A system and operation method for phase change energy storage assisted boiler low load stable combustion

Abstract

The application provides a system and a running method for phase change energy storage auxiliary boiler low-load stable combustion, when the boiler is in medium and high load operation, the inlet of the phase change energy storage device is connected with the flue gas outlet of the economizer, and the outlet of the phase change energy storage device is connected with the front flue inlet of the air preheater; when the boiler is in low load operation, the inlet of the phase change energy storage device is connected with the normal temperature combustion supporting air source, and the outlet of the phase change energy storage device is connected with the burner of the boiler; the system uses the waste heat with moderate grade carried by the flue gas of the boiler in medium and high load operation to heat and store the phase change material, and releases the stored heat when the unit is switched into low load peak regulation working condition, and the released heat is used for heating the combustion supporting air into the boiler, so that the stable ignition and combustion of the pulverized coal are ensured, and the key technical problems such as poor combustion stability and low operation economy of the large capacity coal-fired thermal power unit in low load working condition under the current deep peak regulation demand of the power system are solved.

Description

Technical Field

[0001] This invention belongs to the field of thermal power generation engineering technology, specifically relating to a system and operation method for low-load stable combustion of a phase change energy storage auxiliary boiler. Background Technology

[0002] Guided by my country's strategic goal of "carbon peaking and carbon neutrality," the energy structure is undergoing a profound transformation, with the proportion of intermittent renewable energy generation, represented by wind and solar power, rapidly increasing. To ensure the safe and stable operation of the power system and maximize the absorption of new energy power, traditional coal-fired power units, as the backbone of the power grid, are shifting their role from providing baseload power to providing flexible regulation capabilities, and must undertake more frequent and in-depth peak-shaving tasks. This means that the units need to operate under low-load conditions below their design economic load range for extended periods.

[0003] However, conventional tangential or wall-mounted pulverized coal boilers face severe technical challenges when operating at low loads. When the boiler load decreases, the total fuel input into the furnace decreases, leading to a significant drop in the average furnace temperature. The low-temperature environment drastically worsens the ignition and burnout conditions of pulverized coal, primarily in the following ways: First, the heating rate of pulverized coal particles slows down, and the release of volatiles is delayed, resulting in a longer ignition distance and poorer flame stability. Second, the combustion reaction rate of coke particles is extremely sensitive to temperature; the burnout process becomes very slow at low temperatures, leading to a sharp increase in unburned carbon content in fly ash and slag, reducing boiler efficiency. Unstable combustion can easily cause large fluctuations in furnace pressure, and may even lead to boiler flameout accidents, posing a serious threat to unit safety.

[0004] To address these issues, the conventional approach commonly used in thermal power plants is to introduce auxiliary fuel oil systems (fuel injection for combustion). This involves injecting fuel oil into the burner area, utilizing its easy ignition and stable combustion properties to "support" the pulverized coal flame and maintain furnace temperature. However, this technology has a series of inherent drawbacks: fuel oil prices are much higher than coal prices, and long-term fuel injection for combustion will significantly erode power plant profit margins and reduce their competitiveness in the power ancillary services market; the combustion of fuel oil produces additional pollutants such as sulfur dioxide, nitrogen oxides (NOx), and soot, contradicting increasingly stringent national environmental emission standards and energy conservation and emission reduction policies; it increases the power plant's dependence on external fuel supplies and requires the configuration and maintenance of a complex fuel oil storage, transportation, and injection system.

[0005] To reduce reliance on fuel oil, the industry has explored various non-fuel or low-fuel low-load stable combustion technologies, but each has its limitations. Plasma ignition and stable combustion technology uses high-temperature plasma generated by an electric arc to directly ignite pulverized coal, enabling oil-free ignition and low-load stable combustion in boilers. However, its drawbacks are significant: short electrode material lifespan and high replacement costs lead to heavy maintenance workload and poor reliability; ignition energy is relatively limited, with poor adaptability to different coal types, especially difficult to ignite low-volatile coals such as anthracite; furthermore, the investment cost and operating power consumption (typically 150KW~300KW per burner) of plasma generators and their supporting power systems are high. Oxygen-enriched combustion technology enhances the combustion process by injecting pure oxygen into the combustion air to increase oxygen concentration. However, this technology faces obstacles when applied to low-load stable combustion in boilers. First, oxygen production requires the construction of expensive and energy-intensive air separation units (ASU), which reduces power plant efficiency by 7-9%. Secondly, the high-temperature, oxygen-rich environment causes severe high-temperature oxidation and corrosion to metal components such as boiler water-cooled walls and superheaters, shortening equipment lifespan. While micro-oil ignition technology is an improvement on oil-assisted combustion, aiming to reduce fuel consumption, it does not fundamentally achieve oil-free operation and still raises concerns about fuel costs and related emissions.

