Oxygen-enriched combustion system of circulating fluidized bed boiler and operation method

Abstract

According to the oxygen-enriched combustion system of the circulating fluidized bed boiler and the operation method, under the oxygen-enriched combustion working condition, part of low-temperature and clean flue gas at the tail is led back, mixed with pure oxygen and then fed into a hearth, the total in-furnace gas flow after mixing is kept equivalent to the original total in-furnace air flow, and the total in-furnace gas flow after mixing is kept equivalent to the original total in-furnace air flow; when the mixed gas entering the furnace keeps the oxygen volume concentration equivalent to or slightly higher than that of air, the problems of fluidization, combustion and heat transfer in a hearth can be completely solved, and safe, reliable, economical and environment-friendly operation of a unit can be guaranteed; after air is completely replaced by the mixed gas, the CO2 concentration in the generated flue gas is greatly improved and can reach 80% or above at most, and CO2 can be captured with low cost and high efficiency; according to the invention, free switching between air combustion and oxygen-enriched combustion can be realized.

Description

Technical Field

[0001] The present invention relates to a circulating fluidized bed boiler combustion system and an operating method, in particular to an oxygen-enriched combustion system and an operating method of a circulating fluidized bed boiler. Background Art

[0002] Coal-fired power boilers can reduce carbon emissions through carbon capture, making it a key path to promoting low-carbon energy transition and achieving the "dual carbon" goals. In traditional coal-fired power boiler units, fuel and air are mixed and burned, resulting in flue gas primarily composed of nitrogen (N2) with low CO2 concentrations. This makes back-end CO2 separation and capture very costly. Oxygen-enriched combustion uses a mixture of oxygen and recycled flue gas as an oxidant, significantly increasing the CO2 concentration in the post-combustion flue gas to as high as 80-95%. This significantly reduces back-end CO2 separation and capture costs, creates favorable conditions for subsequent storage or utilization, and effectively reduces CO2 emissions to the atmosphere.

[0003] Circulating fluidized bed boiler is a high-efficiency, low-pollution combustion technology that has developed rapidly in recent decades. It accounts for at least 10% of the installed capacity of coal-fired power plants currently in service. The working principle of the circulating fluidized bed is as follows: a certain proportion of primary air is heated by the preheater and enters the water-cooled air chamber at the bottom of the furnace as primary combustion air and fluidizing air. It enters the furnace through the air distribution plate to fluidize the bed material and form a gas-solid two-phase flow that passes upward through the furnace. The secondary air is heated by the preheater and then sent into the furnace in layers from the dense phase area at the bottom of the furnace to further supplement oxygen, strengthen the disturbance and combustion of air and fuel. The combustion makes the fuel burn more completely, and at the same time cooperates with the primary air to strengthen the upward flow of materials; with the assistance of the coal-spreading air, the coal is fed into the furnace from the coal feeding device evenly distributed in front of the furnace and fully mixed with the high-temperature bed material, primary air and secondary air for combustion. The heat generated by its combustion is carried by the flue gas to the upper part of the furnace for full heat exchange; the flue gas carries the solid materials and flows upward under the main effect of the primary fluidizing air and the assistance of the secondary air. When some materials with larger particle sizes reach the upper part of the furnace, their speed decreases, and under the action of gravity, they fall back to the lower part of the furnace along the side wall of the heating surface of the furnace and continue to participate in the combustion The combustion and fluidization form the internal circulation of the circulating fluidized bed. Some of the finer materials enter the cyclone separator from the flue gas outlet at the upper part of the furnace. In the cyclone separator, most of the solid materials are separated and enter the return material device at the lower part of the cyclone separator. With the assistance of high-pressure fluidizing air, these materials return to the furnace in an orderly manner, forming the external circulation of the circulating fluidized bed. The heat transfer in the furnace mainly relies on the convection heat transfer of the flue gas and the circulating materials and the radiation heat transfer of the high temperature in the furnace. Among them, the upward flow of a large amount of materials in the furnace is the heat source for the upper heating surface of the furnace to absorb heat and the heat transfer of the upper heating surface of the furnace. The main factor of heat transfer is the flow; the clean flue gas separated in the cyclone separator enters the tail flue through the upper outlet flue of the cyclone separator, and the heat is transferred to the heating surface in the tail flue mainly by convection heat transfer. Finally, after passing through the preheater, the flue gas temperature has been reduced to about 130-150℃, and then enters the dust collector, induced draft fan, flue gas purification system to remove dust and pollutants. The clean flue gas is sent to the chimney and discharged into the atmosphere; the circulating fluidized bed boiler furnace is more suitable for oxygen-enriched combustion due to the characteristics of radiation + convection heat transfer.

