Circulating fluidized bed boiler low-nitrogen combustion system with external bed and operation method

Abstract

The invention provides a circulating fluidized bed boiler low-nitrogen combustion system with an external bed. The circulating fluidized bed boiler low-nitrogen combustion system comprises a boiler hearth, a material returning pipe, the external bed and a coal feeding pipe. The external bed is communicated with a boiler hearth through a material returning pipe, a coal feeding port is formed in the external bed, and an outlet of a coal feeding pipe is communicated with the coal feeding port; an external bed fluidization air system is arranged at the bottom of the external bed; smoke generated by combustion reaction after boiler fire coal enters the external bed contains CO, the smoke is in a reducing atmosphere, CO enters a boiler hearth along with the fire coal and circulating material particles and is in an oxidizing atmosphere, the fire coal in the hearth combusts to generate a large amount of NO, CO can react with NO generated in the hearth, and NO is reduced to generate N2; the retention time of the fire coal in the external bed is long, the incomplete combustion reaction of the fire coal is promoted, the CO release amount is increased, the combustion process of the fuel is effectively improved by optimally designing the external structure of the hearth, and therefore the aim of reducing emission of nitrogen oxide of the boiler is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of clean coal utilization, and in particular relates to a low-nitrogen combustion system of a circulating fluidized bed boiler with an external bed and an operation method thereof. Background Art

[0002] As one of the main air pollutants emitted by coal-fired power plants, nitrogen oxides (NOx) have long been harmful to the environment, posing a threat to human health, air quality and the ecosystem that cannot be ignored. In order to meet this challenge, strict "ultra-low emission" standards have been formulated in recent years, which clearly require that the NOx emission concentration of coal-fired power plants must be controlled below 50 mg / m3. This standard places extremely high demands on the environmental protection technology and management level of coal-fired power plants.

[0003] As an advanced coal-fired power generation technology, the circulating fluidized bed (CFB) boiler has the characteristics of segmented air supply and low combustion temperature in the furnace, which makes the original generation of NOx relatively low, which reduces the environmental pressure to a certain extent. However, in order to further reduce NOx emissions, CFB boilers also need to adopt more advanced denitrification technology. Selective non-catalytic reduction (SNCR) denitrification technology is one of them. By setting a denitrification nozzle at the inlet of the separator, the reducing agent (such as urea solution) is sprayed into the flue gas to react chemically with NOx, thereby effectively controlling NOx emissions.

[0004] In addition to SNCR technology, optimizing combustion control is also an important means to reduce the original NOx emissions of CFB boilers. For example, the low bed pressure drop energy-saving CFB boiler technology based on fluid reconstruction can improve the combustion conditions and reducing atmosphere distribution in the furnace by increasing the proportion of fine particle bed material and optimizing the bed material inventory, and further enhance the low nitrogen emission characteristics. At the same time, this technology has also adopted a series of optimization measures, such as optimizing the proportion of primary and secondary air, increasing the position of secondary air ports, implementing flue gas recirculation, and controlling the excess air coefficient at a lower level. These measures are all helpful to further reduce NOx emissions.

[0005] In addition, optimizing the low-nitrogen combustion of CFB boilers is also an effective way to reduce NOx emissions. The transformation mainly includes the transformation of the secondary air system, the transformation of key components (such as hoods and separators), and the transformation of the heating surface. These transformations are mainly based on the principles of reducing the furnace temperature, redistributing the combustion air, and increasing the ash concentration in the furnace, aiming to further optimize the combustion process in the furnace and reduce the generation of NOx. By adopting advanced denitrification technology, optimizing combustion control, and optimizing low-nitrogen combustion, CFB boilers can further reduce NOx emissions to meet more stringent environmental protection requirements. These efforts not only help protect the environment and improve air quality, but also promote the sustainable development of coal-fired power plants.

[0006] In summary, the traditional boiler low-nitrogen combustion transformation method mainly focuses on optimizing the internal structure of the furnace. This strategy does help to reduce the original NOx emissions to a certain extent. By improving the burner design, adjusting the size or shape of the furnace, and optimizing the airflow distribution in the furnace, the formation of high-temperature and oxygen-rich areas is reduced, thereby inhibiting the generation of NOx. However, these methods will inevitably interfere with the original combustion and heat exchange process in practical applications.

