Method for recycling high-temperature flue gas

Abstract

The invention provides a method capable of recycling high-temperature flue gas. The method comprises the steps that the high-temperature flue gas and chloride composite fused salt are subjected to heat exchange so as to store waste heat in the high-temperature flue gas; the medium-temperature flue gas is introduced into the high-temperature chloride composite fused salt to adsorb CO2 in the medium-temperature flue gas, and then the carbon-containing high-temperature chloride composite fused salt is electrolyzed; carrying out heat exchange on the medium-temperature flue gas and nitrate fused salt to obtain medium-temperature nitrate fused salt; carrying out heat exchange on the high-temperature electrolytic chloride composite fused salt and first saturated water vapor and generating power; and carrying out heat exchange on the medium-temperature nitrate fused salt and the second saturated steam and generating electricity to obtain electric energy. According to the built high-temperature flue gas waste heat and CO2 coupling synergistic utilization route, flue gas waste heat stored by chloride composite molten salt provides heat for CO2 molten salt trapping-electrolytic carbon sequestration, meanwhile, the temperature quality of the molten salt in the process is not reduced, high-temperature and high-pressure superheated steam can be further obtained through molten salt / water vapor heat exchange, and then high-efficiency power generation is achieved. And after heat exchange, the chloride composite fused salt can be reused for storing flue gas waste heat, so that cyclic utilization of the fused salt is realized.

Description

Technical Field

[0001] This invention belongs to the field of waste recycling technology, specifically relating to a method for the recycling and reuse of high-temperature flue gas. Background Technology

[0002] The iron and steel metallurgy industry is a major contributor to CO2 emissions and energy consumption, accounting for 16% of my country's total carbon emissions and 11% of its total energy consumption, respectively. Various types of kilns in the iron and steel metallurgy industry emit massive volumes of high-temperature flue gas, especially converter flue gas. Even after vaporization and cooling in the flue, the flue gas temperature can still reach around 800℃, with a CO2 content of about 10%. Therefore, high-temperature flue gas from metallurgical processes is essentially a resource-energy aggregate of CO2 and waste heat. Thus, waste heat recovery and CO2 capture and utilization from metallurgical flue gas are crucial for energy conservation and emission reduction in the iron and steel industry.

[0003] Currently, CO2 capture and utilization and waste heat recovery from high-temperature flue gas in the steel industry mainly employ two relatively independent pathways. The common CO2 capture method using alcohol amines requires external energy input, and alcohol amines are highly volatile, resulting in significant energy losses. Meanwhile, waste heat boiler power generation suffers from low energy utilization and efficiency. It is evident that current approaches do not fully consider the unique characteristics of integrating high-temperature flue gas CO2 resources with waste heat energy. On the one hand, the application value and demand for CO2 enriched by absorption methods in steel enterprises are currently low. In fact, CO2 is a cheap and abundant carbon-based resource; its targeted conversion into high-value-added materials offers significant economic and environmental benefits. However, CO2 has extremely high stability, with a bond energy as high as 750 KJ / mol, requiring a substantial additional energy input for its conversion and utilization. On the other hand, the waste heat energy abundant in high-temperature flue gas has not been efficiently utilized and is severely dissipated.

[0004] Considering the current industrial situation where CO2 capture and utilization in flue gas requires external energy supply and flue gas waste heat dissipation is severe and utilization rate is low, there is an urgent need to innovate and develop technical processes for the synergistic utilization of carbon and heat in high-temperature flue gas in the iron and steel metallurgy industry, so as to maximize energy conservation and emission reduction in the metallurgical process and support the "dual carbon" strategic goal of the iron and steel industry. Summary of the Invention

[0005] Addressing the integrated nature of high-temperature flue gas waste heat, CO2 energy, and resources in the iron and steel metallurgy industry, this invention utilizes molten salt, which combines long-term heat storage and selective CO2 capture, as a medium to design a synergistic utilization route for high-temperature flue gas waste heat and CO2 molten salt. Specifically, a low-cost alkali / alkaline earth metal chloride composite molten salt is used to store the high-temperature flue gas waste heat and further capture CO2 from the flue gas. The CO2 captured by the molten salt is electrolyzed into elemental carbon and O2. After electrolysis, the high-temperature chloride composite molten salt undergoes heat exchange to obtain high-quality superheated steam, which is then used for efficient power generation, providing electricity for CO2 electrolysis and carbon fixation. After heat exchange, the low-temperature chloride composite molten salt is used for heat storage again to achieve a cycle. After the high-temperature flue gas is stored in the chloride composite molten salt, the emitted medium-temperature flue gas undergoes further heat storage in the nitrate molten salt. The stored nitrate molten salt undergoes heat exchange to obtain medium-quality superheated steam, which is then used for efficient power generation.

