A circulating fluidized bed boiler of pure combustion high-alkali fuel

Abstract

The utility model relates to fluidized bed technical field, concretely relates to a kind of circulating fluidized bed boiler of pure combustion high-alkali fuel, including cyclone separator, cyclone separator is communicated hearth by separator inlet flue, and is communicated cooling flue by separator outlet flue, the lower part of cyclone separator still sends back hearth by returner to separated material;The cooling flue is communicated tail flue, and cooling flue and tail flue are provided with several heat utilization devices and flue gas purification device.By improving the structure of boiler, long-period stable operation of boiler under pure combustion high-alkali fuel can be realized under different boiler parameters, the continuous operation time of boiler is greatly improved, and the contamination problem of high-alkali coal is effectively solved.

Description

Technical Field

[0001] This utility model relates to the field of fluidized bed technology, specifically to a circulating fluidized bed boiler that burns only high-alkali fuel. Background Technology

[0002] High-alkali coal refers to coal with a high content of alkali metals (such as potassium and sodium). During the combustion or gasification of high-alkali coal, alkali metals can cause serious contamination problems, which manifest in the following ways:

[0003] 1. Alkali metal volatilization and condensation

[0004] During high-temperature combustion, alkali metals such as sodium and potassium in coal are released in gaseous form. When these metals flow with the flue gas to cooler heating surfaces (such as water-cooled walls and superheaters), they condense and form a viscous molten layer of sulfates (such as Na₂SO₄ and K₂SO₄) or chlorides (such as NaCl). This molten layer traps fly ash particles, forming hard slag, which leads to decreased heat transfer efficiency and pipe wall corrosion.

[0005] 2. Formation of low-melting-point ash slag

[0006] The high ratio (alkali-acid ratio) of alkaline oxides (Na2O, K2O, CaO, etc.) to acidic oxides (SiO2, Al2O3) in high-alkali coal ash leads to a significant decrease in the ash melting point. For example, the actual ash melting point of Zhundong coal may be as low as 750℃, far lower than the conventional measured value (about 1170℃), which exacerbates the tendency to slagging.

[0007] 3. Impact on boiler operation

[0008] Deterioration of heat transfer: The fouling layer hinders heat transfer, resulting in an increase in flue gas temperature (for every 1°C increase, coal consumption increases by approximately 0.059 g / (kW·h)).

[0009] Corrosion risk: The deposition of sulfates and chlorides can cause high-temperature corrosion of metal pipes. For example, after a power plant boiler was co-fired with high-alkali coal, the average thinning rate of the water-cooled wall tubes reached 0.5 mm / month.

[0010] Operational safety: Severe contamination may force the boiler to shut down for cleaning. In some cases, the boiler can only operate at full load for one week before needing maintenance.

[0011] 4. Challenges of Special Coal Types

[0012] Taking Xinjiang Zhundong coal as an example, it has a high alkali metal content and low ash content. When burning, the ash and slag are highly fluid and easily form large areas of slag in the furnace and return feeder, even blocking the flue gas passage.

[0013] The contamination problem of high-alkali coal is mainly caused by its high alkali metal content, involving complex physicochemical reactions. A multi-pronged approach is needed to solve it, addressing issues from coal pretreatment and combustion process optimization to equipment modification. The main technical directions include:

[0014] 1. Pre-combustion treatment

[0015] Dealkali removal technology: CO2-enhanced water washing or acid leaching is used to remove water-soluble and some organic alkali metals from coal, with a maximum removal rate of 97% for sodium.

[0016] Upgrading and modification: Hydrothermal treatment reduces the alkali metal content in coal, but the problem of waste liquid treatment needs to be solved.

[0017] 2. Combustion optimization

[0018] Blending with low-alkali coal: Mixing high-alkali coal with coal containing ≥20% Al2O3 in ash to reduce the alkali-acid ratio.

[0019] Adjust operating parameters: control furnace temperature, optimize air distribution, and reduce alkali metal volatilization.

