Boiler feedwater temperature control system and method

Abstract

This invention provides a boiler feedwater temperature control system that can prevent low-temperature corrosion of economizers and downstream equipment, and improve heat recovery efficiency. [Solution] The boiler feedwater temperature control system comprises an economizer 6 that preheats the boiler feedwater to cool the exhaust gas generated by the combustion of waste, a deaerator 18 that removes oxygen present in the boiler feedwater to the economizer 6, a steam amount adjustment device 32 that adjusts the amount of steam supplied to the deaerator 18, and a control device 31 that controls the pressure of the deaerator 18 according to the exhaust gas temperature after passing through the economizer 6. The control device 31 increases the pressure of the deaerator 18 when the exhaust gas temperature falls to or below a first temperature, and decreases the pressure of the deaerator 18 when the exhaust gas temperature rises to or above a second temperature, setting the second temperature higher than the first temperature.

Description

Technical Field

[0001] The present invention relates to a feed water temperature control system and method for a boiler.

Background Art

[0002] As shown in FIG. 1, a waste incinerator 2 is provided with an exhaust heat recovery boiler 1 for recovering heat from exhaust gas. The exhaust heat recovery boiler 1 includes a superheater 4, an evaporator 5, and an economizer 6. The superheated steam from the superheater 4 is supplied to a steam turbine. The exhaust steam discharged from the steam turbine is condensed by a condenser 15 and stored in a condensate tank 16. The condensate in the condensate tank 16 is supplied to a deaerator 18 by a deaerator feed water pump 17 and used as boiler feed water to the economizer 6. The deaerator 18 has a role of removing oxygen present in the boiler feed water to prevent corrosion inside the pipes of the exhaust heat recovery boiler 1. In order to heat the boiler feed water of the deaerator 18, the deaerator 18 is supplied with steam stored in a steam header 19. The steam header 19 stores a part of the superheated steam and extraction steam from the steam turbine. In some cases, the steam header 19 may be omitted, and a part of the superheated steam and extraction steam from the steam turbine may be directly supplied to the deaerator 18.

[0003] The boiler feed water of the deaerator 18 is supplied to the economizer 6 by a feed water pump. The boiler feed water preheated in the economizer 6 is supplied to a boiler drum 9 through a drum feed water pipe 8. The boiler feed water supplied to the boiler drum 9 circulates naturally or forcibly in the order of a downcomer 10, an evaporator 5, an evaporator tube 11, and the boiler drum 9, and is heated during this process. The steam generated by heating is separated from water in the boiler drum 9, then sent to the superheater 4 through a drum steam pipe 12, further heated in the superheater 4, and then sent to the steam turbine. Note that the superheater 4 may be arranged in a flue 3 or may be arranged in a flue 14. The same applies to the economizer 6. The evaporator 5 may be composed of a water wall or may be arranged as a unit in a flue 3.

[0004] In waste heat recovery boilers, load fluctuations occur due to variations in waste composition and incineration volume. Load fluctuations are unavoidable, and when the load is light, the exhaust gas temperature drops, raising concerns about low-temperature corrosion in the economizer 6 and downstream equipment (exhaust gas treatment equipment, etc.).

[0005] As a solution to low-temperature corrosion, a heat recovery boiler has been proposed in which a bypass duct 21 is installed in the flue 14 where the economizer 6 is located, bypassing the economizer 6, and a damper 22 is provided in the bypass duct 21 to adjust the flow rate of exhaust gas (see Patent Document 1). When the exhaust gas temperature after joining the bypass duct 21 drops below a set X°C under light load conditions, the control device 23 opens the damper 22, allowing exhaust gas to flow into the bypass duct 21. This raises the exhaust gas temperature after joining the bypass duct 21, preventing low-temperature corrosion of downstream equipment. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-11373 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, in conventional heat recovery boilers with bypass ducts, there is a risk that the damper may become stuck due to dust accumulation when the damper is closed, making it unusable when needed. From the perspective of power generation, there is always leakage in the gas bypass (about 1% bypasses continuously), which reduces the heat recovery rate. In addition, more space is required for routing the flue. Furthermore, there was a problem that the exhaust gas temperature drops before joining the bypass duct 21, which promotes low-temperature corrosion of the economizer 6 and flue 14.

