Flue gas extremely-deep waste heat recovery system and method for graded flash evaporation of desulfurization slurry

Abstract

The invention discloses a flue gas extremely-deep waste heat recovery system and method for desulfurization slurry grading flash evaporation, and belongs to the technical field of coal-fired boiler waste heat heat supply. Desulfurization slurry grading flash evaporation, injection pressurization and boiler combustion-supporting air heating and humidifying technologies are combined, deep flue gas waste heat recovery is achieved, the water temperature of a heat supply network is greatly increased, and the process comprises the following steps that desulfurization slurry is firstly fed into a front flash evaporation tank to be subjected to first-stage negative pressure flash evaporation, and flash evaporation steam is used for conducting first-stage preheating on return water of the heat supply network; the first-stage concentrated slurry is sent to an injection flash tank for second-stage negative pressure flash evaporation, flash steam is sent to a negative pressure injector and is driven by heat supply steam to be pressurized into medium-pressure steam, and then second-stage large-temperature-difference heating is conducted on heat supply network return water; sending the second-stage concentrated slurry to a final-stage flash tank for final-stage negative-pressure flash evaporation, heating intermediate water by flash evaporation steam, and returning the final-stage slurry to a desulfurized water system; the intermediate water is fed into the total heat air preheater, and is fed into the boiler after heating, humidifying and overheating the inlet air of the boiler, so that the steam content of flue gas and the temperature of a desulfurization slurry outlet are increased.

Description

Technical Field

[0001] The invention relates to a system and method for recovering extremely deep waste heat of flue gas by graded flash evaporation of desulfurized slurry, belonging to the technical field of waste heat heating of coal-fired boilers. Background Art

[0002] The boiler is accompanied by a large amount of high-temperature flue gas emissions during the heat production process. Recovering the waste heat from the flue gas can not only reduce the heat loss of the boiler, but also reduce fuel consumption and greenhouse gas emissions and other pollutant emissions, and improve the thermal efficiency of the boiler. Therefore, the development of flue gas waste heat recovery technology is an important way to help achieve the "dual carbon" strategic goal. The existing flue gas waste heat recovery system mainly includes three categories: (1) The first category is to use a partition heat exchanger to achieve flue gas waste heat recovery, that is, to use a low-temperature economizer or energy saver. A shell-and-tube heat exchanger with other structures is installed on the boiler outlet flue to recover the flue gas waste heat and heat the boiler feed water or heating return water. This method has a simple system and a relatively low total cost. The disadvantage is that the waste heat recovery rate is low. Affected by changes in the inlet temperature of the heated water and the exhaust temperature, the actual temperature difference is usually only about 20 to 30°C, which is equivalent to about 1% to 2% of the heat generated by the boiler. The latent heat of vaporization of water vapor contained in the flue gas cannot be recovered, so deep heat recovery cannot be achieved. (2) The second category is to use absorption heat pump flue gas deep recovery technology. By using an absorption heat pump to recover flue gas waste heat and heat the return water of the heating network, the flue gas temperature can be greatly reduced to about 30°C, and the waste heat recovery amount is equivalent to 8% to 15% of the heat output of the boiler, so deep heat recovery can be achieved. However, the absorption heat pump needs to consume high-level heat sources such as steam to drive it, and the flue gas will produce a large amount of acidic condensate, so an anti-corrosion heat exchanger is required, and the equipment investment is relatively high. (3) The third type is based on air-to-flue gas full heat exchange recovery technology, which humidifies the air, increases the flue gas dew point temperature, and recovers the condensation heat of the flue gas in the tail flue. This method has a simple system structure, does not require an additional driving heat source, and has a low investment cost. In recent years, the series of patented technologies for "steam-heated boiler flue gas waste heat recovery" jointly developed by Tsinghua University and Beijing Tsinghua Tiangong Energy Technology Research Institute (patent numbers CN104110675B, CN107166420A, CN206929794U, etc.) have been widely promoted and applied and have good application prospects. However, the normal pressure flue gas waste heat spray tower usually uses intermediate water to extract heat from the flue gas. The size of the flue gas waste heat spray tower is large, and a certain amount of resistance will be added on the flue gas side. If the original induced draft fan has insufficient pressure margin, the fan needs to be replaced or a booster induced draft fan needs to be installed. The above situations will increase the cost, and sometimes the conditions for implementation are not available on site.

[0003] In recent years, the second type of flue gas waste heat recovery method has adopted the method of flash evaporation of desulfurized slurry + absorption heat pump, that is, the flue gas waste heat exchanger is abandoned, and the heat in the flue gas is taken out from the desulfurized slurry through the flash tank, and the flash steam is sent to the heat pump for waste heat recovery, and the return water of the heat network or other process water is heated, and the concentrated liquid is returned to the desulfurized circulating water. Its advantages are: no need to modify the flue gas system, reducing the difficulty of on-site implementation; the water quality of the condensed water is good, which is easy to recycle. But the disadvantages are also very obvious: the flue gas temperature can usually only be reduced to 42-45℃, and about half of the flue gas waste heat is recovered. It does not belong to deep heat recovery and can only be regarded as a half-finished project. A large amount of flue gas waste heat still escapes from the flue gas in vain. In the future, secondary transformation will be needed to achieve deep heat recovery. The cost of calculating the unit waste heat recovery is relatively high, and the investment recovery period is long. The fundamental reasons why it cannot reduce the flue gas temperature to 30℃ and achieve deep heat recovery are: first, the desulfurization slurry flash evaporation equipment is a vacuum equipment, and its system integration is relatively complex and has high security requirements. The lower the flash steam temperature, the larger the specific volume, the larger the equipment volume, and the higher the cost; second, more non-condensable gases such as SO2 will escape during the flash evaporation of the desulfurization slurry. The flash steam is sent to the absorption heat pump, and it is difficult to evacuate the heat during the condensation and heat release process in the horizontal evaporator of the heat pump. The absolute pressure during actual operation can only be maintained at about 7-8kPa. It is difficult to continue to increase the vacuum degree with existing equipment and conditions, and it is impossible to further increase the vacuum degree like a normal condenser. Therefore, the saturation temperature of the flash steam can only be reduced to 38-40℃, resulting in the flue gas temperature can only be reduced to 40-45℃; third, this technical method is essentially still a heat pump method, but the heat extraction device is different, and it still requires a lot of driving steam, with high operating costs, which worsens the problem of thermal power flexibility adjustment of the power plant and sometimes seriously affects the technical and economic benefits of the power plant. Summary of the invention

[0004] The purpose and task of the present invention is to construct a new flue gas waste heat recovery integrated system and propose the best operation strategy in view of the inherent technical limitations of the above-mentioned various flue gas waste heat recovery systems and their impact on the economic efficiency of power plants.

