CO2 heat pump system for waste heat recovery and cascade heating and its working method

Abstract

The application belongs to the technical field of heat pump and industrial waste heat recycling, and discloses a CO2 heat pump system for waste heat recycling and cascade heat supply and a working method thereof. In view of the problems that low-temperature waste heat in the steel and iron metallurgical industry cannot be effectively recycled, medium-temperature waste heat has low power generation efficiency, high-temperature heat supply relies on fossil fuels, and the control of the strong coupling system is difficult under variable conditions, the application realizes deep recycling of waste heat in the whole temperature range and simultaneously produces hot air and industrial steam by using the wide temperature slip characteristics of CO2 supercritical heat release, through the cooperation of an ultrahigh-temperature CO2 transcritical cycle heat pump subsystem and a high-temperature steam preparation subsystem, and by using four heat exchangers to recycle high-temperature flue gas, low-temperature flue gas, blast furnace cooling water and blast furnace semi-clean coal gas waste heat. The technical scheme disclosed by the application greatly improves the utilization rate of waste heat in the metallurgical industry.

Description

Technical Field

[0001] This invention belongs to the field of heat pump and industrial waste heat recovery and utilization technology, specifically relating to a CO2 heat pump system for waste heat recovery and cascade heating and its working method. Background Technology

[0002] In the iron and steel metallurgical industry, approximately 50% of the input energy is emitted as waste heat and waste energy in various forms, with the total waste heat resources of the entire industry reaching 350 million tons of standard coal equivalent. The current status of low-grade waste heat recovery and utilization in the iron and steel metallurgical industry is not optimistic; data shows that the high-temperature waste heat recovery rate is about 44.4%, the medium-temperature waste heat recovery rate is about 30.2%, while the low-temperature waste heat recovery rate is less than 2%.

[0003] Currently, the recovery of low-grade waste heat in the iron and steel metallurgical industry mainly relies on three technologies: waste heat boiler steam power generation, organic Rankine cycle power generation, and direct heat exchange utilization. The overall recovery efficiency across the industry is low, primarily due to the inherent limitations of existing technologies and their insufficient compatibility with the characteristics of waste heat resources in the iron and steel industry. Specifically, firstly, waste heat boiler steam power generation, the mainstream application, is based on a steam Rankine cycle. However, due to the vaporization characteristics of water, low-grade waste heat below 160℃ cannot vaporize water to produce saturated steam, making recovery impossible. Even for medium-temperature waste heat, the pressure and temperature of the produced steam are low due to the enthalpy of the heat source. The theoretical Carnot efficiency of the cycle is already limited, and coupled with actual losses such as pipeline heat dissipation, auxiliary equipment power consumption, and mechanical transmission losses, the actual power generation efficiency is only 10%-15%, with most of the heat energy irreversibly dissipated during heat exchange and work. Secondly, while the organic Rankine cycle technology, which can adapt to lower-temperature waste heat, overcomes the water vaporization temperature threshold by using a low-boiling-point organic working fluid, the organic working fluid has poor thermal stability and is prone to thermal decomposition above 200℃. This makes it unable to cover waste heat in the medium- and high-temperature range, and the working fluid's heat transfer performance is weaker than water, resulting in greater irreversible losses during heat exchange. Consequently, the system's net energy efficiency improvement is very limited, and the actual operating efficiency is far below the design value, making large-scale promotion difficult. Thirdly, although direct heat exchange utilization technology has a simple structure and low investment cost, it is limited by the poor heat transfer end, preventing deep recovery of waste heat. A large amount of low-temperature sensible heat is still directly emitted with the flue gas. Furthermore, this technology can only achieve the transfer of heat energy at the same or lower grade, failing to improve heat energy quality. It can only match a small number of nearby low-temperature heating scenarios, resulting in a very limited application scope and an inability to absorb the massive amount of medium- and low-temperature waste heat in the industry. Summary of the Invention

[0004] The purpose of this invention is to provide a CO2 heat pump system and its operating method for waste heat recovery and cascade heating. Utilizing the transcritical cycle characteristics of CO2, it adapts to the cascade utilization needs of high and low temperature waste heat in metallurgy, solving one or more of the technical problems existing in current waste heat recovery schemes in the iron and steel metallurgy industry, such as low waste heat recovery efficiency, the need for large-scale combustion of fossil fuels for supplemental heating, and high overall carbon emissions. The CO2 heat pump system disclosed in this invention can efficiently recover low-grade waste heat from the metallurgical industry, output ultra-high temperature heat sources, and complete cascade heating, thereby improving the comprehensive waste heat recovery and utilization rate, reducing fossil fuel consumption, and lowering carbon emissions from industrial heating.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a CO2 heat pump system for waste heat recovery and cascade heating, comprising: an ultra-high temperature CO2 transcritical cycle heat pump subsystem and a high temperature steam preparation subsystem; wherein, The ultra-high temperature CO2 transcritical cycle heat pump subsystem includes: a compressor, a first gas cooler, a second gas cooler, a third gas cooler, an expander, a first CO2-flue gas heat exchanger, a CO2-cooling water heat exchanger, a CO2-gas heat exchanger, and a second CO2-flue gas heat exchanger; wherein, the outlet of the compressor is connected to the inlet of the expander via the CO2 side channels of the first gas cooler, the second gas cooler, and the third gas cooler; the outlet of the expander is connected to the inlet of the CO2 side channel of the first CO2-flue gas heat exchanger, the CO2 side channel of the CO2-cooling water heat exchanger, and the CO2 side channel of the CO2-gas heat exchanger; the outlets of the CO2 side channels of the first CO2-flue gas heat exchanger, the CO2 side channel of the CO2-cooling water heat exchanger, and the CO2 side channel of the CO2-gas heat exchanger converge and are connected to the inlet of the compressor via the CO2 side channel of the second CO2-flue gas heat exchanger, forming a CO2 circulation loop; The high-temperature steam preparation subsystem includes: a water supply regulating valve, a first three-way valve, a water pump, the third gas cooler, a throttle valve, a flash tank, a second gas cooler, and a bypass valve; wherein, the water supply regulating valve is connected to the first inlet of the first three-way valve, the saturated water outlet of the flash tank is connected to the second inlet of the first three-way valve, the outlet of the first three-way valve is connected to the inlet of the flash tank via the water pump, the water-side channel of the third gas cooler, and the throttle valve, and the saturated steam outlet of the flash tank is connected to the steam outlet via the water-side channel of the second gas cooler; the water-side channel of the second gas cooler is connected in parallel with a bypass pipeline, one end of the bypass pipeline is connected to the connecting pipeline between the flash tank and the second gas cooler, and the other end is connected to the connecting pipeline between the second gas cooler and the steam outlet, and the bypass valve is installed on the bypass pipeline; The first gas cooler has an air-side channel for introducing air to be heated; the second CO2-flue gas heat exchanger has a flue gas-side channel inlet for introducing high-temperature flue gas from the iron and steel metallurgical industry, and its flue gas-side channel outlet is connected to the first CO2-flue gas heat exchanger's flue gas-side channel inlet, while the first CO2-flue gas heat exchanger's flue gas-side channel outlet is used to discharge the heat-exchanged flue gas; a low-temperature flue gas conveying pipe is connected to the connecting pipe between the second CO2-flue gas-side channel outlet and the first CO2-flue gas-side channel inlet, and this low-temperature flue gas conveying pipe is used to introduce low-temperature flue gas from the iron and steel metallurgical industry; the CO2-cooling water heat exchanger has a water-side channel for introducing blast furnace cooling water from the iron and steel metallurgical industry; and the CO2-gas heat exchanger has a gas-side channel for introducing semi-clean blast furnace gas from the iron and steel metallurgical industry.

