Optimized flue gas denitration system of circulating fluidized bed boiler

Abstract

The invention provides an optimized circulating fluidized bed boiler flue gas denitration system, which belongs to the technical field of flue gas denitration, and comprises a boiler, a cyclone separator group, a preheater, a dust remover, an induced draft fan, a chimney, a material returning device, an air chamber, an injection system and a flue gas recirculation device, the cyclone separator group comprises a plurality of cyclone separators; the injection system comprises a static mixer, a first spray gun group and a second spray gun group; and the flue gas recirculation device comprises a recirculation fan and a primary fan. Based on the difference between the inlet and the outlet of the cyclone separator, the ultra-low emission requirement is met through SNCR (selective non-catalytic reduction) dynamic partition injection, flue gas recirculation variable load self-adaption and coal quality-denitration advanced linkage; the problem of NOx fluctuation under variable load and variable coal quality is solved.

Description

Technical Field

[0001] This invention relates to the field of flue gas denitrification technology, and more specifically, to an optimized flue gas denitrification system for circulating fluidized bed boilers. Background Technology

[0002] In the current energy structure dominated by coal-fired power generation, circulating fluidized bed (CFB) boilers have become one of the core equipment for power supply due to their wide fuel adaptability and high combustion efficiency. However, the combustion process of CFB boilers produces a large amount of nitrogen oxides (NOx), which is one of the major air pollutants and poses a great threat to the environment and human health. With increasingly stringent national environmental protection standards, the emission limits for NOx in flue gas from thermal power plants are constantly being raised, and achieving deep NOx emission reduction has become an important issue that coal-fired power plants urgently need to address.

[0003] In recent years, national environmental protection policies have been continuously strengthened, and the control standards for air pollutant emissions from thermal power plants have become increasingly stringent, especially for nitrogen oxides (NOx) emission limits, which have been continuously tightened. Ultra-low emissions (≤50mg / m³) have become a hard indicator for the survival and development of coal-fired units. Against this backdrop, the flue gas denitrification system of circulating fluidized bed (CFB) coal-fired units is facing unprecedented challenges.

[0004] The current flue gas denitrification unit of a certain project, consisting of 2×300MW CFB coal-fired power units, adopts the selective non-catalytic reduction (SNCR) process. Its design boundary clearly requires that the original NOx concentration in the boiler should not exceed 300mg / m³ when the SNCR system is shut down. However, in actual operation, the original NOx concentration of the unit is often in the range of 250-350mg / m³. The denitrification efficiency of the existing SNCR process can only be maintained at 50%-70%. Even under ideal operating conditions, it is difficult to stably control the NOx emission concentration within the ultra-low emission limit (≤50mg / m³), which cannot meet the requirements of current and future environmental protection policies.

[0005] During unit operation, the problems are particularly prominent under low-load conditions. Due to the high oxygen content of the unit under low load, the NOx generation increases significantly. The existing denitrification system lacks effective dynamic adjustment capabilities and is unable to cope with the impact of such changes in operating conditions, resulting in large fluctuations in NOx emission concentrations and an extremely high risk of exceeding standards.

[0006] Meanwhile, the existing SNCR system still has many technical shortcomings. Uneven ammonia injection is common, the mixing effect of urea solution and flue gas is poor, resulting in insufficient local denitrification reaction; the temperature window is prone to shift, which further reduces the denitrification efficiency; and the ammonia slip rate is too high, which not only wastes reducing agent, but may also cause subsequent problems such as air preheater blockage and corrosion, affecting the safe and stable operation of the unit.

[0007] Therefore, it is necessary to develop technologies that can achieve efficient denitrification, adapt to changes in operating conditions, and achieve deep emission reduction while ensuring the normal operation of the unit, so as to further reduce NOx emission concentration to ultra-low limits and meet more stringent environmental protection requirements. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the above-mentioned background technology and to provide an optimized circulating fluidized bed boiler flue gas denitrification system.

[0009] This invention primarily addresses the following issues: 1) Existing SNCR denitrification systems exhibit unstable denitrification efficiency under variable load conditions, making it difficult to stably control NOx emission concentrations within ultra-low limits. This is especially true during low-load operation, where higher oxygen levels in the unit lead to increased initial NOx formation concentrations, impacting denitrification performance; 2) The number and arrangement of spray guns in existing SNCR injection systems are not sufficiently optimized, resulting in inadequate and uneven mixing of urea solution and flue gas, hindering the complete denitrification reaction and reducing efficiency; 3) The urea solution preparation system, flow distribution system, and control system are incompatible with the denitrification requirements after the upgrade, failing to meet the urea solution consumption and precise control requirements after the efficiency improvement upgrade; 4) During boiler combustion, the material flow pattern and circulation state... The following issues were identified: 1) Poor combustion conditions, uneven bed temperature, and unreasonable parameters such as particle size of the combustion gas entering the furnace and the ratio of primary and secondary air, resulting in high initial NOx concentrations and increasing the processing burden on the denitrification system; 2) Poor compatibility between flue gas recirculation and boiler combustion, requiring solutions for reasonable system design, coordinated operation with existing equipment, corrosion prevention, and insulation to ensure effective reduction of oxygen levels in low-load units, thereby reducing initial NOx production concentrations; 3) Poor compatibility between flue gas recirculation and boiler combustion, delayed response to dynamic changes in coal quality leading to unstable NOx emissions, and coupling interference in multi-system linkage control. The coordinated control of the boiler and turbine, and the automatic control of various parameters urgently need optimization to meet relevant industry standards while ensuring stable and efficient system operation.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: an optimized circulating fluidized bed boiler flue gas denitrification system, comprising a boiler, a cyclone separator group connected to the boiler, a preheater connected to the cyclone separator group, a dust collector connected to the preheater, an induced draft fan connected to the dust collector, a chimney connected to the induced draft fan, a return material device connecting the boiler and the cyclone separator group, and a wind chamber located at the boiler inlet, characterized in that: it further comprises an injection system and a flue gas recirculation device; the cyclone separator group comprises multiple cyclone separators;

