Tail gas emission on-line measuring system, measuring method and measuring range dividing method

Abstract

The invention discloses a tail gas emission on-line measurement system, a measurement method and a range division method, and relates to the technical field of motor vehicle and non-road machinery tail gas measurement, the measurement system comprises a light source, a light path switching module, a measurement cell group and a calculation control system; the measuring cell group comprises a plurality of groups of measuring cells with different effective detection optical paths, and gas paths of the measuring cells are connected in series; the light source comprises a laser light source; the light path switching module comprises a plurality of switchable detection light paths, the detection light paths are in one-to-one correspondence with the measuring cells, and laser emitted by the laser light source can enter the corresponding measuring cell through one detection light path during each measurement and then reach the calculation control system; and the calculation control system is used for completing pollution gas concentration calculation, detection light path switching control and laser light source control. The device and the method have the advantages that high-precision and large-span measuring range tail gas pollution gas concentration measurement requirements can be met at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of exhaust gas measurement for motor vehicles and non-road machinery, and in particular to an online exhaust gas emission measurement system, a measurement method and a range division method. Background Art

[0002] In order to measure the actual emissions of exhaust gaseous pollutants under actual road driving of motor vehicles and actual working conditions of non-road machinery, vehicle-mounted exhaust monitoring technology came into being. By installing portable exhaust measurement equipment on the target vehicle, comprehensive monitoring and management of exhaust emissions can be achieved.

[0003] The near-infrared and mid-infrared spectral regions have abundant absorption peaks of exhaust gas pollutants, which makes spectral detection technology an important tool in the field of exhaust monitoring and pollution control. Due to the spectral overlap and cross-interference of multi-component pollutants in exhaust emissions, some spectral detection methods such as non-dispersive infrared absorption (NDIR) and Fourier transform infrared spectroscopy (FTIR) are difficult to overcome the measurement errors caused by spectral overlap, while sensors based on tunable laser absorption spectroscopy (TDLAS) have ultra-high resolution. Therefore, sensors using TDLAS can reduce the interference caused by spectral overlap of coexisting gases in exhaust. Sensors based on the absorption spectrum measurement principle use the Beer-Lambert law as the basis of spectral analysis. Within a certain concentration range, there is a linear relationship between absorbance and gas concentration, and the gas concentration can be inverted by absorbance; in addition, the sensitivity of the sensor is determined by the absorption optical path length. By increasing the absorption optical path length, the absorbance can be enhanced, which can improve the detection limit.

[0004] In the process of exhaust monitoring, a single sensor can no longer meet the measurement of emissions of all types of vehicles: first, due to the large difference in the concentration of pollutants emitted by vehicles with different emission standards, the previous emission standard limits were relatively loose, resulting in higher pollutant emissions from old vehicles, and the new emission standards have reduced emissions of various pollutants by more than 95%, resulting in the sensor's measurement range being unable to meet the actual road tests of vehicles with various emission standards; second, due to differences in emission characteristics, operating conditions, and environmental conditions of different vehicles, there will be significant differences in the concentration of pollutants in the exhaust gas. For example, the use of poor quality fuel will lead to incomplete combustion of the oil-gas mixture when the engine is working, thereby increasing the pollutant content in the exhaust gas; in addition, the post-treatment catalyst needs to be within a certain temperature range to achieve the best catalytic effect. If the temperature is insufficient, the activity of the catalyst will decrease, resulting in the inability to effectively decompose pollutants. For example, in the cold start state, pollutants generated by incomplete combustion are discharged from the vehicle exhaust pipe, resulting in the concentration of pollutants changing from a few ppm to a percentage (%) in a short period of time. Faced with vehicles emitting pollutants in different ranges under different circumstances, the measurement range of a single sensor is limited and cannot simultaneously meet the monitoring needs of high precision and a large span range.