[0007] Specifically, changes in the internal structure of the furnace may affect the stability of the airflow in the furnace, leading to problems such as incomplete combustion or flame deviation, which may not only increase the content of unburned carbon, but also affect the thermal efficiency and output of the boiler. At the same time, interference with the heat exchange process may cause dust accumulation, wear or corrosion on the heating surface, thereby shortening the service life of the boiler and increasing maintenance costs.

[0008] In addition, the traditional method of low-nitrogen combustion transformation of boilers has many disadvantages. First, the construction period is often long and requires shutdown for transformation, which will cause inconvenience to the normal operation of the power plant and may cause interruption or reduction of power supply. Secondly, the investment cost required for the transformation is usually large, including design costs, material costs, construction costs, and subsequent commissioning and maintenance costs, which may increase the operating cost and economic burden of the power plant. In addition, some transformation methods may reduce the thermal efficiency of the boiler, leading to an increase in fuel consumption, which further increases the operating cost.

[0009] Therefore, when carrying out low-nitrogen combustion transformation of boilers, it is necessary to comprehensively consider multiple aspects such as the transformation effect, construction difficulty, investment cost, and the impact on boiler operation and efficiency, in order to develop a transformation plan that is both economical and effective. Summary of the invention

[0010] To solve the above problems, the present invention proposes a low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed, which cleverly avoids direct impact on the combustion process inside the furnace. By optimizing the design of the external structure of the furnace, the fuel combustion process is effectively improved, thereby achieving the purpose of reducing boiler nitrogen oxide emissions. At the same time, the combustion efficiency is also significantly improved, providing a new entry point and idea for boiler low-nitrogen transformation.

[0011] To achieve the above-mentioned objectives, on the one hand, the present invention provides a low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed, comprising a boiler furnace, a return pipe, an external bed and a coal feeding pipe; the external bed is connected to the boiler furnace through the return pipe, a coal feeding port is arranged in the external bed, and the outlet of the coal feeding pipe is connected to the coal feeding port; an external bed fluidizing air system is arranged at the bottom of the external bed.

[0012] Furthermore, part of the boiler's coal or all of the pulverized coal is fed into the external bed through a coal feeding pipe.

[0013] Furthermore, the external bed fluidizing air system is connected to the fluidizing air system of the boiler furnace; air is sent into the external bed through the external bed fluidizing air system.

[0014] Furthermore, an expansion fan is provided in the external bed fluidizing air system.

[0015] Furthermore, at least one coal feeding pipe is provided. When more than one coal feeding pipe is provided, they are located in different chambers of the external bed.

[0016] Furthermore, the ambient temperature in the external bed is 700-1000°C.

[0017] Furthermore, the material return device is arranged below the separator, the external bed is arranged at the bottom of the separator, and the external bed is connected to the material return device.

[0018] Furthermore, pressure and temperature monitoring devices are provided in each chamber of the external bed, and the pressure and temperature monitoring devices are connected to the control system of the coal-fired unit.

[0019] On the other hand, the present invention provides an operating method of the low-nitrogen combustion system of the circulating fluidized bed boiler with an external bed. After the boiler coal enters the external bed, it undergoes a combustion reaction with the fluidized air in the external bed. The flue gas generated by the combustion of the coal in the external bed contains CO, and the flue gas is in a reducing atmosphere. CO enters the boiler furnace together with the coal and circulating material particles through a return pipe. The excess air coefficient in the boiler furnace is greater than 1.1, and it is an oxidizing atmosphere. The combustion of the coal in the furnace produces a large amount of NO, and CO reacts with the NO generated in the furnace to reduce the NO to produce N2.

[0020] Furthermore, as the coal feed rate increases, the fluidizing air volume in the external bed increases.

[0021] Furthermore, when the load is not higher than 50% of the boiler's continuous maximum output, all the coal feed enters the external bed through the coal feed pipe; when the load is higher than 50% of the boiler's continuous maximum output, the coal feed exceeding 50% enters the boiler furnace through the original coal feed system.

