AIR POLLUTION CONTROL SYSTEM

Abstract

An air pollution control system includes adenitration device that removes nitrogen oxide in flue gas from a boiler; a heat transfer tube for recovering part of heat of the flue gas after denitration; a precipitator that removes soot and dust in the flue gas after heat recovery; a desulfurization device that removes sulfur oxide in the flue gas discharged from the precipitator; a heat transfer tube for heating the flue gas discharged from the desulfurization device; a circulation pump that circulates a heat medium between the heat transfer tubes; a heat medium heater provided to the circulation pipe to heat the heat medium; and a control device that controls the heat medium heater based on an ammonia concentration at an outlet of the denitration device. The control device causes the heat medium heater to heat the heat medium when the ammonia concentration is higher than a certain value.

Claims

1. An air pollution control system, comprising: a denitration device configured to remove nitrogen oxide in flue gas from a boiler; a heat recovery unit including a heat transfer tube for recovering part, of heat of the flue gas after denitration; a precipitator configured to remove soot and dust in the flue gas after heat recovery; a desulfurization device configured to remove sulfur oxide in the flue gas discharged from the precipitator; a reheater including a heat, transfer tube for heating the flue gas discharged from the desulfurization device; a heat medium circulation pipe including a heat medium circulation pump configured to circulate a heat, medium between the heat transfer tube of the heat recovery unit and the heat transfer tube of the reheater; a heat medium heater provided to the heat medium circulation pipe to heat the heat medium; a detection device configured to detect a state of ammonia at an outlet of the denitration device; and a control device configured to control the heat medium heater based on a value of an ammonia concentration detected by the detection device, wherein the control device is configured to cause the heat medium heater to heat the heat medium when the ammonia concentration is equal to or higher than a certain value.

2. The air pollution control system according to claim 1, wherein the detection device obtains an ammonia concentration in the flue gas at the outlet of the denitration device.

3. The air pollution control system according to claim 1, wherein the detection device obtains an input amount of ammonia to be supplied to the denitration device.

4. The air pollution control system according to claim 1, wherein the detection device detects a switching of a setting value for inputting ammonia into the flue gas.

5. The air pollution control system according to claim 1, further comprising a soot and dust removal device configured to remove soot and dust sticking to the heat transfer tube of the heat recovery unit, wherein the control device commands operation of the soot and dust removal device while controlling the heat medium heater.

6. The air pollution control system according to claim 1, further comprising a thermometer configured to measure a temperature of the flue gas at an outlet of the heat recovery unit, wherein the control device controls the heat medium heater such that a temperature of the flue gas flowing into the precipitator is an appropriate operating temperature of the precipitator.

Description

Brief Description of Drawings

[0022] FIG. 1 is a schematic diagram illustrating an air pollution control system according to a first embodiment.

[0023] FIG. 2 is an explanatory view illustrating a schematic structure of a heat exchange unit in the air pollution control system.

[0024] FIG. 3 is a graph indicating whether ammonium chloride is in a solid phase precipitation state or a gas state when an ammonia concentration is high (e.g., 4 ppm or more).

[0025] FIG. 4 is a graph indicating whether ammonium chloride is in the solid phase precipitation state or the gas state when the ammonia concentration is low (e.g., 1 ppm or less).

[0026] FIG. 5 is a diagram illustrating a relation between a flue gas temperature ( C.) and an equilibrium concentration (ppm) of hydrogen chloride (HCl) in flue gas in relation to a change in ammonia concentration.

[0027] FIG. 6 is a flowchart illustrating an example of a control operation of the air pollution control system.

[0028] FIG. 7 is a schematic diagram of the air pollution control system according to a second embodiment.

[0029] FIG. 8 is a schematic diagram of a structure of a heat recovery unit.

[0030] FIG. 9 is a schematic diagram illustrating removal of products in the heat recovery unit by a soot blower.

[0031] FIG. 10 is a flowchart illustrating another example of the control operation of the air pollution control system.

DESCRIPTION OF EMBODIMENTS

[0032] The following describes preferred embodiments of the invention in detail with reference to the accompanying drawings. The embodiments do not limit the invention. When multiple embodiments are available, the embodiments include those obtained by combining the respective embodiments.