[0006] To address the aforementioned issues, it is necessary to propose a reasonably designed and effective system and operation method for low-load stable combustion of phase change energy storage auxiliary boilers. Summary of the Invention

[0007] The present invention aims to solve at least one of the technical problems existing in the prior art, and to provide a system and operation method for low-load stable combustion of phase change energy storage auxiliary boiler.

[0008] One aspect of the present invention provides a system for stabilizing combustion at low load in a phase change energy storage auxiliary boiler, comprising a boiler, an economizer, an air preheater, and a phase change energy storage device; wherein the boiler, the economizer, and the air preheater are connected in sequence; When the boiler is operating at medium to high load, the inlet of the phase change energy storage device is connected to the flue gas outlet of the economizer, and the outlet of the phase change energy storage device is connected to the inlet of the front flue of the air preheater. The high-temperature flue gas generated by the boiler flows from the economizer into the phase change energy storage device to release heat, causing the phase change material in the phase change energy storage device to melt and store a large amount of latent heat. The cooled flue gas returns to the front flue of the air preheater. When the boiler is running at low load, the inlet of the phase change energy storage device is connected to a normal temperature combustion air source, and the outlet of the phase change energy storage device is connected to the burner of the boiler. The normal temperature combustion air flows into the phase change energy storage device, absorbs the latent heat released during the solidification of the phase change material, and is heated to high temperature air, and then returns to the burner of the boiler for stable combustion.

[0009] Optionally, it also includes a blower, the inlet of which is connected to a normal temperature combustion air source, and the outlet of which is connected to the inlet of the phase change energy storage device.

[0010] Optionally, the phase change energy storage device is a shell-and-tube heat exchanger structure, wherein the shell side of the shell-and-tube heat exchanger structure is filled with phase change material, and the tube side of the shell-and-tube heat exchanger structure is used for the flow of flue gas or air.

[0011] Optionally, the phase change material is a medium-low temperature mixed molten salt. Optionally, when the boiler is operating at medium to high load, a portion of the high-temperature flue gas at 350°C to 450°C from the economizer outlet is extracted and introduced into the phase change energy storage device.

[0012] Optionally, when the boiler is operating at low load, the ambient temperature combustion air is heated to above 300°C in the phase change energy storage device.

[0013] Optionally, a flue gas duct is provided between the inlet of the phase change energy storage device and the flue gas outlet of the economizer, and a flue gas control valve is provided on the flue gas duct.

[0014] Optionally, an air duct is provided between the inlet of the phase change energy storage device and the ambient temperature combustion air source, and an air control valve is provided on the air duct.

[0015] Optionally, it also includes a controller, which is used to automatically switch the system to energy storage mode or energy release mode according to the load status of the unit.

[0016] Another aspect of the present invention provides an operation method for a phase change energy storage auxiliary boiler system for low-load stable combustion, employing the phase change energy storage auxiliary boiler system for low-load stable combustion described above; the operation method includes: When the boiler is running at medium to high load, the high-temperature flue gas generated by the boiler flows from the economizer into the phase change energy storage device to release heat, causing the phase change material in the phase change energy storage device to melt and store a large amount of latent heat. The cooled flue gas returns to the front flue of the air preheater. When the boiler is running at low load, ambient temperature combustion air flows into the phase change energy storage device. The ambient temperature combustion air absorbs the latent heat released during the solidification process of the phase change material and is heated to high temperature air, which is then returned to the burner of the boiler for stable combustion.