[0004] Since the volume ratio of oxygen in air is approximately 21%, for existing circulating fluidized bed boilers using air combustion, if oxygen-enriched combustion is used, directly replacing air with pure oxygen, the pure oxygen flow rate entering the boiler will be reduced by approximately 79% compared to the air flow rate entering the boiler during air combustion, while providing the same amount of oxygen required for boiler combustion. This reduction in inlet gas flow will cause a large amount of bed material in the furnace to accumulate in the dense phase area at the bottom of the furnace, preventing normal fluidization. The large amount of heat concentrated in the lower furnace can lead to serious accidents, such as safety issues with the fuel and high oxygen concentration, such as excessive combustion temperatures leading to severe fuel coking. At the same time, the significant reduction in gas flow prevents the heat and material from being carried from the lower part to the upper furnace, significantly reducing the material concentration in the main heat transfer zone in the upper furnace, severely degrading heat transfer characteristics, and causing serious problems such as hydrodynamic safety issues and disordered heat distribution within the furnace, ultimately causing the operating circulating fluidized bed boiler unit to fail.

[0005] Currently, a large number of research institutions are studying new pressurized or atmospheric pressure oxygen-enriched combustion circulating fluidized bed technologies, but there are problems in the following aspects: First, the new pressurized / atmospheric pressure oxygen-enriched combustion circulating fluidized bed technologies are currently in a relatively early stage of research and development. Research on the combustion and heat transfer characteristics in the furnace, and research on the layout of the heating surface are still in a relatively early stage, and the current technical maturity is not enough; second, the pressurized oxygen-enriched combustion circulating fluidized bed technology is not suitable for air combustion conditions and can only operate under oxygen-enriched combustion conditions. It has very high requirements on the reliability of the front-end oxygen production equipment. If the front-end oxygen cannot be supplied in time, the unit can only suspend operation. Summary of the Invention

[0006] The purpose of the present invention is to address the above-mentioned deficiencies in the prior art and to provide a circulating fluidized bed boiler oxygen-enriched combustion system and operating method, which can realize free switching between air combustion and oxygen-enriched combustion, and ensure safe, reliable, economical and environmentally friendly operation of the unit. Under oxygen-enriched combustion conditions, the CO2 concentration at the unit outlet can be increased to a predetermined target, facilitating the subsequent efficient capture of CO2.

