U.S. patent application number 14/005691 was filed with the patent office on 2014-01-16 for coal-fired power generating system and coal-fired power generating method.
This patent application is currently assigned to TSUKISHIMA KIKAI CO., LTD.. The applicant listed for this patent is Masaki Kataoka, Toshiyuki Kimura, Takayuki Noguchi. Invention is credited to Masaki Kataoka, Toshiyuki Kimura, Takayuki Noguchi.
Application Number | 20140013746 14/005691 |
Document ID | / |
Family ID | 46930456 |
Filed Date | 2014-01-16 |
United States Patent
Application |
20140013746 |
Kind Code |
A1 |
Kimura; Toshiyuki ; et
al. |
January 16, 2014 |
COAL-FIRED POWER GENERATING SYSTEM AND COAL-FIRED POWER GENERATING
METHOD
Abstract
There is provided a coal-fired power generating system that, in
addition to recovering latent heat of condensation and the like
from a dry exhaust gas of a drying device, which is for predrying
coal, precludes a large variation, from a design value, of the
amount of steam flowing at a final stage of a steam turbine. The
coal-fired power generating system includes an indirect-heating
dryer 1 including a heating medium passage inside its casing, a
coal-fired boiler 3 for combusting coal to generate steam, and a
steam turbine 6 for generating power with the steam from the boiler
3. The dryer dries the coal fed into the casing by performing
indirect heating with steam fed to the heating medium passage. The
coal-fired power generating system heats boiler supply water for
the coal-fired boiler 3 with extracted steam extracted from the
steam turbine 6. The coal-fired power generating system also
includes a line for using part of the extracted steam as heated
steam for the indirect-heating dryer 1, a steam condenser 5 for the
steam turbine 6, a wet scrubber 11 provided on a path of the dry
exhaust gas from the indirect-heating dryer 1, heat recovery heat
exchangers 22 and 24 for performing heat exchanging between
circulating water of the wet scrubber 11 and condensate of the
steam condenser 5. The coal-fired power generating system is
configured to heat boiler supply water by using the condensate
having heat recovered from dry exhaust gas by the heat recovery
heat exchangers 22 and 24.
Inventors: |
Kimura; Toshiyuki; (Tokyo,
JP) ; Noguchi; Takayuki; (Tokyo, JP) ;
Kataoka; Masaki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kimura; Toshiyuki
Noguchi; Takayuki
Kataoka; Masaki |
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
TSUKISHIMA KIKAI CO., LTD.
Tokyo
JP
|
Family ID: |
46930456 |
Appl. No.: |
14/005691 |
Filed: |
February 27, 2012 |
PCT Filed: |
February 27, 2012 |
PCT NO: |
PCT/JP2012/054761 |
371 Date: |
October 4, 2013 |
Current U.S.
Class: |
60/645 ; 60/670;
60/691 |
Current CPC
Class: |
F23K 1/04 20130101; F22D
1/32 20130101; F01K 7/44 20130101; F01K 7/34 20130101 |
Class at
Publication: |
60/645 ; 60/670;
60/691 |
International
Class: |
F01K 7/34 20060101
F01K007/34; F23K 1/04 20060101 F23K001/04; F22D 1/32 20060101
F22D001/32 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2011 |
JP |
2011-067437 |
Claims
1. A coal-fired power generating system, comprising: an
indirect-heating dryer comprising a heating medium passage inside a
casing thereof, the dryer being configured to dry a coal fed into
the casing by performing indirect heating with steam fed to the
heating medium passage; a coal-fired boiler for combusting a dried
coal to generate steam; and a steam turbine for generating power
with the steam from the boiler, the coal-fired power generating
system being configured to heat boiler supply water for the
coal-fired boiler with extracted steam extracted from the steam
turbine, wherein the coal-fired power generating system comprises:
a line for using part of the extracted steam as heated steam for
the indirect-heating dryer; a steam condenser for the steam
turbine; a heat recovery unit provided on a path of a dry exhaust
gas from the indirect-heating dryer, the heat recovery unit being
configured to transfer heat of the dry exhaust gas to condensate of
the steam condenser, the heat recovery unit having a heat recovery
quantity adjusting unit for adjusting a quantity of heat recovery
of the heat recovery unit; and a line for using the condensate
having the heat recovered from the dry exhaust gas by the heat
recovery unit for heating the boiler supply water.
2. The coal-fired power generating system according to claim 1,
wherein the heat recovery unit comprises a wet scrubber provided at
the path of the dry exhaust gas from the indirect-heating dryer and
a heat recovery heat exchanger for performing heat exchanging
between circulating water of the wet scrubber and the condensate of
the steam condenser, and the heat recovery quantity adjusting unit
is configured to adjust the quantity of heat recovery by
controlling an amount of the circulating water of the wet
scrubber.
3. The coal-fired power generating system according to claim 1,
wherein the heat recovery unit comprises a heat pump unit.
