A circulating fluidized bed boiler ash discharge waste heat recovery system and its operation method

Abstract

This invention discloses a circulating fluidized bed boiler ash discharge waste heat recovery system and its operation method, belonging to the technical field of boiler ash discharge waste heat recovery. The system includes an integrated vertical staged reaction tower, an ash discharge inlet device, an ash discharge outlet, a fluidizing air distribution system, a methanol and steam supply system, a gas-solid separator, and a syngas distribution pipeline. The reaction tower is divided into a top dilute phase zone, a middle suspended transition zone, and a bottom dense phase zone from top to bottom. After the high-temperature bottom ash is introduced, it is classified according to particle size under the action of rising airflow. Coarse ash accumulates in the bottom dense phase zone and undergoes catalytic reforming at 250-350℃ to generate syngas, while simultaneously cooling to below 250℃. The syngas is sent into the boiler furnace or stored externally, while fine ash enters the top dilute phase zone to preheat feedwater. The operation method includes S1 startup, S2 normal operation, and S3 shutdown steps. This invention achieves improved ash discharge waste heat quality and continuous cross-time storage.

Description

Technical Field

[0001] This invention relates to the field of waste heat recovery technology for ash discharge from circulating fluidized bed boilers, specifically to a waste heat recovery system for ash discharge from circulating fluidized bed boilers and its operation method. Background Technology

[0002] Circulating fluidized bed (CFB) boilers are widely used in the power and chemical industries due to their advantages such as wide fuel adaptability, high combustion efficiency, and good environmental performance. During operation, CFB boilers generate a large amount of high-temperature bottom ash, with ash discharge temperatures typically reaching 850–950℃. The physical heat loss from ash discharge can account for 1%–3% of the boiler's input heat. Currently, water-cooled drum ash coolers or fluidized bed ash coolers are commonly used in industry to cool the high-temperature ash discharge. Water-cooled drum ash coolers transfer the sensible heat of the ash discharge to the cooling water through indirect heat exchange, and the recovered heat is used for boiler feedwater preheating or heating. Fluidized bed ash coolers, on the other hand, directly exchange heat with the bottom ash through fluidizing air, and the heated air is returned to the furnace for combustion. In addition, some solutions propose using molten salt as an intermediate heat exchange medium to recover the high-temperature waste heat from the ash discharge, or constructing a thermal storage system based on the circulating ash from the CFB boiler.

[0003] However, the existing technologies still have the following shortcomings: First, physical heat exchange methods cannot avoid the degradation of waste heat during the transfer process, resulting in high-grade heat energy being downgraded to low-grade heat energy for utilization, and it is impossible to achieve the conversion of heat energy into chemical energy and its long-term storage. Second, existing staged cooling schemes mostly require external sorting equipment. In addition, to improve the heat exchange effect, some schemes first classify the bottom ash by particle size and then cool them separately. However, these staged cooling schemes mostly require external sorting equipment, and their particle size classification only serves the purpose of classification and treatment, lacking intrinsic coupling with the waste heat recovery process; the system is complex and the investment is high. Third, thermochemical energy storage systems usually require periodic switching between two stages of heat charging and heat dissipation, which is difficult to adapt to the continuous ash discharge operation characteristics of CFB boilers, and cannot simultaneously meet the dual requirements of ash discharge cooling and high-quality chemical energy output during continuous operation. Fourth, the existing system has a single function, focusing only on heat recovery, without synergistic effects with the boiler's own fuel processing and in-furnace denitrification processes. Furthermore, the naturally abundant active components such as CaO in the bottom ash are not utilized, requiring additional catalyst consumption. To address these issues, there is an urgent need to develop a waste heat recovery system and its operating method that integrates in-situ ash discharge classification, thermochemical conversion, and cascade heat exchange. Summary of the Invention

[0004] (a) Technical problems to be solved This invention provides a circulating fluidized bed boiler ash discharge waste heat recovery system and its operation method, aiming to overcome the shortcomings of existing physical heat exchange systems, such as grade decay, complex discrete systems, and inability to continuously store heat. The system includes an integrated vertical staged reaction tower, an ash discharge inlet device, an ash discharge outlet, a fluidizing air distribution system, a methanol and steam supply system, a gas-solid separator, and a syngas distribution pipeline. The reaction tower has a vertical cylindrical structure, internally divided into a top dilute phase zone, a middle suspended transition zone, and a bottom dense phase zone from top to bottom. A fine slag fluidized bed heat exchanger connected to the boiler feedwater pipeline is installed in the top dilute phase zone. High-temperature bottom ash is introduced from the top of the tower and, under the action of rising airflow, is classified in situ according to particle size. The coarse ash, heated by its own temperature, accumulates in the bottom dense phase zone and catalyzes methanol reforming at 250–350°C to generate hydrogen-containing syngas. Simultaneously, the coarse ash cools to below 250°C. The syngas, after gas-solid separation, is sent to the boiler furnace or external gas storage device through distribution pipelines. The fine ash enters the fine ash fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater. After cooling, the bottom ash is discharged from the bottom of the tower. The operation method includes S1 startup steps, S2 normal operation steps (including ash discharge rate control, fluidized bed airflow adjustment, and syngas distribution), and S3 shutdown steps. This invention achieves grade improvement and continuous cross-time storage of waste heat from ash discharge, while also possessing NOx synergistic emission reduction and system self-cleaning functions.

