AU2018202163B2

AU2018202163B2 - Method for operating a steam generator

Info

Publication number: AU2018202163B2
Application number: AU2018202163A
Authority: AU
Inventors: Rainer Hesse; Bernhard Roper
Original assignee: RWE Power AG
Current assignee: RWE Power AG
Priority date: 2012-10-11
Filing date: 2018-03-27
Publication date: 2019-09-19
Anticipated expiration: 2033-10-10
Also published as: PL2906876T3; AU2018202163A1; EP2906876A2; CA2887109C; AU2013328717A1; DE102012019928A1; CN104781606B; EP2906876B2; WO2014057025A2; PL2906876T5; CN104781606A; WO2014057025A3; EP2906876B1; CA2887109A1

Abstract

METHOD FOR OPERATING A STEAM GENERATOR A method for operating a steam generator with a lignite-fired boiler (1) and with at least one first mill (5) for grinding lignite, the method comprising the following method steps: grinding of pit wet raw lignite (7) in the first mill (5), branching off of a drying smoke gas stream (9) out of the boiler (1) and drying of the raw lignite (7) in the first mill (5) in direct contact with the drying smoke gas stream (7), injection of a smoke gas lignite stream (10) out of the first mill (5) into the boiler (1), drying of a subquantity of raw lignite (7) in a separate drying assembly into dry lignite (8) and feeding of the dry lignite (8) from the separate drying assembly into the smoke gas lignite stream (10) of the first mill (5) before burner allocation (6) or into burner allocation (6).

Description

METHOD FOR OPERATING A STEAM GENERATOR

This is a divisional application of Australian Patent Application No. 2013328717, the content of which is incorporated herein by reference in its entirety.

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

A known principle of direct dust injection by means of a coaldust grinding and drying plant is described, for example, in the publication Helmut Effenberger, Dampferzeugung [Steam Generation], Springer-Verlag, ISBN 3-540-64175-0. In such a method, smoke gas sucked back as drying gas is used, which, in the context of the present application, is designated as a drying smoke gas stream. For this purpose, the mill is connected via a smoke gas return to the end of the combustion chamber where the drying smoke gas stream required for drying the raw lignite is extracted at a temperature of between approximately 800°C and approximately 1200°C.

A method of the type initially mentioned is known, for example, from DE 42 03 713 C2. The method comprises the grinding of pit-wet lignite in at least one mill which is acted upon with a drying smoke gas stream which is branched off, downstream of the convection draft, out of the boiler and is introduced into the mill at a temperature of about 350°C. In the mill, the lignite is comminuted to the grain band required for combustion in the boiler and is simultaneously dried in the inert smoke gas atmosphere. Part of the drying smoke gas stream is used as carrier gas for transporting the dried fuel to the burners of the boiler. This combined predrying and grinding of the lignite at a low temperature level is to be judged as beneficial in terms of the efficiency of the power plant.

A further variant of the drying of lignite which has a beneficial effect in terms of an increase in efficiency is the drying of lignite in a separate drying assembly in the form of a fluidized bed drier. Such a method is known, for example, from DE 196 20 047 Al. It is known that a marked increase in efficiency can be achieved by drying the lignite before firing in the steam generator of a power plant. The pit-wet lignite has approximately a water content of 45% to 65% which is reduced by drying to 10% to 25%.

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It is known from DE 195 18 644 C2 to utilize the energy-rich vapors emerging from the drying installation in the drier itself. For this purpose, it is proposed, in DE 195 18 644 C2, to compress at least a substream of the vapor and feed it as heating medium to the heat exchanger, the vapor at least partially condensing, so that a large part of the heat of evaporation of the vapor can be utilized for the desired drying of the fuel.

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

The dry lignite from fluidized bed drying usually has a maximum water content of 25 percent by mass and a mean grain diameter D50 of 0.4 to 0.8 mm, so that it can be introduced directly, if appropriate without regrinding, into the boiler and can be fired there.

The combustion temperature of predried lignite, which is designated below as dry lignite (DL), lies approximately 500°C above that of raw lignite.

