered flue gas in the flue gas desulfurization system.

In the present invention, raw coal is preferably raw lignite, but it can also be raw hard coal. In the present invention, the combustion of the raw coal is simply referred to as coal combustion. Raw coal is a natural product. The percentages by weight cited in the course of the description of the invention always relate to this raw coal which, in addition to pure carbon, also contains other constituents, such as water, surface water and hygroscopic water.

In addition to the actual combustion of the raw coal, the firing process according to the invention also includes the steps of pretreatment and supply of the fuels to the combustion boiler and the steps of processing the combustion waste products. The firing method according to the invention is a special embodiment of the coal combustion known in the prior art as dust firing.

When raw lignite is burnt with dust in the combustion chamber of a steam generator, filter ash is one of the residues. Filter ash is also known as fly ash. The filter ash is in the flue gas emerging from the boiler and, as it is very fine, is literally carried away with the flue gas.

This filter ash is separated from the flue gas produced during coal combustion, for example via an electrostatic precipitator, transported to a silo and, after loading, taken for recycling or disposal.

In addition to the filter ash, when the raw coal is burned, wet ash is produced, which typically falls from the combustion chamber into a funnel slope, is extinguished in a water bath and discharged via a scraper belt.

In principle, filter ash can be recycled, whereby the filter ash can be used, for example, as an aggregate in soil preparation or when backfilling underground facilities. If it is not possible to use the fly ash, it is disposed of in landfills.

For this purpose, the filter ash is examined according to various criteria before it can be recycled or disposed of. If the fly ash is disposed of in a landfill, the provisions of the landfill operator's acceptance criteria or the legal provisions such as the Landfill Ordinance or the requirements for the recycling of mineral waste (LAGA) apply. The procedure is based on landfill classes, the LAGA approval criteria and the definition of key parameters.

A limit value according to the Landfill Ordinance and a key parameter is the barium content of the filter ash eluate. The combustion residues of power stations, such as filter ash, contain easily soluble barium salts which, due to their solubility products, dissolve very easily to easily through an eluting agent such as water. If the specified limit value for barium in a filter ash eluate is exceeded, the permission to dispose of the respective filter ash, for example on a landfill according to landfill class 2, is withdrawn. This means that the respective filter ash can then only be disposed of in a different landfill, for example according to landfill class 3, with considerable additional expense. In addition, there is a risk that the filter ash with an increased elutable barium content will have to be classified from "harmless" to "dangerous" with regard to the waste code number.

There is an approach in the prior art to lower the barium content in the eluate in the fly ash from heating power stations by aging the fly ash under the action of carbon dioxide, with barium compounds being converted into sparingly soluble salts. The time required for this, however, is not justifiable from a logistical and thus economic point of view.

The pamphlet WO 89/07 974 A1 discloses a method for the purification of flue gases that arise when coal is burned in a pulverized coal-fired thermal power station. The aim of the process is to increase the sulfur dioxide absorption capacity of the fly ash in the flue gas by means of its modification. The flue gas resulting from the combustion of coal dust is first fed to a preheater, which returns the heat from the flue gas to the coal combustion. The flue gas then passes through an electrostatic filter in which the fly ash is filtered out of the flue gas and discharged into a vessel in which the fly ash is mixed with water. The resulting aqueous suspension is then fed to a mill and ground therein. The ground, aqueous fly ash suspension is then fed to a further vessel in which the ground, aqueous fly ash suspension is mixed with quicklime or slaked lime and thus reacts to form calcium silicates on the surface of the fly ash particles.

After a waiting period, the resulting mixture is fed to a drying unit, which leaves the mixture in powder form. This powder is blown into a container by means of compressed air, from which the powder is fed to a diverging part of the vessel, in which it is mixed with the flue gas. This is followed in a reaction part by a reaction between the powder and the acidic gases, in particular sulfur dioxide, in the flue gas, which means that sulfur or sulfur compounds are separated from the flue gas.

The pamphlet US 2017/0113085 A1 _ deals with the treatment of fly ash that is produced when coal is burned in a coal-fired power plant. Fly ash containing sodium is first generated by injecting a sorbent containing sodium into the flue gas flow. Then the sodium-containing fly ash is mixed with anhydrite and with an additive, such as. B. strontium hydroxide and / or dolomite limestone and / or iron sulfate, added. The fly ash treated in this way is then dried. The method described is intended to reduce the sodium content in the fly ash, the alkalinity of the fly ash can be reduced and / or the elution of heavy metal (s) such as selenium or arsenic from the fly ash can be reduced.

