EP2906876B2 - Method for operating a steam generator - Google Patents

Description

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

A known principle of direct dust injection by means of a pulverized coal drying plant is, for example, in Publication of Helmut Effenberger, "Steam Generation", Springer-Verlag, ISBN 3-540-64175-0 , described. In such a method, flue gas sucked back is used as the drying 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 to the end of the combustion chamber via a flue gas recirculation, where the drying flue gas stream required for drying the raw lignite is extracted at a temperature between approx. 800 ° C and approx. 1200 ° C.

A method of the type mentioned at the outset is known from, for example DE 42 03 713 C2 C2 or from the DE 43 23 469 A1 known. The method comprises the grinding of humidified brown coal in at least one mill, to which a drying flue gas stream is applied, which is branched off from the boiler behind the convection draft and which is introduced into the mill at a temperature of approximately 350.degree. In the mill, the lignite is crushed to the grain belt required for combustion in the boiler and at the same time dried in the inert flue gas atmosphere. Part of the drying flue gas flow is used as a carrier gas for the transport of the dried fuel to the burners of the boiler. This combined pre-drying and grinding of the brown coal at a low temperature level can be assessed as 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 known for example from the DE 196 20 047 A1 known. It is known that drying the lignite before firing it in the steam generator of a power plant can achieve a significant increase in efficiency. The pit-moist brown coal has a water content of 45% to 65%, which is reduced to 10% to 25% by drying.

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

Further known processes for drying lignite using fluidized bed dryers are, 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 an average grain diameter D50 of 0.4 to 0.8 mm, so that it can be fed into the boiler immediately, if necessary without re-grinding, and burned there.

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

Since the known dust-fired lignite boilers are mainly designed for the burning of raw lignite, i.e. For the firing of raw lignite which has been subjected to mill drying, the dry lignite in the steam generators of the applicant has been co-burned as an additional fuel to increase the efficiency. Co-combustion of around 25% dry lignite from a separate drying unit based on the calorific value of the fuel mixture has proven to be favorable.

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

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

This is precisely why swirl burners should not be built too large. Swirl burners are limited in their size. In general, 60MW thermal power is considered technically feasible. In the case of large brown coal boilers with additional dry brown coal, this requires a correspondingly high number of swirl burners. As a result, this leads to high investment costs.

Dry lignite is a comparatively inexpensive starting and support fuel for coal-fired steam generators. Here, the swirl burners described above with regard to their disadvantages are common, since they ensure a stable flame and good burnout even when the firebox is still cold.

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

The object is achieved by a method according to claim 1.

In the context of the invention, dry brown coal is understood to mean ready-dried brown coal which is converted from the mine-moist, mined product to a dried finished product with a moisture content of 10 to 30% by mass, preferably between 10 and 25% by mass, and an average grain diameter D50 of 0.4 to 10 mm was produced in a drying unit.

Mine-moist raw lignite is to be understood as the unprepared broken and possibly pre-shredded mined raw lignite, which has a moisture content of between 45 and 65 percent by mass.

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

A burner allocation in the sense of the invention is understood to mean a fuel supply line from the mill to an individual burner, a burner group or a burner level. The burner allocation ends where the fuel is ignited by supplying combustion air. Burner allocation can include classifiers and distributors.

The method according to the invention can be summarized to the extent that dry lignite is also burned in the boiler of the steam generator in addition to meal-dried raw lignite, the dry lignite being fed into the carrier gas / fuel mixture from the mill before being allocated or in the allocation to the individual coal burners. In contrast to a previously practiced process variant, the dry lignite is not burned via separate swirl burners 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 sense of the invention is to be understood as a mixture of flue gas, evaporated water and combustion air.

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

Using fire chamber simulation calculations, the applicant was able to find out that, with such a measure, temperature peaks within the boiler in the area of the burners can surprisingly be avoided.

The raw lignite is expediently ground in an atmosphere inertized with the drying flue gas stream. Inerted in the sense of the invention means that the oxygen content in the flue gas drying stream is set to an oxygen content of <12% by volume.