[0020] 3. Equipment Modification

[0021] Boiler design: Use low wall temperature heat exchange surfaces, L-type radiant boilers or circulating fluidized bed gasification technology to reduce alkali metal condensation in high-temperature areas.

[0022] Soot blowing and corrosion protection: Increase the frequency of soot blowing and use corrosion-resistant coating materials.

[0023] Patent CN217004446U proposes a circulating fluidized bed boiler to prevent slagging of high-alkali coal. It utilizes a flue partition wall between the empty flue and the rear flue, and arranges slagging tubes in the rear flue to cool the flue gas to below 600℃, solidifying some alkali metal and alkaline earth metal compounds in the flue gas into solids. This solves the problem of slagging and fouling of the heating surfaces caused by the combustion of high-alkali coal. However, the empty flue is easily fouled by alkali metals during operation. If the fouling layer on the empty flue cannot be removed in time, its heat exchange capacity will be greatly reduced, resulting in a higher inlet flue gas temperature at the tail heating surface. This fails to completely solve the problem of fouling of the heating surface and affects the continuous operation of the boiler.

[0024] Patent CN212226992U proposes a circulating fluidized bed boiler structure with a cooling chamber for burning mixed fuels. A cooling chamber wall is set after the steam-cooled separator to reduce the flue gas temperature. Low-temperature superheaters and medium-temperature superheaters are arranged inside the cooling chamber wall, which can reduce the problem of alkali metal fouling and corrosion, but it fails to completely solve the fouling problem caused by high-alkali fuels.

[0025] It is evident that current technologies have not adequately addressed the contamination problem of high-alkali coal in practical applications. Therefore, it is necessary to propose more reasonable technical solutions to resolve the existing technical issues. Utility Model Content

[0026] To overcome at least one of the aforementioned defects, this utility model proposes a circulating fluidized bed boiler that burns only high-alkali fuel. By improving the boiler structure, it can achieve long-term stable operation of the boiler under different boiler parameters, significantly increasing the continuous operation time of the boiler.

[0027] To achieve the above objectives, the fluidized bed boiler disclosed in this utility model can adopt the following technical solution:

[0028] A circulating fluidized bed boiler that burns only high-alkali fuel includes a cyclone separator. The cyclone separator is connected to the furnace through a separator inlet flue and to a cooling flue through a separator outlet flue. The lower part of the cyclone separator also sends the separated material back to the furnace through a return feeder. The cooling flue is connected to a tail flue, and the cooling flue and tail flue are equipped with several heat utilization devices and flue gas purification devices.

[0029] When using the above-mentioned fluidized bed boiler, the combustion uniformity of high-alkali coal can be improved, while reducing the fouling problem of high-alkali coal.

[0030] Furthermore, the lower part of the furnace is provided with an air chamber, which is connected to an ignition channel. The ignition channel obtains air from the outside as the ignition combustion gas.

[0031] Furthermore, there are various methods for heating the combustion-supporting gas. Here, we optimize and propose one feasible option: an air preheater is installed in the tail flue. The combustion-supporting gas is heated by the air preheater and then transported to the ignition channel. With this scheme, the air is first heated by a heater before being sent to the air preheater in the tail flue of the boiler, which increases the inlet air temperature of the air preheater and reduces the risk of low-temperature corrosion in the cold air inlet section of the air preheater.

[0032] Furthermore, to ensure uniform temperature within the furnace, a heat exchange structure is installed inside the furnace. Various options can be used for this heat exchange structure; here, we optimize and propose one feasible option: an in-furnace screen-type heating surface is installed within the furnace. With this option, the entire furnace adopts a full-film water-cooled structure, with water-cooled partitions, water-cooled screens, and high-temperature superheater heating surfaces arranged within the furnace. This ensures uniform flue gas temperature distribution, controls the furnace bed temperature below 860℃, and reduces alkali metal precipitation at the source.

