Waste heat gradient utilization system of polycrystalline silicon reduction furnace

Abstract

The invention discloses a waste heat gradient utilization system of a polycrystalline silicon reduction furnace, relates to the technical field of polycrystalline silicon production, and mainly aims to improve the waste heat utilization rate. According to the main technical scheme, the waste heat gradient utilization system for the polycrystalline silicon reduction furnace is characterized in that a second flash tank is connected to an inlet of a bell jar cooling water jacket of the reduction furnace through a first conveying pump, and an outlet of the bell jar cooling water jacket of the reduction furnace is connected to a third flash tank; the first flash tank is connected to a tail gas pipe jacket cooling water inlet of the reduction furnace through a second delivery pump, and a tail gas pipe jacket cooling water outlet of the reduction furnace is connected to the first flash tank; the third flash tank is connected to a steam inlet of the generator set through a first pipeline, and the third flash tank is connected to the second flash tank through a second pipeline; the first flash tank is connected to an inlet of a medium-pressure steam user, and an outlet of the medium-pressure steam user is connected to the second flash tank; the second flash tank is connected to an inlet of the low-pressure steam user, and an outlet of the low-pressure steam user is connected to the second flash tank.

Description

Technical Field

[0001] The present invention relates to the technical field of polysilicon production, and particularly to a cascade utilization system for the waste heat of a polysilicon reduction furnace. Background Art

[0002] Currently, the polysilicon production process still mainly uses the improved Siemens method. In a fluidized bed, metallurgical-grade silicon is converted into crude trichlorosilane (TCS) through a hydrogenation reaction. After being purified through multiple distillation stages, it is vaporized and enters a CVD reduction furnace, where it undergoes a chemical vapor deposition reaction with hydrogen on the surface of a heated silicon core (about 1200 °C) under the application of electricity, and is reduced to elemental silicon. The silicon core gradually grows into a polysilicon rod.

[0003] Throughout the reaction cycle, the silicon rod always needs to operate with a high current, so the electricity cost becomes the main cost in polysilicon production. The comprehensive power consumption per kilogram of polysilicon produced is about 55 kW·h, of which the reduction power consumption is about 40 kW·h. Therefore, researching how to further improve the utilization rate of reduction waste heat and reduce the reduction power consumption is of great significance for reducing the production cost of polysilicon.

[0004] During the polysilicon chemical vapor deposition reaction process, most of the electrical energy consumed is converted into heat energy except for a small amount absorbed by the reduction reaction. The heat lost by the reduction furnace is mainly carried away by the cooling water in parts such as the bell jar and the tail gas pipe jacket heat exchanger, and finally flashes into steam in a flash tank for use in the process heating of the whole plant. This is the current mainstream way to utilize the reduction waste heat.

[0005] Currently, according to the different cooling water temperatures, the flashed steam is divided into two grades: low-pressure steam (0.2 MPag ≤ P ≤ 0.5 MPag) and medium-pressure steam (0.8 MPag ≤ P ≤ 1.0 MPag). When there are many reduction furnaces in operation, there is an excess of low-pressure steam. When it exceeds the process heating demand, it needs to be vented, resulting in waste of heat and water. However, the production of medium-pressure steam is less and cannot meet the production demand, and a large amount still needs to be purchased from a thermal power plant, which overall affects the waste heat utilization rate. Summary of the Invention

[0006] In view of this, an embodiment of the present invention provides a cascade utilization system for the waste heat of a polysilicon reduction furnace, and the main purpose is to improve the waste heat utilization rate and reduce the operation cost of the device.

[0007] To achieve the above object, the present invention mainly provides the following technical solutions:

[0008] An embodiment of the present invention provides a cascade utilization system for the waste heat of a polysilicon reduction furnace, and the system includes: a reduction furnace, a first flash tank, a second flash tank, a third flash tank, a first transfer pump, and a second transfer pump;

[0009] The lower end of the second flash tank is connected to the inlet of the bell jar cooling water jacket of the reduction furnace through the first delivery pump, and the outlet of the bell jar cooling water jacket of the reduction furnace is connected to the upper end of the third flash tank;

[0010] The lower end of the first flash tank is connected to the inlet of the tail gas pipe jacket cooling water of the reduction furnace through the second delivery pump, and the outlet of the tail gas pipe jacket cooling water of the reduction furnace is connected to the upper end of the first flash tank;

[0011] The upper end of the third flash tank is connected to the main steam inlet of the generator set through the first pipeline, and the lower end of the third flash tank is connected to the upper end of the second flash tank through the second pipeline;

[0012] The upper end of the first flash tank is connected to the inlet of the medium-pressure steam user, and the outlet of the medium-pressure steam user is connected to the upper end of the second flash tank;

[0013] The upper end of the second flash tank is connected to the inlet of the low-pressure steam user, and the outlet condensate pipe of the low-pressure steam user is connected to the upper end of the second flash tank.

