EP2414731B1 - Method for improving the dynamic behavior of a coal-fired power plant for primary and/or secondary requirements of the power grid operator with respect to the current output into the grid and coal-fired power plant - Google Patents

Description

The invention relates to a method and arrangement for improving the dynamic behavior of a coal-fired power plant at primary and / or secondary requirements of the electricity grid operator to the power delivery to the grid.

Keeping constant the AC frequency in electricity networks is an important task. Deviations from the given frequency can lead to a failure of the connected consumers and the resulting consequential damage.

Deviations from the specified mains frequency value occur especially when the power demand on the power plants connected to the electricity grid suddenly changes, for example because a power plant is disconnected from the grid due to an accident or a large consumer is switched on or because the network configuration or network distribution changes. In order to keep the grid frequency constant or within the specified tolerance range within the framework of the so-called primary control or primary control power, it is necessary to ensure that the generation power and the grid load remain balanced and always generate as much electrical power as the grid load Operation is consumed at the predetermined mains frequency. The primary control is still supported by the secondary control or secondary control power, which compensates for quasi-stationary deviations of both the frequency and the transfer capacity after the sudden adjustment of the power consumed or generated by the primary control.

In order to be able to counteract deviations from the specified power frequency value in the shortest possible time, some national network operators specify in their standards under which conditions or specifications this can be achieved. For example, the British grid operator National Grid Electricity transmission plc, through its document "The Grid Code" Issue 3, states that in the event of a frequency deviation, a power plant connected to the electricity grid would, for example, operate at 65% of its rated power or the primary requirements within 10 seconds by 10 percent of its rated power is increased and thus the frequency deviation is counteracted. This very fast change, which is very large in terms of the scope of services, places great demands on the power plant, especially on a coal-fired power plant.

Large coal-fired power plants are usually formed with coal dust firing in which the coal ground in the coal grinding plant are fed directly via pulverized coal pipes of the combustion chamber of the power plant (so-called "direct" coal dust firing). The treatment of the fuel is one of the main factors for a good combustion, a good efficiency, low emissions and little unburned in the ash to use this by-product. For coal preparation, the coal grinding plant or coal mill must be in a stationary heat and mass flow equilibrium, which means that load changes to the pulverized coal firing and thus the power plant itself can be carried out only slowly and thus a delay occurs when made or required load changes ,

The delay time of the coal mill with changing fuel quantity or task is an essential part of the total system delay time. The delay time of the coal mill can be long according to the raw coal preparation process (depending on fineness, moisture, hardness of the raw coal and the mill load) and therefore has an adverse effect on the delay time of the entire system.

The US 4,332,207 discloses the delivery of coal from a raw coal bunker and through a pulverizer directly into the furnace via burners. In addition, if necessary, the coal can be fed from an independent source through a variable feed into the furnace.

The object of the invention is therefore to provide a method for improving the dynamic behavior of a coal-fired power plant at primary and / or secondary requirements of the electricity network operator to the power delivery to the network, in which the delay time of coal dust firing of the power plant is reduced so that the power plant complies with the specifications or conditions of respective national electricity grid operators. It is a further object of the invention to provide an arrangement for improving the dynamic behavior of a coal-fired power plant at primary and / or secondary requirements of the electricity grid operator to the power delivery to the grid.

The above object is achieved with respect to the method by the totality of the features of claim 1 and with respect to the arrangement by the totality of the features of claim 9.

Advantageous embodiments of the invention can be found in the dependent claims.

• Schaffung der Möglichkeit für Kraftwerksbetreiber, die erforderlichen Zulassungen zum Bauen und Betreiben von Kraftwerken in Übereinstimmung mit den vorgeschriebenen nationalen Netzfrequenzanforderungen zu erhalten.
• Durch Verkauf von Primärregelreserve wird es dem Kraftwerksbetreiber ermöglicht, die Anlage ökonomischer zu betreiben bzw. höhere Gewinne zu erzielen.
• Dem Hersteller bzw. Anbieter derartiger Kraftwerke wird es ermöglicht, diese Kraftwerke auf weltweiten Märkten, z.B. UK, Irland, Frankreich, China, Indien, Singapur etc. anbieten bzw. verkaufen zu können.