[0033] Furthermore, to reduce ash accumulation at the separator inlet flue, an optimization is proposed, and one feasible option is suggested: a purging structure is installed on the separator inlet flue to deliver purging air into the separator inlet flue. When using the above scheme, the purging air for the separator inlet flue can be compressed air, high-pressure fluidizing air, or primary air.

[0034] Furthermore, the structure of the return feeder is optimized to reduce the risk of coking and ash blockage. One feasible optimization option is proposed: a fluidizing air system is installed at the return feeder to supply both fluidizing and coking air. With this solution, compared to conventional return feeders with fluidizing air at the bottom, decoking air is provided in both the rising and falling sections of the return feeder, reducing the risk of coking and ash blockage in the main circulation loop.

[0035] Furthermore, the structure of the cooling flue can be constructed in various forms, and is not limited to a single one. Here, we optimize and propose one feasible option: the cooling flue includes a downward cooling flue and an upward cooling flue; the flue gas purification device includes a hydraulic cleaning device installed in the downward cooling flue; and the heat utilization device includes a water-cooled heating surface, a low-temperature reheater, and a low-temperature superheater installed in the upward cooling flue. When adopting the above scheme, the downward and upward cooling flues are arranged side-by-side and connected at the bottom. The downward cooling flue may contain a screen-type heating surface, or no heating surface, as needed, to quickly reduce the flue gas temperature, adsorb some alkali metals in the high-temperature flue gas onto the flue, and then the hydraulic cleaning device promptly cleans the fouling layer, maintaining the flue's cleanliness and heat exchange capacity, providing a foundation for reducing flue gas temperature and resolving fouling on the tail heating surface.

[0036] Furthermore, ash accumulates in the cooling flue, requiring cleaning. This can be achieved using various methods; here, we optimize and propose one feasible option: Ash hoppers are installed at the bottom of both the downward and upward cooling flues, connected to an ash conveying device. The ash hoppers are offset from directly below the downward cooling flue to prevent contaminants from the downward cooling flue from entering the ash hoppers and the ash conveying device. With this solution, a cooling-type ash conveying device is installed at the ash hopper outlet to cool and discharge the settled ash and fallen contaminants. The ash hopper has an asymmetrical structure, with its interface located below the upward cooling flue to prevent contaminants falling from the downward cooling flue from directly impacting the ash conveying device.

[0037] Furthermore, the design schemes for the heat utilization structure and flue gas purification structure within the cooling flue can be varied, corresponding to multiple arrangements of the cooling flue: the number of upward cooling flues is greater than or equal to two, with at least one upward cooling flue equipped with a water-cooled heating surface and a superheater. A flue gas baffle is installed at the outlet of the flue containing the reheater to regulate the reheat steam temperature. When adopting the above scheme, the upward cooling flue can be arranged as a single flue or a double flue (used when a reheat system is present). When using a single flue, a water-cooled heating surface and a low-temperature superheater are installed within the flue. When using a double flue, the two flues are separated by a partition wall, with water-cooled heating surfaces and low-temperature superheaters arranged on both sides, and water-cooled heating surfaces and low-temperature reheaters arranged on the other side. A flue gas baffle is installed at the outlet of the cooling flue to regulate the reheat steam and improve economic efficiency. The flue gas, which has been cooled and had its alkali metal content reduced by the downward cooling flue, turns 180° to the upward cooling flue. The bottom of the upward cooling flue is equipped with a water-cooled heating surface with a large pitch and low wall temperature, which further reduces the flue gas temperature to below 650°C, completely solving the problem of fouling on the tail convection heating surface.

[0038] Furthermore, the lower part of the furnace is provided with several feeding structures, which are used to uniformly feed materials into the furnace so that the high-alkali fuel can be burned evenly.