[0006] To achieve the above objectives, one aspect of the present invention provides a method for recycling and reusing high-temperature flue gas, comprising the following steps:

[0007] 1) High-temperature flue gas thermal energy storage

[0008] High-temperature flue gas is heat-exchanged with chloride composite molten salt through a heat exchanger to store the waste heat in the high-temperature flue gas, resulting in medium-temperature flue gas and high-temperature chloride composite molten salt. The chloride composite molten salt includes CaO and chloride molten salt, and the chloride molten salt includes two or more of LiCl, NaCl, KCl, and CaCl2.

[0009] 2) CO2 recovery

[0010] The medium-temperature flue gas obtained in step 1) is passed into the high-temperature chloride composite molten salt obtained in step 1) to adsorb CO2 in the medium-temperature flue gas, resulting in low-carbon medium-temperature flue gas and carbon-containing high-temperature chloride composite molten salt. Then, the carbon-containing high-temperature chloride composite molten salt is electrolyzed to obtain elemental carbon, O2 and high-temperature electrolytic chloride composite molten salt.

[0011] 3) Medium-temperature flue gas thermal energy storage

[0012] The low-carbon medium-temperature flue gas obtained in step 2) is passed through a heat exchanger to exchange heat with nitrate molten salt in order to store the residual heat in the medium-temperature flue gas, resulting in low-temperature flue gas and medium-temperature nitrate molten salt.

[0013] 4) Thermal energy utilization

[0014] The high-temperature electrolytic chloride composite molten salt obtained in step 2) is heat-exchanged with the first saturated steam through a heat exchanger to obtain high-quality saturated steam and low-temperature electrolytic chloride composite molten salt.

[0015] The medium-temperature nitrate molten salt obtained in step 3) is heat-exchanged with the second saturated steam through a heat exchanger to obtain medium-quality saturated steam and low-temperature nitrate molten salt.

[0016] The high-quality saturated steam and the medium-quality saturated steam are fed into a steam turbine to generate electricity.

[0017] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the high-temperature flue gas is converter high-temperature flue gas.

[0018] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the temperature of the high-temperature flue gas is above 750°C; and by volume, the high-temperature flue gas contains 5-15% CO2.

[0019] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the chloride molten salt has a melting point ≤600℃, a specific heat capacity ≥1.0J / g·K, a thermal conductivity ≥0.5W / m·K, and a thermal stability ≥800℃.

[0020] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the basic chloride molten salt components and proportions, in molar mass, are selected from any combination of the following:

[0021] 1) CaCl2:NaCl = 40~50:55~45,

[0022] 2) CaCl2:KCl = 20~30:75~65,

[0023] 4) CaCl2:LiCl = 35~45:60~50.

[0024] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the content of CaO in the chloride composite molten salt is 5-15% by molar mass.

[0025] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the nitrate molten salt includes two or more of LiNO3, NaNO3, KNO3, NaNO2, and Ca(NO3)2.

[0026] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, in step 1), the flow rate of the chloride composite molten salt is 90-110 t / h; in step 3), the flow rate of the nitrate molten salt is 90-110 t / h; in step 4), the temperature of the first saturated steam is 160-200℃, the pressure is 0.8-1.2 MPa, and the flow rate is 40-60 t / h, the flow rate of the high-temperature electrolytic chloride composite molten salt is 90-110 t / h, the temperature of the second saturated steam is 160-200℃, the pressure is 0.8-1.2 MPa, and the flow rate is 90-110 t / h, and the flow rate of the medium-temperature nitrate molten salt is 90-110 t / h.

[0027] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the electrolysis temperature is 690-720℃ and the electrolysis voltage is 2-3V.

