EP2906876B1 - Method for operating a steam generator - Google Patents

Description

The invention relates to a method for operating a steam generator with a lignite-fired boiler, with at least one mill for grinding the brown coal.

A well-known principle of direct dust injection by means of a pulverized coal drying plant, for example, in the Publication Helmut Effenberger, "steam generation", Springer-Verlag, ISBN 3-540-64175-0 , described. In such a method is sucked back as the drying gas flue gas, which is referred to in the sense of the present application as a drying flue gas stream. For this purpose, the mill is connected via a flue gas recirculation to the end of the combustion chamber, where the drying flue gas stream required for drying the lignite coal is removed at a temperature between about 800 ° C and about 1200 ° C.

A method of the type mentioned is, for example, from DE 42 03 713 C2 C2 or from the DE 43 23 469 A1 known. The process involves the milling of brown coal wet lignite in at least one mill, which is supplied with a drying flue gas stream, which is branched off from the boiler behind the convection train and is introduced into the mill at a temperature of about 350 ° C. In the mill, the brown coal is comminuted to the grain band required for combustion in the boiler and at the same time dried in the inert flue gas atmosphere. Part of the drying flue gas stream is used as a carrier gas for transporting the dried fuel to the burners of the boiler. This combined predrying and grinding of lignite at a low temperature level is to be considered favorable in terms of the efficiency of the power plant.

Another variant of the drying of lignite, which has a favorable effect in terms of an increase in efficiency, is the drying of lignite in a separate drying unit in the form of a fluidized bed dryer. Such a method is for example from the DE 196 20 047 A1 known. It is known that by drying the brown coal before burning in the Steam generator of a power plant, a significant increase in efficiency can be achieved. The lignite brown coal has a water content of 45% to 65%, which is reduced by drying to 10% to 25%.

From the DE 195 18 644 C2 It is known to use the emerging from the drying plant high-energy vapors in the dryer itself. In the DE 195 18 644 C2 For this purpose, it is proposed to compress at least a partial stream of the vapor and to supply the heat exchanger as a heating medium, wherein the vapor condenses at least partially, so that for the desired drying of the fuel, the heat of vaporization of the vapor can be utilized to a large extent.

Further known processes for drying lignite using fluidized bed dryers are described, for example, in the publications DE 103 19 477 A1 and DE 10 2009 035 062 A1 described.

The dry lignite from the fluidized bed drying usually has a maximum water content of 25 percent by mass and a mean particle diameter D50 0.4 to 0.8 mm, so that it can be added directly, possibly without subsequent grinding in the boiler and burned there.

The combustion temperature of pre-dried lignite, hereinafter referred to as dry lignite (TBK), is about 500 ° C higher than that of lignite.

Since the known lignite-fired lignite boilers are designed predominantly for the burning of lignite coal, ie for the burning of lignite coal, which was subjected to grinding drying, the dry lignite in the steam generators of the applicant was mitverbrannt so far as additional fuel to increase the efficiency. Co-incineration of about 25% dry brown coal from a separate drying aggregate, based on the calorific value of the fuel mixture, has proved favorable.

The dry lignite can be introduced into the steam generator, for example, via so-called swirl burners, which are also used in hard coal firing.

Practical experiments and combustion chamber simulations in the co-incineration of dry lignite coal in raw lignite fired steam generators have shown, however, that in the field of dry lignite burners or swirl burners relatively high temperature peaks occur in the boiler, which can lead to ash softening or ash melting operations and thus to slagging. This in turn reduces the availability of the steam generator.

That's why swirl burners should not be too big. Swirl burners are limited in their size. In general, a thermal power of 60MW is considered technically feasible. For large brown coal boilers with dry lignite auxiliary firing, this requires a correspondingly high number of swirl burners. This leads to high investment costs as a result.

Dry lignite is a comparatively low-cost start-up and support fuel for coal-fired steam generators. In this case, the vortex burners previously described with regard to their disadvantages are customary, since they ensure a stable flame and a good burnout even in still cold fire spaces.