4. The coal-fired power generating system according to claim 2,
wherein the wet scrubber is a two-stage type, and a first heat
recovery heat exchanger corresponding to circulating water of a
first-stage scrubber heats the boiler supply water, a second heat
recovery heat exchanger corresponding to circulating water of a
second-stage scrubber receives the boiler supply water and heats
the boiler supply water to a higher temperature, and the second
heat recovery heat exchanger is a heat pump.
5. The coal-fired power generating system according to claim 1,
wherein a boiler combustion exhaust gas is fed as a carrier gas
into the casing of the indirect-heating dryer.
6. A coal-fired power generating method for a coal-fired power
generating system, the system comprising: an indirect-heating dryer
comprising a heating medium passage inside a casing thereof, the
dryer being configured to dry a coal fed into the casing by
performing indirect heating with steam fed to the heating medium
passage; a coal-fired boiler for combusting a dried coal to
generate steam; and a steam turbine for generating power with the
steam from the boiler, the system being configured to heat boiler
supply water for the coal-fired boiler with extracted steam
extracted from the steam turbine, the method comprising the steps
of: using part of the extracted steam as heated steam for the
indirect-heating dryer and condensing exhaust of the steam turbine
by a steam condenser; providing a heat recovery unit at a path of a
dry exhaust gas from the indirect-heating dryer, the heat recovery
unit being configured to transfer heat of the dry exhaust gas to
condensate of the steam condenser, the heat recovery unit having a
heat recovery quantity adjusting unit for adjusting a quantity of
heat recovery of the heat recovery unit; and using the condensate
having the heat recovered from the dry exhaust gas by the heat
recovery unit for heating the boiler supply water.
7. The coal-fired power generating method according to claim 6,
wherein a boiler combustion exhaust gas is fed as a carrier gas
into the casing of the indirect-heating dryer, and the dry exhaust
gas has a dew point in a range from 80.degree. C. to 95.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a coal-fired power
generating system and a coal-fired power generating method for
predrying and pulverizing coal and feeding the coal to a coal-fired
boiler to drive a steam turbine for power generation.
[0002] In particular, the present invention relates to a coal-fired
power generating system and a coal-fired power generating method
that are designed to, in addition to recovering latent heat of
condensation and the like from a dry exhaust gas of a drying
device, which is for predrying the coal, preclude a large
variation, from a design value, of the amount of steam flowing at a
final stage of a steam turbine.
[0003] More specifically, the present invention is suitable to
suppress a reduction in efficiency of power generation performed
using a low rank coal, such as brown coal, lignite and sub
bituminous coal, for combustion.
BACKGROUND ART
[0004] In recent years, approaches have been devised for new
coal-fired power generating systems to use a high water content
coal, which contains a high proportion of water, as fuel because of
a substantial rise in coal prices.
[0005] In addition, existing coal-fired power generating systems
also tend to change a coal in use to one of a lower rank (with
higher water content). When a low rank coal, such as brown coal,
lignite and sub bituminous coal, is combusted, part of a quantity
of heat of the coal is used for the vaporization of the water
contained in the coal. This may reduce the amount of steam
generated by a boiler commensurately, resulting in a deterioration
of the efficiency of power generation (the amount of power
generation/the quantity of heat of the coal).
[0006] As a solution to this, addition of a drying device to predry
the coal has been known. In this scheme, the quantity of heat of
high-pressure high-temperature steam generated by the boiler is
recovered as power by the steam turbine. During the recovery, part
of the steam, which has reached a medium pressure or a low
pressure, is extracted from the steam turbine. Latent heat of
condensation of the extracted steam is used as a heat source to
predry the coal in the drying device. The dried coal is combusted
at the boiler to improve the efficiency of the power
generation.
[0007] The latent heat of condensation of the extracted steam,
however, has been transferred to a dry exhaust gas produced by the
drying device during the drying of the coal. Releasing the dry
exhaust gas without processing may lead to a loss of effective
heat. Furthermore, extracting part of the medium-pressure or
low-pressure steam from the steam turbine may reduce the amount of
steam flowing at a final stage of the steam turbine, resulting in
an increase in exhaust loss and a reduction in turbine
efficiency.
[0008] In particular, in the case where the drying device has been
added to predry the coal using the extracted steam from the steam
turbine as a heat source because of reasons such as a change of the
type of coal for an existing coal-fired power generating system,
the amount of steam flowing at the final stage of the steam turbine
may be reduced significantly in comparison with a design value in
some cases. Such a significant reduction in the amount of steam may
lead to a reduction in turbine efficiency, serving as an impediment
to a sufficient improvement in efficiency of the power generation
expected through the predrying of the coal.
[0009] Furthermore, coal-fired power generating systems need to be
operated under a low load in response to a reduced power demand at,
for example, nighttime. In this case, the amount of steam flowing
at the final stage of the steam turbine is further reduced. This
may result in vibration and the like, which leads to another
disadvantage of a reduced range of a low load operation in
comparison with traditional techniques.