[0017] The present invention relates to a phase change energy storage auxiliary boiler low-load stable combustion system and operation method. The system stores the excess flue gas waste heat generated by the unit during medium- and high-load operation by setting up a phase change energy storage device. When the unit switches to low-load peak-shaving operation, the stored heat is precisely released to deeply heat the combustion air. Thus, without consuming any additional fossil fuels (such as oil), the boiler can achieve safe, stable, efficient and economical operation, significantly reducing the minimum technical output of the unit, expanding its peak-shaving range and profitability in the electricity market, and providing a new integrated approach for energy-saving and carbon-reduction retrofitting of traditional thermal power units. It realizes the transfer and utilization of heat energy at different points in time, ensuring the safe, stable and efficient operation of the boiler across the entire load range. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a low-load stable combustion system for a phase change energy storage auxiliary boiler according to an embodiment of the present invention; Figure 2 This is a schematic flowchart illustrating the operation method of a phase change energy storage auxiliary boiler system for stable combustion under low load, according to another embodiment of the present invention. Detailed Implementation

[0019] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] like Figure 1 As shown, one aspect of the present invention provides a system for low-load stable combustion of a phase change energy storage auxiliary boiler, comprising a boiler 1, an economizer 2, a phase change energy storage device 3, and an air preheater 4; the boiler 1, the economizer 2, and the air preheater 4 are connected in sequence.

[0021] When the boiler is operating at medium to high load, the energy storage mode is activated. The inlet of the phase change energy storage device 3 is connected to the flue gas outlet of the economizer 2, and the outlet of the phase change energy storage device 3 is connected to the inlet of the front flue of the air preheater 4. The high-temperature flue gas generated by the boiler 1 flows from the economizer 2 into the phase change energy storage device 3 to release heat, causing the phase change material in the phase change energy storage device 3 to melt and store a large amount of latent heat. The cooled flue gas returns to the front flue of the air preheater 4.

[0022] When the boiler is running at low load, the energy release mode is activated. The inlet of the phase change energy storage device 3 is connected to the ambient temperature combustion air source, and the outlet of the phase change energy storage device 3 is connected to the burner of the boiler 1. The ambient temperature combustion air flows into the phase change energy storage device 3 and absorbs the latent heat released during the solidification of the phase change material, and is heated to high temperature air. Then it returns to the burner of the boiler 1 for stable combustion.

[0023] The phase change energy storage system of this invention assists in the stable combustion of a boiler under low load. It utilizes the moderately graded waste heat carried by the flue gas during medium-to-high load operation to heat and store energy in the phase change material. When the unit switches to low-load peak-shaving conditions, the stored heat is released to heat the combustion air entering the furnace, thereby creating a localized high-temperature combustion zone and ensuring stable ignition and combustion of pulverized coal. Taking a conventional 600MW subcritical coal-fired unit as an example, this invention achieves the following effects: 1) It can effectively replace the fuel oil system under low load conditions, reducing the minimum stable operating load of the unit from 40%~50% of the rated load to 25%~30% of the rated load, significantly expanding the peak-shaving range of the unit.

[0024] 2) Significant economic benefits: Calculated based on the unit operating under low-load stable combustion conditions for 4 hours per day, fuel consumption during this period can be completely saved, equivalent to saving approximately 4-6 tons of standard coal per day, resulting in substantial fuel cost savings throughout the year.

[0025] 3) By providing high-temperature combustion air, the combustion organization under low load is improved, the combustion efficiency is increased, and the generation of incomplete combustion products such as CO and carbon content in fly ash is reduced, which has good energy-saving and emission reduction benefits.

[0026] For example, such as Figure 1 As shown, the system also includes a blower 5, the inlet of which is connected to a source of ambient temperature combustion air, and the outlet of which is connected to the inlet of the phase change energy storage device 3. Ambient temperature combustion air is delivered to the phase change energy storage device 3 through the blower 5.

[0027] For example, in this embodiment, the phase change energy storage device 3 can be a shell-and-tube heat exchanger structure, wherein the shell side of the shell-and-tube heat exchanger structure is filled with phase change material, and the tube side of the shell-and-tube heat exchanger structure is used for circulating flue gas or air.

[0028] In this embodiment, the variable energy storage device 3 can be a shell-and-tube heat exchanger structure. The phase change material filling the shell side is beneficial for stable heat storage / release, while the tube side allows flue gas or air to circulate, preventing material leakage and contamination. The shell-and-tube structure has a large heat transfer area and high structural strength, making it suitable for the high-temperature and high-pressure environment of power plant boilers. It also has a long service life and is easy to maintain.