[0006] A further improvement of the technical solution of the present invention is that it also includes: a water replenishment flow PID control module, used to control the opening of the water replenishment regulating valve in accordance with the steam output flow rate; The correlation formula for the steam output flow rate is as follows: ; in, The water supply flow rate for the water supply regulating valve; The high-temperature steam output flow rate for the steam outlet; The mixed water flow rate is the flow rate of the pipeline between the first three-way valve and the water pump; The water supply temperature for the water supply regulating valve; The pressure of the water after it has been pressurized by the water pump; The water-side channel outlet temperature of the third gas cooler; The pressure of the two-phase fluid after the throttle valve; These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0007] A further improvement of the technical solution of the present invention is that it also includes: a steam output pressure PID control module, used to control the opening of the throttle valve according to the steam output pressure correlation. The correlation for the steam production pressure is as follows: ; In the formula, The high-temperature steam output pressure for the steam outlet; , , , These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0008] A further improvement of the technical solution of the present invention is that it also includes: a steam production temperature PID control module, used to control the opening degree of the bypass valve according to the steam production temperature correlation. The correlation for the steam production temperature is as follows: ; In the formula, The high-temperature steam output temperature for the steam outlet; This represents the steam flow rate through the bypass pipe; , , , , These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0009] A further improvement of the technical solution of the present invention is that it also includes: a compressor discharge pressure PID control module, used to control the compressor speed according to the optimal compressor discharge pressure correlation; The optimal compressor discharge pressure correlation is as follows: ; In the formula, The optimal compressor discharge pressure; This refers to the compressor discharge temperature. This refers to the compressor suction temperature. The hot air output temperature at the outlet of the air-side passage of the first gas cooler; , , , These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0010] A further improvement to the technical solution of the present invention is that it further includes: The first flow meter is installed at the steam outlet to detect the high-temperature steam output flow rate at the steam outlet. The second flow meter and the second temperature sensor are installed on the pipeline where the water supply regulating valve is located to detect the water supply flow rate and the water supply temperature of the water supply regulating valve, respectively. The third flow meter is used to install on the pipeline between the first three-way valve and the water pump to detect the flow rate of the mixed water through the pipeline between the first three-way valve and the water pump. The third pressure sensor is installed on the pipeline between the water pump and the third gas cooler to detect the pressure of the water after it has been pressurized by the water pump. The third temperature sensor is installed on the pipeline between the third gas cooler and the throttle valve to detect the water-side channel outlet temperature of the third gas cooler. The fourth pressure sensor is installed on the pipeline between the throttle valve and the flash tank to detect the pressure of the two-phase fluid after the throttle valve; The first pressure sensor is installed at the steam outlet to detect the high-temperature steam output pressure at the steam outlet. The first temperature sensor is installed at the steam outlet to detect the high temperature of the steam produced at the steam outlet. The fourth flow meter is used to install on the bypass line to detect the steam flow rate through the bypass line; The fifth temperature sensor is installed on the pipeline between the compressor and the first gas cooler to detect the compressor's exhaust temperature; The fourth temperature sensor is installed on the pipeline between the second CO2-flue gas heat exchanger and the compressor to detect the compressor's suction temperature. The sixth temperature sensor is installed at the outlet of the air-side channel of the first gas cooler to detect the temperature of the hot air produced at the outlet of the air-side channel of the first gas cooler.

[0011] A further improvement of the technical solution of the present invention is that the temperature of the high-temperature flue gas is 300℃~350℃, the temperature of the low-temperature flue gas is 120℃~180℃, the temperature of the blast furnace cooling water is 60℃~80℃, and the temperature of the blast furnace semi-clean gas is 120℃~180℃.

[0012] A further improvement of the technical solution of the present invention is that the temperature of the heated air output from the wind-side channel of the first gas cooler is above 500°C; and the temperature of the high-temperature steam output from the steam outlet is above 200°C.

[0013] A further improvement to the technical solution of the present invention lies in that, A first shut-off valve is installed at the inlet of the air-side passage of the first gas cooler; A second shut-off valve is installed at the inlet of the flue gas side passage of the second CO2-flue gas heat exchanger; The low-temperature flue gas conveying pipeline is equipped with a third shut-off valve; A fourth shut-off valve is installed at the inlet of the water-side channel of the CO2-cooling water heat exchanger; A fifth shut-off valve is installed at the inlet of the gas side channel of the CO2-gas heat exchanger.

[0014] A second aspect of the present invention provides a method for operating a CO2 heat pump system for waste heat recovery and cascade heating, comprising: After being compressed into a high-temperature, high-pressure supercritical fluid by a compressor, the CO2 working fluid flows sequentially through the CO2 side channels of the first gas cooler, the second gas cooler, and the third gas cooler, gradually releasing heat and cooling down. The cooled CO2 then enters an expander for further expansion and depressurization. The expanded CO2 then enters the CO2 side channels of the first CO2-flue gas heat exchanger, the CO2-cooling water heat exchanger, and the CO2-gas heat exchanger, all connected in parallel, and absorbs mixed flue gas from the iron and steel metallurgical industry. Waste heat from blast furnace gas, blast furnace cooling water, and semi-clean blast furnace gas is used to raise the temperature. After the temperature is raised and the gas is combined, it enters the CO2 side channel of the second CO2-flue gas heat exchanger to absorb the waste heat from the high-temperature flue gas in the iron and steel metallurgical industry. Finally, the heated CO2 flows back to the compressor inlet, completing the closed-loop heat pump cycle. Specifically, the high-temperature flue gas from the iron and steel metallurgical industry first enters the flue gas side channel of the second CO2-flue gas heat exchanger to exchange heat with the CO2 working medium and release heat to cool down. The high-temperature flue gas after releasing heat and cooling down mixes with the low-temperature flue gas from the low-temperature flue gas conveying pipeline to form mixed flue gas. External makeup water enters the first three-way valve through the makeup water regulating valve, where it mixes with the saturated water from the flash tank's saturated water outlet to form mixed water. The mixed water is pressurized by a water pump and sent to the water-side channel of the third gas cooler, where it exchanges heat with the CO2 working fluid to become high-temperature pressurized water. The high-temperature pressurized water is throttled and depressurized by a throttling valve before entering the flash tank to complete flash separation, producing saturated steam and saturated water. The saturated water flows back to the first three-way valve to mix with the external makeup water and is recycled. The saturated steam enters the water-side channel of the second gas cooler, exchanges heat with the CO2 working fluid to increase its temperature, and is then output through the steam supply outlet.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a CO2 heat pump system for waste heat recovery and cascade heating. Specifically, it defines a complete dual-system architecture consisting of a CO2 transcritical cycle heat pump subsystem and a high-temperature steam preparation subsystem. The connection relationship of the CO2-side circulation loop is clearly defined: the compressor outlet is connected to an expander via a series of three gas coolers; the expander outlet is split to three parallel low-temperature heat exchangers (a first CO2-flue gas heat exchanger, a CO2-cooling water heat exchanger, and a CO2-gas heat exchanger); the three paths converge and then return to the compressor inlet via a second CO2-flue gas heat exchanger. Simultaneously... The invention clearly defines the series and parallel connections of the water supply regulating valve, the first three-way valve, the water pump, the third gas cooler, the throttle valve, the flash tank, the second gas cooler, and the bypass valve in the water-side circulation loop. In addition, the invention defines the connection relationships of four types of metallurgical waste heat (high-temperature flue gas mixed with low-temperature flue gas after passing through the second CO2-flue gas heat exchanger and then entering the first CO2-flue gas heat exchanger, blast furnace cooling water entering the CO2-cooling water heat exchanger, and blast furnace semi-clean coal gas entering the CO2-coal gas heat exchanger) and their corresponding heat exchangers, as well as the output path of the first gas cooler's air-side channel to the air to be heated. Through the above technical solution, this invention forms a tiered heat recovery architecture on the waste heat recovery side: "low-temperature stage three-source parallel heat absorption, high-temperature stage series reheating." After the CO2 working fluid expands, it first absorbs the low-grade waste heat from the mixed flue gas, cooling water, and coal gas through three parallel heat exchangers, respectively. After being heated and combined, it further absorbs the medium-temperature waste heat from the high-temperature flue gas through a second CO2-flue gas heat exchanger, achieving deep tiered recovery of waste heat across the entire temperature range. This fundamentally solves the industry problem of the inability to effectively recover low-temperature waste heat (<160℃) in existing technologies. On the heating side, a tiered heat release architecture with three-stage gas coolers in series is adopted, utilizing the wide temperature range of CO2 when releasing heat in the supercritical region. The sliding characteristic sequentially heats air to produce hot air, saturated steam to produce industrial steam, and pressurized water, significantly reducing irreversible losses during the heat exchange process. It also meets the diverse heat demands of the metallurgical industry for hot air and steam, replacing a large amount of fossil fuel combustion and reducing carbon emissions at the source. Finally, the design of a pathway where high-temperature flue gas is first initially recovered by a second CO2-flue gas heat exchanger, then mixed with low-temperature flue gas before entering the first CO2-flue gas heat exchanger for deep heat release, achieves tiered utilization of waste heat from flue gas of different grades. This avoids the direct waste of high-grade waste heat by low-grade cooling media, maximizing the overall recovery rate of flue gas waste heat.