[0011] The spraying system includes a static mixer, a first spray gun group, and a second spray gun group; the static mixer is connected to dilution water and a reducing agent; the first spray gun group is located at the inlet of the cyclone separator and is connected to the static mixer, and includes multiple narrow-angle atomizing nozzles; the second spray gun group is located at the outlet of the cyclone separator and is connected to the static mixer, and includes multiple wide-angle atomizing nozzles.

[0012] The flue gas recirculation device includes a recirculation fan and a primary air fan. One end of the recirculation fan is connected to the flue between the outlet of the induced draft fan and the outlet of the induced draft fan and the outlet of the cyclone separator, and the other end is connected to the inlet of the primary air fan. The inlet of the primary air fan is connected to air, and the outlet is connected to the preheater.

[0013] In the above technical solution, the narrow-angle atomizing nozzle has a nozzle angle of 30 degrees and the atomized particle size is controlled between 50-80μm; the wide-angle atomizing nozzle has a nozzle angle of 60 degrees and the atomized particle size is controlled between 80-120μm.

[0014] In the above technical solution, the first spray gun assembly is connected to the static mixer through a first distribution pipeline, and the pressure of the first distribution pipeline is set to 0.8-1.0 MPa;

[0015] The second spray gun assembly is connected to the static mixer via a second distribution pipeline, and the pressure of the second distribution pipeline is set to 0.6-0.8 MPa.

[0016] In the above technical solution, under low load conditions: 50% of the narrow-angle atomizing nozzles and wide-angle atomizing nozzles are used; under medium load conditions: 75% of the narrow-angle atomizing nozzles are used; under full load conditions: all narrow-angle atomizing nozzles and wide-angle atomizing nozzles are used.

[0017] In the above technical solution, there are 4 cyclone separators; each cyclone separator is equipped with 6 narrow-angle atomizing nozzles and 4 wide-angle atomizing nozzles.

[0018] In the above technical solution, when the NOx concentration in the flue gas is >300mg / m³, the urea solution concentration in the static mixer is increased to 15%; when the NOx concentration in the flue gas is <150mg / m³, the urea solution concentration in the static mixer is reduced to 10%.

[0019] In the above technical solution, there are two primary air fans. The recirculation fan is connected to one primary air fan via a main pipeline after being connected to a pressure transmitter and a temperature sensor, and is connected to the other primary air fan via a bypass pipeline. A flue gas flow meter and a main electric regulating valve are installed on the main pipeline. A flue gas flow meter and a bypass electric regulating valve are installed on the bypass pipeline.

[0020] In the above technical solution, when the bed temperature is >950℃, the circulation volume of the recirculation fan automatically increases to 18-20%; when the bed temperature is <850℃, the circulation volume of the recirculation fan decreases to 10-12%; if the nitrogen content of the coal is >1.5%, the circulation volume of the recirculation fan increases by an additional 2-3%.

[0021] When the circulation volume of the recirculation fan increases by more than 5%, the injection system reduces the urea injection volume by 3-4% in advance; when the grid voltage fluctuates by ±10%, the frequency converter of the recirculation fan automatically switches to the voltage stabilization mode.

[0022] In the above technical solution, the air chamber is connected to the preheater through a flue gas duct branch;

[0023] At high load, the boiler inlet uses high-temperature flue gas from the air chamber; at low load, the air chamber introduces low-temperature flue gas from the preheater; the preheater conditions the flue gas temperature to ensure that the air temperature entering the air chamber is adjustable between 150-200℃.

[0024] In the above technical solution, the flue gas recirculation device further includes a Venturi-type mixing nozzle, which includes a first inlet, a second inlet, a mixing outlet, and a scaling structure; the first inlet is connected to air, the second inlet is connected to a main electric regulating valve and a bypass electric regulating valve, and the mixing outlet is connected to a primary air fan; the scaling structure includes a contraction tube, a throat nozzle, and an expansion tube connected in sequence; the first inlet and the second inlet are connected to the contraction tube inlet; the mixing outlet is connected to the primary air fan; and the inner wall of the throat nozzle is provided with arc-shaped turbulence teeth.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1) Based on the difference between the inlet and outlet of the cyclone separator, this invention achieves a stable denitrification efficiency of ≥80% (compared to 40-60% in traditional systems) and a stable NOx emission concentration of ≤50mg / m³ under different load conditions through SNCR dynamic zonal injection, flue gas recirculation with variable load adaptive design, and coal quality-denitrification advanced linkage. This meets the ultra-low emission requirements. Under low load (30% of rated load), the original NOx concentration is reduced by 15-20% (dual-stage induced draft design), and the fluctuation range of the original concentration is reduced from ±50mg / m³ to ±15mg / m³ (precise control of bed temperature gradient), thus solving the problem of NOx fluctuation under different loads and coal quality.

