Patent Publication Number: US-2016238245-A1

Title: Flue gas heat recovery system

Description:
FIELD 
     The present invention relates to a flue gas heat recovery system which recovers heat of flue gas discharged from a boiler generating steam by combustion of fuel and air. 
     BACKGROUND 
     For example, a coal combustion boiler includes a hollow furnace located in the vertical direction. A plurality of combustion burners are provided in a lower part of the furnace. A flue gas duct is connected with an upper part of the furnace. A heat exchanger is provided in the flue gas duct to recover heat from flue gas. Water is heated by using flue gas generated by combustion within the furnace to generate steam. A gas duct is further connected with the flue gas duct of the coal combustion boiler. An air heater is provided in the gas duct. The air heater heats air by using flue gas to generate heated air, and supplies the heated air to the combustion burners as combustion air. 
     The air heater rotates a heat element to alternately bring a flue gas path and an air path into contact with flue gas and air, respectively, for heat exchange, thereby heating air by using flue gas for generation of heated air. The flue gas discharged from the boiler contains corrosion substances such as sulfurous acid (SO 3 ). Accordingly, the amount of heat recovered by the air heater is limited within such a range that sulfurous acid does not become sulfuric acid by condensation. Even when heat is not recovered by the air heater to such a temperature not causing condensation of sulfurous acid, an area having a temperature lowered to a condensation temperature of sulfurous acid may be produced due to the mechanism of heat exchange realized by rotation of the heat element. This area may cause corrosion or closure of the heat element of the air heater. 
     For example, Patent Literature 1 describes a coal combustion boiler which includes a heat recovery device in place of the rotational air heater. This heat recovery device includes a high-temperature loop and a low-temperature loop. In the high-temperature loop, a high-temperature heat medium circulates to preheat combustion air by using heat recovered from flue gas, while in the low-temperature loop, a low-temperature heat medium circulates to preheat combustion air by using heat recovered from flue gas, and further to reheat flue gas and heat boiler feed water. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Laid-open Patent Publication No. 63-217103 
     SUMMARY 
     Technical Problem 
     In the heat recovery device described above, the heat medium is heated by using heat of flue gas. The heated heat medium heats combustion air, flue gas, and boiler feed water. In this case, the temperature of the flue gas at an inlet of the heat recovery device is 300° C. or higher, while the temperature of the flue gas at an outlet decreases to 100° C. or lower. In this case, a temperature change within the heat recovery device becomes 200° C. or higher. Accordingly, the volume of the flue gas flowing through the heat recovery device considerably changes (decreases), and the flow velocity also considerably changes (decreases). In this condition, the flow velocity of the flue gas increases at the inlet of the heat recovery device. As a result, erosion of a heat exchange tube may be accelerated. On the other hand, the flow velocity of the flue gas decreases at the outlet of the heat recovery device. In this case, heat exchange performance may be lowered, while soot and dust or the like may be accumulated. 
     The present invention has been developed to solve the aforementioned problems. It is an object of the present invention to provide a flue gas heat recovery system capable of increasing durability and heat recovery efficiency. 
     Solution to Problem 
     According to an aspect of the present invention, a flue gas heat recovery system comprises: a hollow furnace located in a vertical direction; a combustion burner that blows fuel gas constituted by a mixture of fuel and combustion air toward the furnace; a flue gas path connected to an upper part of the furnace; a heat exchanging unit disposed in the flue gas path to exchange heat between flue gas and water; a heat recovering unit disposed in the flue gas path on the downstream side of the heat exchanging unit and configured such that a path cross-sectional area of an outlet of the heat recovering unit is made smaller than a path cross-sectional area of an inlet of the heat recovering unit; and a first heat exchanger that heats combustion air by using heat recovered by the heat recovering unit. 
     According to this structure, a heat medium recovers heat from flue gas while the flue gas is passing through the heat recovering unit. In this case, the temperature and volume of the flue gas decrease. However, the heat recovering unit is configured such that the path cross-sectional area of the outlet of the heat recovering unit is made smaller than the path cross-sectional area of the inlet. Accordingly, the flow velocity does not increase when the flue gas having the large volume passes through the inlet. On the other hand, the flow velocity does not decrease when the flue gas having the small volume passes through the outlet. In other words, fluctuations of the flow velocity are reduced even when the temperature and volume of the flue gas decrease during passage through the heat recovering unit. As a result, erosion of the heat exchange tube constituting the heat recovering unit, and deterioration of the heat exchange performance are both avoidable. Accordingly, durability and heat recovery efficiency are improved. 
     Advantageously, in the flue gas heat recovery system, the heat recovering unit is configured such that an inner size of the outlet in the flue gas path is made smaller than an inner size of the inlet. 
     According to this structure, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is determined easily and in a simplified configuration, so that the flue gas flow velocity becomes a velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side. 
     Advantageously, in the flue gas heat recovery system, the heat recovering unit is configured such that density of a heat exchange tube or heat exchange fins at the outlet is made higher than density of the heat exchange tube or the heat exchange fins at the inlet. 
     According to this structure, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, in accordance with a change of the position, shape, and number of the heat exchange tube or the heat exchange fins, without the necessity of changing the configuration of the flue gas path. 
     Advantageously, in the flue gas heat recovery system, the heat recovering unit includes an upstream side high temperature unit and a downstream side low temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit. 
     According to the structure of the heat recovering unit including the high temperature unit and the low temperature unit, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side. 
     Advantageously, in the flue gas heat recovery system, the flue gas path includes a vertical path extending in the vertical direction and a horizontal path connected with a lower part of the vertical path and extending in a horizontal direction, the high temperature unit is disposed in the vertical path, and the low temperature unit is disposed in the horizontal path. 
     According to the structure which positions the high temperature unit in the vertical path and the low temperature unit in the horizontal path, the path cross-sectional area of the inlet of heat exchanger into which high-temperature flue gas flows is determined easily and in a simplified configuration, so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, without the necessity for changing the configuration of the existing flue gas path. 
     Advantageously, in the flue gas heat recovery system, a NOx removal device is provided in the vertical path, and the high temperature unit is disposed below the NOx removal device. 
     According to this structure, the high temperature unit is disposed below the NOx removal device. In this case, flue gas flows into the high temperature unit after removal of harmful substances by the NOx removal device. Accordingly, adhesion of harmful substances to the heat recovering unit is avoidable. 
     Advantageously, in the flue gas heat recovery system, a hopper is provided between the vertical path and the horizontal path and below the high temperature unit. 