It can be provided that the dry lignite is fed from the separate drying unit into the first mill and there is subjected to post-grinding with the raw lignite. The dry lignite is then dried and shredded in an inert atmosphere. This has the procedural advantage that it is not absolutely necessary to inspect and / or regrind the dry lignite behind the drying unit. This eliminates the influence of fluctuating moisture content in the dry brown coal. In general, fluctuations in moisture, bulk density and grain size of the dry lignite are not critical due to the additional mill drying in a mixture with the raw lignite. This also makes it possible to operate a connected drying unit with optimized performance. Finally, it has also been found that the method according to the invention is favorable with regard to the NOx concentration in the flue gas, since the addition of dry lignite to the flue gas lignite stream results in a concentration of the dust content in the burners, so that the carrying gas content in the burners is reduced. As a result, this leads to a lower NOx concentration in the flue gas / exhaust gas than with a conventional furnace, in which the amount of carrier gas is generally higher.

Another advantage is that the controllability of the furnace is improved. If the amount of raw brown coal increases, the first mill only reacts with a longer dead time because the mill has to be moved to a different operating point for the higher amount of coal, or an additional mill has to be put into operation. In contrast, an increase in the amount of dry lignite in a comparatively short time makes a higher amount of fuel available. This improves the control ability of the entire steam generator system.

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

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, a drying unit can be used as a separate drying unit in which the lignite is subjected to direct drying in direct contact with flue gas.

In this case, it is advantageous if the dried brown coal from the second mill is fed as dry brown coal into the flue gas brown coal stream of the first mill in a burner allocation. It can be provided, for example, that the dried lignite from the second mill is fed entirely as dry lignite into the flue gas lignite stream of the first mill, so that the second mill does not directly feed the boiler with a flue gas lignite stream.

The method according to the invention is characterized in particular by the fact that the boiler is fired with tangential firing with jet burners, to which the flue gas lignite stream is allocated. Jet burners are much less susceptible to malfunctions than omnidirectional or swirl burners. Jet burners are also significantly simpler in terms of design. These are based on the principle that the fuel / carrier gas duct is essentially designed as a rectangular duct, which is surrounded by corresponding secondary air ducts. A swirling and swirling of the flue gas / fuel flow or the secondary air flow with corresponding internally acting installations is not necessary.

In a particularly advantageous variant of the method according to the invention, it is provided that the dry lignite is fed from the separate drying unit into a carrier gas recirculation line of the first mill. This makes it possible to carry out the method according to the invention essentially without constructively adapting existing mills. Carrier gas recirculation lines are usually used to control the power of the mill, in that part of the carrier gas is optionally branched off behind the mill and is recirculated through the mill.

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

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

Alternatively, dry lignite can be blown into the first mill with inert gas, for example low-pressure water vapor, or, for example, dry lignite mixed with recirculated carrier gas as the conveying medium. It is expedient to inject dry lignite in such a way that the dry lignite is rapidly mixed with the raw lignite.

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

Finally, because of the tendency of dry lignite to ignite, provision can be made for the dry lignite to be fed to the mill as cold as possible.

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

If a mixture of ground-dried raw lignite and dry lignite is to be fed approximately evenly into all the burners of the boiler, it is advantageous to feed the dry lignite into the flue gas lignite stream immediately behind the first mill. In principle, however, it may also be desirable to set a different fuel concentration in fires arranged one above the other over the height of the steam generator. In any case, for the burnout of the fuel, it is favorable to set a higher fuel concentration at the lower burners, viewed over the height of the steam generator, than at the upper burners. In this case, it is advantageous if the dry lignite, viewed in the direction of flow of the flue gas lignite stream, is added behind the first mill and behind a branch of the flue gas lignite stream to a post-reaction burner or vapor burner, the flue gas lignite stream for the main burners. This results in a concentration of the fuel at the lower burners (main burner), so that low-NOx combustion can be achieved.

For example, beater wheel mills can be used as mills, in which the impact material is impacted in a known manner. So-called wet fan mills or fan beater mills can also be used. When using a beater wheel mill or blower mill as the first mill in the sense of the invention, the dry lignite can be added, for example, in the propeller part of the mill.

In a variant of the method according to the invention it is provided that between 15% and 35% of the total lignite to be burned based on the calorific value at full load of the drying system is subjected to a separate drying unit, whereas the remaining portion of the lignite to be burned is conventionally subjected to direct drying with flue gas during 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 lignite is carried out in at least one fluidized bed dryer.

In particular, when using a fluidized bed dryer, it can be operated in a performance-optimized manner without subsequent grinding using a bowl mill. When grinding dry brown coal in a roller mill, special explosion protection measures must be taken, so that any post-grinding that is not required represents a particular simplification of the process.

The energy of the vapor produced during indirect drying is expediently used at least in part 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.

If the load is increased, the amount of dry lignite fed in can be increased at short notice. Since this dry lignite may not have to go through the mill-drying cycle of the first mill, it is relatively easy to control the output of the steam generator in this way.