The solution according to the invention provides a method and an arrangement for improving the dynamic behavior of a coal-fired power plant at primary and / or secondary requirements of the electricity grid operator for the supply of electricity to the grid, which has the following advantages:

• Provide the opportunity for power plant operators to obtain the necessary approvals for the construction and operation of power plants in accordance with the prescribed national grid frequency requirements.
• The sale of primary control reserve will allow the power plant operator to operate the plant more economically or to achieve higher profits.
• The manufacturer or supplier of such power plants will be able to offer or sell these power plants on worldwide markets, eg UK, Ireland, France, China, India, Singapore etc.

An advantageous embodiment of the invention provides that the storage volume V _Sp having silo in normal operation of the indirect combustion system volume side in about half filled with coal dust for provision and use in increasing the primary and / or secondary requirements of the electricity network operator to the power delivery to the grid and the remaining storage volume is used to receive and store the excess coal dust produced while reducing the primary and / or secondary requirements of the electricity grid operator to the power delivery to the grid.

In an advantageous embodiment of the invention, the increase or decrease of indirectly supplied amount of pulverized coal by a controlled increase or decrease in the throughput of the metering organs. This makes it possible to precisely address the needs or the dynamic behavior of the coal-fired power plant.

An advantageous embodiment provides to increase or reduce the volume flow of the conveying gas blower in a controlled manner when increasing or decreasing the amount of coal dust indirectly supplied. Thus, the smooth entry of coal dust is maintained in the combustion chamber.

It is advantageous that the increase or decrease in the throughput of the metering organs and / or the increase or decrease in the volume flow of the delivery gas blower is effected by the affected by the requirements of the electricity grid block power control of the coal-fired power plant. This measure ensures that, in the event of a frequency change or a requirement in the electricity grid, the grid control of the coal-fired power plant and its firing are influenced immediately and a countermeasure is initiated without loss of time in order to optimize the dynamic behavior of the power plant.

In an advantageous embodiment of the invention, the primary requirement or the primary control is triggered by a remote-controlled signal. In a further advantageous Training the invention, the secondary requirement or the secondary control is also triggered by a remote-controlled signal.

The secondary requirement or the secondary regulation can also be triggered by written or verbal instruction to the operating personnel of the power plant.

Embodiments of the invention with reference to the drawings and the description are explained in more detail.

Fig. 1

einen Auszug aus den britischen Elektrizitätsnetz-Vorschriften (Grid Code (UK)), wobei der Auszug das minimale Anforderungsprofil der Frequenzabhängigkeit für eine 0,5 Hz Frequenzänderung von der Soll-Frequenz aufzeigt (Minimum Frequency Response Requirement Profile for a 0,5 Hz frequency change from Target Frequency),

Fig. 2

einen Auszug aus den britischen Elektrizitätsnetz-Vorschriften (Grid Code (UK)), wobei der Auszug die Interpretation der Primär- und Sekundärregelung bzw. Primär- und Sekundäranforderung (Interpretation of Primary and Secondary Response Values) aufzeigt,

Fig. 3

schematisch dargestellt eine Anordnung zur Verbesserung des dynamischen Verhaltens eines kohlegefeuerten Kraftwerkes bei primären und/oder sekundären Anforderungen des Elektrizitätsnetz-Betreibers an die Stromabgabe in das Netz, wobei die Kohlemahlanlage einschließlich Kohlenstaubleitungen der Feuerung des Kraftwerkes dargestellt ist,

Fig. 4

schematisch dargestellt die Relation einer Leistungserhöhung in Abhängigkeit der Zeit und des Feuerungsverfahrens.

It shows:

Fig. 1

an extract from the British Electricity Grid Regulations (Grid Code (UK)), the extract shows the minimum requirement profile of the frequency dependence for a 0.5 Hz frequency change from the desired frequency (Minimum Frequency Response Requirement Profile for a 0.5 Hz frequency change from target frequency),

Fig. 2

an excerpt from the British Grid Code (UK), the excerpt showing the interpretation of Primary and Secondary Control and Primary and Secondary Response Values, respectively;

Fig. 3

schematically illustrated an arrangement for improving the dynamic behavior of a coal-fired power plant at primary and / or secondary requirements of the electricity network operator to the power delivery to the network, the coal mill including coal dust pipes of the firing of the power plant is shown,

Fig. 4

schematically shows the relation of a power increase as a function of time and the firing process.