     According to this structure, the hopper is disposed below the high temperature unit. Accordingly, particles such as soot and dust contained in flue gas are collectable between the vertical path and the horizontal path. 
     Advantageously, the flue gas heat recovery system further comprises a second heat exchanger that reheats flue gas prior to discharging from a stack by using heat recovered by the heat recovering unit. 
     According to this structure, the second heat exchanger reheats flue gas by using heat recovered by the heat recovering unit. Accordingly, generation of white smoke is avoidable. 
     Advantageously, the flue gas heat recovery system further comprises a third heat exchanger that heats water supplied to the heat exchanging unit by using heat recovered by the heat recovering unit. 
     According to this structure, the third heat exchanger heats water supplied to the heat exchanging unit by using heat recovered by the heat recovering unit. Accordingly, effective utilization of the recovered heat is achievable. 
     Advantageously, the flue gas heat recovery system further comprises a distribution amount control device that adjusts distribution amounts of a heat medium supplied from the heat recovering unit to the first heat exchanger, the second heat exchanger, and the third heat exchanger. 
     According to this structure, the distribution amount control device adjusts the distribution amounts of the heat medium supplied from the heat recovering unit to the respective heat exchangers in accordance with the operation condition of the boiler. Accordingly, this structure makes it possible, while keeping a necessary amount of heat recovered from flue gas for environmental measurements, to utilize the remaining amount of heat recovered for improvement of power generation efficiency and for appropriate operation of the unit. 
     A flue gas heat recovery system according to the present invention includes a heat recovering unit disposed in a flue gas path for recovery of heat from flue gas, and configured such that a path cross-sectional area at an outlet of the heat recovering unit is made smaller than a path cross-sectional area at an inlet of the heat recovering unit. Accordingly, durability and heat recovery efficiency is increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view schematically illustrating a configuration of a boiler which includes a flue gas heat recovery system according to a first embodiment. 
         FIG. 2  is a view schematically illustrating flows of water (steam) and a heat medium included in the flue gas heat recovery system. 
         FIG. 3  is a view schematically illustrating a heat recovering unit and heat exchangers. 
         FIG. 4  is a view schematically illustrating a configuration of the heat recovering unit. 
         FIG. 5  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a second embodiment. 
         FIG. 6  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a third embodiment. 
         FIG. 7  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a fourth embodiment. 
         FIG. 8  is a view schematically illustrating a heat recovering unit included in a flue gas heat recovery system according to a fifth embodiment. 
         FIG. 9  is a view schematically illustrating a fin tube of the heat recovering unit. 
         FIG. 10  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A flue gas heat recovery system according to preferred embodiments according to the present invention is hereinafter described in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments described herein. When there are presented a plurality of embodiments, arbitrary combinations of the respective embodiments are contained in the scope of the present invention. 
     First Embodiment 
       FIG. 1  is a view schematically illustrating a configuration of a boiler which includes a flue gas heat recovery system according to a first embodiment. 
     The boiler in which a flue gas heat recovery system according to the first embodiment is included is a coal combustion boiler which uses pulverized coal as pulverized fuel (fuel), and burns the pulverized fuel by using combustion burners. The flue gas heat recovery system recovers heat generated by the combustion of the pulverized fuel. 
     As illustrated in  FIG. 1 , a boiler  10  in the first embodiment is a conventional boiler which includes a furnace  11 , a combustion device  12 , a flue gas duct (flue gas path)  13 , and a heat exchanging unit  14 . First of all, a general configuration of the boiler  10  is described. 
     The furnace  11  has a hollow and square cylindrical shape, and is located in the vertical direction. A furnace wall constituting the furnace  11  includes a heat exchange tube. 
     The combustion device  12  is disposed in a lower part of the furnace wall constituting the furnace  11 . The combustion device  12  includes a plurality of stages of combustion burners  21 ,  22 ,  23 ,  24 , and  25  attached on the furnace wall. According to this embodiment, five sets, or five stages of the combustion burners  21 ,  22 ,  23 ,  24 , and  25 , each of which contains four burners at equal intervals in the circumferential direction as one set, are provided in the vertical direction. However, the shape of the furnace  11 , the number of the combustion burners for each stage, the number of stages of the combustion burners, and other conditions are not limited to the specific examples in this embodiment, but may be arbitrarily determined. 
     Each of the combustion burners  21 ,  22 ,  23 ,  24 , and  25  uses pulverized coal which is milled coal as solid fuel. Coal pulverizers (pulverizers or mills)  31 ,  32 ,  33 ,  34 , and  35  are connected with the combustion burners  21 ,  22 ,  23 ,  24 , and  25  via pulverized coal supply tubes  26 ,  27 ,  28 ,  29 , and  30 , respectively. While not depicted in the figure, the coal pulverizers  31 ,  32 ,  33 ,  34 , and  35  pulverize coal into a predetermined size when coal is supplied between a plurality of pulverizing rollers and a pulverizing table. Pulverized coal thus produced is classified by using conveyance air (primary air), and supplied to the combustion burners  21 ,  22 ,  23 ,  24 , and  25  via the pulverized coal supply tubes  26 ,  27 ,  28 ,  29 , and  30 . 
     The furnace  11  includes a wind box  36  disposed at attachment positions of the combustion burners  21 ,  22 ,  23 ,  24 , and  25 . The furnace  11  further includes an additional air nozzle  37  on the furnace wall at a position above the attachment positions of the combustion burners  21 ,  22 ,  23 ,  24 , and  25 . One end of an air duct  38  is connected with a blower  39 , while the other end is connected with the wind box  36  and the additional air nozzle  37 . Accordingly, combustion air (secondary air) generated by the blower  39  is supplied to the wind box  36  via the air duct  38 , and further supplied from the wind box  36  to the respective combustion burners  21 ,  22 ,  23 ,  24 , and  25 . The combustion air coming from the air duct  38  is further supplied to the additional air nozzle  37 . 
     The flue gas duct  13  is connected with an upper part of the furnace  11 . The flue gas duct  13  includes a first horizontal path  41  connected with an upper end of the furnace  11 , a first vertical path  42  connected with an end of the first horizontal path  41 , a second horizontal path  43  connected with a lower end of the first vertical path  42 , a second vertical path  44  connected with an end of the second horizontal path  43 , and a gas duct  45  connected with an end of the second vertical path  44 . 