[0014] The object of the present invention and the technical problems to be solved can be further realized by the following technical measures.

[0015] Optionally, it further includes a third delivery pump, and the lower end of the second flash tank is connected to the upper end of the first flash tank through the third delivery pump.

[0016] Optionally, it further includes a hot water bypass pipeline, one end of the hot water bypass pipeline is connected to the outlet of the bell jar cooling water jacket of the reduction furnace, and the other end is connected to the upper end of the second flash tank.

[0017] Optionally, a first liquid level sensor is installed on the third flash tank, a first pressure transmitter and a first regulating valve are installed on the second pipeline, and the first liquid level sensor, the first pressure transmitter and the first regulating valve constitute a first interlock control mechanism.

[0018] Optionally, a second regulating valve, a second pressure transmitter and a third regulating valve are sequentially installed on the first pipeline, and the second pressure transmitter, the second regulating valve and the third regulating valve constitute a second interlock control mechanism.

[0019] Optionally, it further includes a third pipeline, one end of the third pipeline is connected to the outlet steam pipeline of the second flash tank, and the other end is connected to the auxiliary steam inlet of the generator set.

[0020] Optionally, it further includes a condenser and a condensate pump, and the gas flow outlet of the generator set, the condenser, the condensate pump and the upper end of the second flash tank are sequentially connected.

[0021] Optionally, it further includes a fourth regulating valve and a first desuperheating and pressure reducing device. The upper end of the third flash tank, the fourth regulating valve, the first desuperheating and pressure reducing device, and the inlet of the low-pressure steam user are connected in sequence.

[0022] Optionally, it further includes a medium-pressure steam pipe, which is provided with a first branch, a second branch, and a third branch. The first branch is connected to the first pipeline between the second regulating valve and the third regulating valve. The second branch is connected to the inlet of the low-pressure steam user through a second desuperheating and pressure reducing device. The third branch is connected to the inlet of the medium-pressure steam user.

[0023] Optionally, a second liquid level sensor is installed on the first flash tank, and a fifth regulating valve is installed at the outlet of the third delivery pump. The fifth regulating valve and the second liquid level sensor form a third interlock control mechanism.

[0024] By means of the above technical solutions, the present invention has at least the following advantages:

[0025] The hot water discharged from the outlet of the cooling water jacket of the reduction furnace bell jar reaches the third flash tank. The flash pressure of the third flash tank is between the low-pressure steam pressure and the medium-pressure steam pressure (0.4Mpag < P ≤ 0.7Mpag) (hereinafter referred to as "medium-low pressure steam"). The flash pressure of the second flash tank is the low-pressure steam pressure (0.2MPag ≤ P ≤ 0.5MPag). The hot water discharged from the cooling water jacket of the bell jar is preferentially flashed into medium-low pressure steam in the third flash tank, driving the generator set to generate electricity. Compared with low-pressure steam, using this medium-low pressure steam has higher power generation efficiency;

[0026] The hot water that cannot be flashed in the third flash tank enters the second flash tank through the second pipeline, and is partially flashed into low-pressure steam. The low-pressure steam reaches the low-pressure steam user, and the low-pressure steam condensate generated after heat exchange is transported to the second flash tank for recycling;

[0027] In the second flash tank, the hot water cooled after partial flashing is transported to the reduction furnace bell jar jacket through the first delivery pump as a cooling medium;

[0028] The condensate in the first flash tank is transported to the tail gas pipe jacket through the second delivery pump. After heat exchange with the reduction furnace tail gas, it enters the first flash tank. The medium-pressure steam flashed in the first flash tank reaches the medium-pressure steam user, and the medium-pressure steam condensate after heat exchange and cooling enters the second flash tank, and continues to flash into low-pressure steam in the second flash tank.