In an electrical power system, the power generated must be constantly in balance with the load power. Changes in the load of the load or faults in power plants affect this balance and cause frequency deviations in the network to which the machines participating in the primary control or the primary demand react. The primary control or equivalent primary demand ensures, due to its control behavior, the restoration of the balance between generated and consumed power within a few seconds, keeping the frequency within the permissible limits. In the electricity grid, after regulating a sudden change in the power consumed or generated by the primary control or the primary demand, there are quasi-stationary deviations (with respect to the set values) both the frequency Δf and the transfer power ΔPi between the individual control zones. In this context, the secondary control or secondary demand comes into operation, the objective of which is to return the frequency to its desired value and the transfer rates to the agreed values and thus to have the entire activated primary control capacity available as a reserve again.

FIG. 1 shows the Interpretation of Primary and Secondary Control Performance and Primary and Secondary Response Values of the UK Electricity Grid Regulations (Grid Code UK), which are based on a Frequency Change of -0.5 Hz from the target frequency of the electricity grid has to be made. The diagram of Fig. 1 indicates that a power plant connected to the electricity grid in accordance with the primary control P must react with a plant response within a period of time T _Sp of 10 seconds and thereby the power plant output must be increased. The amount of power increase within this period T _Sp depends on the load range with which the power plant is currently operating at the time of the frequency drop. For example, the British Electricity Network regulations set at a specified requested Minimum load (minimum generation) of 65% of the rated power RC (Registered Capacity) of the power plant determines that at this part load the power plant output must be increased within 10 seconds by 10% (percentage A _P ) of the nominal power or capacity RC of the power plant ( please refer FIG. 2 ). According to the FIG. 2 (the abscissa shows the load range (in% of RC) of the power plant, the ordinate shows the primary and secondary control ranges (in% of the RC)) is the increase by 10% of the nominal capacity RC of the power plant between the partial load range of 65 to 80% to guarantee the power plant rating RC. Between the partial load range of 80 to 100% of the power plant rated power RC, the power increase decreases linearly from 10% to 0.

In the case of a frequency overrun or reduction of the primary and / or secondary requirements of the electricity network operator to the power delivery to the grid is in accordance with FIG. 2 It is planned to lower the power plant capacity in the partial load range between 95% and 70% of the nominal power of the power plant within 10 seconds by 10% of the nominal power RC of the power plant. Between the partial load ranges of 70% to 65% of the power plant rated power RC, the power reduction decreases linearly from 10% to approx. 6.5 and between 100% and the partial load range of 95% the power reduction increases linearly from approx. 5% to 10%. FIG. 2 also indicates the minimum load (minimum generation MG) of the power plant required by the UK electricity grid, which is 65% of the rated power of the power plant.

FIG. 3 shows by way of example how these requirements set out in the British Electricity Grid Regulations can be met. For this purpose, the furnace 1 of the power plant according to the invention and not shown, for example, with four coal grinding 2.1, 2.2, 2.3, 2.4 formed, all of which directly fire the combustion chamber of the power plant, not shown (direct firing or direct firing system), wherein at least one of the coal -Mahlanlagen 2.1, 2.2, 2.3, 2.4 is designed such that so that the combustion chamber instead indirectly fired directly (indirect firing or indirect firing system) can be, ie that at least one of the coal grinding 2.1, 2.2, 2.3, 2.4 next to direct firing system is additionally formed with an indirect firing system.

With directly fired or a direct firing system is stated that the coal crushed in the coal grinding plant or coal mill 2.1, 2.2, 2.3, 2.4 coal fed through coal dust lines 3.1, 3.2, 3.3, 3.4 by means of a carrier gas or air directly to the combustion chamber and is burned in it. It can according to FIG. 3 of each coal grinding 2.1, 2.2, 2.3, 2.4 are served in each case a burner level and emanating from the respective coal grinding plants 2.1, 2.2, 2.3, 2.4 outgoing pulverized coal pipes 3.1, 3.2, 3.3, 3.4 each serve the burner, not shown in the respective Corners or side walls of the generally rectangular combustion chamber of the coal-fired power plant.

