Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method and
arrangement for operating a steam power plant having a
fluidized bed furnace.
It is known to burn pulverized coal mixed with
an absorbent in a fluidized bed to provide heat for a
steam generator or boiler. The exhaust gases of the
fluidized bed are relatively sulphur-free, thereby
reducing pollution of the environment.
Presently known fluidized bed systems use
either conventional steam generating processes in which
the combustion occurs at atmospheric pressure or combined
gas-steam processes. The steam and the combined gas-
steam systems both suffer from both economic and technical
disadvantages.
The conventional steam process, in which the
combustion occurs at atmospheric pressure, requires a
relatively large-dimensioned fluidized bed, since the
fluidized bed velocity must be kept low. Therefore,
relatively large and expensive equipment is necessary
to provide for passing of the entire volume of air through
the bed.
The combined gas-steam system, in turn, is
disadvantageous because the operating temperature of the
fluidized bed must not be substantially greater than
800C to prevent reaction of the fuel sulphur with the
absorbent material. The limitation on the operating
temperature of the fluidized bed, according to the
present state of the art, results in a process which is
not economically feasible. The efficiency of the gas-
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steam process is also limited by the necessity of using airpassing through the bed to cool the pipes within the fluidized
bed. In order to maintain the pipe wall temperature within
reasonable limits, a relatively high pressure drop through the
pipes is required. This high pressure drop provides for the
substantial heat transfer needed for cooling the pipes. The
large pressure drop and the relatively low temperature of the air
which flows through the air preheater of the in~tallation ad-
versely effect the efficiency of the fluidized bed. In addition,
a high pipe wall temperature can lead to corrosion and erosion
phenomena. For example, at a pipe wall temperature of approx-
imately 750C, the temperature at the inlet of the gas turbine
is at most 770C, which is insufficient to prevent corrosion with-
in the turbine.
Therefore, it is an object of the present invention to
provide a method and an arrangement for operating a power plant
using a fluidized bed which is not subject to the limitations of
the prior known systems.
The method of the present invention employs a fluidized
bed in which combustion takes place under pressure. Pressurized
air is introduced into the bed to fluidize the fuel and to
pressurize the steam generator. The gases produced ~y the com-
bustion of the fuel within the fluidized bed vaporize feedwater
passing through the steam generator to provide steam for operating
a turbogenerator arrangement. Thus the electrical current gener-
ated by the turbogenerator arrangement is the product of a purely
steam generating process. The combustion gases are also used for
superheating the steam and for preheating the feedwater. Exhaust
gase~ exiting the fluidized bed pass through a precipitator to
remove flyash, with the gases then used to drive a turbine and
compressor arrangement which supplies the pressurized air to the
fluidized bed. The gases, which are then at a temperature of
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about 120-150C, are then passed directly into the atmosphere
through a chimney.
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The apparatus of the present invention consists of
a steam generator heated by a fluidized bed furnace. A steam
turbogenerator having adjoining feedwater preheater~zones
respectively before and after the turbines is employed to
generate electricity. The steam generator or boiler is kept
under pressure by a compressor which supplies air to the
fluidized bed. The compressor is driven by a gas turbine
which uses the exhaust gases of the fluidized bed as a working
fluid. A feedwater heater, typically a multiple pipe heat
exchanger, is arranged within the fluidized bed for generating
steam by extracting the heat of the burned fuel. Additionally,
a superheater and a feedwater preheater are arranged in the
pressure chamber of the steam generator in such a way that
they are exposed to the flue gas rising in the fluidized bed.
In one aspect of the present invention there is
provided a method of producing steam in a steam boiler fired
by a fluidized bed burner to drive a steam turbine, the method
comprising the steps of: supplying fuel and pressurized air from
a rotary compressor to the fluidized bed burner, combusting said
fuel and said pressurized air at said fluidized bed burner to
produce heat at a first zone, supplying feedwater to a preheater
in a second zone in said steam boiler spaced from said fluidized
bed burner; conducting feedwater from said preheater through
an evaporator in said first zone to produce steam, conducting
steam from said evaporator through a superheater in a third
zone in said steam boiler located between said first zone and
said second zone; conducting combustion gases from said first
zone sequentially through said third zone and said second zone
and then through a dust precipitator; driving said rotary
compressor by a gas turbine supplied with relatively cool gases
under pressure from said dust precipitator, the temperatuxe
of the gases in said second zone being substantially less than
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1n said first zone; and driving said steam turbine by steam fromsaid superheater.
