CN106642089A - Combustion method and combustion device for desulfuration and denitration of circulating fluid bed - Google Patents

Abstract

A combustion method and combustion device for desulfurization and denitrification in a circulating fluidized bed, wherein the combustion method includes the steps of: feeding fuel, a desulfurizer, and a combustion air volume higher than the theoretical combustion air volume but lower than the conventional combustion air volume into the lower area of the circulating fluidized bed furnace Combustion wind; the gas-solid mixture produced by combustion in the circulating fluidized bed furnace enters the cyclone separator from the upper part of the furnace, the particles in the gas-solid mixture are separated and sent back to the circulating fluidized bed furnace, and the flue gas in the gas-solid mixture Pass through the center tube at the top of the cyclone separator, and then flow into the tail flue through the outlet flue; and at the height of the circulating fluidized bed furnace from 2.5 times to 4 times the fluidization velocity value of the air distribution plate, the upper part of the furnace and/or cyclone separation The inlet section of the device is provided with a supplementary combustion air nozzle, and the supplementary combustion air is introduced into the supplementary combustion air nozzle. The combustion device of the present invention has the characteristics of simple system, low engineering investment cost, small occupied area and the like.

Description

Combustion method and device for circulating fluidized bed desulfurization and denitrification

technical field

The invention relates to a combustion method and device for a circulating fluidized bed boiler, in particular to a combustion method and device for combined desulfurization and denitrification in a circulating fluidized bed boiler.

Background technique

Sulfur dioxide and nitrogen oxides (NOx) are important air pollutants and important substances that cause acid rain and photochemical pollution. When sulfur dioxide enters the atmosphere and reaches a certain concentration, it will cause harm to humans, animals and plants; more importantly, when the relative humidity is high and there are particulate matter, catalytic oxidation reactions will occur to generate SO ₃ and sulfuric acid mist, which is 10 times more toxic than sulfur dioxide times. The total amount of nitrogen oxide emissions from coal-fired boilers is less than that of sulfur dioxide, but its potential harm to the environment is more serious than that of sulfur dioxide. Among them, NO and NO ₂ are the main components of air pollution, which can cause human poisoning, plant damage, Acid rain, acid mist, etc., and hydrocarbons form photochemical smog, causing damage to the ozone layer. Therefore, it is very important to control the emissions of sulfur dioxide and nitrogen oxides from coal-fired boilers in thermal power plants. China stipulates that newly built, rebuilt and expanded coal-fired power plants must meet and meet the national emission standards for environmental protection; the latest "Emission Standards of Air Pollutants for Thermal Power Plants (GB-13223-2011)" requires that all thermal power plant boilers must implement SO ₂ and NO _x are all lower than the emission standard of 100mg/Nm ³ , but most of the original sulfur dioxide and nitrogen oxide emission concentrations of circulating fluidized bed boilers cannot directly meet the standards. Therefore, the circulating fluidized bed boiler is faced with the problem that it is necessary to further reduce the emission of sulfur dioxide and nitrogen oxides.

At present, there are mainly three types of desulfurization technologies to control the sulfur dioxide produced by coal combustion, namely desulfurization before combustion, desulfurization during combustion and flue gas desulfurization. Flue gas desulfurization is currently the most effective and widely used technology for controlling SO ₂ gas emissions from coal-fired power plants. According to the state of desulfurization agent and desulfurization reactants, flue gas desulfurization can be divided into three categories: wet method, dry method and semi-dry method. The wet desulfurization process is mature and efficient, but the investment and operation costs are expensive; the dry process has the advantages of low investment, simple equipment and easy product handling, but has the problems of low removal efficiency and absorbent utilization. The desulfurization product of semi-dry technology is in a dry state, which is easy to handle, but the system is prone to fouling and clogging, and special equipment is required for the preparation of absorbent, which requires a large investment cost, and the desulfurization efficiency and utilization rate of the absorbent are not as high as those of the wet method. .

