Heat exchanger and method for controlling heat exchanger

A heat exchanger includes a heat recovery unit that causes a heat medium to recover heat from flue gas through first heat exchange by bringing the flue gas into contact with a fin tube; a reheater including a preheating unit configured to preheat flue gas through second heat exchange by bringing the flue gas into contact with a tube, and heating units that heat the flue gas through third heat exchange by bringing the flue gas into contact with the heat medium; and a control unit that calculates a recovered heat quantity to be recovered by the heat recovery unit from the flue gas through the first heat exchange, and that controls temperature of the heat medium after the first heat exchange within a predetermined range.

FIELD

The present invention relates to a heat exchanger and a method for controlling the heat exchanger, for example, a heat exchanger that includes a preheating unit for preheating flue gas introduced into a reheater and a method for controlling the heat exchanger.

BACKGROUND

An air pollution control device is used in thermal power plants and chemical plants. In the air pollution control device, a denitration device, an air preheating unit air heater, a heat recovery unit of a reheating heat exchanger (gas-gas heater), a dry electronic precipitator, a wet desulfurization device, a reheater of the reheating heat exchanger, and a stack are sequentially provided from the upstream side toward the downstream side of a flue gas flow path. A gas-gas heater including a preheating unit that is provided on a flue gas introduction portion of a reheater and that preheats flue gas introduced into the reheater body has been developed as the heat exchanger used in the air pollution control device such as the above (for example, see Patent Literature 1). In the gas-gas heater disclosed in Patent Literature 1, wet flue gas that has passed through the wet desulfurization device is preheated and dried in the preheating unit supplied with a heat medium having been heated by the heat recovery unit and the heating unit. Because the flue gas is dried, it is possible to reduce dust in the flue gas from adhering to the inside of the reheater body and reduce corrosion inside of the reheater body resulting from wet components in the flue gas.

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

In the gas-gas heater disclosed in Patent Literature 1, the heat exchanger is controlled so that the temperature of the flue gas at the flue gas outlet portion of the heat recovery unit and the temperature of the heat medium at the heat medium outlet portion of the reheater will fall within respective predetermined ranges.

However, in the thermal power plants and the like, the gas temperature and the gas flow of the flue gas introduced into the air pollution control device may be reduced by the variation in the power generation load corresponding to the change in the operating conditions. Moreover, the temperature of the heat medium at the heat medium inlet portion of the reheater may be reduced by the change in the heat quantity of the flue gas recovered by the heat recovery unit. When the temperature of the heat medium at the heat medium inlet portion (preheating unit) of the reheater is reduced in this manner, the flue gas in the preheating unit of the reheater will not be sufficiently preheated. Thus, dust accompanying the wet flue gas from the desulfurization device easily adheres to a heat transfer tube in the reheater by using the mist in the flue gas as a binder. Consequently, the gas differential pressure between the inlet portion and the outlet portion of the reheater may be increased and corrosion inside the reheater may be accelerated.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a heat exchanger and a method for controlling the heat exchanger capable of reducing dust in the flue gas from adhering to the inside of the reheater and reducing corrosion of the heat transfer tube, even if the operating conditions have changed.

Solution to Problem

A heat exchanger, comprising: a heat recovery unit that causes a heat medium to recover heat from flue gas from a combustion engine through first heat exchange by bringing the flue gas into contact with a heat transfer tube in which the heat medium flows; a reheater that includes a preheating unit configured to preheat the flue gas after the first heat exchange through second heat exchange by bringing the flue gas after the first heat exchange into contact with the heat transfer tube in which the heat medium after the first heat exchange flows, and a heating unit configured to heat the flue gas after the second heat exchange through third heat exchange by bringing the flue gas after the second heat exchange into contact with the heat medium after the second heat exchange; a circulation line that circulates the heat medium between the heat recovery unit and the reheater; and a control unit that calculates a recovered heat quantity to be recovered by the heat recovery unit from the flue gas through the first heat exchange, and that controls temperature of the heat medium after the first heat exchange within a predetermined range based on the calculated recovered heat quantity.

