AU2012235025A1

AU2012235025A1 - Coal-fired power generation plant and coal-fired power generation method

Info

Publication number: AU2012235025A1
Application number: AU2012235025A
Authority: AU
Inventors: Masaki Kataoka; Toshiyuki Kimura; Takayuki Noguchi
Original assignee: Tsukishima Kikai Co Ltd
Current assignee: Tsukishima Kikai Co Ltd
Priority date: 2011-03-25
Filing date: 2012-02-27
Publication date: 2013-10-10
Anticipated expiration: 2032-02-27
Also published as: KR101879471B1; TW201319474A; US20140013746A1; KR20140002743A; JP5535112B2; WO2012132700A1; TWI544183B; AU2012235025B2; JP2012202607A

Abstract

[Problem] To provide a coal-fired power generation plant that not only recovers latent heat condensation from dry exhaust gas generated by drying facilities that dry coal in advance, but also is configured so that the amount of steam that flows in the final stage of a steam turbine does not materially differ from design values. [Solution] A coal-fired power generation plant comprising an indirect heat dryer (1) that has a heat medium flow channel in a housing and that indirectly heats coal brought into the housing using steam fed to the heat medium flow channel, thus drying the coal; a coal-firing boiler (3) that burns coal to generate steam; and a steam turbine (6) that generates power using the steam from the boiler (3). The boiler feedwater for the coal-firing boiler (3) is waste-heated using the extracted steam extracted from the steam turbine (6). The coal-fired power generation plant is provided with a system for using a portion of the extracted steam as the heating steam of the indirect heat dryer (1); a condenser (5) of the steam turbine (6); a wet scrubber (11) provided with a dry exhaust gas path from the indirect heat dryer (1); and heat recovery and heat exchange devices (22) (24) that perform heat exchange between the circulation water of the wet scrubber (11) and the condensation water of the condenser (5), wherein the condensation water that recovered the heat from the dry exhaust gas through the heat recovery and heat exchange devices (22) (24) is used to waste-heat the boiler feedwater.

Description

DESCRIPTION Title of Invention COAL-FIRED POWER GENERATING SYSTEM AND COAL-FIRED POWER GENERATING METHOD 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. 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. 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 [0002] 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. 1 [0003] 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). [0004] 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. [0005] 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 2 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. [00061 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. [0007] 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 [0008] 3 Patent Literature 1: JP 8-296835 A Patent Literature 2: JP 6-66107 A Summary of Invention Technical Problem [0009] 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 [0010] 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: 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; 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, 4 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 includes: 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. [0011] 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. 5 [0012] 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. [0013] 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. Through the use of a wet scrubber to recover the heat from the dry exhaust gas, the sensible heat of the dry exhaust gas 6 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. [0014] 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. 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. 7 [0015] 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. [0016] 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. [0017] 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 0 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 0 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 8 heat can be further recovered for heating the boiler supply water. [0018] 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. [0019] For example, the temperature of the dry exhaust gas at an outlet of a first-stage scrubber is cooled to approximately 65 0 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. 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, 9 around 30 0 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 0 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 0 C) , which enables the condensate to be further heated. On account of this, the quantity of heat extracted for the drying can be mostly recovered to be used to heat the condensate. [0020] 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. [0021] 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 10 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. [0022] 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: 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; 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 including 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 11 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. [0023] 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 0 C to 95 0 C. Advantageous Effects of Invention [0024] 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 [0025] Fig. 1 is a partially cutout perspective view of a steam tube dryer applied to a first embodiment according to the present invention. Fig. 2 is a schematic view of a regenerative coal-fired power generating system according to the first embodiment of the present invention. 12 Fig. 3 is a schematic view of a regenerative coal-fired power generating system according to a second embodiment of the present invention. Fig. 4 is an enlarged view of a main part of the second embodiment of the present invention. Fig. 5 is a graph of a relationship between the amount of steam at a final stage of a steam turbine and exhaust loss. 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 [0026] 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. [0027] 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 13 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. [0028] 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. [0029] 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 14 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. [0030] 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. [0031] Note that the overall arrangement of the steam tube dryer 1 described above is an example. The present invention is not limited by the foregoing arrangement. [0032] Fig. 2 is a schematic view of a regenerative coal-fired power generating system to which the present embodiment is applied. 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 15 pulverized by the pulverizer 2, is introduced to a coal-fired boiler 3. [0033] 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. 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. [0034] 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 16 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. [0035] 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 S4 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 S6 extracted from the low-pressure steam turbine 8 similarly heats the boiler supply water D2 in the water supply pipeline 12. [0036] 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 S7 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 17 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 Dl. [0037] 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 0 C to 95 0 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. [0038] 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 18 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 Dl that has been generated by the condensation at the steam condenser 5 and fed to the scrubber 11. This boiler supply water Dl 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. 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. 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. [0039] 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 19 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. [0040] The operation of the coal-fired power generating system and coal-fired power generating method according to the present embodiment will now be described. 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 Dl, which is for the boiler 3. [0041] 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 20 steam turbines 6 and 9. [0042] 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 Dl. 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. [0043] 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 2 emissions per unit amount of power generation, and hence, allows a coal-fired power generating system, such as a coal-fired power 21 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. [0044] 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 0 C to 95 0 C. [0045] 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 0 C to 95 0 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. 22 [0046] 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. [0047] 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. 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 Dl with circulating water W of the first-stage scrubber 21A. [0048] In addition, a heat pump unit 27 is positioned for a 23 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 Dl 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. [0049] 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. [0050] 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. 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 24 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. [0051] 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. 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 0 C according to the illustrated graph. [0052] 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 25 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 Dl, the temperature of the exhaust gas at the outlet of the scrubber 21 may be controlled with the amount of boiler supply water Dl allowed through the indirect heat exchanger 22. 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 [0053] The present invention can be applied to a coal-fired power generating system. Reference Signs List [0054] 1 Steam tube dryer (indirect-heating dryer) 3 Boiler 5 Steam condenser 6 First steam turbine 7 High-pressure steam turbine 8 Low-pressure steam turbine 9 Second steam turbine (low-pressure steam turbine) 26 11 Scrubber (heat exchanger) 12 Water supply pipeline 21 Scrubber (heat exchanger) 22 Indirect heat exchanger 27 Heat pump unit 27

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 28 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 any one of claims 1 to 3, 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. 29

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 30 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 0 C to 95 0 C. 31