[0022] Compared with the prior art, the present invention has at least the following beneficial effects: there is no need to lower the furnace temperature, which has little effect on the combustion reaction in the furnace and improves the thermal efficiency of the boiler; a large amount of denitrification reducing agent is generated by the coal itself, which reduces the amount of denitrification reducing agent (ammonia water, urea or ammonia gas, etc.); the coal stays in the external bed for a long time, which promotes the incomplete combustion reaction of the coal and increases the CO release, cleverly avoiding direct impact on the combustion process inside the furnace; and by optimizing the design of the external structure of the furnace, effectively improving the fuel combustion process, thereby achieving the purpose of reducing boiler nitrogen oxide emissions, and also significantly improving the combustion efficiency, providing a new entry point and idea for boiler low-nitrogen transformation, which is worthy of further in-depth research and promotion and application.

[0023] Furthermore, under low load, all the coal enters the external bed through the coal feeding pipe. Under high load, the coal feed exceeding the set amount enters the boiler furnace through the original coal feeding system, which can fully utilize the external bed to generate CO, and the CO then enters the boiler furnace to participate in the redox reaction, effectively reducing nitrogen oxide emissions in all load ranges.

[0024] Furthermore, the air volume entering the external bed can be adjusted by controlling the fan in the fluidized air system of the external bed, and the capacity of the fan can be expanded to increase the amount of CO precipitation and thus control the amount of CO generated in the external bed.

[0025] Furthermore, the external bed fluidizing air system is connected to the fluidizing air system of the boiler furnace so that the air volume between the two can be adjusted and supplemented to each other; the external bed fluidizing air system is connected to the fluidizing air system of the furnace through pipes and valves to form a complete fluidizing air system; the air is pressurized by the fan of the external bed fluidizing air system and then sent into the external bed through a pipe, where the air is mixed with the solid particles to form a fluidized state, which is helpful for cooling and separation of the solid particles.

[0026] Furthermore, by arranging coal feeding pipes in different chambers of the external bed, precise control and distribution of the fuel can be achieved, which can adapt to different load requirements and fuel types and improve combustion efficiency.

[0027] Furthermore, as the coal feed rate increases, the fluidizing air volume in the external bed increases, so that the material is in a stable fluidized state, which helps maintain a normal operating temperature and generates a reducing atmosphere in the external bed. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the overall structure of the present invention.

[0029] In the figure, 1- boiler furnace, 2- return pipe, 3- external bed fluidizing air system, 4- external bed, 5- return device, 6- coal feeding pipe, 7- separator. DETAILED DESCRIPTION

[0030] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the new invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0032] The external bed is mainly used to transfer part of the heat carried by the circulating ash to the heating surface, and it also has the function of circulating ash return; it effectively increases the combustion area of ​​the boiler, improves the boiler's heat production capacity, and ensures the smooth operation of the boiler under high load; the external bed can separate the carbon and oxygen in the exhaust gas during the combustion process, so as to achieve the purpose of improving combustion efficiency; the installation of an external bed helps to reduce NOx emissions and achieve environmental protection and energy saving effects; the ash flow entering the external bed is controllable, and the external bed also has the function of adjusting the combustion chamber temperature and the superheater / reheater steam temperature, especially under low load and variable load conditions.

[0033] The external bed adopts a steel plate structure and is lined with refractory materials. Part of the high-temperature ash separated by the high-temperature separator enters the combustion chamber, and the other part is sent to the external bed. In the external bed, the circulating ash exchanges heat with the heating surface before entering the combustion chamber.