With indirectly fired or an indirect firing system is stated that in the coal grinding plant or coal mill 2.1, 2.2, 2.3, 2.4 comminuted or ground coal on coal dust lines 3.1, 3.2, 3.3, 3.4 discharged and initially directed towards the combustion chamber is then, but in each case one in the pulverized coal pipe 3.1, 3.2, 3.3, 3.4 arranged pulverized coal switch 6 and via storage lines 7.1, 7.2, 7.3, 7.4 a common for all storage lines separator 4 is supplied. In the separator 4, the pulverized coal is separated from the carrier gas or carrier air or separated and fed via a connecting line 8 to a silo 5 and stored therein. About supply lines 9.1, 9.2, 9.3, 9.4 and arranged in these supply lines and controlled metering organs 10, the coal dust from the silo 5 deducted and via a respective feeder 15 and a further pulverized coal switch 13 the pulverized coal pipes 3.1, 3.2, 3.3, 3.4 downstream of the first pulverized coal switches 6 are supplied to be conveyed by these in the combustion chamber. To transport the coal dust drawn off from the silo 5 into the combustion chamber, the feed devices 9.1, 9.2, 9.3, 9.4 located downstream of the delivery members 10 are supplied via a conveying gas line 11 with a conveying gas, for example air, which is supplied by a conveying gas blower 12 is introduced. The feeder 15 may be, for example, an injector, a feed shoe, a dust pump or the like.

The separated in the separator 4 carrier gas or carrier air is discharged via a carrier gas discharge line 14 and fed to the atmosphere, where it is previously cleaned again in a Staubabscheidesystem. The carrier gas may also be introduced into the combustion chamber or into the combustion chamber downstream flue passages of the coal-fired power plant instead of into the atmosphere and freed of dust in the existing dust collection system (e.g., E-filter, baghouse filter or the like) of the power plant.

Deviating from FIG. 3 each of the storage lines 7.1, 7.2, 7.3, 7.4 each have their own separator 4 and its own downstream silo 5, from which then depart the respective supply lines 9.1, 9.2, 9.3, 9.4.

In normal operation of the power plant, the coal grinding plants 2.1, 2.2, 2.3 of the furnace 1 work according to FIG. 3 in such a way that the pulverized coal pulverized therein is fed directly to the combustion chamber for combustion via the respective pulverized coal pipes 3.1, 3.2, 3.3, 3.4. In the coal grinding plant 2.4, which is an example (it may be any other grinding system instead of the grinding plant 2.4) in addition to the direct firing system is additionally formed with an indirect firing system, are in the coal dust lines 3.1, 3.2, 3.3, 3.4 respectively arranged pulverized coal Turnouts 6 and 13 are set such that the pulverized coal pulverized in the coal grinding plant 2.4 is not fed directly to the combustion chamber but via the silo 5 to the combustion chamber. For this purpose, arranged in the supply lines 9.1, 9.2, 9.3, 9.4 metering organs 10 and feeders 15 in operation and conveying gas is provided by the conveying gas line 11 and the conveying gas blower 12 to the feeders 15. The conveying gas takes in the feeders 15 each assigned by the metering organs 10 coal dust and promotes him into the combustion chamber. The mode of operation of the grinding plant 2.4 is such that, as a rule, at the beginning of the operation, the grinding capacity of the grinding plant 2.4 compared to the grinding capacity of the grinding plant 2.1, 2.2, 2.3 or 2.3 compared to the current demand of the grinding capacity of the grinding plant 2.4 or compared to the instantaneous actual performance of the grinding plant 2.4 is increased to fill with the oversupply of crushed fuel, the storage volume V _Sp having silo 5 volume side about half. After the filling of the silo 5, the grinding capacity of the grinding plant 2.4 is equalized with those of the grinding plant 2.1, 2.2, 2.3 or the current demand of the grinding capacity of the grinding plant 2.4. With the exception of the filling process of the silo 5, the discharge or delivery rate of the metering elements 10 corresponds to the volume grinding performance of the grinding plant 2.4, ie after the filling process, the amount of pulverized coal from the silo 5 is discharged as produced by the grinding plant 2.4 and entered into the silo 5 , with smallest losses in the separator 4 are taken into account.

