阅读说明：本技术 一种烟气中co浓度混合取样式在线监测装置及方法 (Mixed sampling type online monitoring device and method for CO concentration in flue gas ) 是由 彭志敏 周佩丽 杜艳君 丁艳军 于 2019-10-25 设计创作，主要内容包括：一种烟气中CO浓度混合取样式在线监测装置及方法,属于可调谐激光二极管吸收光谱技术领域。该装置针对烟气中CO分布不均匀的特点,将省煤器出口烟道分为多个区域,各区域中心位置安装稀释取样探头,混合后的烟气进入赫里奥特池后结合波长调制光谱四次谐波与一次谐波比值法实现高精度免标定在线监测。该装置及方法关键在于对多个区域CO浓度平均值进行监测,其结果更具代表性；采用稀释方式取样由于降低了烟气露点无需做除水处理；采用赫里奥特池长光程进行测量提高了检测精度；采用四次谐波与一次谐波测量方法根据浓度与比值曲线进行免标定测量。该测量装置及方法解决了目前烟气CO测量中取样过程复杂、维护量大、单点测量代表性差等问题。(A mixed sampling type on-line monitoring device and method for CO concentration in flue gas belong to the technical field of tunable laser diode absorption spectrum. The device is directed at the characteristic that CO in the flue gas is not uniformly distributed, an outlet flue of the economizer is divided into a plurality of areas, a dilution sampling probe is installed at the center position of each area, and the mixed flue gas enters a Herriott cell and then is combined with a wavelength modulation spectrum fourth harmonic and first harmonic ratio method to achieve high-precision calibration-free online monitoring. The device and the method have the key points that the average value of the CO concentration in a plurality of areas is monitored, and the result is more representative; the sampling is carried out in a dilution mode, so that the dew point of the flue gas is reduced, and water removal treatment is not needed; the detection precision is improved by adopting the Herriott cell to carry out measurement in a long optical path; and (4) performing calibration-free measurement according to a concentration and ratio curve by adopting a fourth harmonic and first harmonic measurement method. The measuring device and the method solve the problems of complex sampling process, large maintenance amount, poor single-point measurement representativeness and the like in the conventional flue gas CO measurement.)

1. A mixed sampling type online monitoring device for CO concentration in flue gas comprises a signal generation module (1), a laser driving module (2), a tunable semiconductor laser (3), a collimator (4), a Herriott cell gas chamber (8), a photoelectric detector (6), a dilution sampling probe (11), a data acquisition processing module (9), an interferometer (5) and an oscilloscope (7); the signal generation module (1) is connected with the laser driving module (2) through a lead, and the laser driving module (2) is connected with the tunable semiconductor laser (3) through a lead; the tunable semiconductor laser (3) is connected with the collimator (4) through an optical fiber; the dilution sampling probe (11) is connected with the Herriott cell air chamber (8) through an air extraction device (12) and a pipeline; the dilution sampling probes (11) are respectively arranged in different areas of the flue (10); the photoelectric detector (6) is connected with the data acquisition processing module (9) through a coaxial cable; the photoelectric detector (6) is connected with the oscilloscope (7) through a coaxial cable.

2. The method for on-line monitoring of CO concentration in flue gas by mixed sampling according to claim 1 is characterized by comprising the following steps:

1) the flue is divided into regions, a dilution sampling probe (11) is arranged at the center of each region, dry compressed air is adopted to dilute the flue gas, and under the action of an air extraction device (12), the flue gas of each path is mixed and then enters a Herriott cell air chamber (8);

2) calibrating parameters of the tunable semiconductor laser (3), and calculating a relation curve of a harmonic ratio and concentration:

a) selecting a center frequency v₀The CO characteristic absorption spectrum line takes a tunable semiconductor laser (3) as a light source, adjusts the set temperature and current of a laser driving module (2), and adjusts the frequency and amplitude of a signal generating module (1) to enable the tunable semiconductor laser (3) to be at the central frequency v₀High-frequency modulation occurs;

b) the laser after high-frequency modulation enters an interferometer (5) after passing through a collimator (4), and an interference signal is received by a photoelectric detector (6) and input into an oscilloscope (7) for displaying and collecting;

c) after the laser after high-frequency modulation passes through a collimator (4), an original light intensity signal I₀The phase difference phi of frequency modulation and light intensity modulation, a current tuning coefficient epsilon of a tunable semiconductor laser (3) and a light intensity linear modulation coefficient i are obtained by adopting the following formula;

wherein v is the instantaneous frequency of the laser; omega is high-frequency modulation frequency; a is frequency modulation amplitude, a is epsilon A, and A is modulation current amplitude; i is₀Is an original light intensity signal;is the light intensity at the center frequency of the spectral line;