[0016] In this invention, a PID control module for the makeup water flow rate is further added, and a complete mathematical model for the correlation between steam output flow rate and makeup water flow rate (m³) is defined. 32 ) and high-temperature steam output flow rate (m 30 ), Mixed water flow rate (m 37 ), water replenishment temperature (T) 33 ), pressurized water pressure (P) 34), third gas cooler outlet temperature (T) 35 ), pressure after the throttle valve (P) 36 A nonlinear quantitative relationship was established using multiple variables, and the opening of the water supply regulating valve was adjusted in real time through this control module. This invention achieves dynamic and precise tracking of the water supply flow rate to the steam output flow rate, ensuring that the water-side circulation remains balanced under varying operating conditions. This effectively solves the engineering problems of flash tank water level fluctuations and unstable steam output. Simultaneously, this correlation allows the controller to calculate the target water supply amount in advance based on multiple operating boundary conditions, achieving feedforward-feedback coordinated control. Compared to traditional single feedback control, this significantly improves the response speed and control accuracy under varying operating conditions, ensuring that the system maintains a stable dynamic balance in the water-side circulation under different operating conditions.

[0017] In this invention, a PID control module for steam output pressure is further added, and a mathematical model for the correlation of steam output pressure is defined, that is, the pressure after the throttle valve (P) 36 ) and steam outlet pressure (P) 28 ) and pressurized water pressure (P) 34 ), water replenishment flow rate (m 32 ), Mixed water flow rate (m 37 A quantitative relationship was established, and the opening of the throttle valve was adjusted in real time through this control module. The technical solution of this invention achieves stable and precise control of the steam outlet pressure, ensuring that the core process indicator of steam pressure always meets the process requirements set by the user. As a key throttling element in the water-side circulation, the opening of the throttle valve directly determines the working pressure of the flash tank and thus affects the output steam pressure. The pressure correlation quantitatively describes the nonlinear coupling relationship between the flash pressure, the upstream pressurized water pressure, and the proportion of makeup water flow, providing a theoretical basis for the precise control of the throttle valve, overcoming the blindness of traditional experience-based adjustment, and ensuring that the output steam pressure can quickly recover to the set value under various disturbances.

[0018] In this invention, a PID control module for steam output temperature is further added, and a mathematical model for the correlation of steam output temperature is defined, that is, the output temperature (T) 29 ) and mixed water flow rate (m 37 ), pressure ratio (P) 36 / P 34 Bypass flow (m) 41 ), third gas cooler outlet temperature (T) 35A quantitative relationship is established, and the bypass valve opening is adjusted in real time through this control module. This invention achieves flexible and precise steam temperature regulation. By controlling a portion of saturated steam to bypass directly to the steam outlet via a bypass pipeline, it mixes with high-temperature steam heated by the second gas cooler. Without altering the CO2 heat pump cycle, precise steam temperature control is achieved by adjusting the bypass ratio, resulting in a fast response and wide adjustment range. The temperature-correlated quantitative relationship reflects the comprehensive influence of water-side flow rate, water-side pressure, and CO2-side heating capacity on steam temperature. This allows the controller to calculate the optimal bypass valve opening in real time based on multi-dimensional inputs, achieving high-precision closed-loop control of steam temperature and ensuring that the output steam temperature remains stable at the process-required set value under various disturbances.

[0019] In this invention, a compressor discharge pressure PID control module is further added, and a mathematical model for the optimal compressor discharge pressure correlation is defined, that is, the optimal discharge pressure (P) 31 , opt ) and steam outlet temperature (T) 29 ), compressor discharge temperature (T) 39 ), compressor suction temperature (T) 38 ), hot air output temperature (T) 40 A quantitative relationship is established, and the compressor speed is adjusted in real time through this control module. The technical solution of this invention realizes dynamic optimization of the system's global energy efficiency. In the CO2 transcritical cycle, there is an optimal compressor discharge pressure corresponding to the system's global optimal energy efficiency (COP). This optimal compressor discharge pressure is not a fixed value but changes dynamically with the operating conditions. Traditional fixed discharge pressure control strategies cannot adapt to variable operating conditions. The correlational quantitative method of this invention describes the complex nonlinear relationship between the optimal discharge pressure and the steam output temperature, the compressor suction and discharge temperatures, and the hot air output temperature. This allows the system to calculate the current optimal discharge pressure setpoint online according to the real-time operating conditions and accurately track it by adjusting the compressor speed through PID control. This represents a technological advancement from "constant pressure operation" to "dynamic energy efficiency optimization," ensuring that the system always operates at the highest global energy efficiency under different operating conditions, maximizing the heating performance coefficient of the entire heat pump system. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1This is a schematic diagram of the structure of a CO2 heat pump system for waste heat recovery and cascade heating in an embodiment of the present invention. Figure 2 This is a schematic block diagram of a CO2 heat pump system for waste heat recovery and cascade heating in an embodiment of the present invention. Figure 3 This is a modular schematic diagram of the PID control system of the CO2 heat pump system in an embodiment of the present invention; Figure 4 This is a schematic diagram of the control logic of the CO2 heat pump system in an embodiment of the present invention; The reference numerals and their explanations in the figure are as follows: 1. Compressor; 2. First gas cooler; 3. Second gas cooler; 4. Third gas cooler; 5. Expander; 6. First CO2-flue gas heat exchanger; 7. CO2-cooling water heat exchanger; 8. CO2-gas heat exchanger; 9. Second CO2-flue gas heat exchanger; 10. Water pump; 11. Throttling valve; 12. Flash tank; 13. Make-up water regulating valve; 14. First three-way valve; 15. First shut-off valve; 16. Bypass valve; 17. Second three-way valve; 18. Third three-way valve; 19. Second shut-off valve; 20. Third shut-off valve; 21. Fourth three-way valve; 22. 23. Fifth three-way valve; 24. Sixth three-way valve; 25. Seventh three-way valve; 26. Eighth three-way valve; 27. Fourth shut-off valve; 28. Fifth shut-off valve; 29. ​​First pressure sensor; 30. First temperature sensor; 31. First flow meter; 32. Second pressure sensor; 33. Second flow meter; 34. Second temperature sensor; 35. Third pressure sensor; 36. Fourth pressure sensor; 37. Third flow meter; 38. Fourth temperature sensor; 39. Fifth temperature sensor; 40. Sixth temperature sensor; 41. Fourth flow meter. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of protection of the present invention, but rather to illustrate specific selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0023] The following detailed description, in conjunction with the accompanying drawings, outlines some embodiments of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other.

