A near-zero emission system and process suitable for circulating fluidized bed boilers

Abstract

The present invention discloses a near-zero emission system and process applicable to a circulating fluidized bed boiler, which is composed of a circulating fluidized bed boiler, an in-furnace desulfurization system, an out-furnace circulating fluidized bed semi-dry desulfurization system, an SNCR denitrification system, an SCR denitrification system and / or a COA denitrification system. The process is characterized in that: firstly, the circulating fluidized bed boiler is modified to improve the boiler load capacity and the in-furnace desulfurization efficiency, and the original NOx emission concentration is reduced to 100 mg / Nm 3 On this basis, the circulating fluidized bed boiler desulfurization adopts the method of in-furnace desulfurization system + out-furnace desulfurization system to achieve the final SO2 emission concentration of less than 5mg / Nm 3 The circulating fluidized bed boiler denitrification adopts SNCR denitrification system + SCR denitrification system and / or COA denitrification system to achieve the final NOx emission concentration of the boiler less than 8mg / Nm 3 , achieving near-zero emissions goals.

Description

Technical Field

[0001] The present invention belongs to the field of chemical industry and environmental management, and specifically relates to a near-zero emission system and process applicable to a circulating fluidized bed boiler. Background Art

[0002] At present, the desulfurization technologies widely used in circulating fluidized bed boilers include in-furnace desulfurization technology and semi-dry desulfurization technology of circulating fluidized bed outside the furnace. In-furnace desulfurization technology is that the sulfur contained in the coal burned by the boiler reacts with the SO2 formed after oxidation with the calcium oxide in the coal ash or with the added limestone, so that it can be directly desulfurized in the furnace. The semi-dry desulfurization technology of circulating fluidized bed outside the furnace is that CaO reacts with SO2 in the flue gas desulfurization device outside the furnace to remove SO2. In order to achieve the near-zero emission standard, if the circulating fluidized bed boiler uses the in-furnace desulfurization technology alone for desulfurization, the desulfurizer consumption is extremely large, the Ca / S is extremely high, even reaching more than 8, and the SO2 emission is prone to fluctuations, and there is a problem of instantaneous exceeding the standard. At the same time, the large amount of desulfurizer added will cause the boiler efficiency to decrease by 1~3 percentage points, and the amount of ash will increase significantly. If the semi-dry desulfurization technology of circulating fluidized bed outside the furnace is used alone for desulfurization, the consumption of desulfurizer will be large, Ca / S will be high, even reaching more than 2.5, and the SO2 emission capacity will easily fluctuate, resulting in the problem of instantaneous exceeding the standard.

[0003] At present, the denitrification technologies widely used in circulating fluidized bed boilers are selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). The SCR method refers to the reaction of nitrogen oxides in flue gas with denitrification agents (NH3 or urea) under the action of catalysts to produce non-toxic and non-polluting N2 and H2O. Due to the presence of catalysts, the SCR method has high denitrification reaction efficiency, but this method has strict restrictions on the catalyst reaction temperature. The SNCR method does not require the use of catalysts and can reduce nitrogen oxides in the range of 850℃~1000℃. Within this temperature range, the denitrification agent (NH3 or urea) reacts with nitrogen oxides in flue gas to produce N2 and H2O. The SNCR method has a simple system and does not require a catalyst, but it also has strict restrictions on the reaction temperature. In order to achieve near-zero emission standards, if the circulating fluidized bed boiler uses SNCR denitrification technology alone for denitrification, the consumption of denitrification agents will be extremely large, the ammonia nitrogen ratio will be extremely high, even reaching more than 4, and NOx emissions will easily fluctuate, resulting in instantaneous exceeding of the standard. At the same time, the large amount of denitrification agent added will lead to a large ammonia escape rate, causing obvious damage to the equipment and the environment. If SCR denitrification technology is used alone for denitrification, there will be a problem of limited space in the tail flue and insufficient layout. Summary of the invention

[0004] The object of the present invention is to provide a near-zero emission system and process suitable for a circulating fluidized bed boiler.

