Low-concentration gas detection method based on direct absorption spectrum

Abstract

The invention discloses a low-concentration gas detection method based on a direct absorption spectrum, which comprises the following steps of: calculating SO2 concentration by using real-time measured spectral data of detected gas and a detected gas absorption cross section database measured in a laboratory through an ultraviolet differential absorption spectrometry, calculating to obtain an interference spectrum of SO2 on NO2, removing the influence of SO2 on NO2 absorbance, and determining the concentration of the NO2. The absorbance of the zero absorption position is calculated, the influence of spectral energy reduction on measurement is eliminated, the NO2 concentration after cross interference is eliminated is accurately calculated, and the detection precision and the detection lower limit are greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of spectral measurement, and in particular relates to a low-concentration gas detection method based on direct absorption spectroscopy. Background Art

[0002] Currently used for SO2 emission from industrial pollution sources and production processes 2 ,NO,NO 2 According to their principles, online concentration monitoring technologies can be divided into non-dispersive infrared absorption method, electrochemical method, differential optical absorption spectroscopy (DOAS), and chemiluminescence method. A large number of engineering applications and repeated laboratory tests have shown that infrared can no longer meet the needs of ultra-low emission monitoring due to its low measurement accuracy and high detection limit. Electrochemical analysis methods have serious cross-interference and are easily affected by moisture, H 2 S, NO, NO 2 The impact mainly applies to short-term SO 2 Concentration detection; chemiluminescence method measures NO 2 When NO is heated to high temperature in a molybdenum furnace, 2 The conversion efficiency of molybdenum will gradually decrease over time, thus affecting the measurement accuracy. Ultraviolet differential absorption spectroscopy can be used to measure SO 2 , NO, low concentration and high sensitivity detection, but due to SO 2 with NO 2 The absorption spectra overlap seriously and NO 2 The differential absorption signal is weak, making it difficult to achieve NO 2 In addition, as my country's requirements for flue gas emission standards and data measurement stability continue to increase, many manufacturers have adopted "indirect measurement methods" in order to meet emission standards. Some customers who want to meet emission standards have adopted methods such as dismantling molybdenum converters and increasing dilution ratios to reduce pollutant measurement values, which seriously reduces the authenticity and effectiveness of data and greatly increases the difficulty of supervision. Therefore, there is an urgent need for a method that can detect NO 2 The technology achieves long-term stable, low-concentration, high-sensitivity measurement of components, ensuring the long-term stability and reliability of pollutant emission monitoring data. Summary of the invention

[0003] In order to solve the shortcomings of the prior art, the purpose of the present invention is to provide a low-concentration, multi-component gas detection technology based on direct absorption spectroscopy technology. 2 Yes 2 The cross-absorption spectrum of the absorption band and the zero absorption baseline calculation extracted relatively pure NO 2Direct absorption spectral data, the data is not affected by the decrease of light intensity, and the signal-to-noise ratio is significantly higher than NO 2 The signal-to-noise ratio of differential absorption spectroscopy in the ultraviolet band can significantly improve the low concentration NO 2 Improve measurement accuracy and achieve lower detection limits.

[0004] In order to achieve the above object, the present invention adopts the following technical solution:

[0005] The present invention adopts SO 2 Calculate SO from the absorbance in the 280-315 nm band 2 The concentration was calculated by using the absorbance at 260-262nm to calculate the zero absorption baseline, and then the pure NO was obtained by cross-interference spectrum elimination and baseline subtraction. 2 Direct absorption, calculate the real NO 2 concentration.

[0006] A low-concentration gas detection method based on direct absorption spectroscopy comprises the following steps:

[0007] S1. Record the dark current spectrum of the spectrometer:

[0008] Block the light inlet of the spectrometer to prevent any external light from entering the spectrometer, and record the spectral data output by the spectrometer. d ;

[0009] S2. Record the spectrometer background spectrum under the blank sample:

[0010] N is introduced into the gas pool 2 Or in the case of air, record the spectral data output by the spectrometer I 0 ;

[0011] S3. Record the spectrometer measurement spectrum under the sample:

[0012] When the flue gas to be tested is introduced into the gas pool, the spectral data output by the spectrometer is recorded. 1 ;

[0013] S4. Subtract the background spectrum data from the absorption spectrum signal to calculate the absorbance σ of the sample. The calculation formula is as follows:

[0014]

[0015] S5. Calculate zero absorption baseline σ zero :

[0016] Extract the 260-262nm band data σ from the absorbance σ obtained by S4 260to262 , the zero absorption absorbance σ is calculated according to the following formula zero , where n is the array σ 260to262The number of elements:

[0017]