[0028] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the method further includes the step of using the electrical energy obtained in step 4) for electrolysis in step 2).

[0029] As a preferred embodiment of the method for recycling high-temperature flue gas according to the present invention, the method further includes the step of recycling the low-temperature nitrate molten salt obtained in step 4) to step 3) for medium-temperature flue gas thermal energy storage, and / or the step of recycling the low-temperature chloride composite molten salt obtained in step 4) to step 1) for high-temperature flue gas thermal energy storage.

[0030] In this invention, the specific method of heat exchange through a heat exchanger is a conventional operation in the field. Taking "high-temperature flue gas exchanging heat with chloride complex molten salt through a heat exchanger" as an example, specifically, chloride complex molten salt is introduced into the heat exchanger cavity, and high-temperature flue gas is introduced into the outside of the heat exchanger cavity. The two exchange heat through the heat conduction of the heat exchanger wall, and the chloride complex molten salt and high-temperature flue gas are isolated by the heat exchanger wall. Other heat exchanges in this invention can be carried out in the same manner as described above.

[0031] It should be noted that the methods in the technical solution of the present invention do not necessarily need to be performed in the order specified by their sequence numbers. For example, step 3) can be ordered between step 1) and step 2).

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. This invention fully leverages the characteristics of high-temperature flue gas in metallurgy as a source of waste heat and CO2. Using a chloride composite molten salt with multiple functions including heat storage and CO2 capture-electrolysis as a medium, it establishes a coupled and synergistic utilization route for high-temperature flue gas waste heat and CO2. The waste heat stored in the chloride composite molten salt provides heat for CO2 capture-electrolysis carbon fixation. Simultaneously, the molten salt temperature and quality are not reduced during this process. Furthermore, high-temperature, high-pressure superheated steam can be obtained through molten salt / steam heat exchange, leading to high-efficiency power generation. The electricity is then used for CO2 electrolysis of elemental carbon, which is also used as a high-quality reducing agent and carbon additive. After heat exchange, the chloride composite molten salt can be reused to store waste heat from the flue gas, achieving molten salt recycling.

[0034] 2. This invention designs and optimizes a two-stage molten salt thermal storage route for high-temperature flue gas. First, high-temperature chloride composite molten salt is obtained through chloride-based thermal storage, followed by medium-temperature nitrate molten salt through nitrate thermal storage. These are then subjected to heat exchange to obtain high-quality and medium-quality superheated steam, which is then used for power generation. This maximizes the storage and utilization of waste heat in the high-temperature flue gas, resulting in the greatest possible electrical energy. Taking the flue gas from a 120t converter as an example, the high-quality saturated steam obtained based on this invention has a temperature of 400-450℃ and a pressure of 5-11 MPa, generating 8000-9000 kW; the medium-quality saturated steam has a temperature of 350-370℃ and a pressure of 2.5-3.5 MPa, generating 10000-15000 kW.

[0035] 3. The technical route of this invention promotes the low-carbon development of the steel industry through three levels: efficient CO2 molten salt capture-electrolytic carbon fixation for direct carbon emission reduction, efficient utilization of waste heat for indirect carbon emission reduction, and high-quality elemental carbon metallurgy for negative carbon emissions.

[0036] 4. Based on the solution of the present invention, when recovering CO2, the utilization rate of CaO reaches 60-70%, and the CO2 recovery effect is obvious. Attached Figure Description

[0037] Figure 1 A schematic flowchart of a method for recycling high-temperature flue gas according to the present invention. Detailed Implementation

[0038] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] The main process of the specific implementation of this invention is as follows: Figure 1 As shown.

[0040] Example 1

[0041] A method for recycling high-temperature flue gas, wherein the high-temperature flue gas is generated by a 120t converter in one furnace cycle, with a temperature of 850℃ and a CO2 volume fraction of 10%, includes the following steps:

[0042] 1) High-temperature flue gas thermal energy storage

[0043] High-temperature flue gas is heat-exchanged with chloride composite molten salt through a heat exchanger to store the waste heat in the high-temperature flue gas, resulting in medium-temperature flue gas at 575℃ and high-temperature chloride composite molten salt at 720℃. The flow rate of the chloride composite molten salt is 110 t / h, and the flow rate of the high-temperature flue gas is 108 t / h. The chloride composite molten salt comprises, by molar mass, 12% CaO, 45.8% NaCl, and 42.2% CaCl2.