In a further aspect of the present invention there
is provided a steam power plant comprising: a steam boiler
having a fluidized bed burner for generating steam, a rotary
air compressor for supplying pressurized air to said fluidized
bed burner; a gas turbine for driving said rotary air compres-
sor' an evaporator in said steam boiler adjacent said fluidized
bed burner, a superheater and a preheater in said steam boiler
spaced above said fluidized bed burner, said steam boiler
being arranged to conduct gas from said fluidized bed burner
sequentially over said superheater and then over said pre-
heater; a dust precipitator; conduit means for conducting flue
gas from said preheater through said dust precipitator to
said gas turbine, and conduit means both for conducting feed-
water in sequence through said preheater and said evaporator
to produce steam and for conducting said steam through said
superheater to a steam turbine, whereby said gas turbine operates
at a relatively cool temperature as a result of passing the
flue gas over said preheater in said steam boiler before passing
the flue gas through said dust precipitator and said gas tur-
bine.
BRIEF DESCRIPTION OF THE DRAWING
A preferred embodiment of the present invention is
described with reference to the single accompanying drawing
wherein like members bear like reference numerals. The single
drawing is a schematic illustration of a steam power plant ac-
cording to the present invention including a fluidized bed
furnace.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to the single drawing, a steam gener-
ator 1 for a steam power plant contains a fluidized bed fur-
nace 2, the details of which are not illustrated. The fluidized
bed operates in a conventional manner, having a
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bed of pulverized coal to which an absorbent such as, for
example, limestone or dolomite, is added. The fuel is
fluidized within the bed to provide for combustion of the
coal, thereby generating heat for the steam generator.
Superheated steam is generated within the steam
generator 1 and passes from the boiler through a fresh steam
line 3 to a steam turbine 4, wherein the steam is expanded
to release energy. An electrical generator 5 is coupled to
the steam turbine 4 to generate electricity. The spent
steam discharging from the steam turbine 4 is precipitated
in a condenser 6 and the condensate is conveyed by means of
a condensate pump 7 through a preheater zone 8. The pre-
heater zone 8, shown in a simplified manner, subjects boiler
feedwater to a multi-stage preheating process, as condensed
steam is withdrawn through extraction points 9 of the steam
turbine 4. A feed pump 10 conveys the boiler feedwater
through a feed line 11 back to the steam generator 1.
According to the present invention, the combustion
of the pulverized coal within the fluidized bed furnace
takes place under pressure. The combustion air required for
the burning of the fuel is generated outside of the steam
generator 1 in a separate turbine and compressor booster
arrangement. A gas turbine 12 drives a compressor 13 in
which air that is drawn in from the atmosphere at an intake
14 is compressed to, for example, 8-10 bar. During turbine
start-up, a starter motor 16 which can be uncoupled at a
coupling 15 from the turbine after start-up is used to drive
the compressor 13.
The compressed air provided by the compressor 13
is conveyed through a pressure line 17 to a point below the
blower plate 18 of the fluidized bed furnace within the
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steam generator 1. The compressed air flows through the
blower plate 18 and forms an upwardly-directed carrier
stream which lifts the solid fuel and absorbent particles
and thus brings about the fluidized bed state.
The solid particles, typically consisting of
pulverized coal together with an absorber material such as
limestone or dolomite, are introduced into the furnace at 19
and 20 respectively.
The turbulent movement of the fluidized solids
results in the exposure of a relatively large specific
surface area. The high heat capacity of the fuel being
fluidized and the exposed surface area result in high heat
transmission coefficients from the fuel to the heat transfer
surfaces within the steam generator 1. The heat transfer
surfaces are typically multiple pipe heat exchangers, as
illustrated by the pipes of a feedwater heater 21. The
operating temperature of the fluidized bed is advantageously
selected to be approximately 800C, which minimizes the
problems arising from the reaction of the sulphur in the
coal with the absorbent.
