Heat recovery system of reduction furnace
By designing a combined heat recovery system for tail gas and chassis heat exchange units in polysilicon production, and using flange connections to separate the ring pipes, the problem of low heat recovery efficiency in the reduction furnace was solved, and high-efficiency production of high-grade saturated steam was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-11
- Publication Date
- 2026-03-24
AI Technical Summary
In existing polysilicon production processes, it is difficult to maximize heat recovery methods, especially the radiant heat from reduction furnaces, which is difficult to effectively recover and utilize.
Design a heat recovery system for a reduction furnace. By combining a tail gas heat exchange unit and a chassis heat exchange unit, and using flange connections to divide the ring pipe into two parts, the system combines the radiant heat from the tail gas and the chassis to generate higher-grade saturated steam, thereby improving heat recovery efficiency.
It increases the production of higher-grade saturated steam, improves the heat recovery and utilization rate, and has low retrofit costs, requiring no large-scale structural modifications.
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Figure CN224034403U_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of polycrystalline silicon production, and in particular to a heat recovery system of a reduction furnace. BACKGROUND
[0002] At present, the mainstream polycrystalline silicon production process adopts a modified Siemens method. High-purity trichlorosilane is sent into a trichlorosilane vaporizer, and after vaporization, it is overheated. High-purity preheated hydrogen gas from outside the boundary zone is used to control and adjust the feed molar ratio of trichlorosilane gas and hydrogen gas through a flowmeter. The mixed gas is uniformly mixed in a static mixer, enters a reduction furnace through a nozzle, and undergoes a deposition reaction in the furnace to reduce silicon.
[0003] In the related art, different temperature cooling water needs to be used in the reduction section to remove radiant heat, mainly including radiant heat of the furnace cylinder, the bottom plate and the tail gas. The recovered radiant heat is used to flash different levels of steam. At present, the bottom plate cooling water, the furnace cylinder cooling water and the tail gas jacket pipe cooling water of the reduction section are separately supplied with water. The heat recovery of the bottom plate is used to flash low-grade saturated steam, and the heat recovery of the furnace cylinder and the tail gas is used to flash high-grade saturated steam. This recovery mode is difficult to maximize the recovery of heat. CONTENT OF THE UTILITY MODEL
[0004] Therefore, the present application provides a heat recovery system of a reduction furnace to solve the problems in the related art.
[0005] The heat recovery system of the reduction furnace provided by the present application is used to recover heat of the reduction furnace. The reduction furnace includes a furnace cylinder and a bottom plate. The furnace heat recovery system includes a tail gas heat exchange unit and a bottom plate heat exchange unit. The bottom plate heat exchange unit includes a ring pipe and a flange connecting piece. The ring pipe includes a first ring pipe and a second ring pipe. The first ring pipe is arranged close to the furnace cylinder, and the second ring pipe is arranged away from the furnace cylinder. The first ring pipe and the second ring pipe are separated by the flange connecting piece.
[0006] The tail gas heat exchange unit includes a first tail gas jacket and a first flash tank. The first tail gas jacket is connected to the second ring pipe, and the second ring pipe is connected to the first flash tank.
[0007] In a possible implementation, the tail gas heat exchange unit further includes a connecting pipe. The first tail gas jacket is connected to the second ring pipe through the connecting pipe.
[0008] In a possible implementation, the bottom plate heat exchange unit further includes a bottom plate jacket and a second flash tank. The first ring pipe is connected to the bottom plate jacket, and the bottom plate jacket is connected to the second flash tank.
[0009] In a possible implementation, the tail gas heat exchange unit further comprises a second tail gas jacket and a third flash tank, the second tail gas jacket is arranged at one end of the first tail gas jacket away from the furnace cylinder, and the second tail gas jacket is communicated with the third flash tank.
[0010] In a possible implementation, the tail gas heat exchange unit further comprises a third tail gas jacket and a fourth flash tank, the first tail gas jacket, the second tail gas jacket and the third tail gas jacket are arranged in sequence.