[0044] 2) CO2 recovery

[0045] The medium-temperature flue gas obtained in step 1) is passed into the high-temperature chloride composite molten salt obtained in step 1) to adsorb CO2 in the medium-temperature flue gas, resulting in low-carbon medium-temperature flue gas and carbon-containing high-temperature chloride composite molten salt, with a CaO utilization rate of 65%; then the carbon-containing high-temperature chloride composite molten salt is electrolyzed at 720℃ and 2.8V to obtain elemental carbon, O2 and high-temperature electrolytic chloride composite molten salt.

[0046] 3) Medium-temperature flue gas thermal energy storage

[0047] The low-carbon, medium-temperature flue gas obtained in step 2) is heat-exchanged with nitrate molten salt through a heat exchanger to store the residual heat in the medium-temperature flue gas, resulting in low-temperature flue gas at 255°C and medium-temperature nitrate molten salt at 465°C. The flow rate of the nitrate molten salt is 110 t / h, and the flow rate of the medium-temperature flue gas is 108 t / h. The nitrate molten salt comprises, by molar mass, 6.9% NaNO3, 44.2% KNO3, and 48.9% NaNO2.

[0048] 4) Thermal energy utilization

[0049] The high-temperature electrolytic chloride composite molten salt obtained in step 2) is heat-exchanged with 180℃, 1MPa first saturated steam through a heat exchanger to obtain 435℃, 10MPa high-quality saturated steam and 555℃ low-temperature electrolytic chloride composite molten salt. The flow rate of the first saturated steam is 45t / h, and the flow rate of the high-temperature electrolytic chloride composite molten salt is 110t / h. The low-temperature electrolytic chloride composite molten salt can be recycled to step 1) to continue storing the thermal energy of the high-temperature flue gas.

[0050] The intermediate-temperature nitrate molten salt obtained in step 3) is heat-exchanged with second saturated steam at 180°C and 1 MPa through a heat exchanger to obtain medium-quality saturated steam at 355°C and 3 MPa and low-temperature nitrate molten salt at 188°C. The flow rate of the second saturated steam is 110 t / h, and the flow rate of the intermediate-temperature nitrate molten salt is 110 t / h. The low-temperature nitrate molten salt can be recycled to step 3) to continue storing the thermal energy of the intermediate-temperature flue gas.

[0051] The high-quality saturated steam and the medium-quality saturated steam are fed into a steam turbine to generate electricity, with power outputs of 9300 kW and 10500 kW respectively. The electricity is used for electrolysis in step 2).

[0052] Example 2

[0053] A method for recycling high-temperature flue gas, wherein the high-temperature flue gas is generated in one furnace cycle of a 120t converter with a temperature of 800℃ and a CO2 volume fraction of 10%, includes the following steps:

[0054] 1) High-temperature flue gas thermal energy storage

[0055] High-temperature flue gas is heat-exchanged with chloride composite molten salt through a heat exchanger to store the waste heat in the high-temperature flue gas, resulting in medium-temperature flue gas at 571℃ and high-temperature chloride composite molten salt at 700℃. The flow rate of the chloride composite molten salt is 100 t / h, and the flow rate of the high-temperature flue gas is 108 t / h. The chloride composite molten salt comprises, by molar mass, 10% CaO, 46.8% NaCl, and 43.2% CaCl2.

[0056] 2) CO2 recovery

[0057] The medium-temperature flue gas obtained in step 1) is passed into the high-temperature chloride composite molten salt obtained in step 1) to adsorb CO2 in the medium-temperature flue gas, resulting in low-carbon medium-temperature flue gas and carbon-containing high-temperature chloride composite molten salt, with a CaO utilization rate of 68%; then the carbon-containing high-temperature chloride composite molten salt is electrolyzed at 700℃ and 2.8V to obtain elemental carbon, O2 and high-temperature electrolytic chloride composite molten salt.