[0007] In order to achieve the above-mentioned purpose, the oxygen-enriched combustion system of the circulating fluidized bed boiler of the present invention comprises a furnace, an air chamber, an air distribution plate, a coal feeding device, an ignition device, a cyclone separator, a return device, and a tail flue. A preheater is provided in the tail flue, and the end of the tail flue is connected in sequence with a dust collector, an induced draft fan, a tail flue gas purification device and a chimney through a smoke exhaust pipe. The air outlet pipe of the primary fan is connected to the air chamber and the coal feeding device respectively after passing through the preheater, the air outlet pipe of the secondary fan is connected to the dense phase area at the lower part of the furnace after passing through the preheater, the air outlet pipe of the fluidizing fan is connected to the return device, and the air inlet pipes of the primary fan, the secondary fan and the fluidizing fan are all connected to an air inlet pipe, and a valve is provided on the air inlet pipe; it is characterized in that: a recirculating flue gas main pipe is connected to the pipeline between the dust collector and the tail flue gas purification device and a valve is provided on the main pipe. A main flue gas valve is provided on the pipe; the main flue gas pipe is connected to the air inlet pipes of the recirculation primary fan, the recirculation secondary fan and the fluidizing fan respectively through the flue gas branch pipes; the flue gas branch pipes connected to the recirculation primary fan and the recirculation secondary fan are each provided with an oxygen and flue gas mixer, and the oxygen and flue gas mixers are each connected to an oxygen delivery pipe, and the oxygen delivery pipe is each provided with an oxygen regulating damper and an oxygen shut-off damper; the outlet pipe of the recirculation primary fan is connected to the air inlet pipe of the primary fan through a flue gas primary air regulating damper and a flue gas primary air shut-off damper, the outlet pipe of the recirculation secondary fan is connected to the air inlet pipe of the secondary fan through a flue gas secondary air regulating damper and a flue gas secondary air shut-off damper, and the flue gas branch pipe connected to the air inlet pipe of the fluidizing fan is provided with a fluidizing flue gas regulating damper and a fluidizing flue gas shut-off damper; The operating method of the circulating fluidized bed boiler oxygen-enriched combustion system of the present invention comprises the following steps: A) Air combustion: Close the main flue gas valve, two oxygen shut-off dampers, flue gas primary air shut-off dampers, flue gas secondary air shut-off dampers and fluidized flue gas shut-off dampers, open the valves on each air inlet pipe, start the primary fan, secondary fan, fluidized fan and induced draft fan, and send air into the preheater through the primary fan to be heated. The heated primary air is sent into the wind chamber on one side, and through the air distribution plate, the material in the furnace is efficiently fluidized, and at the same time, it reacts with the coal quality fed by the coal feeding device for combustion. The other side is sent to the coal feeding device to feed the coal into the furnace for combustion; air is sent into the preheater through the secondary fan to be heated, and the heated secondary air is sent to the dense phase area at the bottom of the furnace; a small amount of air is sent to the return feeder, and the particles separated by the cyclone separator in the return feeder are sent back to the furnace for continuous circulation; the flue gas enters the dust collector for dust removal, and then is sent to the tail flue gas purification device through the induced draft fan. Finally, the clean flue gas enters the chimney and is discharged into the atmosphere; it is characterized by: B) Oxygen-enriched combustion: close the valves on each air inlet pipe, open the flue gas main valve, two oxygen shut-off dampers, flue gas primary air shut-off dampers, flue gas secondary air shut-off dampers and fluidized flue gas shut-off dampers, start the recirculation primary fan and recirculation secondary fan, adjust the opening of the flue gas primary air regulating damper, flue gas secondary air regulating damper and fluidized flue gas regulating damper, so that 1%-3% of the total amount of recycled flue gas enters the return device through the fluidized fan, and the rest of the recycled flue gas enters the two oxygen and flue gas mixers, adjust the opening of the two oxygen regulating dampers, so that the flue gas and oxygen are mixed to form a recycled mixed gas, which accounts for 35%-50% of the total recycled mixed gas. After passing through the primary fan, it is sent to the preheater