[0005] The specific description of the present invention is: a flue gas extreme depth waste heat recovery system for graded flash evaporation of desulfurized slurry, which is composed of an original boiler and auxiliary subsystem, a desulfurized slurry graded flash evaporation and heating process water subsystem and a combustion-supporting air heating and humidification subsystem. The original boiler and auxiliary subsystem include a boiler body 4, an air preheater 5, a blower 6, a dust collector 7, an induced draft fan 8, an original desulfurization tower 1, an original flue gas inlet pipe 2 and an original slurry pump 3, characterized in that the desulfurized slurry graded flash evaporation and heating process water subsystem includes a pre-flash tank 21, a pre-heater 26, an ejector flash tank 11, an ejector 15, an ejector heater 16, a vacuum pump 18, a final flash tank 31, an intermediate water heater 36 and connecting pipelines and components, and the combustion-supporting air heating and humidification subsystem The system comprises a full-heat air preheater 40, a fresh air superheater 41, an intermediate water pump 42 and connecting pipelines and components, wherein the slurry inlet of the pre-flash tank 21 is connected to the waste heat slurry outlet of the bottom desulfurization slurry pool of the original desulfurization tower 1 through the waste heat slurry pump 10, and is connected to the slurry inlet of the ejection flash tank 11 through the pre-slurry bypass valve 24, and the first-stage concentrated slurry outlet of the pre-flash tank 21 is connected to the slurry inlet of the ejection flash tank 11, and is connected to the slurry inlet of the final flash tank 31 through the ejection slurry bypass valve 34, the second-stage concentrated slurry outlet of the ejection flash tank 11 is connected to the slurry inlet of the final flash tank 31, the final concentrated slurry outlet of the final flash tank 31 is connected to the inlet of the original slurry pump 3, and is connected to the final flash tank 31 through the intermediate slurry bypass valve 44. The slurry inlet of the steam tank 31 is connected; the flash steam pre-defogger 22 and the pre-washing spray layer 23 are arranged on the upper part of the interior of the pre-flash tank 21, and the flash steam outlet is connected to the steam inlet of the pre-heater 26 through the pre-connecting pipe 25, and the condensate outlet of the pre-heater 26 is connected to the inlet of the pre-condensation pump 27, and the outlet of the pre-condensation pump 27 is respectively connected to the inlet of the pre-washing spray layer 23, the ejection washing spray layer 13 and the final-stage washing spray layer 33, and is communicated with the outlet pipe of the pre-exhaust condensate W1; the flash steam ejection demister 12 and the ejection washing spray layer 13 are arranged on the upper part of the interior of the ejection flash tank 11, and the flash steam outlet is connected to the low-pressure steam inlet pipe 15b of the ejector 15, and the high-pressure steam inlet pipe 15a of the ejector 15 is connected to the low-pressure steam inlet pipe 15b of the ejector 15. The ejector regulating valve 14 is connected to the steam supply pipe of the heating steam Q, the medium-pressure exhaust pipe 15c of the ejector 15 is connected to the steam inlet of the ejector heater 16, the condensate outlet of the ejector heater 16 is connected to the inlet of the ejector condensate pump 17, and the outlet of the ejector condensate pump 17 is connected to the outlet pipe of the ejector condensate W; the low-temperature water inlet of the preheater 26 is connected to the water inlet pipe of the return water H1 of the heating network, and the low-temperature water outlet of the preheater 26 is respectively connected to the water inlet pipe of the return water H1 of the heating network, the water return pipe of the return water H2 of the heating network, and the low-temperature water inlet of the ejector heater 16 through the hot water series-parallel valve group 28, and the low-temperature water outlet of the ejector heater 16 is respectively connected to the low-temperature water inlet of the ejector heater 16 and the water return pipe of the return water H2 of the heating network;The flash steam final demister 32 and the final washing spray layer 33 are arranged at the upper part of the final flash tank 31, and the flash steam outlet is connected to the steam inlet of the intermediate water heater 36 through the final connecting pipe 35, and the condensate outlet of the intermediate water heater 36 is connected to the inlet of the intermediate water condensate pump 37, and the outlet of the intermediate water condensate pump 37 is respectively connected to the low-temperature water outlet of the intermediate water heater 36, the spray water inlet of the full heat air preheater 40 and the outlet pipe of the intermediate external condensate W2; the intermediate water heater 36 The low-temperature water inlet of the preheater 40 is connected to the bottom water pool of the full-heat air preheater 40 through the intermediate water pump 42; the outlet of the preheater 26 of the preheater 26 is connected to the outlet of the intermediate water non-condensable gas S2 of the intermediate water heater 36 through the non-condensable gas regulating valve 29, and is connected to the low-pressure steam inlet pipe 15b of the ejector 15; the outlet of the ejector non-condensable gas S1 of the ejector heater 16 is connected to the air inlet of the vacuum pump 18, and the exhaust port of the vacuum pump 18 is connected to the original flue gas inlet pipe 2 of the original desulfurization tower 1; the full-heat air preheater 4 0 is provided with an air inlet for ambient air A0 at the bottom, and an air outlet for humidified air A1 is provided at the top of the full-heat air preheater 40. The air outlet of humidified air A1 is connected to the air inlet of the fresh air superheater 41. The air outlet of the fresh air superheater 41 is connected to the air inlet of the blower 6 through the connecting pipe of the superheated air A2. The air outlet of the blower 6 is connected to the furnace of the boiler body 4 through the air inlet side of the air preheater 5; the exhaust side outlet of the air preheater 5 is connected to the original flue gas inlet pipe 2 through the dust collector 7 and the induced draft fan 8. The original flue gas The upstream of the smoke inlet pipe 2 is the inlet of the original flue gas Y1, the downstream is the smoke inlet of the original desulfurization tower 1, and the top of the original desulfurization tower 1 is the outlet of the clean flue gas Y2; the circulating liquid outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower 1 is respectively connected to the inlet of the original slurry pump 3, the water inlet pipe of the desulfurization makeup water B and the final concentrated slurry outlet of the final flash tank 31, the outlet of the original slurry pump 3 is connected to the circulating slurry inlet of the original desulfurization tower 1, and the sewage outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower 1 is connected to the drainage pipe of the desulfurization wastewater P. ;

[0006] The specific methods and steps of the flue gas extreme deep waste heat recovery system are as follows.

[0007] (i) When the temperature of the return water H1 from the heating network is less than or equal to 35℃.

[0008] (i) When the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump 3 is higher than that of the return water H1 of the heating network by more than 10°C, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is closed, the ejector slurry bypass valve 34 is opened, and the intermediate slurry bypass valve 44 is closed. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the first-stage concentrated slurry is sent to the final flash tank 31 for flashing, and the final concentrated slurry outlet is sent to the inlet of the raw slurry pump 3 and sent to the slurry circulation spraying device of the desulfurization tower to continue the desulfurization operation; and the return water H1 of the heating network is sent to the pre-heater 26 and heated by the flash steam, and then returns to the return water pipe of the return water H2 of the heating network through the series-parallel valve group 28 of the heating network water. ; The intermediate water from the intermediate water pump 42 is sent to the intermediate water heater 36, and is heated by flash steam and then sent to the spray water inlet of the full-heat air preheater 40, and performs direct contact full-heat countercurrent heat exchange with the ambient air A0 flowing in from the lower air inlet inside the full-heat air preheater 40. The intermediate water temperature and flow rate are reduced and then flow out from the water pool at the bottom of the tower, and the humidified air A1 at the outlet of the full-heat air preheater 40 is heated by the fresh air superheater 42 and then enters the blower 6; the front non-condensable gas S and the intermediate water non-condensable gas S2 both pass through the ejector 15 and the ejector heater 16 to enter the outlet of the ejected non-condensable gas S1, and are extracted by the vacuum pump 18 and return to the original desulfurization tower 1 through the original flue gas inlet pipe 2 to continue the desulfurization treatment and discharge.