[0058] 3) Medium-temperature flue gas thermal energy storage

[0059] The low-carbon, medium-temperature flue gas obtained in step 2) is heat-exchanged with nitrate molten salt through a heat exchanger to store the residual heat in the medium-temperature flue gas, resulting in low-temperature flue gas at 264°C and medium-temperature nitrate molten salt at 471°C. The flow rate of the nitrate molten salt is 100 t / h, and the flow rate of the medium-temperature flue gas is 108 t / h. The nitrate molten salt comprises, by molar mass, 6.9% NaNO3, 44.2% KNO3, and 48.9% NaNO2.

[0060] 4) Thermal energy utilization

[0061] The high-temperature electrolytic chloride composite molten salt obtained in step 2) is heat-exchanged with first saturated steam at 180℃ and 1MPa through a heat exchanger to obtain high-quality saturated steam at 442℃ and 11MPa and low-temperature electrolytic chloride composite molten salt at 550℃. The flow rate of the first saturated steam is 50t / h and the flow rate of the high-temperature electrolytic chloride composite molten salt is 100t / h. The low-temperature electrolytic chloride composite molten salt can be recycled to step 1) to continue storing the thermal energy of the high-temperature flue gas.

[0062] The intermediate-temperature nitrate molten salt obtained in step 3) is heat-exchanged with second saturated steam at 180°C and 1 MPa through a heat exchanger to obtain medium-quality saturated steam at 360°C and 3 MPa and low-temperature nitrate molten salt at 190°C. The flow rate of the second saturated steam is 100 t / h and the flow rate of the intermediate-temperature nitrate molten salt is 100 t / h. The low-temperature nitrate molten salt can be recycled to step 3) to continue storing the thermal energy of the intermediate-temperature flue gas.

[0063] The high-quality saturated steam and the medium-quality saturated steam are fed into a steam turbine to generate electricity, with power outputs of 9373 kW and 10865 kW respectively. The electricity is used for electrolysis in step 2).

[0064] Example 3

[0065] A method for recycling high-temperature flue gas, wherein the high-temperature flue gas is generated in one furnace cycle of a 120t converter at a temperature of 820℃ and a CO2 volume fraction of 12%, includes the following steps:

[0066] 1) High-temperature flue gas thermal energy storage

[0067] High-temperature flue gas is heat-exchanged with chloride composite molten salt through a heat exchanger to store the waste heat in the high-temperature flue gas, resulting in medium-temperature flue gas at 575℃ and high-temperature chloride composite molten salt at 705℃. The flow rate of the chloride composite molten salt is 100 t / h, and the flow rate of the high-temperature flue gas is 108 t / h. The chloride composite molten salt comprises, by molar mass, 10% CaO, 46.8% NaCl, and 43.2% CaCl2.

[0068] 2) CO2 recovery

[0069] The medium-temperature flue gas obtained in step 1) is passed into the high-temperature chloride composite molten salt obtained in step 1) to adsorb CO2 in the medium-temperature flue gas, resulting in low-carbon medium-temperature flue gas and carbon-containing high-temperature chloride composite molten salt, with a CaO utilization rate of 69%; then the carbon-containing high-temperature chloride composite molten salt is electrolyzed at 705℃ and 2.7V to obtain elemental carbon, O2 and high-temperature electrolytic chloride composite molten salt.

[0070] 3) Medium-temperature flue gas thermal energy storage

[0071] The low-carbon, medium-temperature flue gas obtained in step 2) is heat-exchanged with nitrate molten salt through a heat exchanger to store the residual heat in the medium-temperature flue gas, resulting in low-temperature flue gas at 270°C and medium-temperature nitrate molten salt at 475°C. The flow rate of the nitrate molten salt is 100 t / h, and the flow rate of the medium-temperature flue gas is 108 t / h. The nitrate molten salt comprises 37% NaNO3, 53% KNO3, and 40% NaNO2 by molar mass.

[0072] 4) Thermal energy utilization

[0073] The high-temperature electrolytic chloride composite molten salt obtained in step 2) is heat-exchanged with first saturated steam at 180℃ and 1MPa through a heat exchanger to obtain high-quality saturated steam at 447℃ and 11MPa and low-temperature electrolytic chloride composite molten salt at 550℃. The flow rate of the first saturated steam is 50t / h and the flow rate of the high-temperature electrolytic chloride composite molten salt is 100t / h. The low-temperature electrolytic chloride composite molten salt can be recycled to step 1) to continue storing the thermal energy of the high-temperature flue gas.

