Method for detecting concentration of low concentration gas based on ultraviolet differential absorption method and ultraviolet direct absorption method and application thereof

Abstract

The application discloses a concentration detection method of low-concentration gas based on ultraviolet differential absorption method and ultraviolet direct absorption method and application thereof, and two concentration values of the measured gas are obtained by using the concentration calculation method of the differential absorption spectrum technology and the direct absorption spectrum technology according to the absorbance, differential absorbance and zero absorption baseline calculation of the measured gas in the characteristic absorption waveband, data composed of the two concentration values are used for first-order fitting, and then the concentration value of the direct absorption spectrum technology is brought into the first-order fitting function to obtain the concentration value. The application has the advantages of the ultraviolet differential absorption method and the ultraviolet direct absorption method, has high data accuracy and small fluctuation, can significantly reduce the fluctuation of the measured value of the measured component, and obtains a lower detection lower limit.

Description

Technical Field

[0001] This invention belongs to the field of spectroscopic measurement technology, specifically relating to a method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption, and its application. Background Technology

[0002] Currently, the technologies used for online monitoring of SO2, NO, NO2, NH3, and H2S concentrations in industrial pollution source flue gas emissions and production processes can be mainly classified according to their principles into non-dispersive infrared absorption method, electrochemical method, differential absorption spectroscopy, ultraviolet direct absorption spectroscopy, and chemiluminescence method. Extensive engineering applications and repeated laboratory tests have shown that: Currently, infrared absorption spectroscopy, due to its low measurement accuracy and high detection limit, cannot meet my country's ultra-low emission monitoring requirements; electrochemical analysis methods suffer from severe cross-interference and are easily affected by moisture, H2S, NO, and NO2, and are mainly suitable for short-term SO2 concentration detection; chemiluminescence spectroscopy requires converting NO2 to NO at high temperatures using a molybdenum furnace before measurement, and the conversion efficiency of molybdenum gradually decreases over time, affecting measurement accuracy; differential absorption spectroscopy can detect low concentrations of components such as SO2, NO, NO2, NH3, and H2S that have characteristic absorption in the ultraviolet light with high sensitivity, but to achieve low concentration monitoring, the common approach is to significantly increase the optical path or improve the signal-to-noise ratio of the spectrometer, both of which lead to significant cost increases and are not cost-effective; direct ultraviolet absorption spectroscopy can measure SO2 and NO2, and its detection limit is lower than that of differential ultraviolet absorption spectroscopy at the same absorption optical path, but its stability is poor and it is easily affected by changes in dust, vibration, and temperature. Therefore, there is an urgent need for a detection technology that can achieve long-term, stable, low-concentration, and highly sensitive measurement of the measured components to ensure the long-term stability and reliability of pollutant emission monitoring data. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption spectroscopy and ultraviolet direct absorption spectroscopy, and its application. The method calculates two concentration values ​​of the gas by using differential absorption spectroscopy and direct absorption spectroscopy, based on the absorbance, differential absorbance, and zero absorption baseline of the gas in its characteristic absorption band. A first-order fit is then performed using the data composed of these two concentration values. Finally, the concentration value from direct absorption spectroscopy is substituted into the first-order fit function. The resulting concentration value combines the advantages of both methods, exhibiting high accuracy and low fluctuation, significantly reducing the measurement fluctuation of the analyte and achieving a lower detection limit.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] The concentration detection method for low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption includes the following steps:

[0006] S1. Record the spectral data output by the spectrometer;

[0007] S2. Subtract the background spectral data from the absorption spectral signal and calculate the absorbance of the sample. ;

[0008] S3. Calculate the concentration of the analyte in the sample using the ultraviolet differential absorption method. ;

[0009] S4. Calculate the zero absorption baseline. ;

[0010] S5. Calculate the concentration of the analyte in the sample using the direct ultraviolet absorption method. ;

[0011] S6. Starting from the same time point, record the concentrations of the analyte calculated in S3 and S5 respectively. and This forms an array with m elements. and Calculate the coefficients k and b of the first-order function using the following formula:

[0012] (5);

[0013] in, ; It is an array with m rows and 1 column; Given an m x 2 array, its first column is connected to the array... The same applies; the second column is all 1s.

[0014] S7. Calculate the true concentration of the analyte after noise reduction using the following formula. :

[0015] (6).

[0016] Preferably, in step S1 above, the specific steps for recording the spectral data output by the spectrometer include:

[0017] (1) Recording the dark current spectrum of the spectrometer:

[0018] Block the light inlet of the spectrometer to prevent external light from entering, and record the dark current spectral data output by the spectrometer. ;

[0019] (2) Record the background spectrum of the spectrometer under the blank sample:

[0020] Record the background spectral data output by the spectrometer when N2 or air is introduced into the gas cell. ;

[0021] (3) Record the spectrometer-measured spectrum of the sample:

[0022] With the flue gas to be tested introduced into the gas chamber, record the measurement spectral data output by the spectrometer. .

[0023] Preferably, in step S2 above, the absorbance of the sample... The calculation formula is as follows:

[0024] (1).