Since the known dust-fired lignite boilers are designed predominantly for the firing of raw lignite, that is to say for the firing of raw lignite which has been subjected to grinding drying, the dry lignite in the applicant's steam generators has hitherto been burnt in accompaniment as additional fuel in order to increase efficiency. The accompanying combustion of a fraction of about 25% of dry lignite from a separate drying assembly has proved beneficial in terms of the calorific value of the fuel mixture.

The dry lignite can be introduced into the steam generator, for example, via what are known as swirl burners, such as are also used in hard coal firing.

However, practical tests and furnace simulations during the accompanying combustion of dry lignite in steam generators fired by raw lignite have shown that relatively high temperature peaks occur in the boiler in the region of the dry lignite burners or swirl burners and may lead to ash softening or ash melting processes and consequently to the formation of slag. This, in turn, reduces the availability of the steam generator.

Precisely because of this, swirl burners should also not be built too large. Swirl burners are limited in respect of their construction size. In general, a thermal power of 60 MW is considered as technically feasible. In the case of large lignite boilers with additional dry lignite

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2018202163 02 Sep 2019 firing, this necessitates a correspondingly large number of swirl burners. This consequently results in high investment costs.

Dry lignite is a comparatively cost-effective starting and supporting fuel for coal-fired steam generators. In this case, it is customary to have swirl burners, the disadvantages of which were described above, since they ensure a stable flame and a good burn-out even in furnaces which are still cold.

There is a need for a method for the accompanying combustion of dry lignite in steam generators fired by raw lignite, said method taking into account the abovementioned problem.

It is an object of the present invention to at least substantially satisfy the above need.

The present invention provides a method for operating a steam generator with a lignitefired 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, 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;

wherein 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.

Dry lignite in the context of the invention is to be understood to mean ready-dried lignite which has been produced from the pit-wet product obtained by mining into a dried finished product with a moisture content of 10 to 30 percent by mass, preferably of between 10 and 25 percent by mass, and with a mean grain diameter D50 of 0.4 to 10 mm in a drying assembly.

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Pit-wet raw lignite is to be understood to mean the untreated broken and, if appropriate, precomminuted raw lignite obtained by mining which has a moisture content of between 45 and 65 percent by mass.

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Grinding-dried raw lignite is to be understood to mean the raw lignite which is ground in a mill, at the same time being dried, into a burner-ready product and of which about 85 to 95 percent by mass has a grain size of less than 1 mm.

Burner allocation is to be understood in the context of the invention to mean a fuel supply line from the mill to an individual burner, to a burner group or to a burner level. Burner allocation ends where the fuel is ignited as a result of the delivery of combustion air. Burner allocation may comprise separators and distribution devices.

The method according to embodiments of the invention may be summarized to the effect that dry lignite is burnt in accompaniment, in addition to grinding-dried raw lignite, in the boiler of the steam generator, the dry lignite being fed to the carrier gas/fuel mixture from the mill before allocation or into allocation to the individual coal burners. Contrary to a method variant practiced hitherto, the dry lignite is not fired in the boiler via separate swirl burners, but instead the dry lignite is added to the smoke gas lignite stream of the mill even upstream of the boiler. Carrier gas is to be understood in the context of the invention to mean a mixture of smoke gas, of evaporated water and of combustion air.

In the proposed method, dry lignite can be diverted via a changeover device from starting burners of the steam generator to the first mill operated with raw lignite, as soon as starting and supporting firing is no longer required.

By means of furnace simulation calculations, the applicant was able to discover that, surprisingly, temperature peaks inside the boiler in the region of the burners can be avoided by means of such a measure.

Expediently, the grinding of the raw lignite takes place in an atmosphere inertized by means of the drying smoke gas stream. Inertized in the context of the invention means that the oxygen fraction in the smoke gas drying stream is set at an oxygen fraction of < 12% by volume.