[0074] The intermediate-temperature nitrate molten salt obtained in step 3) is heat-exchanged with second saturated steam at 180°C and 1 MPa through a heat exchanger to obtain medium-quality saturated steam at 364°C and 3 MPa and low-temperature nitrate molten salt at 193°C. The flow rate of the second saturated steam is 100 t / h and the flow rate of the intermediate-temperature nitrate molten salt is 100 t / h. The low-temperature nitrate molten salt can be recycled to step 3) to continue storing the thermal energy of the intermediate-temperature flue gas.

[0075] The high-quality saturated steam and the medium-quality saturated steam are fed into a steam turbine to generate electricity, with power outputs of 9500 kW and 11000 kW respectively. The electricity is used for electrolysis in step 2).

[0076] Example 4

[0077] A method for recycling high-temperature flue gas, wherein the high-temperature flue gas is generated in one furnace cycle of a 120t converter with a temperature of 760℃ and a CO2 volume fraction of 8%, includes the following steps:

[0078] 1) High-temperature flue gas thermal energy storage

[0079] High-temperature flue gas is heat-exchanged with chloride composite molten salt through a heat exchanger to store the waste heat in the high-temperature flue gas, resulting in medium-temperature flue gas at 590℃ and high-temperature chloride composite molten salt at 702℃. The flow rate of the chloride composite molten salt is 90 t / h, and the flow rate of the high-temperature flue gas is 108 t / h. The chloride composite molten salt comprises, by molar mass, 10% CaO, 46.8% NaCl, and 43.2% CaCl2.

[0080] 2) CO2 recovery

[0081] The medium-temperature flue gas obtained in step 1) is passed into the high-temperature chloride composite molten salt obtained in step 1) to adsorb CO2 in the medium-temperature flue gas, resulting in low-carbon medium-temperature flue gas and carbon-containing high-temperature chloride composite molten salt, with a CaO utilization rate of 62%; then the carbon-containing high-temperature chloride composite molten salt is electrolyzed at 702℃ and 2.9V to obtain elemental carbon, O2 and high-temperature electrolytic chloride composite molten salt;

[0082] 3) Medium-temperature flue gas thermal energy storage

[0083] The low-carbon, medium-temperature flue gas obtained in step 2) is heat-exchanged with nitrate molten salt through a heat exchanger to store the residual heat in the medium-temperature flue gas, resulting in low-temperature flue gas at 280°C and medium-temperature nitrate molten salt at 480°C. The flow rate of the nitrate molten salt is 90 t / h, and the flow rate of the medium-temperature flue gas is 108 t / h. The nitrate molten salt comprises, by molar mass, 37.2% LiNO3, 49.2% KNO3, and 13.6% NaNO3.

[0084] 4) Thermal energy utilization

[0085] The high-temperature electrolytic chloride composite molten salt obtained in step 2) is heat-exchanged with 180℃, 1MPa first saturated steam through a heat exchanger to obtain 420℃, 9MPa high-quality saturated steam and 545℃ low-temperature electrolytic chloride composite molten salt. The flow rate of the first saturated steam is 60t / h, and the flow rate of the high-temperature electrolytic chloride composite molten salt is 90t / h. The low-temperature electrolytic chloride composite molten salt can be recycled to step 1) to continue storing the thermal energy of the high-temperature flue gas.

[0086] The intermediate-temperature nitrate molten salt obtained in step 3) is heat-exchanged with second saturated steam at 180°C and 1 MPa through a heat exchanger to obtain medium-quality saturated steam at 370°C and 3 MPa and low-temperature nitrate molten salt at 195°C. The flow rate of the second saturated steam is 100 t / h and the flow rate of the intermediate-temperature nitrate molten salt is 90 t / h. The low-temperature nitrate molten salt can be recycled to step 3) to continue storing the thermal energy of the intermediate-temperature flue gas.

[0087] The high-quality saturated steam and the medium-quality saturated steam are fed into a steam turbine to generate electricity, with power outputs of 8500 kW and 11500 kW respectively. The electricity is used for electrolysis in step 2).