[0027] 2) Improve system adaptability and stability: This invention can be put into operation after the unit is ignited and coal is fed, as well as during the full load period. It has good adaptability during load adjustment, can operate reliably and stably continuously, has an availability rate of not less than 99%, a service life of 20 years, and does not interfere with boiler operation.

[0028] 3) Optimize resource utilization: The present invention reduces urea consumption by 8-12%, saving ≥500,000 yuan in urea costs per furnace per year; reduces power consumption of flue gas recirculation fan by 10% (variable load adaptive + frequency conversion control), reduces the footprint of desulfurization and denitrification system by 20%, and reduces overall operating costs by 15%; utilizes existing equipment during the transformation (such as retaining the main body of the original SNCR preparation system), reducing transformation costs by 18-22%.

[0029] 4) Improved boiler combustion performance: This invention improves the combustion characteristics of the boiler, increases combustion efficiency, and reduces energy consumption through combustion optimization and adjustment.

[0030] 5) Staged Flue Gas Recirculation: This invention reduces the oxygen content of the primary air by introducing air volume after the induced draft fan, ensuring that the combustion center and combustion temperature are lowered under the same boiler output, avoiding excessive oxygen combustion, reducing NOx concentration from the source, and achieving ultra-low emissions. Furthermore, the flue gas recirculation device of this invention introduces air volume, and the recirculation fan is equipped with a variable frequency fan. An electric regulating valve is installed in the flue, which can adjust the air volume ratio in real time according to the boiler load, realizing the adjustment of air volume ratio under varying operating conditions. The dual-stage induced draft design of this invention further reduces the original NOx concentration by 15-20% at low load (30% of rated load) and reduces fan power consumption by 10%, solving the problem of traditional single-path systems affecting boiler efficiency at high loads.

[0031] 6) Venturi-type mixing nozzle: This invention employs a scaled-down Venturi structure in the flue gas and primary air mixing section. Through fluid dynamics design, it enables the flue gas and air to form a strong turbulent mixing, improving the mixing uniformity to over 95% (compared to only 70% in traditional pipeline mixing). The inner wall of the nozzle is made of arc-shaped turbulence-inducing teeth, which further enhances the turbulence effect while reducing system resistance (resistance coefficient ≤80Pa).

[0032] 7) Enhanced system safety: The flue gas recirculation device of this invention adopts comprehensive anti-corrosion and heat preservation measures, avoiding problems such as pipeline corrosion and liquid accumulation, and extending the service life of the equipment. The optimization of the control system also improves the safety and reliability of system operation. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of the present invention.

[0034] Figure 2 This is a layout diagram of the first and second spray gun groups within the cyclone separator assembly.

[0035] Figure 3 This is a schematic diagram of a Venturi mixing nozzle.

[0036] Among them, 110-boiler, 120-cyclone separator group, 121-cyclone separator, 130-preheater, 140-dust collector, 150-induced draft fan, 160-chimney, 170-return material device, 180-wind chamber, 200-jet system, 210-static mixer, 211-total pressure gauge, 212-flow meter, 220-first spray gun group, 221-narrow angle atomizing nozzle, 230-second spray gun group, 231-wide angle atomizing nozzle, 232-pressure gauge, 240-first distribution pipeline, 250-second distribution pipeline, 300-flue gas recirculation Circulation device, 310-Recirculation fan, 311-Pressure transmitter, 312-Temperature sensor, 320-Primary air fan, 330-Main pipeline, 331-Main electric regulating valve, 340-Bypass pipeline, 341-Bypass electric regulating valve, 350-Flue gas flow meter, 360-Venturi mixing nozzle, 361-First inlet, 362-Second inlet, 363-Mixing outlet, 364-Scaling structure, 3641-Contraction tube, 3642-Throat nozzle, 3643-Expanding tube, 3644-Arc-shaped turbulence teeth, 370-Flue gas induced draft branch. Detailed Implementation

[0037] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but these descriptions are not intended to limit the invention and are merely illustrative. The advantages of the present invention will become clearer and easier to understand through this description.

[0038] Referring to the accompanying drawings, an optimized circulating fluidized bed boiler flue gas denitrification system includes a boiler 110, a cyclone separator group 120 connected to the boiler 110, a preheater 130 connected to the cyclone separator group 120, a dust collector 140 connected to the preheater 130, an induced draft fan 150 connected to the dust collector 140, a chimney 160 connected to the induced draft fan 150, a return material device 170 connecting the boiler 110 and the cyclone separator group 120, and a wind chamber 180 located at the inlet of the boiler 110. The system is characterized by further including an injection system 200 and a flue gas recirculation device 300; the cyclone separator group 120 includes multiple cyclone separators 121.

[0039] The spraying system 200 includes a static mixer 210, a first spray gun assembly 220, and a second spray gun assembly 230. The static mixer 210 is connected to dilution water and a reducing agent. The first spray gun assembly 220 is located at the inlet of the cyclone separator 121 and is connected to the static mixer 210. The first spray gun assembly 220 includes multiple narrow-angle atomizing nozzles 221. The second spray gun assembly 230 is located at the outlet of the cyclone separator 121 and is connected to the static mixer 210. The second spray gun assembly 230 includes multiple wide-angle atomizing nozzles 231.