[0039] Compared with the prior art, some of the beneficial effects of the technical solution disclosed in this utility model include:

[0040] Lower combustion temperatures and uniform bed temperatures are achieved through multi-point uniform feeding on the front wall and the arrangement of heating surfaces inside the furnace, reducing the precipitation of alkali metals at the source. Purge air and descaling air are respectively installed in the separator inlet flue and the return feeder to reduce the risk of fouling in the main circulation loop. A cooling flue is located downstream of the separator to rapidly cool and adsorb some alkali metals in the flue gas. The flue gas temperature is further reduced by water-cooled heating surfaces, ensuring that the inlet flue gas temperature of the tail heating surface avoids the fouling-sensitive temperature range. Hydraulic ash removal and ash conveying devices promptly clean the fouling layer in the cooling flue, resolving fouling issues online. The boiler is equipped with a reheat system, which divides the upward cooling flue into left and right flues. The amount of flue gas flowing through the two flues can be adjusted by regulating the opening of the flue outlet damper. A warm air heater is installed at the air preheater inlet to reduce the risk of low-temperature corrosion of the air preheater. The entire system is not limited by boiler capacity and parameters and is suitable for various high-alkali fuel requirements. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of the embodiments of this utility model, the drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this utility model and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0042] Fig. 1 This is a front view of a circulating fluidized bed boiler that burns only high-alkali fuel.

[0043] Fig. 2 A top view of a circulating fluidized bed boiler that burns only high-alkali fuel—without a reheat system.

[0044] Fig. 3 A top view of a circulating fluidized bed boiler that burns only high-alkali fuels—with a reheat system.

[0045] In the above attached figures, the meanings of each label are as follows:

[0046] 1. Ignition air duct; 2. Air chamber; 3. Furnace; 4. In-furnace screen-type heating surface; 5. Separator inlet flue; 6. Cyclone separator; 7. Return feeder; 8. Separator outlet flue; 9. Downward cooling flue; 10. Ash conveying device; 11. Upward cooling flue; 12. Water-cooled heating surface; 13. Low-temperature stage superheater; 14. Low-temperature stage reheater; 15. Tail flue; 16. High-temperature economizer; 17. SCR denitrification device; 18. Low-temperature economizer; 19. Air preheater; 20. Hydraulic ash removal device; 21. Air heater; 22. Return feeder fluidizing air system; 23. Flue gas damper. Detailed Implementation

[0047] The following description, in conjunction with the accompanying drawings and specific embodiments, further illustrates this embodiment.

[0048] To address the shortcomings of existing fluidized bed boiler structures, the following embodiments are optimized and overcome the defects in the prior art.

[0049] Example

[0050] like Figs. 1-3 As shown, this embodiment provides a circulating fluidized bed boiler that burns pure high-alkali fuel, including a cyclone separator 6. The cyclone separator 6 is connected to the furnace 3 through the separator inlet flue 5 and to the cooling flue through the separator outlet flue 8. The lower part of the cyclone separator 6 also sends the separated material back to the furnace 3 through the return feeder 7. The cooling flue is connected to the tail flue 15, and the cooling flue and the tail flue 15 are equipped with several heat utilization devices and flue gas purification devices.

[0051] Preferably, the heat utilization device installed in the tail flue 15 includes a high-temperature economizer 16, an SCR denitrification device 17, and a low-temperature economizer 18.

[0052] When using the above-mentioned fluidized bed boiler, the combustion uniformity of high-alkali coal can be improved, while reducing the fouling problem of high-alkali coal.

[0053] The lower part of the furnace chamber 3 is provided with an air chamber 2, which is connected to an ignition channel. The ignition channel obtains air from the outside as the ignition combustion gas.

[0054] There are various methods for heating combustion gases. This embodiment optimizes and adopts one feasible option: an air preheater 19 is installed in the tail flue 15. The combustion gas is heated by the air preheater 19 and then transported to the ignition channel. With this scheme, the air is first heated by the air heater 21 and then sent to the air preheater 19 in the tail flue 15 of the boiler, which increases the inlet air temperature of the air preheater 19 and reduces the risk of low-temperature corrosion in the cold air inlet section of the air preheater 19.