[0005] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: a circulating fluidized bed boiler ash discharge waste heat recovery system and its operation method, comprising an integrated vertical staged reaction tower, wherein the reaction tower is a vertical cylindrical structure, and its interior is divided into a top dilute phase zone, a middle suspension transition zone and a bottom dense phase zone from top to bottom, wherein the bottom dense phase zone is a thermochemical reforming reaction zone, and a fine slag fluidized bed heat exchanger is provided in the top dilute phase zone; A slag discharge inlet device is installed at the top of the reaction tower to introduce slag from the circulating fluidized bed boiler into the reaction tower. The slag outlet is located at the bottom of the reaction tower and is used to discharge the cooled bottom slag. A fluidized air distribution system is installed at the bottom of the reaction tower to introduce rising airflow into the reaction tower, so that bottom ash of different particle sizes is classified according to settling velocity in the reaction tower. Coarse ash accumulates in the bottom dense phase zone, medium ash is suspended in the middle suspension transition zone, and fine ash is carried by the airflow to the top dilute phase zone. A methanol and water vapor supply system is connected to the bottom dense phase zone and is used to inject a mixture of methanol and water vapor into the bottom dense phase zone. The mixture undergoes an endothermic reforming reaction under the sensible heat of the coarse slag to generate hydrogen-containing synthesis gas, while simultaneously cooling the coarse slag. The reaction temperature of the bottom dense phase zone is 250-350°C, and the temperature of the coarse slag drops below 250°C after the endothermic reforming reaction. A gas-solid separator is installed at the top outlet of the reaction tower to separate the syngas from the fine residue; Syngas distribution pipeline, which is connected to the gas outlet of the gas-solid separator, is used to send the separated syngas into the boiler furnace or to an external gas storage device. The fine slag fluidized bed heat exchanger is connected to the boiler feedwater pipeline and is used to preheat the boiler feedwater with the waste heat of the separated fine slag.

[0006] Preferably, the slag discharge inlet device includes a slag discharge pipe, a manual gate valve, an electric gate valve, and a distributor. The slag discharge pipe is connected to the slag discharge port of the circulating fluidized bed boiler, and the distributor is located at the outlet end of the slag discharge pipe to evenly disperse the bottom slag into the reaction tower.

[0007] Preferably, the fluidizing air distribution system includes a fluidizing air header, an air chamber, and an air distribution plate. The inlet end of the fluidizing air header is connected to the secondary air source or recirculated flue gas source of the circulating fluidized bed boiler, and its outlet end is connected to the air chamber via a recirculation fan. The air distribution plate is disposed above the air chamber, and air caps are distributed on the air distribution plate.

[0008] Preferably, the supply system includes a methanol storage tank, a demineralized water storage tank, a mixer, a preheater, and an injector. The injector is disposed on the side wall of the bottom dense phase zone and is evenly arranged along the circumference of the reaction tower. The installation height of the injector is above the air distribution plate.

[0009] Preferably, the gas-solid separator is a cyclone separator, and its solid outlet is connected to the inlet of the fine slag fluidized bed heat exchanger; the syngas distribution pipeline includes a reburning pipeline connected to the boiler furnace and an external delivery pipeline connected to the gas storage device, and regulating valves are respectively provided on the reburning pipeline and the external delivery pipeline.

[0010] Preferably, a method for recovering waste heat from ash discharge in a circulating fluidized bed boiler includes the following steps: S1: Start the fluidizing air distribution system to introduce fluidizing air into the reaction tower, open the slag discharge inlet device to allow the high-temperature bottom slag to enter the reaction tower, detect the temperature of the bottom dense phase zone, and when the temperature rises to above 250°C, start the methanol and water vapor supply system to spray a mixture of methanol and water vapor into the bottom dense phase zone. S2: During normal operation, the high-temperature bottom ash is classified in the reaction tower under the action of rising fluidizing air. The coarse ash accumulates in the bottom dense phase zone and catalyzes the methanol reforming reaction at 250-350℃ to generate hydrogen-containing syngas. At the same time, the coarse ash cools down to below 250℃. The syngas rises and passes through the middle suspension transition zone to exchange heat with the middle ash before entering the gas-solid separator. The separated syngas is sent to the boiler furnace or sent for storage through the distribution pipeline. The separated fine ash enters the fine ash fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater. The cooled bottom ash is discharged from the ash discharge outlet at the bottom of the reaction tower. S3: When shutting down, shut off the methanol and steam supply system, continue to supply fluidizing air until all residue in the tower is discharged and the tower temperature drops below 200°C, then shut off the slag discharge inlet device and the fluidizing air distribution system.

[0011] Preferably, in step S2, the injection rates of methanol and water vapor are adjusted according to the slag discharge rate to maintain the temperature of the bottom dense phase zone at 270-290°C. Specifically, the control system uses the opening signal of the electric gate valve of the slag discharge inlet device to represent the slag discharge rate, adjusts the opening of the regulating valve on the supply pipeline according to the preset correspondence between the injection rate and the slag discharge rate, and uses the temperature of the bottom dense phase zone as a feedback signal for correction.

[0012] Preferably, in step S2, the fluidizing air volume is adjusted within the range of 60% to 120% of the rated air volume.

[0013] Preferably, in step S2, the syngas is sent into the boiler furnace reburning zone via a reburning pipeline, or the syngas is sent into the gas storage device via an external transmission pipeline, or the syngas is sent into the boiler furnace reburning zone via a reburning pipeline and into the gas storage device simultaneously via an external transmission pipeline. The flow ratio of the reburning pipeline to the external transmission pipeline is adjusted according to the boiler load and the syngas production.

[0014] Preferably, the molar ratio of water vapor to methanol in the mixture is between 0.5:1 and 5:1; and the temperature of the bottom slag discharged does not exceed 150 degrees Celsius.

[0015] Compared with the prior art, the present invention provides a circulating fluidized bed boiler ash discharge waste heat recovery system and its operation method, which has the following beneficial effects: 1. This invention converts the high-temperature sensible heat of the slag discharge into the chemical energy of the syngas through methanol reforming reaction, thereby achieving the grade improvement and time-transfer storage of thermal energy into chemical energy.

[0016] 2. This invention utilizes rising airflow to classify bottom slag of different particle sizes in situ within a single tower. Coarse slag settles in the high-temperature zone at the bottom to drive an endothermic reforming reaction, while fine slag is carried to the low-temperature zone at the top to preheat the feedwater. The device structure naturally corresponds to the thermodynamic grade gradient, eliminating the need for external sorting equipment and making the system compact.

