Temperature control method for exhaust gas from boiler

Abstract

To prevent generation of drain inside a chimney for passing exhaust gas from a boiler.SOLUTION: There is provided a temperature control method for exhaust gas 15 exhausted from a boiler 11 and having passed through an exhaust gas economizer 21 heating feed water 23 to the boiler 11 by heat exchange. In the temperature control method for the exhaust gas 15, when the temperature of the exhaust gas 15 having passed through the exhaust gas economizer 21 does not lower to less than a set value, the flow direction of the feed water is controlled in such a manner that the flow direction of the exhaust gas 15 and the flow direction of the feed water 23 are opposed to each other in the exhaust gas economizer 21; and when the temperature of the exhaust gas 15 having passed through the exhaust gas economizer 21 lowers to or is predicted to lower less than set temperature, the flow direction of the feed water 23 is controlled in such manner that a flow direction of the exhaust gas 15 and the flow direction of the feed water 23 are in parallel with each other in the exhaust gas economizer 21, consequently the temperature of the waste gas leaving the economizer 21 does not go down to the set value.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for controlling the temperature of exhaust gas from a boiler.

Background Art

[0002] It is known to improve the boiler efficiency by passing the exhaust gas discharged from the boiler through an exhaust gas economizer that heats the feed water to the boiler by heat exchange, thereby transferring the thermal energy possessed by the exhaust gas to the feed water to the boiler (for example, Patent Document 1).

[0003] At this time, in order to improve the boiler efficiency, it is necessary to transfer the thermal energy possessed by the exhaust gas to the feed water as much as possible. On the other hand, the exhaust gas from the boiler is discharged into the atmosphere through the flue and chimney. In this case, the chimney is usually installed in a state exposed to the atmosphere. Therefore, although the high-temperature exhaust gas passes through the inside of the chimney, the chimney is constantly cooled by the atmosphere.

[0004] As a result, when the temperature of the exhaust gas passing through the chimney is too low and falls below a certain value, the water vapor component in the exhaust gas comes into contact with the inner wall surface of the chimney, loses heat, and condenses, which may cause drain to occur inside the chimney. This drain has corrosive components that corrode the materials constituting the chimney. Therefore, the generation of drain inside the chimney must be avoided as much as possible.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Therefore, the technical problem of the present invention is to prevent the generation of drain inside the chimney through which the exhaust gas from the boiler passes.

Means for Solving the Problem

[0007] To solve this problem, the present invention when controlling the temperature of the exhaust gas after passing through an exhaust gas economizer that is discharged from a boiler and heats the feed water to the boiler by heat exchange, when the temperature of the exhaust gas exiting the exhaust gas economizer does not drop below a set value, in the exhaust gas economizer, the flow direction of the feed water is controlled so that the flow direction of the exhaust gas and the flow direction of the feed water are opposite to each other, when the temperature of the exhaust gas exiting the exhaust gas economizer drops below the set value, or when it is predicted that the temperature of the exhaust gas exiting the exhaust gas economizer will drop below the set value, in the exhaust gas economizer, the flow direction of the feed water is controlled so that the flow direction of the exhaust gas and the flow direction of the feed water are parallel to each other, so that the temperature of the exhaust gas exiting the exhaust gas economizer does not drop below the set value.

[0008] In the exhaust gas economizer, when the flow direction of the exhaust gas and the flow direction of the feed water are in a countercurrent pattern where they are opposite to each other, the thermal energy possessed by the exhaust gas can be efficiently transferred to the feed water. On the other hand, when the flow direction of the exhaust gas and the flow direction of the feed water are in a cocurrent pattern where they are parallel to each other, the energy transfer efficiency is lower than that of the countercurrent pattern, and the efficiency as high as that of the countercurrent pattern cannot be obtained.