     The flue gas duct  13  includes the heat exchanging unit  14  in the first horizontal path  41  and the first vertical path  42 . The heat exchanging unit  14  exchanges heat between flue gas generated by combustion within the furnace  11  and water (steam) flowing within the heat exchange tube. The heat exchanging unit  14  includes superheaters  46 ,  47  and  48 , reheaters  49  and  50 , and economizers (economizers)  51  and  52 . 
     The flue gas duct  13  includes a NOx removal device (selectively reducing catalyst)  61  disposed in the second vertical path  44 . The NOx removal device  61  decomposes nitrogen oxide (NOx) contained in flue gas into harmless nitrogen and steam by the function of a catalyst contained in a reducing agent (such as ammonia) 
     The flue gas duct  13  includes an electrostatic precipitator  62 , an induced draft fan  63 , a desulfurization device  64 , and a stack  65  in the gas duct  45 . The electrostatic precipitator  62  charges particles of various types of dust contained in flue gas to attract the particles toward a collecting electrode and collect dust. The desulfurization device  64  is a wet-type desulfurization device that ejects absorbent toward flue gas having entered an absorber and brings the absorbent into contact with the flue gas to absorb and remove sulfur oxide (SO2) gas contained in the flue gas. 
     The boiler  10  according to this embodiment includes a heat recovering unit  71  disposed in the flue gas duct  13  on the downstream side of the heat exchanging unit  14  to recover heat from flue gas. The heat recovering unit  71  includes an upstream side high temperature unit  72  and a downstream side low temperature unit  73 . The high temperature unit  72  and the low temperature unit  73  are directly connected via a heat exchange tube. The boiler  10  further includes a first heat exchanger  74  which heats combustion air by using heat recovered by the heat recovering unit  71 . More specifically, provided between the heat recovering unit  71  and the first heat exchanger  74  is a heat medium circulation path  75  where a heat medium (such as water and steam) circulates. The heat medium circulation path  75  includes a first circulation path  75   a  extending from the high temperature unit  72  to the first heat exchanger  74 , and a second circulation path  75   b  extending from the first heat exchanger  74  to the low temperature unit  73 . A pump  76  is provided in the second circulation path  75   b  to circulate the heat medium. 
     The boiler  10  further includes a second heat exchanger  77  which reheats flue gas prior to discharge from the stack  65  by using heat recovered by the heat recovering unit  71 . The boiler  10  further includes a third heat exchanger  78  (see  FIG. 2 ) which heats feed water supplied to the economizers  51  and  52  of the heat exchanging unit  14  by using heat recovered by the heat recovering unit  71 . 
     According to this structure, pulverized coal is produced by operation of the coal pulverizers  31 ,  32 ,  33 ,  34 , and  35 , and supplied from the pulverized coal supply tubes  26 ,  27 ,  28 ,  29 , and  30  to the combustion burners  21 ,  22 ,  23 ,  24 , and  25  by using conveyance air. The combustion air heated by the first heat exchanger  74  is supplied from the air duct  38  through the wind box  36  to the respective combustion burners  21 ,  22 ,  23 ,  24 , and  25 , and also supplied to the additional air nozzle  37 . The pulverized coal mixture air containing a mixture of the pulverized coal and the conveyance air, and the combustion air are both blown into the furnace  11  and ignited in this condition by the combustion burners  21 ,  22 ,  23 ,  24 , and  25  to produce flames. In addition, additional air is blown through the additional air nozzle  37  into the furnace  11  for combustion control so as to reduce NOx generated by combustion of the pulverized coal. Thereafter, flue gas having passed through the heat exchanging unit  14  of the flue gas duct  13  is discharged through the stack  65  into the atmosphere after removal of NOx by the NOx removal device  61 , particulate substances by the electronic precipitator  62 , and sulfur oxide by the desulfurization device  64 . 
     Flows of water (steam) and the heat medium in the flue gas heat recovery system are hereinafter described.  FIG. 2  is a view schematically illustrating flows of water and the heat medium in the flue gas heat recovery system, while  FIG. 3  is a view schematically illustrating a heat recovery device and heat exchangers. 
     As illustrated in  FIGS. 2 and 3 , the heat recovering unit  71  includes the high temperature unit  72  and the low temperature unit  73 . The high temperature unit  72  and the low temperature unit  73  contain heat exchange tubes  72   a  and  73   a , respectively. One end of the heat exchange tube  72   a  is connected with one end of the heat exchange tube  73   a . The first heat exchanger  74  contains a heat exchange tube  74   a . The other end of the heat exchange tube  72   a  and one end of the heat exchange tube  74   a  are connected via a first circulation path  75   a . The other end of the heat exchange tube  73   a  and the other end of the heat exchange tube  74   a  are connected via a second circulation path  75   b . The pump  76  is provided in the second circulation path  75   b.    
     The second heat exchanger  77  contains a heat exchange tube  77   a . A first branch path  81   a  branched from the first circulation path  75   a  is connected with one end of the heat exchange tube  77   a . A second branch path  81   b  branched from the second circulation path  75   b  is connected with the other end of the heat exchange tube  77   a . The third heat exchanger  78  contains a heat exchange tube  78   a . A third branch path  82   a  branched from the first branch path  81   a  is connected with one end of the heat exchange tube  78   a . A fourth branch path  82   b  branched from the second branch path  81   b  is connected with the other end of the heat exchange tube  78   a.    
     A steam turbine  91  operated by steam generated from the boiler includes a high-pressure turbine  92  and a low-pressure turbine  93 . A water/steam circulation path  94  is provided between the heat exchanging unit  14  of the boiler  10  and the steam turbine  91  to circulate water and steam. The superheaters  46 ,  47 , and  48 , the high-pressure turbine  92 , the reheaters  49  and  50 , the low-pressure turbine  93 , a condenser  95 , the third heat exchanger  78 , a deaerator  96 , a water feed pump  97 , and the economizers  51  and  52  are provided in this order in the water/steam circulation path  94 . 
     Accordingly, flue gas (300° C. to 400° C.) flowing in the flue gas duct  13  proceeds in the order of the high temperature unit  72  and the low temperature unit  73  of the heat recovering unit  71 . During the flow of the flue gas in the flue gas duct  13 , the heat recovering unit  71  recovers heat from the flue gas by using the heat medium. More specifically, the heat medium (65° C. to 100° C.) is circulated in the heat medium circulation path  75  in accordance with operation of the pump  76 , and heated by the heat of the flue gas during circulation. Thereafter, a part of the high-temperature heat medium (100° C. to 350° C.) is supplied to the first heat exchanger  74  to exchange heat between the heat medium circulating in the heat medium circulation path  75  and the air flowing in the air duct  38 . As a result, the air heated by the heat medium becomes high-temperature air, and is supplied to the combustion device  12  as combustion air (200° C. to 330° C.) 
