A spectrophotometric method for detecting ammonia in waste gas of power plant

Abstract

The application discloses a kind of spectrophotometric detection methods for ammonia in power plant exhaust, extract denitration system tail flue ammonia-containing exhaust and cut into main reaction core gas stream and sheath gas protection gas stream;Liquid color reagent is atomized into color developing aerosol group using surface acoustic wave;Main reaction core gas stream carries ammonia into center channel, and mixed with color developing aerosol group to occur across-phase mass transfer and complex color developing reaction, combined with outside sheath gas protection gas stream to form internal liquid drop reaction zone, outside dry gas isolation zone Coaxial laminar flow;Coaxial laminar flow is introduced into hollow-core photonic crystal fiber, and external sheath gas layer makes aerosol group suspended transmission, avoid fiber microstructure pollution;Characteristic absorption wavelength and reference scattering wavelength light beam are alternately emitted and coaxially coupled into center channel.The application realizes the pollution prevention of pneumatic suspended transmission and high-precision optical density attenuation compensation.

Description

Technical Field

[0001] This invention relates to the field of power plant exhaust gas monitoring technology, specifically to a spectrophotometric detection method for ammonia in power plant exhaust gas. Background Technology

[0002] In the process of denitrification of coal-fired power plants, in order to reduce nitrogen oxide emissions, reducing agents such as ammonia or urea are usually injected into the flue gas. The reducing agents that do not participate in the reaction and are discharged with the flue gas will form ammonia escape. Accurately monitoring the ammonia concentration in the exhaust gas is of great significance for evaluating and optimizing the operating efficiency of the denitrification system, preventing blockage and corrosion of downstream equipment pipelines, and controlling environmental pollution.

[0003] Currently, spectrophotometry combined with specific optical waveguide structures is commonly used in industrial settings to monitor ammonia. This method typically involves mixing extracted ammonia-containing waste gas with a specific liquid colorimetric reagent, causing ammonia molecules to undergo mass transfer and reaction in the gas-liquid interphase, generating complexes with specific spectral absorption characteristics. Subsequently, the ammonia concentration is inferred by detecting the attenuation of the optical density through a probe beam penetrating the reaction medium. To further improve the optical sensitivity of trace ammonia detection, existing technologies often incorporate hollow-core photonic crystal fibers with an internal array of micron-sized air holes as miniature optical absorption cells, thereby extending the effective optical path for the interaction between the beam and the absorption medium.

[0004] However, when a two-phase medium containing liquid reactants in a gas-liquid suspension is directly introduced into a microstructured optical fiber for detection, the liquid colorimetric agent or complex droplets are easily deflected outwards and come into physical contact with the inner wall of the optical fiber due to internal flow field disturbances and radial Brownian diffusion. Once the liquid substance adheres to and penetrates the air pores in the inner wall of the microstructured cladding, it triggers capillary condensation, causing the micron-sized air pores to be filled with liquid. After the air pores are filled with liquid, the local equivalent refractive index of the microstructured cladding changes abruptly from the air refractive index to the liquid refractive index, thereby disrupting the original periodic distribution boundary of the photonic crystal cladding. This disruption causes spectral drift or bandgap collapse in the photonic bandgap of the optical fiber, resulting in leakage loss of the probe beam to the outside of the cladding, leading to a decrease in light guiding performance. Ultimately, this makes it difficult for the measurement system to achieve long-term stable monitoring of ammonia concentration in a gas-liquid mixed environment. Summary of the Invention

[0005] The technical problem to be solved by this invention is that in the prior art, when a gas-liquid suspended two-phase medium containing liquid reactants is introduced into a hollow photonic crystal fiber for detection, the droplets are easily affected by the flow field disturbance and radial Brownian diffusion motion, which can easily lead to physical contact with the inner wall of the fiber and cause capillary condensation. This disrupts the periodic distribution boundary of the cladding refractive index, resulting in a decrease in light guiding performance. Consequently, the measurement system is unable to achieve long-term stable monitoring of ammonia concentration in a gas-liquid mixed environment. This invention provides a spectrophotometric detection method for ammonia in power plant exhaust gas.

[0006] This invention provides a spectrophotometric method for detecting ammonia in power plant exhaust gas, comprising the following steps:

[0007] Ammonia-containing exhaust gas is extracted from the tail flue of the denitrification system. After filtering out fly ash particles, the total sampled gas flow is cut into the main reaction core flow that enters the central flow channel of the micro-nozzle, and the sheath gas protection flow that enters the annular flow channel of the micro-nozzle.

[0008] Liquid color developer is pumped into the surface of a piezoelectric microfluidic chip, and surface acoustic wave energy is used to continuously atomize the liquid color developer into a group of color-developing aerosols.

[0009] The main reaction core gas flow carries the ammonia gas to be tested into the central flow channel. The ammonia molecules undergo cross-phase mass transfer and complexation color reaction with the colorimetric aerosol group. The sheath gas protective gas flow outside the central flow channel forms a laminar boundary layer under the action of the internal and external velocity pressure difference in the contraction section. This layer radially constrains and encapsulates the colorimetric aerosol group in the center, forming a coaxial layer fluid with a droplet-containing reaction zone inside and a dry gas isolation zone outside.

[0010] The coaxial layer fluid is continuously introduced into the central hollow channel of the hollow core photonic crystal fiber. The external sheath gas layer acts as a physical isolation barrier, allowing the colorimetric aerosol group to be suspended and transported, avoiding physical contact and capillary condensation with the optical microstructure of the inner wall of the hollow core photonic crystal fiber.

[0011] The beams of characteristic absorption wavelength and reference scattering wavelength are emitted alternately. The beams of characteristic absorption wavelength and reference scattering wavelength are coaxially coupled into the central hollow channel, where they undergo transmission absorption and scattering attenuation with the colorimetric aerosol group.

[0012] The system receives the composite light intensity signal transmitted from the end of a hollow photonic crystal fiber, converts the composite light intensity signal into a continuous analog voltage signal, extracts the transmission voltage signal corresponding to the characteristic absorption wavelength and the transmission voltage signal corresponding to the reference scattering wavelength, calculates the total attenuation absorbance of the characteristic absorption wavelength, and uses the reference scattering wavelength to extract and subtract the baseline scattering loss caused by aerosols, calculates and outputs the concentration value of ammonia in the exhaust gas.

[0013] Preferably, the process of continuously atomizing liquid colorimetric agent into a colorimetric aerosol group using surface acoustic wave energy includes: applying an alternating radio frequency voltage signal to the input pin of the metal interdigital transducer on the surface of the piezoelectric microfluidic chip to excite Rayleigh-type surface acoustic waves on the surface of the piezoelectric substrate; in order to form effective resonance, the center resonant frequency of the alternating radio frequency voltage signal is equal to the ratio of the propagation speed of the Rayleigh-type surface acoustic wave on the surface of the piezoelectric substrate to the periodic distance between the two sets of comb electrodes of the metal interdigital transducer; the Rayleigh-type surface acoustic wave propagates directionally to the acoustic coupling region where the liquid colorimetric agent is spread, and acoustic-solid-liquid multiphase energy coupling occurs to generate unstable surface capillary waves. The peaks of the surface capillary waves break under the continuous action of acoustic radiation pressure, causing the liquid colorimetric agent to peel off from the surface of the piezoelectric substrate and transform into a colorimetric aerosol group.

[0014] Preferably, driving frequency locking and aerosol particle size control are performed during continuous atomization. The process includes: collecting the forward transmission power and reverse reflection power during the transmission of alternating radio frequency voltage signals to calculate the voltage standing wave ratio (VSWR) in the radio frequency transmission channel; extracting the frequency corresponding to the minimum VSWR point as the actual physical resonant frequency of the piezoelectric substrate at the current temperature; and locking the output frequency at the actual physical resonant frequency; modulating the continuous radio frequency signal into a radio frequency pulse train output; and maintaining the average radio frequency power applied to the metal interdigital transducer above the critical power threshold for liquid film instability atomization by adjusting the ratio of the on-time duration to the off-time of a single radio frequency pulse.

[0015] Preferably, the process of dividing the total sampled gas flow into the main reaction core flow and the sheath gas protection flow includes: obtaining the geometric parameters of the cross-sectional area of ​​the central flow channel and the cross-sectional area of ​​the annular flow channel of the micro-nozzle; calculating the ratio of the equivalent cross-sectional area of ​​the annular flow channel to the central flow channel based on the principle of equal velocity matching, as the target allocation ratio of the flow field boundary; calculating the target volumetric flow rate of the main reaction core flow and the sheath gas protection flow respectively based on the dynamically obtained total system intake flow rate and the target allocation ratio; multiplying the target volumetric flow rate by the exhaust gas density coefficient under the calibration conditions, and converting it into the control voltage setting threshold of the corresponding thermal mass flow controller for closed-loop regulation.

[0016] Preferably, the process of forming the coaxial layer fluid includes: the main reaction core gas flow carrying ammonia mixes with the colorimetric aerosol group to form a colorimetric aerosol core flow, which is discharged from the outlet end of the central flow tube and enters the contraction section of the outer constraint tube. Simultaneously, the sheath gas protection gas flow enters the contraction section along the annular flow channel. Under the geometric cross-sectional contraction effect inside the contraction section, the outer sheath gas protection gas flow generates radially inward converging momentum, applying radial aerodynamic pressure to the colorimetric aerosol core flow located at the axial position, forming an aerodynamic focusing effect. To ensure that the colorimetric aerosol core flow is introduced into the hollow photonic crystal fiber without collision, the focusing diameter formed by the colorimetric aerosol core flow in the straight outlet section is controlled to be smaller than the hollow aperture of the hollow photonic crystal fiber. The focusing diameter of the colorimetric aerosol core flow is equal to the inner diameter of the straight outlet section multiplied by the square root of the ratio of the volumetric flow rate of the main reaction core gas flow to the sum of the volumetric flow rates of the main reaction core gas flow and the sheath gas protection gas flow.