[0029] For example, in this embodiment, the phase change material can be a medium-low temperature mixed molten salt. Mixed molten salts have advantages such as moderate phase change temperature (300℃~450℃), high latent heat value, good thermal stability, and controllable cost, making them particularly suitable for matching the waste heat temperature of boiler flue gas. It should be noted that in this embodiment, the specific material of the phase change material is not specifically limited and can be selected according to actual needs.

[0030] For example, when the boiler is operating at medium to high load, a portion of the high-temperature flue gas at 350°C to 450°C from the economizer outlet is introduced into the phase change energy storage device. This temperature range of flue gas, taken from the economizer outlet, represents medium-grade waste heat, has high recovery value, and does not affect the operation of the main boiler system.

[0031] For example, when the boiler is operating at low load, ambient temperature combustion air is heated to over 300°C within the phase change energy storage device. The temperature of the output air during the energy release phase is clearly defined, ensuring a significant improvement in the low-load combustion environment. High-temperature combustion air effectively increases the local temperature of the furnace, promoting pulverized coal ignition and burnout, achieving stable, oil-free combustion.

[0032] For example, a flue gas duct is provided between the inlet of the phase change energy storage device 3 and the flue gas outlet of the economizer 2, and a flue gas control valve is provided on the flue gas duct. The flue gas control valve can control the opening and closing of the flue gas duct, and realize the flexible switching and control of the flue gas flow path through the flue gas control valve, ensuring that the energy storage mode can be started and stopped as needed without affecting the flow rate and resistance of the main flue.

[0033] For example, an air duct is provided between the inlet of the phase change energy storage device 3 and the ambient temperature combustion air source, and an air control valve is provided on the air duct. The air control valve can control the opening and closing of the air duct to ensure that the appropriate air temperature and air volume can be provided according to the combustion requirements during the energy release stage.

[0034] For example, the system also includes a controller for automatically switching the system to energy storage mode or energy release mode based on the unit's load status. This enables intelligent switching of system operating modes, automatically controlling the energy storage and release processes based on boiler load signals, reducing manual intervention, and improving response speed and operational safety. It provides the hardware foundation for power plants to achieve "one-click peak shaving" and enhances the unit's automation level.

[0035] like Figure 2 As shown, another aspect of the present invention provides an operation method S100 for a phase change energy storage auxiliary boiler system for low-load stable combustion, which employs the phase change energy storage auxiliary boiler system for low-load stable combustion described above; wherein, the specific structural features of the phase change energy storage auxiliary boiler system for low-load stable combustion have been described in detail above, and will not be repeated here.

[0036] The operation method S100 of the phase change energy storage auxiliary boiler low-load stable combustion system may specifically include: S110. When the boiler is operating at medium to high load, the high-temperature flue gas generated by the boiler flows from the economizer into the phase change energy storage device to release heat, causing the phase change material in the phase change energy storage device to melt and store a large amount of latent heat. The cooled flue gas returns to the front flue of the air preheater.

[0037] Specifically, when the boiler is running at medium to high load, the system is in energy storage mode. It extracts a portion of the high-temperature flue gas at about 350°C to 450°C from the outlet of the economizer 2 and introduces it into the phase change energy storage device 3. This causes the phase change material (such as medium and low temperature mixed molten salt) inside the device to melt and store a large amount of latent heat. After the high-temperature flue gas releases heat, it becomes low-temperature flue gas and returns to the front flue of the air preheater 4.

[0038] S120. When the boiler is running at low load, ambient temperature combustion air flows into the phase change energy storage device. The ambient temperature combustion air absorbs the latent heat released during the solidification of the phase change material and is heated to high temperature air, which then returns to the burner of the boiler for stable combustion.

[0039] Specifically, when the boiler is running at low load, the system switches to energy release mode and introduces part of the ambient temperature combustion air (primary air or secondary air) from the outlet of the blower 5 into the phase change energy storage device 3. The ambient temperature combustion air absorbs the latent heat released during the solidification of the phase change material and is deeply heated to above 300°C, forming high temperature combustion air which is sent into the burner of the boiler 1, thereby achieving stable combustion under low load.