according to the Lambert beer law, the transmitted light intensity signal I after gas absorption_tWith the original light intensity signal I₀Satisfies the following formula:

wherein tau (v) is a laser transmittance function, α (v) is a laser absorptance function, P is a gas pressure, S (T) is a spectral line intensity, x is a gas concentration, and L is an absorption path length;

for transmitted light intensity signal I_tPerforming harmonic detection, X-axis reference signal V_xAnd Y-axis reference signal V_yThe expression is as follows: :

wherein R is the amplitude of the reference signal, β is the initial phase of the reference signal;

according to the phase-locked detection principle, a transmission light intensity signal I can be obtained_tFirst harmonic X-axis signal X_1fAnd Y-axis signal Y_1fThe expression is as follows:

wherein G is photoelectric amplification factor, first harmonic S_1fThe amplitude is:

fourth harmonic S_4fThe expression is as follows:

d) substituting the frequency modulation amplitude a, the phase difference phi of the frequency modulation and the light intensity modulation calibrated in the step c) and the light intensity linear modulation coefficient i into the following formula, and calculating the CO concentration and the fourth harmonic S under the fixed pressure, the optical path and the temperature_4fAnd the first harmonic S_1fA relation curve between the ratios;

3) adjusting the modulation current amplitude A generated by the signal generation module (1) according to the current tuning coefficient epsilon calibrated in the step 2), the determined modulation amplitude a and the formula a ═ epsilon A, collimating the laser emitted by the tunable semiconductor laser (3) by the collimator (4), then entering a Herriott cell air chamber (8), reflecting for many times in the air chamber, absorbing by gas, then being received by the photoelectric detector (6), and transmitting to the data acquisition processing module (9);

4) the data acquisition processing module (9) sends a signal to the photoelectric detector (6)Detecting harmonic wave to obtain transmitted light intensity signal I_tFourth harmonic of S_4fAnd the first harmonic S_1fAnd the ratio S thereof_4f/S_1fAnd measuring the CO concentration in the flue gas according to a harmonic ratio and concentration curve obtained by calculation of a formula (8).

3. The method for mixed sampling of CO concentration in flue gas according to claim 2, wherein the flue partition in step 1) is to divide the flue into at least 4 regions along the cross section direction of the flue gas.

Technical Field

The invention relates to an online measurement device and method for CO concentration in flue gas, in particular to an online measurement device and method for flue gas CO concentration mixed sampling based on a wavelength modulation spectrum technology combined with dilution sampling and a Herriott cell long-optical-path mode.

Background

The quality of the combustion process in the hearth of the coal-fired power plant directly influences the boiler efficiency, so that the economical problem is caused. With the improvement of the requirement of environmental protection on ultra-low emission in China, the coal-fired power plant adopts a low-nitrogen combustion mode to reduce the emission of nitrogen oxides. The low-nitrogen combustion in the denitration is carried out in a grading and oxygen-poor mode, and air required by the combustion is divided into two stages in the grading mode and is sent into a hearth, a main combustion area and a burnout area. The main combustion zone is in a fuel-rich and oxygen-deficient state, so that the generation of nitrogen oxides is reduced, but the content of CO is increased due to incomplete combustion; after entering the burnout zone, the burnout zone belongs to an oxygen-rich zone, however, because the temperature of the burnout zone is low, although the generation amount of nitrogen oxides is small, the emission concentration of CO is high due to the poor mixing, low temperature and the like of a large amount of unburned gas and coke smoke in the main combustion zone. The emission of CO not only causes the waste of coal resources, but also easily causes high-temperature oxidation, coking and the like of the furnace wall, thereby improving the maintenance cost of the power plant.

At present, a coal-fired boiler mainly monitors the oxygen content of exhaust smoke to predict whether the air supply proportion of fuel is proper or not, whether combustion is complete or not, however, because the oxygen content in air is large, flue air leakage and instrument air leakage can bring large errors to the measurement of the oxygen content, and therefore if the monitoring of incomplete combustion products CO and the monitoring of the oxygen content can be combined, the accuracy and the reliability of the monitoring of combustion working conditions in a hearth can be improved. In addition, the online monitoring of CO is also significant to the safe operation of the coal mill, so that the measurement of the content of CO in the flue gas is very necessary.