[0005] Based on the above purpose, the present invention adopts the following technical solutions:

[0006] A near-zero emission system suitable for a circulating fluidized bed boiler comprises a circulating fluidized bed boiler, a dust collector, an off-furnace circulating fluidized bed semi-dry desulfurization system and a chimney which are sequentially connected by pipelines. The circulating fluidized bed boiler comprises a furnace, a separator and a tail flue. Furnace outlet flues are arranged on both sides of an upper portion of the furnace, flue gas inlets are arranged on both sides of an upper portion of the separator, the furnace is connected to the flue gas inlet of the separator through the furnace outlet flue, a combustion chamber of the furnace is provided with a limestone inlet, the limestone inlet is provided with an in-furnace desulfurization system for spraying limestone powder into the combustion chamber, an SNCR denitration system is arranged on the furnace outlet flue, a denitrification agent inlet is arranged on the tail flue, an SCR denitration system is arranged outside the tail flue corresponding to the denitrification agent inlet, the dust collector is provided with a flue gas outlet and a dust outlet, the flue gas outlet is connected to the chimney through a pipeline, a fly ash recycling system is arranged on the dust outlet pipeline, the fly ash recycling system is connected to the off-furnace circulating fluidized bed semi-dry desulfurization system through a pipeline, a return device is arranged below the separator, and the return device is connected to the combustion chamber of the furnace through a return pipe.

[0007] Furthermore, a primary superheater, a secondary superheater, a primary economizer, a denitrification catalyst layer, a secondary economizer and an air precooler are sequentially arranged in the tail flue from top to bottom, a denitrification agent inlet is arranged on the tail flue corresponding to the denitrification catalyst layer, a branch is arranged on the pipe between the external circulating fluidized bed semi-dry desulfurization system and the chimney, a flue gas recirculation system is arranged on the branch, and the outlet of the flue gas recirculation system is connected with the tail flue corresponding to the air precooler through the pipe; the cross-sectional area of ​​the furnace outlet flue is 4500mm×1185mm, and the separator is a circular separator.

[0008] Furthermore, a COA desulfurization system is provided between the dust collector and the external circulating fluidized bed semi-dry desulfurization system. The COA desulfurization system consists of a pure oxygen storage bin, an ozone reactor, an ozone distribution module and an ozone injection grid which are sequentially connected through pipelines.

[0009] Furthermore, the in-furnace desulfurization system comprises a limestone storage bin, a desulfurizer conveying module, a desulfurizer distribution module and a desulfurizer injection device, and the desulfurizer distribution module is connected to the desulfurizer injection device through a conveying pipeline.

[0010] Furthermore, the SNCR denitrification system and the SCR denitrification system have the same composition, both including a denitrification agent storage bin, a denitrification agent delivery module, a denitrification agent distribution module and a denitrification agent injection device. The denitrification agent distribution module is connected to the desulfurization agent injection device through a delivery pipeline, and the denitrification agent storage bin is filled with ammonia water.

[0011] Furthermore, the fly ash recycling system comprises a fly ash storage bin, a fly ash conveying module, a fly ash distribution module and a fly ash injection device, and the fly ash distribution module is connected to the fly ash injection device through a conveying pipeline.

[0012] Furthermore, the flue gas recirculation system includes an oxygen-depleted flue gas storage bin, an oxygen-depleted flue gas delivery module, an oxygen-depleted flue gas distribution module and an oxygen-depleted flue gas injection device, and the oxygen-depleted flue gas distribution module is connected to the oxygen-depleted flue gas injection device through a delivery pipeline. The primary economizer and the secondary economizer are both H-type economizers.

[0013] A process for near-zero emission of a circulating fluidized bed boiler using the above system is as follows:

[0014] (1) When the boiler coal is burned in the combustion chamber of the circulating fluidized bed boiler, limestone powder is sprayed through the furnace desulfurization system to initially remove sulfur dioxide from the flue gas after combustion;

[0015] (2) After the flue gas undergoes SNCR denitration in the flue gas duct at the furnace outlet, it enters the separator at a speed of 15-25 m / s. The solid biomass material returns to the combustion chamber of the furnace through the return device and the return pipe, and the flue gas enters the tail flue;

[0016] (3) After passing through the primary superheater, secondary superheater and primary economizer, the flue gas temperature is reduced to 380~420℃, and SCR denitrification is carried out in the denitrification catalyst layer. After SCR denitrification, the flue gas enters the secondary economizer and air precooler and is cooled to 130~150℃;

[0017] (4) The flue gas coming out of the tail flue is dedusted by the dust collector, enters the COA denitrification system for denitrification, and then enters the external circulating fluidized bed semi-dry desulfurization system for desulfurization or directly enters the external circulating fluidized bed semi-dry desulfurization system for desulfurization by calcium hydroxide solution; most of the flue gas desulfurized by the external circulating fluidized bed semi-dry desulfurization system is discharged through the chimney, and a small part returns to the flue through the flue gas recirculation system and then enters the furnace to participate in combustion;

[0018] (5) The fly ash accumulated at the bottom of the dust collector enters the external circulating fluidized bed semi-dry desulfurization system through the fly ash recycling system, so that the temperature in the external circulating fluidized bed semi-dry desulfurization system is maintained at 110~130℃.