[0005] The traditional solution to the above problems is to combine sensors with different measuring ranges to expand the measurement capabilities. However, due to the use of sensors with multiple measuring ranges, the system will be complicated, there will be cost and maintenance issues, and it will not be easy to be portable. In addition, switching between multiple sensors will result in errors in measurement consistency.

[0006] Patent document with publication number CN107063773A discloses an exhaust gas measuring device and a control method for the exhaust gas measuring device. The measurement range is divided by diluting high-concentration exhaust gas. This method requires dilution gas when applied, which does not meet the demand for portable measurement. In vehicle-mounted exhaust gas measurement, gas concentration dilution will cause errors. There are multiple solenoid valves, pipelines, mixing channels, etc., with a complex structure. The measurement response time is long when switching the gas path, and the actual exhaust gas emission situation cannot be measured in real time. Corrosive gases such as ammonia and sulfides in the exhaust gas will adsorb and corrode the pipeline, causing serious measurement errors. Summary of the invention

[0007] The technical problem to be solved by the present invention is how to simultaneously meet the requirements of high-precision and large-span measurement of exhaust pollutant gas concentration.

[0008] The present invention solves the above technical problems through the following technical means: an online exhaust emission measurement system, comprising a light source, an optical path switching module, a measuring cell group and a computing control system; the measuring cell group comprises a plurality of measuring cells with different effective detection optical paths, and the gas paths of the measuring cells are connected in series; the light source comprises a laser light source; the optical path switching module comprises a plurality of switchable detection optical paths, and the detection optical paths correspond to the measuring cells one by one. The laser emitted by the laser light source can be emitted into the corresponding measuring cell through one of the detection optical paths for each measurement, and then reaches the computing control system; the computing control system is used to complete the calculation of polluted gas concentration, the switching control of the detection optical path and the control of the laser light source.

[0009] As an optimized technical solution, the optical path switching module includes a dichroic mirror, a beam splitter and a switching device. The laser emitted by the laser light source passes through the dichroic mirror and the beam splitter in sequence. The beam splitter splits the light beam into multiple beams. The switching device is used to switch and block between the light beams separated by the beam splitter.

[0010] As an optimized technical solution, the light source also includes a visible light source, and the visible light emitted by the visible light source first passes through the dichroic mirror to combine with the laser, and then passes through the beam splitter.

[0011] As an optimized technical solution, the optical path switching module also includes a first total reflection mirror, a second total reflection mirror, a third total reflection mirror, a fourth total reflection mirror and a beam combining mirror. The first total reflection mirror is used to reflect the laser emitted by the laser light source, the second total reflection mirror is used to reflect the visible light emitted by the visible light source, the third total reflection mirror is used to reflect the partial light beam after splitting, the fourth total reflection mirror is used to reflect the light beam coming out of the measuring cell, and the beam combining mirror is used to reflect and transmit the light beam coming out of the measuring cell.

[0012] As an optimized technical solution, the switching device includes a rotation drive device and a light shielding plate, and the light shielding plate is installed at the rotation end of the rotation drive device.

[0013] As an optimized technical solution, the laser light source uses a single laser or a combination of multiple lasers.

[0014] As an optimized technical solution, the computing control system includes a photodetector, a preamplifier, an analog-to-digital converter and a computer; the photodetector uses one detector to detect the absorption spectrum signals of different detection optical paths in a time-sharing manner; the preamplifier is used to perform transimpedance amplification and filtering on the current signal output by the photodetector, and output an AC spectrum absorption signal with DC removed and a spectrum absorption signal containing DC; the analog-to-digital converter is used to complete data acquisition of the spectrum absorption signal; the computer completes the calculation of the concentration of polluted gas, the switching control of the detection optical path and the control of the laser light source.