[0017] Preferably, the process of suspending and transporting the colorimetric aerosol group includes: constraining the radial diffusion displacement of the colorimetric aerosol droplets in the gaseous medium during axial transport along the hollow-core photonic crystal fiber; calculating the geometric difference between the inner diameter of the central hollow channel of the hollow-core photonic crystal fiber and the initial focusing diameter of the colorimetric aerosol core flow, and obtaining half of this geometric difference as the initial radial thickness boundary condition of the aerodynamic isolation layer; and controlling the pump to maintain the target average flow velocity so that the radial diffusion displacement is always less than the initial radial thickness of the aerodynamic isolation layer, and the colorimetric aerosol droplets are always suspended in the axial region of the central hollow channel during axial transport.

[0018] Preferably, the process of extracting and subtracting baseline scattering loss caused by aerosols includes: controlling the beams of characteristic absorption wavelength and reference scattering wavelength to emit light alternately in a time-division multiplexing manner; performing high-speed sampling of continuous analog voltage signals and dividing them into a first sampling voltage sequence and a second sampling voltage sequence; performing mean-averaging processing on the sampling voltage sequences; extracting the transmission voltage signals of the characteristic absorption wavelength and the reference scattering wavelength; calculating the total attenuation absorbance at the two wavelengths in combination with the initial reference voltage; determining that the total attenuation absorbance at the reference scattering wavelength is entirely due to Mie scattering by aerosol particles, and using it as the scattering background loss; obtaining the corresponding Egstrom index based on the median particle size parameter of the aerosol, and converting the scattering background loss to the position of the characteristic absorption wavelength; and subtracting the converted scattering background loss value from the total attenuation absorbance of the characteristic absorption wavelength through differential operation to extract the intrinsic absorbance of the chromogenic complex.

[0019] Preferably, the process of calculating and outputting the concentration value of ammonia in the exhaust gas includes: obtaining the diameter of the Gaussian mode field of the light wave energy in the central hollow channel, and calculating the spatial overlap factor between the light field and the reaction core area; calculating the actual molar concentration of ammonia under operating conditions based on the intrinsic absorbance, spatial overlap factor, effective optical path of the absorption medium actually penetrated by the beam, and the aerosol volume fraction calculated by the law of conservation of mass in the gas and liquid phases; the actual molar concentration of ammonia under operating conditions is equal to the intrinsic absorbance of the colorimetric complex divided by the product of the molar absorptivity, the actual physical length of the hollow photonic crystal fiber, and the spatial overlap factor; obtaining the real-time collected thermodynamic temperature, absolute pressure, and absolute humidity parameters of the flue gas, converting the actual molar concentration of ammonia under operating conditions into the dry basis volume concentration under standard conditions, and then filtering and outputting the result.

[0020] The present invention, by adopting the above technical solution, can bring the following beneficial effects:

[0021] 1. This invention utilizes surface acoustic waves to continuously atomize liquid colorimetric reagents into a group of colorimetric aerosols. Combined with the contraction effect of micro-nozzles, the outer sheath gas protective airflow applies radial aerodynamic pressure to the core flow of the colorimetric aerosol carrying the ammonia to be tested, forming a coaxial layer fluid with an internal reaction zone and an external isolation zone. This enables the colorimetric aerosol to maintain aerodynamic suspension and transmission within the hollow photonic crystal fiber, preventing droplets from intruding and filling the air pores in the inner wall of the fiber microstructure cladding, thereby ensuring the integrity of the photonic bandgap and the stability of the light guiding performance.

[0022] 2. This invention controls a dual-wavelength semiconductor laser to alternately emit beams of characteristic absorption wavelength and reference scattering wavelength into a hollow channel. A digital timing generator is used to synchronously separate the sampling voltage sequence from the composite transmitted light intensity signal. The baseline loss caused by Mie scattering is deducted based on the transmission attenuation at the reference scattering wavelength and the Egstrom exponent. This eliminates the light density attenuation interference caused by aerosol particles in a gas-liquid two-phase fluid environment, thereby accurately extracting the intrinsic absorbance to analyze and output objective ammonia concentration data.

[0023] 3. This invention calculates the spatial overlap factor by obtaining the mode field diameter of the beam in the optical fiber and the focusing diameter of the colorimetric aerosol core flow, and constructs a concentration analytical model by combining the effective optical path of the absorption medium and the aerosol volume fraction calculated by the mass conservation law of the gas-liquid two phases. Temperature, absolute pressure and humidity parameters are introduced simultaneously for dry-wet basis conversion, realizing dynamic compensation for absorbance loss caused by aerodynamic focusing and measurement deviation caused by thermodynamic state fluctuations, and thus outputting the dry-based ammonia volume concentration that conforms to the standard state. Attached Figure Description

[0024] Figure 1 This is a flowchart of the spectrophotometric detection method of the present invention;

[0025] Figure 2 This is a diagram of the measurement system and the gas-liquid optical path connection of the present invention;

[0026] Figure 3 This is a schematic diagram of the partial structure and atomization mechanism of the surface acoustic wave atomization unit of the present invention;

[0027] Figure 4 This is a cross-sectional structure and aerodynamic focusing principle diagram of the coaxial nested nozzle of the present invention;

[0028] Figure 5 This is a schematic diagram of the radial cross-section of the hollow-core photonic crystal fiber and the aerodynamic levitation transmission mechanism of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Example 1

[0031] See attached document Figure 1-5 This embodiment provides a measurement system for a spectrophotometric detection method of ammonia in power plant exhaust gas, including a sampling and flow field distribution unit, a surface acoustic wave atomization unit, a coaxial pneumatic focusing reaction cavity, an optical flow coaxial coupling unit, and a photoelectric analysis and closed-loop control unit;

[0032] The inlet of the sampling and flow field distribution unit is connected to the tail flue of the denitrification system. The sampling and flow field distribution unit uses an isodynamic pressure sampling probe, and the flue gas to be measured is introduced into the system in conjunction with a heat tracing pipeline wrapped with an external constant temperature heating jacket. The inner wall material of the heat tracing pipeline is specifically polytetrafluoroethylene or 316L stainless steel, which is used to prevent wall condensation and physical adsorption of high-temperature ammonia-containing waste gas during transmission. The system is equipped with a cyclone dust collector and a microporous filter membrane component with a pore size of 0.1 micrometer to 1 micrometer to perform mechanical and physical interception gas-solid separation treatment on the flue gas. The gas path after dust removal is divided into two independent branches through a fluid manifold. Each branch is equipped with a high-precision mass flow controller connected in series, which constitutes the core reaction gas path and sheath gas protection gas path of the measurement system.

[0033] The surface acoustic wave (SAW) atomization unit is located on the side or downstream of the aforementioned flow field distribution unit. It consists of a micro-injection pump and a piezoelectric microfluidic chip. The substrate material of the piezoelectric microfluidic chip is a lithium niobate piezoelectric crystal. A metal interdigital transducer is photolithographically deposited on the surface of the crystal substrate. The liquid outlet of the micro-injection pump extends to the working beam energy transfer area of ​​the interdigital transducer. An alternating radio frequency electric field excites the metal interdigital transducer to generate surface acoustic waves. Using the acoustic-fluid force formed by solid-liquid interface energy coupling, the liquid colorimetric agent continuously injected by the micro-injection pump is dispersed into an aerosol group with a uniform particle size distribution. Through the SAW atomization unit, the static liquid phase colorimetric agent is transformed into a highly dispersed gas-liquid suspension two-phase fluid.

[0034] The coaxial pneumatic focusing reaction chamber is equipped with a micro-nozzle with a double-layer nested geometry. The central flow channel inside the micro-nozzle is connected to the core reaction gas path and the aerosol output port of the piezoelectric microfluidic chip. The annular flow channel around the micro-nozzle is connected to the sheath gas protection gas path.

[0035] The optical flow coaxial coupling unit includes a detection light source, a hollow-core photonic crystal fiber, and a photoelectric detection terminal. The detection light source is specifically implemented as a time-division multiplexed dual-wavelength semiconductor laser containing a characteristic absorption wavelength and a reference scattering wavelength. The air inlet end of the hollow-core photonic crystal fiber and the outlet of the constriction section of the micro-nozzle are on the same central axis and are seamlessly connected. The cladding region of the hollow-core photonic crystal fiber has a micron-scale periodic bandgap microstructure array with a hollow channel at the center that allows fluid to pass through. A dichroic mirror and a photomultiplier tube are installed on the external optical path of the air outlet end of the hollow-core photonic crystal fiber. A miniature electrostatic dust collector is installed at the end of the exhaust gas path to perform gas-liquid separation and exhaust gas recovery on the aerosol containing colorimetric complexes that has completed photometric detection.

[0036] The above-mentioned spectrophotometric detection method for ammonia in power plant exhaust gas includes the following steps:

[0037] Step S101: The measurement system extracts ammonia-containing waste gas from the tail flue of the denitrification system through the heat tracing pipeline. After filtering out fly ash particles, two independent mass flow controllers divide the total sampled gas flow into the main reaction core airflow entering the central flow channel and the sheath gas protection airflow entering the annular flow channel according to the preset mass flow ratio value.

[0038] In step S102, the photoelectric analysis and closed-loop control unit outputs a radio frequency drive signal to the interdigital transducer, and the micro-injection pump pumps the liquid color developer into the surface of the piezoelectric microfluidic chip. The surface acoustic wave energy continuously atomizes the liquid color developer into submicron-sized colorimetric aerosol groups.

[0039] In step S103, the main reaction core gas flow carries the ammonia gas to be tested into the central flow channel. The ammonia molecules undergo cross-phase mass transfer and complexation color reaction with the color reagent at the aerosol droplet interface. The sheath gas protective gas flow outside the central flow channel forms a laminar boundary layer under the action of the internal and external velocity pressure difference of the micro-nozzle contraction section, which radially constrains and wraps the color aerosol group in the center, forming a core-shell coaxial fluid field with a droplet-containing reaction zone inside and a pure gas-wrapped zone outside.