[0034] Embodiment 1, as Figure 1As shown, a circulating fluidized bed boiler low nitrogen transformation system with an external bed comprises a boiler furnace 1, a return pipe 2, an external bed fluidized air system 3, an external bed 4, a return device 5, a coal feeding pipe 6, and a separator 7; the external bed 4 is connected to the coal feeding pipe 6, the lower part of the external bed 4 is connected to the external bed fluidized air system 3, the external bed 4 is connected to the furnace through the return pipe 2, the top flue gas outlet of the boiler furnace 1 is connected to the separator 7, the return device 5 and the external bed 4 are arranged below the separator 7, and the boiler The coal feeding port of the furnace is moved to the external bed 4; the specific operation is: a coal feeding port is added to the external bed 4 of the circulating fluidized bed boiler, part of the boiler's coal or all of the coal powder is fed into the external bed 4 through the coal feeding pipe 6, and air is fed into the external bed 4 through the external bed fluidizing air system 3. In the external bed 4, the coal powder and air react to generate a large amount of CO, and the CO is fed into the furnace 1 through the return pipe 2 along with the circulating material particles and the coal powder, and reacts with the NO generated in the furnace to generate N2, thereby reducing the content of nitrogen oxides in the flue gas.

[0035] The returner 5 is arranged below the separator 7, the external bed 4 is arranged at the bottom of the separator 7, and the external bed 4 is connected to the returner 5; pressure and temperature monitoring devices are arranged in each chamber of the external bed 4, and the pressure and temperature monitoring devices are connected to the control system of the coal-fired unit; the high-temperature circulating ash separated in the separator 7 can smoothly fall into the returner 5, and at the same time, the external bed 4 is not only closely connected with the separator 7, but also connected with the returner 5, so that after the circulating ash is separated in the separator 7, it can be buffered and adjusted by the returner 5, and then sent to the external bed 4 for further cooling and treatment; pressure and temperature monitoring devices are installed in each chamber of the external bed 4. Real-time monitoring of pressure and temperature changes in the chamber can timely discover and deal with any problems that may affect the normal operation of the external bed, realize refined control, and thus ensure the stability and safety of the entire coal-fired unit.

[0036] What is more advanced is that these pressure and temperature monitoring devices are connected to the coal-fired unit control system. This means that once any abnormal parameters or deviations from the preset values ​​are detected, the control system will immediately receive an alarm and automatically take corresponding adjustment measures. This intelligent control method not only improves the automation level of the coal-fired unit, but also greatly enhances its ability to respond to emergencies.

[0037] An ash distribution chamber, a high-temperature chamber and a low-temperature chamber are arranged in the external bed. The ash distribution chamber, the high-temperature chamber and the low-temperature chamber are internally arranged with buried heating surface tube bundles, such as superheaters, reheaters or evaporating heating surfaces; the superheaters, reheaters or evaporating heating surfaces are arranged according to the different temperatures of the ash to achieve effective heat transfer; the coal feeding pipe 6 of the present invention can be located in different chambers of the external bed. By setting the coal feeding pipes in different chambers, the graded feeding and flexible adjustment of coal powder can be achieved to meet the combustion requirements of the boiler under different working conditions; when the coal feeding pipe 6 is set in the high-temperature chamber, more consideration is given to using high-temperature resistant materials; when the coal feeding pipe 6 is set in the low-temperature chamber, more consideration is given to using anti-corrosion materials.

[0038] By increasing the coal supply in the external bed 4, the preheating and combustion functions of the external bed can be fully utilized to improve the combustion efficiency of the coal powder. The coal supply amount is adjusted according to the boiler load, coal quality and combustion conditions. After increasing the coal supply in the external bed, the primary air volume, secondary air volume and bed temperature need to be adjusted to ensure that the coal powder can be fully burned.

[0039] When the external bed 4 is fed with more coal, the operating temperature directly determines the heat load and combustion effect of the entire boiler. After increasing the amount of coal fed, the bed temperature may rise. It is necessary to adjust the amount of coal fed, the amount of primary air and the amount of secondary air, and control the amount of circulating ash to adjust the bed temperature. According to the changes in load and coal quality, the amount of coal fed should be adjusted in time to ensure that the bed temperature is stable at 700~1000℃. The primary air volume is used to keep the material in a good fluidized state and provide some oxygen for fuel combustion; the secondary air is used to supplement the air required for combustion and disturb the combustion in the upper part of the furnace to promote the full combustion of the fuel; by adjusting the size of the return air, the amount of circulating ash can be controlled, thereby further adjusting the bed temperature.