First Embodiment

[0033] FIG. 1 is a schematic diagram of an air pollution control system according to a first embodiment. As illustrated in FIG. 1, a power generation unit includes a boiler 100 that burns fuel and an air pollution control system 101 that, processes flue gas discharged from the boiler 100. The boiler 100 burns fuel and the like to produce heated gas. A mechanism that converts thermal energy into power absorbs heat from gas heated by the boiler 100. The gas from which heat has been absorbed is discharged to the air pollution control system 101 as the flue gas.

[0034] The air pollution control system 101 removes nitrogen oxide (NOx), soot and dust, and sulfur oxide (SOx) that are included in the flue gas in its process in which the flue gas discharged from the boiler 100 is released from a stack 111. The air pollution control system 101 includes a denitration device 102, an air heater 103, a heat recovery unit 104 of a gas-gas heater, an electronic precipitator 105, a draft fan 106, a desulfurization device 107, a reheater 108 of the gas-gas heater, a circulation pump 109, a heat medium circulation pipe 110, and the stack 111. In FIG. 1, G.sub.0 to G.sub.7 indicate the respective flue gases discharged from the boiler 100 to the stack 111 while T.sub.1 to T.sub.5 indicate the respective flue gas temperatures.

[0035] The flue gas G.sub.0 discharged from the boiler 100 is introduced into the denitration device 102 filled with a catalyst. In the denitration device 102, nitrogen oxide contained in the flue gas G.sub.0 is reduced into water and nitrogen by ammonia gas (NH.sub.3) injected as a reducing agent. The flue gas G.sub.0 is, thus, rendered to be harmless.

[0036] The flue gas G.sub.1 discharged from the denitration device 102 passes through the air heater (AH) 103, and is cooled to, generally, a temperature from 130 C. to 150 C. (preferably from 135 C. to 140 C.).

[0037] After passing through the air heater 103, the flue gas G.sub.2 is introduced into the heat recovery unit 104 of the gas-gas heater. Heat of the flue gas G.sub.2 is recovered through heat exchange with a heat medium (e.g., hot water) 83 flowing in the heat transfer tube (e.g., a finned tube) 115 inserted inside the heat recovery unit 104. After passing through the heat recovery unit 104, the flue gas G.sub.3 has a temperature, generally, from 85 C. to 120 C. (preferably from 90 C. to 100 C.). The flue gas G.sub.3 having such temperature increases soot and dust collection capacity in the electronic precipitator (EP) 105 at a low temperature, for example.

[0038] After passing through the heat recovery unit 104, the flue gas G.sub.3 is introduced into the electronic precipitator 105, where soot and dust are removed.

[0039] After passing through the electronic precipitator 105, pressure of the flue gas G.sub.4 is increased by the draft fan 106 driven by an electric motor. The draft fan 106 is not installed in some cases. The draft fan 106 may be installed at a position where the flue gas G.sub.7 flows after the gas-gas heater reheater 108, in some cases.

[0040] The flue gas G.sub.5 having a pressure increased by the draft fan 106 is introduced into the desulfurization device 107. In the desulfurization device 107, sulfur oxide in the flue gas G.sub.5 is absorbed and removed by an alkaline or weak alkaline absorbent in which limestone is dissolved in a slurry form, for example. When using an absorbent in which limestone is dissolved in a slurry form, the desulfurization device 107 produces gypsum as a by-product. A temperature of the flue gas G.sub.6 after passing through the desulfurization device 107 is reduced to, generally, a temperature of approximately 50 C.

[0041] The flue gas G.sub.6 after the desulfurization device 107 is introduced into the reheater 108 of the gas-gas heater. The reheater 108 heats the flue gas G.sub.6 by heat recovered by the heat recovery unit 104 in a process in which the heat medium 83 is circulated in the paired heat medium circulation pipe 110 by the circulation pump 109. The flue gas G.sub.6 having a temperature of about 50 C. at an outlet of the desulfurization device 107 is reheated by the reheater 108 such that the flue gas G.sub.6 has a temperature of approximately from 85 C. to 110 C. Thereafter, the flue gas G.sub.6 is released into the atmosphere from the stack 111.