[0011] The third tail gas jacket is communicated with the fourth flash tank.
[0012] In a possible implementation, the heat recovery system of the reduction furnace further comprises a furnace cylinder heat exchange unit, the furnace cylinder heat exchange unit comprises a furnace cylinder jacket and a fifth flash tank, and the furnace cylinder jacket is communicated with the fifth flash tank.
[0013] In a possible implementation, the flash pressure of the first flash tank is less than or equal to 1.0 MPa (G), the flash pressure of the second flash tank and the fourth flash tank is less than or equal to 0.2 MPa (G), and the flash pressure of the third flash tank and the fifth flash tank is greater than or equal to 0.4 MPa (G) and less than or equal to 0.6 MPa (G).
[0014] In a possible implementation, the heat recovery system of the reduction furnace further comprises a furnace cylinder heat exchange unit, the furnace cylinder heat exchange unit comprises a furnace cylinder jacket and a sixth flash tank, the tail gas jacket further comprises a second tail gas jacket, the second tail gas jacket is communicated with the furnace cylinder jacket, and the furnace cylinder jacket is communicated with the sixth flash tank.
[0015] In a possible implementation, the tail gas jacket further comprises a third tail gas jacket, the first tail gas jacket, the second tail gas jacket and the third tail gas jacket are arranged in sequence.
[0016] The bottom heat exchange unit further comprises a bottom jacket and a seventh flash tank, the third tail gas jacket is communicated with the first ring pipe, the first ring pipe is communicated with the bottom jacket, and the bottom jacket is communicated with the seventh flash tank.
[0017] In a possible implementation, the flash pressure of the first flash tank is less than or equal to 1.0 MPa (G), the flash pressure of the sixth flash tank is greater than or equal to 0.4 MPa (G) and less than or equal to 0.6 MPa (G), and the flash pressure of the seventh flash tank is less than or equal to 0.2 MPa (G).
[0018] The heat recovery system of the reduction furnace provided by the application comprises a tail gas heat exchange unit and a bottom heat exchange unit, the tail gas heat exchange unit comprises a first tail gas jacket, and the bottom heat exchange unit comprises a ring pipe and a flange connecting piece. The first tail gas jacket is arranged to absorb the radiant heat of the tail gas, the flange connecting piece is arranged to divide the ring pipe into a first ring pipe and a second ring pipe, the second ring pipe is coupled with the first tail gas jacket, and the heat of the tail gas and the heat of part of the bottom are used to jointly produce high-grade saturated steam, so that the yield of the high-grade saturated steam is increased, and the first flash tank is arranged to flash the circulating liquid, which has absorbed the heat of the tail gas and the heat of part of the bottom, into saturated steam. Therefore, the heat recovery utilization rate of the heat recovery system of the reduction furnace provided by the application is high, and the modification cost is low.
[0019] In addition to the technical problems solved by the embodiments of the application described above, the technical features constituting the technical solutions and the beneficial effects brought by the technical features, other technical problems solved by the heat recovery system of the reduction furnace provided by the application, other technical features included in the technical solutions and the beneficial effects brought by the technical features will be further described in detail in the specific embodiments. BRIEF DESCRIPTION OF DRAWINGS
[0020] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.
[0021] Figure 1 The structural schematic diagram of the heat recovery system of the reduction furnace provided by the embodiments of the application;
[0022] Figure 2 The structural schematic diagram of the heat recovery system of the reduction furnace provided by the embodiments of the application.