[0025] Preferably, in step S3 above, the concentration of the component being measured is... The calculation method is as follows:

[0026] The absorbance obtained from step S2 Extract the characteristic absorption band data of the gas being measured to obtain its absorbance. Then, the absorbance is obtained by using the SG filtering function. low-frequency filtered signal Calculated according to the following formula:

[0027] (2);

[0028] in, The base database contains the differential absorbance of the gas being measured at 1 ppm; n is an array. and The number of elements.

[0029] Preferably, the parameters of the aforementioned SG filter function are set with the window number ranging from 3 to 101 and the order from 1 to 5.

[0030] Preferably, in step S4 above, the zero absorption baseline The calculation formula is as follows:

[0031] (3);

[0032] in, The absorbance obtained from step S2 Extract 260-262 nm band data; p is an array. The number of elements.

[0033] Preferably, in step S5 above, the concentration of the analyte in the sample is... The calculation formula is:

[0034] (4)

[0035] in, The absorbance of the analyte at 1 ppm is the base database, where n is an array. and The number of elements.

[0036] Preferably, during the test, the gas pool is heated to a constant temperature of 40~300 ℃ with a temperature control accuracy of ±0.5 ℃; the gas pool flow rate is constant, ranging from 0.1 to 5 L / min, with a flow rate variation range of <0.1 L / min.

[0037] The above concentration detection method is used to detect the concentration of low-concentration gas components in flue gas.

[0038] Preferably, the low-concentration gaseous components in the aforementioned flue gas are one of SO2, NO, NO2, NH3, H2S or Cl2; the concentration of the gaseous components is <200 μmol / mol.

[0039] The advantages of this invention are:

[0040] (1) This invention uses ultraviolet absorption spectroscopy technology. It also utilizes the characteristics of ultraviolet differential absorption spectroscopy technology, which has accurate measurement values, strong anti-interference energy, and small zero-point range drift, as well as the characteristics of ultraviolet direct absorption spectroscopy technology, which has large absorption intensity and small measurement value fluctuation. It also utilizes the characteristic that the measurement values ​​of the two methods have extremely high correlation in a short time. First-order fitting is used to filter out the noise of the measurement values. The concentration of the gas to be measured after noise removal is calculated by the coefficient of the first-order fitting curve and the measurement value of the direct absorption method. Especially under low concentration conditions, it has a low detection limit and high accuracy.

[0041] (2) The present invention can realize accurate measurement of low concentration of the component to be measured in flue gas by a single analysis module. The long-term measurement results are stable and reliable, and the short-term measurement results are stable with small fluctuations, which can better guide industrial production. Attached Figure Description

[0042] Figure 1 This is a flowchart of the detection method of the present invention;

[0043] Figure 2 Based on the database 1ppm SO2 absorbance ;

[0044] Figure 3 Based on the database 1ppm SO2 differential absorbance ;

[0045] Figure 4 The values ​​of SO2 concentration are measured using the differential absorption method, the direct absorption method, and the detection method of the examples, respectively. Detailed Implementation

[0046] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0047] Examples, see Figure 1 The concentration detection method for low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption includes the following steps:

[0048] S1: Recording spectrometer dark current spectrum:

[0049] Block the light inlet of the spectrometer to prevent any external light from entering, and record the spectral data output by the spectrometer. .

[0050] S2: Record the spectrometer background spectrum under blank sample conditions:

[0051] Record the spectral data output by the spectrometer while purging N2 or air into the gas chamber. .

[0052] S3: Record the spectrometer-measured spectrum for unknown samples:

[0053] Record the spectral data output by the spectrometer when the flue gas to be tested is introduced into the gas chamber. .

[0054] S4: Subtract background spectral data from the absorption spectral signal to calculate the absorbance of the unknown sample. :

[0055] use , and The absorbance of the sample is calculated according to formula (1). :

[0056] (1);

[0057] S5. Calculate the SO2 concentration in the sample using the ultraviolet differential absorption method. :

[0058] absorbance obtained from S4 Data in the 280nm–315nm band were extracted to obtain the SO2 absorbance. Based on the basic database of 1 ppm SO2 absorbance The SO2 concentration can be calculated using the following formula. , where n is an array and The number of elements, n=351:

[0059] (2);

[0060] S6. Calculate the zero absorption baseline :

[0061] The absorbance obtained in step S4 Extracting 260-262 nm band data The zero-absorption absorbance is calculated according to formula (2). , where p is an array The number of elements, p=21;

[0062] (3);

[0063] S7. Calculate the concentration of the analyte in the sample using the direct ultraviolet absorption method. :

[0064] Using the absorbance obtained in step S6 and the absorbance of the analyte at 1 ppm in the basic database The concentration of the analyte is calculated using the following formula. :

[0065] (4);

[0066] S8. Starting from the same time point, record the concentrations of the analyte calculated in S6 and S7 respectively. and This forms an array with m = 2400 elements. and Calculate the coefficients k and b of the first-order function using the following formula:

[0067] (5);

[0068] in, It is an array with 2400 rows and 1 column. It is a 2400-row, 2-column array, whose first column is connected to the array... The same applies; the second column is all 1s.