[0055] To ensure uniform temperature within the furnace chamber 3, a heat exchange structure is installed inside. Various heat exchange structures can be adopted; this embodiment optimizes and uses one feasible option: an in-furnace screen-type heating surface 4 is installed within the furnace chamber 3. With this design, the entire furnace chamber 3 employs a full-film water-cooled structure, with water-cooled partitions, water-cooled screens, and high-temperature superheater heating surfaces arranged within it. This ensures uniform flue gas temperature distribution and controls the furnace bed temperature to below 860℃, reducing alkali metal precipitation at its source.

[0056] To reduce ash accumulation at the separator inlet flue 5, this embodiment optimizes the process by employing one feasible option: a purging structure is installed on the separator inlet flue 5 to deliver purging air into the separator inlet flue 5. When using the above solution, the purging air for the separator inlet flue 5 can be compressed air, high-pressure fluidizing air, or primary air.

[0057] To optimize the structure of the return feeder 7 and reduce the risk of coking and ash blockage, this embodiment optimizes the structure by employing one feasible option: a fluidizing air system is installed at the return feeder 7 to supply fluidizing air and coking air. With this solution, compared to a conventional return feeder 7 which has fluidizing air at the bottom, the return feeder 7 also has decoking air in both the rising and falling sections, reducing the risk of coking and ash blockage in the main circulation loop.

[0058] The structure of the cooling flue can be constructed in various forms, and is not limited to a single one. This embodiment optimizes and adopts one feasible option: the cooling flue includes a downflow cooling flue 9 and an upflow cooling flue 11; the flue gas purification device includes a hydraulic cleaning device 20 installed in the downflow cooling flue 9; and the heat utilization device includes a water-cooled heating surface 12, a low-temperature reheater 13, and a low-temperature superheater 14 installed in the upflow cooling flue 11. When adopting the above scheme, the downflow cooling flue 9 and the upflow cooling flue 11 are arranged side-by-side and connected at the bottom. The downflow cooling flue 9 can be equipped with a screen-type heating surface, or no heating surface, as needed, to quickly reduce the flue gas temperature, adsorb some alkali metals in the high-temperature flue gas onto the flue, and then the hydraulic cleaning device 20 promptly cleans the fouling layer, maintaining the flue's cleanliness and heat exchange capacity, providing a foundation for reducing flue gas temperature and resolving fouling on the tail heating surface.

[0059] Preferably, the heat-receiving structure 4 in the cooling flue includes a water-cooled heat-receiving surface 12.

[0060] Ash accumulates in the cooling flue and needs to be cleaned. This can be achieved using various methods, and this embodiment optimizes and adopts one feasible option: Ash hoppers are installed at the bottom of the downflow cooling flue 9 and the upflow cooling flue 11, and the ash hoppers are connected to the ash conveying device 10; the ash hoppers are offset from directly below the downflow cooling flue 9 to prevent contaminants from the downflow cooling flue 9 from entering the ash hopper and the ash conveying device 10. With this method, a cooling-type ash conveying device 10 is installed at the outlet of the ash hopper to cool and discharge the settled ash and fallen contaminants in the ash hopper; the ash hopper has an asymmetrical structure, with its interface located below the upflow cooling flue 11 to prevent contaminants falling from the downflow cooling flue 9 from directly impacting the ash conveying device 10.