[0019] Preferably, the molar ratio of the limestone injection amount to SO2 is ≤1.5, and during the SNCR and SCR denitrification processes, the molar ratio of the denitrification agent injection amount to NOx is ≤1.5.

[0020] This application firstly improves the efficiency of circulating fluidized bed boiler, improves the boiler load capacity and desulfurization efficiency in the furnace, and reduces the original NOx emission concentration to 100mg / Nm 3(6% O2) or less. On this basis, the circulating fluidized bed boiler desulfurization adopts the furnace desulfurization system (the original emission concentration of SO2 is reduced to 500mg / Nm 3 (6% O2) or less) + semi-dry desulfurization system of circulating fluidized bed outside the furnace to achieve the final SO2 emission concentration of less than 5mg / Nm 3 (6% O2), and Ca / S≤1.5. The circulating fluidized bed boiler denitrification adopts SNCR denitrification system + SCR denitrification system and / or COA denitrification system to achieve the final NOx emission concentration of the boiler less than 8mg / Nm 3 (6% O2), and the ammonia nitrogen ratio is ≤1.5. The present invention can flexibly configure the required system according to the current status of the boiler to achieve near-zero emissions of the boiler. It has the advantages of stable system operation, low consumption of denitrification agents and desulfurization agents, and low and stable pollutant emission concentrations. While carrying out near-zero emission transformation of the boiler, it can also simultaneously improve the boiler's load capacity and extend the boiler's operating cycle.

[0021] A small amount of desulfurizer is added in the furnace through the in-furnace desulfurization system to efficiently remove part of SO2; a small amount of denitrifier is added in the furnace through the SNCR denitrification system to efficiently remove part of NOx. The flue gas generated by combustion first passes through the SCR denitrification system at the tail flue of the boiler to further remove NOx, and then passes through the dust collector for dust removal and the semi-dry desulfurization system of the circulating fluidized bed outside the furnace for desulfurization in turn, and finally the qualified flue gas is discharged into the chimney. After the flue gas passes through the dust collector for dust removal, it can also pass through the COA denitrification system to further remove NOx before entering the semi-dry desulfurization system of the circulating fluidized bed outside the furnace for desulfurization.

[0022] Circulating fluidized bed boilers remove sulfur dioxide during the combustion process by adding desulfurizers such as limestone. Due to the low combustion temperature, the nitrogen oxide emissions are also low. In order to adapt to more stringent environmental regulations, this application uses a boiler efficiency improvement device to reduce the original NOx emission concentration of the boiler, and sprays desulfurizers and denitrifiers in different areas of the circulating fluidized bed boiler to further reduce the SO2 and NOx content in the flue gas. In terms of desulfurization, first, the in-furnace desulfurization system sprays a small amount of desulfurizer in the boiler furnace. As long as a lower in-furnace desulfurization efficiency is achieved, the SO2 emission concentration entering the semi-dry desulfurization system of the circulating fluidized bed outside the furnace can be reduced to 500mg / Nm 3 After the flue gas enters the semi-dry desulfurization system of the circulating fluidized bed outside the furnace, since the flue gas still contains a certain amount of desulfurizer and is evenly mixed, only a small amount of water and a very small amount of desulfurizer need to be sprayed into it to reduce the SO2 emission concentration to 5mg / Nm 3 In terms of denitration, a small amount of denitrification agent is first injected into the boiler furnace through the SNCR denitration system. As long as a lower denitration efficiency in the furnace is achieved, the NOx emission concentration entering the SCR denitration system can be reduced to 35mg / Nm 3After the flue gas enters the SCR denitrification system, since the flue gas still contains a certain amount of denitrification agent and is evenly mixed, only a very small amount of denitrification agent needs to be sprayed into it to reduce the NOx emission concentration to 8mg / Nm 3 (6%O2) or less.