The technologies for controlling NOx emissions of circulating fluidized bed boilers can be divided into two categories: one is to reduce the amount of NOx generated in the combustion process through various technical means, that is, low NOx combustion technology; Gas removal, that is, flue gas denitrification technology. The flue gas denitrification technology is complicated, and the investment and operation costs are high, which undoubtedly reduces the market competitiveness of circulating fluidized bed boilers. Traditional low-NOx combustion technologies mainly include low-oxygen combustion, air staged combustion in the furnace, fuel staged combustion, and flue gas recirculation.

In order to meet the requirements of the national environmental protection standards for the emission of air pollutants, the mature technology for the control of nitrogen oxides and sulfur dioxide in large-scale thermal power boilers is to use selective catalytic reduction (SCR) technology or selective non-catalytic reduction (SNCR) technology combined with tail desulfurization. , which is a hierarchical governance method. This method not only occupies a large area, but also has high investment and operating costs, making it difficult to be widely used. Moreover, since the optimum operating temperature of SCR is around 350-450°C, there is still the problem of flue gas reheating after desulfurization. If not run properly, elevated _SO2 levels will poison the SCR catalyst. There have been many related studies at home and abroad, but most of them have technical and economic deficiencies, making it difficult to develop a more practical technology. Therefore, it is the general trend of flue gas purification technology research to develop cheap and efficient new technologies and new equipment that can desulfurize and denitrify at the same time.

Chinese patent application 201310539752.0 proposes a multi-stage, high-efficiency, low-nitrogen combustion method and combustion system for a circulating fluidized bed boiler. Dispersed and even combustion in the furnace. However, this method can only reduce the generation of nitrogen oxides and cannot reduce the emission of sulfur compounds. In addition, Chinese patent application 201310723729.7 provides a combustion method for circulating fluidized bed boilers with low nitrogen oxide emissions. The outlets of the secondary air ducts are set in upper, middle and lower layers, and the range of each layer is limited to a certain height. At the same time, reduce the primary air volume and secondary air volume to keep the oxygen content of the flue gas at 2% to 3%. At the same time, this method has many other requirements, such as keeping the temperature of the bed material low, the thickness of the bed material Keep the material level low, keep the air pressure in the air chamber low, etc. Although this method can effectively reduce the emission of nitrogen oxides, it still cannot meet the requirements of removing sulfur compounds in flue gas at the same time.

Contents of the invention

The object of the present invention is to address the above-mentioned deficiencies, and propose a combustion method and device for simultaneous desulfurization and denitrification in a circulating fluidized bed boiler furnace.

According to one aspect of the present invention, there is provided a combustion method for circulating fluidized bed desulfurization and denitrification, the steps are:

a) Fuel, desulfurizer and combustion air higher than the theoretical combustion air volume but lower than the conventional combustion air volume are fed into the lower area of the circulating fluidized bed furnace to keep the atmosphere in the lower area of the circulating fluidized bed furnace at a reducing level Atmosphere, in which part of the combustion air is passed through the air distribution plate at the bottom of the furnace;

b) The gas-solid mixture produced by combustion in the circulating fluidized bed furnace enters the cyclone separator from the upper part of the furnace, the particles in the gas-solid mixture are separated and sent back to the circulating fluidized bed furnace, and the flue gas in the gas-solid mixture is separated by cyclone The central tube at the top of the device, and then flows into the tail flue through the outlet flue; and

c) At the height of 2.5 to 4 times the fluidization velocity value between the circulating fluidized bed furnace and the air distribution plate, the upper part of the furnace and/or the inlet section of the cyclone separator is provided with a supplementary combustion air nozzle, and the supplementary combustion air nozzle is introduced into the Supplementary combustion air is used to ensure that the sulfur dioxide in the flue gas and the desulfurizer can fully react, and the fluidization velocity unit: length unit/s.