for heating and then sent into the wind chamber, entering the furnace through the air distribution plate, and the other part is sent to the coal feeding device. The oxygen concentration of the recycled primary furnace mixed gas is controlled between 22% and 27%. The recycled secondary furnace gas, which accounts for 50% to 65% of the total recycled mixed gas, is sent to the preheater for heating by the secondary fan and then sent into the furnace from the dense phase area of ​​the furnace. The oxygen concentration of the recycled secondary furnace gas is controlled between 22% and 30% until the recycled mixed gas completely replaces the air and the recycled flue gas completely replaces the original high-pressure fluidized air. The remaining flue gas is sent to the tail flue gas purification device, and the remaining clean flue gas after the CO2 is captured is discharged through the chimney. The present invention, under the oxygen-rich combustion condition, draws back the low-temperature, clean part of the flue gas at the tail, mixes it with pure oxygen and then feeds it into the furnace. The total flow rate of the mixed gas entering the furnace is kept equivalent to the original total flow rate of the air entering the furnace. When the oxygen volume concentration of the mixed gas entering the furnace is kept equivalent to or slightly higher than that of the air, it can completely solve the problems of fluidization, combustion and heat transfer in the upper furnace, and can ensure the safe, reliable, economical and environmentally friendly operation of the unit; and after the mixed gas completely replaces the air, the CO2 concentration in the flue gas generated will be greatly improved, up to more than 80%, which is very beneficial for low-cost and efficient capture of CO2; and the pure oxygen and flue gas are fully mixed before being fed into the furnace, which is more conducive to the uniformity of the oxygen distribution in the furnace. In addition, the oxygen is fed in a staged manner, and the oxygen concentrations in the recycled primary furnace gas and the recycled secondary furnace gas can be controlled respectively, so that the staged combustion of oxygen and fuel in the furnace can be realized, which is beneficial to improving the combustion efficiency in the furnace and controlling the original generation of NOx. As a further improvement of the present invention, a flue gas cooler is provided between the end of the tail flue and the dust collector, and the flue gas cooler is provided with a medium inlet pipe and a medium outlet pipe, and a shut-off valve and a regulating valve are provided on the medium inlet pipe; correspondingly, in the step B), the shut-off valve is opened and the regulating valve is gradually adjusted, and the flue gas at the dust collector inlet is cooled by passing the cooling medium into the flue gas cooler; the flue gas can be cooled to a range within which the dust collector can operate safely, and heat can be recovered to improve the efficiency of the boiler. As a further improvement of the present invention, the outlet pipe of the primary recirculation fan and the outlet pipe of the secondary recirculation fan are both provided with flow transmitters and oxygen concentration transmitters; the outlet pipe of the fluidizing fan is provided with a fluidizing air flow transmitter; corresponding to the step B), the openings of the flue gas primary air regulating damper, the flue gas secondary air regulating damper and the two oxygen regulating dampers are accurately adjusted by means of the flow transmitters and oxygen concentration transmitters, so that the recycled primary furnace gas accounts for 35%-50% of the total recycled mixed gas, and the oxygen concentration of the recycled primary furnace mixed gas is controlled between 22%-27%; the recycled secondary furnace gas accounts for 50%-65% of the total recycled mixed gas, and the oxygen concentration of the recycled secondary furnace gas is controlled between 22%-30%; the opening of the fluidizing flue gas regulating damper is accurately adjusted by means of the fluidizing air flow transmitter, and the circulating flue gas fluidizing air accounts for 1%-3% of the total circulating flue gas; In summary, the present invention can realize the free switching between air combustion and oxygen-enriched combustion, and ensure the safe, reliable, economical and environmentally friendly operation of the unit. Under the oxygen-enriched combustion condition, the CO2 concentration at the unit outlet can be increased to the predetermined target, facilitating the subsequent efficient capture of CO2. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a structural diagram of an embodiment of the circulating fluidized bed boiler oxygen-enriched combustion system of the present invention. DETAILED DESCRIPTION

