Binary molten salt based thermal storage system for fossil power plants

Abstract

The utility model provides a kind of based on binary molten salt's thermal power plant molten salt heat storage system, including heat storage loop, heat release loop and molten salt circulation loop, by process optimization ensure that molten salt heat exchanger inlet medium is always low-temperature steam, completely avoid the risk of liquid water and molten salt direct contact, to break through binary molten salt solidification temperature limit.

Description

Technical Field

[0001] This utility model belongs to the field of molten salt thermal storage technology for thermal power plants, and in particular relates to a molten salt thermal storage system for thermal power plants based on binary molten salt. Background Technology

[0002] In the field of thermal power generation, coal-fired or gas-fired units generate high-temperature, high-pressure steam through boilers to drive steam turbines for power generation. Subcritical boilers require periodic discharge of wastewater containing impurities to a periodic blowdown expansion tank during operation. Supercritical boilers also require high-temperature, high-pressure condensate to be discharged to a boiler condensate expansion tank. After the wastewater or condensate enters the expansion tank, a large amount of low-temperature steam (95-100℃) will flash out due to the sudden pressure drop. This type of steam contains abundant latent heat of vaporization (approximately 2200 kJ / kg), but due to its low temperature and pressure close to atmospheric pressure, traditional technologies cannot efficiently recover it and it can only be directly discharged into the atmosphere.

[0003] I. Limitations of Existing Waste Heat Recovery Technologies

[0004] At present, thermal power plants urgently need to improve their operational flexibility to adapt to the demand for renewable energy consumption. Molten salt thermal energy storage technology, due to its large capacity and long-cycle thermal storage characteristics, has become one of the key paths for the deep peak-shaving retrofit of thermal power units. The core contradiction faced by traditional molten salt thermal energy storage systems in thermal power applications lies in balancing the risk of molten salt solidification with media compatibility.

[0005] 1. Limitations imposed by thermodynamic cycle characteristics:

[0006] Thermal power units use water / steam as the working fluid in their thermal cycle, and the vaporization process needs to be completed in a relatively low temperature range. If condensate is directly used for heat exchange with high-temperature molten salt, the molten salt will solidify and block the heat exchange tubes because the water temperature is much lower than the freezing point of the molten salt.

[0007] 2. The dilemma of molten salt selection:

[0008] Binary molten salts (typically 40% KNO3-60% NaNO3) have advantages such as low melting point (about 220℃), good thermal stability, and controllable cost, but their freezing point is still significantly higher than the water vaporization temperature (only 133℃ at 0.3MPa).

[0009] While ternary molten salts (such as HitecXL, containing Ca / K / Na nitrates) can lower the melting point to about 130°C, they have three major drawbacks:

[0010] The thermal decomposition temperature is reduced (≤500℃), requiring continuous nitrogen gas to be introduced to seal and suppress decomposition, which increases system complexity and operation and maintenance costs;

[0011] Increased high-temperature corrosivity necessitates higher requirements for the material grades of heat exchange equipment.

[0012] The unit heat capacity is about 15% lower than that of binary molten salt, and the amount of molten salt required to store the same amount of heat is 30% higher.

[0013] 3. Existing technical solutions and limitations

[0014] 1) The current mainstream solution adopts a three-stage heating + ternary molten salt architecture, and its typical process is as follows:

[0015] Thermal storage stage: The unit extracts steam or electrically heats molten salt to achieve thermal energy storage;

[0016] Exothermic phase:

[0017] Primary preheating: Condensate → Molten salt heat exchanger → Heating to saturation (Risk: Molten salt sidewall temperature close to freezing point);

[0018] Two-stage evaporation: saturated water / steam mixture → steam drum → saturated steam generation;

[0019] Three-stage superheating: saturated steam → molten salt heat exchanger → superheated steam with qualified output parameters.

[0020] 2) This solution has the following technical bottlenecks:

[0021] The system is too complex: a two-stage heat exchanger, a steam-water separator, and a nitrogen sealing system lead to increased equipment investment.

[0022] Slow start-up response: The time from cold state to full load steam supply is relatively long, which cannot meet the demand for rapid peak shaving;

[0023] Significant operational risks:

[0024] The temperature of the molten salt in the condensate heat exchange section is close to the freezing point, posing a risk of localized solidification leading to pipe rupture.