[0029] Using this system, the step-by-step flashing and utilization of the waste heat of the reduction furnace are realized. The electricity generated by using the medium-low pressure steam can partially make up for the electricity consumption of the polysilicon reduction furnace process. The comprehensive power consumption of the system can be reduced through this system. Description of the Drawings

[0030] Figure 1 This is a schematic structural diagram of a multi - crystal silicon reduction furnace waste heat cascade utilization system provided by an embodiment of the present invention;

[0031] Figure 2 This is another schematic structural diagram of a multi - crystal silicon reduction furnace waste heat cascade utilization system provided by an embodiment of the present invention.

[0032] Reference numerals in the accompanying drawings of the specification include: first flash tank 1, second flash tank 2, third flash tank 3, first transfer pump 4, second transfer pump 5, bell jar cooling water jacket 6, tail gas pipe jacket 7, first pipeline 8, generator set 9, second pipeline 10, medium - pressure steam user 11, third transfer pump 12, hot water bypass pipeline 13, first liquid level sensor 14, first pressure transmitter 15, first regulating valve 16, second regulating valve 17, second pressure transmitter 18, third regulating valve 19, third pipeline 20, condenser 21, condensate pump 22, fourth regulating valve 23, first desuperheating and pressure - reducing device 24, medium - pressure steam pipe 25, second desuperheating and pressure - reducing device 26, second liquid level sensor 27, fifth regulating valve 28. Detailed implementation manners

[0033] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following, in combination with the accompanying drawings and preferred embodiments, details the specific implementation manners, structures, features, and effects of the application according to the present invention. In the following description, different "one embodiment" or "embodiment" do not necessarily refer to the same embodiment. In addition, the specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0035] As Figure 1 shown, a multi - crystal silicon reduction furnace waste heat cascade utilization system provided by an embodiment of the present invention includes: a reduction furnace, a first flash tank 1, a second flash tank 2, a third flash tank 3, a first transfer pump 4, and a second transfer pump 5;

[0036] The lower end of the second flash tank 2 is connected to the inlet of the bell jar cooling water jacket 6 of the reduction furnace through the first transfer pump 4, and the outlet of the bell jar cooling water jacket 6 of the reduction furnace is connected to the upper end of the third flash tank 3;

[0037] The lower end of the first flash tank 1 is connected to the cooling water inlet of the tail gas pipe jacket 7 of the reduction furnace through the second transfer pump 5, and the cooling water outlet of the tail gas pipe jacket 7 of the reduction furnace is connected to the upper end of the first flash tank 1;

[0038] The upper end of the third flash tank 3 is connected to the main steam inlet of the generator set 9 through the first pipeline 8, and the lower end of the third flash tank 3 is connected to the upper end of the second flash tank 2 through the second pipeline 10;

[0039] The upper end of the first flash tank 1 is connected to the inlet of the medium-pressure steam user 11, and the outlet of the medium-pressure steam user 11 is connected to the upper end of the second flash tank 2;

[0040] The upper end of the second flash tank 2 is connected to the inlet of the low-pressure steam user, and the outlet condensate pipe of the low-pressure steam user is connected to the upper end of the second flash tank 2.

[0041] The regulating valves of the inlet hot water pipelines of the first flash tank 1, the second flash tank 2, and the third flash tank 3 are all installed on the side close to the flash tank to ensure that the hot water is flashed as much as possible inside the tank.

[0042] The working process of the polysilicon reduction furnace waste heat cascade utilization system is as follows:

[0043] The hot water discharged from the outlet of the reduction furnace bell jar cooling water jacket 6 reaches the third flash tank 3. The flashing pressure of the third flash tank 3 is the medium-low pressure steam pressure (0.4 Mpag < P ≤ 0.7 Mpag), and the flashing pressure of the second flash tank 2 is the low-pressure steam pressure (0.2 MPag ≤ P ≤ 0.5 MPag). The medium-low pressure steam generated by the third flash tank 3 enters the generator set 9 through the first pipeline 8 to drive the generator set 9 to generate electricity. The hot water discharged from the bell jar cooling water jacket 6 is preferentially flashed into medium-low pressure steam in the third flash tank 3 for power generation by the generator set 9. Compared with low-pressure steam, using this medium-low pressure steam has higher power generation efficiency;