[0069] S9. Calculate the true concentration of the analyte after noise reduction using the following formula. :

[0070] (6);

[0071] During the test, the gas pool was heated and kept at a constant temperature with a temperature control accuracy of ±0.5 ℃, and the gas pool flow rate was kept constant with a flow rate variation range of <0.1 L / min.

[0072] Depend on Figure 4Analysis shows that the SO2 measured values ​​obtained using the differential absorption method fluctuate significantly, but the deviation from the true concentration is small, with a standard deviation of 1.66 ppm and an average error of 0.08 ppm. The SO2 measured values ​​obtained using the direct absorption method fluctuate less, but the measurement error is larger, with a standard deviation of 0.42 ppm and an average error of 6.12 ppm. The detection method used in this embodiment inherits the advantages of both algorithms, achieving a standard deviation of 0.43 ppm and an average error of 0.11 ppm, thus realizing accurate and stable measurement of the analyte.

[0073] The detection method used in this embodiment achieves a detection limit that is only one-quarter of that obtained using the differential absorption method, significantly improving the system's low-concentration detection performance and measurement accuracy. If the detection limit were lowered by increasing the optical path length or improving the spectrometer's signal-to-noise ratio, the optical path length would need to be increased by at least four times, and the spectral signal-to-noise ratio by at least four times, resulting in an additional cost of approximately 10,000 to 30,000 yuan.

[0074] Therefore, by replacing the detection method from differential absorption spectroscopy to the technology described in this invention without changing the system structure, the measurement fluctuations of the measured components can be effectively reduced, and the response time can be improved. This is especially true for the measurement of low-concentration flue gas, where the measurement results are more accurate and stable. Furthermore, by adopting this patented technology, the system can achieve a lower detection limit by equipping it with a short-path and low signal-to-noise ratio spectrometer, effectively reducing system costs. If a long-path and high signal-to-noise ratio spectrometer is used, the system's lowest detection limit can be greatly improved, enabling accurate measurement of extremely low concentrations and providing technical support for future monitoring of ultra-low emission pollutants.

[0075] The foregoing has shown and described 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 way, and all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption, characterized in that, Includes the following steps: S1. Record the spectral data output by the spectrometer; S2. Subtract the background spectral data from the absorption spectral signal and calculate the absorbance of the sample. ; S3. Calculate the concentration of the analyte in the sample using the ultraviolet differential absorption method. ; S4. Calculate the zero absorption baseline. ; S5. Calculate the concentration of the analyte in the sample using the direct ultraviolet absorption method. ; S6. Starting from the same time point, record the concentrations of the analyte calculated in S3 and S5 respectively. and This forms an array with m elements. and Calculate the coefficients k and b of the first-order function using the following formula: （5）； in, ; It is an array with m rows and 1 column; Given an m x 2 array, its first column is connected to the array... The same applies; the second column is all 1s. S7. Calculate the true concentration of the analyte after noise reduction using the following formula. : （6）； In step S1, the specific steps for recording the spectral data output by the spectrometer include: (1) Recording the dark current spectrum of the spectrometer: Block the light inlet of the spectrometer to prevent external light from entering, and record the dark current spectral data output by the spectrometer. ; (2) Record the background spectrum of the spectrometer under the blank sample: Record the background spectral data output by the spectrometer when N2 or air is introduced into the gas cell. ; (3) Record the spectrometer-measured spectrum of the sample: With the flue gas to be tested introduced into the gas chamber, record the measurement spectral data output by the spectrometer. ; In step S2, the absorbance of the sample The calculation formula is as follows: （1）； In step S3, the concentration of the component being measured The calculation method is as follows: The absorbance obtained from step S2 Extract the characteristic absorption band data of the gas being measured to obtain its absorbance. Then, the absorbance is obtained by using the SG filtering function. low-frequency filtered signal Calculated according to the following formula: （2）； in, The base database contains the differential absorbance of the gas being measured at 1 ppm; n is an array. and The number of elements; In step S4, the zero absorption baseline The calculation formula is as follows: （3）； in, The absorbance obtained from step S2 Extract 260-262 nm band data; p is an array. The number of elements; In step S5, the concentration of the analyte in the sample The calculation formula is: （4） in, The absorbance of the analyte at 1 ppm is the base database, where n is an array. and The number of elements.

2. The method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption according to claim 1, characterized in that, The parameters of the SG filter function are set with the window number ranging from 3 to 101 and the order from 1 to 5.

3. The method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption according to claim 1, characterized in that, During the test, the gas pool was heated to a constant temperature of 40~300 ℃ with a temperature control accuracy of ±0.5℃; the gas pool flow rate was constant, ranging from 0.1 to 5 L / min, with a flow rate variation range of <0.1 L / min.

4. The application of the concentration detection method according to claim 1, characterized in that, Used to detect the concentration of low-concentration gas components in flue gas.

5. The application according to claim 4, characterized in that, The low-concentration gaseous components in the flue gas are one of SO2, NO, NO2, NH3, H2S or Cl2; the concentration of the gaseous components is <200 μmol / mol.

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