[0009] The present invention will be further described below with reference to the accompanying drawings.

[0010] like Figure 1 As shown, the oxygen-enriched combustion system of the circulating fluidized bed boiler of this embodiment includes a furnace 1, a wind chamber 2, an air distribution plate 3, a coal feeding device 4, an ignition device 5, a cyclone separator 6, a material return device 7, and a tail flue 8. A preheater 9 is provided in the tail flue. The end of the tail flue 8 is connected to a flue gas cooler 10, a dust collector 11, an induced draft fan 12, a tail flue gas purification device 13 and a chimney 14 in sequence through a flue gas exhaust pipe. The outlet pipe of the primary fan 15 is connected to the bottom of the wind chamber 2 and the coal feeding device 4 respectively after passing through the preheater 9. The outlet pipe of the secondary fan 16 is connected to the furnace after passing through the preheater 9. The lower dense phase zone is connected in layers through several nozzles. The outlet pipe of the fluidizing fan 17 is connected to the return material device 7. The air inlet pipes of the primary fan 15, the secondary fan 16 and the fluidizing fan 17 are all connected to the air inlet pipe, and the air inlet pipe is provided with a valve 18; the flue gas cooler 10 is provided with a medium inlet pipe and a medium outlet pipe, and the medium inlet pipe is provided with a shut-off valve 19 and a regulating valve 20; between the dust collector 11 and the tail flue gas purification device 12, the exhaust pipe in front of the induced draft fan 12 is connected to a recirculating flue gas main pipe 21 and a flue gas main valve 22 is provided on the main pipe; the flue gas main pipe 21 is connected to the exhaust pipe in front of the induced draft fan 12. The flue gas branches are connected to the air inlet pipes of the recirculation primary fan 23, the recirculation secondary fan 24 and the fluidizing fan 17 respectively. The flue gas branches connected to the recirculation primary fan 23 and the recirculation secondary fan 24 are equipped with oxygen and flue gas mixers 25. The oxygen and flue gas mixers 25 are connected to oxygen delivery pipes. Both oxygen delivery pipes are equipped with oxygen regulating dampers 26 and oxygen shut-off dampers 27. The air outlet pipe of the recirculation primary fan 23 is connected to the air inlet pipe of the primary fan 15 through the flue gas primary air regulating damper 28 and the flue gas primary air shut-off damper 29. The outlet pipe of the recirculation primary fan 23 is equipped with a flow transmitter 35 and an oxygen concentration transmitter 36. The outlet pipe of the recirculation secondary fan 24 is connected to the air inlet pipe of the secondary fan 16 via a flue gas secondary air regulating damper 31 and a flue gas secondary air shutoff 32. The outlet pipe of the recirculation secondary fan 24 is equipped with a flow transmitter 37 and an oxygen concentration transmitter 38. The flue gas branch pipe connected to the air inlet pipe of the fluidizing fan 17 is equipped with a fluidizing flue gas regulating damper 33 and a fluidizing flue gas shutoff damper 34. The outlet pipe of the fluidizing fan 17 is equipped with a fluidizing air flow transmitter 39. When in use, both oxygen delivery pipes are connected to an oxygen supply device such as an oxygen concentrator. The exhaust pipe behind the tail flue gas purification device 13 is connected to a CO2 capture system 40. As a conventional configuration of a fluidized bed boiler, a furnace heating surface 46 is provided on the upper part of the furnace, and a superheater 47, a reheater 48, and an economizer 49 and other tail heating surfaces are provided in the tail flue 8 in front of the preheater 9. The operating method of the circulating fluidized bed boiler oxygen-enriched combustion system of the present invention includes the following steps: A) air combustion: closing the flue gas main valve 22, two oxygen shut-off dampers 27, the flue gas primary air shut-off damper 29, the flue gas secondary air shut-off damper 32, and the fluidized flue gas shut-off damper 34, opening the valves 18 on each air inlet pipe, starting the primary fan 15, the secondary fan 16, the fluidizing fan 17, and the induced draft fan 12, one path of air is sent into the preheater 9 through the primary fan 15 to be heated, the heated primary air is sent into the wind chamber 2, and the material in the furnace is efficiently fluidized by the air distribution plate 3, while simultaneously undergoing a combustion reaction with the coal quality sent by the coal feeding device 4, and the other path is sent into the coal feeding device 4 for sending the coal quality into the furnace. 1; another air stream is sent to the preheater 9 through the secondary fan 16 for heating. The heated secondary air is sent to the dense phase area at the bottom of the furnace and then to the furnace 1 in layers. A small amount of air is sent to the return hopper 7 by the fluidizing fan 17. The particles separated by the cyclone separator 6 in the return hopper 7 are sent back to the furnace for continuous circulation and combustion. The clean flue gas enters the tail flue 8 through the top outlet of the cyclone separator 6. After the heat of the flue gas is transferred to the tail heating surface, it enters the dust collector 11 for dust removal. It is then sent to the tail flue gas purification device 13 through the induced draft fan 12. Finally, the clean flue gas enters the chimney 14 and is discharged into the atmosphere. B) Oxygen-enriched