According to a specific embodiment, after step c), step d) is also included: a supplementary combustion air nozzle is also provided outside the center tube and/or the outlet flue, and the supplementary combustion air is introduced to make the carbon monoxide and fly ash in the flue gas Combustibles burn completely.

According to a specific embodiment, in the step b), the air volume for combustion passed into the lower part of the circulating fluidized bed furnace is such that the excess air ratio of the furnace in the lower area is 1.05-1.08.

According to a specific embodiment, the total amount of supplementary combustion air in step c) and step d) accounts for 10%-15% of the total air input into the combustion device.

According to a specific embodiment, the total amount of supplementary combustion air in step c) and step d) makes the total excess air ratio in the combustion device reach 1.15-1.2.

According to a specific embodiment, the air for supplementary combustion in step c) and step d) is provided through a wind collecting box.

According to another aspect of the present invention, a circulating fluidized bed combined desulfurization and denitrification combustion device is provided, including a circulating fluidized bed furnace, a cyclone separator and a tail flue, wherein,

The circulating fluidized bed furnace includes a lower area and an upper area, the lower area opens a first channel for fuel, desulfurizer and combustion air to pass through, and the upper area opens a first channel for the gas-solid mixture produced by combustion to flow out. two channels;

The cyclone separator is communicated with the second channel, and the top of the cyclone separator is provided with a central cylinder for the separated smoke to flow out, and the central cylinder is connected to the tail flue through the outlet flue;

Wherein, at the height of the circulating fluidized bed furnace 2.5 times to 4 times the fluidization velocity value from the air distribution plate, the upper part of the furnace and/or the inlet section of the cyclone separator is provided with a supplementary combustion air nozzle for supplementary combustion. Input, where fluidization velocity unit: length unit/s.

According to a specific embodiment, a post-combustion air nozzle is also provided outside the central tube and/or the outlet flue for the post-combustion air to pass through.

According to a specific embodiment, the combustion device further includes an air collection box connected to each post-combustion air nozzle for providing post-combustion air.

Through the foregoing technical scheme, it can be known that the beneficial effects of the present invention are:

(1) The combustion device of the present invention has the characteristics of simple system, low engineering investment cost, and small footprint, and is suitable for the transformation of existing power plants and industrial boilers and new boilers.

(2) The desulfurizing agent circulates in the fluidized bed for many times, and the gas and solid are strongly turbulently mixed, so the utilization rate of the desulfurizing agent is high. Moreover, the contact time between the desulfurizer and the flue gas is long, so the desulfurization efficiency is high, and the desulfurization products can be comprehensively utilized.

Description of drawings

Fig. 1 is a schematic flowchart of a combustion method for desulfurization and denitrification in a circulating fluidized bed according to an embodiment of the present invention.

Fig. 2 is a schematic flowchart of a combustion method for desulfurization and denitrification in a circulating fluidized bed according to another embodiment of the present invention.

Fig. 3 is a schematic diagram of a combustion device for desulfurization and denitrification in a circulating fluidized bed according to an embodiment of the present invention.

detailed description

In this field, the excess air ratio refers to the ratio of the amount of air actually used during combustion to the amount of air that should be theoretically used when the fuel is fully combusted. Since the mixing of fuel and air in the combustion process cannot achieve the ideal degree of uniformity, when the excess air coefficient is equal to 1, the fuel cannot be completely burned. In the engineering practice of boiler design and operation, the excess air coefficient at the furnace outlet is generally selected as 1.2 or so, so the air consumption when the excess air coefficient is equal to 1 is called the theoretical combustion air volume, and the air consumption when the excess air coefficient is equal to 1.2 is called the conventional combustion air volume. In the present invention, both the air for combustion and the air for post-combustion refer to air.

Among the present invention, the fluidization velocity refers to the commonly used air fluidization velocity in the fluidized bed combustion process, for example, the typical fluidization velocity is 4-6m/s, but the above-mentioned typical fluidization velocity is only an example, and is not used to limit the present invention invention.