[0017] 3. In the continuous slag discharge process, the production of syngas and its return to the furnace for combustion or external delivery are carried out simultaneously without the need for switching between charging and releasing heat stages, thus achieving continuous steady-state operation with high operational flexibility and suitability for variable load conditions.

[0018] 4. This invention introduces syngas into the boiler furnace for re-combustion, utilizing its reducing properties to reduce NOx to N2, achieving in-furnace denitrification while recovering waste heat, thus providing environmental benefits. Simultaneously, CaO in the coarse slag acts as a natural catalyst and CO2 absorbent, and the continuous discharge of slag from the flowing coarse slag bed enables self-cleaning of the reaction chamber. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the method steps of the present invention. Detailed implementation method: The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all 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.

[0020] refer to Figure 1 This invention provides a circulating fluidized bed boiler ash discharge waste heat recovery system and its operation method, aiming to overcome the shortcomings of existing physical heat exchange systems, such as grade decay, complex discrete systems, and inability to continuously store heat. The system includes an integrated vertical staged reaction tower, an ash discharge inlet device, an ash discharge outlet, a fluidized air distribution system, a methanol and steam supply system, a gas-solid separator, and a syngas distribution pipeline. The reaction tower has a vertical cylindrical structure, internally divided into a top dilute phase zone, a middle suspension transition zone, and a bottom dense phase zone from top to bottom. A fine ash fluidized bed heat exchanger connected to the boiler feedwater pipeline is installed in the top dilute phase zone. High-temperature bottom ash is introduced from the top of the tower and, under the action of rising airflow, is in-situ graded according to particle size. Coarse ash accumulates in the bottom dense phase zone and catalyzes a methanol reforming reaction at 250–350°C to generate hydrogen-containing syngas, while simultaneously cooling the coarse ash to below 250°C. In this invention, the high-temperature bottom ash (850–900°C) is used to stabilize the average temperature of the reaction zone at 250–350°C by adjusting the accumulation amount of bottom ash in the dense phase zone and the fluidizing air velocity. This temperature perfectly matches the operating range of commercial methanol reforming catalysts, preventing catalyst sintering. Simultaneously, the bottom ash temperature drops to 350°C after preheating the reaction zone. CaO in the bottom ash still exhibits significant carbon dioxide adsorption capacity (carbonation reaction) at 250–350°C, effectively increasing H2 yield. Syngas, after gas-solid separation, is distributed to the boiler furnace or external gas storage device via distribution pipelines. Fine ash enters the fine ash fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater. After cooling, the bottom ash is discharged from the bottom of the tower. The operation method includes S1 startup step, S2 normal operation step (including ash discharge rate control, fluidizing air velocity adjustment, and syngas distribution), and S3 shutdown step. This invention achieves grade improvement and continuous cross-time storage of waste heat from ash discharge.

[0021] Example 1 Purpose of implementation: The purpose of this embodiment is to verify the operating effect of the system of the present invention under continuous and stable slag discharge conditions in a large circulating fluidized bed boiler unit (300MW class). The focus is on examining the system's efficiency in recovering waste heat from slag discharge, syngas production and quality, NOx synergistic emission reduction effect, and the improvement in the overall thermal efficiency of the system. It is also compared with a traditional water-cooled slag cooler to prove the effectiveness and superiority of the technical solution of the present invention.

[0022] Implementation System: This embodiment uses a 300MW supercritical circulating fluidized bed boiler unit as the application object. The boiler is designed to discharge ash at a rate of 15 tons / hour and a ash discharge temperature of 900℃. The integrated vertical staged reaction tower used has a height of 14m, an inner diameter of 2.8m, and a diameter-to-height ratio of 1:5.

[0023] Six injectors are evenly distributed circumferentially along the sidewall of the dense phase zone at the bottom of the reaction tower, with the injectors installed at a height of 1.0m above the air distribution plate. The supply system includes a methanol storage tank, a demineralized water storage tank, a mixer, a preheater, and the injectors.

[0024] The fluidized air distribution system's main fluidized air pipe connects to the boiler's recirculated flue gas source at its inlet and to the air chamber via a recirculation fan at its outlet. An air distributor plate is positioned above the air chamber, and air caps are distributed on the plate. The ash discharge inlet device is located at the top of the reaction tower and includes an ash discharge pipe, a manual gate valve, an electric gate valve, and a distributor. The ash discharge pipe connects to the boiler's ash discharge port. The ash discharge outlet is located at the bottom of the reaction tower. The gas-solid separator is a cyclone separator, located at the top outlet of the reaction tower. Its solid outlet connects to the inlet of the fine slag fluidized bed heat exchanger in the top dilute phase zone. The syngas distribution pipeline includes a recombustion pipeline connecting to the boiler furnace recombustion zone and an external supply pipeline connecting to the gas storage device. Both pipelines are equipped with regulating valves. The fine slag fluidized bed heat exchanger is connected to the boiler feedwater pipeline. Syngas is sent to the boiler furnace recombustion zone via the recombustion pipeline, where the reducing properties of H2 and CO in the syngas reduce NOx to N2. To ensure the effectiveness of reburning denitrification, the following key conditions need to be controlled: (1) The temperature range of the reburning zone is 850-1050℃. When the temperature of the furnace reburning zone is lower than 850℃, the amount of reburning syngas is reduced by adjusting the external syngas delivery ratio or the boiler operating conditions are adjusted to increase the temperature of the reburning zone; (2) The excess air coefficient of the reburning zone is controlled at 0.8-0.95 to form a suitable reducing atmosphere; (3) The residence time of syngas in the reburning zone is not less than 0.5 seconds; (4) The syngas injection point is set at the inlet of the boiler furnace reburning zone. The injection amount is controlled by the regulating valve on the reburning pipeline. The injection amount is adjusted in real time according to the boiler load and NOx emission concentration. Under the above conditions, H2 and CO in the syngas undergo selective non-catalytic reduction reaction with NOx, which can reduce NOx to N2. The expected NOx emission reduction is 25%-40%. When boiler load fluctuations cause the temperature or oxygen content in the reburning zone to deviate from the above range, the control system ensures that the reburning and denitrification conditions are maintained within the effective range by adjusting the syngas distribution ratio or adjusting the boiler operating parameters.