[0009] The present invention successfully incorporates these points into the temperature control of the exhaust gas from the boiler. That is, when the temperature of the exhaust gas exiting the exhaust gas economizer does not drop below the set value, it is controlled to be in a countercurrent pattern to efficiently transfer the thermal energy possessed by the exhaust gas to the feed water. On the other hand, when the temperature of the exhaust gas exiting the exhaust gas economizer drops below the set value, or when it is predicted that the temperature of the exhaust gas exiting the exhaust gas economizer will drop below the set value, by controlling it to be in a cocurrent pattern, the temperature of the exhaust gas exiting the exhaust gas economizer is prevented from dropping too much, and the generation of drain inside the chimney is prevented.

[0010] According to the present invention, when the temperature of the exhaust gas exiting the exhaust gas economizer drops below the set value, or when it is predicted that the temperature of the exhaust gas exiting the exhaust gas economizer will drop below the set value, it is preferable to pass a part of the feed water through the exhaust gas economizer and supply the remaining part of the feed water directly to the boiler without passing it through the exhaust gas economizer.

[0011] By doing so, compared with the case where the entire amount of feed water is passed through the exhaust gas economizer, the degree to which the thermal energy possessed by the exhaust gas is transferred to the feed water to the boiler further decreases. Therefore, it is possible to more effectively prevent the temperature of the exhaust gas exiting the exhaust gas economizer from dropping too much, and it is possible to surely prevent the generation of drain inside the chimney.

Effect of the Invention

[0012] According to the exhaust gas temperature control method of the boiler of the present invention, it is possible to prevent the generation of drain inside the chimney through which the exhaust gas from the boiler passes.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0014] The boiler 11 shown in Fig. 1 has a main body 12 and a burner 14 for generating a flame 13. An exhaust gas passage 16 for guiding the exhaust gas 15 from this boiler 11 is connected to the boiler 11. Also, a water supply passage 17 to this boiler 11 is connected to the boiler 11.

[0015] In the exhaust gas passage 16, a flue 18 and a chimney 19 are provided. In the middle of the flue 18, an exhaust gas economizer 21 is provided. Inside the exhaust gas economizer 21, a water passage 22 composed of pipes for heat exchange and the like is provided. The exhaust gas 15 from the boiler 11 is sent into the chimney 19 after passing through the flue 18 and the exhaust gas economizer 21, and is discharged into the atmosphere after passing through the chimney 19.

[0016] A water supply pump 24 is provided in the water supply passage 17 for passing the water supply 23. The portion of the water supply passage 17 downstream of the water supply pump 24 is connected to one end of the water passage 22 in the exhaust gas economizer 21. The water supply passage 17 from the other end of the water passage 22 in the exhaust gas economizer 21 is connected to the main body 12 of the boiler 11. The water supply 23 is sent into the water passage 22 of the exhaust gas economizer 21 by the water supply pump 24, and when passing through the water passage 22, it is heat-exchanged with the exhaust gas 15 passing through the exhaust gas economizer 21 and its temperature is raised.

[0017] The exhaust gas 15 heat-exchanged with the water supply 23 inside the water passage 22 is sent into the downstream flue 18 and chimney 19 from the outlet portion 25 of the exhaust gas economizer 21 in a state where its temperature has decreased accordingly.

[0018] An exhaust gas temperature sensor 26 for detecting the temperature of the exhaust gas 15 at this outlet portion 25, that is, the temperature of the exhaust gas 15 sent from the exhaust gas economizer 21 to the chimney 19, is provided at the outlet portion 25 of the exhaust gas economizer 21.

[0019] A water supply temperature sensor 27 for detecting the temperature of the water supply 23 sent into the exhaust gas economizer 21 is provided. In the illustrated example, the water supply temperature sensor 27 is installed on the water supply pump 24.

[0020] A method for controlling the temperature of the exhaust gas 15 in the apparatus of FIG. 1 will be described.