With this configuration, the temperature of the heat medium to be supplied to the preheating unit of the reheater is controlled on the basis of the recovered heat quantity that is recovered by the heat recovery unit from the flue gas introduced into the heat recovery unit. Consequently, it is possible to set the temperature of the heat medium to be supplied to the preheating unit within a predetermined range without delay, according to the change in the recovered heat quantity that is recovered by the heat recovery unit from the flue gas. In this manner, even if the operating conditions of a boiler and the like have changed, it is possible to implement the heat exchanger and the method for controlling the heat exchanger capable of reducing dust in the flue gas from adhering to the inside of the reheater, and reduce corrosion of the heat transfer tube of the preheating unit.

In the heat exchanger according to present invention, it is preferable that the control unit calculates the recovered heat quantity based on at least one type selected from the group consisting of gas temperature of the flue gas introduced into the heat exchanger, a gas flow of the flue gas, and operation load of the combustion engine. With this configuration, the accuracy of the recovered heat quantity calculated by the control unit will be improved. Consequently, it is possible to further reduce dust in the flue gas from adhering to the inside of the reheater and reduce corrosion of the heat transfer tube of the preheating unit.

In the heat exchanger according to present invention, it is preferable that the control unit heats the heat medium after the first heat exchange, when the recovered heat quantity is less than a predetermined value. With this configuration, the heat medium can be heated according to the recovered heat quantity. Consequently, it is possible to reduce dust in the flue gas from adhering to the inside of the reheater and reduce corrosion of the heat transfer tube of the preheating unit.

In the heat exchanger according to present invention, it is preferable that the control unit supplies steam to the heat medium after the first heat exchange from a steam supply unit, and sets the temperature of the heat medium after the first heat exchange within the predetermined range. With this configuration, the heat medium can be heated by steam. Consequently, it is possible to easily heat the heat medium.

In the heat exchanger according to present invention, it is preferable that the circulation line includes a bypass line that bypasses the heat recovery unit, and the control unit sets the temperature of the heat medium within the predetermined range, by circulating the heat medium between the heat recovery unit and the reheater via the bypass line, when the recovered heat quantity exceeds the predetermined value. With this configuration, even if the heat quantity recovered by the heat recovery unit is too large, it is possible to reduce the heat quantity recovered by the heat recovery unit, and set the temperature of the heat medium within a predetermined range.

In the heat exchanger according to present invention, it is preferable that in the reheater, a plurality of heat transfer tubes of the heating unit are arranged in a tetragonal lattice pattern relative to a flowing direction of the flue gas. With this configuration, the gas flow velocity of the flue gas of the heating unit will be improved. Consequently, it is possible to further reduce dust in the flue gas from adhering to the heating unit and reduce corrosion of a pipe.

A method for controlling a heat exchanger that includes a heat recovery unit that causes a heat medium to recover heat from flue gas from a combustion engine through first heat exchange by bringing the flue gas into contact with a heat transfer tube in which the heat medium flows; and a reheater that includes a preheating unit configured to preheat the flue gas after the first heat exchange using heat of the heat medium after the first heat exchange through second heat exchange by bringing the flue gas after the first heat exchange into contact with the heat transfer tube in which the heat medium after the first heat exchange flows, and a heating unit configured to heat the flue gas after the second heat exchange through third heat exchange by bringing the flue gas after the second heat exchange into contact with the heat medium after the second heat exchange, the method for controlling the heat exchanger, comprising: a step of calculating a recovered heat quantity to be recovered by the heat recovery unit from the flue gas through the first heat exchange; and a step of controlling temperature of the heat medium within a predetermined range by heating the heat medium after the first heat exchange, when the calculated recovered heat quantity becomes less than a predetermined value.

With this method, the temperature of the heat medium to be supplied to the preheating unit of the reheater is controlled on the basis of the recovered heat quantity that is recovered by the heat recovery unit from the flue gas introduced into the heat recovery unit. Consequently, it is possible to set the temperature of the heat medium to be supplied to the preheating unit within a predetermined range without delay, according to the change in the recovered heat quantity that is recovered by the heat recovery unit from the flue gas. In this manner, even if the operating conditions of the boiler and the like have changed, it is possible to implement the heat exchanger and the method for controlling the heat exchanger capable of reducing dust in the flue gas from adhering to the inside of the reheater, and reduce corrosion of the heat transfer tube of the preheating unit. With this configuration, even if the heat quantity recovered by the heat recovery unit is too large, it is possible to reduce the heat quantity recovered by the heat recovery unit, and set the temperature of the heat medium within a predetermined range.

