Cascade waste heat recovery system based on membrane deoxidization and corresponding recovery process

Abstract

The invention discloses a stepped waste heat recovery system based on membrane deoxidization, which is characterized in that on the basis of source deoxidization, stepped preheating recovery is arranged, and low-grade waste heat is recovered, so that the energy utilization rate is increased. The boiler flue gas cascade flow direction assembly comprises a boiler, a dust remover, a desulfurizing tower, a condenser and a chimney, the boiler comprises a hearth, a steam pocket and a flue, and a superheater, an economizer and an air pre-heater are sequentially arranged in the outflow direction of the flue; the water step flow direction assembly comprises a chemical water station, a desalted water tank, a membrane deaerator, a deoxygenated water tank and a plurality of step warming and heating mechanisms; the superheated steam utilization assembly comprises a steam turbine and a generator; a flue output port of the boiler is connected with an inlet of the dust remover, an outlet of the dust remover is connected into an inlet of the desulfurizing tower after being cooled, an outlet of the desulfurizing tower is connected with a gas end input port of the condenser, and a gas end output port of the condenser is connected to the bottom of the chimney.

Description

Technical Field

[0001] The present invention relates to the technical field of waste heat recovery systems, and in particular to a cascade waste heat recovery system based on membrane deoxidation. The present invention also provides a recovery process corresponding to the recovery system. Background Art

[0002] In the boiler feed water treatment process, deoxygenation is a very critical link. Oxygen is the main corrosive substance in the boiler feed water system. The oxygen in the feed water system should be removed quickly, otherwise it will corrode the boiler feed water system and components. The corrosive substance iron oxide will enter the boiler and deposit or adhere to the boiler tube wall and heating surface, forming iron scale that is difficult to dissolve and has poor heat transfer. The corroded iron scale will cause pits on the inner wall of the pipeline, increase the resistance coefficient, and when the pipeline corrosion is serious, even pipeline explosion accidents may occur.

[0003] In the fields of electricity, steel, petrochemicals, etc., a large amount of low-grade waste heat is still directly discharged, including the latent heat of saturated wet flue gas after desulfurization, the sensible heat of exhaust gas after dust removal, and the sensible heat of turbine condensate; the existing thermal deaerator is based on the deoxidation principle of Henry's law, and a large amount of steam needs to be introduced for heating and deoxidation, which not only wastes high-grade steam, but also cannot recover low-grade waste heat after the temperature rises, so the heat can only be wasted. Summary of the Invention

[0004] In response to the above problems, the present invention provides a cascade waste heat recovery system based on membrane deoxidation. On the basis of source deoxidation, it arranges cascade preheating recovery to recover low-grade waste heat, thereby improving energy utilization.

[0005] A cascade waste heat recovery system based on membrane deoxidation, characterized in that it comprises:

[0006] The boiler flue gas cascade flow assembly includes a boiler, a dust collector, a desulfurization tower, a condenser, and a chimney. The boiler includes a furnace, a steam drum, and a flue. The outflow direction of the flue is sequentially arranged with a superheater, an economizer, and an air preheater;

[0007] Water cascade flow components, including a chemical water station, a desalted water tank, a membrane deaerator, a deoxygenated water tank, and several step-by-step heating mechanisms;

[0008] and a superheated steam utilization assembly, which includes a steam turbine and a generator;

[0009] The flue outlet of the boiler is connected to the inlet of the dust collector, the outlet of the dust collector is connected to the inlet of the desulfurization tower after cooling, the outlet of the desulfurization tower is connected to the gas end input port of the condenser, and the gas end output port of the condenser is connected to the bottom of the chimney;

[0010] The water output from the chemical water station passes through the desalted water tank and the membrane deaerator in sequence and then enters the water end inlet of the condenser, the water end outlet of the condenser is connected to the input port of the deaerated water tank, the output port of the deaerated water tank is connected to the primary input ports of several step-by-step heating mechanisms, and the high-order output ports of several step-by-step heating mechanisms pass the water into the water input end of the economizer, the water output end of the economizer is connected to the steam drum, the steam drum output end is connected to the steam input end of the superheater, the steam output end of the superheater is connected to the steam input end of the steam turbine, the steam turbine drives the generator to generate electricity, and the low-temperature steam of the steam turbine is connected to the corresponding heating mechanisms of several step-by-step heating mechanisms to heat the deaerated water;

[0011] The gas input end of the membrane deaerator is connected to a nitrogen purge pipeline, and the gas output port of the membrane deaerator discharges a mixed gas of oxygen and nitrogen.