[0021] As shown in FIG. 2(b), FIG. 2 represents a countercurrent configuration in which the flow direction 31 of the exhaust gas 15 inside the exhaust gas economizer 21 and the flow direction 32 of the feed water 23 in the water passage 22 are opposite to each other. FIG. 2(a) is a graph showing the relationship between the position and temperature of the heating surface in FIG. 2(b). The position of the heating surface on the horizontal axis of FIG. 2(a) corresponds one-to-one to the position of the water passage 22 along the flow direction 31 of the exhaust gas 15 in FIG. 2(b).

[0022] As shown in FIG. 2(a), when the exhaust gas 15 passes through the installation part of the water passage 22 shown in FIG. 2(b), its temperature drops from T1 to T2 by heat exchange with the feed water 23. As shown in the figure, the temperature T2 can be lowered to near the temperature t1 of the feed water 23 at the feed water inlet of the water passage 22, for example, near 60°C. When the feed water 23 passes through the water passage 22, its temperature rises from t1 to t2 by heat exchange with the exhaust gas 15.

[0023] As shown in FIG. 3(b), FIG. 3 represents a cocurrent configuration in which the flow direction 31 of the exhaust gas 15 inside the exhaust gas economizer 21 and the flow direction 32 of the feed water 23 in the water passage 22 are parallel to each other. Similar to the case of FIG. 2, FIG. 3(a) is a graph showing the relationship between the position and temperature of the heating surface in FIG. 3(b). The position of the heating surface on the horizontal axis of FIG. 3(a) corresponds one-to-one to the position of the water passage 22 along the flow direction 31 of the exhaust gas 15 in FIG. 3(b).

[0024] As shown in FIG. 3(a), when the exhaust gas 15 passes through the installation part of the water passage 22 shown in FIG. 3(b), its temperature drops from T1 to T2 by heat exchange with the feed water 23. When the feed water 23 passes through the water passage 22, its temperature rises from t1 to t2 by heat exchange with the exhaust gas 15.

[0025] As described above, in the exhaust gas economizer 21, when the flow direction 31 of the exhaust gas 15 and the flow direction 32 of the feed water 23 are in a countercurrent configuration where they face each other as shown in FIG. 2, the thermal energy possessed by the exhaust gas 15 can be efficiently transferred to the feed water 23. In contrast, when the flow direction 31 of the exhaust gas 15 and the flow direction 32 of the feed water 23 are in a parallel flow configuration where they are parallel to each other as shown in FIG. 3, the energy transfer efficiency is lower than that of the countercurrent configuration, and the efficiency as high as that of the countercurrent configuration cannot be obtained.

[0026] The result is manifested in the temperature change of the exhaust gas 15 in FIGS. 2(a) and 3(a). That is, in the case of the countercurrent configuration shown in FIG. 2, the exhaust gas temperatures T1 and T2 change significantly before and after passing through the portion of the water passage 22. In contrast, in the case of the parallel flow configuration shown in FIG. 3, the exhaust gas temperatures T1 and T2 do not change as much as in the case of FIG. 2(a). That is, even if the exhaust gas temperature T1 before passing through the portion of the water passage 22 is the same in the cases of FIG. 2 and FIG. 3, in the case of FIG. 3, the exhaust gas temperature T2 after passing through the portion of the water passage 22 does not decrease as much as in the case of FIG. 2. Therefore, when the exhaust gas temperature T1 before passing through the portion of the water passage 22 is the same, the exhaust gas 15 sent into the chimney 19 becomes hotter in the case of the parallel flow configuration shown in FIG. 3 than in the case of the countercurrent configuration shown in FIG. 2.