[0088] Comparative Example 1

[0089] The difference between this comparative example and Example 1 is that in step 1), the flow rate of the chloride composite molten salt is 70 t / h.

[0090] Compared with Example 1, in this comparative example, the molten salt flow rate is too small when the chloride composite molten salt exchanges heat with the high-temperature flue gas, which is conducive to sufficient heat exchange between the molten salt and the flue gas. Although the temperature of the chloride composite molten salt can reach 780°C, achieving more efficient heat storage, the higher temperature of the chloride composite molten salt is not conducive to subsequent CO2 capture. At this time, the CaO utilization rate is only 30%, which reduces the CO2 capture capacity and further reduces the efficiency of electrolysis of elemental carbon.

[0091] Comparative Example 2

[0092] The difference between this comparative example and Example 2 is that the flow rate of the high-temperature electrolytic chloride composite molten salt is 120 t / h. The pressure of the high-quality superheated steam obtained at this time is 13 MPa.

[0093] Compared with Example 1, in this comparative example, the excessive flow rate of the high-temperature electrolytic chloride composite molten salt resulted in excessively high superheated steam pressure (13 MPa), which caused the liquid phase fraction of the superheated steam to be too high, reaching 12.5%, exceeding the turbine's requirement of less than 10-12% liquid phase fraction. This easily led to the generation of droplets in the turbine, which could cause significant damage to the turbine blades and make it difficult for them to operate safely.

[0094] Comparative Example 3

[0095] The difference between this comparative example and Example 3 is that in step 3), the flow rate of the nitrate molten salt is 125 t / h.

[0096] Compared to Example 3, in this comparative example, due to the excessively high nitrate molten salt flow rate, the flue gas outlet temperature dropped to 190°C, only slightly higher than the lower limit of the low-temperature nitrate inlet temperature of 185°C. At this point, the remaining heat in the flue gas was insufficient to raise the nitrate molten salt temperature, and the medium-temperature nitrate temperature dropped to 440°C, which was not conducive to obtaining superheated steam that met the requirements. Simultaneously, the heat load and required area of ​​the nitrate / flue gas heat exchanger increased significantly, which was detrimental to heat exchanger selection and safe operation.

[0097] Comparative Example 4

[0098] The difference between this comparative example and Example 4 is that in step 4), the flow rate of the second saturated water vapor is 70 t / h.

[0099] Compared with Example 4, this comparative example has a low saturated steam flow rate, which makes it difficult to fully absorb the heat stored in the medium-temperature nitrate molten salt. As a result, after heat exchange, the outlet low-temperature nitrate molten salt temperature still reaches 260°C, causing a great waste of heat.

[0100] Comparative Example 5

[0101] The difference between this comparative example and Example 4 is that, in step 1), the chloride composite molten salt comprises CaO 10%, NaCl 45.8%, and MgCl2 44.2% by molar mass.

[0102] Compared with Example 4, after obtaining high-temperature chloride composite molten salt through chloride composite molten salt thermal storage, it is difficult to further capture CO2 in the medium-temperature flue gas. This is mainly because the CaCO3 generated by the reaction of CaO and CO2 during the capture process will further react with MgCl2 in the molten salt to generate MgCO3 and CaCl2. MgCO3 decomposes into MgO and CO2 at temperatures above 400°C, releasing CO2 again.

[0103] Comparative Example 6

[0104] The difference between this comparative example and Example 2 is that in step 4), the nitrate molten salt is replaced with a chloride molten salt of NaCl-MgCl2 (52-48 mol.%).

[0105] Compared with Example 2, the melting point of NaCl-MgCl2 (52-48 mol.%) chloride molten salt is as high as 505°C, while the temperature of medium-temperature flue gas is 550-600°C. The temperature range between the two is narrow, so it is impossible to fully store the heat in the medium-temperature flue gas, which significantly reduces the utilization rate of flue gas waste heat.

[0106] It should be noted that, based on the above embodiments of the present invention, those skilled in the art can fully realize the scope of the independent claims and dependent claims of the present invention, and the implementation process and methods are the same as those in the above embodiments; and the parts of the present invention not described in detail belong to the well-known technology in the art. However, the protection scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention.