[0018] S6. Calculate the SO in each sample 2 concentration

[0019] Extract the 280-315 nm band data from the absorbance σ obtained by S4 to obtain SO 2 Absorbance According to the basic database 1ppm SO 2 Absorbance SO is calculated according to the following formula 2 concentration Where n is an array and The number of elements:

[0020]

[0021] S7. Using SO 2 Concentration, basic database SO 2 Yes 2 Interference spectra and NO 2 Absorbance-based database for calculating NO 2 concentration

[0022] Extract the 260-470nm band data from the absorbance σ obtained by S4 to obtain NO 2 Absorbance According to the basic database SO 2 Interference spectrum of absorbance in the 260~470nm band SO obtained in S6 2 Concentration and basic database 1ppmNO 2 Absorbance NO was calculated according to the following formula 2 concentration:

[0023]

[0024] Preferably, during the test, the gas pool is heated to a constant temperature with a temperature control accuracy of ±0.5°C, a constant gas pool flow rate, and a flow rate variation range of <0.1L / min.

[0025] Preferably, the SO in the sample 2 and NO 2 The concentrations were all less than 100 μmol / mol.

[0026] The present invention is beneficial in that:

[0027] (1) The present invention uses ultraviolet direct absorption spectroscopy technology to simultaneously extract SO 2 、NO 2 The absorption spectrum in the ultraviolet and visible light bands is basically based on NO 2 The absorption signal intensity of the full-band absorption spectrum is greater than the ultraviolet differential absorbance. The SO 2 with NO 2 The cross-interference caused by the overlapping absorption spectra resulted in the acquisition of pure NO 2 Absorbance signal, with high signal-to-noise ratio, can be used to accurately measure NO 2 Gas concentration, especially at low concentrations, has a low detection limit;

[0028] (2) The present invention can realize the delayed SO 2 、NO 2 The long-term measurement results are stable and reliable, which can better guide industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a flow chart of the low-concentration gas detection technology based on direct absorption spectroscopy technology of the present invention;

[0030] Figure 2 1ppm SO for basic database 2 Absorbance

[0031] Figure 3 For the basic database 1ppm NO 2 Absorbance and 1ppmSO 2 Interference spectrum

[0032] Figure 4 NO with cross interference 2 Absorbance σ NO , zero absorption baseline and NO after removing cross interference 2 Absorbance σ NOpure Comparison chart. DETAILED DESCRIPTION

[0033] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] See also Figure 1 , the low concentration gas detection method based on direct absorption spectroscopy includes the following steps:

[0035] Step 1: Record the spectrometer dark current spectrum:

[0036] Block the light inlet of the spectrometer to prevent any external light from entering the spectrometer, and record the spectral data output by the spectrometer. d .

[0037] Step 2: Record the spectrometer background spectrum under the blank sample:

[0038] N is introduced into the gas pool 2 Or in the case of air, record the spectral data output by the spectrometer I 0 .

[0039] Step 3: Record the spectrometer measurement spectrum of the unknown sample:

[0040] When the flue gas to be tested is introduced into the gas pool, the spectral data output by the spectrometer is recorded. 1 .

[0041] Step 4: Subtract the background spectrum data from the absorption spectrum signal and calculate the absorbance σ of the unknown sample:

[0042] Use I d ,I 0 and I 1 According to formula (1), the sample absorbance σ is calculated as:

[0043]

[0044] Step 5: Calculate the zero absorption baseline σ zero :

[0045] Extract the 253-254 nm band data σ from the absorbance σ obtained in step 4 253to254 According to formula (2), the zero absorption absorbance σ is calculated zero , where k is the array σ 253to254 The number of elements of , k = 11;

[0046]

[0047] Step 6: Calculate the SO in the unknown samples separately 2 concentration

[0048] According to the absorbance σ obtained in step 4, the absorbance data of the 280-315 nm band is extracted from the absorbance to obtain SO 2 Absorbance Combined with zero absorption baseline and basic database 1ppm SO 2 Absorbance (See Figure 2 ), and the SO in the gas pool is calculated using formula (3). 2 concentration Calculate, where n is an array and The number of elements, n = 351;

[0049]

[0050] Step 7: Using SO 2 Concentration, basic database SO 2 Yes 2 Interference spectra and NO 2 Absorbance-based database for calculating NO 2 concentration

[0051] Extract the 330-470nm band data from the absorbance σ obtained in step 4 to obtain NO 2 Absorbance like Figure 4 As shown, according to the basic database SO 2 Interference spectrum of absorbance in the 330~470nm band (See Figure 3 ), SO obtained in step 6 2 Concentration and basic database 1ppm NO 2 Absorbance (See Figure 3 ),like Figure 4 As shown in the figure, after eliminating the cross interference and baseline interference, pure NO can be obtained. 2 Absorbance, calculate NO according to formula (4) 2 concentration, where m is an array and The number of elements, m = 1401;