[0009] (ii) When the temperature difference of the outlet desulfurization slurry of the raw slurry pump 3 is less than 10°C higher than the temperature of the return water H1 of the heating network, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is opened, the ejector slurry bypass valve 34 is closed, and the intermediate slurry bypass valve 44 is closed. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the primary concentrated slurry is sent to the ejector flash tank 11 for flashing again, and then sent to the final flash tank 31 for flashing again through the secondary concentrated slurry outlet of the ejector flash tank 11. The final concentrated slurry outlet is sent to the inlet of the raw slurry pump 3 and sent to the desulfurization tower slurry circulation spraying device to continue the desulfurization operation; and the return water H1 of the heating network is sent to the pre-heater 26 and heated by the flash steam, and then sent to the ejector heater 16 through the heating network water series-parallel valve group 28 and is raised. The compressed flash steam continues to be heated and then returns to the return water pipe of the heat network return water H2; the intermediate water from the intermediate water pump 42 is sent to the intermediate water heater 36, and is sent to the spray water inlet of the full heat air preheater 40 after being heated by the flash steam, and is directly contacted with the ambient air A0 flowing in from the lower air inlet inside the full heat air preheater 40. Full heat countercurrent heat exchange is carried out, and the intermediate water temperature and flow rate are reduced and then flow out from the water pool at the bottom of the tower, and the humidified air A1 at the outlet of the full heat air preheater 40 is heated by the fresh air superheater 42 and then enters the blower 6; the front non-condensable gas S and the intermediate water non-condensable gas S2 are both ejected and pressurized by the ejector 15 and sent to the ejector heater 16, and finally are extracted by the vacuum pump 18 from the outlet of the ejected non-condensable gas S1 and return to the original desulfurization tower 1 through the original flue gas inlet pipe 2 to continue desulfurization treatment and discharge.

[0010] (ii) When the temperature of the return water H1 from the heating network is greater than 35°C.

[0011] (iii) If the temperature of the return water H1 of the heating network is less than 55°C, and the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump 3 is greater than that of the return water H1 of the heating network by more than 10°C, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is opened, the ejector slurry bypass valve 34 is closed, and the intermediate slurry bypass valve 44 is closed. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the primary concentrated slurry is sent to the ejector flash tank 11 for flashing again, and then sent to the final flash tank 31 for flashing again through the secondary concentrated slurry outlet of the ejector flash tank 11. The final concentrated slurry outlet is sent to the inlet of the raw slurry pump 3 and sent to the desulfurization tower slurry circulation spraying device to continue the desulfurization operation; and the return water H1 of the heating network is sent to the pre-heater 26 and heated by the flash steam, and then sent to the hot water series-parallel valve group 28 for flashing again. The flash steam that is sent to the ejector heater 16 and is pressurized is further heated and then returned to the return water pipe of the heat network return water H2; the intermediate water from the intermediate water pump 42 is sent to the intermediate water heater 36, and is heated by the flash steam and then sent to the spray water inlet of the full heat air preheater 40, and is directly contacted with the ambient air A0 flowing in from the lower air inlet inside the full heat air preheater 40. The intermediate water temperature and flow rate are reduced and then flow out from the water pool at the bottom of the tower, and the humidified air A1 at the outlet of the full heat air preheater 40 is heated by the fresh air superheater 42 and then enters the blower 6; the front non-condensable gas S and the intermediate water non-condensable gas S2 are both ejected and pressurized by the ejector 15 and sent to the ejector heater 16, and finally are extracted by the vacuum pump 18 from the outlet of the ejected non-condensable gas S1 and return to the original desulfurization tower 1 through the original flue gas inlet pipe 2 to continue the desulfurization treatment and discharge.

[0012] (iv) If the temperature of the return water H1 of the heating network is greater than or equal to 55°C, and the temperature difference of the outlet desulfurization slurry of the raw slurry pump 3 is greater than that of the return water H1 of the heating network by more than 10°C, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is opened, the ejector slurry bypass valve 34 is closed, the intermediate slurry bypass valve 44 is opened, the valves of the low-temperature water inlet and outlet of the heating network water series-parallel valve group 28 connected to the pre-heater 26 are opened, the intermediate water condensate pump 37 is stopped, and the intermediate water pump 42 is stopped. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the primary concentrated slurry is sent to the ejector flash tank 24. After flash evaporation again in the steam tank 11, it is sent to the inlet of the original slurry pump 3 through the secondary concentrated slurry outlet of the ejector flash tank 11 and the intermediate slurry bypass valve 44, and is sent to the desulfurization tower slurry circulation spraying device to continue the desulfurization operation; and a part of the heat network return water H1 is sent to the pre-heater 26 and heated by the flash steam, and then returns to the return pipe of the heat network return water H2 through the heat network water series-parallel valve group 28, and the other part is sent to the ejector heater 16 and continues to be heated by the pressurized flash steam, and then returns to the return pipe of the heat network return water H2; the full hot air preheater 40 is shut down, and the boiler intake air enters the blower 6 from the original air inlet.

[0013] (v) If the temperature difference of the outlet desulfurization slurry of the raw slurry pump 3 is less than 10°C higher than the temperature of the return water H1 of the heating network, the pre-slurry bypass valve 24 is opened, the valves of the low-temperature water inlet and outlet of the heating network water series-parallel valve group 28 connected to the pre-heater 26 are opened, and the pre-condensation pump 27 is closed. At this time, the pre-flash tank 21 and the pre-heater 26 are shut down, and the rest of the entire waste heat recovery system is implemented in accordance with Article iii and Article iv of the above Article 2.

[0014] (iii) The pre-discharged condensate W1 is preferentially used as the washing water source for the pre-washing spray layer 23, the ejected washing spray layer 13 and the final washing spray layer 33, and eventually replaces a portion of the desulfurization make-up water B to become a part of the desulfurization slurry circulating liquid; the ejected condensate W and the intermediate condensate W2 are respectively connected to the make-up water pipe inlet of the desulfurization make-up water B, the heat network return water H1 and / or the outlet pipe of the pre-discharged condensate W1 and serve as the corresponding make-up water sources for the latter three.

[0015] The preheater 26, the ejector heater 16 and the intermediate water heater 36 respectively adopt a vertical tube-in-tube heat exchange structure, with a condensate hot well at the bottom and a non-condensable gas outlet above the hot well liquid surface.

[0016] The pre-flash tank 21, the ejector flash tank 11 and the final flash tank 31 respectively adopt a spray-type flash structure, a spiral-feeding flash structure or a filler evaporation flash structure.

[0017] The interior of the pre-flash tank 21 and the final flash tank 31 is set as a first-stage heat exchange zone or an N-stage heat exchange zone, where N is greater than or equal to 2. When N-stage heat exchange zones are set, the flash concentrated slurry outlet of each heat exchange zone is connected to the slurry inlet of the next heat exchange zone, and the low-temperature water inlet is connected to the low-temperature water outlet of the next heat exchange zone.

[0018] The ejector 15 adopts a high-efficiency wide-range negative pressure ejector structure with an internal flow cross-sectional area adjusted in nearly equal proportion.

[0019] The vacuum pump 18 adopts a water ring vacuum pump, a water jet steam extractor or a Roots vacuum pump structure.

[0020] The heating steam Q is steam at a pressure greater than normal pressure or negative pressure steam; the outlet pipes of the induced external condensate W and the intermediate external condensate W2 are respectively connected to the feed pipe inlet of the desulfurization feed water B, the heat network return water H1 and / or the outlet pipe of the front external condensate W1.

[0021] The beneficial effects of the present invention are as follows.

[0022] (1) Heat is extracted from the desulfurization slurry without adding a flue gas waste heat exchanger and its flue gas resistance; at the same time, the waste heat of the low-temperature section of the flue gas is humidified by the boiler inlet air through the final flash evaporation through the intermediate water system, and enters the flue gas in the form of latent heat of water vaporization, thereby increasing the water vapor content, energy grade and desulfurization slurry temperature of the flue gas. The first-stage flash evaporation can directly heat the heat network return water, and the second-stage flash evaporation can continue to heat the heat network return water by the ejector-type heat pump composed of the ejector, thereby realizing the deep heat recovery of the flue gas. The flue gas temperature can be reduced to 20-30℃, fundamentally avoiding the inherent shortcomings of the existing desulfurization slurry flash evaporation + absorption heat pump technology, and the waste heat recovery amount can reach 2-3 times.