In the method for controlling a heat exchanger according to present invention, it is preferable that the method further comprising a step of controlling the temperature of the heat medium within the predetermined range by making the heat medium flow in a bypass line that bypasses the heat recovery unit, when the calculated recovered heat quantity exceeds the predetermined value.

Advantageous Effects of Invention

With the present invention, even if the operating conditions have changed, it is possible to implement the heat exchanger and the method for controlling the heat exchanger capable of reducing dust in the flue gas from adhering to the inside of the reheater and reduce corrosion of the heat transfer tube.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is to be understood that the present invention is not limited to the following embodiments and may be implemented by suitably modifying the embodiments. Moreover, the present invention may be implemented by suitably combining the following embodiments. Furthermore, the same reference numerals denote the same components in the embodiments, and the repeated description will be omitted.

First Embodiment

FIG. 1is a schematic diagram of an air pollution control system10according to a first embodiment of the present invention. As illustrated inFIG. 1, the air pollution control system10according to the present embodiment is an air pollution control system that treats flue gas discharged from a thermal power plant, a chemical plant, or the like, and that removes nitrogen oxides (NOx), dust, and sulfur oxides (SOx) included in the flue gas to discharge.

The air pollution control system10according to the present embodiment includes a boiler11installed in a thermal power plant, a chemical plant, and the like, a denitration device12provided at a subsequent stage of the boiler11, an air heater (AH)13provided at a subsequent stage of the denitration device12, and an electronic precipitator14provided at a subsequent stage of the air heater13. The air pollution control system10also includes an air blower15provided at a subsequent stage of the electronic precipitator14, a desulfurization device16provided at a subsequent stage of the air blower15, and a stack17provided at a subsequent stage of the desulfurization device16.

A heat recovery unit21of a heat exchanger20according to the present embodiment is disposed between the air heater13and the electronic precipitator14. A reheater22of the heat exchanger (gas-gas heater)20according to the present embodiment is disposed between the desulfurization device16and the stack17. A fin tube21aas a heat transfer tube in which a heat medium flows is provided inside the heat recovery unit21. The reheater22includes a preheating unit221that preheats the flue gas introduced into the reheater22, a low-temperature heating unit222that heats the flue gas preheated by the preheating unit221, and a high-temperature heating unit223that further heats the flue gas heated by the low-temperature heating unit222. A tube221ais disposed inside the preheating unit221as a heat transfer bare tube. A fin tube222ais disposed inside the low-temperature heating unit222as a heat transfer tube. A fin tube223ais disposed inside the high-temperature heating unit223as a heat transfer tube. A circulation line L that circulates a heat medium M between the heat recovery unit21and the reheater22is provided between the heat recovery unit21and the reheater22. A liquid feeding pump P that circulates the heat medium M in the circulation line L between the heat recovery unit21and the reheater22is provided on the circulation line L. Heat exchange is performed between the heat recovery unit21and the reheater22, using the heat medium M that flows in the circulation line L by the liquid feeding pipe P.

Flue gas G0discharged from the boiler11is introduced into the denitration device12filled with a catalyst. The flue gas G0introduced into the denitration device12is made harmless by reducing the nitrogen oxides included in the flue gas G0to water and nitrogen, with ammonia (NH3) injected into the denitration device12as a reducing agent.

Flue gas G1discharged from the denitration device12is introduced into the air heater (AH)13. For example, the temperature of the flue gas G1introduced into the air heater13is cooled to equal to or more than 130 degrees Celsius and equal to or less than 150 degrees Celsius, by heat exchange with air.

Flue gas G2discharged from the air heater13is introduced into the heat recovery unit21of the heat exchanger (gas-gas heater)20according to the present embodiment. The heat of the flue gas G2introduced into the heat recovery unit21is recovered and cooled by heat exchange with a heat medium (such as water), when the flue gas G2is brought into contact with the fin tube21ain which the heat medium M flows. For example, the temperature of flue gas G3after the heat exchange in the heat recovery unit21is equal to or more than 85 degrees Celsius and equal to or less than 110 degrees Celsius.

The flue gas G3discharged from the heat recovery unit21is introduced into the electronic precipitator (EP)14to remove dust. In this example, dust such as fly ash in the flue gas G3that is cooled by heat exchange in the heat recovery unit21is removed. Consequently, it is possible to improve the dust collection efficiency of the electronic precipitator14.

The air blower15driven by a motor boosts the pressure of flue gas G4discharged from the electronic precipitator14. It is to be understood that the air blower15is not necessarily provided. The air blower15may also be provided at a subsequent stage of the reheater22of the heat exchanger20.