The invention is therefore based on the object to provide a process for the co-incineration of dry lignite in raw lignite-fired steam generators, which takes into account the aforementioned problem.

• Mahlen von grubenfeuchter Rohbraunkohle in der ersten Mühle,
• Abzweigen eines Trocknungs-Rauchgasstroms aus dem Kessel und
• Trocknen der Rohbraunkohle in der ersten Mühle in direktem Kontakt mit dem Trocknungs-Rauchgasstrom,
• Einblasen eines Rauchgas-Braunkohlenstroms aus der ersten Mühle in den Kessel,
• Trocknen einer Teilmenge von Rohbraunkohle durch indirekte Trocknung in einem separaten Trocknungsaggregat zu Trockenbraunkohle und
• Einspeisen der Trockenbraunkohle in den Rauchgas-Braunkohlenstrom der ersten Mühle vor einer Brennerzuteilung oder in eine Brennerzuteilung.

The object is achieved by a method for operating a steam generator with a lignite-fired boiler, with at least one first mill for grinding the lignite, the method comprising the following method steps:

• Grinding of pit-wet raw lignite in the first mill,
• Branching a drying flue gas stream from the boiler and
• Drying the raw lignite in the first mill in direct contact with the drying flue gas stream,
• Blowing a flue gas lignite stream from the first mill into the boiler,
• Drying of a subset of raw lignite by indirect drying in a separate drying unit to dry lignite and
• Feeding the dry lignite into the flue gas lignite stream of the first mill prior to a burner allotment or into a burner allotment.

Dry lignite in the context of the invention is to be understood as meaning lignite which has dried from the mined mining product to a dried finished product having a moisture content of 10 to 30% by mass, preferably between 10 and 25% by mass and a mean grain diameter D50 of 0.4 to 10 mm in a drying unit was produced.

Mine-wet crude lignite refers to the untreated crushed and optionally pre-shred mined lignite that has a moisture content of between 45 and 65% by mass.

Under ground-dried raw lignite is to be understood in a mill with simultaneous drying to a ready-to-burn finished product lignite, of which about 85 to 95 percent by mass has a particle size of less than 1 mm.

A burner allocation in the context of the invention is to be understood as meaning a fuel supply line from the mill to a single burner, a burner group or a burner level. Burner allocation ends where the fuel is ignited by supplying combustion air. Burner allocation may include classifiers and distributors.

The process of the invention can be summarized in that in the boiler of the steam generator in addition to meal-dried raw lignite dry lignite is mitverbrannt, the dry lignite is the suspension gas / fuel mixture from the mill before the allocation or in the allocation to the individual coal burner abandoned. Unlike a previously practiced process variant, the dry lignite is not burned through separate swirl burner in the boiler, but the dry lignite is added to the flue gas lignite stream of the mill before the boiler. Carrying gas in the context of the invention is to be understood as meaning a mixture of flue gas, evaporated water and combustion air.

In the proposed method dry lignite can be redirected from start-up burners of the steam generator to the operated with raw lignite first mill via a switching device as soon as a starting and support firing is no longer needed.

By combustion chamber simulation calculations, the Applicant was able to find that with such a measure, surprisingly, temperature peaks within the boiler in the area of the burners can be avoided.

Appropriately, the grinding of the raw lignite takes place in inerted with the drying flue gas stream atmosphere. Inertized according to the invention means that the oxygen content in the flue gas drying stream is adjusted to an oxygen content of <12% by volume.