[0040] In step S104, the core-shell coaxial fluid field is continuously introduced into the central hollow channel of the hollow photonic crystal fiber. The outer pure gas acts as an aerodynamic physical barrier, causing the colorimetric aerosol droplets at the axial position to be suspended and transported, preventing the tiny droplets from physically contacting and capillary condensing with the optical microstructure of the inner wall of the hollow photonic crystal fiber. The photoelectric resolution and closed-loop control unit controls the time-division multiplexed dual-wavelength semiconductor laser to alternately emit beams of characteristic absorption wavelength and reference scattering wavelength. The beams are coupled into the central hollow channel. Under the lateral confinement of the photonic bandgap, the light waves interact with the colorimetric aerosol group in the propagation direction through transmission and absorption.

[0041] In step S105, the photomultiplier tube receives the composite light intensity signal transmitted from the end of the hollow photonic crystal fiber. The photoelectric analysis and closed-loop control unit synchronously separates and extracts the transmitted light intensity signal of the characteristic absorption wavelength and the transmitted light intensity signal of the reference scattering wavelength according to the alternating triggering sequence of the light source. The photoelectric analysis and closed-loop control unit has a built-in concentration analysis model. By calculating the total attenuation of the characteristic absorption wavelength and extracting and subtracting the baseline scattering loss caused by aerosols using the reference scattering wavelength, it finally calculates and outputs the concentration value of ammonia in the exhaust gas.

[0042] During the concentration analysis process, the measurement system simultaneously operates an anti-saturation adaptive flow rate control mechanism. The photoelectric analysis and closed-loop control unit continuously reads the transmitted light intensity signal at the reference scattering wavelength. When the transmitted light intensity is lower than the set lower limit threshold, it indicates that the aerosol concentration inside the hollow-core photonic crystal fiber is overloaded or condensation has occurred on the tube wall. This lower limit threshold is set based on the reference transmitted light intensity of the fiber when pure sheath gas protective gas flow is introduced. At this time, the photoelectric analysis and closed-loop control unit outputs a negative feedback control signal, while increasing the overall pumping speed of the system and reducing the output power of the radio frequency drive signal to reduce the aerosol concentration entering the hollow-core photonic crystal fiber and accelerate the internal gas discharge speed until the transmitted light intensity at the reference scattering wavelength recovers to above the lower limit threshold. Then, the photoelectric analysis and closed-loop control unit cancels the negative feedback control signal, and the measurement system returns to normal detection status.

[0043] Example 2

[0044] See attached document Figure 1-5This embodiment provides a measurement system for spectrophotometric detection of ammonia in power plant exhaust gas. Under the scheduling of the control unit, the system executes a closed loop of physical transport, chemical colorimetric reaction, and optical signal analysis of gas-liquid-solid multiphase flow. The specific lower-level hardware architecture of the control unit includes a microprocessor, an analog-to-digital converter module, a digital timing generator, and a digital-to-analog output module, which coordinate the timing actions and data flow of each gas path, liquid path, and photoelectric component within the measurement system, including the following steps:

[0045] Step S201: The control unit outputs a constant temperature heating command to the heat tracing pipeline to maintain the pipe wall temperature above the flue gas acid dew point temperature, extracts ammonia-containing waste gas from the flue, and the ammonia-containing waste gas passes through the cyclone dust collector and microporous filter membrane assembly to complete the physical separation of gas phase and solid fly ash. The control unit outputs the given control voltage to the mass flow controller on the two branches through the digital-analog output module, and divides the clean waste gas flow after dust removal into the main reaction core flow and sheath gas protection flow according to the set mass flow ratio value.

[0046] In step S202, the microprocessor outputs a high-frequency square wave pulse to the radio frequency generation circuit via a digital timing generator, generating a radio frequency electrical signal of a preset frequency and applying it to the metal interdigital transducer of the piezoelectric microfluidic chip. The micro-injection pump receives the stepping pulse command and pushes the colorimetric reagent solution to the acoustic coupling region on the surface of the piezoelectric microfluidic chip at a constant rate. The surface acoustic wave energy excited by the interdigital transducer causes capillary wave instability on the liquid film surface, continuously atomizing the liquid colorimetric reagent and converting it into a submicron-sized colorimetric aerosol flow. The control unit locks the median droplet size distribution of the colorimetric aerosol flow by changing the driving frequency and power reference of the radio frequency electrical signal. Specifically, the control unit has a built-in correlation function between the median droplet size distribution and the driving frequency.

[0047]

[0048] In the formula, This represents the median droplet size. The surface tension of the colorimetric reagent solution; The density of the colorimetric reagent solution; This refers to the driving frequency of the radio frequency electrical signal; To establish system calibration constants, the control unit sets the target frequency. To achieve stable control of atomized particle size;

[0049] In step S203, the main reaction core gas flow carrying the ammonia molecules to be tested is introduced into the central channel of the coaxial pneumatic focusing reaction chamber, where it merges with the colorimetric aerosol flow. The ammonia molecules penetrate the gas-liquid interface of the aerosol droplets and complete the complexation colorimetric reaction in the liquid microenvironment. Simultaneously, the sheath gas protective gas flow is introduced into the annular channel around the micro-nozzle. In the constriction section of the micro-nozzle, based on the momentum exchange between the inner and outer fluid layers and the physical constraint of the geometric cavity, the pure sheath gas protective gas flow forms an annular laminar flow wrapping layer outside the colorimetric aerosol flow, constructing a coaxial laminar fluid with a droplet-containing reaction zone inside and a dry gas isolation zone outside.

[0050] In step S204, the coaxial layer fluid is continuously pumped into the central hollow channel of the hollow photonic crystal fiber. The outer sheath layer acts as a physical isolation barrier, preventing the internal droplets from colliding and condensing with the microstructure of the inner wall of the hollow photonic crystal fiber, thus maintaining the bandgap guiding properties of the hollow photonic crystal fiber. The digital timing generator outputs a square wave synchronous pulse to drive the dual-wavelength semiconductor laser to alternately emit a characteristic absorption wavelength beam and a reference scattering wavelength beam. The two probe beams are coaxially coupled into the central hollow channel, propagate along the axial direction for a long distance, and undergo transmission absorption and scattering attenuation with the suspended colorimetric aerosol group.

[0051] In step S205, the composite transmitted beam carrying attenuation information, after being filtered by a dichroic mirror at the end of the optical path to remove stray ambient light, illuminates the cathode target of the photomultiplier tube, converting it into a weak current signal. The transimpedance amplifier circuit within the measurement system converts the current signal into a continuous analog voltage signal, which is then discretized by the analog-to-digital converter module and input to the microprocessor. Based on the phase of the square wave synchronization pulse at the transmitting end, the microprocessor executes a demultiplexing algorithm to accurately extract the transmission voltage signal corresponding to the characteristic absorption wavelength and the transmission voltage signal corresponding to the reference scattering wavelength. The microprocessor then calls the concentration analysis model in the system memory, subtracts the aerosol scattering background loss parameter, calculates and outputs the ammonia concentration data in the exhaust gas.

[0052] In step S206, while outputting concentration data, the microprocessor simultaneously executes an anti-saturation adaptive logic control closed loop, and extracts the transmission voltage signal corresponding to the real-time acquired reference scattering wavelength. and compared with the reference voltage threshold pre-stored in the memory cell. Perform numerical comparisons, reference voltage threshold The calibration is the transmission baseline voltage when only pure sheath gas is introduced into the system in the initial stage, and the logical judgment condition satisfies the formula:

[0053]

[0054] In the formula, This represents the transmission voltage deviation value. This is the transmission voltage signal corresponding to the reference scattering wavelength; The reference voltage threshold is pre-stored in the memory cell;

[0055] When the difference is calculated (in When the system tolerance margin is set, the microprocessor determines whether an aerosol concentration overload or condensation has occurred inside the hollow photonic crystal fiber.

[0056] In step S207, based on the above anomaly determination, the control unit outputs a power attenuation command to the RF generation circuit via the digital-to-analog output module, forcibly reducing the atomization rate of the piezoelectric microfluidic chip; simultaneously, it outputs a frequency upsampling command to the diaphragm pump on the main suction line, increasing the overall suction sweep flow rate of the measurement system. Under the purging of the flow field, redundant droplets in the hollow photonic crystal fiber cavity are discharged, and the microprocessor continuously monitors... Changes, when When the optical state of the cavity returns to normal, the control unit cancels the power attenuation and frequency upsampling commands, and the measurement system restores the preset atomization and pumping parameter configurations to maintain continuous online detection of the system.

[0057] Furthermore, an isodynamic pressure sampling probe is installed at the front end of the sampling and flow field distribution unit, extending into the tail flue of the denitrification system. The inlet cross-section of the isodynamic pressure sampling probe is parallel to the flow direction of the exhaust gas inside the flue. To achieve isodynamic pressure sampling, an S-shaped Pitot tube is fixedly installed on the outer wall of the isodynamic pressure sampling probe. The pressure measuring end face of the S-shaped Pitot tube is at the same horizontal plane as the inlet cross-section of the isodynamic pressure sampling probe. The control unit calculates the flue gas velocity inside the flue by measuring the dynamic pressure difference of the S-shaped Pitot tube. Based on the calculated flue gas velocity, the control unit adjusts the flow rate of the rear exhaust gas path in real time to keep the intake gas velocity of the isodynamic pressure sampling probe consistent with the flue gas velocity. The control logic of the control unit adjusting the exhaust gas flow rate satisfies the following formula:

[0058]

[0059] In the formula, The target pumping volume flow rate of the isodynamic pressure sampling probe; The correction factor for the S-shaped Pitot tube; The area of ​​the air inlet cross section of the isodynamic pressure sampling probe; The dynamic pressure difference measured by the S-type Pitot tube; To correspond to the working density of the gas at the operating temperature, the above hardware configuration and flow tracking logic are used to avoid errors in the concentration distribution of particulate matter in the exhaust gas entering the isodynamic sampling probe due to flow field distortion.