[0025] Implementation steps: S1: Start the fluidizing air distribution system, using recirculated flue gas as the fluidizing air source, and introduce fluidizing air into the reaction tower. The fluidizing air velocity is controlled at 0.9 m / s. Open the electric gate valve of the slag discharge inlet device to allow the 900℃ high-temperature bottom slag to be evenly dispersed into the reaction tower through the slag discharge pipe and the distributor. Detect the temperature of the bottom dense phase zone. After the temperature stabilizes above 250℃, the bottom slag temperature will drop to below 350℃. Start the methanol and steam supply system. Methanol and demineralized water are mixed in the mixer at a steam to methanol molar ratio of 2:1. After being preheated to 180℃ by the preheater, it is injected into the bottom dense phase zone through the injector at a pressure of 0.6 MPa. The methanol injection rate is controlled at 27 kg of methanol per ton of slag discharge.

[0026] S2: During normal operation, the high-temperature bottom slag is classified within the reaction tower under the action of rising fluidizing air. Coarse slag with a particle size greater than 1 mm accumulates in the bottom dense phase zone, forming a high-temperature moving bed at 280–290°C. Medium slag with a particle size of 0.3–1 mm is suspended in the middle suspension transition zone, and fine slag with a particle size less than 0.3 mm is carried by the airflow to the top dilute phase zone. In the bottom dense phase zone, the coarse slag undergoes catalytic methanol steam reforming at 280–290°C to generate hydrogen-containing syngas, while the coarse slag cools down to 295–305°C. The syngas rises through the middle suspension transition zone, exchanges heat with the medium slag, and then enters the gas-solid separator. The separated syngas is distributed through the distribution pipe. The feedwater is distributed as follows: 60% is sent to the boiler furnace via the reburning pipeline, and 40% is sent to the gas storage device via the external pipeline. The separated fine slag enters the fine slag fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater, and the cooled bottom slag is discharged from the slag discharge outlet at the bottom of the reaction tower. During normal operation, the control system uses the opening signal of the electric gate valve of the slag discharge inlet device to represent the slag discharge volume. It adjusts the opening of the regulating valve on the supply pipeline according to the preset correspondence between the injection volume and the slag discharge volume, and uses the temperature of the bottom dense phase zone as a feedback signal for correction, so that the temperature of the bottom dense phase zone is maintained at 270-290℃. The fluidizing air volume is controlled at 90% of the rated air volume.

[0027] S3: When shutting down, shut off the methanol and steam supply system, continue to supply fluidizing air until all residue in the tower is discharged and the tower temperature drops below 200°C, then shut off the slag discharge inlet device and the fluidizing air distribution system.

[0028] Implementation results: Actual measurements show that the syngas production in this embodiment is approximately 930 standard cubic meters per hour, with an H2 volume fraction of approximately 68%. Boiler NOx emissions are reduced by approximately 35%. The boiler feedwater temperature rises by approximately 18°C ​​after preheating in the top fine slag fluidized bed heat exchanger. The final slag discharge temperature is 115–120°C. The overall thermal efficiency of the system is approximately 12 percentage points higher than that of a traditional water-cooled drum slag cooler.

[0029] Example 2 Purpose of implementation: The purpose of this embodiment is to verify the applicability of the system of the present invention to medium-sized circulating fluidized bed boiler units (135MW class), to examine the system operation characteristics under the condition of complete syngas re-burning, and the conversion efficiency and slag cooling effect of the reforming reaction under different water-to-ethanol ratios, so as to provide a basis for the system design of boilers of different capacity levels.

[0030] Implementation System: This embodiment uses a 135MW circulating fluidized bed boiler unit as the application object. The boiler is designed to discharge ash at a rate of 8 tons / hour and a ash discharge temperature of 870℃. The integrated vertical staged reaction tower used has a height of 10m, an inner diameter of 2.0m, and a diameter-to-height ratio of 1:5.

[0031] Four injectors are evenly distributed circumferentially along the sidewall of the dense phase zone at the bottom of the reaction tower, with the injectors installed 0.8m above the air distribution plate. Methanol is used. The inlet end of the fluidizing air header of the fluidizing air distribution system is connected to the boiler's recirculated flue gas source. The remaining system configuration is the same as in Example 1.

[0032] Implementation steps: S1: Start the fluidizing air distribution system, using recirculated flue gas as the fluidizing air source, and introduce fluidizing air into the reaction tower. The fluidizing air velocity is controlled at 1.06 m / s. Open the electric gate valve of the slag discharge inlet device to allow the 870℃ high-temperature bottom slag to be evenly dispersed into the reaction tower through the slag discharge pipe and the distributor. Detect the temperature of the bottom dense phase zone. After the temperature stabilizes above 250℃, start the methanol and steam supply system. Methanol and demineralized water are mixed in the mixer at a steam to methanol molar ratio of 1.5:1. After being preheated to 170℃ by the preheater, it is injected into the bottom dense phase zone through the injector at a pressure of 0.5MPa. The methanol injection rate is controlled at 25 kg of methanol per ton of slag discharge.