[0040] The flue gas recirculation device 300 includes a recirculation fan 310 and a primary air fan 320. One end of the recirculation fan 310 is connected to the flue between the outlet of the induced draft fan 150 and the outlet of the induced draft fan 150 and the cyclone separator 121, and the other end is connected to the inlet of the primary air fan 320. The inlet of the primary air fan 320 is connected to air, and the outlet is connected to the preheater 130.

[0041] The narrow-angle atomizing nozzle 221 has a nozzle angle of 30 degrees and an atomized particle size controlled between 50-80 μm; the wide-angle atomizing nozzle 231 has a nozzle angle of 60 degrees and an atomized particle size controlled between 80-120 μm.

[0042] The first spray gun assembly 220 is connected to the static mixer 210 via the first distribution pipe 240; the second spray gun assembly 230 is connected to the static mixer 210 via the second distribution pipe 250.

[0043] Flow distribution system strategy: Based on the grouping of spray guns and real-time usage in the spray system 200, a "zoned pressure control module" is added to the flow distribution system: the pressure of the first distribution line 240 is set to 0.8-1.0MPa to meet the narrow-angle atomization requirements; the pressure of the second distribution line 250 is set to 0.6-0.8MPa to meet the wide-angle atomization requirements.

[0044] Variable load spray gun dynamic operation mechanism: Load linkage control logic automatically switches the spray gun operation combination according to the real-time load signal of the unit (taken from the DCS system): When under low load conditions (≤50% of rated load): 50% of the narrow-angle atomizing nozzles 221 and wide-angle atomizing nozzles 231 are used to reduce the total consumption of reducing agent; When under medium load conditions (50%-75% of rated load): 75% of the narrow-angle atomizing nozzles 221 and wide-angle atomizing nozzles 231 are used to balance the mixing effect and energy consumption; When under full load conditions (≥75% of rated load): All narrow-angle atomizing nozzles 221 and wide-angle atomizing nozzles 231 are used to ensure sufficient reaction under high NOx concentration.

[0045] There are 4 cyclone separators 121; each cyclone separator 121 is equipped with 6 narrow-angle atomizing nozzles 221 and 4 wide-angle atomizing nozzles 231; there are a total of 24 narrow-angle atomizing nozzles 221 and 16 wide-angle atomizing nozzles 231.

[0046] This invention optimizes the injection system by setting up an SNCR system including a static mixer 210, urea solution dilution, metering and distribution module, a first spray gun group 220, and a second spray gun group 230. Based on the mechanism of "dynamic zoned injection + differentiated atomization", the compatibility of the injection system at the inlet and outlet of the cyclone separator 121 is optimized. This is specifically designed for the high turbulence intensity of the flue gas at the inlet of the cyclone separator 121 (flow velocity 12-15 m / s) and the stable flue gas velocity at the outlet (flow velocity 8-10 m / s). Due to differences in characteristics, while adjusting the number and arrangement of spray guns, a "differentiated atomization design" was adopted. The 24 inlet spray guns used 30° narrow-angle atomizing nozzles 221 to enhance the collision and mixing efficiency of reducing agent droplets and flue gas by utilizing a high turbulence environment, with the atomized particle size controlled at 50-80μm. The 16 outlet spray guns used 60° wide-angle atomizing nozzles 231 with a coverage diameter ≥3m to compensate for insufficient diffusion under low flow velocity by expanding the droplet coverage range, with the atomized particle size controlled at 80-120μm.

[0047] The SNCR system module integrates a "static mixer + variable concentration dilution unit", which can dynamically adjust the dilution ratio according to the NOx concentration in the flue gas to ensure the reaction efficiency of urea solution and flue gas under different loads. Specifically, when the NOx concentration in the flue gas is >300mg / m³, the urea solution concentration in the static mixer 210 is increased to 15%; when the NOx concentration in the flue gas is <150mg / m³, the urea solution concentration in the static mixer 210 is reduced to 10%.

[0048] Urea solution preparation system: An "intelligent redundancy switching logic" is set up to automatically switch the standby pump based on real-time consumption prediction (modeled from the consumption trend of the previous 10 minutes), avoiding the 2-3 minute flow fluctuations that occur with manual switching. Simultaneously, a "pressure balancing valve" is installed in the pump outlet pipeline. When a single pump switches, the balancing valve adjusts its opening in real time according to the pipeline pressure deviation (set threshold ±0.2 bar) to ensure flow fluctuation ≤5%. The dilution water pump pipeline is equipped with a "cross-furnace standby linkage" function, meaning that when one furnace's dilution water pump fails, the standby pump of another furnace can quickly replenish the supply through the cross-furnace pipeline, with a response time ≤1 minute, overcoming the limitations of traditional single-furnace standby.

[0049] Static mixing enhancement: Using a static mixer 210, the uniformity of the diluted urea solution is increased to ≥95% through fluid division-rotation-recombination action, avoiding fluctuations in denitrification efficiency caused by local concentration deviations.

[0050] There are two primary air fans 320.

[0051] The flue gas recirculation device 300 of this invention reuses low-oxygen flue gas by rationally allocating the ratio of primary and secondary air. It reduces the oxygen supply from the primary air, and the low oxygen level in the dense phase region suppresses the boiler bed temperature. By appropriately increasing the secondary air volume to replenish the replaced primary air, the reducing atmosphere in the dense phase region is strengthened. Furthermore, it lowers the primary air pressure head and raises the secondary air pressure head, significantly improving the penetration of the secondary air and achieving staged combustion, ensuring complete fuel combustion. This approach increases the suspended concentration of materials in the dilute phase region and shifts the combustion share upwards, increasing the concentration of unburned carbon monoxide and other reducing gases in some areas, resulting in a significant reduction effect on NOx and promoting subsequent complete combustion of coke particles.