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

According to the general inventive concept of the present invention, a combustion method and device for circulating fluidized bed desulfurization and denitrification, wherein, referring to Fig. 1, the combustion method includes steps:

a) Fuel, desulfurizer and combustion air higher than the theoretical combustion air volume but lower than the conventional combustion air volume are fed into the lower area of the circulating fluidized bed furnace to keep the atmosphere in the lower area of the circulating fluidized bed furnace at a reducing level Atmosphere, in which part of the combustion air is passed through the air distribution plate at the bottom of the furnace;

b) The gas-solid mixture produced by combustion in the circulating fluidized bed furnace enters the cyclone separator from the upper part of the furnace, the particles in the gas-solid mixture are separated and sent back to the circulating fluidized bed furnace, and the flue gas in the gas-solid mixture is separated by cyclone The central tube at the top of the device, and then flows into the tail flue through the outlet flue; and

c) At the height of 2.5 to 4 times the fluidization velocity value between the circulating fluidized bed furnace and the air distribution plate, the upper part of the furnace and/or the inlet section of the cyclone separator is provided with a supplementary combustion air nozzle, and the supplementary combustion air nozzle is introduced into the Supplementary combustion air is used to ensure that the sulfur dioxide in the flue gas and the desulfurizer can fully react, and the fluidization velocity unit: length unit/s.

As preferably, as shown in Figure 2, the above-mentioned combustion method also includes step d) after step c): a post-combustion air nozzle is also provided outside the center tube and/or the outlet flue, and the post-combustion air is introduced to make the flue gas The carbon monoxide and fly ash combustibles in the fuel burn completely.

Referring to Fig. 3, the combustion device includes a circulating fluidized bed furnace 1, a cyclone separator 2 and a tail flue 6, wherein:

The circulating fluidized bed furnace 1 includes a lower area and an upper area, the lower area opens a first channel for fuel, desulfurizer and combustion air to pass through, and the upper area opens a channel for the gas-solid mixture produced by combustion to flow out second channel;

The cyclone separator 2 communicates with the second passage, and the top of the cyclone separator 2 is provided with a central cylinder 4 for the separated flue gas to flow out, and the central cylinder is connected to the tail flue 6 through the outlet flue 5;

Wherein, at the height of 2.5 to 4 times the fluidization velocity value between the circulating fluidized bed furnace and the air distribution plate, the upper part of the furnace and/or the inlet section of the cyclone separator are provided with a supplementary combustion air nozzle 7 for supplementary combustion. In, where fluidization velocity unit: length unit/s.

In the following, the present invention will be explained and illustrated by combining the combustion device and the combustion method.

During the combustion of fuel in the circulating fluidized bed, sulfur dioxide and nitrogen oxides (NOx) are formed, while limestone decomposes to release carbon dioxide. Nitrogen oxides are mainly nitric oxide (NO), whose proportion is as high as 95%. Generally, the nitrogen source of NOx generated by coal combustion can be divided into fuel N and thermal N. The fuel nitrogen comes from the N in the fuel, and the thermal N comes from the N in the combustion air. The thermal N is only possible at high temperatures (above 1100°C). Converted to NOx. For the combustion of coal in a circulating fluidized bed, the formation of nitrogen oxides mainly comes from the nitrogen element in coal. Under normal combustion conditions, nitrogen compounds in coal are first pyrolyzed into intermediate products such as HCN and NH ₃ , which are precipitated from coal together with volatile matter, called volatile matter N, and nitrogen remaining in coke is called In the oxidizing atmosphere, the proportion of nitrogen oxides generated from volatile N during coal combustion in circulating fluidized bed boilers can reach 60%-80%, and the nitrogen oxides generated by coke combustion only account for 20%. %-40%.