In an advantageous variant of the method according to the invention it is provided that the dry lignite is fed from the separate drying unit in the first mill and is subjected there to the raw lignite a Nachmahlung. The dry brown coal is subsequently dried in an inertized atmosphere and comminuted. This has the procedural advantage that a Nachsichtung and / or regrinding the dry lignite behind the drying unit is not essential. The influence of a The fluctuating moisture content of the dry brown coal is thereby eliminated. In general, moisture, bulk density and grain variations of the dry lignite due to the additional mill drying in mixture with the lignite coal are not critical. This also makes it possible to operate a connected drying unit with optimized performance. Finally, it has also been found that the inventive method is favorable in terms of NOx concentration in the flue gas, since by the addition of dry lignite in the flue gas lignite stream concentration of the dust fraction takes place at the burners, so that the Traggasanteil is reduced at the burners. As a result, this leads to a lower NOx concentration in the flue gas / exhaust gas than in a conventional furnace, in which the fuel-related carrying gas amount is usually higher.

Another advantage is that the controllability of the furnace is improved. If the amount of raw lignite is increased, the first mill reacts only with a longer dead time, because the mill for the higher amount of coal has to be moved to another operating point, or an additional mill must be put into operation. By contrast, an increase in the amount of dry lignite coal in a relatively short time means that a larger quantity of fuel is available. This improves the controllability of the entire steam generator system.

Preferably, at least one indirectly heated dryer is provided as a separate drying unit. This can be, for example, a fluidized-bed dryer.

In an alternative variant of the method according to the invention, it is provided that a second mill is used as a separate drying unit in which drying is carried out in an atmosphere rendered inert by a drying flue gas stream. In other words, as a separate drying unit, a drying unit can find application in which the lignite is subjected to direct drying in direct contact with flue gas.

In this case, it is advantageous if the dried lignite from the second mill is fed as dry lignite in the flue gas brown waste stream of the first mill. It can be provided, for example, that the dried lignite from the second mill is completely fed as dry lignite in the flue gas lignite stream of the first mill, so that the second mill is not charged in the usual way unmitelbar the boiler with a flue gas lignite stream.

The inventive method is characterized in particular by the fact that the boiler is fired with a tangential firing with jet burners to which the flue gas brown coal stream is allocated. Jet burners are in operation much less susceptible to interference than omnidirectional burners or swirl burners. Jet burners are also structurally much simpler. These are based on the principle that the fuel / carrier gas channel is formed substantially as a rectangular shaft, which is enclosed in each case by corresponding secondary air ducts. A turbulence and twisting of the flue gas / fuel flow or the secondary air flow with corresponding flow dynamics acting internals is not required.

In a particularly advantageous variant of the method according to the invention, it is provided that the dry brown coal from the separate drying unit is fed into a carrier gas recirculation line of the first mill. This makes it possible to carry out the inventive method substantially without constructive adjustment of existing mills. Carrying gas recirculation lines are commonly used to control the performance of the mill by, if appropriate, diverting a portion of the carrier gas behind the mill and recirculating it through the mill.

If a classifier mill is used as the first mill, the dry brown coal can be fed into a solids return of a mill classifier. Even such a procedure requires hardly any structural adjustments of existing mills.

In a further advantageous variant of the method according to the invention it can be provided that the dry lignite is introduced with at least one screw conveyor in an inlet region of the first mill.

Alternatively, an injection of dry brown coal into the first mill with inert gas, for example low-pressure steam, or, for example, dry lignite mixed with recirculated carrier gas as the pumped medium. It is expedient to blow dry brown coal in such a way that rapid mixing of the dry brown coal with the raw lignite takes place.

Furthermore, it is also possible to separate the dry lignite coal by means of a separator from the fluid when compressed air is used as the conveying medium, so that the dry lignite is introduced without air in the mill, for example via a rotary valve.

Finally, it may be provided because of a tendency to ignite of dry lignite, to supply the dry lignite at a cold spot as possible the mill.

The entry point for the dry brown coal may also be provided after the first mill in the burner allocation.