[0015] A method for dividing the measurement range of an online exhaust emission measurement system, for dividing the measurement range of a measurement cell group of the online exhaust emission measurement system described in any one of the above, comprises the following steps:

[0016] S1, select a measuring cell that can reach the minimum detection limit optical path length as the first measuring cell, pass a variety of concentrations of standard substances gradually increasing from the minimum detection limit concentration to the full range SPAN, and fit the calibration curve of the first measuring cell by polynomial fitting;

[0017] S2, finding the nonlinear turning point in the calibration curve of the first measuring cell, and recording it as the full scale value SPAN1 of the first measuring cell;

[0018] S3, selecting a measuring cell that can reach the optical path length of SPAN1 as the second measuring cell, introducing various concentrations of standard substances gradually increasing from SPAN1 to the full scale SPAN according to step S1, and fitting a calibration curve of the second measuring cell by polynomial;

[0019] S4, finding the nonlinear turning point in the calibration curve of the second measuring cell, and recording it as the full scale value SPAN2 of the second measuring cell;

[0020] S5. If the value of SPAN2 is greater than or equal to the full-scale value SPAN, the full-scale value SPAN is used to replace SPAN2. At this time, the measurement range is divided into a low concentration measurement range [detection limit, SPAN1] and a high concentration measurement range [SPAN1, SPAN]; if the value of SPAN2 is less than the full-scale value SPAN, continue to repeat steps S3 and S4, select the third measurement cell and find out the full-scale value SPAN3 of the third measurement cell, and so on until the linear range of the calibration curve includes the full-scale value SPAN.

[0021] As an optimized technical solution, when fitting the calibration curve, the absorbance is expressed as The calculation formula is:

[0022]

[0023] In the formula, I 0 is the incident light intensity, I t is the transmitted light intensity, k v is the spectral absorption coefficient, L is the absorption path length;

[0024] The absorbance threshold TH1 is determined by the nonlinear turning point of the previous measuring cell, and the standard substance with the concentration corresponding to the threshold point TH1 is introduced into the next measuring cell. The absorbance value TH2 is calculated and the optical path correction coefficient k=TH1 / TH2 is obtained to realize the correction of the absorbance of the next measuring cell.

[0025] A method for measuring exhaust gas emissions online, using an exhaust gas emissions online measurement system as described in any one of the above, comprising the following steps: exhaust gas is sequentially passed into each measuring cell of a measuring cell group and then discharged; during initialization, an optical path switching module is in the detection optical path corresponding to the lowest range measuring cell, and the laser enters the computing control system after passing through the lowest range measuring cell, and the computing control system calculates the absorbance of the absorption spectrum in real time; each time a new absorbance spectrum absorbance is calculated, it is compared with the range of the current measuring cell, when it is judged that the absorbance value is within the range of the current measuring cell, the detection optical path of the optical path switching module remains unchanged, when it is judged that the absorbance value is greater than the maximum range value of the previous measuring cell, the computing control system controls the optical path switching module to switch to the detection optical path corresponding to the next measuring cell, and when it is judged that the absorbance value is less than the minimum range value of the previous measuring cell, the computing control system controls the optical path switching module to switch to the detection optical path corresponding to the previous measuring cell.

[0026] The advantages of the present invention are:

[0027] 1. The measurement system realizes the measurement of a wide range of exhaust gas concentrations by selecting measurement cells with different effective detection optical path lengths for time-sharing multiplexing. It does not require additional sensors or additional dilution gases to dilute the exhaust gas concentration, and can complete the wide range, high-precision, and rapid detection of gaseous pollutants in exhaust gas from ppm to percentage (%) levels.

[0028] 2. After the exhaust gas is sampled, it passes through the measuring cell group in sequence, which is equivalent to that each measuring cell has completed the sampling of the exhaust gas. When switching the detection optical path, it is only necessary to block the unnecessary optical path, and there is no need to change any optical path adjustment mechanism. The structure is simple, stable and reliable, and different ranges can be quickly switched. The same detector is used for detection, and there is no interference with the subsequent preamplifier circuit and the concentration inversion algorithm, which can realize rapid switching of different optical paths.