[0052] The flue gas recirculation device 300 of this invention breaks through the traditional design of a fixed 10-20% recirculation volume and constructs a "variable load recirculation volume adaptive algorithm": it dynamically adjusts according to the boiler bed temperature and coal nitrogen content. When the bed temperature is >950℃ (which easily generates thermal NOx), the recirculation volume of the recirculation fan 310 automatically increases to 18-20%; when the bed temperature is <850℃ (to avoid affecting combustion efficiency), the recirculation volume of the recirculation fan 310 decreases to 10-12%; if the coal nitrogen content is >1.5%, the recirculation volume of the recirculation fan 310 increases by an additional 2-3%; in addition to considering a 10% air volume reserve coefficient, the selection of the recirculation fan 310 adds a "variable frequency speed regulation and air volume feedback closed loop" to ensure that the deviation between the actual recirculation volume and the set value is ≤2%.

[0053] In addition to the electrically operated shut-off valve and regulating valve, the flue gas recirculation pipe of the flue gas recirculation device 300 is equipped with a "high and low load dual-mode switching valve group" at the interface between the flue gas recirculation pipe and the primary air fan 320. The recirculation fan 310 is connected to a pressure transmitter 311 and a temperature sensor 312, and then connected to one primary air fan 320 via a main pipe 330, and to another primary air fan 320 via a bypass pipe 340. A flue gas flow meter 350 and a main electric regulating valve 331 are installed on the main pipe 330; a flue gas flow meter 350 and a main electric regulating valve 331 are installed on the bypass pipe 340. The system includes a bypass electric regulating valve 350 and a bypass electric regulating valve 341. Under high load (≥80% of rated load), the bypass electric regulating valve is linked with the main electric regulating valve to precisely control the flue gas supply through "main flow restriction + bypass gas supply" (deviation ≤3%). At the same time, a silencer is installed on the bypass pipeline (noise reduction ≥25dB) to solve the problem of excessive noise from traditional bypass gas supply (≤85dB, meeting the "Emission Standard for Environmental Noise at the Boundary of Industrial Enterprises"). The manual damper at the inlet of the recirculation fan 310 is upgraded to an "electric and manual dual-purpose damper", which can be manually and quickly adjusted in case of sudden power failure to avoid system shutdown.

[0054] Measurement device installation: A pressure transmitter 311, a temperature sensor 312, and a flue gas flow meter 350 are installed on the outlet flue of the recirculation fan 310 to provide basic data for the normal operation of the flue gas recirculation device 300.

[0055] The flue gas recirculation device 300 further includes a Venturi-type mixing nozzle 360, which includes a first inlet 361, a second inlet 362, a mixing outlet 363, and a scaling structure 364. The first inlet 361 is connected to air, the second inlet 362 is connected to a main electric regulating valve 331 and a bypass electric regulating valve 341, and the mixing outlet 363 is connected to a primary air fan 320. The scaling structure 364 includes a contraction tube 3641, a throat nozzle 3642, and an expansion tube 3643 connected in sequence. The first inlet 361 and the second inlet 362 are connected to the inlet of the contraction tube 3641. The mixing outlet 363 is connected to the primary air fan 320. The inner wall of the throat nozzle 3642 is provided with arc-shaped turbulence-inducing teeth 3644. The scaling structure 364 is used in the flue gas and primary air mixing section. Through fluid dynamics design, the flue gas and air form a strong turbulent mixing, and the mixing uniformity is improved to more than 95% (compared to only 70% for traditional pipeline mixing). The inner wall of the throat nozzle 3642 is made of arc-shaped turbulence teeth 3644, which further enhances the turbulence effect and reduces the system resistance (drag coefficient ≤80Pa).

[0056] Corrosion prevention and insulation measures: To prevent the undesulfurized flue gas from corroding the blades of the recirculation fan 310, the recirculation fan 310 uses corrosion-resistant blades, and corresponding anti-corrosion measures are taken for the blades, rotor, casing and cold air duct of the primary air fan 320; all pipelines of the flue gas recirculation device 300 are insulated with insulation materials to prevent acid condensation due to the decrease in flue gas temperature, which would cause corrosion to the pipelines.

[0057] Control system integration: When the flue gas recirculation device 300 is integrated into the power plant DCS, a new "linkage logic with the SNCR system" is added: when the amount of recirculated flue gas increases by more than 5%, the SNCR system reduces the amount of urea injection by 3-4% in advance to avoid excessive denitrification; the frequency converter control of the recirculation fan 310 is upgraded to "emergency frequency converter protection". When the grid voltage fluctuates by ±10%, the frequency converter automatically switches to the voltage stabilization mode to ensure the fan speed is stable and prevent sudden changes in the recirculation volume.

[0058] The air chamber 180 is connected to the preheater 130 via the flue gas induced draft branch 370; it forms a two-stage induced draft structure with the flue gas recirculation device 300; under high load (≥80% of rated load), the high-temperature flue gas (approximately 350-400℃) in the air chamber 180 is used to maintain the furnace temperature stability by utilizing its heat; under low load (<50% of rated load), it switches to the low-temperature flue gas (approximately 120-150℃) at the outlet of the induced draft fan 150 to enhance the oxygen control effect; the two flue gas paths are temperature-conditioned by the preheater 130 to ensure that the air temperature entering the boiler air chamber is adjustable between 150-200℃.