[0012] It is further characterized by:

[0013] Several step-by-step heating mechanisms include a low-pressure heater, a low-temperature economizer, and a high-pressure heater in sequence along the water flow direction, and a feed water pump is also provided between the low-temperature economizer and the high-pressure heater;

[0014] The output port of the deaeration water tank is connected to the first medium input port of the low-pressure heater through a deaeration water pump, the first medium output port of the low-pressure heater is connected to the first medium input port of the low-temperature economizer, the first medium output port of the low-temperature economizer is connected to the first medium input port of the high-pressure heater after passing through the feed water pump, and the first medium output port of the high-pressure heater is connected to the water input end of the economizer;

[0015] The steam outlet of the steam turbine includes an unused surplus steam outlet and a used exhaust steam outlet, the surplus steam outlet is connected to the second medium input port of the high-pressure heater, the second medium output port of the high-pressure heater is connected to the return port of the deaeration water tank, the exhaust steam outlet is connected to the condenser and then to the second medium input port of the low-pressure heater, and the second medium output port of the low-pressure heater is connected to the return port of the deaeration water tank;

[0016] A nitrogen sealing module is provided on the top of the deoxygenated water tank, and the nitrogen sealing module transports nitrogen through a nitrogen pipeline;

[0017] The flue gas discharged from the dust collector is connected to the second medium input port of the low-temperature economizer, and the second medium output port of the low-temperature economizer then transports the flue gas to the inlet of the desulfurization tower;

[0018] The membrane deaerator includes at least one group of deoxygenation membrane contactors, each group of deoxygenation membrane contactors includes a contactor body, an oxygenated water inlet, a deoxygenated water outlet, a nitrogen inlet, and a waste gas outlet. The water inlets and outlets of several groups of deoxygenation membrane contactors are connected in series, and several groups of nitrogen inlets and waste gas outlets are arranged in parallel. The oxygenated water inlet of the first-stage deoxygenation membrane contactor formed in series is the total water inlet, and the deoxygenated water outlet of the last-stage deoxygenation membrane contactor formed in series is the total water outlet.

[0019] A cascade waste heat recovery process based on membrane deoxygenation is characterized in that it adopts a cascade waste heat recovery system based on membrane deoxygenation, the deionized water pretreated by the water treatment station is stored in the deionized water tank for standby, and is pumped into the membrane deaerator for deoxygenation treatment. The deoxygenated water after deoxygenation absorbs the latent heat of saturated wet flue gas after the desulfurization tower through the condenser and is stored in the deoxygenated water tank. The deoxygenated water tank must be nitrogen-sealed by a nitrogen sealing module, the deoxygenated water absorbs the waste heat of exhaust condensate after the turbine works through the low-pressure heater, and then absorbs part of the waste heat of boiler exhaust through the low-temperature economizer, and then the deoxygenated water is sent to the high-pressure heater through the feed water pump, and the high-pressure heater absorbs the waste heat of extraction steam after the turbine works. After the water is heated, it is sent to the boiler, and the target steam obtained after heating by the boiler is sent to the turbine to work, and the turbine drives the motor to generate electricity.

[0020] It is further characterized by:

[0021] The condenser recovers the condensation latent heat of the saturated wet flue gas after the desulfurization tower, reducing the burden of high slurry temperature in the desulfurization tower;

[0022] The waste heat of the exhaust condensate after the turbine works is recovered through the low-pressure heater and the high-pressure heater. If the dissolved oxygen in the condensate does not meet the standard, the temperature is lowered and the condensate is returned to the deaeration tank to be mixed with the deaerated water to meet the dissolved oxygen standard.