[0023] (2) The ejector heat pump system is composed of the ejector flash tank 11, the ejector 15, the ejector heater 16 and other equipment components to recover a part of the waste heat of the desulfurization slurry. The ejector 15 adopts a high-efficiency wide-range negative pressure ejector structure with nearly proportional adjustment of the internal flow cross-sectional area. As a result, the ejector heat pump can flexibly adapt to changes in technical conditions such as the inlet and outlet temperature, pressure and flow rate on the flash evaporation side of the desulfurization slurry and the return water heating side of the heat network, while always maintaining a high system energy efficiency ratio.

[0024] (3) Direct heat exchange is used to recover the waste heat of the desulfurized slurry to the greatest extent possible, and the return water of the heating network and the inlet air of the boiler are preheated; and the ejector heat pump system can also maintain a very high energy efficiency ratio, thereby greatly reducing the steam consumption of the driving steam, and its steam consumption is only a fraction of the conventional desulfurized slurry flash evaporation + absorption heat pump method. Therefore, the comprehensive energy efficiency ratio of the entire waste heat recovery system can reach 3 to 6 times, which is much higher than the conventional method.

[0025] (4) The non-condensable gas of the pre-heater 26 and the intermediate water heater 36 contains a large amount of SO2, etc., and there is no need to set up a special vacuum pump. Instead, it is directly extracted by the ejector 15, which can maintain a very high vacuum degree (reduced to an absolute pressure of 2 to 4 kPa when necessary). Therefore, the pre-flash tank 21 and the final flash tank 31 can achieve a very high vacuum degree, which is beneficial to significantly reduce the temperature of the desulfurization slurry, and then significantly reduce the exhaust gas temperature, realize extremely deep flue gas waste heat recovery, and recover the flue gas waste heat to the greatest extent.

[0026] (5) The steam side pressure of the ejector heater 16 is relatively high, so the technical requirements for the vacuum pump 18 for extracting the non-condensable gas are reduced; at the same time, the pre-heater 26, the ejector heater 16 and the intermediate water heater 36 respectively adopt a vertical shell-and-tube heat exchange structure, which is also conducive to improving the vacuum pump 18 to extract the non-condensable gas and maintain a higher vacuum degree.

[0027] (6) Using the condensate of flash steam to wash the demister in each flash tank can maintain a higher demisting effect and replenish the desulfurization slurry to replace part or all of the original desulfurization replenishment water B. The latter's replenishment water source is often wastewater with a high chloride content. After using the condensate of flash steam to replace it, it can help to significantly reduce the discharge flow of desulfurization wastewater P, and greatly reduce the secondary pollution and operation and maintenance costs caused by it, thereby improving the operation effect of the desulfurization system.

[0028] (7) The condensate from flash steam can also be used as replenishment water for the heat network return water, which helps to significantly reduce the amount of softened water produced and its cost. At the same time, the comprehensive recycling of condensate can also significantly reduce the consumption of water resources accordingly.

[0029] (8) This scheme and system can be widely applied to the flue gas waste heat recovery heating system of thermal power plants and boiler rooms, promoting the realization of comprehensive technical and economic benefits of energy conservation and environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a system schematic diagram of the present invention.

[0031] Figure 1 The part numbers and names are as follows.

[0032] Original desulfurization tower 1, original flue gas inlet pipe 2, original slurry pump 3, boiler body 4, air preheater 5, blower 6, dust collector 7, induced draft fan 8, waste heat slurry pump 10, ejector flash tank 11, flash steam ejector demister 12, ejector washing spray layer 13, ejector regulating valve 14, ejector 15, high pressure steam inlet pipe 15a, low pressure steam inlet pipe 15b, medium pressure exhaust pipe 15c, ejector heater 16, ejector condensate pump 17, vacuum pump 18, pre-flash tank 21, flash steam pre-demister 22, pre-washing spray layer 23, pre-slurry bypass valve 24, pre-connecting pipe 25, pre-heater 26, pre-condensate pump 27, hot water series-parallel valve group 28, non-condensable gas regulating Valve 29, final flash tank 31, final flash steam demister 32, final washing spray layer 33, induced slurry bypass valve 34, final connecting pipe 35, intermediate water heater 36, intermediate water condensate pump 37, full heat air preheater 40, fresh air superheater 41, intermediate water pump 42, intermediate slurry bypass valve 44, ambient air A0, humidified air A1, superheated air A2, desulfurization makeup water B, heating network return water H1, heating network return water H2, desulfurization wastewater P, heating steam Q, front non-condensable gas S, induced non-condensable gas S1, intermediate water non-condensable gas S2, induced external discharge condensate W, front external discharge condensate W1, intermediate external discharge condensate W2, raw flue gas Y1, clean flue gas Y2. DETAILED DESCRIPTION

[0033] Figure 1 It is a system schematic diagram and embodiment of the present invention.

[0034] Specific embodiments of the present invention are as follows.