The CO concentration in the flue gas in the coal-fired boiler has the following characteristics: the content is higher: the operation data show that the CO concentration of the power plant exceeds 1000ppm at about 1/3 time, and the CO average value is 1000ppm at 1400 hours per year by taking a 600MW unit as an example through accounting, so that about 924 tons of coal are wasted each year. Furthermore, the concentration of CO has a positive correlation with the carbon content in fly ash, which is increased by about 2% if 1000ppm CO is produced, wasting about 1693 tons of coal each year. The fluctuation is severe: the short-time measurement curve of the CO concentration shows that the fluctuation of the CO concentration in the flue gas is severe, the concentration change is rapid, and the concentration can be increased from dozens of ppm to thousands of ppm within two minutes and then decreased to dozens of ppm. Uneven distribution: due to the reasons of uneven air distribution of the hearth, aging of the hearth, inclined flame and the like, although the oxygen distribution is relatively uniform, the CO distribution is greatly changed, so that distributed measurement is needed, the air distribution is adjusted, and the combustion is better controlled.

At present, the online monitoring of CO in flue gas is mainly realized by a flue gas online monitoring System (CEMS), the instrument adopts a non-dispersive infrared principle and combines a sampling mode for measurement, however, in order to ensure that the measured flue gas components are not distorted, a sampling path needs to be subjected to heat tracing and condensation treatment, the sampling process is complex, the problem of lens pollution also exists, and the operation and maintenance amount of the instrument is large. And all instruments used at present adopt single-point measurement mode, measure the representativeness poor, can't be applicable to the inhomogeneous characteristics of CO distribution.

Disclosure of Invention

The invention aims to solve the defects existing in the prior art that oxygen is used as a basis for evaluating combustion efficiency and the problems existing in the prior art of online monitoring of CO in flue gas, and provides a mixed sampling type online monitoring device and method for the concentration of CO in flue gas.

The technical scheme of the invention is as follows:

a mixed sampling type online monitoring device for CO concentration in flue gas comprises a signal generation module, a laser driving module, a tunable semiconductor laser, a collimator, a Herriott cell gas chamber, a photoelectric detector, a dilution sampling probe, a data acquisition processing module, an interferometer and an oscilloscope; the signal generation module is connected with the laser driving module through a lead, and the laser driving module is connected with the tunable semiconductor laser through a lead; the tunable semiconductor laser is connected with the collimator through an optical fiber; the dilution sampling probe is connected with the air chamber of the Herriott cell through an air extraction device and a pipeline; the dilution sampling probes are respectively arranged in different areas of the flue; the photoelectric detector is connected with the data acquisition processing module through a coaxial cable; the photoelectric detector is connected with the oscilloscope through a coaxial cable.

The invention provides a mixed sampling type online monitoring method for CO concentration in flue gas, which is characterized by comprising the following steps:

1) dividing the flue, mounting a dilution sampling probe at the center of each area, diluting the flue gas by adopting dry compressed air, and mixing the flue gas of each path under the action of an air extraction device and then feeding the mixed flue gas into a Herriott cell air chamber;

2) calibrating parameters of the tunable semiconductor laser, and calculating a relation curve of a harmonic ratio and concentration:

a) selecting a center frequency v₀The CO characteristic absorption spectrum line of the tunable semiconductor laser is used as a light source, the set temperature and the current of a laser driving module are adjusted, and the frequency and the amplitude of a signal generating module are adjusted, so that the tunable semiconductor laser has the central frequency v₀High-frequency modulation occurs;

b) the laser after high-frequency modulation enters an interferometer after passing through a collimator, and an interference signal is received by a photoelectric detector and input into an oscilloscope for displaying and collecting;

c) after the laser after high frequency modulation passes through a collimator, an original light intensity signal I₀Receiving the signal by a photoelectric detector, inputting the signal into an oscilloscope for displaying and collecting, and fitting an interference signal and an original light intensity signal by adopting the following formula to obtain a phase difference phi of frequency modulation and light intensity modulation, a current tuning coefficient epsilon of a tunable semiconductor laser and a light intensity linear modulation coefficient i;

wherein v is the instantaneous frequency of the laser; omega is high-frequency modulation frequency; a is frequency modulation amplitude, a is epsilon A, and A is modulation current amplitude; i is₀Is an original light intensity signal; i is₀Is the light intensity at the center frequency of the spectral line;

according to the Lambert beer law, the transmitted light intensity signal I after gas absorption_tWith the original light intensity signal I₀Satisfies the following formula:

wherein tau (v) is a laser transmittance function, α (v) is a laser absorptance function, P is a gas pressure, S (T) is a spectral line intensity, x is a gas concentration, and L is an absorption path length;

is a linear function; h_kThe Fourier coefficient of the laser transmittance function is expressed as:

for transmitted light intensity signal I_tPerforming harmonic detection, X-axis reference signal V_xAnd Y-axis reference signal V_yThe expression is as follows: :