[0033] S2: During normal operation, the high-temperature bottom slag is classified within the reaction tower under the action of rising fluidizing air. The coarse slag accumulates in the bottom dense phase zone, forming a high-temperature moving bed at 255–265°C. The medium slag is suspended in the middle suspension transition zone, and the fine slag is carried by the airflow to the top dilute phase zone. In the bottom dense phase zone, the coarse slag undergoes catalytic methanol steam reforming at 255–265°C to generate hydrogen-containing syngas, while the coarse slag cools down to 280–290°C. The syngas rises, passes through the middle suspension transition zone, exchanges heat with the medium slag, and then enters the gas-solid separator. All the separated syngas passes through the reburning pipeline. The ash is fed into the boiler furnace; the separated fine ash enters the fine ash fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater, and the cooled bottom ash is discharged from the ash discharge outlet at the bottom of the reaction tower; during normal operation, the control system uses the opening signal of the electric gate valve of the ash discharge inlet device to represent the ash discharge volume, adjusts the opening of the regulating valve on the supply pipeline according to the preset correspondence between the injection volume and the ash discharge volume, and uses the temperature of the bottom dense phase zone as a feedback signal for correction, so that the temperature of the bottom dense phase zone is maintained at 255-265℃; the fluidizing air volume is controlled at 100% of the rated air volume.

[0034] S3: When shutting down, shut off the methanol and steam supply system, continue to supply fluidizing air until all residue in the tower is discharged and the tower temperature drops below 200°C, then shut off the slag discharge inlet device and the fluidizing air distribution system.

[0035] Implementation results: Actual measurements showed that the syngas production in this embodiment was approximately 500 standard cubic meters per hour, with an H2 volume fraction of approximately 65%. Boiler NOx emissions were reduced by approximately 30%. The final ash discharge temperature was 120–125°C. Boiler efficiency improved by approximately 1.8 percentage points compared to before the modification.

[0036] Example 3 Purpose of implementation: The purpose of this embodiment is to verify the operational stability and control response capability of the system of the present invention under the condition of large fluctuation in slag discharge volume. The focus is on examining the effectiveness of the "slag discharge rate and injection volume following control" strategy, the ability to maintain the temperature of the bottom dense phase zone, and the actual effect of the dual-function regulation of fluidizing air volume, so as to prove that the system of the present invention has good adaptability to variable load conditions.

[0037] Implementation System: This embodiment uses a 50MW circulating fluidized bed boiler unit as the application object, with a boiler ash discharge rate of 4-12 tons / hour and an ash discharge temperature of 880℃. The integrated vertical staged reaction tower used has a height of 8m, an inner diameter of 1.6m, and a diameter-to-height ratio of 1:5.

[0038] Four injectors are evenly distributed circumferentially along the sidewall of the dense phase zone at the bottom of the reaction tower, with the injectors installed 0.6m above the air distribution plate. Methanol is used. The inlet end of the fluidizing air header of the fluidizing air distribution system is connected to the boiler's recirculated flue gas source. The remaining system configuration is the same as in Example 1.

[0039] Implementation steps: S1: Start the fluidizing air distribution system, using recirculated flue gas as the fluidizing air source, and introduce fluidizing air into the reaction tower. The fluidizing air velocity is controlled at 0.8 m / s. Open the electric gate valve of the slag discharge inlet device to allow the 880℃ high-temperature bottom slag to be evenly dispersed into the reaction tower through the slag discharge pipe and the distributor. Detect the temperature of the bottom dense phase zone. After the temperature stabilizes above 250℃, start the methanol and steam supply system. Methanol and demineralized water are mixed in the mixer at a steam to methanol molar ratio of 2.5:1. After being preheated to 190℃ by the preheater, it is injected into the bottom dense phase zone through the injector at a pressure of 0.7MPa. The initial methanol injection rate is controlled at 28 kg of methanol per ton of slag discharge.

[0040] S2: During normal operation, the high-temperature bottom ash is classified in the reaction tower under the action of rising fluidizing air. The coarse ash accumulates in the bottom dense phase zone to form a high-temperature moving bed of 265-275°C, the medium ash is suspended in the middle suspension transition zone, and the fine ash is carried by the airflow to the top dilute phase zone. The coarse ash undergoes a catalytic methanol steam reforming reaction at 265-275°C in the bottom dense phase zone to generate hydrogen-containing syngas, while the coarse ash cools down to 190-250°C. The syngas rises through the middle suspension transition zone and exchanges heat with the medium ash before entering the gas-solid separator. The separated syngas is distributed through the distribution pipeline. When the boiler load is higher than the set value, the syngas is sent into the boiler furnace through the reburning pipeline. When the syngas production exceeds the reburning consumption, the excess syngas is sent to the gas storage device through the external transmission pipeline. The flow ratio of the reburning pipeline and the external transmission pipeline is adjusted according to the boiler load and syngas production. The separated fine slag enters the fine slag fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater, and the cooled bottom slag is discharged from the slag discharge outlet at the bottom of the reaction tower. During normal operation, the control system uses the opening signal of the electric gate valve of the slag discharge inlet device to represent the slag discharge volume. It adjusts the opening of the regulating valve on the supply pipeline according to the preset correspondence between the injection volume and the slag discharge volume, and uses the temperature of the bottom dense phase zone as a feedback signal for correction, so that the temperature of the bottom dense phase zone is maintained at 270±5℃. The fluidizing air volume is adjusted within the range of 60% to 120% of the rated air volume. When it is necessary to increase the proportion of low-grade heat recovery, the fluidizing air volume is increased so that more fine slag is carried to the top dilute phase zone. When it is necessary to increase the proportion of chemical energy storage, the fluidizing air volume is decreased. In this embodiment, the fluidizing air volume is dynamically adjusted within the range of 70% to 110% of the rated air volume according to the slag discharge volume.

[0041] S3: When shutting down, shut off the methanol and steam supply system, continue to supply fluidizing air until all residue in the tower is discharged and the tower temperature drops below 200°C, then shut off the slag discharge inlet device and the fluidizing air distribution system.

