Low-concentration gas concentration detection method based on ultraviolet differential absorption method and ultraviolet direct absorption method and application thereof

Abstract

The invention discloses a low-concentration gas concentration detection method based on an ultraviolet differential absorption method and an ultraviolet direct absorption method and application of the low-concentration gas concentration detection method. The method comprises the following steps: obtaining two concentration values of a detected gas by utilizing a concentration calculation method of a differential absorption spectrum technology and a direct absorption spectrum technology, carrying out first-order fitting by utilizing data formed by the two concentration values, and substituting the concentration value of the direct absorption spectrum technology into a first-order fitting function to obtain the concentration value. The method has the advantages of an ultraviolet differential absorption method and an ultraviolet direct absorption method, the data accuracy is high, the fluctuation is small, the measured value fluctuation of the detected component can be obviously reduced, and a lower detection lower limit is obtained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of spectrum measurement, and particularly relates to a concentration detection method for low-concentration gas based on ultraviolet differential absorption method and ultraviolet direct absorption method and application thereof. BACKGROUND

[0002] At present, online monitoring technologies for SO2, NO, NO2, NH3 and H2S concentrations in industrial pollution source flue gas emission and production processes can be mainly divided into non-dispersive infrared absorption method, electrochemical method, differential absorption spectrum method, ultraviolet direct absorption spectrum method and chemiluminescence method according to their principles. A large number of engineering applications and repeated laboratory tests show that: at present, the infrared absorption method cannot meet the monitoring demand of ultra-low emission in China due to low measurement accuracy, high detection lower limit and other problems; the electrochemical analysis method is seriously interfered and is easily affected by water, H2S, NO and NO2, and is mainly suitable for short-term SO2 concentration detection; when measuring NO2 by the chemiluminescence method, NO2 needs to be converted into NO at high temperature through a molybdenum furnace and then measured, and the conversion efficiency of molybdenum gradually decreases with time, thereby affecting the measurement accuracy; the differential absorption spectrum method can detect low-concentration and high-sensitivity components such as SO2, NO, NO2, NH3 and H2S which have characteristic absorption in ultraviolet, but if low-concentration monitoring is to be realized, the common implementation scheme is to greatly increase the optical path or improve the signal-to-noise ratio of the spectrometer, and both of the two schemes will bring a large increase in cost and low cost performance; the ultraviolet direct absorption spectrum method can realize the measurement of SO2 and NO2, and the detection lower limit thereof is lower than that of the ultraviolet differential absorption spectrum technology under the same absorption optical path, but the stability thereof is poor and is easily affected by changes in dust, vibration and temperature. Therefore, there is an urgent need for a detection technology which can realize long-term stable low-concentration high-sensitivity measurement of the measured components and ensure long-term stable and reliable pollutant emission monitoring data. SUMMARY

[0003] In order to solve the problems of the prior art, the purpose of the present application is to provide a concentration detection method for low-concentration gas based on ultraviolet differential absorption method and ultraviolet direct absorption method and application thereof, 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, so that the concentration value obtained has the advantages of both methods, the data accuracy is high and the fluctuation is small, the fluctuation of the measured value of the measured component can be significantly reduced, and a lower detection lower limit is obtained.

[0004] In order to realize the above-mentioned purpose, the present application adopts the following technical scheme:

[0005] The concentration detection method of low concentration gas based on the ultraviolet differential absorption method and the ultraviolet direct absorption method, comprising 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 to calculate the absorbance σ of the sample;

[0008] S3, calculate the concentration x of the measured component in the sample by the ultraviolet differential absorption method;

[0009] S4, calculate the zero absorption baseline σ zero ;

[0010] S5, calculate the concentration z of the measured component in the sample by the ultraviolet direct absorption method;

[0011] S6, record the concentrations x and z of the measured component calculated by S3 and S5 respectively from the same time, form arrays X and Z with m elements, and calculate the first order function coefficients k and b according to the following formula:

[0012]

[0013] Wherein, m≥2; X is an array of m rows and 1 column; Y is an array of m rows and 2 columns, the first column is the same as array Z, and the second column is all 1;

[0014] S7, calculate the true concentration C of the measured component after denoising according to the following formula:

[0015]

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

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

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

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

[0020] Record the background spectral data I0 output by the spectrometer under the condition that N2 or air is introduced into the gas cell;

[0021] (3) Record the measurement spectrum of the spectrometer under the sample:

[0022] Record the measurement spectral data I1 output by the spectrometer under the condition that the flue gas to be measured is introduced into the gas cell.