[0008] This invention has been made in view of the above problems, and aims to provide a boiler feedwater temperature control system and method that can prevent low-temperature corrosion of economizers and downstream equipment and improve heat recovery efficiency. [Means for solving the problem]

[0009] To solve the above problems, one aspect of the present invention is a boiler feedwater temperature control system comprising: an economizer that preheats boiler feedwater to cool exhaust gas generated by the combustion of waste; a deaerator that removes oxygen present in the boiler feedwater to the economizer; a steam amount adjustment device that adjusts the amount of steam supplied to the deaerator; and a control device that controls the pressure of the deaerator according to the exhaust gas temperature after passing through the economizer, wherein the control device increases the pressure of the deaerator when the exhaust gas temperature falls to or below a first temperature, and decreases the pressure of the deaerator when the exhaust gas temperature rises to or above a second temperature, and sets the second temperature higher than the first temperature.

[0010] Another aspect of the present invention is a boiler feedwater temperature control method comprising: an economizer preheating boiler feedwater to cool exhaust gas generated by the combustion of waste; a deaerator removing oxygen present in the boiler feedwater to the economizer; a steam amount adjustment device adjusting the amount of steam supplied to the deaerator; and controlling the pressure of the deaerator according to the exhaust gas temperature after passing through the economizer, wherein the pressure of the deaerator is increased when the exhaust gas temperature falls to or below a set first temperature, and the pressure of the deaerator is decreased when the exhaust gas temperature rises to or above a second temperature set higher than the first temperature. [Effects of the Invention]

[0011] According to the present invention, the pressure of the deaerator is controlled according to the exhaust gas temperature after passing through the economizer. When the exhaust gas temperature falls to or below the first temperature, the pressure of the deaerator is increased (i.e., the boiler feedwater temperature is increased). This prevents low-temperature corrosion of the economizer and downstream equipment. Furthermore, when the exhaust gas temperature rises to or above the second temperature, the pressure of the deaerator is decreased (i.e., the boiler feedwater temperature is decreased). This improves the heat recovery rate. Moreover, since the second temperature at which the deaerator pressure is decreased is set higher than the first temperature at which the deaerator pressure is increased, stable operation of the deaerator can be ensured even if the waste quality and incineration amount (i.e., load) of the waste incinerator fluctuates, and consequently, stable operation of the waste heat recovery boiler can be ensured. [Brief explanation of the drawing]

[0012] [Figure 1] This diagram shows a conventional incinerator and a waste heat recovery boiler. [Figure 2] This diagram shows a conventional gas bypass. [Figure 3] This figure shows the boiler feedwater temperature control system according to this embodiment. [Figure 4] This is a flowchart of the control device's processing. [Figure 5] This graph illustrates the switching of the deaerator pressure (i.e., the switching of the boiler feedwater temperature). [Modes for carrying out the invention]

[0013] Hereinafter, a boiler feedwater temperature control system and method according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the boiler feedwater temperature control system and method of the present invention can be embodied in various forms and are not limited to the embodiments described herein. These embodiments are provided with the intention that those skilled in the art will be able to fully understand the invention by making full disclosures in the specification.

[0014] As shown in Figure 3, the boiler feedwater temperature control system according to an embodiment of the present invention comprises an economizer 6, a deaerator 18, a steam volume adjustment device 32, a control device 31, and a controller 35.

[0015] The boiler feedwater temperature control system according to this embodiment is part of the waste heat recovery boiler. As shown in Figure 1, the waste heat recovery boiler 1 includes a superheater 4, an evaporator 5, an economizer 6, a condenser 15, a condensate tank 16, a deaerator feedwater pump 17, a deaerator 18, and a steam reservoir 19.

[0016] As shown in Figure 3, the steam reservoir 19 stores a portion of the superheated steam and extracted steam from the steam turbine. Also as shown in Figure 3, the amount of steam supplied from the steam reservoir 19 to the deaerator 18 is regulated by a steam flow rate adjustment device 32, such as an electric valve. The deaerator 18 is also supplied with condensate from the condensate tank 16 shown in Figure 1 by the deaerator feedwater pump 17. The deaerator 18 removes oxygen present in the boiler feedwater within the deaerator 18. The boiler feedwater for the deaerator 18 is supplied to the economizer 6 by the feedwater pump 33. The economizer 6 preheats the boiler feedwater to cool the exhaust gas generated by the combustion of waste.