[0042] Implementation results: Actual measurements showed that when the slag discharge rate increased by 50% from 8 tons / hour to 12 tons / hour, the control system adjusted the methanol injection rate from 200 kg / hour to 336 kg / hour within 3 minutes. The temperature fluctuation in the bottom dense phase zone did not exceed ±20℃, and the reforming reaction conversion rate remained stable above 85%. When the slag discharge rate increased by 5% from 8 tons / hour to 4 tons / hour, the control system reversed the adjustment to 140 kg / hour, and the temperature fluctuation in the bottom dense phase zone also did not exceed ±20℃. Within a wide range of slag discharge rates from 4 to 12 tons / hour, the system's syngas production ranged from 250 to 750 standard cubic meters / hour, the H2 volume fraction in the syngas was approximately 66%, the slag discharge temperature did not exceed 130℃, and the overall thermal efficiency of the system was improved by approximately 10 to 13 percentage points compared to traditional water-cooled slag coolers.

[0043] Comparative Example 1 Purpose of implementation: The purpose of this comparative example is to provide a conventional technical solution that contrasts with Example 1, namely, using the most common water-cooled drum slag cooler in industry for slag discharge waste heat recovery. By conducting tests under the same boiler slag discharge conditions as Example 1, the differences between the system of the present invention and the conventional technology in terms of slag discharge final temperature, waste heat recovery efficiency, resource utilization level and environmental benefits are directly compared, so as to prove that the technical solution of the present invention has significant progress compared with the prior art.

[0044] Implementation System: This comparative example uses a traditional water-cooled drum ash cooler for waste heat recovery from ash discharge from a circulating fluidized bed boiler. It does not employ an integrated vertical staged reaction tower, thermochemical reforming, or ash particle size classification. The application is the same as in Example 1: a 300MW supercritical circulating fluidized bed boiler unit with an ash discharge rate of 15 tons / hour and a discharge temperature of 900℃. The water-cooled drum ash cooler is a horizontal rotating drum structure with spiral heat exchange tubes installed on the inner wall of the drum. Boiler feedwater (approximately 105℃) flows through these tubes. The drum rotates at a low speed driven by a drive unit. The bottom ash moves forward along the spiral direction with the drum, exchanging heat with the cooling water in the heat exchange tubes through a wall-to-wall heat exchange process.

[0045] Implementation steps: The 900℃ high-temperature bottom ash directly enters the water-cooled drum ash cooler. Driven by a drive unit, the drum rotates at a low speed, and the bottom ash moves forward in a spiral direction within the drum. Cooling water at approximately 105℃ is introduced into the spiral heat exchange tubes on the inner wall of the drum, where it exchanges heat with the moving bottom ash through indirect contact. After absorbing the sensible heat from the bottom ash, the cooling water's temperature rises to approximately 130℃, and it is then piped to the boiler regenerative system. The cooled bottom ash is discharged from the ash cooler outlet. The system operates continuously, with the ash discharge rate and cooling water flow rate maintained at the design values.

[0046] Implementation results: Actual measurements showed that the heat recovered by the cooling water in this comparative study was approximately 2.8 MW, equivalent to about 0.34 tons of standard coal per hour. The physical heat loss from ash discharge accounted for approximately 1.2% of the boiler's input heat. The final ash discharge temperature was 150–180℃. The CaO in the bottom ash was not chemically utilized and was directly discharged as solid waste or used as raw material for building materials. The system produced no syngas and had no NOx emission reduction effect.

[0047] Compared with Example 1: Under the same conditions of 300MW boiler capacity, ash discharge rate of 15 tons / hour, and ash discharge temperature of 900℃, the final ash discharge temperature of Example 1 was 115-120℃, lower than that of Comparative Example 1 (150-180℃), indicating that the system of the present invention has a stronger ash discharge cooling capacity; Example 1 produced 930 standard cubic meters / hour of high-value-added syngas (H2 volume fraction of approximately 68%), while Comparative Example 1 produced no syngas; Example 1 achieved a NOx emission reduction of approximately 35%, while Comparative Example 1 had no NOx emission reduction effect; The overall system thermal efficiency of Example 1 was improved by approximately 12 percentage points, while that of Comparative Example 1 was only about 0.5 percentage points; In Example 1, the CaO in the bottom ash was effectively utilized as a catalyst for the reforming reaction and a CO2 absorbent, while the CaO in the bottom ash of Comparative Example 1 was not utilized. The above comparison fully demonstrates that the present invention, by adopting a technical solution based on in-situ particle size classification of slag discharge and coupling thermochemical reforming with physical step-by-step heat exchange, is significantly superior to traditional water-cooled drum slag coolers in terms of slag discharge waste heat recovery efficiency, resource utilization level, and environmental benefits.

[0048] This invention can be widely applied to the construction or renovation of waste heat recovery systems for circulating fluidized bed boilers in industries such as power and chemical engineering. It is particularly suitable for the utilization of waste heat from ash discharge in in-furnace desulfurization (CFB) boilers (where the bottom ash is naturally rich in CaO). The system of this invention has a compact structure, continuous and stable operation, and requires no external catalyst, thus possessing good industrial application value and market prospects.

[0049] Based on the test results of Examples 1 to 3 and Comparative Example 1, it can be seen that the technical solution of the present invention is superior to the traditional water-cooled drum slag cooler in terms of slag discharge waste heat recovery performance. Regarding the final slag discharge temperature, the slag discharge temperatures of Examples 1 to 3 are 115–120℃, 120–125℃, and no more than 130℃, respectively, all significantly lower than the 150–180℃ of Comparative Example 1. This indicates that the system of the present invention has stronger cooling capacity and can more fully extract the sensible heat from the slag discharge. In terms of resource utilization, Examples 1 to 3 all produce high-quality hydrogen-containing syngas, with yields of 930 standard cubic meters per hour, 500 standard cubic meters per hour, and 250–750 standard cubic meters per hour, respectively, with H2 volume fractions of approximately 68%, 65%, and 66%, respectively. In contrast, Comparative Example 1 produces no syngas, and the waste heat from the slag discharge is only converted into low-grade hot water. In terms of environmental benefits, Examples 1 and 2 achieved NOx emission reductions of approximately 35% and 30%, respectively. Example 3 also demonstrated significant NOx reduction capabilities through syngas reburning, while Comparative Example 1 showed no NOx emission reduction effect. Regarding the improvement in system thermal efficiency, Examples 1 to 3 achieved improvements of approximately 12 percentage points, 1.8 percentage points, and 10–13 percentage points, respectively, while Comparative Example 1 only achieved an improvement of approximately 0.5 percentage points.