[0060] The isodynamic pressure sampling probe's tube body is made of 316L stainless steel, with a PTFE anti-corrosion coating on the inner wall. The probe's tail end is connected to a heat tracing pipeline via a flange. This pipeline has a coaxial, multi-layered structure. A resistance heating wire is wrapped around the outer side of the central airflow channel, and this wire is further wrapped with an aluminum silicate insulation layer. The outermost layer of this insulation layer is extruded with a flame-retardant PVC protective sheath. An armored thermocouple is installed inside the pipeline, with its measuring face in contact with the outer wall of the central airflow channel. The heating temperature of the pipeline is controlled by a control unit. To prevent ammonia and sulfur trioxide in the exhaust gas from reacting in a steam environment to form sticky ammonium bisulfate particles that adhere to the pipeline wall, the control unit sets a temperature limit for the pipeline wall. The following thermodynamic conditions must be met:

[0061]

[0062]

[0063] In the formula, The control unit stores the acid dew point temperature of the ammonia-containing waste gas under the current component partial pressure, based on a preset value derived from the sulfur content of the coal. Constant value; This is the set margin for anti-condensation temperature compensation.

[0064] The exhaust end of the heat tracing pipeline is connected to the inlet of the cyclone dust collector. The cyclone dust collector includes a cylindrical upper cyclone chamber, an inverted conical lower contraction chamber, and a dust collection hopper at the bottom. The inlet of the cyclone dust collector is tangentially arranged to the upper cyclone chamber. The exhaust gas enters the upper cyclone chamber tangentially, generating a rotating vortex. Large-diameter fly ash particles inside the exhaust gas move outward under centrifugal force and collide with the inner wall of the upper cyclone chamber. After losing kinetic energy, the fly ash particles slide down along the inverted conical lower contraction chamber and are collected in the dust collection hopper. The exhaust gas flow after being treated by the cyclone dust collector flows out from the center of the top of the upper cyclone chamber. Exhaust gas exits through the exhaust pipe and is introduced into a microporous filter membrane assembly. The outer shell of the microporous filter membrane assembly is made of high-temperature resistant alloy steel. Inside the microporous filter membrane assembly is a sintered metal mesh filter membrane with a pore size between 0.1 micrometers and 1 micrometer. When exhaust gas passes through the sintered metal mesh filter membrane, submicron-sized dust particles are physically trapped on the windward side of the sintered metal mesh filter membrane. Both the cyclone dust collector and the microporous filter membrane assembly are equipped with cast aluminum heating modules. These cast aluminum heating modules contain embedded electric heating rods. The operating temperature of the cast aluminum heating modules is controlled by the wall temperature setpoint of the heat tracing pipeline. To maintain consistency and keep the exhaust gas at a constant temperature during the physical dust removal process, the high-temperature clean sample gas filtered by the microporous membrane module is then input into the fluid manifold and enters the subsequent gas distribution stage of the measurement system.

[0065] Example 3

[0066] See attached document Figure 1-5 In this embodiment, the high-temperature clean sample gas filtered by the microporous filter membrane assembly is input to the air inlet of the fluid manifold through a stainless steel gas guide tube. The fluid manifold is provided with a stagnation point diversion chamber to buffer the pressure pulsation during the gas flow process. The exhaust end of the fluid manifold is provided with a first exhaust branch pipe and a second exhaust branch pipe in parallel. The first exhaust branch pipe is connected to the core reaction gas path of the measurement system.

[0067] The second exhaust branch pipe is connected to the sheath gas protection gas circuit in the measurement system. A first thermal mass flow controller is connected in series on the first exhaust branch pipe, and a second thermal mass flow controller is connected in series on the second exhaust branch pipe. The first and second thermal mass flow controllers mainly include a capillary heat transfer sensor, a proportional electromagnetic regulating valve, and a preamplifier processing circuit. The capillary heat transfer sensor obtains the instantaneous mass flow signal of the gas by measuring the heat carried away by the gas when it flows through the heating element. The preamplifier processing circuit converts the instantaneous mass flow signal into a standard voltage feedback value and transmits it to the control unit. The control unit outputs a drive voltage to the proportional electromagnetic regulating valve according to the set target flow distribution value to change the valve core opening.

[0068] The control unit performs the following control steps to distribute the flow between the core reaction gas path and the sheath gas protection gas path:

[0069] In step S301, the control unit acquires the geometric parameters of the cross-sectional area of ​​the central flow channel and the cross-sectional area of ​​the annular flow channel of the micro-nozzle. To ensure the formation of a stable laminar aerodynamic boundary within the coaxial aerodynamic focusing reaction chamber and to avoid turbulent mixing between the core flow and the sheath flow at the contact interface caused by velocity differences and shear forces, the control unit calculates the target distribution ratio of the flow field boundary based on the principle of equal velocity matching.

[0070] Step S302, the flow allocation ratio logic built into the control unit satisfies the following mathematical constraint formula:

[0071]

[0072] In the formula, Allocate a percentage to the target traffic; The volumetric flow rate of the sheath gas protective gas flow; The volumetric flow rate of the main reaction core gas flow; The equivalent cross-sectional area of ​​the annular flow channel inside the coaxial pneumatic focusing reaction chamber; This is the equivalent cross-sectional area of ​​the central flow channel inside the coaxial pneumatic focusing reaction chamber;

[0073] Step S303: The control unit dynamically obtains the total system intake flow rate from the front-end isodynamic pressure sampling probe. Combined with the target traffic allocation ratio The specific flow target values ​​for the core reaction gas path and the sheath gas protection gas path are calculated separately, and the specific calculation formulas are as follows:

[0074]

[0075]

[0076] Since the system gas path is in a constant temperature state, the control unit will use the volumetric flow rate of the main reaction core gas flow calculated above. Volumetric flow rate of sheath gas protective gas flow Multiply by the exhaust gas density coefficient under the calibrated operating conditions to convert it into the control voltage setting threshold corresponding to the first thermal mass flow controller and the second thermal mass flow controller;

[0077] In step S304, the first thermal mass flow controller and the second thermal mass flow controller achieve steady-state output under proportional-integral-derivative closed-loop regulation, and introduce the constant flow rate of the main reaction core airflow into the central flow channel of the micro-nozzle, while simultaneously introducing the constant flow rate of the sheath gas protective airflow into the annular flow channel around the micro-nozzle. Through the above hardware configuration and flow distribution control logic, the measurement system establishes a stable coaxial flow field boundary that meets the aerodynamic isolation requirements at the front end of the subsequent optical microstructure.

[0078] The surface acoustic wave atomization unit consists of a reagent supply module and a piezoelectric coupling module. The reagent supply module is configured as a micro-injection pump, and the piezoelectric coupling module is configured as a solid surface acoustic wave microfluidic chip. The liquid supply end of the micro-injection pump is connected to the working surface of the surface acoustic wave microfluidic chip through a capillary infusion tube made of polyetheretherketone material.

[0079] The bottom layer of the surface acoustic wave (SAW) microfluidic chip is a solid piezoelectric substrate. The specific material of the piezoelectric substrate is a 128° YX-cut lithium niobate single crystal with a high electromechanical coupling coefficient. The upper surface of the piezoelectric substrate is deposited with a metal interdigital transducer through photolithography and electron beam evaporation. The electrode material of the metal interdigital transducer is a gold and chromium bilayer film. The metal interdigital transducer includes two sets of intersecting comb-shaped electrodes with a fixed interdigital periodic distance. The intersection area of ​​the two sets of comb-shaped electrodes constitutes the acoustic wave excitation region. A rectangular flat area treated with hydrophilicity is photolithographically etched on one side of the acoustic wave excitation region along the direction of SAW propagation, which constitutes the acoustic wave coupling region.

[0080] The micro-injection pump contains a glass syringe that stores liquid colorimetric reagent. The piston of the glass syringe is mechanically connected to the stepper motor inside the micro-injection pump. The outlet end of the polyetheretherketone capillary infusion line is suspended directly above the acoustic coupling zone. The liquid colorimetric reagent flows out under the thrust and spreads on the surface of the acoustic coupling zone, forming a continuous thin liquid film.

[0081] The physical process of colorimetric reagent transport and atomization performed by the surface acoustic wave microfluidic chip and reagent pumping system can be divided into the following steps:

[0082] In step S401, the control unit sends a control command to the stepper motor of the micro-injection pump. The stepper motor rotates according to a preset pulse frequency, pushing the push piston of the glass syringe. By adjusting the speed parameters of the stepper motor, the liquid color developer is output to the acoustic coupling region of the surface acoustic wave microfluidic chip at a constant volume flow rate, thereby enabling the acoustic coupling region to obtain a constant flow rate of reaction substrate.

[0083] In step S402, the control unit applies an alternating radio frequency voltage signal to the input pin of the metal interdigital transducer. Under the inverse piezoelectric effect, the lithium niobate single-crystal piezoelectric substrate undergoes periodic lattice deformation, exciting Rayleigh-type surface acoustic waves on the surface of the piezoelectric substrate. To form an effective resonance, the input frequency of the alternating radio frequency voltage signal and the physical dimensions of the metal interdigital transducer satisfy the following matching formula:

[0084]

[0085] In the formula, The center resonant frequency of the alternating radio frequency voltage signal; The propagation speed of Rayleigh surface acoustic waves on a 128° YX-cut lithium niobate single crystal surface; The periodic distance between the two sets of comb electrodes of the metal interdigital transducer is the periodic distance between the fingers. Rayleigh surface acoustic waves propagate directionally along the surface of the piezoelectric substrate to the acoustic coupling region and undergo acoustic-solid-liquid multiphase energy coupling with the liquid colorant film covering the acoustic coupling region.

[0086] In step S403, the acoustic energy of the surface acoustic wave penetrates the solid-liquid interface and refracts into the interior of the liquid film, forming an acoustic flow effect. The acoustic flow force generates internal vortices inside the liquid film, accelerating the mixing of color developer solution molecules. The acoustic radiation pressure overcomes the gas-liquid surface tension and liquid viscous resistance of the liquid color developer. Unstable surface capillary waves are generated at the gas-liquid interface of the thin liquid film. The peaks of the surface capillary waves break under the continuous action of acoustic radiation pressure. The liquid color developer is peeled off from the piezoelectric substrate surface and transformed into a uniform submicron-sized aerosol group. The aerosol group generated by atomization leaves the acoustic coupling zone under the guidance of the negative pressure of the gas path and enters the subsequent micro-nozzle flow channel.