Flue gas G5the pressure of which is boosted by the air blower15is introduced into the desulfurization device16. In the desulfurization device16, sulfur oxides in the flue gas G5are absorbed and removed by absorbent in which slurry limestone is dissolved, and gypsum (not illustrated) is generated as a by-product. In this example, flue gas G6discharged from the desulfurization device16absorbs water in the absorbent and becomes wet. The temperature of the flue gas G6is reduced to, for example, about 50 degrees Celsius.

The flue gas G6discharged from the desulfurization device16is introduced into the heat recovery unit21of the heat exchanger (gas-gas heater)20according to the present embodiment. The flue gas G6introduced into the heat recovery unit21is sequentially brought into contact with the tube221a, the fin tube222a, and the fin tube223ain the preheating unit221, the low-temperature heating unit222, and the high-temperature heating unit223, and is heated by heat exchange with the heat medium. In this example, the preheating unit221heats the wet flue gas G6to the temperature exceeding 50 degrees Celsius in advance. Thus, the humidity of the wet flue gas G6is reduced. Consequently, it is possible to prevent dust accompanying the flue gas G6from adhering to the low-temperature heating unit222, and corrosion of the low-temperature heating unit222caused by the mist of the absorbent in the flue gas G6and the like. The flue gas G6the heat of which is exchanged in the reheater22is discharged through the stack17.

FIG. 2is a schematic view of the heat exchanger20according to the present embodiment. As illustrated inFIG. 2, the heat exchanger20according to the present embodiment includes the heat recovery unit21, the reheater22, and a steam supply unit23. The heat recovery unit21heats the heat medium M by causing the heat medium M to recover the heat from the flue gas G2that is introduced from the air heater13, and discharges the cooled flue gas G3after the heat is recovered, to the electronic precipitator14. The reheater22heats the wet flue gas G6introduced from the desulfurization device16with the heat medium M, and discharges the heated flue gas G7to the stack17. The steam supply unit23supplies steam S to the heat medium M in the circulation line L that supplies the heat medium M from the heat recovery unit21toward the reheater22. The heat medium M that is heated by the heat recovery unit21is transmitted to the reheater22by the liquid feeding pump P through the circulation line L. Moreover, the heat medium M that is cooled by the reheater22is transmitted to the heat recovery unit21by the liquid feeding pump P through the circulation line L. The heat medium M is supplied to the circulation line L that supplies the heat medium M from the reheater22toward the heat recovery unit21, from a heat medium tank24as required.

The fin tube21ais disposed inside the heat recovery unit21. The fin tube21ais a heat transfer tube obtained by providing a plurality of fins that are heat sinks on a tube-shaped member. The circulation line L for circulating the heat medium M between the heat recovery unit21and the reheater22is connected to the fin tube21a. The heat exchanger20heats the heat medium M by causing the heat medium M to recover the heat from the flue gas G2by first heat exchange in which the flue gas G2introduced into the heat recovery unit21from the air heater13is brought into contact with the fin tube21a. The heated heat medium M is transmitted toward the reheater22by the liquid feeding pump P provided on the circulation line L.

The reheater22includes the preheating unit221, the low-temperature heating unit222, and the high-temperature heating unit223. The preheating unit221includes the tube221aas the heat transfer bare tube that is a tube-shaped member. The low-temperature heating unit222includes the fin tube222aas the heat transfer tube obtained by providing the fins that are heat sinks on a tube-shaped member. The high-temperature heating unit223includes the fin tube223aas the heat transfer tube obtained by providing the fins that are heat sinks on a tube-shaped member. One end of the tube221ais connected to the circulation line L, and the other end of the tube221ais connected to an end of the fin tube223avia the circulation line L. The other end of the fin tube223ais connected to one end of the fin tube222avia the circulation line L. The other end of the fin tube222ais connected to the circulation line L. In other words, in the reheater22, the heat medium M after the first heat exchange that is supplied from the heat recovery unit21is sequentially transmitted through the preheating unit221, the high-temperature heating unit223, and the low-temperature heating unit222, in the order of the preheating unit221, the high-temperature heating unit223, and the low-temperature heating unit222. The heat medium M supplied to the low-temperature heating unit222is transmitted to the heat recovery unit21through the circulation line L.