[0023] In order to achieve economical and efficient removal, the patented desulfurizer and denitrifier injection device is connected to the flue gas monitoring system, and the injection amount of the desulfurizer and denitrifier is controlled by the flue gas monitoring system. When the SO2 and NOx content in the flue gas increases, the injection amount of the desulfurizer and denitrifier increases accordingly, and when the SO2 and NOx content in the flue gas decreases, the injection amount of the desulfurizer and denitrifier decreases accordingly. At the same time, the desulfurizer and denitrifier injected in different areas can also be controlled in quantity, so as to achieve stable and controllable pollutant control and the lowest operating cost.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. Flexible configuration of required systems according to the current status of the boiler, which can be widely used in new circulating fluidized bed boiler units and the transformation of existing circulating fluidized bed boiler units;

[0026] 2. The pollutant emission concentration is low and stable, achieving near-zero boiler emissions;

[0027] 3. The system operates stably, the consumption of denitrification agent and desulfurization agent is low, and the operating cost is low;

[0028] 4. Improve the boiler's load capacity and extend the boiler's operating cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of the present invention;

[0030] Figure 2 A schematic diagram of the structure of the denitrification system without COA in the present invention;

[0031] Figure 3 It is a flow chart of the furnace desulfurization system of the present invention;

[0032] Figure 4 It is a flow chart of the SNCR denitration system of the present invention;

[0033] Figure 5 This is a flow chart of the SCR denitration system of the present invention;

[0034] Figure 6 It is a flow chart of the COA denitration system of the present invention;

[0035] Figure 7 is a flow chart of the fly ash recycling system of the present invention;

[0036] Figure 8 is a flow chart of the flue gas recirculation system of the present invention;

[0037] In the figure:

[0038] 1. Boiler, 1A. Combustion chamber, 1B. Secondary air duct, 1C. Furnace, 1D. Furnace water cooling screen, 1E. Furnace outlet flue, 1F. Separator, 1G. Return device, 1H. Return pipe, 1I. Tail flue, 1J. Primary superheater, 1K. Secondary superheater, 1L. Primary economizer, 1M. Denitration catalyst, 1N. Secondary economizer, 1P Air precooler, 2. In-furnace desulfurization system, 3. SNCR denitration system, 4. SCR denitration system, 5. Dust collector, 6. COA denitrification system, 7. External circulating fluidized bed semi-dry desulfurization system, 8. Chimney, 9. Fly ash recycling system, 10. Flue gas recycling system, 21. Limestone storage silo, 22. Desulfurizer conveying module, 23. Desulfurizer distribution module, 24. Conveying pipeline, 25. Desulfurizer injection device, 31. Denitrifier storage silo, 32. Denitrifier conveying module, 33. Denitrifier distribution module, 34. Conveying pipeline, 35. Denitrifier injection device, 41. Denitrifier storage silo, 42. Denitrifier conveying module, 43. Denitrifier distribution module, 44. Conveying pipeline, 45. Denitrifier injection device, 61. Pure oxygen storage bin, 62. Ozone generator, 63. Ozone distribution module, 64. Transportation pipeline, 65. Ozone injection grille, 91. Fly ash storage bin 31 (i.e., dust collector 5), 92. Fly ash transportation module, 93. Fly ash distribution module, 94. Transportation pipeline, 95. Fly ash injection device, 101. Oxygen-depleted flue gas storage bin, 102. Oxygen-depleted flue gas transportation module, 103. Oxygen-depleted flue gas distribution module, 104. Transportation pipeline, 105. Oxygen-depleted flue gas injection device. DETAILED DESCRIPTION

[0039] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0040] The injection device in the following embodiments is specifically an injection grid, and the distribution module is used to evenly distribute the amount of desulfurizer or denitrifier, which is an existing conventional equipment. The semi-dry desulfurization system of the external circulating fluidized bed is an existing known mature system.