CITATION LIST
Patent Literature
[0010] Patent Literature 1: JP 8-296835 A [0011] Patent Literature
2: JP 6-66107 A
SUMMARY OF INVENTION
Technical Problem
[0012] The present invention has been achieved in light of the
background described above, and it is an object of the present
invention to provide a coal-fired power generating system and a
coal-fired power generating method that are capable of, in addition
to recovering latent heat of condensation and the like from a dry
exhaust gas of a drying device, which is for predrying coal,
precluding a large variation, from a design value, of an amount of
steam flowing at a final stage of a steam turbine and suppressing a
reduction in efficiency of power generation.
Solution to Problem
[0013] An aspect of the present invention described in claim 1 that
has solved the problems described above is a coal-fired power
generating system, including:
[0014] an indirect-heating dryer including a heating medium passage
inside a casing thereof, the dryer being configured to dry a coal
fed into the casing by performing indirect heating with steam fed
to the heating medium passage;
[0015] a coal-fired boiler for combusting a dried coal to generate
steam; and
[0016] a steam turbine for generating power with the steam from the
boiler,
[0017] the coal-fired power generating system being configured to
heat boiler supply water for the coal-fired boiler with extracted
steam extracted from the steam turbine,
[0018] wherein the coal-fired power generating system includes:
[0019] a line for using part of the extracted steam as heated steam
for the indirect-heating dryer;
[0020] a steam condenser for the steam turbine;
[0021] a heat recovery unit provided on a path of a dry exhaust gas
from the indirect-heating dryer, the heat recovery unit being
configured to transfer heat of the dry exhaust gas to condensate of
the steam condenser, the heat recovery unit having a heat recovery
quantity adjusting unit for adjusting a quantity of heat recovery
of the heat recovery unit; and
[0022] a line for using the condensate having the heat recovered
from the dry exhaust gas by the heat recovery unit for heating the
boiler supply water.
[0023] In a coal-fired power generating system according to claim
1, a drying device predries coal using steam extracted from a steam
turbine as a heat source. From dry exhaust gas discharged by this
drying device, heat is recovered by a heat recovery heat exchanger
to heat boiler supply water for a boiler. The quantity of heat
recovery is adjusted during the heat recovery from the dry exhaust
gas, so that the amount of steam extracted from a low pressure (low
temperature) part of the steam turbine for regeneration can be
reduced or eliminated.
[0024] The amount of steam extracted for the predrying varies
depending on the water content of the coal and a throughput. An
appropriate adjustment, however, on the quantity of heat recovery
from the dry exhaust gas allows an adjustment to the amount of
steam extracted from the low-pressure steam turbine for heating the
boiler supply water. Hence, a reduction in variability of the
amount of extracted steam through a reduction or elimination of the
amount of extracted steam to be extracted from the low-pressure
steam turbine can preclude a large variation, from a design value,
of the amount of steam flowing at a final stage of the steam
turbine. This allows the amount of exhaust from the low-pressure
steam turbine to remain within a tolerable range.
[0025] Accordingly, while the coal-fired power generating system
according to the present invention uses latent heat of condensation
of the extracted steam from the steam turbine as the heat source
for the drying device to predry the coal, the system allows
recovery of the latent heat of condensation and the like from the
dry exhaust gas discharged from the drying device. In addition, the
system can prevent a reduction in efficiency of the low-pressure
steam turbine because the system precludes a large variation, from
a design value, of the amount of steam flowing at the final stage
of the steam turbine.
[0026] Through the use of a wet scrubber to recover the heat from
the dry exhaust gas, the sensible heat of the dry exhaust gas and
the latent heat of condensation of the steam formed by the
vaporization of the water content of the coal can be transferred to
circulating water with high efficiency of heat recovery.
Furthermore, controlling a temperature of the exhaust gas at an
outlet of the wet scrubber facilitates the control to suppress the
amount of extracted steam from the low pressure (low temperature)
part of the steam turbine.
[0027] An aspect of the present invention described in claim 2 is
the coal-fired power generating system according to claim 1,
wherein the heat recovery unit includes a wet scrubber provided at
the path of the dry exhaust gas from the indirect-heating dryer and
a heat recovery heat exchanger for performing heat exchanging
between circulating water of the wet scrubber and the condensate of
the steam condenser, and the heat recovery quantity adjusting unit
is configured to adjust the quantity of heat recovery by
controlling an amount of the circulating water of the wet
scrubber.
[0028] Even though it is possible to use, for example, a dry
exhaust gas--gas of condensate--liquid type shell and tube heat
exchanger in order to recover the heat of the dry exhaust gas, the
wet scrubber, which is a circulating water--condensate--liquid type
heat exchanger, provides significantly higher efficiency of the
heat recovery in comparison. Furthermore, the wet scrubber also
offers ease of control over the amount of circulating water
thereof, and hence, it facilitates forming a heat recovery quantity
adjusting unit.