[0024] Please see Figure 1 and Figure 2 The present invention provides a CO2 heat pump system for waste heat recovery and cascade heating, specifically an ultra-high temperature CO2 heat pump system capable of recovering waste heat from the metallurgical industry and realizing cascade heating. It is used to absorb heat from industrial waste heat to efficiently and stably produce industrial steam above 200°C and heat hot air to 500°C.

[0025] The CO2 heat pump system of this invention specifically includes: a high-temperature compressor 1, a first gas cooler 2, a second gas cooler 3, a third gas cooler 4, an expander 5, a first CO2-flue gas heat exchanger 6, a CO2-cooling water heat exchanger 7, a CO2-gas heat exchanger 8, a second CO2-flue gas heat exchanger 9, a water pump 10, a throttling valve 11, a flash tank 12, a water supply regulating valve 13, a first three-way valve 14, a first shut-off valve 15, a bypass valve 16, a second three-way valve 17, a third three-way valve 18, a second shut-off valve 19, a third shut-off valve 20, a fourth three-way valve 21, a fifth three-way valve 22, a sixth three-way valve 23, a seventh three-way valve 24, and an eighth three-way valve. 25. Fourth shut-off valve; 26. Fifth shut-off valve; 27. First pressure sensor; 28. First temperature sensor; 29. ​​First flow meter; 30. Second pressure sensor; 31. Second flow meter; 32. Second temperature sensor; 33. Third pressure sensor; 34. Third temperature sensor; 35. Fourth pressure sensor; 36. Third flow meter; 37. Fourth temperature sensor; 38. Fifth temperature sensor; 39. Sixth temperature sensor; 40. Fourth flow meter; 41; wherein, the outlet of compressor 1 is connected to the CO2 side inlet of the first gas cooler 2, the CO2 side outlet of the first gas cooler 2 is connected to the CO2 side inlet of the second gas cooler 3, and the CO2 side outlet of the second gas cooler 3... The outlet is connected to the CO2 side inlet of the third gas cooler 4. The CO2 side outlet of the third gas cooler 4 is connected to the inlet of the expander 5. The outlet of the expander 5 is connected to the CO2 side inlet of the first CO2-flue gas heat exchanger 6, CO2-cooling water heat exchanger 7, and CO2-gas heat exchanger 8 via the sixth three-way valve 23 and the eighth three-way valve 25. The CO2 side outlet of the first CO2-flue gas heat exchanger 6, CO2-cooling water heat exchanger 7, and CO2-gas heat exchanger 8 is connected to the CO2 side inlet of the second CO2-flue gas heat exchanger 9 via the fifth three-way valve 22 and the seventh three-way valve 24. The CO2 side outlet of the second CO2-flue gas heat exchanger 9 is connected to the inlet of the compressor 1. This constitutes an ultra-high temperature CO2 transcritical cycle heat pump subsystem; the saturated water outlet of flash tank 12 and the outlet of water supply regulating valve 13 are connected to the inlet of water pump 10 through a first three-way valve 14, the outlet of water pump 10 is connected to the water side inlet of third gas cooler 4, the water side outlet of third gas cooler 4 is connected to the inlet of throttle valve 11, the outlet of throttle valve 11 is connected to the inlet of flash tank 12, the saturated steam outlet of flash tank 12 is connected to the water side inlet of second gas cooler 3 and the inlet of bypass valve 16 through a third three-way valve 18, and the water side outlet of second gas cooler 3 and the outlet of bypass valve 16 are connected to the steam supply outlet through a second three-way valve 17, thus constituting a high temperature steam preparation subsystem;A first shut-off valve 15 is installed on the hot air side inlet pipe of the first gas cooler 2; a second shut-off valve 19 is installed on the flue gas side inlet pipe of the second CO2-flue gas heat exchanger 9; a third shut-off valve 20 is installed on the connection pipe between the flue gas side outlet of the second CO2-flue gas heat exchanger 9 and the flue gas side inlet of the first CO2-flue gas heat exchanger 6 via a fourth three-way valve 21; a fourth shut-off valve 26 is installed on the cooling water side inlet pipe of the CO2-cooling water heat exchanger 7; and a fifth shut-off valve 27 is installed on the gas side inlet pipe of the CO2-gas heat exchanger 8.

[0026] In this embodiment of the invention, a fifth temperature sensor 39 and a second pressure sensor 31 are installed on the connecting pipe between the compressor 1 and the CO2 side channel of the first gas cooler 2, for detecting the temperature (T) of the CO2 working fluid at the compressor exhaust state point. 39 ) and pressure (P) 31 A fourth temperature sensor 38 is installed on the connecting pipe between the CO2 side channel of the second CO2-flue gas heat exchanger 9 and the compressor 1 to detect the temperature (T) of the CO2 working fluid at the compressor suction point. 38 A sixth temperature sensor 40 is installed on the hot air outlet pipe of the first gas cooler 2 to detect the heating temperature (T) of the hot air. 40 The high-temperature steam outlet pipe of the second gas cooler 3 is equipped with a first pressure sensor 28, a first temperature sensor 29, and a first flow meter 30 to detect the pressure (P) at the high-temperature steam outlet. 28 ), temperature (T) 29 ), flow rate (m) 30 A fourth flow meter 41 is installed on the bypass pipeline where the bypass valve 16 is located, used to detect the bypass flow rate (m³) of high-temperature steam. 41 A second temperature sensor 33 and a second flow meter 32 are installed on the pipeline where the water supply regulating valve 13 is located, for detecting the water supply temperature (T). 33 ) and flow rate (m 32 A third flow meter 37 is installed on the connecting pipeline between the first three-way valve 14 and the water pump 10 to detect the flow rate (m³) of the mixed water (saturated water and makeup water) at the outlet of the flash tank. 37 A third pressure sensor 34 is installed on the connecting pipe between the water pump 10 and the water-side channel of the third gas cooler 4 to detect the pressure (P) of the mixed water after it has been pressurized by the water pump. 34 A third temperature sensor 35 is installed on the connecting pipe between the water side channel of the third gas cooler 4 and the throttle valve 11 to detect the temperature (T) of the pressurized water after it absorbs heat through the third gas cooler. 35 A fourth pressure sensor 36 is installed on the connecting pipe between the throttle valve 11 and the inlet of the flash tank 12 to detect the pressure (P) after the pressurized water is throttled. 36 ).