[0041] Example 1

[0042] A near-zero emission system suitable for circulating fluidized bed boilers, such as Figure 1As shown, it includes a circulating fluidized bed boiler 1, a dust collector 5, an external circulating fluidized bed semi-dry desulfurization system 7 and a chimney 8 which are connected in sequence through pipelines. The circulating fluidized bed boiler 1 includes a furnace 1C, a separator 1F and a tail flue 1I. The lower part of the furnace 1C is a combustion chamber 1A. A secondary air duct 1B is arranged on the side wall of the combustion chamber 1A. A furnace water cooling screen 1D is arranged on the upper part of the furnace 1C. Furnace outlet flues 1E are arranged on both sides of the upper part of the furnace 1C. Flue gas inlets are arranged on both sides of the upper part of the separator 1F. The furnace 1C is connected to the flue gas inlet of the separator 1F through the furnace outlet flue 1E. The combustion chamber 1A of the furnace 1C is provided with a limestone inlet. The inlet is provided with an in-furnace desulfurization system 2 for spraying limestone powder into the combustion chamber 1A, the furnace outlet flue 1E is provided with an SNCR denitration system 3, the tail flue 1I is provided with a denitrification agent inlet, and an SCR denitration system 4 is provided outside the tail flue corresponding to the denitrification agent inlet, the dust collector 5 is provided with a flue gas outlet and a dust outlet, the flue gas outlet is connected to the chimney 8 through a pipeline, and the dust outlet pipeline is provided with a fly ash recycling system 9, which is connected to the furnace external circulating fluidized bed semi-dry desulfurization system 7 through a pipeline, and a return device 1G is provided below the separator 1F, and the return device 1G is connected to the combustion chamber 1A of the furnace 1C through a return pipe 1H. The fly ash captured by the dust collector 5 is introduced into the desulfurization tower of the furnace external circulating fluidized bed semi-dry desulfurization system 7 to increase the fly ash concentration in the desulfurization tower and accelerate the material circulation replacement speed in the desulfurization tower.

[0043] The return material device consists of three high-pressure fans, two in operation and one in standby, to ensure the return material air volume.

[0044] In this embodiment, the tail flue 1I is provided with a primary superheater 1J, a secondary superheater 1K, a primary economizer 1L, a denitration catalyst layer 1M, a secondary economizer 1N and an air precooler 1P from top to bottom. The tail flue corresponding to the denitration catalyst layer 1M is provided with a denitrification agent inlet, and a branch is provided on the pipeline between the semi-dry desulfurization system 7 and the chimney 8 outside the furnace. The branch is provided with a flue gas recirculation system 10, and the outlet of the flue gas recirculation system 10 is connected to the tail flue corresponding to the air precooler 1P through a pipeline; the cross-sectional area of ​​the furnace outlet flue 1E is 4500mm×1185mm, and the separator 1F is a circular separator. The dust collector 5 is a bag dust collector, and the primary economizer 1L and the secondary economizer 1N are both H-type economizers. The three-stage light tube economizer is changed to a two-stage H-type economizer to provide installation space for the SCR denitration catalyst. The denitrification catalyst mainly uses TiO2 as a carrier and V2O5 as the main active ingredient.

[0045] In this embodiment, a COA desulfurization system 6 is further provided between the dust collector 5 and the external circulating fluidized bed semi-dry desulfurization system 7. Figure 6As shown, the COA destocking system 6 comprises a pure oxygen storage bin 61, an ozone reactor 62, an ozone distribution module 63 and an ozone injection grid 65 which are sequentially connected by pipelines, and the ozone distribution module 63 and the ozone injection grid 65 are connected by a delivery pipeline 64. The purity of oxygen in the pure oxygen storage bin 61 is ≥99%.

[0046] In this embodiment, Figure 3 As shown, the in-furnace desulfurization system includes a limestone storage bin 21 , a desulfurizer conveying module 22 , a desulfurizer distribution module 23 and a desulfurizer injection device 25 , and the desulfurizer distribution module 23 is connected to the desulfurizer injection device 25 through a conveying pipeline 24 .

[0047] In this embodiment, the SNCR denitration system 3 and the SCR denitration system 4 have the same composition. Figure 4 As shown, the SNCR denitration system 3 includes a denitration agent storage bin 31, a denitration agent delivery module 32, a denitration agent distribution module 33 and a denitration agent injection device 35. The denitration agent distribution module 33 is connected to the desulfurization agent injection device 35 through a delivery pipeline 34. Figure 5 As shown, the SCR denitrification system 4 includes a denitrification agent storage bin 41, a denitrification agent delivery module 42, a denitrification agent distribution module 43 and a denitrification agent injection device 45. The denitrification agent distribution module 43 is connected to the desulfurization agent injection device 45 through a delivery pipeline 44. The denitrification agent storage bins 31 and 41 are filled with 5wt% ammonia water.