The preheating unit221heats the flue gas G6and reduces the humidity of the wet flue gas G6, by second heat exchange in which the wet flue gas G6introduced to the reheater22from the desulfurization device16is brought into contact with the heated heat medium M after the first heat exchange that is supplied from the heat recovery unit21. The preheating unit221also cools the heat medium M. The preheating unit221further supplies the flue gas G6after the second heat exchange and the humidity of which is reduced, to the low-temperature heating unit222and the high-temperature heating unit223. The preheating unit221also supplies the cooled heat medium M after the second heat exchange to the high-temperature heating unit223.

The low-temperature heating unit222further heats the flue gas G6by third heat exchange in which the flue gas G6supplied from the preheating unit221is brought into contact with the heat medium M supplied from the high-temperature heating unit223. The low-temperature heating unit222also cools the heat medium M. In this example, the wet flue gas G6is heated by the preheating unit221and is turned into the flue gas G6the humidity of which is reduced. Consequently, it is possible to prevent dust accompanying the flue gas G6from adhering to the tube221aof the preheating unit221, and corrosion of the tube221aof the preheating unit221caused by the mist. Moreover, the low-temperature heating unit222supplies the flue gas G6after the third heat exchange to the high-temperature heating unit223, and supplies the cooled heat medium M after the third heat exchange to the heat recovery unit21.

The high-temperature heating unit223further heats the flue gas G6that is heated by fourth heat exchange in which the heated flue gas G6that is supplied from the low-temperature heating unit222is brought into contact with the heat medium M after the second heat exchange that is supplied from the preheating unit221. The high-temperature heating unit223also cools the heat medium M. The high-temperature heating unit223further supplies the flue gas G7after the third heat exchange to the stack17, and supplies the cooled heat medium M after the third heat exchange to the heat recovery unit21. In this example, the flue gas G6supplied from the preheating unit221is heated to sufficient temperature by the low-temperature heating unit222and the high-temperature heating unit223. Thus, it is possible to prevent white smoke generated from flue gas G7that is discharged toward the stack17.

The steam supply unit (heating unit)23supplies the steam S toward a heat exchanging unit25that is provided on the circulation line L for supplying the heat medium M toward the reheater22from the heat recovery unit21through a steam supply line L1. A flow control valve V1for controlling the flow of steam supplied to the heat exchanging unit25from the steam supply unit23is provided on the steam supply line L1. In this manner, when the steam supply unit23supplies the steam S to the heat medium M and heats the heat medium M that flows in the circulation line L, even if the heat quantity recovered by the heat recovery unit21from the flue gas G0that is supplied from the boiler11is not sufficient, the heat medium M supplied to the preheating unit221can be heated to a predetermined temperature range. Thus, the heat exchanger20can sufficiently heat the wet flue gas G6in the preheating unit221of the reheater22. Consequently, it is possible to prevent dust in the flue gas G6from adhering to the tube221aof the preheating unit221, and corrosion of the tube221acaused by the mist.

The heat exchanger20according to the present embodiment includes a flue gas measurement unit31and a control unit32. The flue gas measurement unit31is provided on an introduction portion of the flue gas G2that is introduced to the heat recovery unit21from the air heater13, in the heat recovery unit21. The control unit32controls the temperature of the heat medium M that flows in the circulation line L on the basis of a measurement value measured by the flue gas measurement unit31.

The flue gas measurement unit31measures the gas flow of the flue gas G2introduced into the heat recovery unit21, the gas temperature of the flue gas G2, and the like and transmits the measurement values to the control unit32. The control unit32calculates the recovered heat quantity that is to be recovered to the heat medium M from the flue gas G2through the first heat exchange by the heat exchanger20, from introduction conditions of the flue gas G2to the heat recovery unit21. The introduction conditions are based on the various measurement values transmitted from the flue gas measurement unit31; air volume supplied to an induced draft fan (IDF, not illustrated) that blows the flue gas G0after combustion from the boiler11, a boost up fan (BUF, not illustrated) provided on the desulfurization device16, as well as the boiler11; combustion load of the boiler11; and the like. The control unit32then controls the flow of the steam S supplied to the heat medium M by the steam supply unit23and the flow control valve V1so that the temperature at an outlet portion of the circulation line L from the heat recovery unit21that is measured by a temperature measurement device T1will fall within a predetermined range, on the basis of the calculated recovered heat quantity. In this manner, the heat exchanger20can speedily calculate the recovered heat quantity through the first heat exchange by the heat exchanger20, on the basis of the introduction conditions of the flue gas G2to the heat recovery unit21that is calculated by the control unit32. Because it is possible to set temperature T2at the outlet portion of the heat recovery unit21and temperature T3at the outlet portion of the reheater22within a predetermined range, even if the heat quantity recovered by the heat exchanger20is changed, it is possible to set the temperature of the heat medium M to be supplied to the preheating unit221of the reheater22within a predetermined range at an early stage. Consequently, it is possible to prevent dust from adhering to the tube221aof the preheating unit221, and corrosion of the tube221aof the preheating unit221at an early stage.