[0052]

[0053] If the NO before cross-interference is removed 2 Absorbance spectrum Calculate the NO concentration before removing cross interference The results are as follows:

[0054]

[0055] The results show that only 20 ppm of SO 2 and a zero absorption baseline with an intensity of only 0.0012 will cause NO 2 The concentration deviation is nearly 22.6ppm, which is a very large factor affecting the accuracy of low-concentration measurements. However, this technology can effectively solve the measurement inaccuracies caused by cross-interference and zero absorption baseline changes, and can also achieve SO 2 、NO 2 accurate measurement.

[0056] In combination with the present invention, the core gas measurement pool of the developed system adopts constant temperature heating, the gas pool temperature control accuracy is ±0.5℃, the gas pool flow is constant, and the flow change range is <0.1L / min. The system adopts ultraviolet differential absorption spectroscopy to carry out NO 2 During measurement, its zero point fluctuates greatly, and the detection limit is about 0.5ppm (the test method is "Technical Guidelines for the Formulation and Revision of Environmental Monitoring Analysis Method Standards HJ168-2020"). After adopting this invention patent, NO 2 The fluctuation of the measured value is significantly reduced, and the detection limit is reduced to 0.2ppm. In addition, the light intensity changes caused by factors such as ambient temperature changes, light intensity energy decrease or vibration, NO 2 The absolute value of the difference between the measured value and the true value is less than 1ppm.

[0057] It can be seen that without changing the system structure, 2 After the measurement method is replaced by the differential absorption spectroscopy method with the technology introduced in this invention, NO can be effectively reduced. 2 The measurement fluctuation improves the stability of long-term measurement and eliminates SO 2 Yes 2 The measurement error caused by the measurement, especially for the measurement of low concentration flue gas, the measurement result is more accurate and stable, and NO 2 After adopting the direct measurement method, the data deviation problem caused by human modification of system accessories is eliminated at the system level, which can provide technical support for obtaining long-term and realistic pollutant emission limits.

[0058] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form, and any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A low-concentration gas detection method based on direct absorption spectroscopy, characterized in that: The following steps are involved: S1. Record spectrometer dark current spectrum I d ; S2, record the spectrometer background spectrum I0 under the blank sample; S3, recording the spectrometer measurement spectrum I1 under the sample; S4. Subtract the background spectrum data from the absorption spectrum signal to calculate the absorbance σ of the sample. The calculation formula is as follows: S5. Calculate zero absorption baseline σ zero : The band data σ is obtained from step S4 260to262 , the zero absorption absorbance σ is calculated according to the following formula zero , where n is the array σ 260to262 The number of elements: S6. Calculate the SO2 concentration in the samples respectively The SO2 absorbance is obtained from step S4 According to the basic database 1ppm SO2 absorbance The SO2 concentration can be calculated by the following formula Where n is the array σ SO2 and The number of elements: S7. Calculate NO2 concentration using SO2 concentration, basic database SO2 interference spectrum on NO2 and NO2 absorbance basic database Obtain NO2 absorbance from step S4 According to the basic database, the interference spectrum of SO2 absorbance in the 260nm~470nm band SO2 concentration obtained in S6 and the basic database 1ppm NO2 absorbance The NO2 concentration was calculated according to the following formula:

2. A low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: The specific steps of step S1 are: blocking the light inlet of the spectrometer to prevent any external light from entering the spectrometer, and recording the spectral data output by the spectrometer.

3. A low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: The specific steps of step S2 are: introducing N2 or air into the gas cell and recording the spectral data output by the spectrometer.

4. A low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: The specific steps of step S3 are: introducing the flue gas to be tested into the gas pool, and recording the spectral data output by the spectrometer.

5. The low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: In step S5, σ 260to262 It is the 260-262 nm band data extracted from the absorbance σ obtained in step S4.

6. A low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: In step S6, SO2 absorbance It is the 280-315 nm band data extracted from the absorbance σ obtained in step S4.

7. A low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: In step S7, NO2 absorbance It is the 260-470 nm band data extracted from the absorbance σ obtained in step S4.

8. The low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: During the test, the gas pool uses heating to maintain a constant temperature with a temperature control accuracy of ±0.5°C.

9. The low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: During the test, the flow rate of the gas pool varied within a range of <0.1L / min.

10. The low-concentration gas detection method based on direct absorption spectroscopy according to claim 1, characterized in that: The concentrations of SO2 and NO2 in the sample are both less than 100 μmol / mol.

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