[0042] FIG. 2 is an explanatory view illustrating a schematic structure of a heat exchange unit of the air pollution control system. As illustrated in FIG. 2, the heat exchange unit includes the heat medium circulation pipe 110 in which the heat medium 83 is circulated between the heat recovery unit 104 and the reheater 108. The heat medium 83 is circulated between the heat recovery unit 104 and the reheater 108 through the heat medium circulation pipe 110. A plurality of fins 115a are provided on the heat transfer tube (e.g., finned tube) 115 provided on the surface of the heat medium circulation pipe 110 provided inside each of the heat recovery unit 104 and the reheater 108. The heat medium circulation pipe 110 is provided with a heat medium heater 86. The heat medium heater 86 complements energy equivalent to a reduced temperature of the heat medium 83 due to heat radiation during the circulation by heating the heat medium 83 with steam 87. The heat medium heater 86, thus, can adjust a medium temperature of the heat medium 83.

[0043] The heat medium 83 is supplied to the heat medium circulation pipe 110 from a heat, medium tank 88. The heat medium 83 is circulated in the heat medium circulation pipe 110 by the circulation pump 105. A supply of the steam 87 is adjusted by a control valve V.sub.1 in accordance with a gas temperature of the flue gas G.sub.6 from the desulfurization device 107. The heat medium 83 fed to the reheater 108 is supplied to the heat recovery unit 104 by a control valve V.sub.2 in accordance with a gas temperature of the flue gas G.sub.3 discharged from the heat recovery unit 104, resulting in a supply of the heat medium 83 fed to the reheater 108 being adjusted. The flue gas G.sub.7 discharged from the reheater 108 is supplied to the stack 111. The gas supplied to the stack 111 is discharged externally as the flue gas G.sub.8.

[0044] The following describes the air pollution control system when ammonia is excessively input into the denitration device and ammonia slip occurs with reference to FIGS. 1 and 2. As illustrated in FIG. 1, the air pollution control system 101 according to the embodiment includes: the denitration device 102 that removes nitrogen oxide in the flue gas G.sub.0 discharged from the boiler 100; the air heater 103 that exchanges heat of the flue gas G.sub.1 after the denitration with air introduced into the boiler 100; the heat recovery unit 104 including the heat transfer tube 115 that recovers part of heat of the flue gas G.sub.2 after the heat exchange; the electronic precipitator 105 that removes soot and dust in the flue gas G.sub.3 after the heat recovery; the desulfurization device 107 that removes sulfur oxide in the flue gas G.sub.5 discharged from the electronic precipitator 105; the reheater 103 including the heat transfer tube 115 that heats the flue gas G.sub.6 discharged from the desulfurization device 107; the heat medium circulation pipe 110 provided with the circulation pump 109 that circulates the heat medium 83 between the heat transfer tube 115 in the heat recovery unit 104 and the heat transfer tube 115 in the reheater 108; the heat medium heater 86 that is provided to the heat medium circulation pipe 110 and heats the heat medium 83; an ammonia (NH.sub.3) meter 200, which is a detection device detecting a state of ammonia at the outlet of the denitration device 102; and a control device 201 that, controls the heat medium heater 86 on the basis of a value of the ammonia concentration detected by the ammonia meter 200. The control device 201 causes the heat medium heater 86 to heat the heat medium 83 when the ammonia concentration is equal to or higher than a certain value.

[0045] In the embodiment, an ammonia (NH.sub.3) concentration at the outlet of the denitration device 102 is measured by the ammonia meter 200. When the catalyst of the denitration device 102 is not deteriorated, an ammonia concentration at the outlet of the denitration device 102 is typically around 1 ppm. When ammonia is excessively injected in the denitration processing due to a reduction in processing capability caused by the deterioration of the denitration catalyst in long time use, an ammonia concentration at the outlet of the denitration device 102 is increased to 10 ppm to 15 ppm or more in some cases. When an increase in ammonia concentration is equal to or higher than a certain value (e.g., 10 ppm to 15 ppm or more), the control device 201 issues a command to input steam 87 into the heat medium heater 86 provided to the heat medium circulation pipe 110 on the outlet side of the heat recovery unit 104. As a result, a temperature of the circulating heat medium 83 is increased, thereby increasing a temperature of the heat transfer tube in the heat, recovery unit 104 of the gas-gas heater. As a result, a gas temperature in the heat recovery unit 104 is increased, thereby preventing the production of ammonium chloride (NH.sub.4Cl) and preventing a product caused by ammonium chloride from sticking to the heat, transfer tube 115.

