Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for operating a steam generator
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
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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%.
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
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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 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.
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The object on which the invention is based, therefore,
is to provide a method for the accompanying combustion
of dry lignite in steam generators fired by raw
lignite, said method taking into account the
abovementioned problem.
The object is achieved by means of a method for
operating a steam generator with a lignite-fired boiler
and 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 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 by indirect
drying in a separate drying assembly to form dry
lignite, and
- Feeding of the dry lignite into the smoke gas lignite
stream of the first mill before burner allocation or
into burner allocation.
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.
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
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of between 45 and 65 percent by mass.
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 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.
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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 advantageous variant 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 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 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
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in which the fuel-related 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 advantageous 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 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 an especially advantageous variant 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 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 further advantageous variant 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
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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.
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 advantageous 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
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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 lignite in a roller-type bowl mill,
care must be taken to have special explosion protection
measures, and therefore any regrinding dispensed with
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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 an expedient and advantageous variant 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 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
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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.
- 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 I. The
steam generated with the boiler 1 can be expanded in a
steam turbine for the purpose of generating electrical
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energy, or alternatively the steam may also be used as
process heat in other coupled processes.
4 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.
9 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 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 1 as burn-out air
14.
The raw lignite 7 which comes from precomminution is
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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:
1 Boiler
2 Furnace
3 Convection part
4 Suction draft
Beating wheel mill
6 Burner allocation
7 Raw lignite
8 Dry lignite
9 Drying smoke gas stream
Smoke gas. lignite stream
11 Combustion air
12 Combustion air preheater
13 Smoke gas stream
14 Burn-out air
Carrier gas recirculation line
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