[0023] The sensible heat of the flue gas before the desulfurization tower is recovered through the low-temperature economizer to improve the thermal efficiency of the boiler.

[0024] It is further characterized by:

[0025] After passing through the condenser, the water is heated to 35°C and enters the deaeration water tank. The temperature of the water entering the first medium input port of the low-pressure heater is 40°C-45°C, the temperature of the water entering the first medium input port of the low-temperature economizer is 45°C-50°C, the temperature of the water entering the first medium input port of the high-pressure heater is 65°C-70°C, the temperature of the water entering the water input end of the economizer is 104°C-158°C, the temperature of the water flowing out of the water output end of the economizer is 150°C-180°C, the steam discharged from the excess steam outlet of the steam turbine is 150°C-180°C, and the temperature of the water formed by the steam discharged from the exhaust steam outlet of the steam turbine after passing through the condenser is 50°C-55°C; the temperature of the water flowing into the deaeration water tank through the reflux port is 40°C-45°C;

[0026] The temperature of the flue gas flowing out of the flue is 130°C-140°C, the temperature of the flue gas flowing out of the dust collector is 120°C-130°C, the temperature of the flue gas flowing out of the low-temperature economizer is 95°C-105°C, the temperature of the flue gas introduced into the condenser inlet by the desulfurization tower through the induced draft fan is 55°C-60°C, and the temperature of the flue gas discharged after passing through the condenser is 50°C-55°C;

[0027] After passing through the membrane deaerator, the dissolved oxygen at room temperature is less than 5ppb.

[0028] Traditional thermal deaerators will fluctuate according to the boiler load, and the deoxidation effect is unstable. In order to prevent cavitation at the feedwater pump inlet, the thermal deaerator must be placed at a high position; the membrane deaerator of this system can be placed at a low position, rationally utilizing the floor space; this system can flexibly adjust the boiler feedwater temperature according to the boiler load, improve the boiler thermal efficiency, and the system is stable and reliable; users use this system to perform room-temperature deoxidation at the source of desalted water, eliminating the thermal deaerator, and achieving zero consumption of high-quality steam, which greatly reduces operating costs and eradicates the problems of cavitation at the feedwater pump inlet and oxygen corrosion of the delivery pipeline. The system also operates stably, greatly saving operation and maintenance costs, and providing users with new ideas for reintegrating waste heat resources, optimizing systems, and increasing efficiency and production. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic block diagram of the system connection corresponding to the present invention;

[0030] Figure 2 This is the process roadmap corresponding to the present invention;

[0031] Figure 3 This is a schematic structural diagram of the membrane deaerator used in the present invention;

[0032] Figure 4 It is a structural schematic diagram of the deoxidation membrane contactor of the present invention;

[0033] The names corresponding to the serial numbers in the figure are as follows:

[0034] Boiler 10, furnace 11, steam drum 12, flue 13, superheater 14, economizer 15, air preheater 16, circulating pump 17, blower 18, dust collector 20, desulfurization tower 30, induced draft fan 31, condenser 40, chimney 50, chemical water station 60, desalted water tank 70, desalted water pump 71, membrane deaerator 80, deaerated water tank 90, deaerated water pump 91, nitrogen blanketing module 92, low-pressure heater 100, low-temperature economizer 110, feedwater pump 111, high-pressure heater 120, nitrogen pipeline 130, steam turbine 140, generator 150, condenser 160, steam trap 161, nitrogen purge pipeline 170, exhaust gas discharge pipeline 180, water ring vacuum pump 181, bypass passage 190, valve 191, deoxidation membrane contactor 200;

[0035] Contactor body 1, oxygenated water inlet 2, deoxygenated water outlet 3, nitrogen inlet 4, exhaust gas outlet 5. DETAILED DESCRIPTION

[0036] A cascade waste heat recovery system based on membrane deoxidation, see Figure 1-Figure 4 , which includes a boiler flue gas cascade flow component, a water cascade flow component, and a superheated steam utilization component;

[0037] The boiler flue gas cascade flow assembly includes a boiler 10, a dust collector 20, a desulfurization tower 30, a condenser 40, and a chimney 50. The boiler 10 includes a furnace 11, a steam drum 12, and a flue 13. The outflow direction of the flue 13 is sequentially arranged with a superheater 14, an economizer 15, and an air preheater 16. A circulating pump 17 injects air into the steam drum 12, and a blower 18 injects air into the air preheater 16.