[0059] Variable diameter duct adjustment: The duct cross-sectional area is automatically adjusted according to load changes. Under high load, the flow area is increased (reducing resistance), and under low load, the cross-sectional area is decreased (increasing flow velocity). The adjustment mechanism uses an electric regulating valve, and the opening degree can be adjusted within 10%-100%, thereby adjusting the cross-sectional area of ​​the flue gas duct. The response time is ≤5 seconds, and stepless adjustment of 10%-100% of the rated air volume can be achieved.

[0060] In practical use, the present invention also includes combustion optimization adjustment, control system optimization, and intelligent injection adaptive adjustment system;

[0061] Combustion optimization adjustments:

[0062] Through three major innovations—systematization of cold-state baseline data, refinement of hot-state air distribution control, and multi-objective synergy of in-depth adjustment—a closed-loop optimization system for the entire process from cold-state commissioning to hot-state operation has been constructed, enabling efficient and low-NOx combustion in the boiler.

[0063] Cold-state testing: Conducting cold-state tests and air volume calibrations closely related to combustion optimization and adjustment, including boiler air volume (primary air, secondary air) calibration tests, air distribution plate empty bed resistance tests, critical fluidization air volume tests, air distribution uniformity tests, and material bed resistance characteristic tests, to provide accurate operating parameters for hot-state operation.

[0064] Hot-state testing and optimization trials: Conduct air preheater leakage measurement tests, flue gas temperature measurement tests, oxygen content calibration tests, and optimization trials for bed temperature operating parameters (air volume, primary and secondary air ratio, bed material thickness, etc.), as well as boiler upper and lower secondary air optimization trials, and provide operation cards for relevant operating parameters. The contractor shall provide and install the lower secondary air branch pipe adjusting valve to achieve effective adjustment of the upper and lower secondary air.

[0065] Deep combustion adjustment: Optimize material flow and circulation, improve boiler combustion characteristics, control combustion temperature uniformity (control bed temperature uniformity), control the particle size of combustion entering the furnace, adjust the ratio of primary and secondary air rates, carry out return material circulation adjustment, and comprehensive adjustment of in-furnace desulfurization and denitrification, etc., to further effectively reduce the original NOx emissions.

[0066] Specifically as follows:

[0067]

[0068] Control system optimization:

[0069] Control system optimization scheme: After setting up the metering module, construct a multi-parameter coupled control logic of "NOx concentration-load-temperature" in the DCS: taking the outlet NOx concentration (target value ≤50mg / m³) as the core, combined with the boiler load (e.g., adjust the urea injection amount by 3-5% in advance for every 5% change in load) and the outlet temperature of the cyclone separator 121 (if the temperature is <850℃, the furnace temperature should be adjusted first through the flue gas recirculation device 300, or the secondary air ratio should be increased by adjusting the linkage combustion to automatically increase the urea concentration by 2-3% to compensate for the reaction activity), to achieve automatic NOx control; analog quantity adjustment, in addition to meeting the requirements of "D In addition to the requirements of L / T657-2015, a new "self-tuning function for adjustment parameters" has been added. When changes in coal quality cause NOx fluctuations >10%, the system automatically corrects the PID parameters (proportional coefficient and integral time) to avoid repeated manual adjustments. The automatic tracking function of the injection system has been upgraded to "load-temperature dual triggering". That is, when the load changes by ±3% or the temperature changes by ±20℃, the number of spray guns and the injection volume are automatically adjusted. The zone activation and deactivation function can be refined to the spray gun group of a single cyclone separator 121 (if the NOx at the outlet of a certain cyclone separator 121 is too high, only the spray gun corresponding to that cyclone separator 121 needs to be activated).

[0070] Boiler-Turbine Coordination Control Optimization: To address the coupled impact of flue gas recirculation and combustion adjustment on boiler-turbine parameters, a "multivariable coupling decoupling algorithm" was added to the logic optimization: the interference of recirculated flue gas volume on primary air pressure and the interference of coal feed rate on main steam pressure are decoupled. For example, when the recirculated flue gas volume increases by 10%, the primary air fan pressure is automatically increased by 5% to compensate; when the coal feed rate increases by 5%, the main steam pressure regulating valve is opened 2-3% in advance to avoid parameter fluctuations (main steam pressure deviation ≤ 0.2MPa, primary air pressure deviation ≤ 0.5kPa); a "coupling interference test item" was added to the analog dynamic disturbance test report to verify the decoupling effect.

[0071] Intelligent injection adaptive adjustment system:

[0072] Intelligent sensing module for spray guns: Multi-parameter sensors are installed on each spray gun to monitor parameters such as flue gas temperature, flow rate, NOx concentration, urea solution atomization particle size, and spray pressure in the spray area in real time. The sensors are made of high-temperature and corrosion-resistant materials to ensure stable operation in harsh environments of 700-1100℃, with a data acquisition frequency of no less than 10Hz.