If a mixture of ground-burnt lignite and dry lignite is to be charged approximately evenly all burners of the boiler, it is advantageous to feed the lignite already in the first mill or in the flue gas lignite stream immediately behind the first mill. In principle, however, it may also be desirable to set a different fuel concentration at firing above the height of the steam generator. In any case, it is favorable for the combustion of the fuel to set a higher fuel concentration at the lower burners, which are considered above the height of the steam generator, than in the case of the upper burners. In this case, it is favorable if, viewed in the flow direction of the flue gas brown coal stream, the dry brown coal is behind the first one Mill and behind a branch of the flue gas lignite stream to a Nachreaktionsbrenner or Brüdenbrenner, the flue gas lignite stream for the main burner is added. As a result, a concentration of the fuel at the lower burners (main burner) is achieved, so that a low NOx combustion is achieved.

As mills, for example, impact wheel mills can be used in which takes place in a known manner an impact stress of the ground material. Likewise, so-called wet fan mills or blower mills can be used. When using a beater mill or blower mill as the first mill in the context of the invention, the dry brown coal can be added, for example, in the proposal part of the mill.

In a variant of the method according to the invention, it is provided that between 15% and 35% of the lignite based on the calorific value at full load of the plant to be burned lignite drying in a separate drying unit is subjected, whereas the remaining portion of the lignite to be burnt conventionally a direct drying with flue gas is subjected to grinding in the first mill.

In a preferred variant of the method according to the invention it is provided that the indirect drying of the brown coal is carried out in at least one fluidized bed dryer.

In particular, when using a fluidized bed dryer this can be operated optimized performance without a subsequent grinding by roller mill. When grinding dry lignite in a vertical roller mill, special explosion protection measures must be taken into account so that any additional regrinding is a special procedural simplification.

Conveniently, the energy of the resulting in the indirect drying vapor is at least partially used for preheating the combustion air and / or the boiler feed water.

In an expedient and advantageous variant of the method according to the invention, it is provided that the amount of dry brown coal fed in is regulated as a function of the load of the steam generator.

With increased load, the amount of dry lignite fed in can be increased in the short term. Since this dry lignite may not have to go through the Mahldrocknungszyklus the first mill, can be relatively easily accomplish this way a power control of the steam generator.

• Durch eine Trockenbraunkohlezufuhr direkt in die Rohbraunkohlemühlen oder direkt stromabwärts des Mühlenaustrages, aber vor dem Ende einer Brennerzuteilung, kann die maximale Temperatur im Brennergürtelbereich des Kessels gegenüber einer Trockenbraunkohlenzufuhr über separate Drallbrenner deutlich abgesenkt werden. Durch die Vermeidung solcher Temperaturspitzen kann eine übermäßige Schlackebildung zuverlässig verhindert werden. Bekanntlich hat die Braunkohle eine mineralogische Zusammensetzung, welche schlackebildende mineralische Bestandteile umfasst, die insbesondere bei höheren Temperaturen zur Bildung von Ablagerungen im Kessel führen.
• Durch die Zugabe der Trockenbraunkohle in die Mühlen oder hinter einer Mühle, aber vor dem Ende einer Brennerzuteilung, können alle Vorteile einer Tangentialfeuerung in Bezug auf die Durchmischung des Brennstoffs mit der Verbrennungsluft genutzt werden. Bei Tangentialfeuerungen bietet sich die Verwendung von Strahlbrennern an, da diese den Brennstoff relativ weit in den Feuerraum tragen und dort verbrennen, so dass die Genauigkeit der Brennstoffzuteilung an einzelne Brenner unkritisch ist.

• Feuchte, Schüttdichte und Körnungsschwankungen der Trockenbraunkohle sind aufgrund der Mahltrocknung in Mischung mit der Rohbraunkohle unkritisch.
• Es ist ein leistungsoptimierter Betrieb einer angeschlossenen Wirbelschichttrocknung ohne besondere Kontrolle von Restfeuchte und Korngrößenverteilung der Trockenbraunkohle möglich.
• Durch eine Erhöhung der Staubkonzentration im Traggas an den Brennern ist eine NOx-Reduzierung im Rauchgas möglich.