[0023] Explanation of reference signs:
[0024] 100-tail gas heat exchange unit; 110-first tail gas jacket; 120-first flash tank; 130-connection pipe; 140-second tail gas jacket; 150-third flash tank; 160-third tail gas jacket; 170-fourth flash tank; 200-bottom heat exchange unit; 210-ring pipe; 211-first ring pipe; 212-second ring pipe; 220-flange connecting piece; 230-bottom jacket; 240-second flash tank; 250-seventh flash tank; 300-furnace shell heat exchange unit; 310-furnace shell jacket; 320-fifth flash tank; 330-sixth flash tank. DETAILED DESCRIPTION
[0025] In order to make the objects, technical solutions and advantages of the present application clearer, the following will be combined with the drawings of the preferred embodiments of the present application to describe the technical solutions in the embodiments of the present application in more detail. In the drawings, identical or similar reference numerals denote identical or similar components or components with identical or similar functions throughout. The described embodiments are part of the embodiments of the present application, rather than all the embodiments. The embodiments described below by reference to the drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work under the premise of the present application, belong to the scope of protection of the present application. The embodiments of the present application will be described in detail below with reference to the drawings.
[0026] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood in a broad sense, for example, it can be fixed connection, or indirect connection through an intermediate medium, or internal communication of two elements or interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.
[0027] In the description of the present application, it should be understood that the terms "upper", "lower", "front", "rear", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.
[0028] The terms "first", "second", "third" (if any) in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.
[0029] In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or display including a series of steps or units does not have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or displays.
[0030] At present, the mainstream polysilicon production process adopts the modified Siemens method. High-purity trichlorosilane is sent into a trichlorosilane vaporizer, and is vaporized by hydrogen gas in proportion through bubbling. High-purity preheated hydrogen gas from outside the boundary layer is controlled and adjusted by a flowmeter to adjust the feed molar ratio of trichlorosilane gas and hydrogen gas. The mixed gas is uniformly mixed in a static mixer, enters a reduction furnace through a nozzle, and undergoes a deposition reaction in the furnace to reduce silicon.
[0031] In the related art, different temperature cooling water needs to be used in the reduction section to remove radiant heat, mainly including radiant heat of the furnace cylinder, the bottom plate and the tail gas. The recovered radiant heat is used to flash different levels of steam. At present, the bottom plate cooling water and the jacket cooling water of the reduction section are separately returned to the water. The heat recovery of the bottom plate is used to flash low-grade saturated steam, and the heat recovery of the tail gas is used to flash high-grade saturated steam. This recovery mode is difficult to effectively recover heat.
[0032] In view of the above problems, the present application provides a heat recovery system of a reduction furnace, which comprises a tail gas heat exchange unit and a bottom plate heat exchange unit. The tail gas heat exchange unit comprises a first tail gas jacket and a first flash tank, and the bottom plate heat exchange unit comprises a flange connecting piece and a ring pipe. The ring pipe is divided into a first ring pipe and a second ring pipe by the flange connecting piece, and the first tail gas jacket is connected with the second ring pipe, so that the circulating liquid first absorbs the radiant heat of the tail gas, then absorbs the radiant heat of the bottom plate, and then enters the first flash tank to be flashed into high-grade saturated steam. In this way, the radiant heat of the tail gas and the radiant heat of the bottom plate can be coupled to improve the yield of high-grade saturated steam, thereby improving the recovery utilization rate of radiant heat. Moreover, no large structural modification is needed for the existing reduction furnace. Only the ring pipe needs to be disconnected into the first ring pipe and the second ring pipe by the flange connecting piece, so the modification cost is low.
[0033] The specific implementation of the heat recovery system of the reduction furnace of the embodiments of the present application will be described in detail below with reference to the accompanying drawings.
[0034] Referring to FIGS. 1 to 3, Figure 1 and Figure 2 The heat recovery system of the reduction furnace provided by the present application is used to recover the heat of the reduction furnace. The reduction furnace comprises a furnace cylinder and a bottom plate. The furnace heat recovery system comprises a tail gas heat exchange unit 100 and a bottom plate heat exchange unit 200. The bottom plate heat exchange unit 200 comprises a ring pipe 210 and a flange connecting piece 220. The ring pipe 210 comprises a first ring pipe 211 and a second ring pipe 212. The first ring pipe 211 is arranged close to the furnace cylinder, and the second ring pipe 212 is arranged away from the furnace cylinder. The first ring pipe 211 and the second ring pipe 212 are disconnected by the flange connecting piece 220.