[0038] The water cascade flow assembly includes a chemical water station 60, a desalted water tank 70, a membrane deaerator 80, a deoxygenated water tank 90, and several step-by-step heating mechanisms. The step-by-step heating mechanisms include a low-pressure heater 100, a low-temperature economizer 110, and a high-pressure heater 120 in sequence along the water flow direction. A feedwater pump 111 is also provided between the low-temperature economizer 110 and the high-pressure heater 120.

[0039] and the superheated steam utilization assembly includes a steam turbine 140 and a generator 150;

[0040] The flue outlet of the boiler 10 is connected to the inlet of the dust collector 20. The flue gas discharged from the dust collector 20 is connected to the second medium inlet of the low-temperature economizer 110. The second medium outlet of the low-temperature economizer 110 then transports the flue gas to the inlet of the desulfurization tower 30. The outlet of the desulfurization tower 30 is connected to the induced draft fan 31 and then to the gas end inlet of the condenser 40. The gas end outlet of the condenser 40 is connected to the bottom of the chimney 50.

[0041] The water output from the chemical water station 60 passes through the desalted water tank 70 and the membrane deaerator 80 in sequence and is then connected to the water inlet of the condenser 40. A desalted water pump 71 is provided between the desalted water tank 70 and the membrane deaerator 80. The water outlet of the condenser 40 is connected to the input port of the deaerated water tank 90. ​​The output port of the deaerated water tank 90 is connected to the first medium input port of the low-pressure heater 100 through the deaerated water pump 91. The first medium output port of the low-pressure heater 100 is connected to the first medium input port of the low-temperature economizer 110. The first medium output port of the low-temperature economizer 110 is connected to the first medium input port of the high-pressure heater 120 through the feed water pump 111. The first medium output port of the high-pressure heater 120 is connected to the water input port of the economizer 15.

[0042] The water output end of the economizer 15 is connected to the steam drum 12, the output end of the steam drum 12 is connected to the steam input end of the superheater 14, the steam output end of the superheater 14 is connected to the steam input end of the steam turbine 140, the steam turbine 140 drives the generator 150 to generate electricity, the steam outlet of the steam turbine 140 includes an outlet for unused surplus steam and an outlet for used exhaust steam, the surplus steam outlet is connected to the second medium input port of the high-pressure heater 120, the second medium output port of the high-pressure heater 120 is connected to the return port of the deaeration water tank 90, the exhaust steam outlet is connected to the condenser 160, then to the steam trap 161, and then to the second medium input port of the low-pressure heater 100, and the second medium output port of the low-pressure heater 100 is connected to the return port of the deaeration water tank 90;

[0043] The gas input end of the membrane deaerator 80 is connected to the nitrogen purge pipeline 170, and the gas output port of the membrane deaerator 80 discharges the oxygen and nitrogen mixed gas through the waste gas discharge pipeline 180;

[0044] A nitrogen sealing module 92 is provided on the top of the deaeration water tank 90 , and the nitrogen sealing module 92 delivers nitrogen through a nitrogen pipeline 130 , and the nitrogen pipeline 130 simultaneously delivers nitrogen to a nitrogen purge pipeline 170 . A water ring vacuum pump 181 is also integrated on the exhaust gas discharge pipeline 180 .

[0045] During specific implementation, the membrane deaerator 80 includes two groups of deoxygenation membrane contactors 200, each group of deoxygenation membrane contactors 200 includes a contactor body 1, an oxygenated water inlet 2, a deoxygenated water outlet 3, a nitrogen inlet 4, and a waste gas outlet 5. The water inlets and outlets of the two groups of deoxygenation membrane contactors 80 are connected in series in sequence, and the two groups of nitrogen inlets 4 and waste gas outlets 5 are arranged in parallel. The oxygenated water inlet of the first-stage deoxygenation membrane contactor formed in series is the total water inlet, and the deoxygenated water outlet of the second-stage deoxygenation membrane contactor formed in series is the total water outlet.