[0046] In the embodiment, the temperature of the heat medium heater 86 is adjusted by input of the steam 87. Other than the input of the steam 87, the temperature of the circulating heat medium 83 can be controlled by adjusting the circulation pump 109 that adjusts a flow of the circulating heat medium 83.

[0047] Examples of the detection device other than control operation of monitoring by the ammonia meter 200 detecting an ammonia concentration include control operation at timing when the input amount of ammonia is changed or a setting value of input amount of ammonia is changed, and measurement data of an ammonia concentration by manual measurement. When the denitration processing in the denitration device 102 is insufficient, a NOx measurement value of a NOx meter, which is not illustrated, is increased to a value equal to or higher than a specified value. In such a case where the NOx value is increased, an input amount of ammonia or timing for setting input amount of ammonia is changed. When such a change is made, an amount of ammonia slip is estimated and the temperature of the heat medium 83 can be adjusted.

[0048] This makes it possible to adjust the heat medium heater 86, to control the temperature of the heat medium circulating in the heat recovery unit 104, to prevent the production of ammonium chloride (NH.sub.4Cl), and to prevent ammonium chloride from sticking to the heat transfer tube 115, even in a plant in which the ammonia meter 200 is not installed.

[0049] As described above, in the embodiment, the ammonia meter 200 detects an increase in ammonia concentration and the control device 201 adjusts the heat medium heater 86 when the ammonia concentration is equal to or higher than a certain value on the basis of the detection data. This adjustment causes a gas atmosphere temperature in the heat recovery unit 104 to be out of an ammonium chloride production temperature region. As a result, ammonium chloride can be prevented from sticking to the heat transfer tube in the heat recovery unit 104 during plant operation.

[0050] The following describes a relation between a flue gas temperature ( C.) in the heat recovery unit 104 and an equilibrium concentration (ppm) of hydrogen chloride (HCl) in flue gas with reference to FIGS. 3 and 4.

[0051] FIG. 3 is a graph indicating whether ammonium chloride is in a solid phase precipitation state or a gas state when the ammonia concentration is high (e.g., 4 ppm or more). FIG. 4 is a graph indicating whether ammonium chloride is in the solid phase precipitation state or the gas state when the ammonia concentration is low (e.g., 1 ppm or less). FIGS. 3 and 4 each illustrate a case where the concentration of hydrogen chloride introduced into the heat recovery unit 104 is an average concentration (e.g., 10 ppm to 20 ppm) in coal combustion plants.

[0052] It is known that whether ammonium chloride is in a precipitation state or a non-precipitation state is determined by equilibrium calculation with conditions of an ammonia concentration, a flue gas temperature, and a hydrogen chloride concentration in an equilibrium reaction. As illustrated in FIGS. 3 and 4, in the equilibrium reaction, whether ammonium chloride (NH.sub.4Cl) is in the precipitation state or the non-precipitation state is determined by the conditions of the ammonia concentration, the flue gas temperature, and hydrogen chloride (HCl) concentration. Under the conditions that are represented in a region on an upper left, side of the equilibrium curve (solid line), ammonium chloride (NH.sub.4Cl) precipitates, while under the conditions that are represented in a region on a lower right, side of the equilibrium curve, ammonia slip and hydrogen chloride (HCl) are in a gas state and, thus, pass through the heat recovery unit 104. As an ammonia concentration is reduced, the solid production region is reduced. The reduction in ammonia concentration further prevents the production of ammonium chloride (NH.sub.4Cl).

[0053] Specifically, as illustrated in FIG. 3, when the hydrogen chloride concentration at the inlet of the heat recovery unit 104 is the average concentration, the ammonia concentration is high, and the flue gas temperature at the outlet of the heat recovery unit 104 is 90 C. to 95 C., for example, ammonium chloride (NH.sub.4Cl) is plotted in the solid region.

[0054] In contrast, when the hydrogen chloride concentration at the inlet of the heat recovery unit 104 is the average concentration, the ammonia concentration is low, and the flue gas temperature at the outlet of the heat recovery unit 104 is 90 C. to 95 C., for example, ammonium chloride (NH.sub.4Cl) is plotted in the gas region.