[0044] The hot water that cannot be flashed in the third flash tank 3 enters the second flash tank 2 through the second pipeline 10 and is partially flashed into low-pressure steam. The low-pressure steam reaches the low-pressure steam user, and the low-pressure steam condensate generated after heat exchange is transported to the second flash tank 2 for recycling;

[0045] In the second flash tank 2, the hot water cooled after partial flashing is transported to the reduction furnace bell jar jacket 6 as a cooling medium through the first transfer pump 4;

[0046] The condensate in the first flash tank 1 is transported to the tail gas pipe jacket 7 through the second transfer pump 5. After heat exchange with the reduction furnace tail gas, it enters the first flash tank 1. The medium-pressure steam flashed in the first flash tank 1 reaches the medium-pressure steam user 11, and the medium-pressure steam condensate after heat exchange and cooling enters the second flash tank 2 to continue flashing into low-pressure steam in the second flash tank 2.

[0047] Using this system, the step-by-step flashing and utilization of the waste heat of the reduction furnace are realized. Among them, the electricity generated by using medium-low pressure steam can partially make up for the power consumption of the polysilicon reduction furnace process, and the system comprehensive power consumption can be reduced through this system.

[0048] Specifically, the generator set 9 adopts a condensing generator set. All the generated electricity of the generator set 9 is used for the self - use of the polysilicon production device without being connected to the grid. Through this process improvement, the comprehensive power consumption and operation cost of the system are further reduced.

[0049] Specifically, by using this system, the heat loss caused by the external discharge of low - pressure steam is reduced, the relatively high - level heat energy in the bell - jar cooling water is reasonably utilized, and the waste heat utilization rate of the reduction furnace is significantly improved.

[0050] Specifically, by using this system, the water loss caused by the external discharge of steam is reduced, the noise pollution during the external discharge of steam is eliminated, and the working environment is improved.

[0051] As Figure 1 or Figure 2 shown, in the specific implementation manner, it further includes a third transfer pump 12. The lower end of the second flash tank 2 is connected to the upper end of the first flash tank 1 through the third transfer pump 12.

[0052] In this implementation manner, specifically, because in the first flash tank 1, after the medium - pressure steam reaches the medium - pressure steam user 11 and condenses into medium - pressure condensate, it reaches the second flash tank 2, which will inevitably increase the amount of condensate in the second flash tank 2. The third transfer pump 12 supplements part of the condensate to the first flash tank 1 to ensure the stable liquid level of the first flash tank 1 and provide continuous cooling water for the tail gas pipe jacket 7.

[0053] As Figure 1 shown, in the specific implementation manner, it further includes a hot - water bypass pipeline 13. One end of the hot - water bypass pipeline 13 is connected to the outlet of the bell - jar cooling water jacket 6 of the reduction furnace, and the other end is connected to the upper end of the second flash tank 2.

[0054] In this implementation manner, specifically, when the low - pressure steam required by the low - pressure steam user is insufficient, part of the hot water discharged from the outlet of the bell - jar cooling water jacket 6 of the reduction furnace can directly enter the second flash tank 2 through the hot - water bypass pipeline 13 to increase the low - pressure steam flashing amount, so as to achieve the balance of low - pressure steam and medium - low - pressure steam.

[0055] As Figure 1 or Figure 2 shown, in the specific implementation manner, the third flash tank 3 is equipped with a first liquid - level sensor 14, and the second pipeline 10 is equipped with a first pressure transmitter 15 and a first regulating valve 16. The first liquid - level sensor 14, the first pressure transmitter 15 and the first regulating valve 16 constitute a first interlock control mechanism.

[0056] In this embodiment, specifically, when the first pressure transmitter 15 monitors that the pressure before the valve of the first regulating valve 16 is greater than or equal to the pressure of the third flash tank 3 (i.e., the saturated vapor pressure corresponding to the current hot water temperature), the first regulating valve 16, the controller, and the first liquid level sensor 14 form an integral feedback control loop to stabilize the liquid level of the third flash tank 3; when the first pressure transmitter 15 monitors that the pressure before the valve of the first regulating valve 16 is less than the pressure of the third flash tank 3, the first regulating valve 16, the controller, and the first pressure transmitter 15 form an integral feedback control loop to adjust the pressure before the valve to be greater than or equal to the pressure of the third flash tank 3.