     On the other hand, a part of the high-temperature heat medium generated by the heat recovering unit  71  is supplied to the second heat exchanger  77  to exchange heat between the high-temperature heat medium and the flue gas (40° C. to 70° C.) discharged from the desulfurization device  64 . The flue gas (80° C. to 100° C.) reheated by the heat medium is supplied to the stack  65 . Furthermore, a part of the high-temperature heat medium generated by the heat recovering unit  71  is supplied to the third heat exchanger  78  to exchange heat between the high-temperature heat medium and the water (30° C. to 70° C.) flowing in the water/steam circulation path  94 . The water heated by the heat medium becomes high-temperature water (60° C. to 100° C.), and is supplied to the economizers  51  and  52 . 
     On the other hand, water supplied from the water feed pump  97  is preheated by the economizers  51  and  52 , and supplied to a not-shown steam drum. This water is heated during supply to the respective heat exchange tubes of the furnace wall to become saturated steam, and supplied to the steam drum in a state of saturated steam. The saturated steam of the steam drum is introduced into the superheaters  46 ,  47 , and  48 , and superheated by flue gas. The superheated steam generated by the superheaters  46 ,  47 , and  48  is supplied to the high-pressure turbine  92  to drive the high-pressure turbine  92 . Steam discharged from the high-pressure turbine  92  is introduced into the reheaters  49  and  50  and again superheated, and then supplied to the low-pressure turbine  93  to drive the low-pressure turbine  93 . Steam discharged from the low-pressure turbine  93  is cooled by the condenser  95  to become condensed water. This condensed water is heated by the third heat exchanger  78 , and returned to the economizers  51  and  52  after removal of remaining oxygen by the deaerator  96 . 
     In this description, the third heat exchanger  78  is provided such that the heat exchange tube  78   a  and the water/steam circulation path  94  are disposed close to each other to heat water supplied to the economizers  51  and  52  of the heat exchanging unit  14  by using heat recovered by the heat recovering unit  71 . However, configuration other than this example may be adopted. As illustrated in  FIG. 3 , a third heat exchanger  79  may be provided to heat feed water supplied to the economizers  51  and  52  of the heat exchanging unit  14  by using heat recovered by the heat recovering unit  71 , in a state that the water/steam circulation path  94  is connected with the third branch path  82   a  and the fourth branch path  82   b.    
     The heat recovering unit  71  is hereinafter detailed.  FIG. 4  is a view schematically illustrating a configuration of the heat recovering unit. 
     As illustrated in  FIG. 4 , the heat recovering unit  71  includes the high temperature unit  72  and the low temperature unit  73 . The high temperature unit  72  is disposed in the second vertical path (vertical path)  44 , while the low temperature unit  73  is disposed in the gas duct (horizontal path)  45 . The NOx removal device  61  is provided in the second vertical path  44 . The high temperature unit  72  is disposed below the NOx removal device  61  at a position away therefrom by a predetermined distance. The lower part of the second vertical path  44  is connected with a base end of the gas duct  45  while bended at right angles. A hopper  66  is provided in a connection portion (bended portion) between the second vertical path  44  and the gas duct  45 . The hopper  66  is disposed below the NOx removal device  61  and the high temperature unit  72 , and positioned on the side of the low temperature unit  73 . 
     The heat recovering unit  71  is configured such that a path cross-sectional area of an outlet of the heat recovering unit  71  is made smaller than a path cross-sectional area of an inlet of the heat recovering unit  71 . More specifically, an inlet  72 A is formed in an upper part of the high temperature unit  72  positioned in the second vertical path  44 , while an outlet  72 B is formed in a lower part of the high temperature unit  72 . On the other hand, an inlet  73 A is formed at one end of the low temperature unit  73 , while an outlet  73 B is formed at the other end of the low temperature unit  73 . Accordingly, an inlet of the heat recovering unit  71  constituted by the high temperature unit  72  and the low temperature unit  73  corresponds to the inlet  72 A of the high temperature unit  72 , while an outlet of the heat recovering unit  71  corresponds to the outlet  73 B of the low temperature unit  73 . 
     The path cross-sectional area in this context is an area of a cross section of the second vertical path  44  or the gas duct  45  in the flue gas duct  13  taken perpendicularly to the flow direction of flue gas. This cross-sectional area is an area through which flue gas is allowed to flow. More specifically, the path cross-sectional area is an area of a cross section taken perpendicularly to the flow direction of flue gas in the second vertical path  44  or the gas duct  45  after removal of closed portions by the heat exchange tube and fins. 
     In a specific configuration, an inner size of the outlet  73 B of the heat recovering unit  71  in the flue gas duct  13  is made smaller than an inner size of the inlet  72 A. Each of the second vertical path  44  and the gas duct  45  is constituted by a casing having a rectangular box-shaped cross section. The high temperature unit  72  contains the heat exchange tube  72   a , while the low temperature unit  73  contains the heat exchange tube  73   a . An inner size of the gas duct  45  is made smaller than an inner size of the second vertical path  44 . Densities of the heat exchange tubes  72   a  and  73   a  of the high temperature unit  72  and the low temperature unit  73  are equalized. More specifically, the heat exchange tubes  72   a  and  73   a  are bended such that piping is curved a plurality of times. In this case, respective pipe portions are positioned adjacent to each other with predetermined distances left therebetween for arrangement at uniform intervals. 
     The inner size in this context is an inside area of the second vertical path  44  or the gas duct  45  in the flue gas duct  13 . When the cross-sectional shape of the second vertical path  44  or the gas duct  45  is rectangular, the inner size is an area calculated as the product of the inner height and the inner width. When the cross-sectional shape of the second vertical path  44  or the gas duct  45  is circular, the inner size is an area calculated by the product of the diameter and the circular constant. In other words, the inner size is an area calculated without consideration of portions closed by the heat exchange tubes and fins disposed inside the second vertical path  44  or the gas duct  45 . 