In the combustion process in the circulating fluidized bed furnace, it is mainly divided into dense phase zone combustion and dilute phase zone combustion. In the dense-phase area of the circulating fluidized bed, when the coal enters the furnace, it undergoes high-temperature pyrolysis in the furnace and precipitates volatile matter. The volatile matter contains sulfur dioxide, and N-containing components such as precursors of nitrogen oxides such as HCN and NH ₃ , volatilize Partial N encounters the oxygen in the combustion-supporting air sent into the furnace, and is quickly oxidized into NO. Because there is not enough excess combustion air in the furnace, a certain concentration of CO is produced during the combustion process. The main reaction equation of the combustion process in the dense phase zone is as follows:

S+O ₂ ->SO ₂

C+O ₂ -> CO ₂ , CO

Fuel-N->NH ₃ , HCN…

NH ₃ , HCN+O ₂ ->NO

Fuel-N+O ₂ ->NO

CaCO ₃ ->CaO+CO ₂

CaO+SO ₂ +1/2O ₂ ->CaSO4

In the dilute phase zone of the circulating fluidized bed, the coke after coal pyrolysis continues to burn with oxygen, and CO ₂ and CO are generated without sufficient excess combustion air. In addition, the NO produced in the dense-phase region is reduced by CO and coke in the flue gas in the dilute-phase region, which is the main way to control nitrogen oxide emissions. The main reaction equation of the combustion process in the dilute phase zone is as follows:

C+O ₂ -> CO ₂ , CO

NO+CO->CO ₂ +N ₂

NO+C->CO ₂ +N ₂

CaO+SO ₂ +1/2O ₂ ->CaSO4

It can be seen that as long as the atmosphere of circulating fluidized bed combustion can be controlled to be a reducing atmosphere, the generation of nitrogen oxides can be suppressed, but if the combustion is always carried out under a reducing atmosphere, it means that the combustion is insufficient and the combustion efficiency is low. The imported limestone cannot achieve the effect of complete desulfurization, and the sulfur dioxide produced by combustion cannot be fully suppressed.

In the embodiment shown in Figure 3, at the height of the circulating fluidized bed furnace 1 from the air distribution plate 2.5 times to 4 times the fluidization velocity value (fluidization velocity unit: length unit/s) and the upper part of the furnace, cyclone separation The device inlet, the central cylinder and the central cylinder outlet flue are provided with a supplementary combustion air nozzle 7, and the supplementary combustion air nozzle 7 and the external passage pass the supplementary combustion air into the furnace 1, the cyclone separator 2 inlet section and the central cylinder 4.

In the process of realizing the combustion method of the embodiment of the present invention, as shown in Fig. 1 and Fig. 2, fuel and desulfurizer are introduced into the lower part of the circulating fluidized bed furnace 1 respectively, and the amount of combustion air is higher than the theoretical combustion air amount but lower than the conventional Combustion air with the amount of combustion air suppresses the nitrogen oxides in the combustion products in the lower part of the circulating fluidized bed furnace 1, and produces flue gas containing sulfur dioxide, a small amount of nitrogen oxides, carbon monoxide and a certain amount of coke particles. Mixture; the gas-solid mixture produced by the circulating fluidized bed furnace 1 enters the cyclone separator 2, and most of the particles in the gas-solid mixture are separated, and are re-sent through the return feeder 3 and the return port on the circulating fluidized bed furnace 1 Returning to the circulating fluidized bed furnace 1, the flue gas entrains a small amount of fine particles into the central tube 4; through the supplementary combustion air nozzle 7 to the furnace 1, it is 2.5 to 4 times the fluidization velocity value from the air distribution plate (fluidization velocity unit: length unit/ s) At the height and/or the upper part of the furnace, and/or the inlet section of the cyclone separator 2, the central tube 4 and/or the outlet flue 5, the air for supplementary combustion is introduced to make the sulfur dioxide, carbon monoxide and fly ash in the flue gas remain Combustible components such as carbon are completely burned.