Claims

1. A method for recycling and reusing high-temperature flue gas, characterized in that, Includes the following steps: 1) High-temperature flue gas thermal energy storage High-temperature flue gas is heat-exchanged with chloride composite molten salt through a heat exchanger to store the waste heat in the high-temperature flue gas, resulting in medium-temperature flue gas and high-temperature chloride composite molten salt. The chloride composite molten salt includes CaO and chloride molten salt, and the chloride molten salt includes two or more of LiCl, NaCl, KCl, and CaCl2. 2) CO2 recovery The medium-temperature flue gas obtained in step 1) is passed into the high-temperature chloride composite molten salt obtained in step 1) to adsorb CO2 in the medium-temperature flue gas, resulting in low-carbon medium-temperature flue gas and carbon-containing high-temperature chloride composite molten salt. Then, the carbon-containing high-temperature chloride composite molten salt is electrolyzed to obtain elemental carbon, O2 and high-temperature electrolytic chloride composite molten salt. 3) Medium-temperature flue gas thermal energy storage The low-carbon medium-temperature flue gas obtained in step 2) is passed through a heat exchanger to exchange heat with nitrate molten salt in order to store the residual heat in the medium-temperature flue gas, resulting in low-temperature flue gas and medium-temperature nitrate molten salt. 4) Thermal energy utilization The high-temperature electrolytic chloride composite molten salt obtained in step 2) is heat-exchanged with the first saturated steam through a heat exchanger to obtain high-quality saturated steam and low-temperature electrolytic chloride composite molten salt. The medium-temperature nitrate molten salt obtained in step 3) is heat-exchanged with the second saturated steam through a heat exchanger to obtain medium-quality saturated steam and low-temperature nitrate molten salt. The high-quality saturated steam and the medium-quality saturated steam are fed into a steam turbine to generate electricity.

2. The method as described in claim 1, characterized in that, The temperature of the high-temperature flue gas is above 750°C; by volume, the high-temperature flue gas contains 5-15% CO2.

3. The method as described in claim 1, characterized in that, The chloride molten salt has a melting point ≤600℃, specific heat capacity ≥1.0J / g·K, thermal conductivity ≥0.5W / m·K, and thermal stability ≥800℃.

4. The method according to any one of claims 1-3, characterized in that, The basic chloride molten salt composition and proportions, on a molar mass basis, are selected from any combination of the following: 1) CaCl2:NaCl = 40~50:55~45, 2) CaCl2:KCl = 20~30:75~65, 4) CaCl2:LiCl = 35~45:60~50.

5. The method as described in claim 4, characterized in that, The CaO content in the chloride complex molten salt is 5-15% by molar mass.

6. The method as described in any one of claims 5, characterized in that, The nitrate molten salt includes two or more of LiNO3, NaNO3, KNO3, NaNO2, and Ca(NO3)2.

7. The method as described in claim 5 or 6, characterized in that, In step 1), the flow rate of the chloride composite molten salt is 90-110 t / h; in step 3), the flow rate of the nitrate molten salt is 90-110 t / h; in step 4), the temperature of the first saturated steam is 160-200℃, the pressure is 0.8-1.2 MPa, and the flow rate is 40-60 t / h, the flow rate of the high-temperature electrolytic chloride composite molten salt is 90-110 t / h, the temperature of the second saturated steam is 160-200℃, the pressure is 0.8-1.2 MPa, and the flow rate is 90-110 t / h, and the flow rate of the medium-temperature nitrate molten salt is 90-110 t / h.

8. The method according to any one of claims 1-3 and 5-7, characterized in that, The electrolysis temperature is 690-720℃, and the electrolysis voltage is 2-3V.

9. The method according to any one of claims 1-3 and 5-7, characterized in that, The method further includes the step of using the electrical energy obtained in step 4) for electrolysis in step 2).

10. The method according to any one of claims 1-3 and 5-7, characterized in that, The method further includes the step of recycling the low-temperature nitrate molten salt obtained in step 4) to step 3) for medium-temperature flue gas thermal energy storage, and / or the step of recycling the low-temperature chloride complex molten salt obtained in step 4) to step 1) for high-temperature flue gas thermal energy storage.

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