     Flue gas having passed through the NOx removal device  61  in the second vertical path  44  is high-temperature flue gas in the range from 300° C. to 400° C. While passing through the high temperature unit  72  of the heat recovering unit  71 , the high-temperature flue gas heats the heat medium flowing in the heat exchange tube  72   a . As a result, the temperature and volume of the flue gas decrease. While passing through the low temperature unit  73  after the high temperature unit  72 , the flue gas heats the heat medium flowing in the heat exchange tube  73   a . As a result, the temperature and volume of the flue gas decrease. The flue gas having passed through the heat recovering unit  71  (low temperature unit  73 ) in the gas duct  45  becomes low-temperature flue gas in the range from 80° C. to 120° C. On the other hand, the heat medium in the range from 65° C. to 100° C. introduced into the high temperature unit  72  of the heat recovering unit  71  is heated to 100° C. to 300° C., and discharged from the low temperature unit  73 . 
     In this case, flue gas having a high temperature and a large volume passes through the high temperature unit  72  having a large path cross-sectional area in the second vertical path  44 , while flue gas having a low temperature and smaller volume passes through the low temperature unit  73  having a small cross-sectional area in the gas duct  45 . Accordingly, the flow velocity of the flue gas having the high temperature and the large volume does not increase while passing through the high temperature unit  72 . On the other hand, the flow velocity of the flue gas having the low temperature and the small volume does not decrease while passing through the low temperature unit  73 . In other words, the flow velocity of the flue gas is maintained substantially uniform with reduced fluctuations even when the temperature and volume of the flue gas gradually decrease during passage of the flue gas through the heat recovering unit  71 . 
     As described above, the flue gas heat recovery system according to the first embodiment includes the furnace  11 , the combustion device  12 , the flue gas duct  13 , and the heat exchanging unit  14 . The flue gas heat recovery system further includes the heat recovering unit  71  disposed in the flue gas duct  13  on the downstream side of the heat exchanging unit  14  to recover heat from flue gas. The heat recovering unit  71  is configured such that the path cross-sectional area of the outlet  73 B is made smaller than the path cross-sectional area of the inlet  72 A. The flue gas heat recovery system further includes the first heat exchanger  74  which heats combustion air by using heat recovered by the heat recovering unit  71 . 
     In this case, the fluctuations of the flow velocity of flue gas in the high temperature unit  72  and the low temperature unit  73  is reduced even when the temperature and volume of the flue gas decrease during passage through the heat recovering unit  71 . As a result, the flow velocity does not become excessively high, wherefore erosion of the heat exchange tubes  72   a  and  73   a  constituting the heat recovering unit  71  are avoidable. 
     According to the flue gas heat recovery system of the first embodiment, the inner size of the outlet  73 B of the heat recovering unit  71  in the flue gas duct  13  is made smaller than the inner size of the inlet  72 A. According to this structure, the path cross-sectional area of the heat exchanger inlet  72 A into which high-temperature flue gas flows is determined easily and in a simplified configuration, so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side. 
     According to the flue gas heat recovery system of the first embodiment, the heat recovering unit  71  includes the upstream side high temperature unit  72  and the downstream side low temperature unit  73 . The path cross-sectional area of the low temperature unit  73  is made smaller than the path cross-sectional area of the high temperature unit  72 . In the structure of the heat recovering unit  71  constituted by the high temperature unit  72  and the low temperature unit  73 , the path cross-sectional area of the inlet  72 A of the heat recovering unit  71  into which high-temperature flue gas flows is determined easily, so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for recovering heat efficiency also from low-temperature flue gas after heat exchange on the upstream side. 
     According to the flue gas heat recovery system of the first embodiment, the flue gas duct  13  includes the second vertical path  44  extending in the vertical direction, and the horizontal gas duct  45  connected with the lower part of the second vertical path  44  and extending in the horizontal direction. The high temperature unit  72  is disposed in the second vertical path  44 , while the low temperature unit  73  is disposed in the gas duct  45 . Accordingly, the path cross-sectional area of the heat exchanger inlet  72 A into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, without the necessity for changing the configuration of the existing flue gas duct  13 . 
     According to the flue gas heat recovery system of the first embodiment, the NOx removal device  61  is provided in the second vertical path  44 . The high temperature unit  72  is disposed below the NOx removal device  61 . In this case, flue gas flows into the high temperature unit  72  after removal of harmful substances by the NOx removal device  61 . Accordingly, adhesion of harmful substances to the heat recovering unit  71  is avoidable. 
     According to the flue gas heat recovery system of the first embodiment, the hopper  66  is provided between the second vertical path  44  and the gas duct  45  and below the high temperature unit  72 . Accordingly, the hopper  66  disposed below the high temperature unit  72  is capable of collecting particles such as soot and dust contained in flue gas between the second vertical path  44  and the gas duct  45 . 
     The flue gas heat recovery system of the first embodiment includes the second heat exchanger  77  which reheats flue gas prior to discharge from the stack  65  by using heat recovered by the heat recovering unit  71 . The second heat exchanger  77  prevents generation of white smoke by reheating the flue gas using heat recovered by the heat recovering unit  71 . 
     The flue gas heat recovery system according to the first embodiment includes the third heat exchanger  78  ( 79 ) which heats water supplied to the heat exchanging unit  14  by using heat recovered by the heat recovering unit  71 . The third heat exchanger  78  ( 79 ) achieves effective utilization of recovered heat by using the heat recovered by the heat recovering unit  71  as heat for heating the water supplied to the heat exchanging unit  14 . 
     Second Embodiment 
       FIG. 5  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a second embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein. 
     As illustrated in  FIG. 5 , the flue gas heat recovery system according to the second embodiment includes a distribution amount control device which adjusts distribution amounts of a heat medium supplied from the heat recovering unit  71  to the first heat exchanger  74 , the second heat exchanger  77 , and the third heat exchanger  78 . 
     The heat recovering unit  71  includes the high temperature unit  72  and the low temperature unit  73 . The high temperature unit  72  and the low temperature unit  73  contain the heat exchange tubes  72   a  and  73   a , respectively. One end of the heat exchange tube  72   a  is connected with one end of the heat exchange tube  73   a . The other end of the heat exchange tube  72   a  is connected with the first circulation path  75   a , while the other end of the heat exchange tube  73   a  is connected with the second circulation path  75   b . The first circulation path  75   a  and the second circulation path  75   b  are connected via a bypass path  101 . A flow rate control valve  102  is provided in the bypass path  101 , while a flow rate control valve  103  is provided in the second circulation path  75   b  on the low temperature unit  73  side with respect to a connection portion of the bypass path  101 . 