[0057] Specifically, control the pressure before the valve of the regulating valve (the first regulating valve 16) at the inlet of the second flash tank 2 to be higher than the saturated vapor pressure of the hot water at the outlet of the third flash tank 3 to ensure that the hot water does not vaporize before entering the second flash tank 2, so as to avoid water hammer and water shock phenomena; at the same time, in order for the hot water to have sufficient flashing space, it is also necessary to keep the liquid level of the third flash tank 3 stable.

[0058] As Figure 1 Or Figure 2 As shown, in the specific embodiment, the second regulating valve 17, the second pressure transmitter 18, and the third regulating valve 19 (the load regulating valve of the generator set 9) are sequentially installed on the first pipeline 8, and the second pressure transmitter 18, the second regulating valve 17, and the third regulating valve 19 form a second interlocking control mechanism.

[0059] In this embodiment, specifically, the medium and low pressure steam discharged from the third flash tank 3 through the first pipeline 8 serves as the main steam of the generator set 9, and the inlet pressure of the generator set 9 is controlled in split range by the second regulating valve 17 and the third regulating valve 19. When the inlet pressure is higher than the steam set pressure of the generator set 9, the second regulating valve 17 is closed slightly; when the inlet pressure is lower than the steam set pressure of the generator set 9, the third regulating valve 19 is closed slightly to reduce the load of the generator set 9.

[0060] As Figure 1 As shown, in the specific embodiment, it further includes a third pipeline 20. One end of the third pipeline 20 is connected to the steam pipeline at the outlet of the second flash tank 2, and the other end is connected to the auxiliary steam inlet of the generator set 9.

[0061] In this embodiment, specifically, the low pressure steam discharged from the second flash tank 2 is used for heating in the existing polysilicon reduction furnace process (low pressure steam user), and the excess low pressure steam reaches the auxiliary steam inlet of the generator set 9 through the third pipeline 20.

[0062] As Figure 1 Or Figure 2 As shown, in the specific embodiment, it further includes a condenser 21 and a condensate pump 22. The gas outlet of the generator set 9, the condenser 21, the condensate pump 22, and the upper end of the second flash tank 2 are sequentially connected.

[0063] In this embodiment, specifically, after the steam expands and does work stage by stage in the generator set 9, the steam pressure and temperature decrease, and finally it enters the condenser, condenses into water, and is pumped to the second flash tank 2 by the condensate pump 22 for recycling.

[0064] As Figure 2 shown, in the specific embodiment, it further includes a fourth regulating valve 23 and a first desuperheating and pressure reducing device 24. The upper end of the third flash tank 3, the fourth regulating valve 23, the first desuperheating and pressure reducing device 24 and the inlet of the low-pressure steam user are connected in sequence.

[0065] As Figure 2 shown, in the case where all the hot water discharged from the outlet of the reduction furnace bell jar cooling water jacket 6 first enters the third flash tank 3 for flashing, if the output of the low-pressure steam flashed by the second flash tank 2 cannot meet the demand of the low-pressure steam user, the medium and low-pressure steam in the third flash tank 3 sequentially passes through the fourth regulating valve 23 and the first desuperheating and pressure reducing device 24 to reach the low-pressure steam user, maintaining the balance of steam consumption.

[0066] When the above design is adopted, there is no surplus in the output of low-pressure steam, the second flash tank 2 cannot supply auxiliary steam to the generator set 9 anymore, and there is no need to set up the third pipeline 20.

[0067] As Figure 1 or Figure 2 shown, in the specific embodiment, it further includes a medium-pressure steam pipe 25. The medium-pressure steam pipe 25 is provided with a first branch, a second branch and a third branch. The first branch is connected to the first pipeline 8 between the second regulating valve 17 and the third regulating valve 19. The second branch is connected to the inlet of the low-pressure steam user through a second desuperheating and pressure reducing device 26. The third branch is connected to the inlet of the medium-pressure steam user 11.

[0068] In this embodiment, specifically, one end of the medium-pressure steam pipe 25 is connected to the externally purchased steam pipe network, and the other end is connected to the first branch, the second branch and the third branch;

[0069] When the reduction furnace device is started up, steam is supplied to the medium-pressure steam pipe 25 through the externally purchased steam pipe network, so as to supply starting steam to the generator set 9, the low-pressure steam user and the medium-pressure steam user 11. After multiple reduction furnaces are in normal production, the externally purchased steam pipe network is closed.