Next, a method for controlling the heat exchanger20according to the present embodiment will be described in detail with reference toFIG. 3.FIG. 3is a flow chart of the method for controlling the heat exchanger20according to the present embodiment. As illustrated inFIG. 3, the method for controlling the heat exchanger20according to the present embodiment includes a first step of calculating the heat quantity recovered by the heat exchanger20, a second step of determining whether the calculated recovered heat quantity is less than a predetermined value, a third step of starting supplying the steam S to the heat medium M when the calculated recovered heat quantity is less than the predetermined value, and a fourth step (step ST14) of stopping supplying the steam S to the heat medium M when the calculated recovered heat quantity exceeds the predetermined value.

First, when the operation of the heat exchanger20has started, the control unit32calculates the recovered heat quantity that is to be recovered to the heat medium M from the flue gas G2through the first heat exchange by the heat exchanger20, from introduction conditions of the flue gas G2to the heat recovery unit21(step ST11). The introduction conditions are based on the various measurement values transmitted from the flue gas measurement unit31; air volume supplied to the induced draft fan (IDF, not illustrated) that blows the flue gas G0after combustion from the boiler11, the boost up fan (BUF, not illustrated) provided on the desulfurization device16, as well as the boiler11; combustion load of the boiler11; and the like.

Next, the control unit32determines whether the calculated recovered heat quantity is less than a predetermined value, by comparing the calculated recovered heat quantity with a predetermined threshold set in advance (step ST12). When the calculated recovered heat quantity is less than the predetermined value (Yes at step ST12), the control unit32starts supplying the steam S from the steam supply unit23, and supplies the steam S to the heat medium M in the circulation line L by opening the flow control valve V1of the steam supply line L1(step ST13). In this manner, it is possible to set the temperature of the heat medium M to be supplied to the preheating unit221of the reheater22within a predetermined range. Consequently, it is possible to prevent dust parts in the flue gas G6from adhering to the tube221aof the preheating unit221, and corrosion of the tube221aof the preheating unit221. Moreover, when the calculated recovered heat quantity exceeds the predetermined value (No at step ST12), the control unit32stops supplying the steam S from the steam supply unit23and stops supplying the steam S to the heat medium M in the circulation line L by closing the flow control valve V1of the steam supply line L1(step ST14).

As described above, with the above embodiment, the temperature of the heat medium M to be supplied to the preheating unit221of the reheater22is controlled on the basis of the recovered heat quantity that is recovered by the heat recovery unit21from the flue gas G2introduced into the heat recovery unit21. Consequently, it is possible to set the temperature of the heat medium M to be supplied to the preheating unit221within a predetermined range without delay, according to the change in the recovered heat quantity that is recovered by the heat recovery unit21from the flue gas G2. In this manner, even if the operating conditions of the boiler11and the like have changed, it is possible to implement the heat exchanger and the method for controlling the heat exchanger capable of reducing dust in the flue gas G6from adhering to the inside of the reheater22, and reduce corrosion of the tube221aof the preheating unit221.

Second Embodiment

Next, a second embodiment of the present invention will be described. In the following, points different from those in the above-described first embodiment are mainly described, and the repeated description will be omitted.