[0048] In this embodiment, Figure 7 As shown, the fly ash recycling system 9 includes a fly ash storage bin 91 (i.e., a dust collector 5), a fly ash conveying module 92, a fly ash distribution module 93, and a fly ash injection device 95. The fly ash distribution module 93 is connected to the fly ash injection device 95 through a conveying pipeline 94. The fly ash injection device 95 is connected to the flue gas inlet of the desulfurization tower of the semi-dry desulfurization system 7 of the circulating fluidized bed outside the furnace, and the original emission concentration of SO2 and NOx is reduced by increasing the amount of water sprayed on the material in the desulfurization tower.

[0049] In this embodiment, Figure 8 As shown, the flue gas recirculation system 10 includes an oxygen-depleted flue gas storage bin 101, an oxygen-depleted flue gas delivery module 102, an oxygen-depleted flue gas distribution module 103 and an oxygen-depleted flue gas injection device 105, and the oxygen-depleted flue gas distribution module 103 is connected to the oxygen-depleted flue gas injection device 105 through a delivery pipeline 104. The purpose of the flue gas recirculation system 10 is to reduce the original emission concentration of SO2 and NOx by reducing the oxygen content in the flue gas.

[0050] A process for near-zero emission of a circulating fluidized bed boiler using the above system is as follows:

[0051] (1) When the boiler coal is burned in the combustion chamber 1A of the circulating fluidized bed boiler, limestone powder is sprayed through the furnace desulfurization system 2 to initially remove sulfur dioxide from the flue gas after combustion;

[0052] (2) After the flue gas undergoes SNCR denitration in the furnace outlet flue 1E, it enters the separator 1F at a speed of 15-25 m / s. The solid biomass material returns to the combustion chamber 1A of the furnace 1C through the return device 1G and the return pipe 1H, and the flue gas enters the tail flue 1I;

[0053] (3) After passing through the primary superheater 1J, the secondary superheater 1K and the primary economizer 1L, the flue gas temperature is reduced to 380~420℃, and SCR denitrification is carried out in the denitrification catalyst layer 1M. After SCR denitrification, the flue gas enters the secondary economizer 1N and the air precooler 1P and is cooled to 130~150℃;

[0054] (4) The flue gas coming out of the tail flue is dedusted by the dust collector 5, enters the COA denitrification system 6 for denitrification, and then enters the external circulating fluidized bed semi-dry desulfurization system 7 for desulfurization by calcium hydroxide solution, or directly enters the external circulating fluidized bed semi-dry desulfurization system 7 for desulfurization by calcium hydroxide solution; most of the flue gas desulfurized by the external circulating fluidized bed semi-dry desulfurization system 7 is discharged through the chimney 8, and a small part is returned to the tail flue through the flue gas recirculation system 10 and then enters the 1C furnace to participate in combustion;

[0055] (5) The fly ash accumulated at the bottom of the dust collector 5 enters the external circulating fluidized bed semi-dry desulfurization system 7 through the fly ash recycling system 9, so that the temperature in the external circulating fluidized bed semi-dry desulfurization system 7 is maintained at 110-130°C.

[0056] Preferably, the molar ratio of the limestone injection amount to SO2 is ≤1.5, and during the SNCR and SCR denitrification processes, the molar ratio of the denitrification agent injection amount to NOx is ≤1.5.

[0057] The boiler reduces the original NOx emission concentration to 100mg / Nm through the improved circulating fluidized bed boiler. 3 The fuel and air are burned in the boiler to generate the required heat. At the same time, a small amount of desulfurizer is added to the furnace through the furnace desulfurization system 2 to efficiently remove part of the SO2, and the SO2 emission concentration is reduced to 500mg / Nm 3 (6% O2) or less; a small amount of denitrification agent is added to the furnace through the SNCR denitrification system 3 to efficiently remove part of the NOx, and the NOx emission concentration is reduced to 35mg / Nm 3 The flue gas generated by combustion first passes through the boiler tail flue SCR denitrification system 4 to further remove NOx, and the NOx emission concentration is reduced to 8mg / Nm 3(6% O2) or less. Then it passes through the dust collector 5 for dust removal and the circulating fluidized bed semi-dry desulfurization system 7 for desulfurization outside the furnace, and the SO2 emission concentration is reduced to 5mg / Nm 3 (6% O2) or less, and the final qualified flue gas is discharged into the chimney 8. After the flue gas is dedusted by the dust collector 5, it is further de-NOxed by the COA denitrification system 6 before entering the semi-dry desulfurization system 7 outside the furnace for desulfurization, and the NOx emission concentration is reduced to 5mg / Nm 3 (6% O2) or less. The entire desulfurization and denitrification system is flexibly switched into operation according to emission requirements and operating costs. The molar ratio of the desulfurization agent injection amount to SO2 is ≤1.5. The molar ratio of the denitrification agent injection amount to NOx is ≤1.5.