In the case of a change in frequency or a drop in frequency or a frequency undershoot of, for example, 0.5 Hz of the electricity network is on the grid control of the electricity grid or their primary and / or secondary requirements of the electricity network operator to the power delivery to the grid, the block power control of the coal-fired power plant influenced, which substantially increases the amount of coal discharged through the metering elements 10 from the silo 5 and the combustion chamber indirectly supplied pulverized coal compared to the instantaneous actual performance or against the by the coal grinding plants 2.1, 2.2, 2.3 each supplied coal dust. It can be placed in the combustion chamber for burning the coal dust stored in the silo 5 for these purposes in the shortest possible time and thus contribute significantly to the part of the furnace to improve the dynamic behavior of the coal-fired power plant. The current actual power refers to the power or the partial load with which the coal-fired power plant is currently operated and from which also the combustion chamber supplied amount of fuel and thus the respective throughput of the individual coal grinding 2.1, 2.2, 2.3, 2.4 is dependent ,

In the case of a frequency overshoot, for example, 0.5 Hz of the electricity network is on the grid control of the electricity grid or their primary and / or secondary requirements of the electricity network operator to the power in the Network affects the block power control of the coal-fired power plant, the amount of discharged through the metering elements 10 from the silo 5 and the combustion chamber indirectly supplied pulverized coal compared to the instantaneous actual performance or compared to the coal Mahlonlagen 2.1, 2.2, 2.3 respectively supplied amounts of coal dust significantly reduced, and thus substantially contributed to the improvement of the dynamic behavior of the coal-fired power plant as well as the increase in the amount of coal dust by the furnace. In this case, provided by the grinding plant 2.4 during this process and unneeded, ie excess, coal dust in the silo 5 cached.

To realize the improvement of the dynamic behavior of a coal-fired power plant, the coal grinding plant 2.4 downstream silo 5 is designed and formed with a corresponding capacity or storage volume V _Sp for the coal dust to be stored. But there are also other coal grinding plants in FIG. 3 exemplary four coal grinding plants 2.1, 2.2, 2.3, 2.4 are each formed with an indirect combustion system and thus with a silo 5 for storing coal dust. If, by way of example, two, three or all four coal grinding plants 2.1, 2.2, 2.3, 2.4 are additionally formed with an indirect firing system or an indirect firing system, then the entire required storage volume or the absorption capacity V _Sp of pulverized coal can be reduced to the existing number of silos 5 or the storage volume V _Sp are increased by the increased number of silos 5. The additional training of several grinding plants with indirect combustion systems and thus increased coal dust storage capacity in the silos 5, the dynamics of the fuel allocation of the coal-fired power plant can be further improved if necessary. Through this fuel-side improvement of the dynamics and the primary and secondary reserve of the coal-fired power plant can be improved or raised.

The storage volume V _{Sp of} the silo 5 is designed such that during normal operation, ie at steady state, the storage volume V _{Sp of} the silo 5 is filled in about half and thereby has stored enough coal dust, in the event of Frequency drop or a primary and / or secondary requirement of the electricity network operator to the power delivery to the network, ie a transient state, to bring an increased amount of pulverized coal in the combustion chamber to improve the dynamic behavior of the power plant. On the other hand, the silo 5 must still have enough storage capacity to bring in the case of frequency exceeding or a primary and / or secondary requirement of the electricity network operator to the power delivery to the network, so again a transient state, a reduced amount of coal dust in the combustion chamber can and absorb the stored during the transient condition of the grinding plant 2.4 excess amount of coal dust in the silo 5 and store it.

In addition to the silo or silos 5, the metering devices 10, the feeders 15 and the coal dust lines (feed lines 9.1, 9.2, 9.3, 9.4 and coal dust lines 3.1, 3.2, 3.3, 3.4) downstream of the silo or the silo 5 up to the Combustion chamber dimensionally appropriate to be formed in order to pass the required amounts of fuel in the required short time and to be able to supply the combustion chamber. The conveying gas or carrying air required for this purpose is brought in by the conveying gas line 11 and regulated by means of the conveying gas blower 12.