FIG. 4is a schematic view of the heat exchanger20according to the second embodiment of the present invention. As illustrated inFIG. 4, the heat exchanger20according to the present embodiment includes a bypass line L2provided between the circulation line L for supplying the heat medium M to the heat recovery unit21from the reheater22, and the circulation line L for supplying the heat medium M to the reheater22from the heat recovery unit21. A flow control valve V2for adjusting the flow of the heat medium M that flows in the bypass line L2is provided on the bypass line L2. The flow control valve V2is openable and closable by the control unit32. In other words, in the heat exchanger20according to the present embodiment, the control unit32adjusts the opening degree of the flow control valve V2according to the heat quantity recovered by the heat recovery unit21that is calculated by the control unit32. Thus, it is possible to control the flow of the heat medium M that flows in the bypass line L2. Consequently, even if the gas flow and the gas temperature of the flue gas G2to be supplied from the air heater13is high, it is possible to prevent excessive heat recovery by the heat recovery unit21and control the recovered heat quantity to be recovered to the heat medium M within a predetermined range. The other configurations are the same as those in the heat exchanger20according to the first embodiment described above, and the description thereof will be omitted.

Next, the method for controlling the heat exchanger20according to the present embodiment will be described in detail with reference toFIG. 5.FIG. 5is a flow chart of the method for controlling the heat exchanger20according to the present embodiment. As illustrated inFIG. 5, the method for controlling the heat exchanger20according to the present embodiment includes a first step of calculating the heat quantity recovered by the heat exchanger20, a second step of determining whether the calculated recovered heat quantity is less than a predetermined range, a third step of starting supplying the steam S to the heat medium M when the calculated recovered heat quantity is less than the predetermined range, a fourth step of stopping supplying the steam S to the heat medium M when the calculated recovered heat quantity falls within the predetermined range, a fifth step of determining whether the calculated recovered heat quantity exceeds the predetermined range, a sixth step of opening the bypass line L2when the calculated recovered heat quantity exceeds the predetermined range, and a seventh step of closing the bypass line L2when the calculated recovered heat quantity falls within the predetermined range.

First, when the operation of the heat exchanger20has started, the control unit32calculates the recovered heat quantity that is to be recovered to the heat medium M from the flue gas G2through the first heat exchange by the heat exchanger20, from introduction conditions of the flue gas G2to the heat recovery unit21(step ST21). The introduction conditions are based on the various measurement values transmitted from the flue gas measurement unit31; air volume supplied to the induced draft fan (IDF, not illustrated) that blows the flue gas G0after combustion from the boiler11, the boost up fan (BUF, not illustrated) provided on the desulfurization device16, as well as the boiler11; combustion load of the boiler11; and the like.

The control unit32then determines whether the calculated recovered heat quantity is less than the predetermined range by comparing the calculated recovered heat quantity with a predetermined threshold set in advance (step ST22). When the calculated recovered heat quantity is less than the predetermined range (Yes at step ST22), the control unit32starts supplying the steam S from the steam supply unit23, and supplies the steam S to the heat medium M in the circulation line L by opening the flow control valve V1of the steam supply line L1(step ST23). In this manner, it is possible to set the temperature of the heat medium M supplied to the preheating unit221of the reheater22within a predetermined range. Consequently, it is possible prevent the dust parts in the flue gas G6from adhering to the tube221aof the preheating unit221, and corrosion of the tube221aof the preheating unit221. Moreover, when the calculated recovered heat quantity exceeds the predetermined range (No at step ST22), the control unit32stops supplying the steam S from the steam supply unit23, and stops supplying the steam S to the heat medium M in the circulation line L by closing the flow control valve V1of the steam supply line L1(step ST24).

The control unit32then determines whether the calculated recovered heat quantity exceeds the predetermined range by comparing the calculated recovered heat quantity with a predetermined threshold set in advance (step ST25). When the calculated recovered heat quantity exceeds the predetermined range (Yes at step ST25), the control unit32opens the flow control valve V2of the bypass line L2and circulates a part of the heat medium M to the reheater22through the bypass line L2without via the heat recovery unit21(step ST26). In this manner, the heat exchanger20can prevent the excessive recovery of heat quantity from the flue gas G2to be introduced into the heat recovery unit21. Consequently, it is possible to set the temperature of the heat medium M to be supplied to the preheating unit221of the reheater22within a predetermined range. It is also possible to prevent the dust parts in the flue gas G6from adhering to the tube221aof the preheating unit221, and corrosion of the tube221aof the preheating unit221. Moreover, when the calculated recovered heat quantity does not fall within the predetermined range (No at step ST25), the control unit32closes the flow control valve V2of the bypass line L2, and circulates the heat medium M between the reheater22and the heat recovery unit21without via the bypass line L2(step ST27).