[0017] The control device 31 controls the pressure of the deaerator 18 according to the exhaust gas temperature after passing through the economizer 6. The control device 31 receives the exhaust gas temperature after passing through the economizer 6 as measured by a thermometer 34 installed in the flue 14. When the exhaust gas temperature after passing through the economizer 6 is below or equal to the first temperature, the control device 31 increases the pressure of the deaerator 18, and when the exhaust gas temperature is above or equal to the second temperature, or higher than the second temperature, it decreases the pressure of the deaerator 18. The second temperature is set to a temperature higher than the first temperature. More precisely, when the exhaust gas temperature is below or equal to the first temperature, the control device 31 increases the set value of the deaerator 18 pressure from the first pressure to the second pressure, and when the exhaust gas temperature is above or equal to the second temperature, or higher than the second temperature, it decreases the set value of the deaerator 18 pressure from the second pressure to the first pressure. The control device 31 is, for example, a central control device for an incinerator.

[0018] The controller 35 performs PID control or the like on the opening degree of the steam amount adjusting device 32 so that the pressure of the deaerator 18 follows the set value of the pressure calculated by the control device 31. The set value of the pressure calculated by the control device 31 is input to the controller 35. Further, the pressure of the deaerator 18 measured by the pressure gauge 36 installed in the deaerator 18 is input to the controller 35.

[0019] Figure 4 shows a flowchart of the processing of the control device 31. The exhaust gas temperature T_PV after passing through the economizer 6 measured by the thermometer 34 is input to the control device 31 (S1). The control device 31 obtains the time average of the exhaust gas temperature T_PV, and when the time average of the exhaust gas temperature T_PV is less than or equal to the first temperature or less than the first temperature (S2), it is determined whether this state continues for, for example, 3 to 7 seconds by an on-delay timer (S3). If it continues, the set value P_SV of the pressure is increased from the first pressure to the second pressure (S4). The first temperature is set to 140 °C or higher, preferably 150 °C or higher, for example 155 °C. The first pressure is, for example, 0.2 MPaG, and the second pressure is, for example, 0.3 MPaG.

[0020] When the time average of the exhaust gas temperature T_PV is higher than or equal to the second temperature or higher than the second temperature (S5), the control device 31 determines whether this state continues for, for example, 3 to 7 seconds by an on-delay timer (S6). If it continues, the set value P_SV of the pressure is decreased from the second pressure to the first pressure (S7). The second temperature is set to 200 °C or lower, preferably 180 °C or lower, for example 166 °C.

[0021] Also, since there is a relationship between the feed water temperature of the boiler = the saturation temperature of the deaerator 18, the feed water temperature of the boiler can be obtained from the pressure of the deaerator 18. The feed water temperature of the boiler is the first feed water temperature (= 133 °C) when the pressure of the deaerator 18 is the first pressure (= 0.2 MPaG), and the second feed water temperature (= 143 °C) when the pressure of the deaerator 18 is the second pressure (= 0.3 MPaG).

[0022] Figure 5 is a graph showing the relationship between boiler heat input (=load), economizer outlet exhaust gas temperature (=exhaust gas temperature after passing through the economizer), and the first and second feedwater temperatures (=feedwater temperature). Boiler heat input is expressed as the product of the lower heating value of waste and the amount of waste incinerated, and it fluctuates moment by moment. When boiler heat input increases, the economizer outlet exhaust gas temperature increases. On the other hand, when boiler heat input decreases, the economizer outlet exhaust gas temperature decreases. Also, when the boiler feedwater temperature is raised from the first feedwater temperature (=133°C) to the second feedwater temperature (=143°C), the economizer outlet exhaust gas temperature increases. On the other hand, when the boiler feedwater temperature is lowered from the second feedwater temperature (=143°C) to the first feedwater temperature (=133°C), the economizer outlet exhaust gas temperature decreases.