[0029] In an aspect of the present invention described in claim 2,
the coal-fired power generating system according to claim 1
includes the heat recovery unit including a wet scrubber provided
on a path of the dry exhaust gas from the indirect-heating dryer
and a heat recovery heat exchanger for performing heat exchanging
between the circulating water of the wet scrubber and the
condensate of the steam condenser. The heat recovery quantity
adjusting unit is configured to adjust the quantity of heat
recovery by controlling the amount of the circulating water of the
wet scrubber.
[0030] An aspect of the present invention described in claim 3 is
the coal-fired power generating system according to claim 1,
wherein the heat recovery unit includes a heat pump unit.
[0031] In the case where a high water content coal, such as brown
coal, lignite and the like, is subjected to the drying processing
by the drying device until the coal has a low water content, the
temperature of the dry exhaust gas is typically at 100.degree. C.
or lower, and hence, through the heat recovery and the exchanging
of the heat with the condensate, the temperature of the condensate
cannot be increased to 100.degree. C. or higher. Thus, the quantity
of heat of the dry exhaust gas cannot be sufficiently recovered as
a result. By using a heat pump unit to convert the low-temperature
waste heat, which cannot be sufficiently recovered, to a
high-temperature heat source, the heat can be further recovered for
heating the boiler supply water.
[0032] An aspect of the present invention described in claim 4 is
the coal-fired power generating system according to claim 1 or 2,
wherein the wet scrubber is a two-stage type, and a first heat
recovery heat exchanger corresponding to circulating water of a
first-stage scrubber heats the boiler supply water, a second heat
recovery heat exchanger corresponding to circulating water of a
second-stage scrubber receives the boiler supply water and heats
the boiler supply water to a higher temperature, and the second
heat recovery heat exchanger is a heat pump.
[0033] For example, the temperature of the dry exhaust gas at an
outlet of a first-stage scrubber is cooled to approximately
65.degree. C. The sensible heat and the latent heat of condensation
of the dry exhaust gas are transferred to circulating water of the
first-stage scrubber. Heat exchanging is performed between the
circulating water of the first-stage scrubber and the condensate to
heat the condensate. At this point in time, the condensate has a
temperature equal to or lower than that of the dry exhaust gas.
[0034] The dry exhaust gas at the outlet of the first-stage
scrubber is allowed to enter a second-stage scrubber. The dry
exhaust gas is cooled, so that the temperature of the dry exhaust
gas at an outlet of the second-stage scrubber is, for example,
around 30.degree. C. The sensible heat and the latent heat of
condensation of the dry exhaust gas are transferred to circulating
water of the second-stage scrubber. The circulating water of the
second-stage scrubber has a maximum temperature of 65.degree. C.,
and hence, the condensate cannot be heated in this condition. Here,
a heat pump that uses the circulating water of the second-stage
scrubber as a heat source is introduced to achieve recovery of
high-temperature liquid (for example, 120.degree. C.), which
enables the condensate to be further heated.
[0035] On account of this, the quantity of heat extracted for the
drying can be mostly recovered to be used to heat the
condensate.
[0036] An aspect of the present invention described in claim 5 is
the coal-fired power generating system according to claim 1,
wherein a boiler combustion exhaust gas is fed as a carrier gas
into the casing of the indirect-heating dryer.
[0037] The feeding of the boiler combustion exhaust gas into the
casing of the indirect-heating dryer as the carrier gas allows
recovery of the sensible heat of the exhaust gas from the boiler
and the latent heat of condensation of steam contained in the
boiler exhaust gas. This enables the recovery of a quantity of heat
larger than the quantity of heat extracted for the drying, which
enables heating of the condensate. This not only conserves energy,
but also can achieve a reduction of low-pressure (low-temperature)
steam used for the regeneration by an amount equal to or more than
the amount of extracted steam for the drying, leading to an
expanded range of a low load operation.
[0038] An aspect of the present invention described in claim 6 is a
coal-fired power generating method for a coal-fired power
generating system, the system including:
[0039] an indirect-heating dryer including a heating medium passage
inside a casing thereof, the dryer being configured to dry a coal
fed into the casing by performing indirect heating with steam fed
to the heating medium passage;
[0040] a coal-fired boiler for combusting a dried coal to generate
steam; and
[0041] a steam turbine for generating power with the steam from the
boiler,
[0042] the system being configured to heat boiler supply water for
the coal-fired boiler with extracted steam extracted from the steam
turbine,
[0043] the method including the steps of:
[0044] using part of the extracted steam as heated steam for the
indirect-heating dryer and condensing exhaust of the steam turbine
by a steam condenser;
[0045] providing a heat recovery unit at a path of a dry exhaust
gas from the indirect-heating dryer, the heat recovery unit being
configured to transfer heat of the dry exhaust gas to condensate of
the steam condenser, the heat recovery unit having a heat recovery
quantity adjusting unit for adjusting a quantity of heat recovery
of the heat recovery unit; and
[0046] using the condensate having the heat recovered from the dry
exhaust gas by the heat recovery unit for heating the boiler supply
water.