wherein R is the amplitude of the reference signal, β is the initial phase of the reference signal;

according to the phase-locked detection principle, the transmitted light intensity I can be obtained_tFirst harmonic X-axis signal X_1fAnd Y-axis signal Y_1fThe expression is as follows:

wherein G is photoelectric amplification factor, first harmonic S_1fThe amplitude is:

fourth harmonic S_4fThe expression is as follows:

d) substituting the frequency modulation amplitude a, the phase difference phi of the frequency modulation and the light intensity modulation calibrated in the step c) and the light intensity linear modulation coefficient i into the following formula, and calculating the CO concentration and the fourth harmonic S under the fixed pressure, the optical path and the temperature_4fAnd the first harmonic S_1fA relation curve between the ratios;

3) adjusting the modulation current amplitude A generated by the signal generation module according to the current tuning coefficient epsilon calibrated in the step 2), the determined modulation amplitude a and the formula a ═ epsilon A, collimating the laser emitted by the tunable semiconductor laser by a collimator, then entering a gas chamber of the Herriott cell, reflecting for many times in the gas chamber, absorbing by gas, then receiving by a photoelectric detector, and transmitting to the data acquisition and processing module;

4) the data acquisition processing module performs harmonic detection on the photoelectric detector signal to obtain a transmission light intensity signal I_tFourth harmonic of S_4fAnd the first harmonic S_1fAnd the ratio S thereof_4f/S_1fAnd measuring the CO concentration in the flue gas according to a harmonic ratio and concentration curve obtained by calculation of a formula (8).

In the technical scheme, the flue partition in the step 1) is to divide the flue into at least 4 areas along the cross section direction of the flue gas.

The method has the advantages that a dilution sampling and Herriott cell long optical path mode is adopted, the distributed measurement is carried out on CO in the flue gas based on a fourth harmonic ratio first harmonic method in a wavelength modulation spectrum technology, and compared with other methods, ① has the advantages that a flue is partitioned, CO mixed sampling measurement can be achieved, the method is suitable for the characteristic of uneven CO distribution and is more representative than single-point measurement, ② sampling is carried out by adopting a dilution method, the water concentration is reduced in the dilution sampling process, the dew point of the flue gas is reduced, tracing and water removal treatment are not needed, the sampling process is simple, the operation maintenance amount is small, the CO concentration of the flue gas is reduced in the ③ dilution sampling process, the measurement sensitivity is improved by adopting the Herriott cell long optical path mode, the measurement lower limit is reduced, ④ a wavelength modulation fourth harmonic ratio first harmonic method is adopted, the background signal is small, the signal-to-noise ratio is high, meanwhile, the first harmonic is adopted for normalization, the influence of the light intensity is eliminated, and finally concentration measurement is achieved according to a relation curve of the harmonic ratio and the concentration.

Drawings

FIG. 1 is a system schematic for CO measurement according to the present invention.

FIG. 2 is a schematic diagram of tunable semiconductor wavelength calibration for CO measurement according to the present invention.

FIG. 3 is a schematic diagram of the CO measurement tunable semiconductor light intensity calibration according to the present invention.

FIG. 4 shows the calibration result of the tunable semiconductor laser for CO measurement according to the present invention.

FIG. 5 is a relationship curve of harmonic ratio and concentration obtained by CO measurement according to calibration parameters of a tunable semiconductor laser.

FIG. 6 is CO concentration monitoring data for actual operation of a power plant in accordance with the present invention.

In the figure: 1-a signal generating module; 2-laser driving module; 3-a tunable semiconductor laser; 4-a collimator; 5-an interferometer; 6-a photodetector; 7-an oscilloscope; 8-Herriott cell chamber; 9-a data acquisition processing module; 10-flue; 11-dilution sampling probe; 12-air extraction device.

Detailed Description

The invention will be further described with reference to the accompanying drawings.

As shown in fig. 1, fig. 2 and fig. 3, the mixed sampling type online monitoring device for CO concentration in flue gas provided by the invention mainly comprises a signal generation module 1, a laser driving module 2, a tunable semiconductor laser 3, a collimator 4, a herriott cell gas chamber 8, a photoelectric detector 6, a dilution sampling probe 11, a data acquisition processing module 9, an interferometer 5 and an oscilloscope 7; the signal generation module 1 is connected with the laser driving module 2 through a lead, and the laser driving module 2 is connected with the tunable semiconductor laser 3 through a lead; the tunable semiconductor laser 3 is connected with the collimator 4 through an optical fiber; the dilution sampling probe 11 is connected with the Herriott cell air chamber 8 through an air extraction device 12 and a pipeline; the dilution sampling probes 11 are respectively arranged in different areas of the flue 10; the photoelectric detector 6 is connected with the data acquisition processing module 9 through a coaxial cable; the photodetector 6 is connected with the oscilloscope 7 through a coaxial cable.