[0035] The specific description of the embodiment of the present invention is: a flue gas extreme depth waste heat recovery system for graded flash evaporation of desulfurized slurry, which is composed of an original boiler and auxiliary machine subsystem, a desulfurized slurry graded flash evaporation and heating process water subsystem and a combustion-supporting air heating and humidification subsystem. The original boiler and auxiliary machine subsystem include a boiler body 4, an air preheater 5, a blower 6, a dust collector 7, an induced draft fan 8, an original desulfurization tower 1, an original flue gas inlet pipe 2 and an original slurry pump 3, characterized in that the desulfurized slurry graded flash evaporation and heating process water subsystem includes a pre-flash tank 21, a pre-heater 26, an ejector flash tank 11, an ejector 15, an ejector heater 16, a vacuum pump 18, a final flash tank 31, an intermediate water heater 36 and connecting pipelines and components, and the combustion-supporting air heating and humidification subsystem The subsystem includes a full-heat air preheater 40, a fresh air superheater 41, an intermediate water pump 42 and connecting pipelines and components, wherein the slurry inlet of the pre-flash tank 21 is connected to the waste heat slurry outlet of the bottom desulfurization slurry pool of the original desulfurization tower 1 through the waste heat slurry pump 10, and is connected to the slurry inlet of the ejection flash tank 11 through the pre-slurry bypass valve 24, and the first-stage concentrated slurry outlet of the pre-flash tank 21 is connected to the slurry inlet of the ejection flash tank 11, and is connected to the slurry inlet of the final flash tank 31 through the ejection slurry bypass valve 34, the second-stage concentrated slurry outlet of the ejection flash tank 11 is connected to the slurry inlet of the final flash tank 31, the final concentrated slurry outlet of the final flash tank 31 is connected to the inlet of the original slurry pump 3, and is connected to the final slurry outlet through the intermediate slurry bypass valve 44. The slurry inlet of the first-stage flash tank 31 is connected; the flash steam pre-defogger 22 and the pre-washing spray layer 23 are arranged on the upper part of the interior of the pre-flash tank 21, and the flash steam outlet is connected to the steam inlet of the pre-heater 26 through the pre-connecting pipe 25, and the condensate outlet of the pre-heater 26 is connected to the inlet of the pre-condensation pump 27, and the outlet of the pre-condensation pump 27 is respectively connected to the inlet of the pre-washing spray layer 23, the ejection washing spray layer 13 and the final-stage washing spray layer 33, and is communicated with the outlet pipe of the pre-exhaust condensate W1; the flash steam ejection demister 12 and the ejection washing spray layer 13 are arranged on the upper part of the interior of the ejection flash tank 11, and the flash steam outlet is connected to the low-pressure steam inlet pipe 15b of the ejector 15, and the high-pressure steam inlet pipe 15a of the ejector 15 The ejector 15 is connected to the steam supply pipe of the heating steam Q through the ejector regulating valve 14, the medium-pressure exhaust pipe 15c of the ejector 15 is connected to the steam inlet of the ejector heater 16, the condensate outlet of the ejector heater 16 is connected to the inlet of the ejector condensate pump 17, and the outlet of the ejector condensate pump 17 is connected to the outlet pipe of the ejector condensate W; the low-temperature water inlet of the pre-heater 26 is connected to the water inlet pipe of the return water H1 of the heating network, the low-temperature water outlet of the pre-heater 26 is respectively connected to the water inlet pipe of the return water H1 of the heating network, the water return pipe of the return water H2 of the heating network and the low-temperature water inlet of the ejector heater 16 through the hot water series-parallel valve group 28, and the low-temperature water outlet of the ejector heater 16 is respectively connected to the low-temperature water inlet of the ejector heater 16 and the water return pipe of the return water H2 of the heating network;The flash steam final demister 32 and the final washing spray layer 33 are arranged at the upper part of the final flash tank 31, and the flash steam outlet is connected to the steam inlet of the intermediate water heater 36 through the final connecting pipe 35, and the condensate outlet of the intermediate water heater 36 is connected to the inlet of the intermediate water condensate pump 37, and the outlet of the intermediate water condensate pump 37 is respectively connected to the low-temperature water outlet of the intermediate water heater 36, the spray water inlet of the full heat air preheater 40 and the outlet pipe of the intermediate external condensate W2; the intermediate water heater 36 The low-temperature water inlet of the preheater 40 is connected to the bottom water pool of the full-heat air preheater 40 through the intermediate water pump 42; the outlet of the preheater 26 of the preheater 26 is connected to the outlet of the intermediate water non-condensable gas S2 of the intermediate water heater 36 through the non-condensable gas regulating valve 29, and is connected to the low-pressure steam inlet pipe 15b of the ejector 15; the outlet of the ejector non-condensable gas S1 of the ejector heater 16 is connected to the air inlet of the vacuum pump 18, and the exhaust port of the vacuum pump 18 is connected to the original flue gas inlet pipe 2 of the original desulfurization tower 1; the full-heat air preheater 4 0 is provided with an air inlet for ambient air A0 at the bottom, and an air outlet for humidified air A1 is provided at the top of the full-heat air preheater 40. The air outlet of humidified air A1 is connected to the air inlet of the fresh air superheater 41. The air outlet of the fresh air superheater 41 is connected to the air inlet of the blower 6 through the connecting pipe of the superheated air A2. The air outlet of the blower 6 is connected to the furnace of the boiler body 4 through the air inlet side of the air preheater 5; the exhaust side outlet of the air preheater 5 is connected to the original flue gas inlet pipe 2 through the dust collector 7 and the induced draft fan 8. The original flue gas The upstream of the smoke inlet pipe 2 is the inlet of the original flue gas Y1, the downstream is the smoke inlet of the original desulfurization tower 1, and the top of the original desulfurization tower 1 is the outlet of the clean flue gas Y2; the circulating liquid outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower 1 is respectively connected to the inlet of the original slurry pump 3, the water inlet pipe of the desulfurization makeup water B and the final concentrated slurry outlet of the final flash tank 31, the outlet of the original slurry pump 3 is connected to the circulating slurry inlet of the original desulfurization tower 1, and the sewage outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower 1 is connected to the drainage pipe of the desulfurization wastewater P. ;

[0036] The specific methods and steps of the flue gas extreme deep waste heat recovery system are as follows.

[0037] (i) When the temperature of the return water H1 from the heating network is less than or equal to 35℃.

[0038] (i) When the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump 3 is higher than that of the return water H1 of the heating network by more than 10°C, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is closed, the ejector slurry bypass valve 34 is opened, and the intermediate slurry bypass valve 44 is closed. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the first-stage concentrated slurry is sent to the final flash tank 31 for flashing, and the final concentrated slurry outlet is sent to the inlet of the raw slurry pump 3 and sent to the slurry circulation spraying device of the desulfurization tower to continue the desulfurization operation; and the return water H1 of the heating network is sent to the pre-heater 26 and heated by the flash steam, and then returns to the return water pipe of the return water H2 of the heating network through the series-parallel valve group 28 of the heating network water. ; The intermediate water from the intermediate water pump 42 is sent to the intermediate water heater 36, and is heated by flash steam and then sent to the spray water inlet of the full-heat air preheater 40, and performs direct contact full-heat countercurrent heat exchange with the ambient air A0 flowing in from the lower air inlet inside the full-heat air preheater 40. The intermediate water temperature and flow rate are reduced and then flow out from the water pool at the bottom of the tower, and the humidified air A1 at the outlet of the full-heat air preheater 40 is heated by the fresh air superheater 42 and then enters the blower 6; the front non-condensable gas S and the intermediate water non-condensable gas S2 both pass through the ejector 15 and the ejector heater 16 to enter the outlet of the ejected non-condensable gas S1, and are extracted by the vacuum pump 18 and return to the original desulfurization tower 1 through the original flue gas inlet pipe 2 to continue the desulfurization treatment and discharge.

[0039] (ii) When the temperature difference of the outlet desulfurization slurry of the raw slurry pump 3 is less than 10°C higher than the temperature of the return water H1 of the heating network, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is opened, the ejector slurry bypass valve 34 is closed, and the intermediate slurry bypass valve 44 is closed. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the primary concentrated slurry is sent to the ejector flash tank 11 for flashing again, and then sent to the final flash tank 31 for flashing again through the secondary concentrated slurry outlet of the ejector flash tank 11. The final concentrated slurry outlet is sent to the inlet of the raw slurry pump 3 and sent to the desulfurization tower slurry circulation spraying device to continue the desulfurization operation; and the return water H1 of the heating network is sent to the pre-heater 26 and heated by the flash steam, and then sent to the ejector heater 16 through the heating network water series-parallel valve group 28 and is raised. The compressed flash steam continues to be heated and then returns to the return water pipe of the heat network return water H2; the intermediate water from the intermediate water pump 42 is sent to the intermediate water heater 36, and is sent to the spray water inlet of the full heat air preheater 40 after being heated by the flash steam, and is directly contacted with the ambient air A0 flowing in from the lower air inlet inside the full heat air preheater 40. Full heat countercurrent heat exchange is carried out, and the intermediate water temperature and flow rate are reduced and then flow out from the water pool at the bottom of the tower, and the humidified air A1 at the outlet of the full heat air preheater 40 is heated by the fresh air superheater 42 and then enters the blower 6; the front non-condensable gas S and the intermediate water non-condensable gas S2 are both ejected and pressurized by the ejector 15 and sent to the ejector heater 16, and finally are extracted by the vacuum pump 18 from the outlet of the ejected non-condensable gas S1 and return to the original desulfurization tower 1 through the original flue gas inlet pipe 2 to continue desulfurization treatment and discharge.

[0040] (ii) When the temperature of the return water H1 from the heating network is greater than 35°C.

[0041] (iii) If the temperature of the return water H1 of the heating network is less than 55°C, and the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump 3 is greater than that of the return water H1 of the heating network by more than 10°C, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is opened, the ejector slurry bypass valve 34 is closed, and the intermediate slurry bypass valve 44 is closed. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the primary concentrated slurry is sent to the ejector flash tank 11 for flashing again, and then sent to the final flash tank 31 for flashing again through the secondary concentrated slurry outlet of the ejector flash tank 11. The final concentrated slurry outlet is sent to the inlet of the raw slurry pump 3 and sent to the desulfurization tower slurry circulation spraying device to continue the desulfurization operation; and the return water H1 of the heating network is sent to the pre-heater 26 and heated by the flash steam, and then sent to the hot water series-parallel valve group 28 for flashing again. The flash steam that is sent to the ejector heater 16 and is pressurized is further heated and then returned to the return water pipe of the heat network return water H2; the intermediate water from the intermediate water pump 42 is sent to the intermediate water heater 36, and is heated by the flash steam and then sent to the spray water inlet of the full heat air preheater 40, and is directly contacted with the ambient air A0 flowing in from the lower air inlet inside the full heat air preheater 40. The intermediate water temperature and flow rate are reduced and then flow out from the water pool at the bottom of the tower, and the humidified air A1 at the outlet of the full heat air preheater 40 is heated by the fresh air superheater 42 and then enters the blower 6; the front non-condensable gas S and the intermediate water non-condensable gas S2 are both ejected and pressurized by the ejector 15 and sent to the ejector heater 16, and finally are extracted by the vacuum pump 18 from the outlet of the ejected non-condensable gas S1 and return to the original desulfurization tower 1 through the original flue gas inlet pipe 2 to continue the desulfurization treatment and discharge.

