Waste heat recovery boiler and feedwater bypass control method therefor

Abstract

To provide a waste heat recovery boiler capable of ensuring operation of a boiler within a wide range of loads while avoiding a risk of steaming before merging with a bypass route due to flowing feedwater into the bypass route and / or a risk of nozzle damage to a boiler drum feedwater pipe base part after merging with the bypass route.SOLUTION: A waste heat recovery boiler 21 comprises: an economizer 6 that preheats feedwater to cool exhaust gas; a bypass route 18 that bypasses the feedwater to the economizer 6; a feedwater amount adjustment device 19 that adjusts a feedwater amount in the bypass route 18; and a control device 22 that controls the feedwater amount adjustment device 19 based on at least an exhaust gas temperature (1) after passing through the economizer 6, and at least one of an economizer outlet feedwater temperature (2) before merging with the bypass route 18 and a feedwater temperature (3) after merging with the bypass route 18.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a heat recovery steam generator, and more particularly to a heat recovery steam generator provided with a bypass path for bypassing feedwater to an economizer. [Background technology]

[0002] Conventionally, heat recovery boilers (hereinafter simply referred to as boilers) that recover heat from the exhaust gas of incinerators such as waste incinerators have been known. As shown in Figure 1, boiler 1 is configured with a superheater 4, an evaporator 5, and an economizer 6 arranged in the exhaust gas discharged from incinerator 2 and flowing through flues 3 and 14. Superheater 4 may be arranged in flues 3 or 14, as may economizer 6. Evaporator 5 may be composed of a water-cooled wall as shown in Figure 1, or may be arranged in flues 3.

[0003] Feedwater is supplied from the feedwater pump to the economizer 6 via the economizer feedwater pipe 7, where it is preheated and then supplied to the boiler drum 9 (hereinafter simply referred to as the drum 9) via the drum feedwater pipe 8. The feedwater supplied to the drum 9 is naturally or forcibly circulated through the downcomer pipe 10, the evaporator 5, the evaporation pipe 11, and the drum 9 in that order, and is heated during this process. The steam generated by heating is separated from the water in the drum 9 and then sent to the superheater 4 via the drum steam pipe 12, where it is further heated and then supplied to the steam turbine via the steam pipe 13 for use in power generation, etc.

[0004] In the boiler 1, load fluctuations occur due to changes in the waste quality and incineration volume of the incinerator, for example. Under light loads, the exhaust gas temperature drops, which poses a problem of low-temperature corrosion occurring in the economizer 6 and its downstream equipment (exhaust gas treatment equipment).

[0005] To solve this problem and ensure operation of the boiler 1 over a wide range of loads, a boiler 1 has been proposed in which a bypass duct 15 that bypasses the economizer 6 is installed in a flue 14 (see FIG. 1) in which the economizer 6 is located, and a damper 16 that adjusts the flow rate of the exhaust gas is provided in the bypass duct 15, as shown in FIG. 2 (see Patent Document 1). In the boiler 1 described in Patent Document 1, when the temperature of the exhaust gas drops after merging into the bypass under light load conditions, the exhaust gas is caused to flow into the bypass duct 15 by a control device 17 and a damper 16, thereby raising the temperature of the exhaust gas after merging into the bypass. This makes it possible to prevent low-temperature corrosion of the economizer 6 and its downstream equipment (exhaust gas treatment equipment), and ensures operation of the boiler 1 over a wide range of loads. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Figure 1 of JP 2013-11373 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, conventional bypass ducts 15 have the following issues. Because exhaust gas flows through the bypass duct 15 under light loads, the approach temperature of the water supply to the drum 9 becomes high (the difference with the drum temperature becomes small). Also, while it is naturally advantageous for heat recovery to keep the damper 16 of the bypass duct 15 fully closed, there is a risk of dust accumulating and adhering to the damper 16 if it is not used. Furthermore, from the perspective of power generation, there is always a leak in the damper 16 (for example, about 1% is continuously bypassed), which reduces the heat recovery rate and power generation. In addition, since a flue must be routed, space is required for its placement.