After passing over the feedwater heater 21, the
hot flue gases within the steam generator transfer a portion
of their heat content to a superheater 22 and a further
portion to a preheater 23, before exiting the steam gener-
ator 1. The superheater 22 provides an additional stage for
heating the steam generated in the feedwater heater 21
before entering the steam turbine 4, while the preheater 23
heats recirculated feedwater discharged from the turbine.
The series of heat exchangers 21, 22 and 23 within the
boiler a~l are in the flow path of the combustion gases of
the fluidized bed on the steam turbine side.
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The electrical current generated by the generator
5 is the product of a purely steam generating process. The
steam process permits maximum use of the heat of the burned
fuel by providing for preheating of the steam and feedwater,
which results in a relatively high thermal efficiency within
the steam power plant. The use of a fluidized bed which is
operated under pressure and that is water-cooled permits a
reduction in the size and cost of the fluidized bed instal-
lation in comparison with the known installations, all of
which operate with combustion at atmospheric pressure and
with an air-cooled fluidized bed.
After passing over the preheater 23, the flue
gases enter a dust precipitator 24. In the present inven-
tion, the dust precipitator 24 is typically a simple cen-
trifugal precipitator, since the flue gases are at a tempe-
rature of only about 400C. At such a temperature, corro-
sion and erosion problems are minimized. In the precipi-
tator, the flue gases are mechanically separated from ash
particles which are entrained in the stream of gas rising in
the fluidized bed during the combustion process.
The precipitator apparatus has substantially
smaller dimensions compared to that of known installations,
because of the relatively simple construction of such a
centrifugal device. Additionally, the gas volume which
flows through the precipitator is relatively small as a
result of the pressurizing operation within the fluidized
bed, which reduces the size of the bed and therefore de-
creases the volume of combustion air introduced into the
bed.
After exiting the dust precipitator 24, the flue
gases pass through a gas line 25 to the gas turbine 12. The
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gas turbine 12 is a pure expansion turbine and forms part of
the booster arrangement. The residual heat content of the
flue gases is released within the gas turbine 12 to drive
the connected compressor 13. A low entry temperature for
the flue gases supplied to the gas turbine (typically less
than 400C) is feasible because the expansion turbine only
drives the compressor 13 and does not contribute to the
generation of electricity. Corresponding to this relatively
low entry temperature is an off-gas exit temperature of
about 120-150C. These off-gases can be passed directly
into the atmosphere, at 26, through a chimney (which is not
illustrated~ because of the low temperature. In conven-
tional arrangements, the exhaust gases first have to be
admitted to a heat exchanger downstream from the expansion
turbine before being released into the atmosphere.
The usual downstream heat exchanger is, in the
present invention, the preheater 23. Since the downstream
heat exchanger in the present invention is incorporated
within the pressure chamber of the steam generator 1, an
optimum preheating of the steam in the steam turbine circuit
becomes possible while the off-gas temperature remains
sufficiently low. The steam preheating and the low off-gas
temperature results in a correspondingly high efficiency of
the installation of the present invention.
The present invention is not to be restricted to
what is disclosed in the drawing and in the foregoing speci-
fication. For example, the steam turbine circuit could
readily be provided with a reheater. Also, a heavy fuel oil
furnace could be provided instead of the coal furnace, in
which case heavy fuel oil, instead of coal, is burnt with
the absorbent in the fluidized bed~ The arrangement of the
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apparatus in the steam generator and the booster system
would be the same and hence the process would remain within
the scope of the invention.
The principles, preferred embodiments and modes of
operation of the present invention have been described in
the foregoing specification. The invention which is intended
to be protected herein, however, is not to be construed as
limited to the particular forms disclosed, since these are
to be regarded as illustrative rather than restrictive.
Variations and changes may be made by those skilled in the
art without departing from the spirit of the present inven-
tion.