     The first heat exchanger  74  contains the heat exchange tube  74   a . The first circulation path  75   a  is connected with an end of the heat exchange tube  72   a . A flow rate control valve  104  is provided in the first circulation path  75   a  on the first heat exchanger  74  side. The second heat exchanger  77  contains the heat exchange tube  77   a . The first branch path  81   a  is connected with an end of the heat exchange tube  77   a . A flow rate control valve  105  is provided in the first branch path  81   a  on the second heat exchanger  77  side. The third heat exchanger  78  contains the heat exchange tube  78   a . The third branch path  82   a  is connected with an end of the heat exchange tube  78   a . A flow rate control valve  106  is provided in the third branch path  82   a  on the third heat exchanger  78  side. 
     A temperature sensor  107  is provided to measure a temperature of the flue gas on the outlet  73 B side of the low temperature unit  73  of the heat recovering unit  71 . A temperature sensor  108  is provided to measure a temperature of the combustion air on the outlet side of the first heat exchanger  74 . A temperature sensor  109  is provided to measure a temperature of the flue gas on the outlet side of the second heat exchanger  77 . A temperature sensor  110  is provided to measure a temperature of the heat medium on the outlet side of the second heat exchanger  77 . A temperature sensor  111  is provided to measure a temperature of the feed water on the outlet side of the third heat exchanger  78 . 
     A control device  100  controls opening positions of the flow rate control valves  102 ,  103 ,  104 ,  105 , and  106  based on the measurement results of the temperature sensors  107 ,  108 ,  109 ,  110 , and  111  to adjust distribution amounts of the heat medium supplied from the heat recovering unit  71  to the respective heat exchangers  74 ,  77 , and  78 . Generally, the opening positions of the flow rate control valves  102  and  103  are adjusted such that the temperature of the flue gas on the outlet  73 B side of the heat recovering unit  71  becomes a predetermined temperature (85° C. to 120° C.) to increase or decrease the supply amount of the heat medium to the respective heat exchangers  74 ,  77 , and  78 . The opening position of the flow rate control valve  104  is adjusted such that the temperature of the combustion air on the outlet side of the first heat exchanger  74  becomes a predetermined temperature (200° C. to 330° C.) to increase or decrease the supply amount of the heat medium to the first heat exchanger  74 . The opening position of the flow rate control valve  105  is adjusted such that the temperature of the flue gas on the outlet side of the second heat exchanger  77  becomes a predetermined temperature (80° C. to 100° C.), and that the temperature of the heat medium on the outlet side of the second heat exchanger  77  becomes a predetermined temperature (80° C. to 95° C.), to increase or decrease the supply amount of the heat medium to the second heat exchanger  77 . The opening position of the flow rate control valve  106  is adjusted such that the temperature of the feed water on the outlet side of the third heat exchanger  78  becomes a predetermined temperature (60° C. to 100° C.) to increase or decrease the supply amount of the heat medium to the third heat exchanger  78 . 
     As described above, the flue gas heat recovery system of the second embodiment includes the flow rate control valves  102 ,  103 ,  104 ,  105 , and  106 , and the control device  100  for controlling these valves as a distribution amount control device for adjusting the distribution amounts of the heat medium supplied from the heat recovering unit  71  to the first heat exchanger  74 , the second heat exchanger  77 , and the third heat exchanger  78  ( 79 ) 
     Accordingly, the distribution amount control device makes it possible, while keeping a necessary amount of heat recovered from flue gas for environmental measurements, to utilize the remaining amount of heat recovered for improvement of power generation efficiency, by control of the distribution amounts of the heat medium supplied from the heat recovering unit  71  to the respective heat exchangers  74 ,  77 , and  78  ( 79 ) in accordance with the operation condition of the boiler  10 . 
     Third Embodiment 
       FIG. 6  is a view schematically illustrating a heat recovering unit and heat exchangers of a flue gas heat recovery system according to a third embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiments have been given similar reference numbers, and detailed explanations of these components are not repeated herein. 
     As illustrated in  FIG. 6 , the flue gas heat recovery system according to the third embodiment includes a distribution amount control device which adjusts distribution amounts of the heat medium supplied from the heat recovering unit  71  to the first heat exchanger  74 , the second heat exchanger  77 , and the third heat exchanger  78 . 
     The first heat exchanger  74  includes a bypass path  121  which connects respective ends of the heat exchange tube  74   a , i.e., the inlet and outlet of the heat exchange tube  74   a . A flow rate control valve  122  is provided in the bypass path  121 . The second heat exchanger  77  includes a bypass path  123  which connects respective ends of the heat exchange tube  77   a , i.e., the inlet and outlet of the heat exchange tube  77   a . A flow rate control valve  124  is provided in the bypass path  123 . The third heat exchanger  78  includes a bypass path  125  which connects respective ends of the heat exchange tube  78   a , i.e., the inlet and outlet of the heat exchange tube  78   a . A flow rate control valve  126  is provided in the bypass path  125 . 
     The control device  100  controls opening positions of the flow rate control valves  122 ,  124 , and  126  based on the measurement results of the temperature sensors  107 ,  108 ,  109 ,  110 , and  111  to adjust distribution amounts of the heat medium supplied from the heat recovering unit  71  to the respective heat exchangers  74 ,  77 , and  78 . In other words, the control device  100  controls the flow rates of the heat medium bypassing the respective heat exchangers  74 ,  77 , and  78  by adjusting the opening positions of the flow rate control valves  122 ,  124 , and  126 . Other control by the control device  100  for adjusting the distribution amounts of the heat medium is similar to the corresponding control in the second embodiment. 
     As described above, the flue gas heat recovery system according to the third embodiment includes the flow rate control valves  102 ,  103 ,  104 ,  105 ,  106 ,  122 ,  124 , and  126 , and the control device  100  for controlling these valves as the distribution amount control device for adjusting distribution amounts of the heat medium supplied from the heat recovering unit  71  to the first heat exchanger  74 , the second heat exchanger  77 , and the third heat exchanger  78 . 
     Accordingly, it is possible to utilize appropriately heat recovered from flue gas for appropriate operation of the boiler  10  with the distribution amount control device adjusting the distribution amounts of the heat medium supplied from the heat recovering unit  71  to the respective heat exchangers  74 ,  77 , and  78  in accordance with the operation condition of the boiler  10 . 
     Moreover, the flue gas heat recovery system according to the third embodiment controls the flow rates of the heat medium bypassing the heat exchange tube  74   a  of the first heat exchanger  74 , the heat exchange tube  77   a  of the second heat exchanger  77 , and the heat exchange tube  78   a  of the third heat exchanger  78  by adjusting the opening positions of the flow rate control valves  122 ,  124 , and  126 . Accordingly, the flue gas heat recovery system according to the third embodiment achieves efficient distribution of the heat medium in accordance with the amounts of heat required by the respective heat exchangers  74 ,  77 , and  78 . 