combustion: close the valves 18 on each air inlet pipe, open the flue gas main valve 22, the two oxygen shut-off dampers 27, the flue gas primary air shut-off damper 29, the flue gas secondary air shut-off damper 32 and the fluidized flue gas shut-off damper 34, start the recirculation primary fan 23 and the recirculation secondary fan 24, the recirculation primary fan 23, the recirculation secondary fan 24 and the fluidizing fan 17 extract 65%-75% of the total flue gas to become recirculated flue gas, and accurately adjust the openings of the flue gas primary air regulating damper 28, the flue gas secondary air regulating damper 31 and the fluidizing flue gas regulating damper 33 through the fluidizing air flow transmitter 39, the flow transmitter 35 and the flow transmitter 37, so that 1%-3% of the total circulating flue gas enters the return device 7 through the fluidizing fan 17, and the rest of the circulating flue gas enters the two oxygen Together with the flue gas mixer 25, the openings of the two oxygen regulating dampers 26 are accurately adjusted through the oxygen concentration transmitter 36 and the oxygen concentration transmitter 38, so that the circulating flue gas and oxygen are mixed to form a recycled mixed gas. The recycled primary furnace gas, which accounts for 35%-50% of the total recycled mixed gas, is sent to the preheater 9 for heating after passing through the primary fan 15, and is then sent from the wind chamber 2 to enter the furnace 1 through the air distribution plate 3. The other part is sent to the coal feeding device 4. The oxygen concentration of the recycled primary furnace mixed gas is controlled between 22% and 27%. The recycled secondary furnace gas, which accounts for 50%-65% of the total recycled mixed gas, is sent to the preheater 9 for heating after passing through the secondary fan 16, and is then sent into the furnace 1 from the dense phase area of ​​the furnace in layers. The oxygen concentration of the recycled secondary furnace gas is controlled between 22% and 30%.At the same time, the shut-off valve 19 is opened and the regulating valve 20 is gradually adjusted, and a cooling medium is introduced through the medium inlet pipe of the flue gas cooler 10 to cool the flue gas at the inlet of the dust collector 11 until the recirculated mixed gas completely replaces the air and the recirculated flue gas completely replaces the original high-pressure fluidizing air; the remaining flue gas is sent to the tail flue gas purification device 13, and the clean flue gas is sent to the CO2 capture system 40 for CO2 capture. The remaining clean flue gas after capture is discharged through the chimney 14; The present invention is no different from a conventional circulating fluidized bed boiler in air combustion conditions. In oxygen-enriched combustion conditions, the low-temperature, clean part of the flue gas at the tail is brought back, mixed with pure oxygen and then fed into the furnace 1. The total flow rate of the mixed gas entering the furnace is kept equivalent to the original total flow rate of the air entering the furnace. When the oxygen volume concentration of the mixed gas entering the furnace is kept equivalent to or slightly higher than that of the air, it can completely solve the problems of fluidization, combustion and heat transfer in the upper furnace, and can ensure the safe, reliable, economical and environmentally friendly operation of the unit; and after the mixed gas completely replaces the air, the CO2 concentration in the flue gas generated will be greatly improved, and can reach up to more than 80%, which is very conducive to the subsequent low-cost and efficient capture of CO2; and pure oxygen and flue gas are mixed before entering the furnace. The oxygen and flue gas are first fully mixed in the oxygen and flue gas mixer 25, which is more conducive to the uniform distribution of oxygen in the furnace. In addition, the oxygen is introduced in a staged manner, which can control the oxygen concentration in the recycled primary furnace gas and the recycled secondary furnace gas respectively, so as to achieve staged combustion of oxygen and fuel in the furnace, which is beneficial to improving the combustion efficiency in the furnace and controlling the initial generation of NOx. The above two combustion conditions can be switched freely: if pure oxygen cannot be supplied in time, air combustion can be used; if the amount of pure oxygen is insufficient for oxygen-enriched combustion, partial oxygen-enriched combustion, that is, a combustion condition of pure oxygen + recycled flue gas + air, can be used; if the amount of pure oxygen is sufficient, full oxygen-enriched combustion can be used to meet the requirements of the new power system for the flexibility of coal-fired power generation. Since the temperature of the extracted recirculated flue gas is usually 120℃-150℃, compared with the air temperature of the original air combustion condition, the temperature of the cooling medium entering the original preheater is greatly increased, which will cause the exhaust temperature of the preheater outlet to be greatly increased. The flue gas cooler 10 cools the flue gas at the dust collector inlet to a range where the dust collector can operate safely, recovers heat, and improves boiler efficiency. The above embodiments have been described, but it should be understood that the above embodiments are only for the purpose of illustration and description, and are not intended to limit the present invention to the scope of the described embodiments.