[0073] AI Decision-Making Algorithm Model: Based on massive operational data, a neural network algorithm model is constructed. According to real-time sensor data, combined with parameters such as boiler load and coal quality characteristics, it automatically calculates the optimal injection angle, flow rate, and atomizing medium ratio. For example, when a local flue gas temperature deviates from the optimal reaction temperature window (850-1050℃), the algorithm will adjust the injection parameters of the corresponding area's spray guns within 0.5 seconds to ensure denitrification reaction efficiency. Dynamic Flow Distribution Matrix: Electromagnetic metering and distribution valve groups are used to replace traditional mechanical valves. Through the control signals output by the AI ​​model, independent closed-loop control of the flow rate of each spray gun is achieved, with a flow regulation accuracy of ±1%. The valve group response time is less than 0.1 seconds, enabling rapid adaptation to boiler load fluctuations (±10% rated load / minute).

[0074] Achieving automated control: This invention achieves "remote control + automatic adjustment" through DCS, reducing the intensity of manual operation by 70%; the AI ​​decision algorithm has a response time of 0.3 seconds, and the multivariable decoupling control ensures that parameter fluctuations are ≤5%, meeting the requirements of standards such as "DL / T657-2015" and "DL / T1213-2013", and supports offline upgrades and working condition adaptation, adapting to future changes in coal types and upgrades in environmental protection standards.

[0075] Intelligent injection system: This invention uses real-time sensing and AI decision-making to maintain a denitrification efficiency of over 80% at full load (compared to 40-60% for traditional systems), and reduces urea consumption by 8-12%.

[0076] Example

[0077] Taking a 2×300MW CFB coal-fired power unit of a certain project as an example, this technical solution will be implemented in detail.

[0078] The original denitrification system used the traditional SNCR process, which had the following problems: large fluctuations in denitrification efficiency (40%-60%), and NOx emissions were prone to exceed the standard (>100mg / m³) at low loads; the urea preparation system was a standby unit with a single furnace, and had to be shut down in case of failure; the flue gas recirculation volume was fixed (15%), the amount of thermal NOx generated at high loads was large, and the combustion efficiency loss at low loads was >2%.

[0079] The present invention is used to modify the SNCR denitrification system: all 24 spray guns at the inlet of the cyclone separator group 120 of each furnace are replaced and their arrangement is optimized. At the same time, 16 spray guns are added at the outlet of the cyclone separator group 120, and a metering and distribution system, a dilution system and an injection system are added. The optimal position and number of spray guns are determined through CFD numerical simulation and physical model cold state test to ensure that the urea solution is fully mixed with the flue gas.

[0080] Adjust the flow distribution system and control system to enable each sub-zone module to operate and control independently, achieving automatic flow control and zoned activation / deactivation functions. The injection volume is automatically adjusted based on boiler load and outlet NOx concentration changes. For low load (≤150MW), 12+8 units are activated; for medium load (150-250MW), 18+12 units are activated; and for full load (≥250MW), all 24+16 units are activated.

[0081] The urea solution preparation system was checked and it was found that the existing system could not meet the urea solution consumption requirements under high load. Therefore, a new urea solution preparation unit was added. The outlet pipelines of the four urea solution delivery pumps and dilution water pumps were modified to connect the outlet pipelines and improve the reliability of the system operation.

[0082] Install the flue gas recirculation device 300 of this invention: According to the design requirements, flue gas is drawn out from the clean flue gas duct at the outlet of the induced draft fan 150, and a flue gas recirculation fan 310 (air volume of 140,000 Nm³ / h, using corrosion-resistant blades) and matching pipelines are installed. The pipeline material is consistent with the existing pipeline material in the plant. Manual dampers, electric adjustable dampers, flue gas bypass and regulating dampers are installed at the corresponding positions. Temperature, pressure and flow measurement devices are installed on the flue gas duct at the outlet of the recirculation fan 310, and the pipeline is insulated. Connect the flue gas recirculation device 300 to the power plant's existing DCS system to realize remote control and frequency conversion regulation.

[0083] Combustion optimization and adjustment implementation:

[0084] Conduct cold-state tests, including boiler air volume calibration and air distribution plate empty bed resistance tests, to provide accurate parameters for hot-state operation.

[0085] Conduct hot-state tests and optimization adjustments, measure air preheater leakage and flue gas temperature, adjust primary and secondary air ratios and parameters such as upper and lower secondary air, develop operation cards, and guide operators in operation.

[0086] Install the lower secondary air branch pipe adjustment valve, and through deep combustion adjustment, optimize the material flow and circulation state, control the bed temperature uniformity, and reduce the original NOx formation concentration.

[0087] Control system optimization implementation:

[0088] The parameters of the coordinated control of the boiler and turbine were optimized and adjusted logically. The analog dynamic disturbance test was completed, and the test results met the requirements of "DL / T657-2015 Acceptance Test Procedure for Analog Control System of Thermal Power Plant".

[0089] Optimization and dynamic testing of the RB control of important auxiliary equipment were carried out, and the test results met the requirements of the "DL / T1213-2013 Technical Specification for Load Reduction of Auxiliary Equipment in Thermal Power Generating Units".

[0090] After implementation, the NOx emission concentration of the 2×300MW CFB coal-fired power unit stabilized below 35-50 mg / m³, meeting the ultra-low emission requirements. The system operated stably and reliably, achieving good environmental and economic benefits.

[0091] All other unspecified parts belong to the prior art.