     Fourth Embodiment 
       FIG. 7  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a fourth embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein. 
     As illustrated in  FIG. 7 , the flue gas heat recovery system according to the fourth embodiment includes a distribution amount control device which adjusts distribution amounts from the heat recovering unit  71  to the first heat exchanger  74 , the second heat exchanger  77 , and the third heat exchanger  78 . 
     The first heat exchanger  74  contains the heat exchange tubes  74   a  and  74   b . The first circulation path  75   a  is connected with an end of the heat exchange tube  74   a . A bypass path  131  branched from the first circulation path  75   a  is connected between the heat exchange tubes  74   a  and  74   b . The flow rate control valve  104  is provided in the first circulation path  75   a . A flow rate control valve  132  is provided in the bypass path  131 . The second heat exchanger  77  contains the heat exchange tubes  77   a  and  77   b . The first branch path  81   a  is connected with an end of the heat exchange tube  77   a . A bypass path  133  branched from the first branch path  81   a  is connected between the heat exchange tubes  77   a  and  77   b . The flow rate control valve  105  is provided in the first branch path  81   a . A flow rate control valve  134  is provided in the bypass path  133 . The third heat exchanger  78  contains the heat exchange tubes  78   a  and  78   b . The third branch path  82   a  is connected with an end of the heat exchange tube  78   a . A bypass path  135  branched from the third branch path  82   a  is connected between the heat exchange tubes  78   a  and  78   b . The flow rate control valve  106  is provided in the third branch path  82   a . A flow rate control valve  136  is provided in the bypass path  135 . 
     The control device  100  controls opening positions of flow rate control valves  132 ,  134 , and  136  based on the measurement results of the temperature sensors  107 ,  108 ,  109 ,  110 , and  111  to adjust distribution amounts of the heat medium supplied from the heat recovering unit  71  to the respective heat exchangers  74 ,  77 , and  78 . In other words, the control device  100  controls the flow rates of the heat medium bypassing the respective heat exchangers  74 ,  77 , and  78  by adjusting the opening positions of the flow rate control valves  132 ,  134 , and  136 . Other control by the control device  100  for adjusting the distribution amounts of the heat medium is similar to the corresponding control in the third embodiment. 
     As described above, the flue gas heat recovery system according to the fourth embodiment includes the flow rate control valves  102 ,  103 ,  104 ,  105 ,  106 ,  122 ,  124 ,  126 ,  132 ,  134 , and  136  and the control device  100  for controlling these valves as the distribution amount control device for adjusting distribution amounts of the heat medium supplied from the heat recovering unit  71  to the first heat exchanger  74 , the second heat exchanger  77 , and the third heat exchanger  78 . 
     Accordingly, it is possible to utilize appropriately heat recovered from flue gas for appropriate operation of the boiler  10  with the distribution amount control device adjusting the distribution amounts of the heat medium supplied from the heat recovering unit  71  to the respective heat exchangers  74 ,  77 , and  78  in accordance with the operation condition of the boiler  10 . 
     Moreover, the flue gas heat recovery system according to the fourth embodiment contains the heat exchangers  74 ,  77  and  78  which are composed of two heat exchange tubes  74   a  and  74   b , two heat exchange tubes  77   a  and  77   b  and two heat exchange tubes  78   a  and  78   b  respectively. And the flue gas heat recovery system selectively uses the heat exchange tubes  74   a  and  74   b  of the first heat exchanger  74 , the heat exchange tubes  77   a  and  77   b  of the second heat exchanger  77 , and the heat exchange tubes  78   a  and  78   b  of the third heat exchanger  78  for control of the flow rates of the heat medium by controlling the opening positions of the flow rate control valves  132 ,  134 , and  136  provided between the two heat exchange tubes  74   a  and  74   b  of the heat exchanger  74 , between the two heat exchange tubes  77   a  and  77   b  of the heat exchanger  77 , and between the two heat exchange tubes  78   a  and  78   b  of the heat exchanger  78 , respectively. Accordingly, the flue gas heat recovery system according to the fourth embodiment achieves efficient distribution of the heat medium in accordance with the amounts of heat required by the respective heat exchangers  74 ,  77 , and  78 . 
     Fifth Embodiment 
       FIG. 8  is a view schematically illustrating a heat recovering unit included in a flue gas heat recovery system according to a fifth embodiment, while  FIG. 9  is a view schematically illustrating a fin tube of the heat recovering unit. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein. 
     As illustrated in  FIG. 8 , the heat recovering unit  140  of the flue gas heat recovery system according to the fifth embodiment is configured such that a path cross-sectional area of an outlet  140 B of the heat recovering unit  140  is made smaller than a path cross-sectional area of an inlet  140 A of the heat recovering unit  140 . More specifically, the heat recovering unit  140  is provided in the gas duct  45  of the flue gas duct  13 . The density of a heat exchange tube  140   a  or heat exchange fins  140   b  at the outlet  140 B is set higher than the density of the heat exchange tube  140   a  or the heat exchange fins  140   b  at the inlet  140 A. 
     For example, the density of the heat exchange fins  140   b  is set higher in the direction from the inlet  140 A to the outlet  140 B of the heat recovering unit  140  as illustrated in  FIG. 9 . The heat exchange tube  140   a  is bended such that piping is curved at a plurality of points. In this case, respective piping portions are disposed adjacent to each other with predetermined distances left therebetween for arrangement at uniform intervals. On the other hand, the heat exchange fins  140   b  are fixed to an outer circumference of the heat exchange tube  140   a . The distances between the respective heat exchange fins  140   b  decrease in the direction from the inlet  140 A to the outlet  140 B, while the number of the heat exchange fins  140   b  increases in the direction from the inlet  140 A to the outlet  140 B. The heat exchange fins  140   b  close a part of the gas duct  45 . Accordingly, the flue gas path cross-sectional area of the heat recovering unit  140  decreases in the direction from the inlet  140 A to the outlet  140 B. 