[0040] The operation method of the phase change energy storage assisted boiler low-load stable combustion system of the present invention combines advanced thermal storage technology with the optimization of traditional boiler combustion process, which can effectively improve the operational flexibility and economy of the unit, and provide a brand-new integrated approach for energy-saving and carbon-reduction retrofitting of traditional thermal power units. Taking a conventional 600MW subcritical coal-fired unit as an example for analysis, the present invention can achieve the following effects: 1) It can effectively replace the fuel oil system under low load conditions, reducing the minimum stable operating load of the unit from 40%~50% of the rated load to 25%~30% of the rated load, significantly expanding the peak-shaving range of the unit.

[0041] 2) Significant economic benefits: Calculated based on the unit operating under low-load stable combustion conditions for 4 hours per day, fuel consumption during this period can be completely saved, equivalent to saving approximately 4-6 tons of standard coal per day, resulting in substantial fuel cost savings throughout the year.

[0042] 3) By providing high-temperature combustion air, the combustion organization under low load is improved, the combustion efficiency is increased, and the generation of incomplete combustion products such as CO and carbon content in fly ash is reduced, which has good energy-saving and emission reduction benefits.

[0043] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A system for low-load stable combustion of a phase change energy storage auxiliary boiler, characterized in that, It includes a boiler, an economizer, an air preheater, and a phase change energy storage device; the boiler, the economizer, and the air preheater are connected in sequence. When the boiler is operating at medium to high load, the inlet of the phase change energy storage device is connected to the flue gas outlet of the economizer, and the outlet of the phase change energy storage device is connected to the inlet of the front flue of the air preheater. The high-temperature flue gas generated by the boiler flows from the economizer into the phase change energy storage device to release heat, causing the phase change material in the phase change energy storage device to melt and store a large amount of latent heat. The cooled flue gas returns to the front flue of the air preheater. When the boiler is running at low load, the inlet of the phase change energy storage device is connected to a normal temperature combustion air source, and the outlet of the phase change energy storage device is connected to the burner of the boiler. The normal temperature combustion air flows into the phase change energy storage device, absorbs the latent heat released during the solidification of the phase change material, and is heated to high temperature air, and then returns to the burner of the boiler for stable combustion.

2. The system according to claim 1, characterized in that, It also includes a blower, the inlet of which is connected to a normal temperature combustion air source, and the outlet of which is connected to the inlet of the phase change energy storage device.

3. The system according to claim 1, characterized in that, The phase change energy storage device is a shell-and-tube heat exchanger structure, wherein the shell side of the shell-and-tube heat exchanger structure is filled with phase change material, and the tube side of the shell-and-tube heat exchanger structure is used for the flow of flue gas or air.

4. The system according to any one of claims 1 to 3, characterized in that, The phase change material is a medium-low temperature mixed molten salt.

5. The system according to any one of claims 1 to 3, characterized in that, When the boiler is operating at medium to high load, a portion of the high-temperature flue gas at 350°C to 450°C from the economizer outlet is introduced into the phase change energy storage device.

6. The system according to any one of claims 1 to 3, characterized in that, When the boiler is operating at low load, the ambient temperature combustion air is heated to over 300°C in the phase change energy storage device.

7. The system according to any one of claims 1 to 3, characterized in that, A flue gas duct is provided between the inlet of the phase change energy storage device and the flue gas outlet of the economizer, and a flue gas control valve is provided on the flue gas duct.

8. The system according to any one of claims 1 to 3, characterized in that, An air duct is provided between the inlet of the phase change energy storage device and the ambient temperature combustion air source, and an air control valve is provided on the air duct.

9. The system according to any one of claims 1 to 3, characterized in that, It also includes a controller, which is used to automatically switch the system to energy storage mode or energy release mode according to the load status of the unit.

10. A method for operating a phase change energy storage auxiliary boiler system for stable combustion under low load, characterized in that, The system employs the phase change energy storage auxiliary boiler low-load stable combustion system according to any one of claims 1 to 9; the operation method includes: When the boiler is running at medium to high load, the high-temperature flue gas generated by the boiler flows from the economizer into the phase change energy storage device to release heat, causing the phase change material in the phase change energy storage device to melt and store a large amount of latent heat. The cooled flue gas returns to the front flue of the air preheater. When the boiler is running at low load, ambient temperature combustion air flows into the phase change energy storage device. The ambient temperature combustion air absorbs the latent heat released during the solidification process of the phase change material and is heated to high temperature air, which is then returned to the burner of the boiler for stable combustion.

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