[0008] In order to solve the above-mentioned problems with the bypass duct, it is conceivable to provide a bypass path 18 that bypasses the feedwater to the economizer 6, as shown in Figure 3, and to provide a feedwater amount adjustment device 19 such as a control valve in the bypass path 18. Then, when the exhaust gas temperature (1) after passing through the economizer 6 drops under light load conditions, the control device 20 and the feedwater amount adjustment device 19 cause the feedwater to flow into the bypass path 18, thereby raising the temperature of the exhaust gas after passing through the economizer 6. This makes it possible to prevent low-temperature corrosion of the economizer 6 and its downstream equipment (exhaust gas treatment equipment), and ensures operation of the boiler 1 over a wide range of loads.

[0009] However, on the other hand, when the amount of water supplied to the bypass passage 18 is increased, the temperature (2) of the feedwater at the outlet of the economizer before merging with the bypass passage 18 increases as the amount of water supplied to the economizer 6 decreases. Therefore, there is a risk of steaming, in which steam evaporates at the outlet of the economizer before merging with the bypass passage 18.

[0010] Furthermore, increasing the amount of feedwater in the bypass path 18 increases the amount of unheated feedwater passing through the bypass path 18, lowering the feedwater temperature (3) after merging with the bypass path 18. As a result, a temperature difference occurs with the drum water supply pipe base where saturated steam is stored, and this temperature difference generates thermal stress, which poses a risk of damaging the nozzle of the drum water supply pipe base.

[0011] The present invention has been made in view of the above-mentioned problems, and aims to provide a heat recovery boiler and a feedwater bypass control method thereof that can ensure boiler operation over a wide range of loads while avoiding the risk of steaming before merging with the bypass route due to feedwater flowing through the bypass route and / or the risk of nozzle damage to the boiler drum feedwater pipe base after merging with the bypass route. [Means for solving the problem]

[0012] In order to solve the above problems, one aspect of the present invention is a waste heat recovery boiler including: an economizer that preheats feedwater to cool exhaust gas; a bypass passage that bypasses the feedwater to the economizer; a feedwater amount adjustment device that adjusts the amount of feedwater to the bypass passage and / or the economizer; and a control device that controls the feedwater amount adjustment device based on at least the exhaust gas temperature after passing through the economizer and at least one of the feedwater temperature at the economizer outlet before joining the bypass passage or the feedwater temperature after joining the bypass passage.

[0013] Another aspect of the present invention is a feedwater bypass control method for a heat recovery boiler including an economizer that preheats feedwater to cool exhaust gas and a bypass path that bypasses the feedwater to the economizer, the method controlling the amount of feedwater to the bypass path and / or the economizer based on at least the exhaust gas temperature after passing through the economizer and at least one of the feedwater temperature at the economizer outlet before merging with the bypass path and the feedwater temperature after merging with the bypass path.

[0014] In the present invention, the feedwater amount adjuster may be provided in the bypass passage, at the inlet or outlet of the bypass passage, or in the economizer feedwater pipe after the bypass passage branches and before the bypass passage joins. The feedwater amount adjuster may adjust the amount of feedwater in the bypass passage, or the amount of feedwater in the economizer, or the amount of feedwater in both the bypass passage and the economizer. The adjustment valve serving as the feedwater amount adjuster is preferably of the aperture control type, but may also be of the open / close type.

[0015] Furthermore, in the present invention, the control device may control the feedwater amount adjustment device based on the exhaust gas temperature (1) after passing through the economizer and the feedwater temperature (2) at the outlet of the economizer before merging with the bypass passage, or may control the feedwater amount adjustment device based on the exhaust gas temperature (1) and the feedwater temperature (3) after merging with the bypass passage, or may control the feedwater amount adjustment device based on the exhaust gas temperature (1), the feedwater temperature (2) at the outlet of the economizer, and the feedwater temperature (3) after merging with the bypass passage. [Effects of the Invention]