[0025] A malfunction in the nitrogen system will cause molten salt decomposition, producing harmful gases such as NOx;

[0026] Economic constraints: Nitrogen consumption and molten salt replenishment costs significantly increase annual operation and maintenance expenses;

[0027] Slow start-up process: The condensate goes through three stages: preheating, evaporation, and superheating. The system has many processes and cannot respond to peak-shaving needs in a timely manner.

[0028] 4. Industry Pain Points and Innovation Needs

[0029] The current proposal is to "promote the commercial application of thermal power unit coupled molten salt thermal energy storage technology," but the existing technology faces obstacles in its promotion due to the aforementioned shortcomings. An innovative solution that meets the following core requirements is urgently needed:

[0030] Molten salt selection optimization: adopting binary molten salt with better thermal stability and eliminating the nitrogen sealing system;

[0031] Simplified heat exchange process: Reduce the number of heat exchange stages and avoid the risk of direct contact between water and molten salt;

[0032] Rapid start-up capability: Achieves rated steam supply from a cold state in a short time;

[0033] Improved economic efficiency: Reduce heat exchange equipment and lower operating costs. Utility Model Content

[0034] This invention aims to address the aforementioned core needs by providing a molten salt thermal power plant system based on binary molten salt. Through process optimization, it ensures that the inlet medium of the molten salt heat exchanger is always low-temperature steam, completely eliminating the risk of direct contact between liquid water and molten salt, thus overcoming the solidification temperature limitation of binary molten salt. The system eliminates the nitrogen sealing device, simplifies the heat exchange process, enables rapid start-up, and significantly reduces investment and maintenance costs.

[0035] To achieve the above-mentioned utility model objectives, this utility model provides a molten salt thermal storage system for thermal power plants based on binary molten salt, including a thermal storage circuit, a heat release circuit, and a molten salt circulation circuit;

[0036] The thermal storage circuit includes a boiler reheater outlet hot section steam pipe, a molten salt heater, and a hot salt tank connected in sequence.

[0037] The heat release circuit includes an auxiliary steam shut-off valve, a condensate vaporizer, a steam booster, a molten salt cooler, a deaerator heating steam shut-off valve, a deaerator, a feedwater pump, and a steam recirculation shut-off valve on the boiler auxiliary steam pipeline. The auxiliary steam shut-off valve, condensate vaporizer, steam booster, and molten salt cooler are connected in sequence. The outlet side of the molten salt cooler is connected to two branches. One branch is connected to the external steam supply pipe, and the other branch is connected to two sub-branches. The first sub-branches are connected in sequence to the deaerator heating steam shut-off valve, the deaerator, the feedwater pump, and the condensate vaporizer. The second sub-branches are connected to the steam recirculation shut-off valve and the condensate vaporizer.

[0038] The molten salt circulation loop includes a cold salt tank, a hot salt tank, a cold molten salt pump, a hot molten salt pump, a molten salt heater, and a molten salt cooler, wherein the cold salt tank, the cold molten salt pump, the molten salt heater, the hot molten salt pump, the hot salt tank, the molten salt cooler, and the cold salt tank are connected in sequence to form a loop.

[0039] Furthermore, the cold salt tank and the hot salt tank use binary molten salt: 60% NaNO3 + 40% KNO3.

[0040] Furthermore, the cold salt tank stores molten salt at 290°C, and the hot salt tank stores molten salt at 550°C;

[0041] This utility model can achieve the following technical effects:

[0042] 1) Binary molten salt replaces ternary molten salt: By using steam as the heat exchange medium throughout the process, the risk of contact with liquid water is avoided, the nitrogen sealing system is eliminated, and the system stability is improved.

[0043] 2) Simplified heat exchange process: Traditional solutions require three-stage heat exchangers (preheating, evaporation, and superheating), while this invention only requires a single-stage molten salt heater. Steam parameters are adjusted through a condensate vaporizer, reducing equipment investment.

[0044] 3) Rapid start-up: The use of auxiliary steam ensures that steam can be supplied at any time even during a cold start-up process.

[0045] 4) Cascaded utilization of energy: Cooling steam during the thermal storage stage is recovered to the high-pressure heater to improve the overall thermal efficiency of the unit. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of a molten salt thermal power plant based on binary molten salt.