[0042] (iv) If the temperature of the return water H1 of the heating network is greater than or equal to 55°C, and the temperature difference of the outlet desulfurization slurry of the raw slurry pump 3 is greater than that of the return water H1 of the heating network by more than 10°C, the pre-slurry bypass valve 24 is closed, the ejector regulating valve 14 is opened, the ejector slurry bypass valve 34 is closed, the intermediate slurry bypass valve 44 is opened, the valves of the low-temperature water inlet and outlet of the heating network water series-parallel valve group 28 connected to the pre-heater 26 are opened, the intermediate water condensate pump 37 is stopped, and the intermediate water pump 42 is stopped. After the slurry sent by the waste heat slurry pump 10 is flashed in the pre-flash tank 21, the primary concentrated slurry is sent to the ejector flash tank 24. After flash evaporation again in the steam tank 11, it is sent to the inlet of the original slurry pump 3 through the secondary concentrated slurry outlet of the ejector flash tank 11 and the intermediate slurry bypass valve 44, and is sent to the desulfurization tower slurry circulation spraying device to continue the desulfurization operation; and a part of the heat network return water H1 is sent to the pre-heater 26 and heated by the flash steam, and then returns to the return pipe of the heat network return water H2 through the heat network water series-parallel valve group 28, and the other part is sent to the ejector heater 16 and continues to be heated by the pressurized flash steam, and then returns to the return pipe of the heat network return water H2; the full hot air preheater 40 is shut down, and the boiler intake air enters the blower 6 from the original air inlet.

[0043] (v) If the temperature difference of the outlet desulfurization slurry of the raw slurry pump 3 is less than 10°C higher than the temperature of the return water H1 of the heating network, the pre-slurry bypass valve 24 is opened, the valves of the low-temperature water inlet and outlet of the heating network water series-parallel valve group 28 connected to the pre-heater 26 are opened, and the pre-condensation pump 27 is closed. At this time, the pre-flash tank 21 and the pre-heater 26 are shut down, and the rest of the entire waste heat recovery system is implemented in accordance with Article iii and Article iv of the above Article 2.

[0044] (iii) The pre-discharged condensate W1 is preferentially used as the washing water source for the pre-washing spray layer 23, the ejected washing spray layer 13 and the final washing spray layer 33, and eventually replaces a portion of the desulfurization make-up water B to become a part of the desulfurization slurry circulating liquid; the ejected condensate W and the intermediate condensate W2 are respectively connected to the make-up water pipe inlet of the desulfurization make-up water B, the heat network return water H1 and / or the outlet pipe of the pre-discharged condensate W1 and serve as the corresponding make-up water sources for the latter three.

[0045] The preheater 26, the ejector heater 16 and the intermediate water heater 36 respectively adopt a vertical tube-in-tube heat exchange structure, with a condensate hot well at the bottom and a non-condensable gas outlet above the hot well liquid surface.

[0046] The pre-flash tank 21, the ejector flash tank 11 and the final flash tank 31 respectively adopt a spray-type flash structure, a spiral-feeding flash structure or a filler evaporation flash structure.

[0047] The interior of the pre-flash tank 21 and the final flash tank 31 is set as a first-stage heat exchange zone or an N-stage heat exchange zone, where N is greater than or equal to 2. When N-stage heat exchange zones are set, the flash concentrated slurry outlet of each heat exchange zone is connected to the slurry inlet of the next heat exchange zone, and the low-temperature water inlet is connected to the low-temperature water outlet of the next heat exchange zone.

[0048] The ejector 15 adopts a high-efficiency wide-range negative pressure ejector structure with an internal flow cross-sectional area adjusted in nearly equal proportion.

[0049] The vacuum pump 18 adopts a water ring vacuum pump, a water jet steam extractor or a Roots vacuum pump structure.

[0050] The heating steam Q is steam at a pressure greater than normal pressure or negative pressure steam; the outlet pipes of the induced external condensate W and the intermediate external condensate W2 are respectively connected to the feed pipe inlet of the desulfurization feed water B, the heat network return water H1 and / or the outlet pipe of the front external condensate W1.

[0051] It should be noted that the present invention is based on key technologies such as multi-stage flash evaporation of desulfurization slurry, boiler air heating and humidification, ejector heat pump pressurization and heat exchange of flash steam, ejector vacuumization, and the use of clean condensate as desulfurization makeup water to reduce the discharge flow of desulfurization wastewater. A whole set of new integrated systems and operation strategies for ultra-deep flue gas waste heat recovery by flash evaporation of desulfurization slurry are proposed. According to this overall solution, there may be different specific implementation measures and specific implementation devices with different structures. The above-mentioned specific implementation method is only one of them. Any other similar simple deformation implementation methods, such as simple deformation of flash steam and heat exchanger and simple adjustment of related pipelines, or simple changes in vacuumization method and condensate recovery method, all fall within the protection scope of the present invention.