[0040] At the same time, the bed pressure can reflect the height of the external bed layer and the fluidization state of the material. After increasing the coal feed, the bed pressure can be controlled by continuous or regular slag discharge to keep the bed pressure within the allowable range; during operation, the changes in bed pressure are monitored in real time. The air volume of the external bed fluidization air system can be achieved by adjusting the speed of the fan and the valve on the pipeline; the air volume can be adjusted in real time according to parameters such as the temperature and pressure in the external bed, as well as the load changes of the boiler; the air pressure can be stabilized by adjusting the fan inlet baffle and balancing the output between the running fans.

[0041] When the boiler furnace is operating, the circulating material is used to transfer mass and heat, bringing heat to the entire furnace, reducing the temperature gradient between the upper and lower parts of the furnace, and increasing the range of load regulation; the load is adjusted by changing the coal feed rate and the amount of circulating ash; an increase in the coal feed rate can increase the thermal load of the boiler, while an increase in the amount of circulating ash can enhance the heat transfer effect of the boiler.

[0042] A flow monitoring device is provided in the coal feeding pipe 6, and a wind pressure and air volume monitoring device is provided in the external bed fluidized air system 3 to monitor the coal feeding amount and air volume entering the external bed in real time to ensure that the coal feeding amount and air volume match and meet the combustion requirements of the boiler. A vibrator for anti-blocking is provided at the coal feeding pipe 6, or a purge air duct is connected.

[0043] An over-temperature protection device may also be provided in the coal feeding area of ​​the external bed 4, for example, a water spray cooling device or an emergency shutdown device.

[0044] The coal feeding pipe 6 is arranged in the external bed 4, and the number may be one or more. Optionally, the coal is fed into the coal dropping pipe by a coal feeding device, such as a belt-type, buried scraper-type or screw conveyor, and then falls into the external bed 4 based on the gravity of the coal itself; at the same time, an external bed fluidizing air system 3 is also arranged in the external bed 4; The amount of air entering the external bed 4 can be adjusted by controlling the fan in the external bed fluidizing air system 3, thereby controlling the amount of CO generated in the external bed 4.

[0045] Embodiment 2 provides an operation method of the circulating fluidized bed boiler low-nitrogen combustion system with an external bed, wherein the coal enters the boiler coal bunker through the coal conveying system, and a branch is branched from the coal bunker to the coal feeding port of the external bed; the ambient temperature in the external bed 4 is 700-1000°C, and the boiler coal enters the external bed 4 and reacts with the fluidized air in the external bed 4 to cause a combustion reaction. Since the amount of fluidized air in the external bed is small, the excess air coefficient in the external bed is much less than 1, and the flue gas generated by the combustion of the coal in the external bed 4 contains a large amount of CO, and the flue gas is a reducing atmosphere. Subsequently, a large amount of CO generated by the combustion in the external bed 4 enters the boiler furnace 1 together with the coal and the circulating material particles through the return pipe 2. The excess air coefficient in the boiler furnace 1 is greater than 1.1, and it is an oxidizing atmosphere, and the combustion of the coal in the furnace produces a large amount of NO. As the CO generated in the external bed enters the furnace, CO will react with the NO generated in the furnace to reduce NO to N2.

[0046] 2CO+2NO=2CO2+N2 Compared with the coal feed to the boiler, the fluidizing air volume in the external bed is smaller. When all the boiler coal is fed into the boiler through the coal feed port in the external bed, the external bed will always be in a reducing atmosphere, and as the coal feed increases, the fluidizing air volume in the external bed also increases, and more carbon monoxide will be released in the external bed.

[0047] As a further optimization, when the load is not higher than 50% of the boiler's continuous maximum output, all the coal feed enters the external bed through the coal feed pipe 6; when the load is higher than 50% of the boiler's continuous maximum output, the coal feed exceeding 50% enters the boiler furnace 1 through the original coal feed system.