[0016] According to the present invention, the amount of feedwater to the bypass path and / or economizer is controlled based on the temperature of the exhaust gas after passing through the economizer, thereby ensuring boiler operation over a wide range of loads. Furthermore, the amount of feedwater to the bypass path and / or economizer is controlled based on at least one of the feedwater temperature at the economizer outlet before merging with the bypass path and the feedwater temperature after merging with the bypass path. This avoids the risk of steaming at the economizer outlet before merging with the bypass path and / or the risk of damage to the nozzle of the boiler drum feedwater pipe base after merging with the bypass path, which may be caused by an increase in the amount of feedwater to the bypass path. Furthermore, the bypass path through which feedwater flows requires less space than a conventional bypass duct, so there are fewer restrictions on its placement. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a system diagram of a conventional incinerator and a waste heat recovery boiler. [Figure 2] FIG. 1 is a system diagram of a conventional waste heat recovery boiler. [Figure 3] FIG. 1 is a system diagram of a heat recovery steam generator equipped with a feedwater bypass path. [Figure 4] 1 is a system diagram of a heat recovery steam generator according to a first embodiment of the present invention. [Figure 5] FIG. 5 is a system diagram of a heat recovery steam generator according to a second embodiment of the present invention. [Figure 6] FIG. 10 is a system diagram of a heat recovery steam generator according to a third embodiment of the present invention. [Figure 7] FIG. 10 is a system diagram of a heat recovery steam generator according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, a heat recovery steam generator and a feedwater bypass control method thereof according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the heat recovery steam generator and the feedwater bypass control method thereof according to the present invention can be embodied in various forms and are not limited to the embodiments described in the specification. The present embodiment is provided with the intention of enabling those skilled in the art to fully understand the invention by fully disclosing the specification. (First embodiment)

[0019] Fig. 4 shows a heat recovery steam generator 21 according to a first embodiment of the present invention. Similar to the heat recovery steam generator shown in Fig. 3, the heat recovery steam generator 21 of this embodiment includes a bypass path 18 that bypasses the feedwater to the economizer 6, an aperture-controlled adjustment valve 19 provided in the bypass path 18, and a control device 22 that controls the adjustment valve 19.

[0020] In the heat recovery steam generator 21 of this embodiment, when the exhaust gas temperature (1) after passing through the economizer 6 drops under light load conditions, the control device 22 increases the aperture of the control valve 19, causing feed water to flow through the bypass path 18 and reducing the amount of water fed to the economizer 6. This makes it possible to increase the temperature of the exhaust gas after passing through the economizer 6. On the other hand, when the exhaust gas temperature (1) after passing through the economizer 6 rises under high load conditions, the control device 22 decreases the aperture of the control valve 19 and increases the amount of water fed to the economizer 6. This makes it possible to decrease the temperature of the exhaust gas after passing through the economizer 6.

[0021] However, if the amount of water supplied to the bypass path 18 is increased, the economizer outlet feedwater temperature (2) before merging with the bypass path 18 will rise as the amount of water supplied to the economizer 6 decreases, resulting in a risk of steaming. To avoid the risk of steaming, when the economizer outlet feedwater temperature (2) increases, the control device 22 reduces the opening of the control valve 19 to increase the amount of water supplied to the economizer 6. This reduces the economizer outlet feedwater temperature (2).

[0022] Furthermore, increasing the amount of feedwater in bypass path 18 increases the amount of unheated feedwater passing through bypass path 18, lowering the feedwater temperature (3) after merging with bypass path 18. As a result, a temperature difference occurs with the drum feedwater pipe base where saturated steam is stored, causing thermal stress due to the temperature difference and risking nozzle damage. To avoid the risk of nozzle damage, control device 22 reduces the opening of control valve 19 and increases the amount of feedwater to economizer 6. This raises the feedwater temperature (3) after merging with bypass path 18.

[0023] The control method of the control device 22 will be described below. Reference numeral 31 denotes a temperature detector that detects the exhaust gas temperature (1) after passing through the economizer 6 and converts it into an electrical signal. SV=X°C is a set value for setting the exhaust gas temperature (1) to a temperature that is desired to be maintained. The temperature detection signal from the temperature detector 31 is input to the control device 22 together with the set value SV=X°C. The control device 22 controls the control valve 19 so that the exhaust gas temperature (1) after passing through the economizer 6 becomes the set value X°C. In other words, the control device 22 calculates the deviation SV-PV between the set value SV=X°C and the current temperature PV detected by the temperature detector 31, calculates the opening MVX of the control valve 19 using PID control based on the deviation, and outputs a signal of the calculated opening MVX to the control valve 19.

[0024] In this way, the control device 22 controls the adjustment valve 19 so that the exhaust gas temperature (1) after passing through the economizer 6 becomes the set value X°C. However, if the amount of water supply to the bypass path 18 is increased during light loads, there is a risk of steaming due to an increase in the economizer outlet water supply temperature (2) as described above, and a risk of damage to the nozzle of the drum water supply pipe base due to a decrease in the water supply temperature (3) after merging with the bypass path 18.