It is therefore the object of the present invention to reduce the soluble barium content in the fly ash produced in power plants in an economical manner.

This task is achieved by a firing process in a power plant, such as a thermal power plant, an industrial power plant, a condensing power plant or a coal-fired power plant, which has at least one mill, at least one steam generator connected to the at least one mill with a combustion chamber and a flue gas cleaning system connected downstream of the steam generator, which has at least has a filter device and at least one flue gas desulphurisation system (FGD) downstream of the filter device, with raw coal being ground to coal dust in the at least one mill, the coal dust being burned in the combustion chamber, the resulting heat generating steam in the steam generator and the in The flue gas produced in the combustion chamber is passed into the flue gas cleaning system, where the flue gas is filtered in the filter device, whereby filter ash is produced as a waste product, and from the filtered flue gas in the flue gas desulfurization system (FGD) sulfur u nd / or at least one sulfur compound is deposited, and wherein a pumpable, sulphate-containing sludge and / or a pumpable, sulphate-containing solution or suspension, which and / or which is referred to in the present invention as thin sludge regardless of the type of its extraction, the at least one mill is fed, is ground therein together with the raw coal, is fed to the combustion chamber and is burned with the raw coal.

As a result of this process, the easily soluble, environmentally harmful barium content in the eluate of the filter ash can be reduced by adding the thin sludge to such an extent that the filter ash can be classified as harmless when it is recycled or disposed of. The sulphate in the thin sludge, for example calcium sulphate, forms a chemical bond with the barium in the raw coal during the combustion process, with the content of easily soluble barium compounds in the filter ash being reduced. This process produces barium sulfate, which is difficult to dissolve, instead. As a result of this connection, barium is less soluble from the filter ash and therefore cannot be eluted from the filter ash in water and is therefore also undetectable. The barium sulphate does not go to the analytical stage during elution Verification still in reality after the power plant waste has been deposited in a landfill as a result of rain events in solution. This means that barium cannot penetrate the groundwater.

Due to its pumpability, the thin sludge can simply be fed to the at least one mill and from there to the combustion cycle, for example via pipes and the use of pumps. In addition, the thin sludge is easy to dose and can therefore be added to the raw coal to be burned at a certain, advantageous percentage.

In the at least one mill, the thin sludge is ground with the raw coal. The resulting grist is blown into the combustion chamber of a boiler and burned in a suspension. In the present invention, for example, one or more wet fan mill (s) can be used as the mill (s). Before it is fed to the at least one mill, the raw coal can also be pre-comminuted in a pre-comminution, such as an impact hammer mill.

In addition to raw coal and thin sludge, other substances such as sewage sludge and / or substitute fuels can also be burned in the combustion chamber.

In a particularly advantageous embodiment of the method according to the invention, the thin sludge that is fed to the at least one mill, is ground therein, is fed to the combustion chamber and is burned together with the coal dust, is obtained by separating and _{washing SO 2 in the flue gas desulphurisation system} is converted to gypsum with a limestone product, the gypsum is dewatered and the thin sludge is precipitated and separated from a dewatering suspension that is created in the process.

In the flue gas desulfurization system of the power plant used in this process execution, sulfur dioxide components are released from the flue gas during a washing process in a quencher and absorber with a limestone powder suspension. During the associated gypsum production, its washing process and gypsum drainage, the above-mentioned drainage suspension is created, which is a suspension containing sulfate. In a wastewater treatment plant (ARA) of the power plant, the drainage suspension is then treated by removing the thin sludge precipitated from the dewatering suspension with at least one precipitant and separated as thin sludge in the compact clarifier, with clear water being produced as waste water.

A first part of this thin sludge is fed back to the absorber. However, a second part of the thin sludge is discharged. This second part of the thin sludge used to be fed either to a wet ash removal system in the power plant or to a vacuum belt filter in the power plant, which was used to remove gypsum.