[0023] As shown in Figure 5, when the exhaust gas temperature at the economizer outlet falls below or below the first temperature (=155°C), the control device 31 raises the pressure of the deaerator 18 from the first pressure (=0.2 MaPG) to the second pressure (=0.3 MaPG) and raises the boiler feedwater temperature from the first feedwater temperature (=133°C) to the second feedwater temperature (=143°C). This raises the exhaust gas temperature at the economizer outlet, preventing low-temperature corrosion of the economizer 6 and its downstream equipment.

[0024] On the other hand, when the exhaust gas temperature at the economizer outlet rises above the second temperature (=166°C) or above the second temperature (=166°C), the control device 31 lowers the pressure in the deaerator 18 from the second pressure (=0.3MaPG) to the first pressure (=0.2MaPG) and lowers the boiler feedwater temperature from the second feedwater temperature (=143°C) to the first feedwater temperature (=133°C). This improves the heat recovery rate of the economizer 6.

[0025] Here, the second temperature (=166°C) used to lower the pressure of the deaerator 18 is set higher than the first temperature (=155°C) used to raise the pressure of the deaerator 18. This ensures stable operation of the deaerator 18 even when the boiler heat input fluctuates, and consequently, stable operation of the waste heat recovery boiler.

[0026] Furthermore, by setting the first temperature to a predetermined temperature of 140°C or higher, preferably 150°C or higher, it is possible to prevent the tube wall temperature of the economizer 6 from dropping below the acid dew point temperature, thereby preventing low-temperature corrosion of the economizer 6 and its downstream equipment. The acid dew point temperature is the temperature at which sulfuric acid gas (H2SO4) condenses into a liquid when it comes into contact with a low-temperature metal surface. If the tube wall temperature of the economizer 6 drops below the acid dew point temperature, there is a concern that the economizer 6 and its downstream equipment may undergo low-temperature corrosion.

[0027] By setting the second temperature to a predetermined temperature of 200°C or lower, preferably 180°C or lower, the time during which the boiler feedwater temperature is switched to the first feedwater temperature (=133°C) can be extended, thereby improving the heat recovery rate of the economizer 6.

[0028] In the above embodiment, the exhaust gas temperature after passing through the economizer is measured using a thermometer, but the exhaust gas temperature after passing through the economizer may also be estimated from the heat exchange rate of the economizer. The heat exchange rate of the economizer can be determined from the exhaust gas temperature before passing through the economizer, the exhaust gas flow rate, the boiler feedwater temperature, the feedwater flow rate, etc. In addition to the boiler feedwater temperature control described above, a bypass duct that bypasses the economizer or a bypass feedwater pipe that bypasses the economizer may also be installed. [Explanation of symbols]

[0029] 6…Economizer 18… Deaeration device 31...Control device 32... Steam volume adjustment device

Claims

1. The system comprises an economizer that preheats the boiler feedwater to cool the exhaust gas generated by the combustion of waste, a deaerator that removes oxygen present in the boiler feedwater to the economizer, a steam amount adjustment device that adjusts the amount of steam supplied to the deaerator, and a control device that controls the pressure of the deaerator according to the exhaust gas temperature after passing through the economizer. The control device is When the exhaust gas temperature falls to or below the first temperature, the pressure of the deaerator is increased. When the exhaust gas temperature rises to or above the second temperature, the pressure in the deaerator is reduced. A boiler feedwater temperature control system in which the second temperature is set to a temperature higher than the first temperature.

2. The first temperature is set to a predetermined temperature of 140°C or higher. The boiler feedwater temperature control system according to claim 1, characterized in that the second temperature is set to a predetermined temperature of 200°C or less.

3. The economizer preheats the boiler feedwater to cool the exhaust gas generated by the combustion of waste. The deaerator removes oxygen present in the boiler feedwater to the economizer. The steam volume adjustment device adjusts the amount of steam supplied to the deaerator. A boiler feedwater temperature control method that controls the pressure of the deaerator according to the exhaust gas temperature after passing through the economizer, When the exhaust gas temperature falls to or below the set first temperature, the pressure of the deaerator is increased. A boiler feedwater temperature control method that reduces the pressure of the deaerator when the exhaust gas temperature rises to or above a second temperature set higher than the first temperature, or when it rises above the second temperature.

Images