[0025] To avoid these risks, the control device 22 performs low-select control. Reference numeral 32 denotes a temperature detector that detects the economizer outlet feedwater temperature (2) before merging with the bypass path 18 and converts it into an electrical signal. SV=Y°C is the upper limit set value above which the risk of steaming does not occur. The temperature detection signal from the temperature detector 32 is input to the control device 22 together with the set value SV=Y°C.

[0026] Reference numeral 33 denotes a temperature detector that detects the feedwater temperature (3) after merging with the bypass route 18 and converts it into an electrical signal. SV=Z°C is the lower limit set value below which there is no risk of nozzle damage. The temperature detection signal from the temperature detector 33 is input to the control device 22 together with the set value SV=Z°C.

[0027] The control device 22 calculates the deviation SV-PV between the temperature set value SV=Y°C and the current temperature PV detected by the temperature detector 32, and calculates the opening degree MVY of the control valve 19 using PID control based on the deviation. Similarly, the control device 22 calculates the deviation SV-PV between SV=Z°C and the current temperature PV detected by the temperature detector 33, and calculates the opening degree MVZ of the control valve 19 using PID control based on the deviation.

[0028] Then, the control device 22 selects the smallest opening from the three calculated openings MVX, MVY, and MVZ, and outputs a signal for the selected opening to the control valve 19. By performing low select control in this manner, the control valve 19 is normally controlled so that the exhaust gas temperature (1) after passing through the economizer 6 becomes the set value X°C, but an increase in the opening of the control valve 19 is locked so that the economizer outlet feedwater temperature (2) before merging with the bypass path 18 does not exceed the upper set value Y°C, and so that the feedwater temperature (3) after merging with the bypass path 18 does not fall below the lower set value Z°C. This makes it possible to avoid the risk of steaming and the risk of damage to the nozzle of the drum feedwater pipe base.

[0029] Note that MVY and MVZ are used solely for risk avoidance control. When MVY or MVZ is selected, the opening of the control valve 19 cannot be increased, and the exhaust gas temperature (1) after passing through the economizer 6 drops, but no problem occurs if the boiler 21 is designed with this margin in mind. (Second embodiment)

[0030] Fig. 5 shows a heat recovery steam generator 23 according to a second embodiment of the present invention. Similar to the heat recovery steam generator 21 of the first embodiment shown in Fig. 4, the heat recovery steam generator 23 of this embodiment includes a bypass path 18 that bypasses the feedwater to the economizer 6. In the heat recovery steam generator 23 of the second embodiment, an opening-controlled adjustment valve 24 (three-way valve) is provided at the inlet of the bypass path 18.

[0031] The control method of the control valve 24 (three-way valve) by the control device 22 is substantially the same as that of the heat recovery steam generator 21 of the first embodiment. That is, the control device 22 controls the control valve 24 so that the exhaust gas temperature (1) after passing through the economizer 6 becomes a set value X°C. Specifically, the control device 22 calculates the deviation SV-PV between the set value SV=X°C and the current temperature PV detected by the temperature detector 31, calculates the opening MVX of the control valve 24 (opening of the bypass path 18) using PID control based on the deviation, and outputs a signal of the calculated opening MVX to the control valve 24.

[0032] The control device 22 also performs low-select control. That is, the control device 22 calculates the deviation SV-PV between the temperature set value SV=Y°C and the current temperature PV detected by the temperature detector 32, and calculates the opening MVY of the control valve 24 (the opening of the bypass path 18) using PID control based on the deviation. Similarly, the control device 22 calculates the deviation SV-PV between SV=Z°C and the current temperature PV detected by the temperature detector 33, and calculates the opening MVZ of the control valve 19 (the opening of the bypass path 18) using PID control based on the deviation. The control device 22 then selects the smallest opening of the three calculated openings MVX, MVY, and MVZ, and outputs a signal of the selected opening to the control valve 24 (the opening of the bypass path 18). (Third embodiment)

[0033] Fig. 6 shows a heat recovery steam generator 25 according to a third embodiment of the present invention. Similar to the heat recovery steam generator 21 according to the first embodiment shown in Fig. 4, the heat recovery steam generator 25 according to this embodiment includes a bypass path 18 that bypasses the feedwater to the economizer 6, an aperture-controlled adjustment valve 19 provided in the bypass path 18, and a control device 22 that controls the adjustment valve 19. These components are substantially the same as those of the heat recovery steam generator 21 according to the first embodiment, and therefore the same reference numerals are used and a description thereof will be omitted.