[0047] An aspect of the present invention described in claim 7 is
the coal-fired power generating method according to claim 6,
wherein a boiler combustion exhaust gas is fed as a carrier gas
into the casing of the indirect-heating dryer, and the dry exhaust
gas has a dew point in a range from 80.degree. C. to 95.degree.
C.
Advantageous Effects of Invention
[0048] The present invention is capable of, in addition to
recovering latent heat of condensation and the like from a dry
exhaust gas of a drying device, which is for predrying coal,
precluding a large variation, from a design value, of the amount of
steam flowing at a final stage of a steam turbine and suppressing a
reduction in efficiency of power generation.
BRIEF DESCRIPTION OF DRAWINGS
[0049] FIG. 1 is a partially cutout perspective view of a steam
tube dryer applied to a first embodiment according to the present
invention.
[0050] FIG. 2 is a schematic view of a regenerative coal-fired
power generating system according to the first embodiment of the
present invention.
[0051] FIG. 3 is a schematic view of a regenerative coal-fired
power generating system according to a second embodiment of the
present invention.
[0052] FIG. 4 is an enlarged view of a main part of the second
embodiment of the present invention.
[0053] FIG. 5 is a graph of a relationship between the amount of
steam at a final stage of a steam turbine and exhaust loss.
[0054] FIG. 6 is a graph of a relationship between the temperature
of the dry exhaust gas after heat recover and the ratio of the
amount of steam flowing at the final stage of the turbine with no
predry device.
DESCRIPTION OF EMBODIMENT
[0055] A first embodiment of a coal-fired power generating system
and a coal-fired power generating method according to the present
invention will be described with reference to the drawings. Prior
to the description of the present embodiment, a steam tube dryer
will be described in advance with reference to FIG. 1 to deepen
understanding. The steam tube dryer will be described as an example
of indirect-heating rotary dryers that can be used suitably as a
drying device applied to some embodiments of the present
invention.
[0056] With reference to FIG. 1, a steam tube dryer 1 includes a
rotating shell 30, a plurality of heating tubes 31, a rotary joint
50, a heating medium inlet nozzle 51, and a heating medium outlet
nozzle 52. The rotating shell 30 is configured to be rotatable
about a shaft center thereof. The plurality of heating tubes 31 is
arranged inside the rotating shell 30 between both of its end
plates in parallel with the shaft center. Extracted steam S7 is fed
from the outside through the heating medium inlet nozzle 51, which
is attached to the rotary joint 50, to be supplied to the heating
tubes 31 as heated steam. The heated steam flows through each
heating tube 31 and then a drain D of the heated steam is
discharged through the heating medium outlet nozzle 52.
[0057] The steam tube dryer 1 is also provided with a feeder, which
is not shown and includes a screw or the like for feeding a
material to be processed into the rotating shell 30. The material
to be processed, which includes coal WC and an organic material
that contain water, is introduced from an inlet nozzle 53 of the
feeder into the rotating shell 30 at its one end side, and is dried
as it is in contact with the heating tubes 31 heated by the heated
steam. In addition, the rotating shell 30 is installed with a
downward inclination, so that the material is moved in a direction
to an outlet nozzle 54 successively and smoothly and the material
processed is discharged continuously from the rotating shell 30 at
its other end side.
[0058] As illustrated in FIG. 1, the rotating shell 30 is installed
on base tables 36 and supported, through tires 34, by two sets of
support rollers 35 and 35 disposed at an interval from each other
and in parallel with the shaft center of the rotating shell 30. The
width between the two sets of support rollers 35 and 35 and their
angles of inclination in a longitudinal direction are selected in
agreement with the downward inclination and a diameter of the
rotating shell 30.
[0059] In order to rotate the rotating shell 30, a driven gear 40
is provided on a circumference of the rotating shell 30 to be in
mesh with a driving gear 43, so that the torque of a motor 41
transmitted through a speed reducer 42 causes the driven gear 40 to
rotate about the shaft center of the rotating shell 30.
Furthermore, a carrier gas CG is introduced through a carrier gas
inlet nozzle 61 into the rotating shell 30. The carrier gas CG
entrains steam formed by the vaporization of the water contained in
the coal or the organic material, which is the material to be
processed, and is discharged through a carrier gas outlet nozzle 62
as a dry exhaust gas DEG.
[0060] Note that the overall arrangement of the steam tube dryer 1
described above is an example. The present invention is not united
by the foregoing arrangement.
[0061] FIG. 2 is a schematic view of a regenerative coal-fired
power generating system to which the present embodiment is
applied.
[0062] As illustrated in FIG. 2, dried coal DC, which has, been
dried and discharged by the steam tube dryer 1, is introduced to a
pulverizer 2. The pulverized dried coal DC, which has been
pulverized by the pulverizer 2, is introduced to a coal-fired
boiler 3.