In the present invention, this second part of the thin sludge is pumped via at least one metering line by means of at least one pump to the at least one mill, that is to say to the at least one coal mill. The as yet unmilled raw coal is wetted with the thin sludge either in the metering line, preferably just before the mill, or in the mill. The raw coal is ground to dust with the thin sludge in the mill and then blown into the combustion chamber or combustion chamber for combustion. The filter ash produced during the combustion process absorbs the sulphate and binds barium. A strong reduction in the soluble barium content can then be detected in the filter ash eluate.

This form of conducting the process according to the invention has several advantageous effects. On the one hand, the thin sludge that arises during the flue gas scrubbing, i.e. in the flue gas desulphurization system used here, can be immediately and advantageously further recycled and fed directly from a waste outlet of the wastewater treatment plant to the at least one mill and used in the coal combustion to reduce barium in the resulting filter ash.

_{The thin sludge produced by the SO 2} separation that takes place in the flue gas desulphurization system used to have to be disposed of in a suitable manner, which is why, as stated above, it was placed on plaster tape, for example. However, the thin sludge applied to the plaster of paris quickly solidified to form a layer on the plaster of paris, making the plaster of paris removed from the plaster of paris tape more difficult to drain and in some cases had higher residual moisture levels, which can be detrimental to the quality of the plaster of paris. For example, longer drying times must then be planned in the recycling process. Such a procedure is no longer necessary since the thin sludge is added to the coal combustion according to the invention. Corresponding As a side effect of the present invention, the gypsum quality of the gypsum produced _{with the aid of SO 2 separation can also be improved.}

In another disposal variant, also mentioned above, the thin sludge was previously abandoned on top of the wet ash that also occurs in the coal combustion process. However, this resulted in a higher water consumption for cleaning the swing drainer, with which the wet ash mixed with the thin sludge is drained before the wet ash can be transported and transported away for recycling or disposal. This negative effect can also be avoided with the method according to the invention.

The treatment of the drainage suspension preferably includes adding at least one precipitant to the drainage suspension and / or passing the drainage suspension through a compact clarifier.

In this embodiment of the invention, the thin sludge is also obtained directly from the flue gas scrubbing that takes place in the power station anyway, by drawing the thin sludge from the dewatering suspension resulting from the formation of gypsum, precipitating it and then clarifying it. The precipitation can take place with the at least one precipitating agent, that is to say by adding one or more chemicals to the dewatering suspension. Additionally or alternatively, the clarification can be carried out in the compact clarifier.

The at least one precipitant forms a bond with certain constituents of the dewatering suspension. The resulting binding products have an increased specific weight and settle as thin sludge.

The compact clarifier can be designed, for example, in the form of a basin into which the dewatering suspension or the dewatering suspension mixed with the at least one precipitant is passed, a lamella arrangement being provided in the upper region of the basin.

However, in simple variants of the present invention it is possible to omit the at least one precipitant and / or to bypass the compact clarifier.

However, there are also power plants in which no S02 separator is used, which washes the SO ₂ out of the flue gas in a wet scrubber so that no thin sludge is produced. Such power plants use dry noise gas processing for flue gas desulphurization, for example. In addition, there may be malfunctions in the flue gas desulfurization system, which lead to the fact that thin sludge - at least temporarily - cannot be extracted from the waste water treatment system of the flue gas desulfurization system. In a further variant of the method according to the invention, it is therefore provided that the thin sludge is at least partially produced as a model suspension and added to the coal combustion. In this embodiment of the present invention, the thin sludge can at least partially be obtained by stirring at least one powdery or dusty carrier material with at least one sulfate and water to form the thin sludge.

If the sulphate content of the thin sludge obtained from the flue gas desulphurisation system and / or chemically produced is too low, in certain embodiments of the present invention this thin sludge can be mixed with further sulphate, i.e. with sulphate-containing raw materials and / or chemicals or even pure sulphate.

In the method according to the invention, the proportion of the thin sludge supplied to the combustion chamber relative to the raw coal supplied to the combustion chamber is preferably at least 0.1% by weight and at most 10% by weight.

In a preferred embodiment of the method according to the invention, the proportion of the thin sludge fed to the combustion chamber relative to the raw coal fed to the combustion chamber is at least 0.5% by weight and at most 2% by weight.

It has been found to be particularly favorable if the proportion of the thin sludge supplied to the combustion chamber relative to the raw coal supplied to the combustion chamber is less than 1% by weight.