[0046] When it is monitored that the dissolved oxygen in the water at the front end of the water inlet of the membrane deaerator 80 is less than 5ppb, the water does not need to pass through the membrane deaerator 80, and is directly opened through the valve 191 on the bypass passage 190 to enter the water end inlet of the condenser 40. When it is monitored that the dissolved oxygen in the water at the front end of the water inlet of the membrane deaerator 80 is not less than 5ppb, the valve 191 on the bypass passage 190 is closed, and the water flows into the water end inlet of the condenser 40 after passing through the membrane deaerator 80.

[0047] A cascade waste heat recovery process based on membrane deoxidation, see Figure 2It adopts a cascade waste heat recovery system based on membrane deoxidation. The deionized water pre-treated by the chemical water station is stored in the deionized water tank for standby use, and is pumped into the membrane deaerator for deoxygenation treatment. The deoxygenated water after deoxygenation absorbs the latent heat of the saturated wet flue gas after the desulfurization tower through the condenser and is stored in the deoxygenated water tank. The deoxygenated water tank must be nitrogen-sealed by the nitrogen sealing module. The deoxygenated water absorbs the waste heat of the exhaust condensate after the turbine works through the low-pressure heater, and then absorbs part of the waste heat of the boiler exhaust through the low-temperature economizer. After that, the deoxygenated water is sent to the high-pressure heater through the feed water pump, and the high-pressure heater absorbs the waste heat of the extraction steam after the turbine works. After the water is heated, it is sent to the boiler, and the target steam obtained after heating by the boiler is sent to the turbine to work, and the turbine drives the motor to generate electricity.

[0048] The condenser recovers the condensation latent heat of the saturated wet flue gas after the desulfurization tower, reducing the burden of high slurry temperature in the desulfurization tower;

[0049] The waste heat of the exhaust condensate after the turbine works is recovered through the low-pressure heater and the high-pressure heater. If the dissolved oxygen in the condensate does not meet the standard, the temperature is lowered and the condensate is returned to the deaeration tank to be mixed with the deaerated water to meet the dissolved oxygen standard.

[0050] The sensible heat of the flue gas before the desulfurization tower is recovered through the low-temperature economizer to improve the thermal efficiency of the boiler.

[0051] During specific implementation, the water is heated to 35°C after passing through the condenser and enters the deaeration water tank. The temperature of the water entering the first medium input port of the low-pressure heater is 40°C-45°C, the temperature of the water entering the first medium input port of the low-temperature economizer is 45°C-50°C, the temperature of the water entering the first medium input port of the high-pressure heater is 65°C-70°C, the temperature of the water entering the water input end of the economizer is 104°C-158°C, the temperature of the water flowing out of the water output end of the economizer is 150°C-180°C, the steam discharged from the excess steam outlet of the steam turbine is 150°C-180°C, and the temperature of the water formed by the steam discharged from the exhaust steam outlet of the steam turbine after passing through the condenser is 50°C-55°C; the temperature of the water flowing into the deaeration water tank through the return port is 40°C-45°C;

[0052] The temperature of the flue gas flowing out of the flue is 135°C, the temperature of the flue gas flowing out of the dust collector is 125°C, the temperature of the flue gas flowing out of the low-temperature economizer is 100°C, the temperature of the flue gas introduced into the condenser inlet by the desulfurization tower through the induced draft fan is 55°C-60°C, and the temperature of the flue gas discharged after passing through the condenser is 50°-55°C; after passing through the membrane deaerator, the dissolved oxygen at room temperature is <5ppb; when it is monitored that the dissolved oxygen of the water at the front end of the water inlet of the membrane deaerator is <5ppb, the water does not need to pass through the membrane deaerator, and is directly opened through the valve on the bypass passage to enter the water end inlet of the condenser; when it is monitored that the dissolved oxygen of the water at the front end of the water inlet of the membrane deaerator is not less than 5ppb, the valve on the bypass passage is closed, and the water flows into the water end inlet of the condenser after passing through the membrane deaerator.