[0035] The tail gas heat exchange unit 100 comprises a first tail gas jacket 110 and a first flash tank 120. The first tail gas jacket 110 is connected to the second ring pipe 212, and the second ring pipe 212 is connected to the first flash tank 120.
[0036] The heat recovery system of the reduction furnace provided by the present application comprises a tail gas heat exchange unit 100 and a bottom heat exchange unit 200. The tail gas heat exchange unit 100 comprises a first tail gas jacket 110, and the bottom heat exchange unit 200 comprises a ring pipe 210 and a flange connecting piece 220. The first tail gas jacket 110 is arranged to absorb the radiant heat of the tail gas, and the flange connecting piece 220 is arranged to divide the ring pipe 210 into a first ring pipe 211 and a second ring pipe 212, so that the second ring pipe 212 is coupled to the first tail gas jacket 110, and the heat of the tail gas and part of the heat of the bottom is used to jointly produce high-grade saturated steam, thereby increasing the yield of the high-grade saturated steam. The first flash tank 120 is arranged to flash the circulating liquid, which has absorbed the heat of the tail gas and part of the heat of the bottom, into saturated steam. Thus, the heat recovery system of the reduction furnace provided by the present application has a high heat recovery utilization rate and a low modification cost.
[0037] Referring to Figure 1 and Figure 2 , in a possible implementation manner, the heat recovery system of the reduction furnace further comprises a connecting pipe 130, and the first tail gas jacket 110 is in communication with the second ring pipe 212 through the connecting pipe 130.
[0038] In this way, the circulating liquid enters the first tail gas jacket 110 from the end of the first tail gas jacket 110 away from the furnace cylinder to absorb the radiant heat of the tail gas, and then enters the second ring pipe 212 through the connecting pipe 130 to absorb the radiant heat of the bottom, and then enters the first flash tank 120, so that the circulating liquid, which has absorbed the radiant heat of the tail gas and the radiant heat of the bottom, is flashed into high-grade saturated steam.
[0039] Referring to Figure 1 , in some embodiments, the bottom heat exchange unit 200 further comprises a bottom jacket 230 and a second flash tank 240, the first ring pipe 211 is in communication with the bottom jacket 230, and the bottom jacket 230 is in communication with the second flash tank 240.
[0040] In this way, the circulating liquid can enter from the first ring pipe 211, then enter the bottom jacket 230, and then absorb the radiant heat of the bottom in the process of flowing, and then the circulating liquid enters the second flash tank 240, so that the circulating liquid, which has absorbed the radiant heat of the bottom, can be flashed into intermediate-grade saturated steam.
[0041] Referring to Figure 1 , in a possible implementation manner, the tail gas heat exchange unit 100 further comprises a second tail gas jacket 140 and a third flash tank 150, the second tail gas jacket 140 is arranged at the end of the first tail gas jacket 110 away from the furnace cylinder, and the second tail gas jacket 140 is in communication with the third flash tank 150.
[0042] In this way, the circulating liquid enters the second tail gas jacket 140 from the end of the second tail gas jacket 140 away from the first tail gas jacket 110 to absorb the radiant heat of the tail gas, and then enters the second flash tank 240, so that the circulating liquid absorbing the radiant heat of the tail gas is flashed into saturated steam of an intermediate grade. The radiant heat of the tail gas can be recovered and utilized in stages, and the radiant heat of the first tail gas jacket 110 is used to produce saturated steam of a higher grade, and the radiant heat of the second tail gas jacket 140 is used to produce saturated steam of an intermediate grade.
[0043] Referring to Figure 1 In a possible implementation, as shown in the figure, the tail gas heat exchange unit 100 further includes a third tail gas jacket 160 and a fourth flash tank 170, the first tail gas jacket 110, the second tail gas jacket 140 and the third tail gas jacket 160 are sequentially arranged, and the third tail gas jacket 160 is communicated with the fourth flash tank 170.