[0058] Example 2

[0059] Figure 2 The near-zero emission system for circulating fluidized bed boiler shown in the figure comprises a circulating fluidized bed boiler 1, an in-furnace desulfurization system 2, a SNCR denitrification system 3 and an SCR denitrification system 4, a dust collector 5, an out-furnace circulating fluidized bed semi-dry desulfurization system 7, a chimney 8, a fly ash recycling system 9 and a flue gas recycling system 10. That is, the COA system is omitted (the denitrification cost of the COA system is too high and is generally only used as a backup system), and the rest is the same as in Example 1. The original emission concentration of NOx is reduced to 100 mg / Nm by the boiler efficiency improvement device 3 The fuel and air are burned in the boiler to generate the required heat. At the same time, a small amount of desulfurizer is added to the furnace through the furnace desulfurization system 2 to efficiently remove part of the SO2, and the SO2 emission concentration is reduced to 500mg / Nm 3 (6% O2) or less; a small amount of denitrification agent is added to the furnace through the SNCR denitrification system 3 to efficiently remove part of the NOx, and the NOx emission concentration is reduced to 35mg / Nm 3 The flue gas generated by combustion first passes through the boiler tail flue SCR denitrification system 4 to further remove NOx, and the NOx emission concentration is reduced to 8mg / Nm 3 (6% O2) or less. Then it passes through the dust collector 5 for dust removal, the circulating fluidized bed semi-dry desulfurization system 7 for desulfurization outside the furnace and the fly ash recycling system 9. The SO2 emission concentration is reduced to 5mg / Nm 3 (6% O2) or less, and the final qualified flue gas is discharged into the chimney 8. The whole set of desulfurization and denitrification system is flexibly switched into operation according to emission requirements and operating costs. The molar ratio of desulfurization agent injection to SO2 is ≤1.5. The molar ratio of denitrification agent injection to NOx is ≤1.5.

[0060] The above implementation cases are only used to illustrate the preferred implementation modes of the present invention, but the present invention is not limited to the above implementation modes. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention within the scope of knowledge possessed by ordinary technicians in the field should be deemed to be within the scope of protection of this application.

Claims

1. A near-zero emission system for a circulating fluidized bed boiler, characterized in that: It comprises a circulating fluidized bed boiler, a dust collector, an off-furnace circulating fluidized bed semi-dry desulfurization system and a chimney which are connected in sequence by pipelines. The circulating fluidized bed boiler comprises a furnace, a separator and a tail flue. Furnace outlet flues are arranged on both sides of the upper part of the furnace. Flue gas inlets are arranged on both sides of the upper part of the separator. The furnace is connected to the flue gas inlet of the separator through the furnace outlet flue. A limestone inlet is arranged in the combustion chamber of the furnace. The limestone inlet is provided with an in-furnace desulfurization system for spraying limestone powder into the combustion chamber. An SNCR denitration system is arranged on the furnace outlet flue. A denitrification agent inlet is arranged in the tail flue. An SCR denitration system is arranged outside the tail flue corresponding to the denitrification agent inlet. The dust collector is provided with a flue gas outlet and a dust outlet. The flue gas outlet is connected to the chimney through a pipeline. A fly ash recycling system is arranged on the dust outlet pipeline. The fly ash recycling system is connected to the off-furnace circulating fluidized bed semi-dry desulfurization system through a pipeline. A return device is arranged below the separator. The return device is connected to the combustion chamber of the furnace through a return pipe. The tail flue is provided with a primary superheater, a secondary superheater, a primary economizer, a denitration catalyst layer, a secondary economizer and an air precooler in order from top to bottom; a denitration agent inlet is provided on the tail flue corresponding to the denitration catalyst layer; a branch is provided on the pipeline between the external circulating fluidized bed semi-dry desulfurization system and the chimney; a flue gas recirculation system is provided on the branch; and an outlet of the flue gas recirculation system is connected to the tail flue corresponding to the air precooler through a pipeline; A COA denitration system is also provided between the dust collector and the external circulating fluidized bed semi-dry desulfurization system. The COA denitration system comprises a pure oxygen storage bin, an ozone reactor, an ozone distribution module and an ozone injection grid which are sequentially connected through pipelines.