[0063] In the case where the coal WC, which is of a low rank (high
water content), is supplied to the steam tube dryer 1, extracted
steam of a first steam turbine 6, which will be described
hereinafter, is used as a heat source to allow the steam tube dryer
1 to perform the preliminary drying of the coal WC and obtain the
dried coal DC.
[0064] This drying operation involves the dry exhaust gas DEG
discharged from the other end side of the steam tube dryer 1. The
dried coal DC is pulverized by the pulverizer 2 as appropriate
while being dried. An exhaust gas EG2 from the pulverizer 2 then
entrains the pulverized material and is supplied to the boiler 3 to
be combusted by a burner, not shown.
[0065] The boiler 3 is provided with three heat exchangers, which
are a first heat exchanging unit 3A to a third heat exchanging unit
3C. Steam generated by the boiler 3 as a heating medium is fed to a
second heat exchanging unit 3B, among the heat exchangers, to be
reheated. This reheated superheated steam S1 is supplied to a
high-pressure steam turbine 7 of the first steam turbine 6 in order
to drive the high-pressure steam turbine 7. The high-pressure steam
turbine 7 is coupled to a low-pressure steam turbine 8 and is also
connected to a power generator 6A, such that the high-pressure
steam turbine 7 and the low-pressure steam turbine 8 are driven to
rotate in conjunction with each other to recover a quantity of heat
and cause the power generator 6A of the first steam turbine 6 to
generate electric power.
[0066] Here, steam is extracted from the high-pressure steam
turbine 7. Part of this steam heats boiler supply water D2, which
is for the boiler 3, as flows of the extracted steam S2 and S3 in a
water supply pipeline 12. The remaining extracted steam 34 is
returned to the first heat exchanging unit 3A of the boiler 3 to be
reheated into re-superheated steam S5, which is supplied to the
low-pressure steam turbine 8 to be used as driving power. In
addition, part of extracted steam 36 extracted from the
low-pressure steam turbine 8 similarly heats the boiler supply
water D2 in the water supply pipeline 12.
[0067] Meanwhile, another flow extracted from the low-pressure
steam turbine 8 of the first steam turbine 6, namely the extracted
steam S7, is used as the heat source for the steam tube dryer 1.
The extracted steam 37 is also fed to a second steam turbine 9,
which is a low-pressure steam turbine, to allow a power generator
9A associated with the second steam turbine 9 to generate electric
power. Subsequently, flows of extracted steam S8 and S9, which are
part of extracted steam from the second steam turbine 9, are merged
with the drain D discharged by the steam tube dryer 1 to be
supplied to the water supply pipeline 12. Concurrently, the flows
of the extracted steam S8 and S9 are supplied directly to the water
supply pipeline 12 to similarly heat the boiler supply water D2.
Another flow from the second steam turbine 9, namely extracted
steam S10, is fed to a steam condenser 5 that performs heat
exchanging using sea water as cooling water. The steam condenser 5
condenses the extracted steam S10 into boiler supply water D1.
[0068] In addition, air from the outside is fed to the third heat
exchanging unit 3C of the boiler 3 to be heated. The air is then
fed into the boiler 3 to assist the combustion of the dried coal
DC. Part of an exhaust gas discharged by the boiler 3 is used as
the carrier gas CG of the steam tube dryer 1 and concurrently used
as a boiler combustion exhaust gas EG2 and fed to the pulverizer 2.
The remainder of the exhaust gas EG1 is discharged to the outside.
Note that, in the present embodiment, the exhaust gas discharged by
the boiler 3 is supplied as the carrier gas CG of the steam tube
dryer 1 such that the dew point of the dry exhaust gas DEG is in a
range from 80.degree. C. to 95.degree. C. Alternatively, an inert
gas, such as air and nitrogen, may be used such that the dew point
of the dry exhaust gas DEG is in this temperature range.
[0069] The steam condenser 5 is connected to a heat recovery unit.
In particular, as illustrated in FIGS. 3 and 4, the steam condenser
5 is connected to a wet scrubber 11, which is used as a heat
recovery unit. As the water contained in the coal WC is vaporized
in the steam tube dryer 1, the dry exhaust gas DEG discharged
therefrom is allowed to pass through this scrubber 11. The dry
exhaust gas DEG is cooled to a predetermined temperature with
circulating water in the scrubber 11. Conversely, the sensible heat
of the dry exhaust gas DEG and the latent heat of condensation of
the steam formed by the vaporization of the water contained in the
coal WC are temporarily transferred to the circulating water.
Subsequently, heat exchanging is performed, for the quantity of the
heat that has been transferred, with the boiler supply water D1
that has been generated by the condensation at the steam condenser
5 and fed to the scrubber 11. This boiler supply water D1 becomes
the boiler supply water D2. In this manner, the scrubber 11
performs heat exchanging with the dry exhaust gas DEG from the
steam tube dryer 1 to recover the heat from the dry exhaust gas
DEG.