[0053] Traditional thermal deaerators will fluctuate according to the boiler load, and the deoxidation effect is unstable. In order to prevent cavitation at the feedwater pump inlet, the thermal deaerator must be placed at a high position; the membrane deaerator of this system can be placed at a low position, rationally utilizing the floor space; this system can flexibly adjust the boiler feedwater temperature according to the boiler load, improve the boiler thermal efficiency, and the system is stable and reliable; users use this system to perform room-temperature deoxidation at the source of desalted water, eliminating the thermal deaerator, and achieving zero consumption of high-quality steam, which greatly reduces operating costs and eradicates the problems of cavitation at the feedwater pump inlet and oxygen corrosion of the delivery pipeline. The system also operates stably, greatly saving operation and maintenance costs, and providing users with new ideas for reintegrating waste heat resources, optimizing systems, and increasing efficiency and production.

[0054] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0055] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A cascade waste heat recovery system based on membrane deoxidation, characterized in that: It includes: A boiler flue gas cascade flow assembly, comprising a boiler, a dust collector, a desulfurization tower, a condenser, and a chimney. The boiler comprises a furnace, a steam drum, and a flue. The outflow direction of the flue is sequentially arranged with a superheater, an economizer, and an air preheater; Water cascade flow components, including a chemical water station, a desalted water tank, a membrane deaerator, a deoxygenated water tank, and a plurality of step-by-step heating mechanisms; and a superheated steam utilization assembly, which includes a steam turbine and a generator; The flue outlet of the boiler is connected to the inlet of the dust collector, the outlet of the dust collector is connected to the inlet of the desulfurization tower after cooling, the outlet of the desulfurization tower is connected to the gas end input port of the condenser, and the gas end output port of the condenser is connected to the bottom of the chimney; The water output from the chemical water station passes through the desalting water tank and the membrane deaerator in sequence and then is connected to the water end inlet of the condenser. The water end outlet of the condenser is connected to the input port of the deaeration water tank. The output port of the deaeration water tank is connected to the primary input ports of several step-up heating mechanisms. The high-order output ports of several step-up heating mechanisms pass the water into the water input end of the economizer. The water output end of the economizer is connected to the steam drum. The output end of the steam drum is connected to the steam input end of the superheater. The steam output end of the superheater is connected to the steam input end of the steam turbine. The steam turbine drives the generator to generate electricity. The low-temperature steam of the steam turbine is connected to the corresponding heating mechanisms of several step-up heating mechanisms to heat the deoxygenated water. The gas input end of the membrane deaerator is connected to a nitrogen purge pipeline, and the gas output port of the membrane deaerator discharges a mixed gas of oxygen and nitrogen.

2. The cascade waste heat recovery system based on membrane deoxidation according to claim 1 is characterized in that: Several step-by-step heating mechanisms include a low-pressure heater, a low-temperature economizer, and a high-pressure heater in sequence along the water flow direction, and a feed water pump is also arranged between the low-temperature economizer and the high-pressure heater.

3. The cascade waste heat recovery system based on membrane deoxidation according to claim 2 is characterized in that: The output port of the deaeration water tank is connected to the first medium input port of the low-pressure heater through a deaeration water pump, the first medium output port of the low-pressure heater is connected to the first medium input port of the low-temperature economizer, the first medium output port of the low-temperature economizer is connected to the first medium input port of the high-pressure heater through the feed water pump, and the first medium output port of the high-pressure heater is connected to the water input end of the economizer.

4. The cascade waste heat recovery system based on membrane deoxidation according to claim 3 is characterized in that: The steam outlet of the steam turbine includes an unused surplus steam outlet and a used exhaust steam outlet. The surplus steam outlet is connected to the second medium input port of the high-pressure heater, and the second medium output port of the high-pressure heater is connected to the reflux port of the deaeration water tank. The exhaust steam outlet is connected to the condenser and then to the second medium input port of the low-pressure heater, and the second medium output port of the low-pressure heater is connected to the reflux port of the deaeration water tank.