[0050] Furthermore, Example 3 verified the operational stability of the system under conditions of significant fluctuations in slag discharge (4–12 tons / hour, with a variation range of ±50%). The control system was able to automatically adjust the injection volume within 3 minutes, the temperature fluctuation in the bottom dense phase zone was controlled within ±20℃, and the reforming reaction conversion rate was maintained above 85%. This fully demonstrates that the present invention is suitable not only for stable load conditions but also for variable load operation scenarios. In contrast, Comparative Example 1 lacked adaptive adjustment capability to slag discharge fluctuations, and the final slag discharge temperature fluctuated significantly with load changes. In summary, the present invention deeply integrates in-situ particle size classification of slag discharge, thermochemical reforming energy storage, and physical cascade heat exchange through an integrated vertical staged reaction tower. It demonstrates significant technological advancements in terms of waste heat recovery depth, product added value, environmental synergy, and operational adaptability, and has promising prospects for industrial application.

[0051] Supplementary explanation regarding the performance and mechanism of CaO participating in reforming reactions in the bottom dense phase region, specifically: (I) Catalytic activity and selectivity of CaO Pure CaO exhibits limited direct catalytic activity for the reforming of lower alcohols, but it demonstrates significant synergistic effects as an alkaline promoter and CO2 adsorbent. Adding 15 wt.% CaO to a Ni-based catalyst increases the H2 yield to 73.58% and the CO selectivity to 68.93%. The presence of CaO enhances the active NiO component in the catalyst and improves its reducibility and dispersibility. In adsorption-enhanced reforming systems, in-situ CO2 removal using CaO-based materials can achieve an H2 molar fraction approaching 90%, significantly improving the selectivity for H2 and CO2, while correspondingly reducing the selectivity for byproducts CO and CH4.

[0052] This invention fully utilizes the naturally occurring CaO (and inherent silicate, aluminate, and other components in the slag) in the bottom slag. Under high-temperature conditions of 250~900℃, CaO mainly participates in the reaction through the following pathways: (1) It consumes the CO2 generated by the reforming reaction in situ through carbonation, pulling the reaction equilibrium towards hydrogen production; (2) CaO acts as an alkaline site to promote the water-gas shift reaction, further converting CO into H2; (3) CaO forms complexes with SiO2, Al2O3, and other components in the slag, providing an alkaline microenvironment for the reforming reaction and assisting in the breaking of C-C bonds and CH bonds in low-carbon alcohol molecules. Under the above synergistic effect, the reforming reaction of this invention is expected to achieve a methanol conversion rate of ≥85% and a hydrogen gas integral of ≥65%.

[0053] (ii) CaO's ability to resist carbon deposition CaO inhibits carbon deposition in two ways. First, CaO removes CO2 promptly through carbonation, promoting the forward reaction of the water-gas shift reaction and reducing the partial pressure of CO in the gas phase, thereby reducing carbon deposition caused by the CO disproportionation reaction (2CO→C+CO2). Second, as an alkaline additive, CaO can modify the surface of acidic supports, limiting hydrogen transfer reactions and subsequent carbon deposition. Literature reports that in CaO-modified catalyst systems, the deactivation coefficient can be reduced by 3.8 to 4.3 times, and the carbon deposition rate significantly decreases. Based on this, this invention controls the water-to-ethanol ratio (0.5:1 to 5:1, preferably 2:1 to 3:1) to increase the partial pressure of water vapor to promote the decarbonization reaction. Combined with the continuous flow and slag renewal of the coarse slag in the bottom dense phase zone, it makes it difficult for carbon deposition precursors to accumulate in the reaction chamber, thus maintaining the "self-cleaning" state of the reaction zone.

[0054] (III) Cyclic stability and effective lifetime of CaO under reaction conditions The stability of CaO-based materials in repeated reaction-regeneration cycles is a recognized technical challenge. Its deactivation is mainly attributed to: (1) volume changes during carbonation / calcination cycles leading to structural collapse; (2) CaO grains growing by sintering at high temperatures; and (3) the generated CaCO3 layer may bury active sites.

[0055] However, this invention has two essential characteristics that distinguish it from traditional adsorption-enhanced reforming processes, so that the "lifetime" problem of CaO does not pose a technical obstacle: Firstly, the continuous slag removal and renewal mechanism. This invention operates in a single-pass continuous manner. The coarse slag in the bottom dense phase zone continuously moves downwards in a moving bed form under the action of fluidizing air and is discharged, while newly introduced high-temperature bottom slag continuously replenishes CaO. The residence time of the coarse slag in the dense phase zone is controlled by the fluidizing air volume (adjustable within 60%~120% of the rated air volume), typically controlled at 10~30 minutes. Within this timescale, the carbonation conversion rate of CaO is within a kinetically controllable range and has not yet entered the severe deactivation stage caused by repeated cycles. The CaCO3-containing coarse slag after the reaction is discharged from the system through the slag outlet without the need for regeneration cycle, fundamentally avoiding the inherent defect of rapid deactivation of traditional CaO adsorbents due to repeated calcination and regeneration.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A circulating fluidized bed boiler ash discharge waste heat recovery system, characterized in that, It includes an integrated vertical staged reaction tower, which is a vertical cylindrical structure. Its interior consists of a top dilute phase zone, a middle suspension transition zone, and a bottom dense phase zone from top to bottom. The bottom dense phase zone is a thermochemical reforming reaction zone, and a fine slag fluidized bed heat exchanger is installed in the top dilute phase zone. A slag discharge inlet device is installed at the top of the reaction tower to introduce slag from the circulating fluidized bed boiler into the reaction tower. The slag outlet is located at the bottom of the reaction tower and is used to discharge the cooled bottom slag. A fluidized air distribution system is installed at the bottom of the reaction tower to introduce rising airflow into the reaction tower, so that bottom ash of different particle sizes is classified according to settling velocity in the reaction tower. Coarse ash accumulates in the bottom dense phase zone, medium ash is suspended in the middle suspension transition zone, and fine ash is carried by the airflow to the top dilute phase zone. A methanol and water vapor supply system is connected to the bottom dense phase zone and is used to inject a mixture of methanol and water vapor into the bottom dense phase zone. The mixture undergoes an endothermic reforming reaction under the sensible heat of the coarse slag to generate hydrogen-containing synthesis gas, while simultaneously cooling the coarse slag. The reaction temperature of the bottom dense phase zone is 250-350°C, and the temperature of the coarse slag drops below 250°C after the endothermic reforming reaction. A gas-solid separator is installed at the top outlet of the reaction tower to separate the syngas from the fine residue; Syngas distribution pipeline, which is connected to the gas outlet of the gas-solid separator, is used to send the separated syngas into the boiler furnace or to an external gas storage device. The fine slag fluidized bed heat exchanger is connected to the boiler feedwater pipeline and is used to preheat the boiler feedwater with the waste heat of the separated fine slag.