The invention provides an online monitoring method for CO concentration mixing sampling in flue gas, which comprises the following steps:

1) dividing a flue 10 into at least 4 areas along the cross section direction of flue gas, installing a dilution sampling probe 11 at the center of each area, diluting the flue gas by adopting dry compressed air, and mixing the flue gas of each path under the action of an air extractor 12 and then feeding the mixed flue gas into a Herriott cell air chamber 8;

2) calibrating parameters of the tunable semiconductor laser, and calculating a relation curve of a harmonic ratio and concentration:

a) selecting a center frequency v₀The CO characteristic absorption spectrum line takes the tunable semiconductor laser 3 as a light source, adjusts the set temperature and current of the laser driving module 2, and adjusts the frequency and amplitude of the signal generating module 1 to ensure that the tunable semiconductor laser 3 has the central frequency v₀High-frequency modulation occurs;

b) the laser after high-frequency modulation enters an interferometer 5 after passing through a collimator 4, and an interference signal is received by a photoelectric detector 6 and input into an oscilloscope 7 for displaying and collecting;

c) after the laser after high frequency modulation passes through the collimator 4, the original light intensity signal I₀The phase difference phi of frequency modulation and light intensity modulation, a current tuning coefficient epsilon of a tunable semiconductor laser and a light intensity linear modulation coefficient i are obtained by adopting the following formula to fit an interference signal and an original light intensity signal;

wherein v is the instantaneous frequency of the laser; omega is high-frequency modulation frequency; a is frequency modulation amplitude, a is epsilon A, and A is modulation current amplitude; i is₀Is an original light intensity signal; i is₀Is the light intensity at the center frequency of the spectral line;

according to the Lambert beer law, the transmitted light intensity signal I after gas absorption_tWith the original light intensity signal I₀Satisfies the following formula:

wherein tau (v) is a laser transmittance function, α (v) is a laser absorptance function, P is a gas pressure, S (T) is a spectral line intensity, x is a gas concentration, and L is an absorption path length;

is a linear function; h_kThe Fourier coefficient of the laser transmittance function is expressed as:

substituting the incident light intensity expression in the formula (1) into the formula (2) to obtain a transmitted light intensity signal I after gas absorption_tThe expression is as follows:

transmitted light intensity signal I in the above formula_tThe respective frequency components of (a) are expressed as follows:

due to the symmetry of the molecular absorption lines, the odd-numbered terms in equation (3) are all 0 at the center frequency of the line, i.e., H₁、H₃、H₅… are all 0. Therefore, the frequency multiplication component and the even frequency multiplication component in equation (5) can be simplified as follows:

for transmitted light intensity signal I_tPerforming harmonic detection, X-axis reference signal V_xAnd Y-axis reference signal V_yThe expression is as follows:

wherein R is the amplitude of the reference signal, β is the initial phase of the reference signal;

according to the phase-locked detection principle, the transmitted light intensity I can be obtained_tFirst harmonic X-axis signal X_1fAnd Y-axis signal Y_1fThe expression is as follows:

wherein G is photoelectric amplification factor, first harmonic S_1fThe amplitude is:

in the same way, the fourth harmonic S_4fThe expression is as follows:

d) substituting the frequency modulation amplitude a, the phase difference phi of the frequency modulation and the light intensity modulation calibrated in the step c) and the light intensity linear modulation coefficient i into the following formula, and calculating the CO concentration and the fourth harmonic S under the fixed pressure, the optical path and the temperature_4fAnd the first harmonic S_1fA relation curve between the ratios;

3) adjusting the modulation current amplitude A generated by the signal generating module 1 according to the current tuning coefficient epsilon calibrated in the step 2), the determined modulation amplitude a and the formula a ═ epsilon A, collimating the laser emitted by the tunable semiconductor laser 3 by the collimator 4, then entering the air chamber 8 of the Herriott cell, reflecting for many times in the air chamber, absorbing by the gas, then receiving by the photoelectric detector 6, and transmitting to the data acquisition processing module 9;

4) the data acquisition processing module 9 carries out harmonic detection on the signal of the photoelectric detector 6 to obtain a transmission light intensity signal I_tFourth harmonic of S_4fAnd the first harmonic S_1fAnd the ratio S thereof_4f/S_1fAnd measuring the CO concentration in the flue gas according to the harmonic ratio and concentration curve calculated by the formula (11).