[0027] Depending on its structure and other factors, there is a set value for the temperature of the exhaust gas 15 sent from the boiler 11 to prevent the generation of condensate inside the chimney 19. When operating in a countercurrent mode to efficiently operate the exhaust gas economizer 21, when the exhaust gas temperature detected by the exhaust gas temperature sensor 26 is equal to or higher than the set value, specifically, when it is actually equal to or higher than the set value, or when it is predicted to be equal to or higher than the set value, the operation in the countercurrent mode is continued as it is by controlling with a control device (not shown). On the other hand, when the exhaust gas temperature detected by the exhaust gas temperature sensor 26 is lower than the set value, or when it is predicted that the temperature of the exhaust gas 15 exiting the exhaust gas economizer 21 will drop below the set value due to the operating conditions of the boiler 11 or other factors, the operation is switched from the countercurrent mode to the parallel flow mode by the above-mentioned control device. As a result, the temperature of the exhaust gas 15 sent into the chimney 19 can be made equal to or higher than the set value, and the generation of condensate inside the chimney 19 can be prevented. After that, when the exhaust gas temperature detected by the exhaust gas temperature sensor 26 exceeds the set value by a certain degree, or when it is predicted that the exhaust gas temperature will exceed the set value by a certain degree, it is possible to switch the operation of the exhaust gas economizer 21 back to the countercurrent mode without any problems.

[0028] As a specific example of the set value, for example, 120°C can be cited. This set value varies depending on the structure and specifications of the chimney 19, but 120°C is a typical value. If the exhaust gas economizer 21 is used effectively, it is possible to further lower the temperature of the exhaust gas 15 exiting the exhaust gas economizer 21. However, in the present invention, the temperature of the exhaust gas 15 exiting the exhaust gas economizer 21 is not lowered too much to prevent the generation of corrosive condensate in the chimney 19.

[0029] When the exhaust gas temperature detected by the exhaust gas temperature sensor 26 is less than the set value, or when it is predicted that the temperature of the exhaust gas exiting the exhaust gas economizer will drop below the set value, examples include when the boiler load decreases or when it is predicted that the boiler load will decrease. Also, the temperature of the exhaust gas 15 from the exhaust gas economizer 21 varies depending on the temperature and flow rate of the feed water 23. For this reason, by referring to the detection results of the illustrated feed water temperature sensor 27 and the detection results of a feed water flow rate sensor (not shown), it is possible to more efficiently switch between countercurrent mode operation and cocurrent mode operation.

[0030] Alternatively, instead of the above, when the exhaust gas temperature is less than the set value, or when it is predicted that the exhaust gas temperature will be less than the set value, it is also possible to control to reduce the flow rate of the feed water 23 while continuing the operation in countercurrent mode. However, since the flow rate of the feed water 23 varies depending on the boiler load, there may be cases where the flow rate of the feed water 23 cannot be reduced for the control of the temperature of the exhaust gas 15. In contrast, when switching between countercurrent mode operation and cocurrent mode operation in the present invention, without changing the flow rate of the feed water 23, as shown in FIGS. 2 and 3, only the flow direction 32 of the feed water 23 in the exhaust gas economizer 21 is changed, so that the generation of drain in the chimney 19 can be simply and surely prevented.

[0031] Moreover, since it switches between countercurrent mode operation and cocurrent mode operation, it is possible to prevent the generation of drain in the chimney 19 without significantly changing the structure of the exhaust gas passage 16 and the feed water passage 17. In contrast, in principle, it is also possible to prevent the generation of drain in the chimney 19 without lowering the exhaust gas temperature to the chimney 19 by changing the amount of exhaust gas 15 passing through the exhaust gas economizer 21. However, in that case, there are drawbacks such as the need for a special structure for bypassing the exhaust gas from the exhaust gas economizer, resulting in a significant structural change.

[0032] A configuration example for switching between countercurrent mode operation and cocurrent mode operation will be described.