[0048] Based on the scheme described in the present invention, a circulating fluidized bed coal-fired boiler without an external bed can be transformed into a circulating fluidized bed coal-fired boiler with an external bed 4 having a coal feeding pipe 6; the inventive concept is used to transform existing coal-fired units, and only the external structure of the furnace is changed, and the operating parameters are appropriately changed, so as to achieve the goal of energy conservation and emission reduction.

[0049] In summary, the present invention provides a low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed, comprising a boiler furnace, a return pipe, an external bed and a coal feeding pipe; the external bed is connected to the boiler furnace through the return pipe, a coal feeding port is arranged in the external bed, and the outlet of the coal feeding pipe is connected to the coal feeding port; an external bed fluidizing air system is arranged at the bottom of the external bed; after the boiler coal enters the external bed, it undergoes combustion reaction with the fluidizing air in the external bed, the flue gas generated by the combustion of the coal in the external bed contains CO, and the flue gas is in a reducing atmosphere, CO enters the boiler furnace together with the coal and circulating material particles through the return pipe, and is in an oxidizing atmosphere, the combustion of the coal in the furnace produces a large amount of NO, CO reacts with the NO generated in the furnace, and reduces the NO to generate N2, thereby reducing the emission of nitrogen oxides, and by optimizing the design of the external structure of the furnace, the fuel combustion process is effectively improved, thereby achieving the purpose of reducing the emission of nitrogen oxides in the boiler, and at the same time significantly improving the combustion efficiency.

[0050] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed, characterized in that: The invention comprises a boiler furnace (1), a return pipe (2), an external bed (4) and a coal feeding pipe (6); the external bed (4) is connected to the boiler furnace (1) via the return pipe (2); a coal feeding port is provided in the external bed (4); and an outlet of the coal feeding pipe (6) is connected to the coal feeding port; and an external bed fluidizing air system (3) is provided at the bottom of the external bed (4).

2. The low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed according to claim 1, characterized in that: Part of the boiler's fuel coal or all of the pulverized coal is fed into the external bed (4) via a coal feeding pipe (6).

3. The low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed according to claim 1, characterized in that: The external bed fluidizing air system (3) is connected to the fluidizing air system of the boiler furnace (1); air is sent into the external bed (4) through the external bed fluidizing air system (3).

4. The low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed according to claim 3, characterized in that: An expansion fan is provided in the external bed fluidizing air system (3).

5. The low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed according to claim 1, characterized in that: At least one coal feeding pipe (6) is provided. When more than one coal feeding pipe (6) is provided, they are located in different chambers of the external bed (4).

6. The low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed according to claim 1, characterized in that: The ambient temperature in the external bed (4) is 700-1000°C.

7. The low-nitrogen combustion system for a circulating fluidized bed boiler with an external bed according to claim 1, characterized in that: The return device (5) is arranged below the separator (7), the external bed (4) is arranged at the bottom of the separator (7), and the external bed (4) is connected to the return device (5); pressure and temperature monitoring devices are arranged in each chamber of the external bed (4), and the pressure and temperature monitoring devices are connected to the control system of the coal-fired unit.

8. The method for operating a low-nitrogen combustion system of a circulating fluidized bed boiler with an external bed according to any one of claims 1 to 7, characterized in that: After the boiler coal enters the external bed (4), it undergoes a combustion reaction with the fluidized air in the external bed (4). The flue gas generated by the combustion of the coal in the external bed (4) contains CO, and the flue gas is in a reducing atmosphere. CO enters the boiler furnace (1) along with the coal and the circulating material particles through the return pipe (2). The boiler furnace (1) is in an oxidizing atmosphere. The combustion of the coal in the furnace produces a large amount of NO. CO reacts with the NO generated in the furnace to reduce the NO to generate N2.

9. The operating method according to claim 8, characterized in that: As the coal feed rate increases, the fluidizing air volume in the external bed (4) increases.

10. The operating method according to claim 8, characterized in that: When the load is not higher than 50% of the continuous maximum output of the boiler, all the coal feed enters the external bed (4) through the coal feed pipe (6); when the load is higher than 50% of the continuous maximum output of the boiler, the coal feed exceeding 50% enters the boiler furnace (1) through the original coal feed system.

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