[0027] In this embodiment of the invention, during the operation of the ultra-high temperature CO2 heat pump system capable of recovering waste heat from the metallurgical industry and achieving cascaded heating, the CO2 working fluid is compressed by the compressor to form a high-temperature, high-pressure supercritical fluid. It then flows sequentially through the first, second, and third gas coolers, gradually releasing heat to achieve cascaded heat energy release. The cooled CO2 then enters the expander for expansion and depressurization. Subsequently, it is distributed through a three-way valve to the parallel-connected first CO2-flue gas heat exchanger, CO2-cooling water heat exchanger, and CO2-gas heat exchanger, respectively absorbing low-grade waste heat from 150°C low-temperature flue gas, 80°C cooling water, and 150°C gas. After heating and merging, the CO2 is introduced into the second CO2-flue gas heat exchanger to further absorb the medium-temperature waste heat from 350°C high-temperature flue gas. Finally, the heated CO2 flows back to the compressor inlet to complete the closed-loop heat pump cycle; waste heat recovery. The high-temperature flue gas on the first side first enters the second CO2-flue gas heat exchanger for initial heat release and cooling. Then, it mixes with the low-temperature flue gas and enters the first CO2-flue gas heat exchanger for further heat release, achieving cascaded recovery of waste heat across the entire temperature range of 80℃-350℃. Cooling water and coal gas release their own waste heat in their respective heat exchangers. On the heat output side, the first gas cooler directly heats the air, producing industrial hot air above 500℃. In the high-temperature steam preparation subsystem, external makeup water mixes with saturated water discharged from the flash tank, is pressurized by a water pump, and sent to the third gas cooler for heating into high-temperature pressurized water. This water then passes through a throttling valve to reduce pressure before entering the flash tank for flash separation. The resulting saturated steam is further heated to industrial steam above 200℃ in the second gas cooler, while the saturated water separated by flash separation is returned to the water pump inlet for reuse. Throughout the system operation, parameter stability adjustment and dynamic energy efficiency optimization can be achieved through a four-way coordinated PID control loop.

[0028] The workflow of the system in this embodiment of the invention is as follows: High-temperature flue gas first passes through a second CO2-flue gas heat exchanger to release heat and cool down, then mixes with low-temperature flue gas before entering the first CO2-flue gas heat exchanger for further heat release and cooling. Blast furnace cooling water passes through a CO2-cooling water heat exchanger to release heat and cool down, heating the low-temperature CO2 to approximately 70°C. Semi-clean blast furnace gas passes through a CO2-gas heat exchanger to release heat and cool down, heating the low-temperature CO2 to approximately 140°C.

[0029] Please see Figure 3 and Figure 4 In this embodiment of the invention, the system control and optimization mainly revolve around the four key parameters of compressor exhaust pressure and steam supply temperature, pressure and flow rate for coordinated optimization and dynamic optimization. By constructing an optimal setpoint feedforward model based on operating boundary conditions and combining it with multi-loop PID feedback control, the actuators at all levels are adjusted in real time so that the above parameters always track their dynamic optimal values, thereby ensuring that the system can stably operate at the highest global energy efficiency under different operating conditions.

[0030] In this embodiment of the invention, the adaptive adjustment of the compressor discharge pressure includes: the compressor forming the core regulating actuator, which, during operation, adjusts the compressor speed to control the impeller's work capacity on the CO2 working fluid, thereby precisely regulating the discharge pressure. Specifically, the compressor discharge pressure (P) 31 ) and compressor suction temperature (T) 38 ), compressor discharge temperature (T) 39 ), the high-temperature flue gas output temperature of the first gas cooler (T) 40 ), the high-temperature steam output temperature of the second gas cooler (T) 29 To optimize the compressor discharge pressure and achieve the optimal Coefficient of Performance (COP) of the system, a correlation model is constructed between the intermediate temperature and these variables, with the following correlation equation:

[0031] In the formula, The optimal compressor discharge pressure is expressed in MPa; T 38 For compressor suction temperature, T 39 For compressor discharge temperature, T 40 The output temperature of the high-temperature hot air after being heated by the first gas cooler, T 29 The temperature of the high-temperature steam produced after heating by the second gas cooler is expressed in °C. These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0032] Based on this, T is dynamically adjusted according to different operating conditions. 38 T 39 T 40 T 29 The boundary parameters are controlled by the compressor speed, and the controller's control objective is to stabilize the compressor discharge pressure at the set value calculated by the optimization model.

[0033] Based on the real-time feedback of the calculated system COP, the proportional control coefficient of the PID control module is optimized and adjusted. Integral adjustment coefficient and differential adjustment coefficient Each time the system completes a control cycle, it compares the deviation between the compressor discharge pressure setpoint and the actual measured value with a preset accuracy threshold. If the deviation exceeds the threshold, this deviation is used as the input to the controller. The PID controller calculates the corresponding control quantity based on this deviation, and by coordinating and adjusting the compressor speed, it precisely controls the compressor discharge pressure, ensuring that the system always operates within its optimal energy efficiency range.

[0034] The compressor discharge pressure is calculated using the differential method by the PID controller. The formula for calculating the compressor's dynamic speed n is as follows:

[0035] In the formula, The difference between the actual exhaust pressure and the set exhaust pressure, where n is the number of calculations. , and These are the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively.

[0036] Based on the current operating conditions, the optimal compressor discharge pressure setpoint (P) under the current system operating conditions is calculated in real time according to the optimal compressor discharge pressure correlation model. 31,opt To achieve rapid and accurate tracking of this setpoint, the proportional, integral, and derivative adjustment parameters are respectively set to [values ​​to be filled in]. , , The controller's output is directly used to adjust the compressor speed.

[0037] In this embodiment of the invention, the adaptive adjustment of the makeup water flow rate includes: setting the makeup water flow rate to the steam flow rate that the high-temperature steam preparation subsystem can generate under specific operating conditions, so as to achieve dynamic balance in the water-side circulation. The high-temperature steam flow rate (m30) is related to the mixed water flow rate (m37), makeup water temperature (T33), pressurized water pressure (P34), the water-side outlet temperature of the third gas cooler (T35), and the pressure after the throttle valve (P36), etc. A correlation model is established between the high-temperature steam flow rate and the above boundary conditions and operating parameters. The correlation formula can be expressed as:

[0038] In the formula, For the water supply regulating valve, the water supply flow rate, m 30 For the high-temperature steam output flow rate at the steam outlet, m 37 The flow rate of the mixed water in the pipeline between the first three-way valve and the water pump is expressed in kg / s; T 33 For the water supply regulating valve, the water supply temperature, T 35 P represents the water-side outlet temperature of the third gas cooler, in °C. 34 The pressure of the water after being pressurized by the water pump, P 36 This represents the pressure of the two-phase fluid after the throttle valve, in MPa. These are empirical coefficients determined through system characteristic analysis and fitting of experimental data. Specifically, these empirical coefficients were obtained as follows: First, a thermodynamic simulation model of the CO2 heat pump system was established, covering the temperature, pressure, and flow rate range of the system's normal operation; then, multiple sets of empirical coefficients under different operating conditions were obtained through batch calculations using the simulation model. , , , , , The sample dataset; substituting the sample dataset into the steam output flow correlation equation, performing multivariate nonlinear regression fitting, and solving for the result. The specific value to be taken.