[0044] In this way, the circulating liquid enters the third tail gas jacket 160 from the end of the third tail gas jacket 160 away from the second tail gas jacket 140 to absorb the radiant heat of the tail gas, and then enters the fourth flash tank 170, so that the circulating liquid absorbing the radiant heat of the tail gas is flashed into saturated steam of a lower grade. The radiant heat of the tail gas can be recovered and utilized in stages, and the radiant heat of the tail gas can be used to produce saturated steam of different grades according to different temperatures, thereby improving the heat recovery rate.
[0045] Referring to Figure 1 In some embodiments, the heat recovery system of the reducing furnace further includes a furnace shell heat exchange unit 300, and the furnace shell heat exchange unit 300 includes a furnace shell jacket 310 and a fifth flash tank 320, and the furnace shell jacket 310 is communicated with the fifth flash tank 320.
[0046] In this way, the radiant heat of the furnace shell can be absorbed by the circulating liquid in the furnace shell jacket 310, and then enter the fifth flash tank 320 to be flashed into saturated steam of an intermediate grade.
[0047] In a possible implementation, the flash pressure of the first flash tank 120 is less than or equal to 1.0 MPa (G), the flash pressure of the second flash tank 240 and the fourth flash tank 170 is less than or equal to 0.2 MPa (G), and the flash pressure of the third flash tank 150 and the fifth flash tank 320 is greater than or equal to 0.4 MPa (G) and less than or equal to 0.6 MPa (G).
[0048] For example, the flash pressure of the first flash tank 120 can be 1.0 MPa (G), the flash pressure of the second flash tank 240 and the fourth flash tank 170 can both be 0.2 MPa (G), and the flash pressure of the third flash tank 150 and the fifth flash tank 320 can both be 0.4 MPa (G).
[0049] Referring to Figure 2 As shown in FIG. 1, in a possible implementation, the heat recovery system of the reduction furnace further comprises a furnace shell heat exchange unit 300, the furnace shell heat exchange unit 300 comprises a furnace shell jacket 310 and a sixth flash tank 330, the tail gas jacket further comprises a second tail gas jacket 140, the second tail gas jacket 140 is communicated with the furnace shell jacket 310, and the furnace shell jacket 310 is communicated with the sixth flash tank 330.
[0050] In the above arrangement, the radiation heat of the furnace shell and the radiation heat of the tail gas can be coupled to jointly produce the intermediate-grade saturated steam, that is, the circulating liquid can first enter the second tail gas jacket 140 to absorb the radiation heat of the tail gas, then enter the furnace shell jacket 310 to absorb the radiation heat of the furnace shell, and then enter the sixth flash tank 330 to be flashed, so as to obtain the intermediate-grade saturated steam. In this way, the yield of the intermediate-grade saturated steam can be improved.
[0051] Referring to Figure 2 As shown in FIG. 1, in some embodiments, the tail gas jacket further comprises a third tail gas jacket 160, and the first tail gas jacket 110, the second tail gas jacket 140 and the third tail gas jacket 160 are arranged in sequence.
[0052] The bottom heat exchange unit 200 further comprises a bottom jacket 230 and a seventh flash tank 250, the third tail gas jacket 160 is communicated with the first annular pipe 211, the first annular pipe 211 is communicated with the bottom jacket 230, and the bottom jacket 230 is communicated with the seventh flash tank 250.
[0053] In the above arrangement, the radiation heat of the bottom shell and the radiation heat of the tail gas can be coupled to jointly produce the lower-grade saturated steam, that is, the circulating liquid can first enter the third tail gas jacket 160 to absorb the radiation heat of the tail gas, then enter the bottom jacket 230 to absorb the radiation heat of the bottom shell, and then enter the seventh flash tank 250 to be flashed, so as to obtain the lower-grade saturated steam. In this way, the yield of the lower-grade saturated steam can be improved.