[0055] When the flue gas temperature at the outlet of the heat recovery unit 104 is 90 C. to 95 C., for example, and the ammonia concentration is high (e.g., 4 ppm or more) these conditions fall within the upper region from the boundary of solid phase precipitation as illustrated in FIG. 3, and solid phase precipitation of ammonium chloride occurs in the heat recovery unit 104.

[0056] The following describes a relation between a threshold of the ammonium concentration causing the sticking of ammonium chloride and the hydrogen chloride concentration. FIG. 5 is a diagram illustrating a relation between the flue gas temperature ( C.) and the equilibrium concentration (ppm) of hydrogen chloride (HCl) in flue gas in relation to a change in ammonia concentration. In FIG. 5, a change in solid region and gas region is illustrated in relation to each of samples A to E having different ammonia concentrations (e.g., in a range from 1 ppm to 20 ppm).

[0057] A gas temperature region in the heat recovery unit 104 is approximately from 95 C. to 150 C. When the gas temperature is set to an environment of near 100 C., a threshold of the ammonium concentration causing ammonium chloride to be formed (what is called sticking to the heat transfer tube 115) can be determined from FIG. 5 on the basis of the past case where the gas temperature region when ammonium chloride had been stuck to the heat transfer tube 115 was a low temperature region (95 C. to 105 C.).

[0058] In FIG. 5, the calculation is made using the hydrogen chloride concentration (10 ppm to 20 ppm) generally contained in flue gas. In a practical plant, an estimated value based on a chlorine concentration in coal serving as fuel used in the plant can be substituted as the hydrogen chloride concentration. The hydrogen chloride concentration is calculated from the Cl concentration value of fuel as the hydrogen chloride concentration of the flue gas G.sub.1 at the outlet of the denitration device 102. The hydrogen chloride concentration is measured using a hydrogen chloride (HCl) meter in some cases.

[0059] In the embodiment, for preliminarily preventing an increase in gas differential pressure in the heat recovery unit 104 due to the sticking of a product composed of ammonium chloride to the heat transfer tube 115 in the heat recovery unit 104, the ammonia (NH.sub.3) meter 200 is installed that measures the ammonium concentration in the flue gas at the outlet of the denitration device 102, and the control device 201 commands control of an increase in temperature of the heat medium 83 circulating in the heat recovery unit 104 on the basis of the measurement value of the ammonia concentration detected by the ammonia meter 200, thereby causing the gas temperature of an inner environment of the heat recovery unit 104 to be out of the ammonium chloride production temperature region. As a result, ammonium chloride can be prevented from sticking to the heat transfer tube 115 in the heat recovery unit 104 during plant operation.

[0060] As a result of reduction in production of ammonium chloride, sticking of a product composed of ammonium chloride to the heat transfer tube 115 is reduced. Consequently, this can prevent an increase in gas differential pressure in the heat transfer tube 115 in the heat recovery unit 104.

[0061] The following describes an example of processing of the air pollution control system with reference to FIG. 6. FIG. 6 is a flowchart illustrating an example of a control operation of the air pollution control system. The air pollution control system repeatedly performs the processing illustrated in FIG. 6 during the operation of the power generation plant. For example, the air pollution control system performs the processing illustrated in FIG. 6 at a certain time interval or at every time measurement information is acquired.

[0062] The control device 201 acquires the measurement information at step S12. The control device 201 acquires the results of measurement by the ammonia meter 200 and the gas temperature meter via communication. A hydrogen chloride concentration (hereinafter described as an HCl concentration) 202 is based on calculation data from the Cl concentration value of the fuel or on measurement data from the hydrogen chloride (HCl) meter.

[0063] After acquiring the measurement information at step S12, the control device 201 determines whether the ammonia concentration is equal to or higher than a certain value at which ammonium chloride precipitates in solid phase from the ammonia concentration, the HCl concentration 202, and the gas temperature of the flue gas G.sub.2 at step S14. If it is determined that the ammonia concentration is the concentration at which ammonium chloride precipitates in solid phase (Yes at step S14), the control device 201 outputs a command to operate the heat medium heater 86 at step S16, and thereafter ends the processing. If it is determined that the ammonia concentration is not the concentration at which ammonium chloride precipitates in solid phase (No at step S14), the control device 201 ends the processing.