[0033] As shown in FIGS. 4, 5, and 2(b), the portion of the water supply path 17 downstream of the water supply pump 24 shown in FIG. 1 is connected to the countercurrent water supply path 35 to the exhaust gas economizer 21. The countercurrent water supply path 35 is connected to the downstream end 36 along the flow direction 31 of the exhaust gas 15 in the water passage 22 of the exhaust gas economizer 21. An upstream end 37 along the flow direction 31 of the exhaust gas 15 in the water passage 22 of the exhaust gas economizer 21 is connected to the countercurrent water supply discharge path 38 from the exhaust gas economizer 21. A portion of the water supply path 17 connecting the exhaust gas economizer 21 and the boiler 11 is connected to the countercurrent water supply discharge path 38. A first on-off valve 39 is provided in the countercurrent water supply path 35, and a second on-off valve 40 is provided in the countercurrent water supply discharge path 38.

[0034] A parallel flow water supply path 41 and a parallel flow water supply discharge path 42 are provided between the countercurrent water supply path 35 and the countercurrent water supply discharge path 38. One end of the parallel flow water supply path 41 is connected to a portion upstream of the first on-off valve 39 in the countercurrent water supply path 35, and the other end is connected to a portion upstream of the second on-off valve 40 in the countercurrent water supply discharge path 38. A three-way valve 43 and a third on-off valve 44 are provided in the parallel flow water supply path 41. The third on-off valve 44 is installed closer to the upstream end 37 along the flow direction 31 of the exhaust gas 15 in the water passage 22 than the three-way valve 43. One end of the parallel flow water supply discharge path 42 is connected to a portion downstream of the first on-off valve 39 in the countercurrent water supply path 35, and the other end is connected to a portion downstream of the second on-off valve 40 in the countercurrent water supply discharge path 38. A fourth on-off valve 45 is provided in the parallel flow water supply discharge path 42.

[0035] The three-way valve 43 has its first port 46 communicating with the counter-flow water supply path 35 via the parallel-flow water supply path 41, and its second port 47 communicating with the third on-off valve 44 via the parallel-flow water supply path 41. The third port 48 of the three-way valve 43 communicates, via the bypass path 49, with a portion of the parallel-flow water discharge path 42 that is farther from the downstream end 36 along the flow direction 31 of the exhaust gas 15 in the water passage 22 than the fourth on-off valve 45. A check valve 50 for preventing backflow is provided in the bypass path 49. The three-way valve 43 can distribute the water supply 23 that has flowed into the first port 46 through the counter-flow water supply path 35 and the parallel-flow water supply path 41 to the second port 47 and the third port 48.

[0036] In such a configuration, when performing counter-flow operation, as shown in FIG. 4, the first on-off valve 39 and the second on-off valve 40 are opened, and the third on-off valve 44 and the fourth on-off valve 45 are closed. Then, due to the operation of the water supply pump 24 shown in FIG. 1, the water supply 23 enters the counter-flow water supply path 35 from the water supply path 17. The water supply 23 that has entered the counter-flow water supply path 35 enters the water passage 22 of the exhaust gas economizer 21 from the downstream end 36 along the flow direction 31 of the exhaust gas 15 in the water passage 22 through the first on-off valve 39, as shown in FIGS. 4 and 2(b), and flows inside the water passage 22 from the downstream side to the upstream side along the flow direction 31 of the exhaust gas 15 in the exhaust gas economizer 21, along the flow direction 32 that is opposite to the flow direction 31 of the exhaust gas 15. As a result, the water supply 23 receives thermal energy from the exhaust gas 15 through heat exchange with the exhaust gas 15 and its temperature rises. The water supply 23 with the increased temperature is supplied to the boiler 11 shown in FIG. 1 from the upstream end 37 along the flow direction 31 of the exhaust gas 15 in the water passage 22 through the second on-off valve 40, the counter-flow water discharge path 38, and the water supply path 17.