Claims

1. A flue gas extreme-depth waste heat recovery system for desulfurization slurry graded flash evaporation, comprising an original boiler and auxiliary subsystem, a desulfurization slurry graded flash evaporation and heating process water subsystem and a combustion-supporting air heating and humidification subsystem, wherein the original boiler and auxiliary subsystem comprises a boiler body (4), an air preheater (5), a blower (6), a dust collector (7), an induced draft fan (8), an original desulfurization tower (1), an original flue gas inlet pipe (2) and an original slurry pump (3), characterized in that: The desulfurization slurry graded flash evaporation and heating process water subsystem comprises a pre-flash tank (21), a pre-heater (26), an ejector flash tank (11), an ejector (15), an ejector heater (16), a vacuum pump (18), a final flash tank (31), an intermediate water heater (36) and connecting pipelines and components; the combustion air heating and humidification subsystem comprises a full-heat air preheater (40), a fresh air superheater (41), an intermediate water pump (42) and connecting pipelines and components, wherein the slurry inlet of the pre-flash tank (21) is connected to the waste heat slurry outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower (1) through a waste heat slurry pump (10), and is connected to the slurry inlet of the ejector flash tank (11) through a pre-slurry bypass valve (24). The first concentrated slurry outlet of the pre-flash tank (21) is connected to the slurry inlet of the ejector flash tank (11), and is connected to the slurry inlet of the final flash tank (31) through an ejector slurry bypass valve (34); the second concentrated slurry outlet of the ejector flash tank (11) is connected to the slurry inlet of the final flash tank (31); the final concentrated slurry outlet of the final flash tank (31) is connected to the inlet of the raw slurry pump (3), and is connected to the slurry inlet of the final flash tank (31) through an intermediate slurry bypass valve (44); a flash steam pre-demister (22) and a pre-washing spray layer (23) are provided at the upper part of the pre-flash tank (21); the flash steam outlet of the pre-heater (26) is connected to the steam outlet of the pre-heater (26) through a pre-connecting pipe (25). The condensate outlet of the preheater (26) is connected to the inlet of the pre-condensate pump (27); the outlet of the pre-condensate pump (27) is respectively connected to the inlets of the pre-washing spray layer (23), the ejector washing spray layer (13) and the final-stage washing spray layer (33), and is communicated with the outlet pipe of the pre-exhaust condensate (W1); the upper part of the ejector flash tank (11) is provided with a flash steam ejector demister (12) and an ejector washing spray layer (13); the flash steam outlet is connected to the low-pressure steam inlet pipe (15b) of the ejector (15); the high-pressure steam inlet pipe (15a) of the ejector (15) is communicated with the steam supply pipe of the heating steam (Q) via the ejector regulating valve (14); the medium-pressure steam exhaust pipe (15c) of the ejector (15) is connected to the steam supply pipe of the heating steam (Q) via the ejector regulating valve (14); the medium-pressure steam exhaust pipe (15c) of the ejector (15) is connected to the steam supply pipe of the heating steam (Q) via the ejector regulating valve (14); the medium-pressure steam exhaust pipe (15d) of the ejector (15) is connected to the steam supply pipe of the heating steam (Q) via the ejector regulating valve (14); the medium-pressure steam exhaust pipe (15d) of the ejector (15) is connected to the steam supply pipe of the heating steam (Q) via the ejector regulating valve (14); the medium-pressure steam exhaust pipe (15c ... ) is connected to the steam inlet of the ejector heater (16), the condensate outlet of the ejector heater (16) is connected to the inlet of the ejector condensate pump (17), and the outlet of the ejector condensate pump (17) is connected to the outlet pipe of the ejector condensate (W); the low-temperature water inlet of the pre-heater (26) is connected to the water inlet pipe of the heat network return water (H1), the low-temperature water outlet of the pre-heater (26) is respectively connected to the water inlet pipe of the heat network return water (H1), the water return pipe of the heat network return water return (H2) and the low-temperature water inlet of the ejector heater (16) through the heat network water series-parallel valve group (28), and the low-temperature water outlet of the ejector heater (16) is respectively connected to the low-temperature water inlet of the ejector heater (16) and the water return pipe of the heat network return water return (H2);The upper part of the inner part of the final flash tank (31) is provided with a flash steam final demister (32) and a final washing spray layer (33), wherein the flash steam outlet is connected to the steam inlet of the intermediate water heater (36) through the final connecting pipe (35), the condensate outlet of the intermediate water heater (36) is connected to the inlet of the intermediate water condensate pump (37), and the outlet of the intermediate water condensate pump (37) is respectively connected to the low-temperature water outlet of the intermediate water heater (36), the spray water inlet of the full heat air preheater (40) and the outlet pipe of the intermediate external condensate water (W2); the low-temperature water of the intermediate water heater (36) is connected to the inlet of the intermediate water condensate pump (37). The inlet is connected to the water pool at the bottom of the full-heat air preheater (40) through the intermediate water pump (42); the outlet of the preheater (26) of the preheater (26) is connected to the outlet of the intermediate water non-condensable gas (S2) of the intermediate water heater (36) through the non-condensable gas regulating valve (29), and is connected to the low-pressure steam inlet pipe (15b) of the ejector (15); the outlet of the ejector heater (16) of the ejector non-condensable gas (S1) is connected to the air inlet of the vacuum pump (18), and the exhaust port of the vacuum pump (18) is connected to the original flue gas inlet pipe (2) of the original desulfurization tower (1); the full-heat air preheater (40 ) is provided with an air inlet for ambient air (A0) at the bottom, and an air outlet for humidified air (A1) is provided at the top of the full-heat air preheater (40). The air outlet of the humidified air (A1) is connected to the air inlet of the fresh air superheater (41). The air outlet of the fresh air superheater (41) is connected to the air inlet of the blower (6) through a connecting pipe for superheated air (A2). The air outlet of the blower (6) is connected to the furnace of the boiler body (4) through the air inlet side of the air preheater (5). The exhaust side outlet of the air preheater (5) is connected to the original flue gas inlet pipe (2) through a dust collector (7) and an induced draft fan (8). The upstream of the flue gas inlet pipe (2) is the inlet of the original flue gas (Y1), the downstream is the inlet of the original desulfurization tower (1), and the top of the original desulfurization tower (1) is the outlet of the clean flue gas (Y2); the circulating liquid outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower (1) is respectively connected to the inlet of the original slurry pump (3), the water inlet pipe of the desulfurization makeup water (B) and the final concentrated slurry outlet of the final flash tank (31), the outlet of the original slurry pump (3) is connected to the circulating slurry inlet of the original desulfurization tower (1), and the sewage outlet of the desulfurization slurry pool at the bottom of the original desulfurization tower (1) is connected to the drainage pipe of the desulfurization wastewater (P). ; 2. A flue gas extreme-depth waste heat recovery system for desulfurization slurry graded flash distillation as claimed in claim 1, characterized in that The specific method and steps of the flue gas extreme-depth waste heat recovery system are as follows: (I) When the temperature of the return water (H1) of the heating network is less than or equal to 35℃: (i) When the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump (3) is higher than that of the return water (H1) of the heating network by more than 10°C, the pre-slurry bypass valve (24) is closed, the ejector regulating valve (14) is closed, the ejector slurry bypass valve (34) is opened, and the intermediate slurry bypass valve (44) is closed. The slurry sent by the waste heat slurry pump (10) is flashed in the pre-flash tank (21), and the first-stage concentrated slurry is sent to the final flash tank (31) for flashing. The final concentrated slurry outlet is sent to the inlet of the raw slurry pump (3) and sent to the desulfurization tower slurry circulation spray device to continue the desulfurization operation; and the return water (H1) of the heating network is sent to the pre-heater (26) and heated by the flash steam, and then returns to the return water return pipe of the heating network (H2) through the heating network water series-parallel valve group (28); The intermediate water of the intermediate water pump (42) is sent to the intermediate water heater (36), and is heated by flash steam and then sent to the spray water inlet of the full heat air preheater (40). In the full heat air preheater (40), the intermediate water is subjected to direct contact full heat countercurrent heat exchange with the ambient air (A0) flowing in from the lower air inlet. The intermediate water temperature and flow rate are reduced and then the intermediate water flows out from the water pool at the bottom of the tower. The humidified air (A1) at the air outlet of the full heat air preheater (40) is heated by the fresh air superheater (42) and then enters the blower (6). The front non-condensable gas (S) and the intermediate water non-condensable gas (S2) are both passed through the ejector (15) and the ejector heater (16) and enter the ejector non-condensable gas (S1) outlet, and are extracted by the vacuum pump (18) and returned to the original desulfurization tower (1) through the original flue gas inlet pipe (2) to continue desulfurization treatment and discharge. (ii) When the temperature difference between the outlet desulfurized slurry temperature of the raw slurry pump (3) and the temperature of the return water (H1) of the heating network is less than 10°C, the pre-slurry bypass valve (24) is closed, the ejector regulating valve (14) is opened, the ejector slurry bypass valve (34) is closed, and the intermediate slurry bypass valve (44) is closed. The slurry sent by the waste heat slurry pump (10) is flashed in the pre-flash tank (21), and the primary concentrated slurry is sent to the ejector flash tank (11). After flashing again, the secondary concentrated slurry outlet of the ejector flash tank (11) is sent to the final flash tank (31) for flashing again. The final concentrated slurry outlet is sent to the inlet of the original slurry pump (3) and sent to the desulfurization tower slurry circulation spray device to continue the desulfurization operation; and the return water (H1) of the heating network is sent to the pre-heater (26) and heated by the flash steam, and then sent to the ejector heater (16) through the heating network water series-parallel valve group (28) and pressurized. The flash steam continues to heat the intermediate water, and then returns to the return water return pipe of the heat network return water (H2); the intermediate water from the intermediate water pump (42) is sent to the intermediate water heater (36), and is heated by the flash steam and then sent to the spray water inlet of the full heat air preheater (40), and is directly contacted with the ambient air (A0) flowing in from the lower air inlet inside the full heat air preheater (40) for full heat countercurrent heat exchange. The intermediate water temperature and flow rate are reduced and then it flows out from the water pool at the bottom of the tower, and the full heat The humidified air (A1) at the air outlet of the air preheater (40) is heated by the fresh air superheater (42) and then enters the blower (6); the front non-condensable gas (S) and the intermediate water non-condensable gas (S2) are both ejected and pressurized by the ejector (15) and sent to the ejector heater (16); finally, the ejected non-condensable gas (S1) is extracted by the vacuum pump (18) from the outlet and returns to the original desulfurization tower (1) through the original flue gas inlet pipe (2) to continue desulfurization treatment and discharge; (ii) When the temperature of the return water (H1) of the heating network is greater than 35°C: (iii) If the temperature of the return water (H1) of the heating network is less than 55°C, and the temperature difference of the desulfurized slurry at the outlet of the original slurry pump (3) is greater than the temperature of the return water (H1) of the heating network by more than 10°C, the pre-slurry bypass valve (24) is closed, the ejector regulating valve (14) is opened, the ejector slurry bypass valve (34) is closed, and the intermediate slurry bypass valve (44) is closed. The slurry delivered by the waste heat slurry pump (10) is flashed in the pre-flash tank (21). The first-stage concentrated slurry is sent to the ejector flash tank (11) for flash evaporation again, and then sent to the final flash tank (31) for flash evaporation again through the second-stage concentrated slurry outlet of the ejector flash tank (11). The final concentrated slurry outlet is sent to the inlet of the raw slurry pump (3) and sent to the desulfurization tower slurry circulation spray device to continue the desulfurization operation; and the heat network return water (H1) is sent to the pre-heater (26) and heated by the flash steam, and then sent to the ejector flash tank (11) through the heat network water series-parallel valve group (28). The intermediate water from the intermediate water pump (42) is sent to the intermediate water heater (36), and after being heated by the flash steam, it is sent to the spray water inlet of the full heat air preheater (40), and directly contacts the ambient air (A0) flowing in from the lower air inlet inside the full heat air preheater (40) for full heat countercurrent heat exchange. After the intermediate water temperature and flow rate are reduced, it is discharged from the water pool at the bottom of the tower. The humidified air (A1) at the outlet of the full-heat air preheater (40) is heated by the fresh air superheater (42) and then enters the blower (6); the front non-condensable gas (S) and the intermediate water non-condensable gas (S2) are both ejected and pressurized by the ejector (15) and sent to the ejector heater (16); finally, the ejected non-condensable gas (S1) is extracted by the vacuum pump (18) from the outlet and returns to the original desulfurization tower (1) through the original flue gas inlet pipe (2) to continue desulfurization treatment and discharge; (iv) If the temperature of the return water (H1) of the heating network is greater than or equal to 55°C, and the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump (3) is greater than or equal to 10°C higher than the temperature of the return water (H1) of the heating network, the pre-slurry bypass valve (24) is closed, the ejector regulating valve (14) is opened, the ejector slurry bypass valve (34) is closed, the intermediate slurry bypass valve (44) is opened, the valves of the low-temperature water inlet and outlet of the pre-heater (26) of the heating network water series-parallel valve group (28) are opened, the intermediate water condensate pump (37) is stopped, and the intermediate water pump (42) is stopped. After the slurry sent by the waste heat slurry pump (10) is flashed in the pre-flash tank (21), the primary concentrated slurry is sent to the ejector. After flashing again in the flash tank (11), the concentrated slurry is sent to the inlet of the raw slurry pump (3) through the secondary concentrated slurry outlet of the ejector flash tank (11) and the intermediate slurry bypass valve (44), and then sent to the desulfurization tower slurry circulation spray device to continue the desulfurization operation; a part of the heat network return water (H1) is sent to the preheater (26) and heated by the flash steam, and then returns to the return water pipe of the heat network return water (H2) through the heat network water series-parallel valve group (28), and the other part is sent to the ejector heater (16) and continues to be heated by the pressurized flash steam, and then returns to the return water pipe of the heat network return water (H2); the full hot air preheater (40) is stopped, and the boiler inlet air enters the blower (6) from the original air inlet; (v) If the temperature difference of the desulfurized slurry at the outlet of the raw slurry pump (3) is less than 10°C higher than the temperature of the return water (H1) of the heating network, the pre-slurry bypass valve (24) is opened, the valves of the heating network water series-parallel valve group (28) connected to the low-temperature water inlet and outlet of the pre-heater (26) are opened, and the pre-condensation pump (27) is closed. At this time, the pre-flash tank (21) and the pre-heater (26) are stopped, and the rest of the waste heat recovery system is implemented in accordance with the above-mentioned items (iii) and (iv) of item (ii); (iii) The pre-discharged condensate (W1) is preferentially used as the washing water source for the pre-washing spray layer (23), the ejector washing spray layer (13) and the final washing spray layer (33), and eventually replaces a portion of the desulfurization makeup water (B) to become a part of the desulfurization slurry circulating liquid; the ejector condensate (W) and the intermediate condensate (W2) are respectively connected to the makeup water pipe inlet of the desulfurization makeup water (B), the heat network return water (H1) and / or the outlet pipe of the pre-discharged condensate (W1) and serve as the corresponding makeup water sources for the latter three.