[0064] The control device 201 can promptly detect whether ammonium chloride precipitates in solid phase by performing the processing illustrated in FIG. 6 every time the control device 201 acquires the measurement, information (states of operation parameters relating to the production of ammonium chloride). When it is detected that the ammonia concentration is the concentration at which ammonium chloride precipitates in solid phase, the heat medium heater 86 is caused to be operated to increase the inner temperature of the heat recovery unit 104, thereby making it possible to prevent the production of ammonium chloride.

[0065] The control device 201 can perform the processing illustrated in FIG. 6 using various detection units. When detecting whether ammonium chloride precipitates in solid phase using parameters of the detection results of the ammonia meter 200 measuring the ammonia concentration and a thermometer, and the HCl concentration 202, i.e., states of the operation parameters relating to the production of ammonium chloride, the control device 201 can determine whether ammonium chloride precipitates in solid phase on the basis of the relation illustrated in FIG. 5, i.e., by performing the determination on the basis of the equilibrium reaction condition.

[0066] As described above, the air pollution control system detects the state of ammonia concentration, i.e., whether ammonium chloride precipitates in solid phase. When the ammonia concentration is equal to or higher than a certain value, the air pollution control system causes the temperature of the heat medium 83 circulating in the heat recovery unit 104 to be increased by the adjustment of the heat medium heater 86 so as to increase the gas temperature in the heat recovery unit 104, thereby making it possible to prevent the production of ammonium chloride.

Second Embodiment

[0067] FIG. 7 is a schematic diagram of the air pollution control system according to a second embodiment. As illustrated in FIG. 7, a gas thermometer that measures the gas temperature T.sub.3 of the flue gas G.sub.3 at the outlet of the heat recovery unit 104 is further provided in the air pollution control system in the first embodiment. The control device 201 can control the heat medium heater 86 such that the gas temperature T.sub.3 of the flue gas G.sub.3 flowing into the electronic precipitator 105 is an appropriate operating temperature of the electronic precipitator 105.

[0068] When the heat medium 83 is heated by the heat medium heater 86, the temperature of the heat medium 83 introduced into the heat recovery unit 104 is increased. In a case where the low temperature electronic precipitator 105 is used as the precipitator, the upper limit value of the operating temperature is about 120 C. The operation of the heat medium heater 86 is, thus, controlled such that the gas temperature T.sub.3 is maintained under the upper limit value. As a result, the low temperature electronic precipitator 105 can be appropriately and continuously operated.

Third Embodiment

[0069] FIG. 8 is a schematic diagram of a structure of the heat recovery unit. As illustrated in FIG. 8, the heat transfer tube is bundled inside the heat recovery unit 104.

[0070] The heat transfer tube is separated into three bundles, i.e., a high temperature bundle 104a, a medium temperature bundle 104b, and a low temperature bundle 104c from a flue gas inlet side, for example. In the three separated bundles, a temperature of gas passing through the heat transfer tube 115 in the low temperature bundle 104c is in a lower temperature region than that of gas passing through the heat transfer tube 115 in the high temperature bundle 104a as a result of heat exchange.

[0071] The control device 201 includes a soot and dust removal device configured to remove soot and dust sticking to the heat transfer tube of the heat recovery unit 104 in addition to the control of the heat medium heater 86 in the first embodiment, to control the heat medium heater 86 and command operation of the soot and dust removal device.

[0072] FIG. 9 is a schematic diagram illustrating removal of products in the heat recovery unit by a soot blower. As illustrated in FIG. 9, a soot blower 210 serving as the soot and dust removal device is installed in the heat transfer tube 115 in the low temperature bundle 104c. The soot blower 210 is composed of a lance tube 212 provided with a nozzle 211 at the tip thereof and a housing 213 that houses the lance tube 212 such that the lance tube 212 is attached in a detachable manner. Products 215 composed of ammonium chloride sticking to the heat transfer tube 115 in the low temperature bundle 104c can be removed by steam or compressed air jetted from the nozzle 211.