In an embodiment of the method according to the invention, there is provision whereby the dry lignite from the separate drying assembly is fed into the first mill and is subjected there to regrinding together with the raw lignite. In this case, the dry lignite is redried and recomminuted in an inertized atmosphere. The advantage of this in terms of the method is that

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2018202163 27 Mar 2018 reseparation and/or regrinding of the dry lignite downstream of the drying assembly are/is not absolutely necessary. The influence of a fluctuating moisture content of the dry lignite is thereby eliminated. In most general terms, fluctuations in the moisture, bulk density and granulation of the dry lignite are uncritical because of the additional grinding drying in mixture with the raw lignite. It is thereby also possible to operate a connected drying assembly with optimized power. Finally, it also became apparent that the method according to embodiments of the invention is beneficial in terms of the NOx concentration in the smoke gas, since, by dry lignite being added to the smoke gas lignite stream, a concentration of the dust fraction of the burners takes place, so that the carrier gas fraction of the burners is reduced. As a result, this leads to a lower NOx concentration in the smoke gas/exhaust gas than during conventional firing in which the fuelrelated carrier gas quantity is usually higher.

A further advantage is that the regulatability of firing is improved. When the raw lignite quantity is increased, the first mill reacts only with a relatively long idle time, because the mill has to be moved to another operating point for the higher coal quantity or an additional mill has to be put into operation. By contrast, by the dry lignite quantity being increased, a higher fuel quantity is available in a comparatively short time. This improves the regulatability of the overall steam generator plant.

Preferably, at least one indirectly heated drier is provided as a separate drying assembly. This may be, for example, a fluidized bed drier.

In an alternative variant of the method according to the invention, there is provision whereby the separate drying assembly used is a second mill in which drying is carried out in an atmosphere inertized by a drying smoke gas stream. In other words, a separate drying assembly used may be a drying assembly in which the lignite is subjected to direct drying in direct contact with smoke gas.

In this case, it is preferable if the dried lignite from the second mill is fed as dry lignite into the smoke gas lignite stream of the first mill. In this case, for example, there may be provision whereby the dried lignite from the second mill is fed completely as dry lignite into the smoke gas lignite stream of the first mill, so that the second mill does not directly charge the boiler with a smoke gas lignite stream in the usual way.

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The method according to embodiments of the invention is distinguished, in particular, in that the boiler is fired by tangential firing with jet burners, to which the smoke gas lignite stream is allocated. Jet burners are substantially less susceptible to faults during operation than circular jet burners or swirl burners. Furthermore, jet burners are also substantially simpler in structural terms. These are based on the principle that the fuel/carrier gas duct is designed essentially as a rectangular shaft which is surrounded in each case by corresponding secondary air ducts. Eddying and swirling of the smoke gas/fuel stream or of the secondary air stream by means of corresponding fittings having a flow-dynamic action are unnecessary.

In a preferred embodiment of the method according to the invention, there is provision whereby the dry lignite from the separate drying assembly is fed into a carrier gas recirculation line of the first mill. It is thereby possible to carry out the method according to embodiments of the invention essentially without any structural adaptation of existing mills. Carrier gas recirculation lines are conventionally used for increasing the power of the mill in that part of the carrier gas, where appropriate, is branched off downstream of the mill and is recirculated through the mill.

If the first mill used is a separator mill, the dry lignite can be fed into a solids return of a mill separator. Even such a procedure requires scarcely any structural adaptations of existing mills.

In a preferred embodiment of the method according to the invention, there may be provision whereby the dry lignite is introduced into an entry region of the first mill by means of at least one worm conveyor.

Alternatively, injection of dry lignite into the first mill by means of inert gas, for example low-pressure steam, or, for example, of dry lignite in mixture with recirculated carrier gas as conveying medium may take place. Expediently, injection of dry lignite takes place such that rapid intermixing of the dry lignite with the raw lignite occurs.

Furthermore, it is also possible to separate the dry lignite from the conveying medium by means of an interceptor if the conveying medium used is compressed air, so that the dry lignite is introduced without air into the mill, for example via a cellular wheel sluice.

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Finally, because of the tendency of dry lignite to ignite, there may be provision for feeding the dry lignite to the mill at as cold a point as possible.

The introduction point for the dry lignite may also be provided downstream of the first mill in the burner allocation.