5. The cascade waste heat recovery system based on membrane deoxidation according to claim 1 is characterized in that: A nitrogen sealing module is arranged on the top of the deoxygenated water tank, and the nitrogen sealing module transports nitrogen through a nitrogen pipeline.

6. The cascade waste heat recovery system based on membrane deoxidation according to claim 2 is characterized in that: The flue gas discharged from the dust collector is connected to the second medium input port of the low-temperature economizer, and the second medium output port of the low-temperature economizer then transports the flue gas to the inlet of the desulfurization tower.

7. The cascade waste heat recovery system based on membrane deoxidation according to claim 1 is characterized in that: The membrane deaerator comprises at least one group of deoxygenation membrane contactors, each group of deoxygenation membrane contactors comprises a contactor body, an oxygenated water inlet, a deoxygenated water outlet, a nitrogen inlet, and a waste gas outlet. The water inlets and outlets of several groups of deoxygenation membrane contactors are connected in series in sequence, and several groups of nitrogen inlets and waste gas outlets are arranged in parallel. The oxygenated water inlet of the first-stage deoxygenation membrane contactor formed in series is the total water inlet, and the deoxygenated water outlet of the last-stage deoxygenation membrane contactor formed in series is the total water outlet.

8. A cascade waste heat recovery process based on membrane deoxidation, which adopts the cascade waste heat recovery system based on membrane deoxidation as claimed in any one of claims 1 to 7, characterized in that: The deionized water pre-treated by the chemical water station is stored in the deionized water tank for standby use, and is pumped into the membrane deaerator for deoxygenation treatment through the deionized water pump. The deoxygenated water after deoxygenation absorbs the latent heat of the saturated wet flue gas after the desulfurization tower through the condenser and is stored in the deoxygenated water tank. The deoxygenated water tank must be nitrogen-sealed by the nitrogen sealing module. The deoxygenated water absorbs the waste heat of the exhaust condensate after the turbine works through the low-pressure heater, and then absorbs part of the waste heat of the boiler exhaust gas through the low-temperature economizer. After that, the deoxygenated water is sent to the high-pressure heater through the feed water pump, and the high-pressure heater absorbs the waste heat of the extraction steam after the turbine works. After the water is heated, it is sent to the boiler, and the target steam obtained after heating by the boiler is sent to the steam turbine to work, and the steam turbine drives the motor to generate electricity.

9. The cascade waste heat recovery process based on membrane deoxidation according to claim 8, characterized in that: After passing through the condenser, the water is heated to 35°C and enters the deaeration water tank. The temperature of the water entering the first medium input port of the low-pressure heater is 40°C-45°C, the temperature of the water entering the first medium input port of the low-temperature economizer is 45°C-50°C, the temperature of the water entering the first medium input port of the high-pressure heater is 65°C-70°C, the temperature of the water entering the water input end of the economizer is 104°C-158°C, the temperature of the water flowing out from the water output end of the economizer is 150°C-180°C, the steam discharged from the excess steam outlet of the turbine is 150°C-180°C, and the temperature of the water formed by the steam discharged from the exhaust steam outlet of the turbine after passing through the condenser is 50°-55°C; the temperature of the water flowing in through the reflux port of the deaeration water tank is 40°C-45°C.

10. The cascade waste heat recovery process based on membrane deoxidation according to claim 8, characterized in that: The temperature of the flue gas flowing out of the flue is 130°C-140°C, the temperature of the flue gas flowing out of the dust collector is 120°C-130°C, the temperature of the flue gas flowing out of the low-temperature economizer is 95°C-105°C, the temperature of the flue gas introduced into the condenser inlet by the desulfurization tower through the induced draft fan is 55°C-60°C, and the temperature of the flue gas discharged after passing through the condenser is 50°-55°C; After passing through the membrane deaerator, the dissolved oxygen at room temperature is less than 5ppb.

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