[0061] The design schemes for the heat utilization structure and flue gas purification structure within the cooling flue can be varied, and the corresponding arrangement of the cooling flue can also be varied: the number of upward cooling flues 11 is greater than or equal to two, and at least one upward cooling flue 11 is equipped with a water-cooled heating surface 12 and a superheater. A flue gas baffle 23 is installed at the flue outlet where the reheater is located, and the flue gas baffle 23 is used to regulate the reheat steam temperature. When adopting the above scheme, the upward cooling flue 11 can be arranged as a single flue or a double flue (used when there is a reheat system) as needed. When a single flue is used, a water-cooled heating surface 12 and a low-temperature stage superheater 13 are installed in the flue. When a double flue is used, the two flues are separated by a partition wall, and the two flues are respectively equipped with a water-cooled heating surface 12 and a low-temperature stage superheater 13 and a water-cooled heating surface 12 and a low-temperature stage reheater 14. A flue gas baffle 23 is arranged at the cooling flue outlet to regulate the reheat steam and improve economic efficiency. The flue gas, cooled and with reduced alkali metal content by the downward cooling flue 9, turns 180° to the upward cooling flue 11. The bottom of the upward cooling flue 11 is equipped with a large-pitch, low-wall-temperature water-cooled heating surface 12, which further reduces the flue gas temperature to below 650°C, completely solving the problem of fouling on the tail convection heating surface.

[0062] The lower part of the furnace 3 is provided with several feeding structures, which are used to uniformly feed materials into the furnace 3 so that the high-alkali fuel can be burned evenly.

[0063] The above are the embodiments listed in this example. However, this example is not limited to the optional embodiments described above. Those skilled in the art can arbitrarily combine the above methods to obtain other various embodiments. Anyone can derive other various forms of embodiments under the guidance of this example. The above specific embodiments should not be construed as limiting the scope of protection of this example. The scope of protection of this example should be defined in the claims.

Claims

1. A circulating fluidized bed boiler that burns only high-alkali fuel, characterized in that: The system includes a cyclone separator (6), which is connected to the furnace (3) through the separator inlet flue (5) and to the cooling flue through the separator outlet flue (8). The lower part of the cyclone separator (6) also sends the separated material back to the furnace (3) through a return feeder (7). The cooling flue is connected to the tail flue (15), and the cooling flue and the tail flue (15) are equipped with several heat utilization devices and flue gas purification devices. The separator inlet flue (5) is provided with a purging structure, which is used to deliver purging air into the separator inlet flue (5); A fluidizing air system is provided at the return feeder (7) to deliver fluidizing air and coking air to the return feeder (7); The cooling flue includes a downflow cooling flue (9) and an upflow cooling flue (11). The flue gas purification device includes a hydraulic cleaning device (20) installed in the downflow cooling flue (9). The heat utilization device includes a water-cooled heating surface (12), a low-temperature reheater (13), and a low-temperature superheater (14) installed in the upflow cooling flue (11). The bottom of the downflow cooling flue (9) and the upflow cooling flue (11) are provided with ash hoppers, which are connected to the ash conveying device (10). The ash hoppers are offset from the bottom of the downflow cooling flue (9) to prevent contaminants from the downflow cooling flue (9) from entering the ash hoppers and the ash conveying device (10).

2. The circulating fluidized bed boiler for burning pure high-alkali fuel according to claim 1, characterized in that: The furnace chamber (3) is provided with a wind chamber (2) at the bottom. The wind chamber (2) is connected to the ignition channel, which obtains air from the outside as the ignition combustion gas.

3. The circulating fluidized bed boiler for burning pure high-alkali fuel according to claim 2, characterized in that: An air preheater (19) is installed in the tail flue (15). The combustion gas is heated by the air preheater (19) and then transported to the ignition channel.

4. The circulating fluidized bed boiler for burning pure high-alkali fuel according to claim 1, characterized in that: The furnace chamber (3) is equipped with an in-furnace screen-type heating surface (4).

5. The circulating fluidized bed boiler for burning pure high-alkali fuel according to claim 1, characterized in that: The number of upward cooling flues (11) is greater than or equal to two, and at least one upward cooling flue (11) is provided with a water-cooled heating surface (12) and a superheater. A flue gas baffle (23) is provided at the outlet of the flue where the reheater is provided. The flue gas baffle (23) is used to regulate the reheat steam.

6. The circulating fluidized bed boiler for burning pure high-alkali fuel according to claim 1, characterized in that: The lower part of the furnace (3) is provided with several feeding structures, which are used to feed materials evenly into the furnace (3).

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