If a mixture of grinding-dried raw lignite and dry lignite is to be fed approximately uniformly to all the burners of the boiler, it is preferable to feed the dry lignite already into the first mill or into the smoke gas lignite stream directly after the first mill. In principle, however, it may also be desirable to set a different fuel concentration on burners arranged one above the other over the height of the steam generator. For the burn-out of the fuel, it is in any event beneficial to set a higher fuel concentration on the lower burners, as seen over the height of the steam generator, than on the upper burners. In this case, it is beneficial if, as seen in the direction of flow of the smoke gas lignite stream, the dry lignite is added to the smoke gas lignite stream for the main burners after the first mill and after a branch-off of the smoke gas lignite stream to a postreaction burner or vapor burner. As a result, concentration of the fuel at the lower burners (main burners) is achieved, so that low-NOx combustion is achievable.

The mills used may be, for example, beating wheel mills in which impact stress upon the grinding stock takes place in a known way. Wet fan mills or blower-type beater mills, as they are known, may likewise be used. If a beating wheel mill or blower-type beater mill is used as the first mill in the context of the invention, the dry lignite may be added, for example, in the pre-beater part of the mill.

In a variant of the method according to the invention, there is provision whereby between 15% and 35% of the overall lignite to be burnt in respect of the calorific value when the plant is under full load is subjected to drying in a separate drying assembly, whereas the remaining fraction of the lignite to be burnt is subjected conventionally to direct drying by smoke gas during grinding in the first mill.

In a preferred variant of the method according to the invention, there is provision whereby the indirect drying of the lignite is carried out in at least one fluidized bed drier.

Particularly when a fluidized bed drier is used, this can be operated with optimized power without regrinding by means of a roller-type bowl mill. In the case of grinding of dry

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2018202163 27 Mar 2018 lignite in a roller-type bowl mill, care must be taken to have special explosion protection measures, and therefore any regrinding dispensed with constitutes a special simplification in terms of the method.

Expediently, 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.

In a preferred embodiment of the method according to the invention, there is provision whereby the quantity of dry lignite fed in is regulated as a function of the load of the steam generator.

With an increased load, the quantity of dry lignite fed in can be briefly increased. Since this dry lignite sometimes does not have to pass through the grinding drying cycle of the first mill, regulation of the power of the steam generator can be implemented relatively simply in this way.

The advantages of at least preferred embodiments of the method according to the invention can be summarized as follows:

- By dry lignite being fed directly into the raw lignite mills or directly downstream of the mill discharge, but before the end of burner allocation, the maximum temperature in the burner belt region of the boiler can be lowered markedly, as compared with a feed of dry lignite via separate swirl burners. By such temperature peaks being avoided, excessive slag formation can be reliably prevented. As is known, lignite has a mineralogical composition which comprises slag-forming mineral constituents which, particularly at higher temperatures, lead to the formation of deposits in the boiler.

- By the dry lignite being added to the mills or after a mill, but before the end of burner allocation, all the advantages of tangential firing in terms of the full mixing of the fuel with the combustion air can be utilized. In tangential firing, it is appropriate to use jet burners, since these carry the fuel relatively far into the furnace and burn it there, so that the accuracy of fuel allocation to individual burners is uncritical.

- Fluctuations in the moisture, bulk density and granulation of the dry lignite are uncritical because of grinding drying in mixture with the raw lignite.

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- Power-optimized operation of connected fluidized bed drying is possible without special checks of residual moisture and grain size distribution of the dry lignite.

- By the dust concentration in the carrier gas at the burners being increased, a NOx reduction in the smoke gas is possible.

The invention is explained below by means of an exemplary embodiment, with reference to the accompanying drawing.

The method flowchart illustrated in the figure shows a boiler 1 with a furnace 2 and with a convection part 3. The convection part 3 comprises in a known way heating surfaces, by means of which convective heat transfer to the heating medium takes place. Inside the furnace 2, dried dust-like lignite is fired via dust burners which are designed as jet burners. In the furnace 2, the radiant heat is transmitted to the heating medium circulating in the steam circuit of the boiler 1. The steam generated with the boiler 1 can be expanded in a steam turbine for the purpose of generating electrical energy, or alternatively the steam may also be used as process heat in other coupled processes.

designates a suction draft, via which the smoke gas is conveyed out of the boiler 1 via a smoke gas purification device to a chimney.