[0047] The components include: 1. Cold molten salt storage tank; 2. Hot molten salt storage tank; 3. Cold molten salt pump; 4. Hot molten salt pump; 5. Molten salt heater; 6. Auxiliary steam shut-off valve; 7. Steam recirculation shut-off valve; 8. Condensate vaporizer; 9. Steam booster; 10. Molten salt cooler; 11. Deaerator heating steam shut-off valve; 12. Feed water pump; 13. Deaerator. Detailed Implementation

[0048] To enable those skilled in the art to better understand the technical solution of this utility model, the preferred embodiments of this utility model are described below in conjunction with specific examples. However, it should be understood that the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent. For better illustration of this embodiment, some components in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable that some well-known structures and their descriptions may be omitted in the drawings for those skilled in the art. The positional relationships described in the drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0049] The embodiments of this utility model are described in detail below with reference to the accompanying drawings. This system is applicable to large and medium-sized thermal power generating units and consists of a heat storage circuit, a heat release circuit, and a molten salt circulation circuit. The connection relationships and parameters of each component strictly follow the principles of thermodynamics and fluid mechanics. The core is to avoid direct contact between liquid water and molten salt throughout the entire process by using steam as the medium, ensuring that the inlet temperature of the molten salt heat exchanger is ≥270℃, and completely eliminating the risk of binary molten salt solidification.

[0050] 1. System Composition

[0051] 1) Molten salt circulation loop

[0052] The molten salt circulation loop includes a cold salt tank 1, a hot salt tank 2, a cold molten salt pump 3, a hot molten salt pump 4, a molten salt heater 5, and a molten salt cooler 10, wherein the cold salt tank 1, the cold molten salt pump 3, the molten salt heater 5, the hot molten salt pump 4, the hot salt tank 2, the molten salt cooler 10, and the cold salt tank 1 are connected in sequence to form a loop.

[0053] Molten salt working medium: binary molten salt (60% NaNO3 + 40% KNO3), melting point 220℃, working temperature range 290–550℃.

[0054] Tank configuration: Dual tank design, cold salt tank 1 stores molten salt at 290℃, and hot salt tank 2 stores molten salt at 550℃; nitrogen sealing system is eliminated.

[0055] Circulation equipment: Cold molten salt pump 3 drives molten salt to molten salt heater 5, and hot molten salt pump 4 delivers high-temperature molten salt to molten salt cooler 10.

[0056] 2) Thermal storage loop

[0057] The thermal storage circuit includes a boiler reheater outlet hot section steam pipe, a molten salt heater 5, and a hot salt tank 2 connected in sequence.

[0058] Heat source interface: Connects to the hot section steam pipe (566℃) at the boiler reheater outlet.

[0059] Molten salt heater 5: High-temperature steam and cold molten salt (290℃) exchange heat in a countercurrent manner. The molten salt is heated to 550℃ and stored in hot salt tank 2. The steam is cooled to 310℃.

[0060] Waste heat recovery: Cooled steam is fed into the high-pressure heater of the turbine's regenerative system to heat the unit's feedwater, thus achieving waste heat recovery.

[0061] 3) Heat dissipation circuit

[0062] The heat release circuit includes an auxiliary steam shut-off valve 6, a condensate vaporizer 8, a steam booster 9, a molten salt cooler 10, a deaerator heating steam shut-off valve 11, a deaerator 13, a feedwater pump 12, and a steam recirculation shut-off valve 7 on the boiler auxiliary steam pipeline. The auxiliary steam shut-off valve 6, the condensate vaporizer 8, the steam booster 9, and the molten salt cooler 10 are connected in sequence. The outlet side of the molten salt cooler 10 is connected to two branches. One branch is connected to the external steam supply pipe, and the other branch is connected to two sub-branches. The first sub-branches are connected in sequence to the deaerator heating steam shut-off valve 11, the deaerator 13, the feedwater pump 12, and the condensate vaporizer 8. The second sub-branches are connected to the steam recirculation shut-off valve 7 and the condensate vaporizer 8.

[0063] Start-up phase:

[0064] Open the auxiliary steam shut-off valve 6 to introduce 0.4MPa, 280℃ auxiliary steam into the molten salt cooler 10. After being heated to superheated steam, it is divided into two paths:

[0065] One path enters the deaerator 13 via the deaerator heating steam shut-off valve 11, preheating the feedwater to 170°C.