FIG. 4 schematically shows the dynamic behavior of a direct and an indirect firing or of a direct and an indirect firing system of a coal-fired power plant. While the increase of the steam generator power from L ₀ to L ₁ in direct firing from t ₀ requires the time t ₂ , the increase of the same steam generator power in indirect firing from t ₀ requires only the time t, and thus comes with an ideal, jump-shaped Increase within a time t ₀ (step response) much closer. By the inventive method or the inventive arrangement, at least one of the coal grinding 2.1, 2.2, 2.3, 2.4 next to the direct firing with an indirect firing form and operate as indirect firing and a frequency change in the electricity grid or a primary and / or or secondary Request of the electricity network operator to the power delivery to the network to increase or decrease the amount of discharged from the silo 5 and the combustion chamber indirectly supplied coal dust against the indirectly supplied amount of pulverized coal at a stable grid frequency, the dynamic behavior of the firing FIG. 4 and thus also the system response, ie the dynamic behavior of the coal-fired power plant can be substantially improved. Increasing the steam generator capacity from L ₀ to L ₁ represents a percentage A _{P of} the power plant rated power RC, for example an increase of 10% of the power plant rated power RC.

In the case of maintenance or failure of a metering element 10 or a feeder 15 of the indirect firing system to the coal grinding 2.4, the coal grinding 2.4 can be operated as a direct firing system by the coal dust switches 6 and 13 converted and the coal dust through the pulverized coal pipes 3.1, 3.2, 3.3, 3.4 is fed directly to the combustion chamber and the silo 5 and the allocating organs 10 and the feeders 15 thus bypassed (bypass). If further coal grinding plants 2.1, 2.2, 2.3 are additionally constructed with an indirect firing system, then one or more coal grinding plants can be converted to the operation as an indirect firing system by conversion of the pulverized coal switches 6 and 13 and thus the indirect firing system in maintenance temporarily replace the coal grinding plant 2.4.

Of course, with the inventive method and the arrangement according to the invention with regard to the primary and secondary control and the primary and secondary demand and therefrom the system response or with regard to the improved dynamic behavior of a coal-fired power plant not only the exemplary British Electricity Grid regulations and their requirements are respected or met, but also other national or international regulations that require a fast or improved dynamic behavior of a coal-fired power plant. For this purpose, only if necessary, the storage volume V _Sp of the silos or 5 and the Throughput of the metering organs 10 and / or the feeders 15 and / or the conveying gas blower 12 are adapted to the regulations.

LIST OF REFERENCE NUMBERS

1

heating

2.1

coal grinding plant

2.2

coal grinding plant

2.3

coal grinding plant

2.2

coal grinding plant

3.1

Pulverized coal conduit

3.2

Pulverized coal conduit

3.3

Pulverized coal conduit

3.4

Pulverized coal conduit

4

separators

5

silo

6

Coal dust Soft

7.1

storage line

7.2

storage line

7.3

storage line

7.4

storage line

8th

connecting line

9.1

feed pipe

9.2

feed pipe

9.3

feed pipe

9.4

feed pipe

10

given organ

11

Conveying gas line

12

Conveying gas blower

13

Coal dust Soft

14

Carrier gas-discharge

15

feeder

Claims (9)