[0034] In the heat recovery boiler 25 of the third embodiment, a boiler feedwater control valve 26 for controlling the level of the drum 9 is provided between the feedwater pump and the economizer 6. Controlling the level of the drum 9 means controlling the amount of water feed to the drum with the boiler feedwater control valve 26 to control the water level of the drum 9. For controlling the level of the drum 9, well-known one-element control or three-element control may be used. One-element control is simple feedback control using only the water level of the drum 9. Three-element control is control using three elements: the amount of water feed to the drum, the water level of the drum 9, and the steam flow rate. (Fourth embodiment)

[0035] FIG. 7 shows a heat recovery steam generator 27 according to a fourth embodiment of the present invention. In the heat recovery steam generator 27 according to this embodiment, multiple economizers 6 are provided, and a bypass passage 18 bypasses only the inlet-side economizers 6. The bypass passage 18 is provided with a degree-of-opening control valve 19. The bypass passage 18, the control valve 19, and the control device 22 are generally identical in configuration to those of the heat recovery steam generator 21 according to the first embodiment, and therefore the same reference numerals are used and their description will be omitted. The temperature detector 31 that detects the exhaust gas temperature (1) after passing through the economizers 6 and the temperature detector 32 that detects the economizer outlet feedwater temperature (2) before joining the bypass passage 18 are also generally identical in configuration to those of the heat recovery steam generator 21 according to the first embodiment. In the heat recovery steam generator 27 according to the fourth embodiment, a temperature detector 33 that detects the feedwater temperature (3) after joining the bypass passage 18 is provided downstream of the outlet-side economizer 6 near the drum 9. In the heat recovery boiler 27 of the fourth embodiment, the inlet side economizer 6 may be configured with two or more economizers arranged in series, or the outlet side economizer 6 may be configured with two or more economizers arranged in series.

[0036] Furthermore, the present invention is not limited to being embodied in the above-described embodiment, and can be embodied in other embodiments without departing from the spirit of the present invention.

[0037] For example, although the incinerator 2 shown in FIG. 1 is a stoker-type incinerator, the incinerator 2 may also be a fluidized bed incinerator, a kiln-type incinerator, a gasification melting furnace, or an ash melting furnace.

[0038] Furthermore, although the economizer 6 shown in FIG. 1 is a separate economizer installed in a flue 14 separate from the boiler body, the economizer 6 may be disposed in the boiler body. [Explanation of symbols]

[0039] 6…Cost saving device 18...Bypass route 19, 24...Control valve (water supply volume control device) 21, 23, 25, 27... Waste heat recovery boiler 22...Control device

Claims

1. an economizer that preheats feedwater and cools exhaust gas; a bypass path for bypassing the water supply to the economizer; a water supply amount adjusting device for adjusting the amount of water supplied to the bypass path and / or the economizer; a control device that controls the feedwater amount adjustment device based on at least the exhaust gas temperature after passing through the economizer, and at least one of the feedwater temperature at the economizer outlet before joining the bypass route and the feedwater temperature after joining the bypass route.

2. 2. The heat recovery boiler according to claim 1, wherein the control device controls the feedwater amount adjustment device based on the exhaust gas temperature after passing through the economizer, the feedwater temperature at the economizer outlet before joining the bypass passage, and the feedwater temperature after joining the bypass passage.

3. 2. The heat recovery boiler according to claim 1, wherein the control device selects the smallest opening degree of the feedwater amount adjustment device among opening degrees of the feedwater amount adjustment device calculated based on at least the exhaust gas temperature after passing through the economizer and at least one of the feedwater temperature at the economizer outlet before joining the bypass route and the feedwater temperature after joining the bypass route.

4. 3. The heat recovery boiler according to claim 2, wherein the control device selects the smallest opening degree of the feedwater amount adjustment device among opening degrees of the feedwater amount adjustment device calculated based on each of the exhaust gas temperature after passing through the economizer, the feedwater temperature at the economizer outlet before joining the bypass route, and the feedwater temperature after joining the bypass route.

5. A feedwater bypass control method for a waste heat recovery boiler including an economizer that preheats feedwater to cool exhaust gas, and a bypass path that bypasses the feedwater to the economizer, comprising: A feedwater bypass control method for controlling the amount of water supplied to the bypass path and / or the economizer based on at least the exhaust gas temperature after passing through the economizer, and at least one of the feedwater temperature at the economizer outlet before joining the bypass path and the feedwater temperature after joining the bypass path.

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