The advantages of the process according to the invention can be summarized as follows:

• By supplying a dry lignite feed directly into the raw lignite coal mills or directly downstream of the mill discharge, but before the end of a burner allocation, the maximum temperature in the burner belt area of the boiler can be significantly lowered over a dry lignite feed via separate swirl burners. By avoiding such temperature peaks, excessive slag formation can be reliably prevented. As is known, the brown coal has a mineralogical composition which comprises slag-forming mineral constituents which lead to the formation of deposits in the boiler, especially at relatively high temperatures.
• By adding the dry lignite into the mills or behind a mill, but before the end of a burner allotment, all the advantages of a tangential firing with respect to the mixing of the fuel with the combustion air can be utilized. Tangential firing offers the use of jet burners, as these are the fuel carry relatively far into the combustion chamber and burn there, so that the accuracy of the fuel allocation to individual burner is not critical.

• Moisture, bulk density and grain fluctuations of the dry lignite are uncritical due to the mill drying in mixture with the lignite coal.
• It is a performance-optimized operation of a connected fluidized bed drying without special control of residual moisture and grain size distribution of the dry lignite possible.
• By increasing the dust concentration in the carrier gas at the burners NOx reduction in the flue gas is possible.

The invention will be explained below with reference to an embodiment with reference to the accompanying drawings.

The process flow diagram shown in the figure shows a boiler 1 with a combustion chamber 2 and a convection part 3. The convection part 3 comprises in a known manner heating surfaces, by means of which a convective heat transfer takes place in the heating medium. Within the combustion chamber 2 is dried dust-like lignite burned on dust burners, which are designed as jet burners. In the combustion chamber 2, the radiant heat is transferred to the circulating in the steam cycle of the boiler 1 heating medium. The steam generated by the boiler 1 can be expanded in a steam turbine to generate electrical energy, alternatively, the steam can also be used as process heat in other coupled processes.

4 denotes a suction, via which the flue gas is conveyed from the boiler 1 via a flue gas cleaning device to a chimney.

The fuel in the form of dry brown coal and raw lignite coal is ground in a beater mill 5 and distributed over a burner allocation 6 on several jet burners.

In the illustrated embodiment, the beater wheel mill 5 on the one hand pit-wet raw lignite 7 and on the other hand dry lignite 8 supplied from a drying unit, not shown. The dry lignite 8 is added either to a carrier gas recirculation line 15 or the beater wheel mill 5 directly.

9 with a drying flue gas stream is referred to, which is taken at the upper end of the furnace 2 of the boiler 1 at a temperature of between about 800 ° C and 1200 ° C and the beater wheel 5 is supplied.

From the beater mill 5, a flue gas lignite stream 10 is supplied to the burner allocation 6, behind the flue gas lignite stream 10 combustion air 11 is added. The combustion air 11 is taken from the atmosphere and preheated via a combustion air preheater 12. The combustion air preheater 12 is operated with the flue gas stream 13 from the convection part 3 of the boiler 1. A portion of the combustion air 11 is added to the drying flue gas stream 9 in front of the beater wheel mill 5, a further part is added to the boiler 1 as burnout air 14.

The raw lignite 7, which comes from a pre-crushing, the beater 5 is abandoned, the raw lignite 7 is crushed in the beater 5 in direct contact with the extracted from the combustion chamber 2 flue gas in mixture with combustion air 11 under impact stress. Dry lignite 8 from a fluidized bed dryer, not shown, is also given the beater wheel mill 5 with a water content of, for example, between 10 and 30 percent by mass. This is entered, for example, in a solids return of a mill classifier or at a suitable location directly in the mill or in the Traggasrezirkulationsleitung 15 of the impact wheel 5. In the beater mill 5, the raw lignite 7 is together with the Dry lignite 8 reground and after-dried and intimately mixed. The flue gas lignite stream 10 (carrier gas stream) leaving the impact wheel mill 5 comprises, for example, about 15% to 35%, preferably about 20% to 25% dry lignite 8 based on the calorific value of the total fuel used at full load of the boiler.