[0037] When operating in the co-current mode, as shown in Fig. 5, the third on-off valve 44 and the fourth on-off valve 45 are opened, while the first on-off valve 39 and the second on-off valve 40 are closed. The three-way valve 43 is set so that all of the feed water 23 entering the first port 46 is distributed to the second port 47. Then, as shown in Figs. 5 and 3(b), the feed water 23 from the water supply passage 17 enters the co-current feed water supply passage 41, and all of it reaches the upstream end 37 in the water passage 22 through the three-way valve 43 and the third on-off valve 44. Then the feed water 23 enters the water passage 22 and flows through the inside of the water passage 22 in the flow direction 32 parallel to the flow direction 31 of the exhaust gas 15 from the upstream side to the downstream side along the flow direction 31 of the exhaust gas 15 in the exhaust gas economizer 21. At this time, the feed water 23 receives less thermal energy from the exhaust gas 15 and is heated to a lower temperature than in the case of the counter-current mode operation described above. As a result, the exhaust gas 15 is discharged from the exhaust gas economizer 21 at a higher temperature than in the case of the counter-current mode operation and is sent into the chimney 19.

[0038] If all of the feed water 23 is sent into the water passage 22 of the exhaust gas economizer 21 as described above, and the temperature of the exhaust gas 15 discharged from the exhaust gas economizer 21 does not rise sufficiently to the required temperature, that is, if the temperature of the exhaust gas 15 is below the set value, the operation described below is performed. That is, the three-way valve 43 is controlled so that a part of the feed water 23 entering the co-current feed water supply passage 41 is sent from the second port 47 into the water passage 22, and the remainder of the feed water 23 entering the co-current feed water supply passage 41 is not sent into the water passage 22 but is sent from the third port 48 through the bypass passage 49 into the co-current feed water discharge passage 42. By doing so, the amount of the feed water 23 flowing through the water passage 22 can be reduced, and the temperature of the exhaust gas 15 discharged from the exhaust gas economizer 21 can be raised to the required temperature. Moreover, the total amount of the feed water 23 that has passed through the water passage 22 and the feed water 23 that has passed through the bypass passage 49 can be supplied to the boiler 11. Therefore, even though the amount of the feed water 23 for heat exchange in the exhaust gas economizer 21 is reduced, the required amount of the feed water 23 can be supplied to the boiler 11.

[0039] The above configuration example for switching between the countercurrent mode of operation and the cocurrent mode of operation is merely an example of a configuration example that can be used in the present invention. For this switching, any other configuration can be adopted as long as it exhibits the above-described functions.

Explanation of Reference Numerals

[0040] 11 Boiler 15 Exhaust Gas 21 Exhaust Gas Economizer 22 Water Passage 23 Feed Water 31 Flow Direction 32 Flow Direction 35 Countercurrent Feed Water Supply Path 38 Countercurrent Feed Water Discharge Path 41 Cocurrent Feed Water Supply Path 42 Cocurrent Feed Water Discharge Path 43 Three-Way Valve 49 Bypass Path

Claims

1. When controlling the temperature of the exhaust gas after passing through an exhaust gas economizer that discharges from a boiler and heats the feed water to the boiler by heat exchange, when the temperature of the exhaust gas exiting the exhaust gas economizer does not drop below the set value, in the exhaust gas economizer, the flow direction of the feed water is controlled so that the flow direction of the exhaust gas and the flow direction of the feed water are opposite to each other, when the temperature of the exhaust gas exiting the exhaust gas economizer drops below the set value, or when it is predicted that the temperature of the exhaust gas exiting the exhaust gas economizer will drop below the set value, in the exhaust gas economizer, the flow direction of the feed water is controlled so that the flow direction of the exhaust gas and the flow direction of the feed water are parallel to each other, so that the temperature of the exhaust gas exiting the exhaust gas economizer does not drop below the set value. A method for controlling the temperature of exhaust gas from a boiler, characterized in that.

2. When the temperature of the exhaust gas exiting the exhaust gas economizer drops below the set value, or when it is predicted that the temperature of the exhaust gas exiting the exhaust gas economizer will drop below the set value, a part of the feed water is passed through the exhaust gas economizer, and the remaining part of the feed water is directly supplied to the boiler without passing through the exhaust gas economizer. The method for controlling the temperature of exhaust gas from a boiler according to claim 1, characterized in that.

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