1. Method for improving the dynamic behavior of a coal-fired power plant for primary and/or secondary requirements of the power grid operator with respect to the current output into the grid, wherein the power plant has a nominal output (RC) and is operated with a furnace (1) comprising at least one firing box for the firing of the fuel, at least two coal grinding plants (2.1, 2.2, 2.3, 2.4) for the grinding of the fuel having a direct firing system, wherein, in the direct firing systems, coal dust lines (3.1, 3.2, 3.3, 3.4), which originate from the grinding plants (2.1, 2.2, 2.3, 2.4), serve the associated burners, wherein at least one of these coal grinding plants (2.4) comprises an additional indirect firing system, wherein the indirect firing system has at least one silo (5) and apportioning organs (10), and the coal dust is indirectly fed via the indirect firing system to the firing box and the further coal grinding plant(s) (2.1, 2.2, 2.3) directly feeds the coal dust to the firing box via the direct firing system, and wherein upon an increase of the primary and/or secondary requirements of the power grid operator with respect to the current output into the grid the coal dust quantity indirectly fed in via silo (5) and apportioning organs (10) compared with the present actual output or compared with the coal dust quantity fed in through each of the coal grinding plant(s) (2.1, 2.2, 2.3, 2.4) is increased and in the process coal dust stocked in the silo (5) is withdrawn and introduced into the firing box, and wherein upon a reduction of the primary and/or secondary requirements of the power grid operator with respect to the current output into the grid the coal dust quantity indirectly fed in via silo (5) and apportioning organs (10) is reduced compared with the present actual output or compared with the coal dust quantity fed in through each of the coal grinding plant (s) (2.1, 2.2, 2.3, 2.4) is reduced and in the process coal dust excessively produced by the grinding plant (2.4) stored in the silo (5) characterized in that, in the case of indirect feeding, the coal dust is separated from a carrier gas in a common separator (4) and fed to the silo (5) via a coal dust switch (6) arranged in each of the coal dust lines (3.1, 3.2, 3.3, 3.4) and via storage lines (7.1, 7.2, 7.3, 7.4), and in that the coal dust is fed to the coal dust lines (3.1, 3.2, 3.3, 3.4) downstream of the first coal dust switch (6) via feed lines (9.1, 9.2, 9.3, 9.4) and apportioning organs (10) arranged in the feed lines (9.1, 9.2, 9.3, 9.4), via in each case a charging device (15) and a further coal dust switch (13).
2. Method according to Claim 1, characterized in that the silo (5) has a storage volume (V_Sp) and in normal operation of the indirect firing system with regard to volume is filled to about half with coal dust for stocking and use upon an increase of the primary and/or secondary requirements of the power grid operator with respect to the power output into the grid and the remaining storage volume is used for receiving and storing the excess produced coal dust upon reduction of the primary and/or secondary requirements of the power grid operator with respect to the current output into the grid.
3. Method according to Claim 1, characterized in that the increase or reduction of the indirectly fed-in coal dust quantity is effected through controlled increase or reduction of the throughput rate of the apportioning organs (10).
4. Method according to Claim 1, characterized in that upon an increase or a reduction of the indirectly fed-in coal dust quantity the volumetric flow of a conveying gas blower (12) is increased or reduced in a controlled manner.
5. Method according to Claim 3 or 4, characterized in that the increase or the reduction of the throughput rate of the apportioning organs (10) and/or the increase or reduction of the volumetric flow of the conveying gas blower (12) is brought about by the block output control of the coal-fired power plant influenced by the requirements of the power grid.
6. Method according to Claim 1, characterized in that the primary requirement is triggered through a remote-controlled signal.
7. Method according to Claim 1, characterized in that the secondary requirement is triggered through a remote-controlled signal.
8. Method according to Claim 1, characterized in that the secondary requirement is triggered through written or oral instruction to the operating personnel of the power plant.
9. Coal-fired power plant, wherein the power plant has a nominal output (RC) and is designed with a furnace (1) which substantially comprises at least one firing box for the firing of the fuel, at least two coal grinding plants (2.1, 2.2, 2.3, 2.4) for the grinding of the fuel comprising a direct firing system, wherein, in the direct firing systems, coal dust lines (3.1, 3.2, 3.3, 3.4), which originate from the grinding plants (2.1, 2.2, 2.3, 2.4), serve the associated burners, wherein at least one of these coal grinding plants (2.4) comprises an additional indirect firing system, wherein the indirect firing system has at least one silo (5) and apportioning organs (10), and the coal dust can be indirectly fed via the indirect firing system to the firing box and with the further coal grinding plant(s) (2.1, 2.2, 2.3) the firing box can be directly fed with coal dust via the direct firing system, characterized in that in in each coal dust line (3.1, 3.2, 3.3, 3.4) of the indirect firing systems there is a first coal dust switch (6), in that the first coal dust switches (6) are connected to storage lines (7.1, 7.2, 7.3, 7.4), which open out in a common separator (4), in that the separator (4) is connected to the silo (5) via a connecting line (8), in that the silo (5) is connected via feed lines (9.1, 9.2, 9.3, 9.4) and further coal dust switches (13) to the coal dust lines (3.1, 3.2, 3.3, 3.4) downstream of the first coal dust switches (6), and in that in the feed lines (9.1, 9.2, 9.3, 9.4) there are respectively an apportioning organ (10) and a charging device (15).

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