According to one embodiment of the present invention, the amount of fuel and combustion air passed into the circulating fluidized bed furnace 1 is such that the excess air ratio is 1.05-1.08, so that the lower part of the circulating fluidized bed boiler 1 can be maintained In the reducing atmosphere, it is ensured that most of the combustible components are completely combusted in the furnace, and the remaining combustible components are less, which can be completely burned in the furnace, the cyclone separator inlet section and the central cylinder.

In an exemplary embodiment, the amount of supplemental combustion air introduced into the upper part of the furnace 1 and the inlet section of the cyclone separator 2, the central cylinder 4 and the outlet flue 5 accounts for 10%-15% of the total air volume of the combustion device. %, when the air volume passes through the upper part of the furnace, the inlet section of the cyclone separator, the central tube and the outlet flue, the flow of flue gas is strengthened, and the mixing of flue gas and supplementary combustion air is improved by optimizing the structure and arrangement of the supplementary combustion port , to achieve the purpose of improving reaction and combustion efficiency. To the furnace 1 at a height 2.5 to 4 times the fluidization velocity value (fluidization velocity unit: length unit/s) from the air distribution plate and/or the upper part of the furnace, and/or the inlet section of the cyclone separator 2, the central cylinder 4 and /or the amount of supplementary combustion air passed into the outlet flue 5 makes the excess air ratio in the combustion device reach 1.15-1.2. In this way, the combustible components contained in the gas-solid mixture discharged from the circulating fluidized bed furnace 1 can be fully combusted, thereby ensuring the combustion efficiency of the boiler.

According to a preferred embodiment of the present invention, the circulating fluidized bed furnace is 2.5 times to 4 times the height of the fluidization velocity value (fluidization velocity unit: length unit/s) and the upper part of the furnace through the air collecting box to the circulating fluidized bed furnace from the air distribution plate. The inlet section of the separator, the central cylinder and the outlet flue of the central cylinder are fed with air for supplementary combustion. The air collection box includes: an air inlet connected to the outside, and a flow of air that is 2.5 to 4 times the distance from the circulating fluidized bed furnace to the air distribution plate. The height of the fluidization velocity value (fluidization velocity unit: length unit/s) and the upper part of the furnace, the inlet section of the cyclone separator, the central cylinder and the air outlet connected to the central cylinder outlet flue.

In the combustion method according to the embodiment of the present invention, the circulating fluidized bed is made The combustion in the lower part of the furnace is in a low-oxygen state (the excess air coefficient of conventional combustion is 1.2), forming a reducing atmosphere, but a certain concentration of CO (concentration is 1000-2000ppm) is produced accordingly, and the NO produced by combustion is reduced by the CO in the flue gas , thereby suppressing the emission of nitrogen oxides; and the combination of combustion air volume and supplementary combustion air volume ensures the balance between low emissions of sulfur dioxide and nitrogen oxides in flue gas and combustion efficiency. Further, in the combustion method described in the embodiment of the present invention, the supply position of supplementary combustion air is also set at a fluidization velocity value 2.5 to 4 times from the circulating fluidized bed furnace to the air distribution plate (fluidization velocity unit: length unit/ The height of s) and/or the upper part of the furnace, and/or the inlet section of the cyclone separator provide an oxygen-enriched area for the reaction of sulfur dioxide in the flue gas and circulating ash with the desulfurizer, ensuring sufficient oxygen for the desulfurization reaction in the furnace and reaction time. At the same time, a supplementary combustion port is also set in the central cylinder and/or the outlet flue, which provides enough combustion space and time for the carbon monoxide in the flue gas and the residual carbon in the fly ash, and finally ensures the formation of nitrogen oxides and sulfur dioxide in the furnace at the same time On the basis of control, the unburned combustibles in the flue gas are fully converted to achieve high combustion efficiency.