Claims

1. An optimized circulating fluidized bed boiler flue gas denitrification system, comprising a boiler (110), a cyclone separator group (120) connected to the boiler (110), a preheater (130) connected to the cyclone separator group (120), a dust collector (140) connected to the preheater (130), an induced draft fan (150) connected to the dust collector (140), a chimney (160) connected to the induced draft fan (150), a return material device (170) connecting the boiler (110) and the cyclone separator group (120), and a wind chamber (180) disposed at the inlet of the boiler (110), characterized in that: It also includes an injection system (200) and a flue gas recirculation device (300); the cyclone separator assembly (120) includes a plurality of cyclone separators (121); The spraying system (200) includes a static mixer (210), a first spray gun assembly (220), and a second spray gun assembly (230). The static mixer (210) is connected to dilution water and a reducing agent. The first spray gun assembly (220) is located at the inlet of the cyclone separator (121) and is connected to the static mixer (210). The first spray gun assembly (220) includes multiple narrow-angle atomizing nozzles (221). The second spray gun assembly (230) is located at the outlet of the cyclone separator (121) and is connected to the static mixer (210). The second spray gun assembly (230) includes multiple wide-angle atomizing nozzles (231). The flue gas recirculation device (300) includes a recirculation fan (310) and a primary air fan (320). One end of the recirculation fan (310) is connected to the flue between the outlet of the induced draft fan (150) and the outlet of the induced draft fan (150) and the cyclone separator (121), and the other end is connected to the inlet of the primary air fan (320). The inlet of the primary air fan (320) is connected to air, and the outlet is connected to the preheater (130).

2. The optimized circulating fluidized bed boiler flue gas denitrification system according to claim 1, characterized in that: The narrow-angle atomizing nozzle (221) has a nozzle angle of 30 degrees and an atomized particle size controlled at 50-80μm; the wide-angle atomizing nozzle (231) has a nozzle angle of 60 degrees and an atomized particle size controlled at 80-120μm.

3. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 2, characterized in that: The first spray gun assembly (220) is connected to the static mixer (210) through the first distribution pipeline (240), and the pressure of the first distribution pipeline (240) is set to 0.8-1.0 MPa; The second spray gun assembly (230) is connected to the static mixer (210) via the second distribution line (250), and the pressure of the second distribution line (250) is set to 0.6-0.8 MPa.

4. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 3, characterized in that: When operating under low load conditions: 50% of the narrow-angle atomizing nozzles (221) and wide-angle atomizing nozzles (231) are used; when operating under medium load conditions: 75% of the narrow-angle atomizing nozzles (221) and wide-angle atomizing nozzles (231) are used; when operating under full load conditions: all narrow-angle atomizing nozzles (221) and wide-angle atomizing nozzles (231) are used.

5. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 4, characterized in that: There are four cyclone separators (121); each cyclone separator (121) is equipped with six narrow-angle atomizing nozzles (221) and four wide-angle atomizing nozzles (231).

6. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 5, characterized in that: When the NOx concentration in the flue gas is >300mg / m³, the urea solution concentration in the static mixer (210) is increased to 15%; when the NOx concentration in the flue gas is <150mg / m³, the urea solution concentration in the static mixer (210) is reduced to 10%.

7. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 1, characterized in that: There are two primary air fans (320). The recirculation fan (310) is connected to a pressure transmitter (311) and a temperature sensor (312) and then connected to one primary air fan (320) through a main pipeline (330) and to the other primary air fan (320) through a bypass pipeline (340). A flue gas flow meter (350) and a main electric regulating valve (331) are installed on the main pipeline (330). A flue gas flow meter (350) and a bypass electric regulating valve (341) are installed on the bypass pipeline (340).

8. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 7, characterized in that: When the bed temperature is >950℃, the circulation volume of the recirculation fan (310) automatically increases to 18-20%; when the bed temperature is <850℃, the circulation volume of the recirculation fan (310) decreases to 10-12%; if the nitrogen content of the coal is >1.5%, the circulation volume of the recirculation fan (310) increases by an additional 2-3%. When the circulation volume of the recirculation fan (310) increases by more than 5%, the injection system (200) reduces the urea injection volume by 3-4% in advance; when the grid voltage fluctuates by ±10%, the frequency converter of the recirculation fan (310) automatically switches to the voltage stabilization mode.

9. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 8, characterized in that: The air chamber (180) is connected to the preheater (130) through the flue gas duct (370); At high load, the boiler (110) inlet uses high-temperature flue gas from the air chamber (180); at low load, the air chamber (180) introduces low-temperature flue gas from the preheater (130); the preheater (130) conditions the flue gas to ensure that the air temperature entering the air chamber (180) is adjustable between 150-200℃.

10. An optimized circulating fluidized bed boiler flue gas denitrification system according to claim 9, characterized in that: The flue gas recirculation device (300) further includes a Venturi mixing nozzle (360), which includes a first inlet (361), a second inlet (362), a mixing outlet (363), and a scaling structure (364). The first inlet (361) is connected to air, the second inlet (362) is connected to a main electric regulating valve (331) and a bypass electric regulating valve (341), and the mixing outlet (363) is connected to a primary air fan (320). The scaling structure (364) includes a contraction tube (3641), a throat nozzle (3642), and an expansion tube (3643) connected in sequence. The first inlet (361) and the second inlet (362) are connected to the inlet of the contraction tube (3641). The mixing outlet (363) is connected to the primary air fan (320). The inner wall of the throat nozzle (3642) is provided with arc-shaped turbulence teeth (3644).

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