It is also advantageous if, in the method according to the invention, the thin sludge is fed to at least two mills connected to the combustion chamber, for example located opposite one another. This means that the thin sludge can still be dosed in the event of repairs or the unavailability of a mill.

The method according to the invention is also particularly efficient if the thin sludge is fed to the at least one mill together with the raw coal. In this way, the raw coal can be mixed with the thin sludge at an early stage in the process, with which a particularly high barium reduction in the filter ash can be achieved.

An advantageous embodiment of the method according to the invention is described below with reference to FIG Figure 1 explained in more detail, whereby
Figure 1 shows schematically components of a region of a power plant 1. In the power plant 1, raw coal 4, such as lignite or hard coal, is burned in a firing process, here a dust firing process, with steam 13, such as water vapor, saturated steam or superheated steam, being generated which is used as an energy carrier becomes.

The power plant 1 has a combustion chamber with a combustion chamber 2. In the exemplary embodiment shown, several, here four, mills 3 are coupled to the combustion chamber 2. In the mills 3, the raw coal 4, which is fed to the mills 3 via conveyor belts and / or bunkers and / or distributors, is ground to form coal dust. Before it is fed to the mill (s), the raw coal 4 can also be pre-comminuted in a pre-comminution, not shown here, such as an impact hammer mill.

In the present invention, the mills 3 are also supplied with thin sludge 10 via pipes and / or hose lines. The thin sludge 10 is a pumpable, sulfate-containing sludge and / or a pumpable, sulfate-containing solution or suspension. In the mills 3, the raw coal 4 is ground together with the thin sludge 10 with which the raw coal 4 is added on the conveying path to the mills 3 and / or in the mills 3 themselves.

In other, not shown embodiments of the present invention, only one mill 3 or a different number of mills 3 can be used. Furthermore, it is basically sufficient if the thin sludge 10 is fed to at least one of the mills 3.

In addition to the raw coal 4 and the thin sludge 10, the at least one mill 3 can also contain other substances, such as sewage sludge and / or substitute fuel such as. B. garbage and / or biofuel such. B. wood are supplied, which are then co-incinerated with the raw coal 4 ground to coal dust and the thin sludge 10.

The sulphate-containing coal dust is blown from the mills 3 into the combustion chamber 2 and burned in suspension therein.

In the exemplary embodiment shown, the proportion of the thin sludge 10 to the raw coal 4 during combustion is less than 1% by weight, but can also be greater in other embodiments of the invention, but is at least 0.1% by weight.

In the firing method according to the invention, treated water and / or deionized water 5 is furthermore fed to a steam generator 12 coupled to the combustion chamber 2 after degassing by means of pumps 15 and heated to form steam 13 during the combustion process.

The steam 13 is then fed, for example, to a turbine, not shown here, which in turn can feed a district heating system with heating network water.

The flue gas 14a produced during the combustion process is discharged from the combustion chamber 2 and fed to a flue gas cleaning system of the power plant 1. The wet ash 16, which also occurs during the combustion process, is discharged from the combustion chamber 2 downwards.

The flue gas cleaning system is shown in FIG Figure 1 The illustrated embodiment of the invention has a filter device 6 and a flue gas desulfurization system 8 connected to the filter device 6, but in other embodiments of the present invention can have significantly more components, such as HCl absorbers, activated carbon filters, nitrogen oxide removers, etc.

The filter device 6 is an electrostatic filter in the embodiment shown, but can also be another suitable filter device, such as a fabric filter device, with which suspended particles or dust particles contained in the flue gas 14a, the so-called filter ash 7, are filtered out of the flue gas 14a can. The filter ash 7 is discharged as a waste product of the filter device 6, collected and later either disposed of in a landfill or, for example, used as concrete aggregate or in other recycling channels.

The filter ash 7 obtained in the process according to the invention has only a harmless proportion of easily soluble barium compounds. This is due to the sulphate-containing thin sludge 10 fed to the incineration. The sulphate component of the thin sludge 10 forms barium sulphate with the barium contained in the raw coal 4, which is sparingly soluble in the filter ash 7 and so that the filter ash 7 does not pose any risk to the environment when it is stored a landfill or other recycling routes.

In the in Figure 1 The embodiment shown, the thin sludge 10 is obtained in a wastewater treatment plant 9 connected to the flue gas desulphurization plant 8.