In addition, since the inlet position of supplementary combustion air is set at a height of 2.5 to 4 times the fluidization velocity value (fluidization velocity unit: length unit/s) from the circulating fluidized bed furnace to the air distribution plate and/or the furnace The upper part, and/or the inlet section of the cyclone separator, the central cylinder and/or the outlet flue, so no additional combustion space (such as the post-combustion chamber) is needed, which saves equipment cost and floor space, and can be passed through small-scale The modification implements the combustion method of the embodiment of the present invention on a conventional circulating fluidized bed combustion device, which expands the application range of the method.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Within the spirit and principles of the present invention, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present invention.

Claims (9)

1. A combustion method for circulating fluidized bed desulfurization and denitrification, characterized in that it comprises steps: a) Fuel, desulfurizer and combustion air higher than the theoretical combustion air volume but lower than the conventional combustion air volume are fed into the lower area of the circulating fluidized bed furnace to keep the atmosphere in the lower area of the circulating fluidized bed furnace at a reducing level Atmosphere, in which part of the combustion air is passed through the air distribution plate at the bottom of the furnace; b) The gas-solid mixture produced by combustion in the circulating fluidized bed furnace enters the cyclone separator from the upper part of the furnace, the particles in the gas-solid mixture are separated and sent back to the circulating fluidized bed furnace, and the flue gas in the gas-solid mixture is separated by cyclone The central tube at the top of the device, and then flows into the tail flue through the outlet flue; and c) At the height of 2.5 to 4 times the fluidization velocity value between the circulating fluidized bed furnace and the air distribution plate, the upper part of the furnace and/or the inlet section of the cyclone separator is provided with a supplementary combustion air nozzle, and the supplementary combustion air nozzle is introduced into the Supplementary combustion air is used to ensure that the sulfur dioxide in the flue gas and the desulfurizer can fully react, and the fluidization velocity unit: length unit/s. 2. The combustion method according to claim 1, characterized in that, after step c), step d) is also included: a supplementary combustion air nozzle is also provided outside the central tube and/or the outlet flue, and the supplementary combustion air is introduced , so that the carbon monoxide and fly ash combustibles in the flue gas can be completely burned. 3. combustion method according to claim 1, is characterized in that, In the step b), the air volume for combustion passed into the lower part of the circulating fluidized bed furnace is such that the excess air ratio of the furnace in the lower area is 1.05-1.08. 4. combustion method according to claim 1, is characterized in that, The total amount of supplementary combustion air in step c) and step d) accounts for 10%-15% of the total air input into the combustion device. 5. combustion method according to claim 1, is characterized in that, The total amount of supplemental combustion air in step c) and step d) makes the total excess air ratio in the combustion device reach 1.15-1.2. 6. The combustion method according to claim 1, characterized in that, The air for supplementary combustion in step c) and step d) is provided through the wind collecting box. 7. A circulating fluidized bed combined desulfurization and denitrification combustion device, comprising a circulating fluidized bed furnace, a cyclone separator and a tail flue, characterized in that: The circulating fluidized bed furnace includes a lower area and an upper area, the lower area opens a first channel for fuel, desulfurizer and combustion air to pass through, and the upper area opens a first channel for the gas-solid mixture produced by combustion to flow out. two channels; The cyclone separator is communicated with the second channel, and the top of the cyclone separator is provided with a central cylinder for the separated smoke to flow out, and the central cylinder is connected to the tail flue through the outlet flue; Wherein, at the height of the circulating fluidized bed furnace 2.5 times to 4 times the fluidization velocity value from the air distribution plate, the upper part of the furnace and/or the inlet section of the cyclone separator is provided with a supplementary combustion air nozzle for supplementary combustion. Input, where fluidization velocity unit: length unit/s. 8. The combustion device according to claim 7, characterized in that a post-combustion air nozzle is also provided outside the central tube and/or the outlet flue for the post-combustion air to flow in. 9. The combustion device according to claim 7, characterized in that, the combustion device further comprises an air collection box connected to each post-combustion air nozzle for providing post-combustion air.