3. The system for recovering flue gas extremely deep waste heat by graded flash distillation of desulfurized slurry according to claim 1, characterized in that The preheater (26), the ejector heater (16) and the intermediate water heater (36) respectively adopt a vertical tube-in-tube heat exchange structure, a condensate water hot well is arranged at the bottom, and a non-condensable gas discharge port is arranged above the hot well liquid surface.

4. The system for recovering flue gas extremely deep waste heat by graded flash distillation of desulfurized slurry according to claim 1, characterized in that The pre-flash tank (21), the ejector flash tank (11) and the final flash tank (31) respectively adopt a spray-type flash structure, a spiral-feeding flash structure or a filler evaporation flash structure.

5. The system for recovering flue gas extremely deep waste heat by graded flash distillation of desulfurized slurry according to claim 1, characterized in that The interior of the pre-flash tank (21) and the final flash tank (31) is configured as a first-stage heat exchange zone or an N-stage heat exchange zone, wherein N is greater than or equal to 2. When N-stage heat exchange zones are configured, the flash concentrated slurry outlet of each stage heat exchange zone is connected to the slurry inlet of the next stage heat exchange zone, and the low-temperature water inlet is connected to the low-temperature water outlet of the next stage heat exchange zone.

6. The system for recovering flue gas extremely deep waste heat by graded flash distillation of desulfurized slurry according to claim 1, characterized in that The ejector (15) adopts a high-efficiency wide-range negative pressure ejector structure in which the internal flow cross-sectional area is adjusted in nearly equal proportion.

7. The system for recovering flue gas extremely deep waste heat by graded flash distillation of desulfurized slurry according to claim 1, characterized in that The vacuum pump (18) adopts a water ring vacuum pump, a water jet steam extractor or a Roots vacuum pump structure.

8. The system for recovering flue gas extremely deep waste heat by graded flash distillation of desulfurized slurry according to claim 1, characterized in that The heating steam (Q) is steam with a pressure greater than normal pressure or negative pressure steam; the outlet pipes of the induced external condensate (W) and the intermediate external condensate (W2) are respectively connected to the feed pipe inlet of the desulfurization feed water (B), the heat network return water (H1) and / or the outlet pipe of the front external condensate (W1).

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