The fuel in the form of dry lignite and raw lignite is ground in a beating wheel mill 5 and is distributed to a plurality of jet burners via a burner allocation 6.

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

designates a drying smoke gas stream which is extracted at the upper end of the furnace 2 of the boiler 1 at a temperature of between about 800°C and 1200°C and is delivered to the beating wheel mill 5.

A smoke gas lignite stream 10 is delivered from the beating wheel mill 5 to the burner allocation 6, after which combustion air 11 is added to the smoke gas lignite stream 10. The

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2018202163 27 Mar 2018 combustion air 11 is extracted from the atmosphere and is preheated via a combustion air preheater 12. The combustion air preheater 12 is operated by means of the smoke gas stream 13 from the convection part 3 of the boiler 1. Part of the combustion air 11 is added to the drying smoke gas stream 9 upstream of the beating wheel mill 5 and a further part is added to the boiler as bum-out air 14.

The raw lignite 7 which comes from precomminution is fed to the beating wheel mill 5, the raw lignite 7 in the beating wheel mill 5 being comminuted by impact stress in direct contact with the smoke gas, extracted from the furnace 2, in mixture with combustion air 11. Dry lignite 8 from a fluidized bed drier, not illustrated, is likewise fed with a water content of, for example, between 10 and 30 percent by mass to the beating wheel mill 5. This is introduced, for example, into a solids return of a mill separator or at a suitable point directly into the mill or into the carrier gas recirculation line 15 of the beating wheel mill 5. In the beating wheel mill 5, the raw lignite 7, together with the dry lignite 8, is reground and redried and also intimately mixed. The smoke gas lignite stream 10 (carrier gas stream) leaving the beating wheel mill 5 comprises, for example, about 15% to 35%, preferably about 20% to 25%, of dry lignite 8 with respect to the calorific value of the overall fuel used when the boiler is under full load.

Alternatively to this procedure, it is possible to add the dry lignite 8 to the smoke gas lignite stream 10 after the beating wheel mill 5, but before the end of fuel allocation 6.

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

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List of reference symbols:

Boiler

Furnace

Convection part

Suction draft

Beating wheel mill

Burner allocation

Raw lignite

Dry lignite

Drying smoke gas stream

Smoke gas lignite stream

Combustion air

Combustion air preheater

Smoke gas stream

Bum-out air

Carrier gas recirculation line

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Claims

1. A 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:

2. The method as claimed in claim 1, wherein at least one indirectly heated drier is provided as a separate drying assembly.

3. The method as claimed in claim 1, wherein 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. The method as claimed in claim 3, wherein 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. The method as claimed in any one of claims 1 to 4, wherein 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. The method as claimed in any one of claims 1 to 5, wherein the boiler is fired by means of jet burners to which the smoke gas lignite stream is allocated.

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7. The method as claimed in any one of claims 1 to 6, wherein the dry lignite is fed into a carrier gas recirculation line of the first mill.

8. The method as claimed in any one of claims 1 to 6, wherein the dry lignite is fed into a solids return of a mill separator of the first mill.

9. The method as claimed in any one of claims 1 to 6, wherein the dry lignite is introduced into an entry region of the first mill by means of at least one worm conveyor.

10. The method as claimed in any one of claims 1 to 9, wherein at least one beating wheel mill is used as the first mill.

11. The method as claimed in any one of claims 1 to 10, wherein 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. The method as claimed in any one of claims 1 or 2 or 6 to 11, wherein the indirect drying of the lignite is carried out in at least one fluidized bed drier.

13. The method as claimed in claim 12, wherein 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. The method as claimed in any one of claims 1 to 13, wherein the quantity of dry lignite fed in is regulated as a function of the load requirement for the firing power of the steam generator.