[0070] As Figure 1 or Figure 2 shown, in the specific embodiment, a second liquid level sensor 27 is installed on the first flash tank 1, and a fifth regulating valve 28 is installed at the outlet of the third delivery pump 12. The fifth regulating valve 28 and the second liquid level sensor 27 form a third interlock control mechanism.

[0071] In this embodiment, specifically, in order to stabilize the liquid level of the first flash tank 1, an integral feedback control is performed by the third interlock control mechanism composed of the second liquid level sensor 27, the controller and the fifth regulating valve 28, so that the liquid level of the first flash tank 1 is stabilized at a set threshold value, thereby ensuring that the flash space of the first flash tank 1 meets the set requirements.

[0072] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A polysilicon reduction furnace waste heat cascade utilization system, characterized in that, Including: Reduction furnace, first flash tank, second flash tank, third flash tank, first transfer pump and second transfer pump; The lower end of the second flash tank is connected to the inlet of the bell jar cooling water jacket of the reduction furnace through the first transfer pump, and the outlet of the bell jar cooling water jacket of the reduction furnace is connected to the upper end of the third flash tank; The lower end of the first flash tank is connected to the inlet of the tail gas pipe jacket cooling water of the reduction furnace through the second transfer pump, and the outlet of the tail gas pipe jacket cooling water of the reduction furnace is connected to the upper end of the first flash tank; The upper end of the third flash tank is connected to the main steam inlet of the generator set through the first pipeline, and the lower end of the third flash tank is connected to the upper end of the second flash tank through the second pipeline; The upper end of the first flash tank is connected to the inlet of the medium-pressure steam user, and the outlet of the medium-pressure steam user is connected to the upper end of the second flash tank; The upper end of the second flash tank is connected to the inlet of the low-pressure steam user, and the outlet condensate pipe of the low-pressure steam user is connected to the upper end of the second flash tank.

2. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 1, characterized in that It further includes a third transfer pump, and the lower end of the second flash tank is connected to the upper end of the first flash tank through the third transfer pump.

3. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 1, characterized in that It further includes a hot water bypass pipeline, one end of the hot water bypass pipeline is connected to the outlet of the bell jar cooling water jacket of the reduction furnace, and the other end is connected to the upper end of the second flash tank.

4. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 1, characterized in that The third flash tank is equipped with a first liquid level sensor, and the second pipeline is equipped with a first pressure transmitter and a first regulating valve. The first liquid level sensor, the first pressure transmitter and the first regulating valve constitute a first interlock control mechanism.

5. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 1, characterized in that The first pipeline is successively installed with a second regulating valve, a second pressure transmitter and a third regulating valve. The second pressure transmitter, the second regulating valve and the third regulating valve constitute a second interlock control mechanism.

6. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 3, characterized in that It further includes a third pipeline, one end of the third pipeline is connected to the outlet steam pipeline of the second flash tank, and the other end is connected to the auxiliary steam inlet of the generator set.

7. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 1, characterized in that It further includes a condenser and a condensate pump, and the gas flow outlet of the generator set, the condenser, the condensate pump and the upper end of the second flash tank are successively connected.

8. The multi-crystalline silicon reduction furnace waste heat cascade utilization system according to claim 1, characterized in that It further includes a fourth regulating valve and a first desuperheating and pressure reducing device, and the upper end of the third flash tank, the fourth regulating valve, the first desuperheating and pressure reducing device and the inlet of the low-pressure steam user are successively connected.

9. The polysilicon reduction furnace waste heat cascade utilization system according to claim 5, wherein, it further includes a medium-pressure steam pipe, the medium-pressure steam pipe is provided with a first branch, a second branch and a third branch, the first branch is connected to the first pipe between the second regulating valve and the third regulating valve, the second branch is connected to the inlet of the low-pressure steam user through a second desuperheating and pressure-reducing device, and the third branch is connected to the inlet of the medium-pressure steam user.

10. The polysilicon reduction furnace waste heat cascade utilization system according to claim 2, wherein, a second liquid level sensor is installed on the first flash tank, a fifth regulating valve is installed at the outlet of the third delivery pump, and the fifth regulating valve and the second liquid level sensor form a third interlock control mechanism.

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