     High-temperature flue gas heats a heat medium flowing in the heat exchange tube  140   a  while passing through the heat recovering unit  140 . As a result, the temperature and volume of the flue gas decrease, wherefore the flue gas having passed through the heat recovering unit  140  becomes low-temperature flue gas. According to this structure, flue gas having a high temperature and a large volume passes through the large path cross-sectional area on the inlet  140 A side, while flue gas having a low temperature and a small volume passes through the small path cross-sectional area on the outlet  140 B side. Accordingly, the flow velocity does not increase when the flue gas having the large volume and the high temperature passes through the inlet  140 A. On the other hand, the flow velocity does not decrease when the flue gas having the small volume and the low temperature passes through the outlet  140 B. In other words, fluctuations of the flow velocity are reduced even when the temperature and volume of the flue gas decrease during passage of the flue gas through the heat recovering unit  140 . 
     According to the flue gas heat recovery system of the fifth embodiment, therefore, the density of the heat exchange tube  140   a  or the heat exchange fins  140   b  at the outlet  140 B of the heat recovering unit  140  is set higher than the density of the heat exchange tube  140   a  or the heat exchange fins  140   b  at the inlet  140 A. 
     Accordingly, the path cross-sectional area of the heat exchanger inlet  140 A into which high-temperature flue gas flows is easily determined so that the flue gas flow velocity becomes a flow velocity appropriate for reducing erosion of the heat exchange tubes, and for efficiently recovering heat also from low-temperature flue gas after heat exchange on the upstream side, in accordance with a change of the position, shape, and number of the heat exchange tube  140   a  or the heat exchange fins  140   b , without the necessity of changing the configuration of the flue gas duct  13 . 
     Sixth Embodiment 
       FIG. 10  is a view schematically illustrating a heat recovering unit and heat exchangers included in a flue gas heat recovery system according to a sixth embodiment. Components in this embodiment having functions similar to the corresponding functions of the components in the foregoing embodiment have been given similar reference numbers, and detailed explanations of these components are not repeated herein. 
     As illustrated in  FIG. 10 , the flue gas heat recovery system according to the sixth embodiment includes the heat recovering unit  71  equipped with the high temperature unit  72  and the low temperature unit  73  for recovering heat from flue gas, the first heat exchanger  74  for heating combustion air by using heat recovered by the heat recovering unit  71 , the second heat exchanger  77  for reheating flue gas discharged from the stack  65  by using heat recovered by the heat recovering unit  71 , and the third heat exchanger  78  for heating feed water supplied to the economizers  51  and  52  of the heat exchanging unit  14  by using heat recovered by the heat recovering unit  71 . 
     As illustrated in  FIG. 10 , an end of the heat exchange tube  72   a  of the heat recovering unit  71  is connected with one end of the heat exchange tube  74   a  of the first heat exchanger  74  via the first circulation path  75   a . The other end of the heat exchange tube  74   a  of the first heat exchanger  74  is connected with one end of the heat exchange tube  77   a  of the second heat exchanger  77  via a first connection path  151 . The other end of the heat exchange tube  77   a  of the second heat exchanger  77  is connected with one end of the heat exchange tube  78   a  of the third heat exchanger  78  via a second connection path  152 . The other end of the heat exchange tube  78   a  of the third heat exchanger  78  is connected with an end of the heat exchange tube  73   a  of the heat recovering unit  71  via the second circulation path  75   b.    
     In this case, the high-temperature heat medium after passing through the heat recovering unit  71  is supplied to the first heat exchanger  74  to heat air flowing through the air duct  38  and generate high-temperature combustion air. The high-temperature heat medium discharged from the first heat exchanger  74  is supplied to the second heat exchanger  77  to reheat flue gas. The high-temperature medium discharged from the second heat exchanger  77  is supplied to the third heat exchanger  78  to heat feed water. 
     As described above, the flue gas heat recovery system according to the sixth embodiment includes the first heat exchanger  74  for heating combustion air by using heat recovered by the heat recovering unit  71 , the second heat exchanger  77  for reheating flue gas discharged from the stack  65  by using heat recovered by the heat recovering unit  71 , and the third heat exchanger  78  for heating feed water supplied to the heat exchanging unit  14  by using heat recovered by the heat recovering unit  71 . The first, second, and third heat exchangers  74 ,  77 , and  78  are disposed in series. 
     In this case, the heat medium is sequentially delivered in accordance with necessary temperatures of the combustion air, flue gas, and feed water to achieve appropriate supply and heating of the heat medium. Accordingly, the configuration of the supply piping of the heat medium is simplified, wherefore the system is small-sized. 
     According to the first through third embodiments described herein, the heat recovering unit  71  includes the high temperature unit  72  and the low temperature unit  73 . However, the number of the units included in the heat recovering unit  71  is not limited to two, but may be three or larger. According to the first through third embodiments described herein, the high temperature unit  72  is disposed in the second vertical path  44 , while the low temperature unit  73  is disposed in the horizontal gas duct  45 . However, both the high temperature unit  72  and the low temperature unit  73  may be disposed in the second vertical path  44 , or in the gas duct  45 . In this case, the inner size of the flue gas duct may be gradually decreased, or the density of the heat exchange tube or the heat exchange fins may be gradually increased. 
     According to the respective embodiments, the three heat exchangers  74 ,  77 , and  78  are provided for the heat recovering unit  71 . However, all of these heat exchangers are not required to be equipped. For example, such a structure which includes only the first heat exchanger  74  may be adopted. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  Boiler 
               11  Furnace 
               12  Combustion device 
               13  Flue gas duct 
               14  Heat exchanging unit 
               21 ,  22 ,  23 ,  24 ,  25  Combustion burner 
               36  Wind box 
               37  Additional air nozzle 
               38  Air duct 
               39  Blower 
               41  First horizontal path 
               42  First vertical path 
               43  Second horizontal path 
               44  Second vertical path 
               45  Gas duct 
               46 ,  47 ,  48  Superheater 
               49 ,  50  Reheater 
               51 ,  52  Economizer 
               61  NOx removal device 
               62  Electrostatic precipitator 
               64  Desulfurization device 
               65  Stack 
               66  Hopper 
               71 ,  140  Heat recovering unit 
               72  High temperature unit 
               72 A,  73 A,  140 A Inlet 
               72 B,  73 B,  140 B Outlet 
               73  Low temperature unit 
               74  First heat exchanger 
               76  Pump 
               77  Second heat exchanger 
               78 ,  79  Third heat exchanger 
               100  Control device 
               102 ,  103 ,  104 ,  105 ,  106 ,  122 ,  124 ,  126 ,  132 ,  134 ,  136  Flow rate control valve (distribution amount control device) 
               107 ,  108 ,  109 ,  110 ,  111  Temperature sensor