[0087] When the surface acoustic wave microfluidic chip continuously performs atomization, the interdigital transducer and the piezoelectric substrate will generate Joule heat due to high-frequency oscillation. Temperature changes will cause microscopic deformation of the lattice spacing of the piezoelectric substrate, causing temperature drift of the inherent resonant frequency of the interdigital transducer. If the frequency of the radio frequency drive signal is not adjusted accordingly, it will lead to a decrease in acoustic wave conversion efficiency, resulting in a wider aerosol particle size distribution, or causing large un-atomized droplets to enter the optical gas path.

[0088] To achieve dynamic locking of the driving frequency and maintain a constant aerosol particle size distribution, the measurement system is equipped with a direct digital frequency synthesizer, an RF power amplifier, and a bidirectional directional coupler inside the control unit. The direct digital frequency synthesizer is used to generate the initial electrical signal in the RF band, the RF power amplifier is used to boost the power level of the initial electrical signal and output it to the interdigital transducer, and the bidirectional directional coupler is connected in series on the RF transmission line between the RF power amplifier and the interdigital transducer.

[0089] The physical process by which the control unit performs drive frequency locking and aerosol particle size control can be divided into the following steps:

[0090] In step S501, the control unit sends a sweep command to the direct digital frequency synthesizer. The direct digital frequency synthesizer outputs a step sweep signal within a set bandwidth range with the preset target atomization frequency as the center frequency. The bidirectional directional coupler synchronously collects the forward transmission power and the reverse reflection power reflected back by the metal interdigital transducer during the radio frequency signal transmission process.

[0091] In step S502, the analog-to-digital conversion module built into the control unit reads the voltage values ​​of the forward transmission power and the reverse reflection power. The control unit calculates the voltage standing wave ratio (VSWR) in the RF transmission channel based on the forward transmission power and the reverse reflection power. The calculation logic satisfies the following formula:

[0092]

[0093] In the formula, The voltage standing wave ratio (VSWR) of the radio frequency transmission channel; Forward transmission power measured for a bidirectional directional coupler; The reverse reflected power measured by the bidirectional directional coupler;

[0094] Step S503: The control unit compares the voltage standing wave ratio (VSWR) values ​​at different sweep frequency points, extracts the frequency corresponding to the minimum VSWR point, determines that the frequency corresponding to the minimum VSWR point is the actual physical resonant frequency of the piezoelectric substrate at the current temperature, and locks the output frequency of the direct digital frequency synthesizer to the actual physical resonant frequency to complete the dynamic matching of acoustic impedance.

[0095] In step S504, based on the locked drive frequency, to prevent changes in the physical parameters such as the surface tension and density of the liquid colorant in the acoustic coupling region due to local temperature rise caused by constant power output, the control unit performs pulse width modulation on the RF power amplifier. The control unit modulates the continuous RF signal into an RF pulse train for output, and the average output power and pulse parameters satisfy the following formula:

[0096]

[0097] In the formula, The average radio frequency power applied to the metal interdigital transducer; This represents the peak envelope power of the radio frequency pulse train. The duration of a single radio frequency pulse; The off interval of a single radio frequency pulse.

[0098] In step S505, the control unit adjusts the on-time. With the shutdown interval The ratio of the average RF power to the average RF power Above the critical power threshold for liquid film instability and atomization, the control unit stores the critical power threshold. When the control unit outputs a radio frequency pulse train, it forces the control parameters to meet logical constraints: Critical power threshold The measurement system is pre-written into the control unit's storage module during the factory calibration stage, depending on the real-time volumetric flow rate and surface tension of the liquid color developer.

[0099] The alternating on and off process of the radio frequency pulse train provides a time window for heat dissipation of the piezoelectric substrate. The constant substrate temperature and the locked resonant frequency work together to ensure that the aerosol droplets stripped from the liquid film surface maintain a nano-to-submicron particle size distribution, which meets the aerodynamic requirements of the subsequent hollow-core photonic crystal fiber for low scattering measurement environment.

[0100] The core hardware of the coaxial pneumatic focusing reaction chamber is a coaxial nested nozzle. The coaxial nested nozzle adopts a double-layer coaxial tube structure, specifically including an internal central flow tube and an outer constraint tube sleeved outside the central flow tube. The internal cavity of the central flow tube forms a central flow channel, and the annular gap between the outer wall of the central flow tube and the inner wall of the outer constraint tube forms an annular flow channel. The air inlet of the central flow tube is connected to the main reaction core gas path of the measurement system and the aerosol output port of the surface acoustic wave microfluidic chip. The air inlet of the outer constraint tube is connected to the sheath gas protection gas path of the measurement system.

[0101] The outer confinement tube is divided into a guide section, a contraction section and a straight outlet section along the direction of fluid movement. The guide section is a cylindrical structure with a constant diameter, the contraction section is a truncated cone structure with an inner diameter that gradually decreases along the direction of fluid movement, and the straight outlet section is a micro-circular tube with a fixed inner diameter. The outlet end of the central flow tube is located inside the outer confinement tube, and the outlet end face of the central flow tube is flush with the starting section of the contraction section. The solid materials of the central flow tube and the outer confinement tube are polytetrafluoroethylene or polished 316L stainless steel to reduce the frictional disturbance of the fluid boundary layer caused by the surface roughness of the inner wall of the pipeline.

[0102] The physical process by which the measurement system achieves aerodynamic focusing and flow field constraint through coaxial nested nozzles can be divided into the following steps:

[0103] In step S601, the main reaction core airflow carries the ammonia gas to be tested into the central flow channel and simultaneously merges into the colorimetric aerosol group generated by the surface acoustic wave atomization unit. The ammonia molecules and aerosol droplets undergo spatial mixing and gas-liquid mass transfer contact in the central flow channel.

[0104] In step S602, the sheath gas protective airflow is introduced into the annular flow channel. In order to maintain the stability of the downstream flow field interface, the control unit adjusts the gas flow rate to ensure that the actual Reynolds number of the gas in the central flow channel and the annular flow channel is always less than the critical Reynolds number threshold stored in the system, thereby ensuring that the two fluids remain in a stable laminar flow state before merging.

[0105] In step S603, the colorimetric aerosol core flow is discharged from the outlet end of the central flow pipe and enters the contraction section of the outer constraint pipe. Simultaneously, the sheath gas protective flow enters the contraction section along the annular flow channel. Under the geometric cross-sectional contraction effect inside the contraction section, the outer sheath gas protective flow generates radially inward converging momentum. The radial momentum applies radial aerodynamic pressure to the colorimetric aerosol core flow located at the axial position, forming an aerodynamic focusing effect. The flow diameter of the colorimetric aerosol core flow is physically compressed.

[0106] In step S604, the focused composite fluid enters the straight outlet section. Due to the wall thickness of the central flow tube, wake interference will occur at the confluence of the inner and outer fluids. To eliminate the shear stress caused by the velocity gradient, the control unit presets a flow distribution ratio to make the axial velocities of the colorimetric aerosol core flow and the sheath gas protective flow tend to be consistent in the straight outlet section. To ensure that the colorimetric aerosol core flow can be introduced into the subsequent hollow photonic crystal fiber without collision, the control unit has built-in pneumatic focusing diameter calculation logic. The focusing diameter formed by the colorimetric aerosol core flow in the straight outlet section satisfies the following formula:

[0107]

[0108] In the formula, The focusing diameter is the core flow diameter of the colorimetric aerosol. The inner diameter of the straight exit section; The volumetric flow rate of the sheath gas protective gas flow; To control the volumetric flow rate of the sheath gas protective gas flow, the control unit adjusts the aforementioned volumetric flow rate parameters to adjust the focusing diameter. Smaller than the hollow core aperture of the subsequent hollow core photonic crystal fiber;

[0109] In step S605, after the flow field rectification and boundary layer development in the straight outlet section, a stable laminar interface is established inside the composite fluid, forming a coaxial layer fluid with a droplet-containing reaction zone inside and a dry gas isolation zone outside. The outer diameter of the reaction zone containing droplets inside the coaxial layer fluid is constrained within the inner diameter of the straight outlet section. The sheath gas boundary layer prevents the droplets from radially diffusing outward or making contact with the wall.

[0110] After establishing a coaxial laminar flow field within the coaxial pneumatic focusing reaction chamber, the gas flow focusing process and the transphase mass transfer and colorimetric reaction occur simultaneously within the coaxial pneumatic focusing reaction chamber. The physicochemical processes of transphase mass transfer and colorimetric reaction within the coaxial laminar flow field can be divided into the following steps:

[0111] In step S701, the main reaction core airflow and the sheath gas protective airflow maintain a stable stratified coaxial layer fluid within the straight outlet section of the micro-nozzle. The sheath gas protective airflow forms a pure gas phase isolation boundary around the coaxial layer fluid. The pure gas phase isolation boundary restricts the radial Brownian motion and lateral diffusion of the internal colorimetric aerosol droplets. The colorimetric aerosol group is constrained by the hydrodynamic space in the axial region of the coaxial layer fluid, forming a reaction core region with a clear physical boundary.

[0112] In step S702, the ammonia molecules to be tested carried in the main reaction core gas flow are in a mixed state with the colorimetric aerosol group in the reaction core area. Thanks to the background presence of gaseous water molecules in the extracted high-temperature ammonia-containing waste gas, a high local humidity in the gas phase is maintained in the coaxial pneumatic focusing reaction chamber, which effectively suppresses the evaporation of water from the droplets and ensures that they remain in a stable liquid phase state throughout the transmission process. At the same time, the submicron-sized colorimetric aerosol droplets generated by surface acoustic wave atomization provide a huge gas-liquid interphase contact area in the reaction core area. The small aerosol droplet size significantly enlarges the interphase contact interface, which is beneficial to accelerating the mass transfer process.