[0054] In a possible implementation, the flash pressure of the first flash tank 120 is less than or equal to 1.0 MPa (G), the flash pressure of the sixth flash tank 330 is greater than or equal to 0.4 MPa (G) and less than or equal to 0.6 MPa (G), and the flash pressure of the seventh flash tank 250 is less than or equal to 0.2 MPa (G).
[0055] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.
Claims
1. A heat recovery system for a reduction furnace, used to recover heat from the reduction furnace, the reduction furnace comprising a furnace cylinder and a chassis, characterized in that, The heat recovery system of the reduction furnace includes a tail gas heat exchange unit and a chassis heat exchange unit. The chassis heat exchange unit includes a ring pipe and a flange connector. The ring pipe includes a first ring pipe and a second ring pipe. The first ring pipe is located close to the chassis, and the second ring pipe is located away from the chassis. The first ring pipe and the second ring pipe are separated by the flange connector. The exhaust gas heat exchange unit includes a first exhaust gas jacket and a first flash tank. The first exhaust gas jacket is connected to the second ring pipe, and the second ring pipe is connected to the first flash tank.
2. The heat recovery system for the reduction furnace according to claim 1, characterized in that, It also includes a connecting pipe, through which the first exhaust gas jacket is connected to the second annular pipe.
3. The heat recovery system for the reduction furnace according to claim 1 or 2, characterized in that, The chassis heat exchange unit also includes a chassis jacket and a second flash tank, with the first annular pipe connected to the chassis jacket and the chassis jacket connected to the second flash tank.
4. The heat recovery system for the reduction furnace according to claim 3, characterized in that, The exhaust gas heat exchange unit further includes a second exhaust gas jacket and a third flash tank. The second exhaust gas jacket is disposed at the end of the first exhaust gas jacket away from the furnace cylinder, and the second exhaust gas jacket is connected to the third flash tank.
5. The heat recovery system for the reduction furnace according to claim 4, characterized in that, The exhaust gas heat exchange unit further includes a third exhaust gas jacket and a fourth flash tank, with the first exhaust gas jacket, the second exhaust gas jacket and the third exhaust gas jacket arranged sequentially. The third exhaust gas jacket is connected to the fourth flash tank.
6. The heat recovery system for the reduction furnace according to claim 5, characterized in that, It also includes a furnace tube heat exchange unit, which includes a furnace tube jacket and a fifth flash tank, with the furnace tube jacket connected to the fifth flash tank.
7. The heat recovery system for the reduction furnace according to claim 6, characterized in that, The flash pressure of the first flash tank is less than or equal to 1.0 MPa(G), the flash pressure of the second and fourth flash tanks is less than or equal to 0.2 MPa(G), and the flash pressure of the third and fifth flash tanks is greater than or equal to 0.4 MPa(G) and less than or equal to 0.6 MPa(G).
8. The heat recovery system for the reduction furnace according to claim 1 or 2, characterized in that, It also includes a furnace tube heat exchange unit, which includes a furnace tube jacket and a sixth flash tank. The tail gas jacket also includes a second tail gas jacket, which is connected to the furnace tube jacket and the furnace tube jacket is connected to the sixth flash tank.
9. The heat recovery system for the reduction furnace according to claim 8, characterized in that, The exhaust gas jacket also includes a third exhaust gas jacket, and the first exhaust gas jacket, the second exhaust gas jacket and the third exhaust gas jacket are arranged in sequence; The chassis heat exchange unit also includes a chassis jacket and a seventh flash tank. The third exhaust gas jacket is connected to the first annular pipe, the first annular pipe is connected to the chassis jacket, and the chassis jacket is connected to the seventh flash tank.
10. The heat recovery system for the reduction furnace according to claim 9, characterized in that, The flash pressure of the first flash tank is less than or equal to 1.0 MPa(G), the flash pressure of the sixth flash tank is greater than or equal to 0.4 MPa(G) and less than or equal to 0.6 MPa(G), and the flash pressure of the seventh flash tank is less than or equal to 0.2 MPa(G).