2. The near-zero emission system for circulating fluidized bed boiler according to claim 1, characterized in that: The cross-sectional area of ​​the furnace outlet flue is 4500 mm×1185 mm, and the separator is a circular separator.

3. The near-zero emission system for circulating fluidized bed boiler according to claim 1, characterized in that: The in-furnace desulfurization system comprises a limestone storage bin, a desulfurizer conveying module, a desulfurizer distribution module and a desulfurizer injection device, and the desulfurizer distribution module is connected to the desulfurizer injection device through a conveying pipeline.

4. The near-zero emission system for circulating fluidized bed boilers according to claim 1, characterized in that: The SNCR denitration system and the SCR denitration system have the same composition, both including a denitration agent storage bin, a denitration agent delivery module, a denitration agent distribution module and a denitration agent injection device. The denitration agent distribution module is connected to the desulfurization agent injection device through a delivery pipeline, and the denitration agent storage bin is filled with ammonia water.

5. The near-zero emission system for circulating fluidized bed boilers according to claim 1 is characterized in that: The fly ash recycling system comprises a fly ash storage bin, a fly ash conveying module, a fly ash distribution module and a fly ash injection device. The fly ash distribution module is connected with the fly ash injection device through a conveying pipeline.

6. The near-zero emission system for circulating fluidized bed boiler according to claim 1, characterized in that: The flue gas recirculation system comprises an oxygen-depleted flue gas storage bin, an oxygen-depleted flue gas conveying module, an oxygen-depleted flue gas distribution module and an oxygen-depleted flue gas injection device. The oxygen-depleted flue gas distribution module is connected to the oxygen-depleted flue gas injection device through a conveying pipeline.

7. The near-zero emission system for circulating fluidized bed boilers according to claim 1, characterized in that: The primary economizer and the secondary economizer are both H-type economizers.

8. A process for near-zero emission of a circulating fluidized bed boiler using the system according to any one of claims 1 to 7, characterized in that: The process is as follows: When the boiler coal is burned in the combustion chamber of the circulating fluidized bed boiler, the sulfur dioxide in the flue gas after combustion is initially removed by spraying limestone powder through the desulfurization system in the furnace; After the flue gas undergoes SNCR denitrification in the flue gas duct at the furnace outlet, it enters the separator at a speed of 15-25 m / s. The solid biomass material returns to the combustion chamber of the furnace through the return device and the return pipe, and the flue gas enters the tail flue. After passing through the primary superheater, secondary superheater and primary economizer, the flue gas temperature is reduced to 380~420℃, and SCR denitrification is carried out in the denitrification catalyst layer. After SCR denitrification, the flue gas enters the secondary economizer and air precooler and is cooled to 130~150℃; The flue gas coming out of the tail flue is dedusted by the dust collector, enters the COA denitrification system for denitrification, and then enters the external circulating fluidized bed semi-dry desulfurization system for desulfurization or directly enters the external circulating fluidized bed semi-dry desulfurization system for desulfurization by calcium hydroxide solution; most of the flue gas desulfurized by the external circulating fluidized bed semi-dry desulfurization system is discharged through the chimney, and a small part returns to the flue through the flue gas recirculation system and then enters the furnace to participate in combustion; The fly ash accumulated at the bottom of the dust collector enters the external circulating fluidized bed semi-dry desulfurization system through the fly ash recycling system, so that the temperature in the external circulating fluidized bed semi-dry desulfurization system is maintained at 110~130℃.

9. The near-zero emission process of a circulating fluidized bed boiler according to claim 8, characterized in that: The molar ratio of limestone injection to SO2 is ≤1.

5. During SNCR and SCR denitrification processes, the molar ratio of denitrification agent injection to NOx is ≤1.5.

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