[0113] In step S703, driven by the concentration gradient, ammonia molecules diffuse directionally from the gas phase to the gas-liquid interface of the colorimetric aerosol droplets. The ammonia molecules penetrate the gas-liquid interface and dissolve into the droplets. This cross-phase mass transfer process obeys the two-film mass transfer theory, and the concentration difference of ammonia molecules between the gas and liquid phases drives the continuous cross-phase transfer.

[0114] In step S704, the ammonia molecules dissolved inside the aerosol droplets chemically complex with the color-developing agent substrate in the liquid phase. The ammonia molecules react with hypochlorite ions and salicylate ions in an alkaline microenvironment to generate an indophenol blue complex with specific light absorption properties. Since the concentration of the pre-placed color-developing agent substrate in the aerosol droplets is much greater than the concentration of ammonia molecules entering the phase, the color-developing complexation reaction in this microscopic liquid phase system exhibits a pseudo-first-order chemical kinetic.

[0115] In step S705, as the coaxial layer fluid continues to flow towards the coupling port of the hollow photonic crystal fiber, the number of complex molecules inside the colorimetric aerosol accumulates over time. The control unit stores the time constant required for the colorimetric complexation reaction to reach kinetic equilibrium. The control unit constrains the flow residence time of the coaxial layer fluid before entering the hollow photonic crystal fiber by regulating the sum of the flow rates of the main reaction core airflow and the sheath gas protective airflow. The calculation logic of the flow residence time satisfies the following formula:

[0116]

[0117] In the formula, The residence time of the fluid in the coaxial layer; This refers to the inner diameter of the straight outlet section of the micro-nozzle; The effective physical transmission tube length from the straight exit section to the end face of the hollow photonic crystal fiber; The volumetric flow rate of the sheath gas protective gas flow; The control unit controls the total flow rate parameter to regulate the volumetric flow rate of the sheath gas protective gas flow, thereby controlling the flow residence time. The time constant is greater than that required for the colorimetric complexation reaction to reach kinetic equilibrium, allowing the colorimetric aerosol group to complete the steady-state transformation of the chemical colorimetric process before being introduced into the hollow photonic crystal fiber.

[0118] The air inlet end face of the hollow photonic crystal fiber is seamlessly connected to the straight outlet section of the coaxial nested nozzle. The hollow photonic crystal fiber includes a central hollow channel, a microstructure cladding surrounding the central hollow channel, and an outermost solid quartz shell in its radial structure. The microstructure cladding is composed of a multi-layered array of micron-sized air holes arranged in a periodic manner. Due to the difference in refractive index between air and the quartz matrix, the microstructure cladding forms a photonic bandgap at the optical level, which confines the probe beam to the central hollow channel for transmission.

[0119] The physical process of a measurement system performing pneumatic levitation transmission and bandgap protection within a hollow photonic crystal fiber can be divided into the following steps:

[0120] In step S801, the coaxial layer fluid output from the coaxial pneumatic focusing reaction chamber is continuously introduced into the central hollow channel of the hollow photonic crystal fiber. The colorimetric aerosol core flow is located in the axial region of the central hollow channel, and the sheath gas protective flow forms an aerodynamic isolation layer between the colorimetric aerosol core flow and the inner wall of the microstructure cladding.

[0121] In step S802, during the axial propagation along the hollow photonic crystal fiber, the colorimetric aerosol droplets undergo radial Brownian diffusion motion in the gaseous medium. The control unit constrains the radial diffusion displacement of the aerosol droplets by setting fluid parameters, ensuring that the radial diffusion displacement is less than the radial thickness of the aerodynamic isolation layer. The boundary conditions for the radial diffusion displacement of the colorimetric aerosol droplets satisfy the following dynamic relationship:

[0122]

[0123] In the formula, The radial diffusion displacement of the chromogenic aerosol droplets during transport; Boltzmann's constant; The thermodynamic temperature of the gas inside the central hollow channel; This is the actual physical length of the hollow-core photonic crystal fiber; The dynamic viscosity of the gas; The median particle size of the chromogenic aerosol droplets; The average flow velocity of the coaxial layer fluid along the axial direction; The initial radial thickness of the aerodynamic isolation layer. The calculation formula is:

[0124]

[0125] In the formula, The inner diameter of the central hollow channel of the hollow-core photonic crystal fiber; To determine the focusing diameter of the colorimetric aerosol core flow, the control unit calculates the initial radial thickness of the aerodynamic isolation layer as a boundary condition and maintains the target average flow velocity by adjusting the pump. The above inequality constraints are satisfied.

[0126] Step S803: Based on the above dynamic constraints, the colorimetric aerosol droplets are always suspended in the central hollow channel during axial transport, which minimizes the risk of physical contact between them and the inner wall of the microstructure cladding. The presence of the aerodynamic isolation layer cuts off the path of liquid substances to migrate to the outer wall, preventing capillary condensation of the colorimetric aerosol droplets on the surface of the micron-sized air pores.

[0127] In step S804, if the liquid substance adheres to the inner wall of the microstructure cladding, the liquid medium will invade and fill part of the micron-sized air holes. After the micron-sized air holes are filled by liquid, the local equivalent refractive index of the micron-sized air holes changes abruptly from the refractive index of air to the refractive index of liquid, which causes the original periodic distribution boundary of the refractive index of the microstructure cladding to be destroyed. The destruction of the periodic distribution boundary of the refractive index will cause spectral drift of the photonic bandgap or collapse of the bandgap structure, causing leakage loss of the probe beam to the outside of the cladding. The aerodynamic suspension mechanism provided in this embodiment maintains the pure gas phase state of the internal lattice structure of the microstructure cladding, ensuring the integrity of the photonic bandgap and the stability of the light guiding performance.

[0128] In step S805, the light beam emitted by the probe light source is coupled into the central hollow channel through the lens group. Under the lateral confinement of the photonic bandgap, the light wave energy is concentrated in the axial region of the central hollow channel. Along the axial direction, it interacts with the colorimetric aerosol group suspended in the axial region of the central hollow channel through transmission and absorption. The gas outlet end of the hollow photonic crystal fiber outputs an optical signal carrying concentration attenuation information, thus completing the photometric detection of the target component in the gaseous suspension phase.

[0129] The optical path coaxial coupling unit includes a detection light source, a collimating and focusing lens group, a hollow photonic crystal fiber, a dichroic mirror, and a spectrophotometer. The detection light source is specifically implemented to include a central wavelength of... Characteristic absorption spectral lines and center wavelength The semiconductor laser group with reference scattering spectrum line, the collimating and focusing lens group is set between the probe light source and the air inlet end face of the hollow photonic crystal fiber, the dichroic mirror is set on the external optical path of the air outlet end of the hollow photonic crystal fiber, and the spectrophotometer detection module includes a photomultiplier tube corresponding to the output optical path of the dichroic mirror.

[0130] After being processed by the collimating and focusing lens group, the composite probe beam emitted by the probe light source is compressed into a converging beam. The cross-sectional diameter of the converging beam at the air inlet end face of the hollow photonic crystal fiber is smaller than the inner diameter of the central hollow channel of the hollow photonic crystal fiber. The converging beam couples into the interior of the hollow photonic crystal fiber along the axial direction of the central hollow channel. When the probe beam propagates inside the fiber, the center of the optical field distribution of the probe beam coincides with the spatial position of the suspended transmission colorimetric aerosol core flow.

[0131] The specific process of the measurement system performing optical path coaxial coupling and anti-saturation adaptive cleaning closed loop can be divided into the following steps:

[0132] Step S901: The control unit drives the detection light source to alternately emit wavelengths according to a preset timing sequence. and The light pulse propagates along the axis of the hollow photonic crystal fiber and undergoes transmission absorption with the colorimetric aerosol. The photomultiplier tube receives the transmitted light signal after it has been split by a dichroic mirror. The control unit, through its internal synchronous triggering timing, splits the continuous electrical signal output by the photomultiplier tube into corresponding characteristic absorption wavelengths on the time axis. Transmitted voltage signal and the transmission voltage signal corresponding to the reference scattering wavelength λ2 ;

[0133] Step S902: The control unit monitors the second sampling voltage in real time. The control unit will sample the second voltage. Compared with the pre-stored lower limit threshold of light intensity in memory Numerical comparison was performed to determine the lower limit threshold of light intensity. The setting logic satisfies the following relation:

[0134]

[0135] In the formula, The reference voltage for the measurement system when pure air is introduced; The preset maximum allowable attenuation coefficient for the optical path is calibrated by the control unit based on the critical attenuation rate of the photonic bandgap failure of the hollow-core photonic crystal fiber.

[0136] Step S903, when the control unit determines the second sampling voltage When the anti-saturation adaptive cleaning program is triggered, the control unit determines that aerosol particles have accumulated inside the hollow core photonic crystal fiber or the microstructure cladding has adhered to the inner wall.

[0137] In step S904, with the cleaning process activated, the control unit sends a modulation command to the RF power amplifier via the digital-to-analog converter interface to reduce the average RF power of the metal interdigital transducer. This reduces the rate at which the surface acoustic wave microfluidic chip generates colored aerosols. Simultaneously, the control unit sends an acceleration command to the pump in the main extraction circuit. The change in the exhaust flow rate of the pump satisfies the following adjustment logic:

[0138]

[0139] In the formula, This refers to the instantaneous purge flow rate after the cleaning process is triggered; A flow rate enhancement factor greater than 1; and These are the volumetric flow rates of the sheath gas protective gas flow and the volumetric flow rate of the sheath gas protective gas flow in the measurement mode, respectively.

[0140] Step S905, instantaneous purge flow Under shearing action, the droplets retained in the central hollow channel are expelled, and the control unit continuously detects the second sampling voltage. When satisfied At that time, the control unit determines that the cleaning is complete; among which, The preset hysteresis voltage is used to prevent the control logic from fluctuating frequently near the threshold point;

[0141] In step S906, the control unit cancels the power reduction command and the speed increase command, restores the RF power amplifier and the air pump to the set parameters in the measurement mode, and the measurement system resumes continuous monitoring of ammonia concentration.