The flue gas desulfurization system 8 is a so-called SO ₂ absorber in the embodiment shown, which can be designed, for example, in the form of an absorber tower. In the SO ₂ absorber, the filtered flue gas 14b coming from the filter device 6 is subjected to wet scrubbing. _{In the wet scrubbing, SO 2} , among other things, is separated from the filtered flue gas 14b; this reacts with limestone powder or possibly with quicklime or chalk to form gypsum. To separate the gypsum, the resulting gypsum-containing suspension is passed, for example, to a water-permeable vacuum belt filter on which gypsum settles and is dehydrated. The dewatering suspension collected under the vacuum belt filter in at least one collecting container of the wastewater treatment plant 9 contains not only water but also the sulphate-containing thin sludge 10.

A first part of the drainage suspension is fed back into the washing cycle of the SO _{2 absorber through the waste water treatment plant 9.} A second part of the dewatering suspension, the thin sludge 10, is pumped to the mills 3 via corresponding feed lines 11.

To separate the thin sludge 10, at least one precipitant can be added to the dewatering suspension and / or the dewatering suspension can be added through a compact clarifier and the thin sludge 10 can be statically separated from wastewater.

The cleaned flue gas 14c is optionally also heated and then released into the atmosphere through a chimney 17 with the aid of a flue gas fan.

Claims (10)

1. Combustion method in a power plant (1) comprising at least one mill (3), at least one steam generator (12) with a combustion chamber (2) connected to the at least one mill (3), and a flue gas cleaning system connected downstream of the steam generator (12) which comprises at least one filter device (6) and at least one flue gas desulfurization system (8) connected downstream of the filter device (6), wherein
   the at least one mill (3) mills raw coal (4) into coal dust, the coal dust is burned in the combustion chamber (2), steam (13) is generated in the steam generator (12) by the heat resulting thereof, and the flue gas (14a) produced in the combustion chamber (2) is fed into the flue gas cleaning system, where the flue gas (14a) is filtered in the at least one filter device (6), whereby filter ash (7) is obtained as a waste product, and sulfur and / or at least one sulfur compound is separated from the filtered flue gas (14b) in the flue gas desulfurization system (8),
   characterized in that
   a pumpable, sulfate-containing slurry and / or a pumpable, sulfate-containing suspension or solution, which in the present invention is referred to as thin slurry (10), is fed to the at least one mill (3), where it is milled together with the raw coal (4), fed to the combustion chamber (2) and burned with the coal dust.
2. Method according to claim 1, characterized in that the thin slurry (10) is at least partially obtained by separating SO₂ from the filtered flue gas (14b) in a washing process in the flue gas desulfurization system (8) and converting it to gypsum with a limestone product, quicklime or chalk, dewatering the gypsum, and separating the thin slurry (10) from a dewatering suspension resulting thereof.
3. Method according to claim 2, characterized in that separating the thin slurry (10) from the dewatering suspension includes adding at least one precipitation agent to the dewatering suspension and / or passing the dewatering suspension through a compact clarifier.
4. Method according to at least one of the preceding claims, characterized in that the thin slurry (10) is produced at least partially independently from flue gas desulfurization as a model suspension and / or solution, wherein at least one powdery or dusty carrier material with at least one sulfate and water is stirred to the thin slurry.
5. Method according to at least one of the preceding claims, characterized in that the thin slurry (10) is additionally mixed with sulfate.
6. Method according to at least one of the preceding claims, characterized in that the proportion of the thin slurry (10) fed to the combustion chamber (2) is at least 0.1% weight/weight and at most 10% weight/weight relatively to the raw coal (4) fed to the combustion chamber (2).
7. Method according to at least one of the preceding claims, characterized in that the proportion of the thin slurry (10) fed to the combustion chamber (2) is at least 0.5% weight/weight and at most 2% weight/weight relatively to the raw coal (4) fed to the combustion chamber (2).
8. Method according to at least one of the preceding claims, characterized in that the proportion of the thin slurry (10) fed to the combustion chamber (2) is less than 1% weight/weight relatively to the raw coal (4) fed to the combustion chamber (2).
9. Method according to at least one of the preceding claims, characterized in that the thin slurry (10) is fed to the at least one mill (3) together with the raw coal (4).
10. Method according to at least one of the preceding claims, characterized in that the thin slurry (10) is fed to at least two mills (3) connected to the combustion chamber (2).

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