[0023] Preferably, in the aforementioned step S2, the absorbance σ of the sample is calculated according to the following formula:

[0024]

[0025] Preferably, in the aforementioned step S3, the concentration x of the measured component is calculated according to the following formula:

[0026] The characteristic absorption wavelength data of the measured gas is extracted from the absorbance σ obtained in step S2, and the absorbance σ is obtained again by using the SG filter function, and the low-frequency filtered signal σ of the absorbance σ is obtained Low , which is calculated according to the following formula:

[0027]

[0028] wherein δ is the differential absorbance of 1 ppm of the measured gas in the basic database; and n is the number of elements of the array σ and δ.

[0029] Preferably, the parameters of the aforementioned SG filter function are set as follows: the window number is set to be in the range of 3-101, and the order is set to be in the range of 1-5.

[0030] Preferably, in the aforementioned step S4, the zero-absorption baseline σ zero is calculated according to the following formula:

[0031]

[0032] wherein σ 260to262 is the data of the wavelength band of 260-262 nm extracted from the absorbance σ obtained in step S2; and n is the number of elements of the array σ 260to262 .

[0033] Preferably, in the aforementioned step S5, the concentration z of the measured component in the sample is calculated according to the following formula:

[0034]

[0035] wherein σ lab is the absorbance of 1 ppm of the measured component in the basic database; and n is the number of elements of the array σ and σ lab .

[0036] Preferably, during the test, the gas cell is heated and kept at a constant temperature, the temperature is 40-300℃, and the temperature control accuracy is ±0.5℃; the flow rate of the gas cell is constant, the flow rate is 0.1-5 L / min, and the flow rate variation range is <0.1 L / min.

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

[0038] Preferably, the low concentration gas component in the foregoing flue gas is one of SO2, NO, NO2, NH3, H2S or Cl2.

[0039] The concentration of the gas component is < 200 μmol / mol.

[0040] The present application has the advantages of:

[0041] (1) The present application uses ultraviolet absorption spectroscopy technology, which simultaneously uses the characteristics of ultraviolet differential absorption spectroscopy technology, such as accurate measurement value, strong anti-interference energy, and small zero point range drift, and the characteristics of ultraviolet direct absorption spectroscopy technology, such as large absorption intensity and small measurement value fluctuation. The correlation of the measurement values of the two methods is extremely high in a short time. First-order fitting is used to filter the noise of the measurement values. The concentration of the measured gas after denoising is calculated by the first-order fitting curve coefficient and the direct absorption method measurement value. Especially under low concentration conditions, it has a lower detection limit and higher accuracy.

[0042] (2) The present application can realize accurate measurement of low concentration of the measured component of flue gas by a single analysis module. The long-term measurement result is stable and reliable, the short-term measurement result is stable and has small fluctuation, and it can better guide industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The flowchart of the detection method of the present application is shown in Figure

[0044] Figure 2 The basis database 1 ppm SO2 absorbance is shown in Figure

[0045] Figure 3 The basis database 1 ppm SO2 differential absorbance is shown in Figure

[0046] Figure 4 The SO2 concentration measurement values obtained by the differential absorption method, the direct absorption method and the detection method of the present application are shown in Figure DETAILED DESCRIPTION

[0047] The present application will be specifically introduced below in combination with the drawings and specific examples.

[0048] The example is described in detail in Figure 1 , the concentration detection method of low concentration gas based on ultraviolet differential absorption method and ultraviolet direct absorption method includes the following steps:

[0049] S1: Record the dark current spectrum of the spectrometer:

[0050] Plug the light inlet of the spectrometer to prevent any external light from entering the spectrometer, and record the spectral data I d .

[0051] S2: Record the spectrometer background spectrum under the blank sample:

[0052] Record the spectrum data I0 output by the spectrometer under the condition that N2 or air is introduced into the gas cell.

[0053] S3: Record the spectrometer measurement spectrum under the unknown sample:

[0054] Record the spectrum data I1 output by the spectrometer under the condition that the flue gas to be measured is introduced into the gas cell.