[0066] Another route directly supplies steam to the outside. When the required steam pressure is higher than that of the auxiliary steam, the steam booster 9 can be turned on to increase the pressure.

[0067] Main exothermic phase:

[0068] Condensate vaporizer 8: Receives 170°C feedwater from deaerator 13 and 540°C superheated steam from molten salt cooler 10, utilizes the superheat of the steam to vaporize the feedwater, and mixes to generate low superheated steam (temperature ≥270°C).

[0069] Steam booster 9: Pressurizes the mixed steam to the target pressure and sends it to the molten salt cooler 10 for heating to 540°C. The steam booster can be centrifugal or axial flow type.

[0070] Steam supply path: After heating, the steam can be returned to the unit's thermal system or supplied to external users.

[0071] Steam source switching: When the feedwater in the deaerator 13 meets the requirements for deaeration and heating, the auxiliary steam shut-off valve 6 is shut off, the steam recirculation shut-off valve 7 is opened, and the feedwater pump 12 is turned on.

[0072] 2. Key Control Strategies

[0073] Temperature protection mechanism:

[0074] By adjusting the desuperheating water volume and steam flow rate, the inlet steam temperature of the molten salt cooler 10 is always ≥270℃, and the temperature difference between the low-temperature side of the molten salt and the working fluid is <20℃, thus avoiding the risk of solidification.

[0075] Fast startup logic:

[0076] During cold start-up, auxiliary steam branch 6 is put into operation first, and has the ability to supply steam to the outside immediately; after the feedwater temperature of deaerator 13 reaches the standard, switch to the main heat release process and shut down the auxiliary steam branch.

[0077] 3. Workflow Example

[0078] Thermal storage process:

[0079] The boiler hot section steam (566℃) enters the molten salt heater 5, where it exchanges heat with the cold molten salt (290℃). The molten salt is then heated to 550℃ and stored in the hot salt tank 2. The cooled steam (310℃) is then recovered to the high-pressure heater.

[0080] Exothermic process:

[0081] Start-up phase: Auxiliary steam (280℃) is heated to 540℃ by molten salt cooler 10, part of which is used for steam supply and part for preheating deaerator feedwater.

[0082] Steady-state period: qualified feedwater (170℃) from the deaerator and high-temperature steam (540℃) are mixed and vaporized in the condensate vaporizer 8, and after being pressurized by the booster 9, they are heated to 540℃ by the molten salt cooler 10 and supplied to the outside.

Claims

1. A molten salt thermal storage system for thermal power plants based on binary molten salt, characterized in that: This includes a heat storage circuit, a heat release circuit, and a molten salt circulation circuit; The thermal storage circuit includes a boiler reheater outlet hot section steam pipe, a molten salt heater (5), and a hot salt tank (2) connected in sequence. The heat release circuit includes an auxiliary steam shut-off valve (6), a condensate vaporizer (8), a steam booster (9), a molten salt cooler (10), a deaerator heating steam shut-off valve (11), a deaerator (13), a feedwater pump (12), and a steam recirculation shut-off valve (7) on the boiler auxiliary steam pipeline. The auxiliary steam shut-off valve (6), the condensate vaporizer (8), the steam booster (9), and the molten salt cooler (10) are connected in sequence. The outlet side of the molten salt cooler (10) is connected to two branches. One branch is connected to the external steam supply pipe, and the other branch is connected to two branch lines. The first branch line is connected in sequence to the deaerator heating steam shut-off valve (11), the deaerator (13), the feedwater pump (12), and the condensate vaporizer (8). The second branch line is connected to the steam recirculation shut-off valve (7) and the condensate vaporizer (8). The molten salt circulation loop includes a cold salt tank (1), a hot salt tank (2), a cold molten salt pump (3), a hot molten salt pump (4), a molten salt heater (5), and a molten salt cooler (10), wherein the cold salt tank (1), the cold molten salt pump (3), the molten salt heater (5), the hot molten salt pump (4), the hot salt tank (2), the molten salt cooler (10), and the cold salt tank (1) are connected in sequence to form a loop.

2. The molten salt thermal storage system for thermal power plants based on binary molten salt according to claim 1, characterized in that: The cold salt tank (1) stores molten salt at 290°C, and the hot salt tank (2) stores molten salt at 550°C.

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