[0070] In the case where the wet scrubber 11 is used as the heat
recovery unit, as evident in FIG. 4, a circulating pump for
adjusting the amount of the circulating water may be configured to
mainly function as a heat recovery quantity adjusting unit.
[0071] Note that the heat recovery unit is not limited to the wet
scrubber 11 and, as described above, a shell and tube type heat
exchanger may be used, for example.
[0072] Furthermore, the scrubber 11 is connected through the water
supply pipeline 12 to the boiler 3, so that the boiler supply water
D2 is fed to the boiler 3 and deaerated by a deaerator 10 disposed
on the pipeline 12 along the way. Here, the flows of the extracted
steam S2, S3, S6, S8, and S9, which constitute part of the steam
discharged by the steam turbines 7, 8, and 9, are introduced to the
water supply pipeline 12 along the way to heat the boiler supply
water D2. In other words, the steam is extracted from each of the
steam turbines 7, 8, and 9 to heat the boiler supply water D2 to a
predetermined temperature.
[0073] The operation of the coal-fired power generating system and
coal-fired power generating method according to the present
embodiment will now be described.
[0074] The coal-fired power generating system according to the
present embodiment uses a regeneration scheme in which the steam
extracted from the steam, turbines 6 and 9 is used to heat the
boiler supply water D2, which is for the boiler 3. Note that, in
the present embodiment, the extracted steam S7 extracted from the
steam turbine 6 is used as the heat source to allow the steam tube
dryer 1 to predry the coal WC. Subsequently, the steam tube dryer 1
discharges the dry exhaust gas DEG, and the heat from the dry
exhaust gas DEG is recovered by the scrubber 11, which is the heat
exchanger, to heat the boiler supply water D1, which is for the
boiler 3.
[0075] Here, the scrubber 11 adjusts the quantity of heat recovery
during the heat recovery from the dry exhaust gas DEG, leading to a
reduction in the amount of the flows of extracted steam S6, S8, and
S9 from the low-pressure steam turbines 8 and 9, which are
low-pressure and low-temperature parts within the steam turbines 6
and 9.
[0076] The amount of steam extracted for the predrying varies
depending on the water content of the coal WC and a throughput. An
appropriate adjustment, however, to change the quantity of heat
recovery by the scrubber 11 from the dry exhaust gas DEG allows an
adjustment to the temperature of the steam extracted from the
low-pressure steam turbines 8 and 9 for heating the boiler supply
water D1. As a result, a reduction in variability of the amount of
extracted steam through a reduction or elimination of the amount of
extracted steam extracted from the low-pressure steam turbines 8
and 9 precludes a large variation, from a design value, of the
amount of steam flowing at the final stage of the steam turbine.
This allows the amount of exhaust from the low-pressure steam
turbines 8 and 9 to remain, within a tolerable range.
[0077] As described above, while the coal-fired power generating
system according to the present embodiment uses the latent heat of
condensation of the extracted steam S7 from the steam turbine 6 as
a heat source for the steam tube dryer 1 to predry the coal WC, the
system allows recovery of the latent heat of condensation and the
like from the dry exhaust gas DEG discharged from the steam tube
dryer 1. Accordingly, the present embodiment can reduce coal
consumption and CO.sub.2 emissions per unit amount of power
generation, and hence, allows a coal-fired power generating system,
such as a coal-fired power plant, to generate power more
efficiently. In addition to recovering the latent heat of
condensation and the like from the dry exhaust gas DEG, the present
embodiment can prevent a reduction in efficiency of the
low-pressure steam turbines 8 and 9 because it precludes a large
variation, from a design value, of the amount of steam flowing at
the final stage of the steam turbine.
[0078] Moreover, in the present embodiment, a boiler combustion
exhaust gas selected from a group of an inert gas, such as air and
nitrogen, and the boiler combustion exhaust gas is supplied to the
steam tube dryer 1 as the carrier gas CG. The dew point of the dry
exhaust gas DEG is in a range from 80.degree. C. to 95.degree.
C.
[0079] Here, a higher dew point of the dry exhaust gas DEG from the
steam tube dryer 1 results in a less amount of the dry exhaust gas,
and hence, a compact size of the processing device for the dry
exhaust gas and an increased quantity of heat that can be recovered
from the dry exhaust gas DEG. This, however, entails a reduced
temperature difference between the temperature of the coal inside
the steam tube dryer 1 and the temperature of the steam for
heating, degrading the drying capacity of the steam tube dryer 1.
Thus, it is preferable that the dew point of the dry exhaust gas
DEG is in a range from 80.degree. C. to 95.degree. C. on the basis
of the relationship between the quantity of the heat recovery and
the drying capacity, although also depending on the water content
and an amount of the coal WC to be dried.