[0142] The measurement system uses a digital timing generator built into the control unit to synchronize the detection light source and the back-end spectroscopic detection module, extracting the intrinsic absorbance of the colorimetric complex from the composite signal containing aerosol background scattering loss. In this embodiment, the specific process of dual-wavelength time-division multiplexing detection and signal extraction is divided into the following steps:

[0143] Step S1001: The control unit sends a high-frequency pulse trigger signal to the detection light source, and the center wavelength of the first laser output inside the detection light source is... The characteristic absorption spectral lines, the center wavelength of the second laser output is The reference scattering spectrum is used, and a digital timing generator controls the first and second lasers to emit light alternately in a time-division multiplexed manner during a single measurement cycle. Inside, The duration of the light pulse is , The duration of the light pulse is ,and and They do not overlap on the timeline. light pulse and A guard interval is set between the light pulses. ;

[0144] In step S1002, the photomultiplier tube at the end of the hollow-core photonic crystal fiber converts the received light intensity signal into a continuous analog voltage signal. The control unit performs high-speed sampling of the analog voltage signal through the analog-to-digital conversion module. The sampling frequency satisfies the Nyquist sampling law. According to the synchronous trigger phase of the digital timing generator, the control unit divides the sampling sequence into a first sampling voltage sequence corresponding to the first laser turn-on period and a second sampling voltage sequence corresponding to the second laser turn-on period.

[0145] In step S1003, the control unit performs averaging processing on the first sampled voltage sequence and the second sampled voltage sequence respectively to remove random photoelectric noise. The averaged voltage values ​​are defined as the transmission voltage signals corresponding to the characteristic absorption wavelengths. and the transmission voltage signal corresponding to the reference scattering wavelength ;

[0146] Step S1004: The control unit extracts the initial reference voltage of the characteristic absorption wavelength stored internally in the measurement system. and reference scattering wavelength initial reference voltage The initial reference voltage is pre-measured and stored by the measurement system under calibration conditions with clean sample gas introduced and no colorimetric aerosol. The control unit calculates the total attenuation absorbance of the measurement system at two wavelengths based on the transmission voltage signal and the initial reference voltage. The formula for calculating the total attenuation absorbance is:

[0147]

[0148]

[0149] In the formula, This represents the total attenuation absorbance at the characteristic absorption wavelength; The total attenuation absorbance at the reference scattering wavelength;

[0150] Step S1005, the control unit executes the scattering loss extraction algorithm, due to the reference scattering wavelength. Selecting a wavelength outside the absorption peak of the chromogenic complex, the control unit determines that the total attenuation absorbance at the reference scattering wavelength originates entirely from Mie scattering by aerosol particles, i.e., the scattering background loss at the reference scattering wavelength. Thanks to the highly concentrated monodispersity (uniform particle size distribution) of the submicron-sized colorimetric aerosol groups generated by the aforementioned surface acoustic wave atomization unit, the aerosol particles entering the hollow-core photonic crystal fiber can strictly satisfy the theoretical premise of Mie scattering distribution, effectively eliminating the calculation error caused by the multidispersive system. The control unit determines that the scattering background loss at the characteristic absorption wavelength and the transmission attenuation at the reference scattering wavelength are wavelength-dependent, and the scattering loss extraction logic satisfies the following physical and mathematical formula:

[0151]

[0152] In the formula, To be converted to the characteristic absorption wavelength The background scattering loss value at the location; The control unit stores an empirical mapping table between the median aerosol particle size and the Egstrom index. Based on the median aerosol particle size parameter output by the surface acoustic wave microfluidic chip, the control unit retrieves the corresponding index from the empirical mapping table. value;

[0153] In step S1006, the control unit calculates the total attenuation absorbance from the characteristic absorption wavelength through differential operation. After subtracting the scattering background loss value calculated above, the intrinsic absorbance of the colorimetric complex is extracted. :

[0154]

[0155] Through the aforementioned time-division multiplexing and cross-wavelength mapping logic, the measurement system eliminates the interference of aerosol particles within the hollow-core photonic crystal fiber on absorbance detection, providing absorbance input values ​​for subsequent quantitative analysis of ammonia concentration.

[0156] The measurement system executes algorithm instructions through the microprocessor built into the control unit to convert the optical parameters extracted from the front end into standard concentration values ​​applicable to engineering, and eliminates measurement deviations caused by flow field fluctuations and changes in thermodynamic state in real time. In this embodiment, the specific execution process of the dynamic compensation algorithm and the ammonia concentration analysis model is divided into the following steps:

[0157] Step S1101: The control unit acquires the spatial distribution parameters of the fundamental mode optical field of the probe beam in the hollow core photonic crystal fiber. In the central hollow channel of the hollow core photonic crystal fiber, the light wave energy distribution follows a Gaussian mode field distribution. The control unit reads the mode field diameter parameters of the probe beam at the characteristic absorption wavelength preset in the internal memory, and calls the focusing diameter of the colorimetric aerosol core flow calculated by the front-end flow field distribution module. The control unit determines that only photons located within the physical boundary of the colorimetric aerosol core flow can have an effective absorption interaction with the colorimetric complex.

[0158] Step S1102: The control unit calculates the spatial overlap factor between the light field and the reaction core region. The spatial overlap factor is used to compensate for the absorbance loss caused by the absorption medium cross-sectional area being smaller than the beam cross-sectional area due to aerodynamic focusing. The calculation logic of the spatial overlap factor satisfies the following formula:

[0159]

[0160] In the formula, The spatial overlap factor between the light field and the reaction core region; The focusing diameter is the core flow diameter of the colorimetric aerosol. To detect the mode field diameter of the beam at the characteristic absorption wavelength, the control unit sets constraints to ensure that it remains stable under any operating conditions. This leads to the spatial overlap factor. ;

[0161] Step S1103: The control unit calculates the actual molar concentration of ammonia under the operating conditions of the measurement system based on the intrinsic absorbance of the extracted colorimetric complex and the spatial overlap factor. The actual molar concentration of ammonia under the operating conditions represents the molar amount of ammonia molecules contained in a unit volume of the main reaction core gas flow. Since the colorimetric reaction occurs inside the aerosol droplets, the effective optical path of the absorption medium actually penetrated by the beam is reduced by the volume fraction of the aerosol. Based on the extended form of the Lambert-Beer law for colorimetric aerosols and the law of conservation of mass in the gas-liquid two-phase system, the calculation logic of the actual molar concentration of ammonia under the operating conditions satisfies the following derivation process:

[0162] First, the intrinsic absorbance of the aerosol suspension system satisfies:

[0163]

[0164] In the formula, The molar absorptivity of the chromogenic complex at its characteristic absorption wavelength is given. The molar concentration of the colorimetric complex generated inside the liquid colorimetric aerosol; This refers to the output volumetric flow rate of the liquid colorimetric reagent. The volumetric flow rate of the core reaction gas path; Characterizes the volume fraction of chromogenic aerosols in the core reaction gas stream;

[0165] Secondly, according to the principle of mass conservation in interphase mass transfer, ammonia molecules in the gas phase transfer to the liquid phase and reach dynamic equilibrium, satisfying the following:

[0166]

[0167] Substituting the above mass conservation relationship into the absorbance formula, we obtain the final calculation model for the molar concentration under operating conditions:

[0168]

[0169] In the formula, This represents the actual molar concentration of ammonia under operating conditions. The intrinsic absorbance of the colorimetric complex extracted by differential extraction in the previous step; The molar absorptivity of the colorimetric complex at the characteristic absorption wavelength; This is the actual physical length of the hollow-core photonic crystal fiber; As the spatial overlap factor between the light field and the reaction core region, the above derivation model directly and equivalently maps the absorbance based on the liquid phase reaction to the target concentration within the original gas phase volume.

[0170] Step S1104: The control unit acquires real-time thermodynamic data from the temperature and pressure sensors integrated in the gas path of the measurement system, and obtains the absolute humidity parameter of the flue gas fed back by the front-end preprocessing module. Based on thermodynamic state compensation and dry-to-wet basis conversion logic, the control unit converts the molar concentration under operating conditions into the dry-to-volume concentration under standard conditions. The formula for calculating the dry-to-volume concentration is:

[0171]

[0172] In the formula, The dry basis ammonia volume concentration under standard conditions is expressed in parts per million (ppm). This represents the actual molar concentration of ammonia under operating conditions. It is the ideal gas constant; The real-time thermodynamic temperature of the fluid inside the central hollow channel; The real-time absolute pressure within the central hollow channel; The mole fraction of water vapor in the flue gas being measured;

[0173] In step S1105, the control unit stores the calculated dry-based ammonia volume concentration into the cache register and calls the first-order hysteresis filtering algorithm to smooth the concentration data on the continuous time series. The filtered concentration value is output to the external denitrification distributed control system through the digital interface or analog current loop to provide continuous ammonia escape monitoring data.

Claims

1. A spectrophotometric method for detecting ammonia in power plant exhaust gas, characterized in that, The process includes the following steps: extracting ammonia-containing exhaust gas from the tail flue of the denitrification system, filtering out fly ash particles, and then cutting the total sampled gas flow into the main reaction core gas flow entering the central channel of the micro-nozzle, and the sheath gas protection gas flow entering the annular channel of the micro-nozzle. Liquid color developer is pumped into the surface of a piezoelectric microfluidic chip, and surface acoustic wave energy is used to continuously atomize the liquid color developer into a group of color-developing aerosols. The main reaction core gas flow carries the ammonia gas to be tested into the central flow channel. The ammonia molecules undergo cross-phase mass transfer and complexation color reaction with the colorimetric aerosol group. The sheath gas protective gas flow outside the central flow channel forms a laminar boundary layer under the action of the internal and external velocity pressure difference in the contraction section. This layer radially constrains and encapsulates the colorimetric aerosol group in the center, forming a coaxial layer fluid with a droplet-containing reaction zone inside and a dry gas isolation zone outside. The coaxial layer fluid is continuously introduced into the central hollow channel of the hollow photonic crystal fiber. The external sheath gas layer acts as a physical isolation barrier, allowing the colorimetric aerosol group to be suspended and transported, effectively suppressing the probability of physical contact and capillary condensation between the aerosol group and the optical microstructure of the inner wall of the hollow photonic crystal fiber. The light beams with characteristic absorption wavelength and reference scattering wavelength are emitted alternately. The light beams with characteristic absorption wavelength and reference scattering wavelength are coaxially coupled into the central hollow channel and interact with the colorimetric aerosol group through transmission absorption and scattering attenuation. The system receives the composite light intensity signal transmitted from the end of the hollow photonic crystal fiber, converts the composite light intensity signal into a continuous analog voltage signal, extracts the transmission voltage signal corresponding to the characteristic absorption wavelength and the transmission voltage signal corresponding to the reference scattering wavelength, calculates the total attenuation absorbance of the characteristic absorption wavelength, and uses the reference scattering wavelength to extract and subtract the baseline scattering loss caused by aerosols, calculates and outputs the concentration value of ammonia in the exhaust gas.

2. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 1, characterized in that, The process of continuously atomizing the liquid colorimetric agent into a colorimetric aerosol group using surface acoustic wave energy is as follows: An alternating radio frequency (RF) voltage signal is applied to the input pin of the metal interdigital transducer on the surface of the piezoelectric microfluidic chip, exciting Rayleigh-type surface acoustic waves on the piezoelectric substrate surface. For effective resonance to occur, the input frequency of the alternating RF voltage signal and the physical dimensions of the metal interdigital transducer satisfy a matching formula: In the formula, The center resonant frequency of the alternating radio frequency voltage signal; The velocity of Rayleigh surface acoustic waves propagating on the surface of a piezoelectric substrate; The interdigital period distance between the two sets of comb electrodes of the metal interdigital transducer. The Rayleigh-type surface acoustic wave propagates directionally to the acoustic coupling region where the liquid color developer is spread, and acoustic-solid-liquid multiphase energy coupling occurs, generating unstable surface capillary waves. The peaks of the surface capillary waves break under the continuous action of acoustic radiation pressure, and the liquid color developer is peeled off from the surface of the piezoelectric substrate and transformed into the color-developing aerosol group.

3. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 2, characterized in that, During continuous atomization, drive frequency locking and aerosol particle size control are performed. The specific process is as follows: The forward transmission power and reverse reflection power during the transmission of the alternating radio frequency voltage signal are collected to calculate the voltage standing wave ratio in the radio frequency transmission channel. The frequency corresponding to the minimum point of the voltage standing wave ratio is extracted as the actual physical resonant frequency of the piezoelectric substrate at the current temperature, and the output frequency is locked at the actual physical resonant frequency. The continuous radio frequency signal is modulated into a radio frequency pulse train output. By adjusting the ratio of the on-time to the off-time of a single radio frequency pulse, the average radio frequency power applied to the metal interdigital transducer is forced to remain above the critical power threshold for liquid film instability and atomization.

4. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 1, characterized in that, The process of cutting the total sampling gas flow into the main reaction core gas flow entering the central channel of the micro-nozzle and the sheath gas protective gas flow entering the annular channel of the micro-nozzle is as follows: Obtain the geometric parameters of the cross-sectional area of ​​the central flow channel and the cross-sectional area of ​​the annular flow channel of the micro-nozzle, and calculate the ratio of the equivalent cross-sectional area of ​​the annular flow channel and the central flow channel based on the principle of equal flow velocity matching, as the target allocation ratio of the flow field boundary. Based on the dynamically acquired total system intake flow rate and the target allocation ratio, the target volumetric flow rates of the main reaction core airflow and the sheath gas protection airflow are calculated respectively. The calculated target volumetric flow rate is multiplied by the exhaust gas density coefficient under calibrated conditions, and then converted into the control voltage setting threshold of the corresponding thermal mass flow controller for closed-loop regulation.

5. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 1, characterized in that, The specific aerodynamic focusing process for forming a coaxial fluid layer with an internal droplet-containing reaction zone and an external dry gas isolation zone is as follows: The main reaction core gas flow carries ammonia and mixes with the colorimetric aerosol group to form a colorimetric aerosol core flow, which is discharged from the outlet end of the central flow pipe and enters the constriction section of the outer constraint pipe. Simultaneously, the sheath gas protective gas flow enters the constriction section along the annular flow channel. Under the contraction of the geometric cross section inside the contraction section, the sheath gas protective airflow on the outside generates radially inward converging momentum, which applies radial aerodynamic pressure to the colorimetric aerosol core flow located at the axial position, forming an aerodynamic focusing effect; After focusing, the composite fluid enters the straight outlet section for flow field rectification and boundary layer development, establishing a stable laminar interface to form the coaxial layer fluid. The outer diameter of the reaction zone containing droplets within the coaxial layer fluid is physically constrained within the inner diameter of the straight outlet section.

6. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 5, characterized in that, The process by which the ammonia molecules undergo cross-phase mass transfer and complexation colorimetric reaction with the colorimetric aerosol group includes: Driven by the concentration gradient, ammonia molecules diffuse directionally from the gas phase to the gas-liquid interface of the colorimetric aerosol droplets, penetrate the gas-liquid interface and dissolve into the interior of the droplets, where they chemically complex with the colorimetric reagent and substrate in the liquid phase to form indophenol blue complexes. The total flow rate of the main reaction core airflow and the sheath gas protective airflow is controlled to constrain the flow residence time of the coaxial layer fluid before entering the hollow photonic crystal fiber, so that the flow residence time is greater than the time constant required for the colorimetric complexation reaction to reach kinetic equilibrium.

7. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 1, characterized in that, The external sheath layer acts as a physical isolation barrier, enabling the chromogenic aerosol group to be suspended and transported, specifically including: During the axial propagation along the hollow photonic crystal fiber, the radial diffusion displacement of the constrained colorimetric aerosol droplets undergoes radial Brownian diffusion motion in the gaseous medium. Calculate the geometric difference between the inner diameter of the central hollow channel of the hollow-core photonic crystal fiber and the initial focusing diameter of the colorimetric aerosol core flow, and obtain half of this geometric difference as the initial radial thickness boundary condition of the aerodynamic isolation layer. By adjusting the air pump to maintain the target average flow velocity, the radial diffusion displacement is always less than the initial radial thickness of the aerodynamic isolation layer, and the colorimetric aerosol droplets are always suspended in the central hollow channel's axial region during axial transmission.

8. The spectrophotometric detection method for ammonia in power plant exhaust gas according to claim 1, characterized in that, The process of calculating the total attenuation absorbance of the characteristic absorption wavelength and extracting and subtracting the baseline scattering loss caused by aerosols using the reference scattering wavelength is as follows: The beams controlling the characteristic absorption wavelength and the reference scattering wavelength emit light alternately in a time-division multiplexing manner, and the continuous analog voltage signal is sampled at high speed and divided into corresponding first sampled voltage sequences and second sampled voltage sequences; The sampled voltage sequence is averaged to extract the transmission voltage signal at the corresponding characteristic absorption wavelength and the transmission voltage signal at the corresponding reference scattering wavelength. Combined with the initial reference voltage under calibration, the total attenuation absorbance at the two wavelengths is calculated. The total attenuation absorbance at the reference scattering wavelength is determined to originate entirely from Mie scattering by aerosol particles, which is considered as scattering background loss. The corresponding Egstrom index is obtained by querying the median particle size parameter of the aerosol, and the scattering background loss is converted to the characteristic absorption wavelength position. Through differential operation, the converted scattering background loss value is subtracted from the total attenuation absorbance at the characteristic absorption wavelength to extract the intrinsic absorbance of the colorimetric complex.

9. A spectrophotometric method for detecting ammonia in power plant exhaust gas according to claim 8, characterized in that, The specific process for calculating and outputting the ammonia concentration value in the exhaust gas is as follows: The diameter of the Gaussian mode field within the central hollow channel is obtained to obtain the light wave energy, and the spatial overlap factor between the light field and the reaction core region is calculated to compensate for the absorbance loss caused by the aerodynamic focusing having a cross-sectional area of ​​the absorbing medium smaller than that of the beam. Based on the extracted intrinsic absorbance, combined with the spatial overlap factor, the effective optical path of the absorption medium actually penetrated by the beam, and the aerosol volume fraction calculated by the law of conservation of mass in the gas-liquid two-phase system, the actual molar concentration of ammonia under the operating conditions is calculated. The calculation model satisfies the following formula: In the formula, This represents the actual molar concentration of ammonia under operating conditions. The intrinsic absorbance of the colorimetric complex; The molar absorptivity of the colorimetric complex at the characteristic absorption wavelength; This is the actual physical length of the hollow-core photonic crystal fiber; The spatial overlap factor between the light field and the reaction core region; The system acquires real-time thermodynamic temperature, absolute pressure, and absolute humidity parameters of flue gas, converts the actual molar concentration of ammonia under operating conditions into the dry basis volume concentration under standard conditions, and then filters and outputs the result.

10. A spectrophotometric method for detecting ammonia in power plant exhaust gas according to claim 8, characterized in that, It also includes the step of synchronously running an anti-saturation adaptive cleaning closed loop during the detection process: The transmission voltage signal of the corresponding reference scattering wavelength is monitored in real time and compared with the pre-stored lower limit threshold of light intensity. When the transmission voltage signal of the corresponding reference scattering wavelength is determined to be lower than the lower limit threshold of light intensity, it is determined that there is an aerosol concentration overload or condensation inside the hollow photonic crystal fiber, triggering the anti-saturation adaptive cleaning procedure. When the cleaning process is active, a modulation command is sent to the RF power amplifier to reduce the average RF power and decrease the rate at which colorimetric aerosols are generated. At the same time, an acceleration command is sent to the pump in the main pumping circuit to increase the instantaneous purging flow rate and discharge the redundant droplets in the central hollow channel; The transmission voltage signal corresponding to the reference scattering wavelength is continuously monitored. When the transmission voltage signal recovers and meets the threshold condition including the set hysteresis voltage, the power reduction command and speed-up command are canceled, and continuous monitoring is resumed.

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