[0055] S4: Subtract the background spectrum data from the absorption spectrum signal to calculate the absorbance σ of the unknown sample:

[0056] Use I d , I0 and I1 to calculate the sample absorbance σ according to formula (1):

[0057]

[0058] S5, calculate the SO2 concentration in the sample by ultraviolet differential absorption method

[0059] Extract the SO2 absorbance σ from the absorbance σ obtained in S4 in the wavelength range of 280 nm to 315 nm According to the basic database 1 ppm SO2 absorbance Calculate the SO2 concentration according to the following formula Where n is the number of elements in the array and n = 351:

[0060]

[0061] S6, calculate the zero absorption baseline σ zero :

[0062] Extract the wavelength range of 260-262 nm from the absorbance σ obtained in step four σ 260to262 , calculate the zero absorption absorbance σ according to formula (2) zero , where n is the number of elements in the array σ 260to262 n = 21;

[0063]

[0064] S7, calculate the concentration of the measured component in the sample by ultraviolet direct absorption method

[0065] Use the absorbance σ obtained in S6 and the absorbance of 1 ppm of the measured component in the basic database Calculate the concentration z of the measured component according to the following formula:

[0066]

[0067] S8. Starting from the same time point, record the concentrations of the analyte calculated in S6 and S7 respectively. and Form an array with m = 2400 elements. and Calculate the coefficients k and b of the first-order function using the following formula:

[0068]

[0069] 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.

[0070] S9. Calculate the true concentration of the analyte after noise reduction using the following formula.

[0071]

[0072] 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 <0.1L / min.

[0073] Depend on Figure 4 Analysis 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.

[0074] 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.

[0075] Therefore, without changing the system structure, the detection method is replaced from the differential absorption spectroscopy to the technology introduced in the application, the measurement fluctuation of the measured component can be effectively reduced, the response time is improved, especially for the measurement of low concentration flue gas, the measurement result is more accurate and stable. And after adopting the patent technology, the system can realize a lower detection lower limit by carrying a short optical path and a low signal-to-noise ratio spectrometer, and the system cost can be effectively reduced. If a long optical path and a high signal-to-noise ratio spectrometer are carried, the lowest detection lower limit of the system can be greatly improved, accurate measurement of extremely low concentration can be realized, and technical support is provided for future ultra-low emission pollutant emission monitoring.

[0076] The basic principles, main features and advantages of the application are shown and described above. Those skilled in the art should understand that the above examples do not limit the application in any form, and any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the application.

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 σ of the sample; S3. Calculate the concentration x of the analyte in the sample using the ultraviolet differential absorption method; S4. Calculate the zero absorption baseline σ zero ; S5. Calculate the concentration z of the analyte in the sample using the direct ultraviolet absorption method; S6. Starting from the same time point, record the concentrations x and z of the analyte calculated in S3 and S5 respectively, forming arrays X and Z with m elements. Calculate the first-order function coefficients k and b according to the following formula: Where m≥2; X is an array of m rows and 1 column; Y is an array of m rows and 2 columns, whose first column is the same as array Z, and whose second column is all 1; S7. Calculate the true concentration C of the analyte after noise reduction using the following formula:

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, 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 the spectrometer, and record the dark current spectral data I output by the spectrometer. d ; (2) Record the background spectrum of the spectrometer under the blank sample: Record the background spectral data I0 output by the spectrometer when N2 or air is introduced into the gas cell. (3) Record the spectrometer-measured spectrum under the sample: When the flue gas to be tested is introduced into the gas pool, the measured spectral data I1 output by the spectrometer is recorded.

3. The method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption according to claim 2, characterized in that, In step S2, the formula for calculating the absorbance σ of the sample is as follows:

4. 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, In step S3, the concentration x of the measured component is calculated as follows: Extract the characteristic absorption band data of the gas to be measured from the absorbance σ obtained in step S2 to obtain its absorbance σ, and then obtain the low-frequency filtered signal σ of absorbance σ through the SG filtering function. Low Calculated according to the following formula: Where δ is the differential absorbance of the gas being measured at 1 ppm in the basic database; n is the number of elements in the arrays σ and δ.

5. The method for detecting the concentration of low-concentration gases based on ultraviolet differential absorption and ultraviolet direct absorption according to claim 4, 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.

6. 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, In step S4, the zero absorption baseline σ zero The calculation formula is as follows: Where, σ 260to262 To extract 260-262nm band data from the absorbance σ obtained in step S2; n is an array σ 260to262 The number of elements.

7. 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, In step S5, the formula for calculating the concentration z of the analyte in the sample is: Where, σ lab The absorbance of the analyte at 1 ppm is given by the basic database, where n is an array of σ and σ'. lab The number of elements.

8. 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 chamber was heated and kept at 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 variation range of <0.1 L / min.

9. 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.

10. The application according to claim 9, 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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