Claims

1. A circulating fluidized bed boiler oxygen-enriched combustion system, comprising a furnace, an air chamber, an air distribution plate, a coal feeder, an ignition device, a cyclone separator, a material return device, and a tail flue. The tail flue is provided with a preheater. The end of the tail flue is connected in sequence to a dust collector, an induced draft fan, a tail flue gas purification device, and a chimney through a smoke exhaust pipe. The air outlet pipe of the primary fan is connected to the air chamber and the coal feeder after passing through the preheater. The air outlet pipe of the secondary fan is connected to the dense phase area below the furnace after passing through the preheater. The air outlet pipe of the fluidizing fan is connected to the material return device. The air inlet pipes of the primary fan, secondary fan, and fluidizing fan are all connected to an air inlet pipe, and the air inlet pipes are all provided with valves. The characteristics are as follows: The pipeline between the dust collector and the tail flue gas purification device is connected to a recirculating flue gas main pipe and a flue gas main valve is provided on the main pipe; the flue gas main pipe is connected to the air inlet pipes of the recirculating primary fan, the recirculating secondary fan and the fluidizing fan through each flue gas branch pipe respectively, and the flue gas branch pipes connected to the recirculating primary fan and the recirculating secondary fan are provided with oxygen and flue gas mixers, and the oxygen and flue gas mixers are connected to oxygen delivery pipes, and the oxygen delivery pipes are provided with oxygen regulating baffles and oxygen shut-off baffles; the air outlet pipe of the recirculating primary fan is connected to the air inlet pipe of the primary fan through the flue gas primary air regulating baffle and the flue gas primary air shut-off baffle, and the air outlet pipe of the recirculating secondary fan is connected to the air inlet pipe of the secondary fan through the flue gas secondary air regulating baffle and the flue gas secondary air shut-off baffle, and the flue gas branch pipe connected to the air inlet pipe of the fluidizing fan is provided with a fluidizing flue gas regulating baffle and a fluidizing flue gas shut-off baffle.

2. The circulating fluidized bed boiler oxygen-enriched combustion system according to claim 1, characterized in that: A flue gas cooler is provided between the end of the tail flue and the dust collector. The flue gas cooler is provided with a medium inlet pipe and a medium outlet pipe. A shut-off valve and a regulating valve are provided on the medium inlet pipe.

3. The circulating fluidized bed boiler oxygen-enriched combustion system according to claim 1 or 2, characterized in that: The air outlet pipe of the recirculation primary fan and the air outlet pipe of the recirculation secondary fan are both provided with a flow transmitter and an oxygen concentration transmitter.

4. The circulating fluidized bed boiler oxygen-enriched combustion system according to claim 3, characterized in that: A fluidizing air flow transmitter is provided on the air outlet pipe of the fluidizing fan.