As described above, according to the present embodiment, the temperature of the heat medium M to be supplied to the preheating unit221of the reheater22and the supply of the heat medium M to the heat recovery unit21are controlled, on the basis of whether the recovered heat quantity that is recovered by the heat recovery unit21from the flue gas G2introduced into the heat recovery unit21falls within a predetermined range. Consequently, even if the recovered heat quantity that is recovered by the heat recovery unit21from the flue gas G2is equal to or more than the predetermined value, it is possible to set the temperature of the heat medium M to be supplied to the preheating unit221within a predetermined range without delay, according to the change in the recovered heat quantity that is recovered by the heat recovery unit21from the flue gas G2. In this manner, even if the operating conditions of the boiler11and the like have changed, it is possible to implement the heat exchanger and the method for controlling the heat exchanger capable of reducing dust in the flue gas G6from adhering to the inside of the reheater22, and reduce corrosion of the tube221aof the preheating unit221.

The arrangement configuration of the tube221aof the preheating unit221, the fin tube222aof the low-temperature heating unit222, and the fin tube223aof the high-temperature heating unit223in the reheater22of the first embodiment and the second embodiment described above is not particularly limited as long as it is possible to heat the flue gas G6to be introduced into the reheater22to a predetermined temperature.

FIG. 6Ais a diagram illustrating an example of a configuration of the reheater22.FIG. 6Ais a schematic view of a vertical section of a plurality of the tubes221a, the fin tubes222a, and the fin tubes223aof the preheating unit221, the low-temperature heating unit222, and the high-temperature heating unit223of the reheater22relative to the extending direction of the tubes221a, the fin tubes222a, and the fin tubes223a.

As illustrated inFIG. 6A, the tubes221a, the fin tubes222a, and the fin tubes223aof the preheating unit221, the low-temperature heating unit222, and the high-temperature heating unit223may be arranged in a lattice pattern relative to the flowing direction of the flue gas G6and the flue gas G7in the sectional view, respectively. By arranging in this manner, the contact area of the tubes221a, the fin tubes222a, and the fin tubes223arelative to the flue gas G6that is introduced into the reheater22is increased. Consequently, it is possible to efficiently heat the flue gas G6and discharge the flue gas G6as the flue gas G7.

FIG. 6Bis a diagram illustrating another example of the configuration of the reheater22. Similar toFIG. 6A,FIG. 6Bis a schematic view of a vertical section of the tubes221a, the fin tubes222a, and the fin tubes223aof the preheating unit221, the low-temperature heating unit222, and the high-temperature heating unit223of the reheater22relative to the extending direction of the tubes221a, the fin tubes222a, and the fin tubes223a.

In the example illustrated inFIG. 6B, the tubes221aand the fin tubes223aof the preheating unit221and the high-temperature heating unit223are arranged in a lattice pattern relative to the flowing direction of the flue gas G6and the flue gas G7in the sectional view, respectively. Moreover, the fin tubes222aof the low-temperature heating unit222are arranged in a tetragonal lattice pattern relative to the flowing direction of the flue gas G6and the flue gas G7in the sectional view, respectively. By arranging in this manner, the contact area of the tubes221aand the fin tubes223arelative to the flue gas G6introduced into the reheater22is increased, and a sufficient contact area can be obtained. Moreover, in the low-temperature heating unit222, the contact area between the flue gas G6and the fin tubes222acan be moderately reduced, thereby improving the flow velocity of the flue gas G6that passes through the low-temperature heating unit222. Thus, it is possible to efficiently heat the flue gas G6and discharge the flue gas G6as the flue gas G7. In this manner, it is possible to prevent dust in the flue gas G6from adhering to the fin tube222aof the low-temperature heating unit222, and corrosion of the fin tube222aof the low-temperature heating unit222caused by the mist, while securing sufficient heat exchange efficiency in the preheating unit221and the high-temperature heating unit223. In the example illustrated inFIG. 6B, only the fin tubes222aof the low-temperature heating unit222are arranged in a tetragonal lattice pattern. However, the fin tubes223aof the high-temperature heating unit223may also be arranged in a tetragonal lattice pattern. In this case, the gas flow velocity of the flue gas G6that flows through the high-temperature heating unit223is further improved. Consequently, it is possible to efficiently heat the flue gas G6and discharge the flue gas G6as the flue gas G7. As a result, it is also possible to further reduce dust from adhering to the fin tube222aof the low-temperature heating unit222and reduce corrosion of the fin tube222aof the low-temperature heating unit222caused by the mist.

REFERENCE SIGNS LIST