2. The circulating fluidized bed boiler ash discharge waste heat recovery system according to claim 1, characterized in that: The slag discharge inlet device includes a slag discharge pipe, a manual gate valve, an electric gate valve, and a material distributor. The slag discharge pipe is connected to the slag discharge port of the circulating fluidized bed boiler, and the material distributor is located at the outlet end of the slag discharge pipe to evenly disperse the bottom slag into the reaction tower.

3. The circulating fluidized bed boiler ash discharge waste heat recovery system according to claim 1, characterized in that: The fluidizing air distribution system includes a fluidizing air header, an air chamber, and an air distribution plate. The inlet end of the fluidizing air header is connected to the secondary air source or recirculated flue gas source of the circulating fluidized bed boiler, and its outlet end is connected to the air chamber via a recirculation fan. The air distribution plate is located above the air chamber, and air caps are distributed on the air distribution plate.

4. The circulating fluidized bed boiler ash discharge waste heat recovery system according to claim 1, characterized in that: The supply system includes a methanol storage tank, a demineralized water storage tank, a mixer, a preheater, and an injector. The injector is located on the side wall of the bottom dense phase zone and is evenly arranged around the circumference of the reaction tower. The installation height of the injector is above the air distribution plate.

5. A circulating fluidized bed boiler ash discharge waste heat recovery system according to claim 1, characterized in that: The gas-solid separator is a cyclone separator, and its solid outlet is connected to the inlet of the fine slag fluidized bed heat exchanger; the syngas distribution pipeline includes a reburning pipeline connected to the boiler furnace and an external delivery pipeline connected to the gas storage device, and regulating valves are respectively provided on the reburning pipeline and the external delivery pipeline.

6. A method for recovering waste heat from ash discharge in a circulating fluidized bed boiler, used in the system described in any one of claims 1-5, characterized in that: Includes the following steps: S1: Start the fluidizing air distribution system to introduce fluidizing air into the reaction tower, open the slag discharge inlet device to allow the high-temperature bottom slag to enter the reaction tower, detect the temperature of the bottom dense phase zone, and when the temperature rises to above 250°C, start the methanol and water vapor supply system to spray a mixture of methanol and water vapor into the bottom dense phase zone. S2: During normal operation, the high-temperature bottom ash is classified in the reaction tower under the action of rising fluidizing air. The coarse ash accumulates in the bottom dense phase zone and catalyzes the methanol reforming reaction at 250-350℃ to generate hydrogen-containing syngas. At the same time, the coarse ash cools down to below 250℃. The syngas rises and passes through the middle suspension transition zone to exchange heat with the middle ash before entering the gas-solid separator. The separated syngas is sent to the boiler furnace or sent for storage through the distribution pipeline. The separated fine ash enters the fine ash fluidized bed heat exchanger in the top dilute phase zone to preheat the boiler feedwater. The cooled bottom ash is discharged from the ash discharge outlet at the bottom of the reaction tower. S3: When shutting down, shut off the methanol and steam supply system, continue to supply fluidizing air until all residue in the tower is discharged and the tower temperature drops below 200°C, then shut off the slag discharge inlet device and the fluidizing air distribution system.

7. The method for recovering waste heat from ash discharge in a circulating fluidized bed boiler according to claim 6, characterized in that: In step S2, the injection rates of methanol and water vapor are adjusted according to the slag discharge rate to maintain the temperature of the bottom dense phase zone at 270-290°C. Specifically, the control system uses the opening signal of the electric gate valve of the slag discharge inlet device to represent the slag discharge rate, adjusts the opening of the regulating valve on the supply pipeline according to the preset correspondence between the injection rate and the slag discharge rate, and uses the temperature of the bottom dense phase zone as a feedback signal for correction.

8. The method for recovering waste heat from ash discharge in a circulating fluidized bed boiler according to claim 6, characterized in that: In step S2, the fluidizing air volume is adjusted within the range of 60% to 120% of the rated air volume.

9. The method for recovering waste heat from ash discharge in a circulating fluidized bed boiler according to claim 6, characterized in that: In step S2, the syngas is sent into the reburning zone of the boiler furnace via the reburning pipeline, or the syngas is sent into the gas storage device via the external transmission pipeline, or the syngas is sent into the reburning zone of the boiler furnace via the reburning pipeline and into the gas storage device via the external transmission pipeline at the same time. The flow ratio of the reburning pipeline and the external transmission pipeline is adjusted according to the boiler load and the syngas production.

10. The method for recovering waste heat from ash discharge in a circulating fluidized bed boiler according to claim 6, characterized in that: The molar ratio of water vapor to methanol in the mixture is between 0.5:1 and 5:1; the temperature of the bottom slag discharged does not exceed 150 degrees Celsius.

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