5. A method for operating an oxygen-enriched combustion system of a circulating fluidized bed boiler, comprising the following steps: A) Air Combustion: Close the main flue gas valve, two oxygen shutoff dampers, flue gas primary air shutoff dampers, flue gas secondary air shutoff dampers, and fluidized flue gas shutoff dampers. Open the valves on the air inlet pipes and start the primary fan, secondary fan, fluidized fan, and induced draft fan. Air is sent into the preheater through the primary fan for heating. The heated primary air is sent into the air chamber one way, passes through the air distribution plate, and effectively fluidizes the materials in the furnace. It also reacts with the coal fed by the coal feeding device for combustion. Another way is sent to the coal feeding device to feed the coal into the furnace for combustion. Air is sent into the preheater through the secondary fan for heating. The heated secondary air is sent to the dense phase area below the furnace. A small amount of air is sent to the return feeder, and the particles separated by the cyclone separator in the return feeder are returned to the furnace for continuous circulation. The flue gas enters the dust collector for dust removal, and is then sent to the tail flue gas purification device through the induced draft fan. Finally, the clean flue gas enters the chimney and is discharged into the atmosphere. This is characterized by: B) Oxygen-enriched combustion: Close the valves on each air inlet pipe, open the flue gas main valve, two oxygen shut-off dampers, flue gas primary air shut-off dampers, flue gas secondary air shut-off dampers and fluidized flue gas shut-off dampers, start the recirculation primary fan and recirculation secondary fan, adjust the opening of the flue gas primary air regulating damper, flue gas secondary air regulating damper and fluidized flue gas regulating damper, so that 1%-3% of the total amount of recycled flue gas enters the return device through the fluidized fan, and the rest of the recycled flue gas enters the two oxygen and flue gas mixers, adjust the opening of the two oxygen regulating dampers, so that the flue gas and oxygen are mixed to form a recycled mixed gas, which accounts for 35%-50% of the total recycled mixed gas. The gas is sent to the preheater for heating after passing through the primary fan, and then sent to the furnace from the wind chamber through the air distribution plate. The other part is sent to the coal feeding device, and the oxygen concentration of the recycled primary furnace mixed gas is controlled between 22% and 27%; the recycled secondary furnace gas, which accounts for 50% to 65% of the total recycled mixed gas, is sent to the preheater for heating after passing through the secondary fan, and then sent to the furnace from the dense phase area of ​​the furnace. The oxygen concentration of the recycled secondary furnace gas is controlled between 22% and 30%; until the recycled mixed gas completely replaces the air and the recycled flue gas completely replaces the original high-pressure fluidized air; the remaining flue gas is sent to the tail flue gas purification device, and the remaining clean flue gas after capture is discharged through the chimney.

6. The method for operating the circulating fluidized bed boiler oxygen-enriched combustion system according to claim 5, characterized in that: In the step B), the shut-off valve is opened and the regulating valve is gradually adjusted, and the flue gas at the dust collector inlet is cooled by introducing a cooling medium into the flue gas cooler.

7. The method for operating the circulating fluidized bed boiler oxygen-enriched combustion system according to claim 5 or 6, characterized in that: In step B), the openings of the flue gas primary air regulating damper, the flue gas secondary air regulating damper, and the two oxygen regulating dampers are accurately adjusted by means of the flow transmitters and the oxygen concentration transmitter, so that the recycled primary furnace gas accounts for 35%-50% of the total recycled mixed gas, and the oxygen concentration in the recycled primary furnace mixed gas is controlled between 22%-27%; the recycled secondary furnace gas accounts for 50%-65% of the total recycled mixed gas, and the oxygen concentration in the recycled secondary furnace gas is controlled between 22%-30%; and the opening of the fluidized flue gas regulating damper is accurately adjusted by means of the fluidizing air flow transmitter, so that the circulating flue gas fluidizing air accounts for 1%-3% of the total circulating flue gas.

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