According to the invention, the dry brown coal 8 is added to the flue gas brown coal stream 10 behind the impact wheel mill 5, but before the end of the burner allocation 6.

In the described embodiment, the combustion air 11 is preheated by means of the flue gas stream 13 in the combustion air preheater 12. The invention is to be understood that instead of a combustion air preheating 12 with flue gas combustion air preheating is used, which is operated with low-temperature heat from the drying of the dry lignite.

LIST OF REFERENCE NUMBERS

1

boiler

2

firebox

3

convection

4

downdraft

5

Beater mill

6

Brenner allocation

7

raw lignite

8th

Dry lignite

9

Drying flue gas stream

10

Flue gas lignite power

11

combustion air

12

combustion air preheater

13

Flue gas stream

14

burn-out

15

Traggasrezirkulationsleitung

Claims (15)

1. Method for operating a steam generator with a lignite-fired boiler and with at least one first mill for grinding lignite, the method comprising the following method steps:

   - Grinding of pit-wet raw lignite in the first mill,

   - Branching off of a drying smoke gas stream out of the boiler and

   - Drying of the raw lignite in the first mill in direct contact with the drying smoke gas stream,

   - Injection of a smoke gas lignite stream out of the first mill into the boiler,

   and being characterized by the method steps:

   - Drying of a subquantity of raw lignite in a separate drying assembly into dry lignite and

   - Feeding of the dry lignite from the separate drying assembly into the smoke gas lignite stream of the first mill before burner allocation or into burner allocation.
2. Method according to Claim 1, characterized in that at least one indirectly heated drier is provided as a separate drying assembly.
3. Method according to Claim 1, characterized in that at least one second mill, in which drying is carried out in contact with the drying smoke gas stream in an inertized atmosphere, is provided as a separate drying assembly.
4. Method according to Claim 3, characterized in that the dried lignite from the second mill is fed at least partially as dry lignite into the smoke gas lignite stream of the first mill.
5. Method according to Claims 1 to 4, characterized in that the dry lignite from the separate drying assembly is fed into the mill and is subjected there, together with the raw lignite, to a regrinding.
6. Method according to one of Claims 1 to 5, characterized in that the boiler is fired by means of jet burners to which the smoke gas lignite stream is allocated.
7. Method according to one of Claims 1 to 6, characterized in that the dry lignite is fed into a carrier gas recirculation line of the first mill.
8. Method according to one of Claims 1 to 6, characterized in that the dry lignite is fed into a solids return of a mill separator of the first mill.
9. Method according to one of Claims 1 to 6, characterized in that the dry lignite is introduced into an entry region of the first mill by means of at least one worm conveyor.
10. Method according to one of Claims 1 to 9, characterized in that at least one beating wheel mill is used as the first mill.
11. Method according to one of Claims 1 to 10, characterized in that between 15% and 35% of the overall lignite to be burnt with respect to the calorific value of the lignite when the steam generator is under full load is subjected to drying in a separate drying assembly.
12. Method according to one of Claims 1 or 2 or 6 to 11, characterized in that the indirect drying of the lignite is carried out in at least one fluidized bed drier.
13. Method according to Claim 12, characterized in that the energy of the vapor occurring during indirect drying is utilized at least partially for preheating the combustion air and/or the boiler feed water.
14. Method according to one of Claims 1 to 13, characterized in that the dry lignite is diverted from starting burners of the steam generator into the smoke gas lignite stream of the first mill by means of a changeover device, as soon as starting and/or supporting firing for the steam generator is no longer required.
15. Method according to one of Claims 1 to 14, characterized in that the quantity of dry lignite fed in is regulated as a function of the load requirement for the firing power of the steam generator.

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