[0039] Using the opening degree of the water supply regulating valve as the control variable, the controller outputs the setpoint of the high-temperature steam flow rate. Based on the dynamic characteristics of the system under coupled operating conditions, the proportional, integral, and derivative parameters of the PID control module are adjusted online. After the system completes one control cycle, if the absolute value of the deviation between the setpoint of the water supply flow rate and the actual measured value of the high-temperature steam flow rate is greater than the preset control accuracy threshold, this deviation is input as an error signal to the PID controller. The controller calculates the control quantity based on the error signal and adjusts the opening degree of the water supply regulating valve to change the water supply flow rate, thereby precisely adjusting the water supply flow rate to be equal to the high-temperature steam output flow rate. As the PID parameters are adaptively adjusted to approach the optimal value, the measured water supply flow rate of the system will gradually approach and eventually stabilize near the target setpoint until the absolute value of the error is less than the control accuracy threshold. This control loop can adjust the control parameters and valve opening in real time according to different operating conditions, so that the water supply flow rate is stabilized at the dynamic optimal value, thereby ensuring that the system performance is always in the best state. The PID controller for replenishing water flow uses the differential method for calculation. The formula for calculating the dynamic adjustment opening of the replenishing water regulating valve is as follows:

[0040] In the formula, The difference between the actual water supply flow rate and the set water supply flow rate, where n is the number of calculations. , and These are the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively.

[0041] Based on the current operating conditions, the water replenishment flow rate (m) under the current system operating conditions is calculated in real time according to the water replenishment flow correlation model. 32 To achieve rapid and accurate tracking of this setpoint, the proportional, integral, and derivative adjustment parameters are set to specific values. , , .

[0042] In this embodiment of the invention, the adaptive adjustment of the high-temperature steam production pressure includes: high-temperature steam production pressure (P) 28The setpoint, defined by the user based on process requirements, is the output target that the system must guarantee. This setpoint is provided by the user according to production needs, and the system's control objective is to stabilize the actual output pressure at this fixed setpoint. In the system, the high-temperature steam output pressure is primarily related to the makeup water flow rate (m³ / s). 32 ), Mixed water flow rate (m 37 ), pressurized water pressure (P) 34 ) and other factors affecting high-temperature steam pressure (P) 28 Closely related to the above key variables, a correlation model is established between exhaust pressure and the above key variables. The correlation formula can be expressed as:

[0043] In the formula, P 36 The pressure of the two-phase fluid after the throttle valve, P 28 The high-temperature steam pressure, P, after being heated by the second gas cooler 34 The pressure of water after being pressurized by a water pump, expressed in MPa; m 32 For the water supply regulating valve, the water supply flow rate, m 37 The flow rate of the mixed water after passing through the first three-way valve is expressed in kg / s. These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0044] Using the valve opening of the throttle valve as the control variable, the controller outputs the setpoint for the high-temperature steam output pressure, and adjusts the PID control module according to the system's dynamic characteristics. , and Three parameters. After one cycle, if the absolute value of the deviation between the setpoint and the actual measured value of the high-temperature steam output pressure exceeds the preset control accuracy threshold, this deviation is input as an error signal to the PID controller. The controller calculates the control quantity based on the error signal and adjusts the valve opening of the throttle valve to change the high-temperature steam output pressure. As the PID parameters adaptively adjust towards the optimal value, the measured high-temperature steam output pressure will gradually approach and stabilize near the target setpoint until the absolute value of the error is less than the control accuracy threshold. This control loop can adjust the valve opening of the throttle valve in real time according to different operating conditions to stabilize the high-temperature steam output pressure at the setpoint. The high-temperature steam pressure PID controller uses the differential method for calculation. The calculation formula for the dynamic adjustment opening v of the throttle valve 11 is as follows:

[0045] In the formula, The value of n represents the difference between the actual high-temperature steam output pressure and the set high-temperature steam output pressure, where n is the number of calculations. , and These are the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively.

[0046] Based on the current operating conditions, the required high-temperature steam output pressure under these conditions is the system setpoint, and the setpoint values ​​for the proportional, integral, and derivative control parameters are respectively set to... , , .

[0047] In this embodiment of the invention, the adaptive adjustment of the high-temperature steam production temperature includes: high-temperature steam production temperature (T 29 The setpoint, defined by the user based on process requirements, is the output target that the system must guarantee. This setpoint is provided by the user according to production needs, and the system's control objective is to drive the actual output temperature to stabilize at this fixed setpoint. Since the high-temperature steam output temperature is affected by multiple coupled factors, a correlation model between the high-temperature steam output temperature and key state parameters is established to meet stringent process specifications. Its expression is:

[0048] In the formula, P 34 The pressure of the water after being pressurized by the water pump, P 36 T represents the pressure of the two-phase fluid after the throttle valve, in MPa. 35 For the water-side outlet temperature of the third gas cooler, T 29 The temperature of the high-temperature steam produced after heating in the second gas cooler, in °C; m 37 The mixed water flow rate (m) after passing through the first three-way valve 41 The steam flow rate through the bypass valve of the second gas cooler is expressed in kg / s. These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

[0049] Using the opening degree of the bypass valve as the control variable, the controller outputs the setpoint for the high-temperature steam output temperature, and adjusts the PID control module according to the system's dynamic characteristics. , and Three parameters. After one cycle, if the absolute value of the deviation between the setpoint and the actual measured high-temperature steam output temperature exceeds the preset control accuracy threshold, this deviation is input as an error signal to the PID controller. The controller calculates the control quantity based on the error signal and adjusts the opening of the bypass valve to change the high-temperature steam bypass flow rate, thereby controlling the high-temperature steam output temperature. As the PID parameters adaptively adjust towards the optimal value, the measured high-temperature steam output temperature will gradually approach and stabilize near the target setpoint until the absolute value of the error is less than the control accuracy threshold. This control loop can adjust the opening of the bypass valve in real time according to different operating conditions to stabilize the high-temperature steam output temperature at the setpoint. The high-temperature steam output temperature PID controller uses the differential method for calculation, and the calculation formula for the dynamic adjustment opening of the bypass valve is as follows:

[0050] In the formula, The value represents the difference between the setpoint and the actual temperature of the high-temperature steam output, where n is the number of calculations. , and These are the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively.

[0051] Based on the current operating conditions, the required high-temperature steam output temperature under these conditions is the system setpoint, and the setpoint values ​​for the proportional, integral, and derivative control parameters are respectively set to... , , .

[0052] This invention discloses an ultra-high temperature CO2 heat pump system capable of stably heating 500℃ hot air and simultaneously producing industrial steam above 200℃ under conditions of abundant and diverse low-temperature waste heat in the metallurgical industry. Addressing the characteristics of the metallurgical industry's diverse waste heat types and wide temperature range, a low-temperature end heat exchange architecture with multiple heat exchangers connected in series and parallel is employed. Waste heat is divided into four categories for recovery: 150℃ low-temperature flue gas, 350℃ high-temperature flue gas, 80℃ cooling water, and 150℃ coal gas. This allows for deep utilization of waste heat across the entire temperature range of 80℃-350℃, significantly improving the industry's waste heat recovery rate. Furthermore, the high-temperature heat release end of the CO2 heat pump cycle employs a three-stage gas cooler connected in series to achieve tiered heating. Utilizing the wide temperature glide characteristic of CO2 during supercritical heat release, hot air, saturated steam, and pressurized water are heated sequentially to meet the diverse heat demands of the metallurgical industry and significantly reduce irreversible losses during the heating process. The heat exchange between the waste heat recovery side and the heating side is mainly based on the basic principle of the heat exchanger, that is, two fluids with different temperatures transfer heat through the solid wall without direct mixing, so that heat is transferred from the high-temperature fluid to the low-temperature fluid, and follows the principle of energy conservation. The heat exchange temperature difference between the two fluids is generally greater than 10℃. The most important link driving the entire CO2 heat pump cycle is to use the compressor to consume external mechanical energy to compress the low-temperature and low-pressure CO2 working fluid to a high-temperature and high-pressure supercritical state. During the gas compression process, the internal energy also increases.