[0080] In addition, the use of the boiler exhaust gas as the
carrier gas CG of the steam tube dryer 1, similarly to the present
embodiment, allows the recovery of the sensible heat of the boiler
exhaust gas and the latent heat of condensation of steam contained
in the boiler exhaust gas. This not only conserves energy, but also
can achieve a reduction of low pressure (low temperature) steam for
regeneration by an amount equal to or more than the amount of
extracted steam for the drying, leading to an expanded range of the
low load operation.
[0081] With reference to FIGS. 3 and 4, a coal-fired power
generating system and a coal-fired power generating method
according to a second embodiment of the present invention will now
be described. Note that like reference figures are used for the
components described in the first embodiment and a description of
these components is excluded.
[0082] In the first embodiment, the scrubber 11 is used as the heat
exchanger, whereas, in the present embodiment, a two-stage scrubber
21 is used as the heat exchanger as illustrated in FIGS. 3 and 4.
An indirect heat exchanger 22 is positioned for a first-stage
scrubber 21A, which is one of the two stages. The indirect heat
exchanger 22 is configured to heat the boiler supply water D1 with
circulating water W of the first-stage scrubber 21A.
[0083] In addition, a heat pump unit 27 is positioned for a
second-stage scrubber 21B as illustrated in FIG. 4. The heat pump
unit 27 includes an evaporator 24, a compressor 25, and a condenser
26. The boiler supply water D1 fed from the indirect heat exchanger
22 of the first-stage scrubber 21A is further heated with
circulating water W to eventually become the boiler supply water
D2.
[0084] As described above, the boiler supply water D2 is heated in
two stages. This arrangement according to the present embodiment
enables efficient heat recovery from the dry exhaust gas DEG to
optimally heat the boiler supply water D2. In addition, this
arrangement allows adjustment of the quantity of heat recovery
during the heat recovery by the scrubber 11, which reduces the
amount of extracted steam from the low-pressure steam turbines 8
and 9. Here, the heat pump unit 27 is used to increase the
temperature of heat recovered from the circulating fluid of the
second-stage scrubber and then to heat the boiler supply water D2,
which allows more effective recovery of the quantity of heat.
[0085] With reference to FIG. 5, a relationship between the amount
of steam at the final stage of the steam turbine and exhaust loss
will be described.
[0086] The exhaust loss is small in proximity to a design point P,
whereas the exhaust loss increases with the amount of steam either
increased or decreased from the design point P, resulting in a
reduction in turbine efficiency and a reduction in efficiency of
power generation. As a result, it is understood that the efficiency
of a steam turbine is improved by maintaining small variability of
the amount of extracted steam from the steam turbine and precluding
a large variation, from a design value, of the amount of steam
flowing at the final stage of the steam turbine.
[0087] With reference to FIG. 6, a relationship between the
temperature of the dry exhaust gas after the heat recovery and the
ratio of the amount of steam flowing at the final stage of the
steam turbine in the embodiments described above to that with no
predry device will now be described.
[0088] When coal of 65% water content is dried to 10% using the
extracted steam from the steam turbine as a heat source with a
constant amount of power generation, this ratio decreases as the
temperature of the dry exhaust gas after the heat recovery
increases, and the ratio falls below 100% at approximately
70.degree. C. according to the illustrated graph.
[0089] Note that the amount of extracted steam from the
low-pressure steam turbine may be determined by installing a
flowmeter in an exhaust line into which the extracted steam is
exhausted and taking measurements with this flowmeter, or by
measuring the amount of water condensed by the steam condenser 5.
In addition, the process of adjusting the quantity of heat recovery
from the dry exhaust gas DEG is not particularly limited. For
example, similarly to the embodiments described above, the dry
exhaust gas DEG is preferably allowed to pass through the scrubber
11 or 21, such that the sensible heat of the dry exhaust gas DEG
and the latent heat of condensation of the dry steam are
transferred to the circulating water in circulation. In the case
where the indirect heat exchanging is performed between the
circulating water and the boiler supply water D1, the temperature
of the exhaust gas at the outlet of the scrubber 21 may be
controlled with the amount of boiler supply water D1 allowed
through the indirect heat exchanger 22.
[0090] As described above, some embodiments according to the
present invention have been described. The present invention,
however, is not limited by these embodiments, and various
modifications can be made without departing from the spirit of the
present invention.
INDUSTRIAL APPLICABILITY
[0091] The present invention can be applied to a coal-fired power
generating system.
REFERENCE SIGNS LIST
[0092] 1 Steam tube dryer (indirect-heating dryer) [0093] 3 Boiler
[0094] 5 Steam condenser [0095] 6 First steam turbine [0096] 7
High-pressure steam turbine [0097] 8 Low-pressure steam turbine
[0098] 9 Second steam turbine (low-pressure steam turbine) [0099]
11 Scrubber (heat exchanger) [0100] 12 Water supply pipeline [0101]
21 Scrubber (heat exchanger) [0102] 22 Indirect heat exchanger
[0103] 27 Heat pump unit
* * * * *