[0073] The following describes another example of the processing of the air pollution control system with reference to FIG. 10. FIG. 10 is a flowchart illustrating another example of the control operation of the air pollution control system. The air pollution control system repeatedly performs the processing illustrated in FIG. 10 during the operation of the power generation plant. For example, the air pollution control system performs the processing illustrated in FIG. 10 at a certain time interval or at every time when measurement information is acquired.

[0074] The control device 201 acquires the measurement information at step S22. The control device 201 acquires the results of measurement by the ammonia meter 200 and the gas temperature meter via communication. The HCl concentration 202 is calculation data from the Cl concentration value of the fuel or measurement data from the hydrogen chloride (HCl) meter.

[0075] After acquiring the measurement information at step S22, the control device 201 determines whether the ammonia concentration is equal to or higher than a certain value at which ammonium chloride precipitates in solid phase from the ammonia concentration, the HCl concentration, and the gas temperature of the flue gas G.sub.2 at step S24. If it is determined that the ammonia concentration is the concentration at which ammonium chloride precipitates in solid phase (Yes at step S24), the control device 201 outputs a command to operate the heat medium heater 86 and a command to operate the soot blower at step S26, and thereafter ends the processing. If it is determined that the ammonia concentration is not the concentration at which ammonium chloride precipitates in solid phase (No at step S24), the control device 201 ends the processing.

[0076] The control device 201 can promptly detect whether ammonium chloride precipitates in solid phase by performing the processing illustrated in FIG. 10 every time the control device 201 acquires the measurement information (states of operation parameters relating to the production of ammonium chloride). When it is detected that the ammonia concentration is the concentration at which ammonium chloride precipitates in solid phase, the heat medium heater 86 is caused to be operated to increase the inner temperature of the heat recovery unit 104, thereby making it possible to prevent the production of ammonium chloride. The control device 201 causes the soot blower 210 to operate such that the nozzle 211 jets steam or compressed air in addition to causing the heat medium heater 86 to operate. The heat medium heater 86 is operated to increase an inner temperature of the heat, recovery unit 104, thereby preventing the production of ammonium chloride. In addition, the soot blower 210 is operated, thereby making it possible to remove the products 215 composed of ammonium chloride sticking to the heat transfer tube 115 in the bundle.

[0077] When the ammonia concentration is increased in the first embodiment, in the third embodiment, the soot blower 210 is additionally operated so as to prevent soot and dust caused by the products 215 from sticking to the heat transfer tube 115 and to remove the soot and dust. Consequently, this can prevent an increase in gas differential pressure.

[0078] As described above, when it is determined that the ammonia concentration is equal to or higher than a certain value on the basis of the measurement result of the ammonia concentration detected by the ammonia meter 200 on a front side (e.g., the outlet of the denitration device 102) of the heat recovery unit 104, the heat medium heater 86 is operated and the soot blower 210 is immediately operated. This makes it possible to preliminarily prevent an increase in gas differential pressure in the heat recovery unit 104 due to the sticking of the products 215 composed of ammonium chloride to the heat transfer tube 115 in the heat recovery unit 104.

[0079] As illustrated in FIG. 8, the operation of the soot blower 210 is concentrated to the low temperature bundle 104c, which is in a low temperature region where sticking of ammonium, chloride occurs, thereby making it possible to prevent ammonium chloride from sticking to the heat transfer tube 115 in the low temperature bundle 104c during plant operation.

[0080] When the ammonia concentration, which is a condition of the precipitation of ammonium chloride, at the outlet of the denitration device 102 indicates the certain value, the operation frequency of the soot blower 210 is increased to remove the products 215 composed of ammonium chloride, thereby making it possible to prevent ammonium chloride from sticking to the heat transfer tube 115.

REFERENCE SIGNS LIST

[0081] 83 Heat medium

[0082] 86 Heat medium heater

[0083] 87 Steam

[0084] 100 Boiler

[0085] 101 Air pollution control system

[0086] 111 Stack

[0087] 102 Denitration device

[0088] 103 Air heater

[0089] 104 Heat recovery unit

[0090] 105 Electronic precipitator

[0091] 106 Draft fan

[0092] 107 Desulfurization device

[0093] 100 Reheater

[0094] 109 Circulation pump

[0095] 110 Heat medium circulation pipe

[0096] 111 Stack

[0097] 200 Ammonia (NH.sub.3) meter

[0098] 201 Control device

[0099] 202 HCl concentration