[0053] This invention categorizes waste heat from the metallurgical industry into four types based on different parameters and temperatures: 150°C low-temperature flue gas, 350°C high-temperature flue gas, 80°C cooling water, and 150°C coal gas. These types of gas then enter a first CO2-flue gas heat exchanger, a second CO2-flue gas heat exchanger, a CO2-cooling water heat exchanger, and a CO2-coal gas heat exchanger, respectively, for heat exchange. The first, second, and third CO2-flue gas heat exchangers are connected in parallel as a low-temperature stage waste heat source, heating the low-temperature CO2 to approximately 120°C. The second CO2-flue gas heat exchanger serves as a high-temperature stage waste heat source, gradually heating the CO2 to above 300°C. This invention utilizes waste heat from the metallurgical industry, extracting low-grade heat energy from low-temperature heat sources and converting it into high-grade industrial-grade heat energy above 500°C. This provides a promising technical solution for replacing traditional waste heat recovery methods such as waste heat boiler power generation, and for achieving deep decarbonization in the heating sector.

[0054] To address the strongly coupled and multivariable characteristics of ultra-high temperature heat pump systems, an intelligent control method based on dynamic optimal setpoint feedforward and multivariable collaborative control is proposed. By solving for the globally optimal compressor discharge pressure under different operating conditions online, and combining this with a multi-loop PID algorithm to collaboratively control actuators such as the compressor, water supply regulating valve, water-side throttling valve, and bypass valve, the system is ensured to always operate at its optimal energy efficiency point while maintaining stable steam parameter output. The control algorithm of this invention possesses rapid response and precise adjustment capabilities. The system adopts a multi-loop PID controller architecture, which, through real-time monitoring of operating parameters and boundary conditions, including temperature and pressure, can rapidly adjust to changes in external environment or user load, ensuring that all steam production parameters remain stable within the setpoints while maintaining key system parameters within their efficient and safe ranges.

[0055] The iron and steel metallurgical industry has a large demand for medium- and high-temperature heat. For example, the 300℃-600℃ hot air required for blast furnace hot blast stove combustion air preheating, ladle medium-temperature baking, and pulverized coal high-temperature preheating, as well as the 1.6MPa-2MPa saturated steam required for RH / VD vacuum degassing, coking crude benzene, and various pipeline heating, has traditionally relied almost entirely on coke oven gas, blast furnace gas, or natural gas combustion for supply. This not only results in high energy consumption and carbon emissions but also wastes energy. Ultra-high temperature CO2 heat pumps can recover low-grade waste heat of 70-380℃ and upgrade it into 200-600℃ hot air or 200-300℃ saturated / superheated steam. This perfectly matches the waste heat resource characteristics and heat demand of the iron and steel metallurgical industry, achieving cascaded energy utilization, replacing a large amount of fossil fuel combustion, reducing carbon emissions at the source, and solving the industry problems of high difficulty in recovering low-grade waste heat and high carbon emissions from process heat. This invention discloses an ultra-high temperature CO2 heat pump system and its optimized control method capable of recovering waste heat from the iron and steel metallurgical industry and achieving cascaded heating. It aims to address the industry pain points of extremely low low-grade waste heat recovery rates and the reliance on fossil fuels for traditional medium- and high-temperature heating, resulting in significant energy waste. The system mainly consists of an ultra-high temperature CO2 transcritical cycle heat pump subsystem and a high-temperature steam preparation subsystem. It employs a low-temperature end heat exchange architecture with multiple heat exchangers connected in series and parallel, enabling deep recovery of waste heat from flue gas, cooling water, and coal gas across a temperature range of 80℃-350℃. At the high-temperature end, utilizing the wide temperature glide characteristics of CO2 supercritical heat release, cascaded heating is achieved through a three-stage gas cooler series connection, simultaneously producing industrial hot air above 500℃ and industrial steam above 200℃. Furthermore, this invention proposes an intelligent control strategy based on dynamic optimal setpoint feedforward and multi-loop PID collaborative control. By establishing a correlation model of key operating parameters, it tracks the globally optimal compressor exhaust pressure in real time and coordinates the adjustment of steam flow, pressure, and temperature. The technical solution disclosed in this invention can significantly improve the utilization rate of waste heat in the iron and steel metallurgical industry, replace the combustion of a large amount of fossil fuels, reduce carbon emissions from the source, and provide an engineering-feasible technical solution for deep decarbonization in the field of industrial heating.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A CO2 heat pump system for waste heat recovery and cascade heating, characterized in that, include: An ultra-high temperature CO2 transcritical cycle heat pump subsystem and a high-temperature steam preparation subsystem; among which... The ultra-high temperature CO2 transcritical cycle heat pump subsystem includes: a compressor (1), a first gas cooler (2), a second gas cooler (3), a third gas cooler (4), an expander (5), a first CO2-flue gas heat exchanger (6), a CO2-cooling water heat exchanger (7), a CO2-gas heat exchanger (8), and a second CO2-flue gas heat exchanger (9); wherein, the outlet of the compressor (1) is connected to the inlet of the expander (5) via the CO2 side channel of the first gas cooler (2), the CO2 side channel of the second gas cooler (3), and the CO2 side channel of the third gas cooler (4). The outlet of the expander (5) is connected to the CO2 side channel inlet of the first CO2-flue gas heat exchanger (6), the CO2 side channel inlet of the CO2-cooling water heat exchanger (7), and the CO2 side channel inlet of the CO2-gas heat exchanger (8). The CO2 side channel outlet of the first CO2-flue gas heat exchanger (6), the CO2 side channel outlet of the CO2-cooling water heat exchanger (7), and the CO2 side channel outlet of the CO2-gas heat exchanger (8) converge and then connect to the inlet of the compressor (1) through the CO2 side channel of the second CO2-flue gas heat exchanger (9), forming a CO2 circulation loop. The high-temperature steam preparation subsystem includes: a water supply regulating valve (13), a first three-way valve (14), a water pump (10), the third gas cooler (4), a throttle valve (11), a flash tank (12), the second gas cooler (3), and a bypass valve (16); wherein, the water supply regulating valve (13) is connected to the first inlet of the first three-way valve (14), the saturated water outlet of the flash tank (12) is connected to the second inlet of the first three-way valve (14), and the outlet of the first three-way valve (14) is connected to the water pump (10) and the third gas cooler (4). The water-side channel, the throttle valve (11), and the inlet of the flash tank (12) are connected. The saturated steam outlet of the flash tank (12) is connected to the steam outlet via the water-side channel of the second gas cooler (3). A bypass pipe is connected in parallel to the water-side channel of the second gas cooler (3). One end of the bypass pipe is connected to the connecting pipe between the flash tank (12) and the second gas cooler (3), and the other end is connected to the connecting pipe between the second gas cooler (3) and the steam outlet. The bypass valve (16) is installed on the bypass pipe. The first gas cooler (2) has an air-side channel for introducing air to be heated; the second CO2-flue gas heat exchanger (9) has a flue gas-side channel inlet for introducing high-temperature flue gas from the iron and steel metallurgical industry; the second CO2-flue gas heat exchanger (9) has a flue gas-side channel outlet connected to the flue gas-side channel inlet of the first CO2-flue gas heat exchanger (6); the first CO2-flue gas heat exchanger (6) has a flue gas-side channel outlet for discharging the heat-exchanged flue gas; a low-temperature flue gas conveying pipe is connected to the connecting pipe between the second CO2-flue gas heat exchanger (9) and the first CO2-flue gas heat exchanger (6); the low-temperature flue gas conveying pipe is used to introduce low-temperature flue gas from the iron and steel metallurgical industry; the CO2-cooling water heat exchanger (7) has a water-side channel for introducing blast furnace cooling water from the iron and steel metallurgical industry; and the CO2-gas heat exchanger (8) has a gas-side channel for introducing semi-clean blast furnace gas from the iron and steel metallurgical industry.

2. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 1, characterized in that, Also includes: The water supply flow PID control module is used to control the opening degree of the water supply regulating valve (13) in accordance with the steam output flow rate; The correlation formula for the steam output flow rate is as follows: ； in, The water supply flow rate for the water supply regulating valve; The high-temperature steam output flow rate for the steam outlet; The mixed water flow rate is the flow rate of the pipeline between the first three-way valve and the water pump; The water supply temperature for the water supply regulating valve; The pressure of the water after it has been pressurized by the water pump; The water-side channel outlet temperature of the third gas cooler; The pressure of the two-phase fluid after the throttle valve; These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

3. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 2, characterized in that, Also includes: The steam output pressure PID control module is used to control the opening of the throttle valve (11) according to the steam output pressure correlation. The correlation for the steam production pressure is as follows: ； In the formula, The high-temperature steam output pressure for the steam outlet; , , , These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

4. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 3, characterized in that, Also includes: The steam output temperature PID control module is used to control the opening of the bypass valve according to the steam output temperature correlation. The correlation for the steam production temperature is as follows: ； In the formula, The high-temperature steam output temperature for the steam outlet; This represents the steam flow rate through the bypass pipe; , , , , These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

5. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 4, characterized in that, Also includes: The compressor discharge pressure PID control module is used to control the speed of the compressor (1) according to the optimal compressor discharge pressure correlation. The optimal compressor discharge pressure correlation is as follows: ； In the formula, The optimal compressor discharge pressure; This refers to the compressor discharge temperature. This refers to the compressor suction temperature. The hot air output temperature at the outlet of the air-side passage of the first gas cooler; , , , These are the empirical coefficients determined through system characteristic analysis and fitting of experimental data.

6. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 1, characterized in that, Also includes: The first flow meter (30) is used to be installed at the steam outlet to detect the high-temperature steam output flow rate at the steam outlet; The second flow meter (32) and the second temperature sensor (33) are used to be installed on the pipeline where the water supply regulating valve (13) is located, so as to detect the water supply flow rate and the water supply temperature of the water supply regulating valve (13) respectively. The third flow meter (37) is used to be installed on the pipeline between the first three-way valve (14) and the water pump (10) to detect the flow rate of mixed water through the pipeline between the first three-way valve (14) and the water pump (10); The third pressure sensor (34) is installed on the pipeline between the water pump (10) and the third gas cooler (4) to detect the pressure of the water after it has been pressurized by the water pump (10); The third temperature sensor (35) is installed on the pipeline between the third gas cooler (4) and the throttle valve (11) to detect the water-side channel outlet temperature of the third gas cooler (4); The fourth pressure sensor (36) is used to be installed on the pipeline between the throttle valve (11) and the flash tank (12) to detect the pressure of the two-phase fluid after the throttle valve (11); The first pressure sensor (28) is used to be installed at the steam outlet to detect the high-temperature steam output pressure at the steam outlet; The first temperature sensor (29) is installed at the steam outlet to detect the high temperature of the steam produced at the steam outlet. The fourth flow meter (41) is used to be installed on the bypass line to detect the steam flow rate through the bypass line; The fifth temperature sensor (39) is used to be installed on the pipeline between the compressor (1) and the first gas cooler (2) to detect the exhaust temperature of the compressor (1); The fourth temperature sensor (38) is used to be installed on the pipeline between the second CO2-flue gas heat exchanger (9) and the compressor (1) to detect the suction temperature of the compressor (1); The sixth temperature sensor (40) is used to be installed at the outlet of the air-side channel of the first gas cooler (2) to detect the hot air output temperature at the outlet of the air-side channel of the first gas cooler (2).

7. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 1, characterized in that, The temperature of the high-temperature flue gas is 300℃~350℃, the temperature of the low-temperature flue gas is 120℃~180℃, the temperature of the blast furnace cooling water is 60℃~80℃, and the temperature of the blast furnace semi-clean gas is 120℃~180℃.

8. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 1, characterized in that, The temperature of the heated air output from the wind-side channel of the first gas cooler (2) is above 500°C; the temperature of the high-temperature steam output from the steam outlet is above 200°C.

9. The CO2 heat pump system for waste heat recovery and cascade heating according to claim 1, characterized in that, A first shut-off valve (15) is provided at the inlet of the air-side passage of the first gas cooler (2); A second shut-off valve (19) is provided at the flue gas side passage inlet of the second CO2-flue gas heat exchanger (9); The low-temperature flue gas conveying pipeline is equipped with a third shut-off valve (20). A fourth shut-off valve (26) is provided at the inlet of the water side channel of the CO2-cooling water heat exchanger (7). The CO2-gas heat exchanger (8) is equipped with a fifth shut-off valve (27) at the gas side channel inlet.

10. A method for operating a CO2 heat pump system for waste heat recovery and cascade heating as described in claim 1, characterized in that, include: After being compressed into a high-temperature, high-pressure supercritical fluid by the compressor (1), the CO2 working fluid flows sequentially through the CO2 side channel of the first gas cooler (2), the CO2 side channel of the second gas cooler (3), and the CO2 side channel of the third gas cooler (4), gradually releasing heat and cooling down. The CO2 after releasing heat and cooling down enters the expander (5) to expand and reduce pressure. The expanded CO2 then enters the CO2 side channel of the first CO2-flue gas heat exchanger (6), the CO2 side channel of the CO2-cooling water heat exchanger (7), and the CO2-gas heat exchanger (8), which are set up in parallel, and absorbs heat from the steel. The waste heat from the mixed flue gas, the waste heat from the blast furnace cooling water, and the waste heat from the semi-clean blast furnace gas in the iron and steel metallurgical industry are heated and then merged into the CO2 side channel of the second CO2-flue gas heat exchanger (9) to absorb the waste heat from the high-temperature flue gas in the iron and steel metallurgical industry. Finally, the heated CO2 flows back to the inlet of the compressor (1) to complete the closed heat pump cycle. Among them, the high-temperature flue gas from the iron and steel metallurgical industry first enters the flue gas side channel of the second CO2-flue gas heat exchanger to exchange heat with the CO2 working medium and release heat to cool down. The high-temperature flue gas after releasing heat and cooling down is mixed with the low-temperature flue gas from the low-temperature flue gas conveying pipeline to form mixed flue gas. External makeup water enters the first three-way valve (14) through the makeup water regulating valve (13) and mixes with the saturated water from the saturated water outlet of the flash tank (12) to form mixed water; the mixed water is pressurized by the water pump (10) and sent to the water side channel of the third gas cooler (4) to exchange heat with the CO2 working fluid and become high-temperature pressurized water; the high-temperature pressurized water is throttled and depressurized by the throttling valve (11) and enters the flash tank (12) to complete the flash separation and generate saturated steam and saturated water; the saturated water flows back to the first three-way valve (14) to mix with the external makeup water and is recycled; the saturated steam is introduced into the water side channel of the second gas cooler (3), exchanged heat with the CO2 working fluid and is then output through the steam supply outlet.

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