• Durch eine Trockenbraunkohlezufuhr 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 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 method according to the invention can be summarized as follows:

• By supplying dry brown coal directly downstream of the mill discharge, but before the end of the burner allocation, the maximum temperature in the burner belt area of the boiler can be significantly reduced compared to supplying dry brown coal via separate swirl burners. By avoiding such temperature peaks, excessive slag formation can be reliably prevented. As is known, lignite has a mineralogical composition which comprises slag-forming mineral components, which lead to the formation of deposits in the boiler, in particular at higher temperatures.
• By adding the dry lignite behind a mill, but before the end of a burner allocation, all the advantages of tangential firing in relation to the mixing of the fuel with the combustion air can be used. In the case of tangential furnaces, the use of jet burners is recommended, since they carry the fuel relatively far into the combustion chamber and burn there, so that the accuracy of the fuel allocation to individual burners is not critical.
• Moisture, bulk density and grain size fluctuations of the dry lignite are not critical due to the mill drying in a mixture with the raw lignite.
• Performance-optimized operation of a connected fluidized bed dryer is possible without special control of residual moisture and particle size distribution of the dry lignite.
• By increasing the dust concentration in the carrier gas on the burners, a NOx reduction in the flue gas is possible.

The figure is not an embodiment of the invention.

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 convective heat transfer into the heating medium takes place. Dried dusty lignite is burned within the combustion chamber 2 by means of dust burners which are designed as jet burners. In the combustion chamber 2, the radiant heat is transferred to the heating medium circulating in the steam cycle of the boiler 1. The water vapor generated by the boiler 1 can be expanded in a steam turbine for the purpose of generating electrical energy, alternatively the steam can also be used as process heat in other coupled processes.

4 with a suction draft is designated, through 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 lignite and raw lignite is ground in a beater wheel mill 5 and distributed to several jet burners via a burner allocation 6.

In the example shown, the impact wheel mill 5 is fed on the one hand with pit-moist raw lignite 7 and on the other hand with dry lignite 8 from a drying unit (not shown). The dry lignite 8 is added, for example, to an inert gas recirculation line.

9 with a drying flue gas stream is designated, which is removed at the upper end of the combustion chamber 2 of the boiler 1 with a temperature of between about 800 ° C and 1200 ° C and fed to the beater wheel mill 5.

A flue gas lignite stream 10 is delivered from the beater wheel mill 5 to the burner allocation 6, behind which the combustion air 11 is added to the flue gas lignite stream 10. 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. Part of the combustion air 11 is added to the drying flue gas stream 9 in front of the beater wheel mill 5, another part is added to the boiler 1 as burnout air 14.

The raw lignite 7, which comes from a preliminary comminution, is fed to the beater wheel mill 5, the raw lignite 7 in the beater wheel mill 5 being comminuted in direct contact with the flue gas removed from the combustion chamber 2 in a mixture with combustion air 11 under impact stress. Dry lignite 8 from a fluidized bed dryer, not shown, is also fed to 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 point directly in the mill or in the inert gas recirculation line of the beater wheel mill 5. In the beater wheel mill 5, the raw lignite 7 is re-ground together with the dry lignite 8 and then dried and thoroughly mixed. The flue gas lignite stream 10 (carrier gas stream) leaving the beater wheel mill 5 comprises, for example, approximately 15% to 35%, preferably approximately 20% to 25% dry lignite 8 based on the calorific value of the entire fuel used under full load of the boiler.

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

The combustion air 11 is preheated in the combustion air preheater 12 by means of the flue gas stream 13. The invention is to be understood in such a way that, instead of a combustion air preheater 12 with flue gas, a combustion air preheater 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 (12)

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 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 one of Claims 1 to 4, characterized in that the boiler is fired by means of jet burners to which the smoke gas lignite stream is allocated.
6. Method according to one of Claims 1 to 5, characterized in that the dry lignite is fed into a carrier gas recirculation line of the first mill.
7. Method according to one of Claims 1 to 6, characterized in that at least one beating wheel mill is used as the first mill.
8. Method according to one of Claims 1 to 7, 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.
9. Method according to one of Claims 1 or 2 or 5 to 11, characterized in that the indirect drying of the lignite is carried out in at least one fluidized bed drier.
10. Method according to Claim 9, 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.
11. Method according to one of Claims 1 to 10, 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.
12. Method according to one of Claims 1 to 11, 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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