[0029] 3. Multiple measurement ranges are divided by calibration, and the calibration curve of each section is linear. Therefore, the measurement system always works in the linear area, the measurement is more accurate, and the process of on-site calibration quality control is simplified. In the past, multi-point calibration was required to fit a nonlinear calibration curve beyond the linear range. However, the measurement system of the present invention is linear over the entire range, and on-site quality control only needs to calibrate the zero point and SPAN point to complete the calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the structure of the online exhaust emission measurement system according to an embodiment of the present invention.

[0031] Figure 2 The present invention is a flowchart of a method for dividing a measurement range of an online exhaust emission measurement system according to an embodiment of the present invention.

[0032] Figure 3 Spectral absorption curve diagram of an embodiment of the present invention.

[0033] Figure 4 This is an absorbance curve diagram of an embodiment of the present invention.

[0034] Figure 5 The graph is a graph showing the relationship between absorbance and concentration of a standard substance according to an embodiment of the present invention.

[0035] Figure 6 A schematic diagram of setting an absorbance threshold and dividing a high concentration range into a low concentration range according to an embodiment of the present invention.

[0036] Figure 7 Schematic diagram of the error between the ideal optical path and the actual optical path according to an embodiment of the present invention.

[0037] Figure 8 The figure is a flow chart of the online exhaust emission measurement method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the protection scope of the present invention.

[0039] like Figure 1 As shown, an embodiment of the present invention discloses an online exhaust emission measurement system, including a light source 1, an optical path switching module 2, a measurement cell group 3 and a computing control system 4.

[0040] The measuring cell group 1 includes a plurality of measuring cells with different effective detection optical path lengths, and the gas paths of the measuring cells are connected in series through gas pipes. Taking two groups of measuring cells as an example, they include a first measuring cell 11 and a second measuring cell 12. The first measuring cell 11 is a long optical path multiple reflection measuring cell, and the effective detection optical path length is in the range of several meters to several tens of meters, and the detection sensitivity is high, which is suitable for measuring polluted gases in a low concentration range; the second measuring cell 12 is a short optical path measuring cell, and the effective optical path length is in the range of several centimeters to several tens of centimeters, and the detection sensitivity is low, which is suitable for measuring polluted gases in a high concentration range.

[0041] The light source 2 includes a laser light source 21 and a visible light source 22. The laser light source 21 is used to measure the concentration of pollutants in the exhaust gas, and the visible light source 22 serves as an indicator. The laser light source 21 can be a DFB tunable semiconductor laser with an output wavelength in the near-infrared band, which is suitable for measuring CO in the exhaust gas. x NH 3 The concentration of polluting gases such as nitrogen and nitrogen can also be measured by using quantum cascade lasers or inter-band cascade lasers with output wavelengths in the mid-infrared band, which are suitable for measuring NO in exhaust gas.x , CO x 、SO x NH 3 The laser light source 21 may use a single laser to measure the concentration of one pollutant gas; or a plurality of lasers may be used in combination to measure the concentration of multiple pollutants.

[0042] The optical path switching module 3 has a plurality of switchable detection optical paths, each of which corresponds to a measuring cell. The laser light emitted by the laser light source 21 can be emitted into the corresponding measuring cell through one of the detection optical paths for each measurement, and then reaches the computing control system 4 .

[0043] The optical path switching module 3 includes a dichroic mirror 31, a beam splitter 32, a first total reflection mirror 33, a second total reflection mirror 34, a third total reflection mirror 35, a fourth total reflection mirror 36, a beam combining mirror 37 and a switching device 38; the laser light emitted by the laser light source 21 passes through the dichroic mirror 31 and the beam splitter 32 in sequence, the visible light emitted by the visible light source 22 first passes through the dichroic mirror 31 to combine with the laser light, and then passes through the beam splitter 32, and the beam splitter 32 divides the light beam into multiple beams; the first total reflection mirror 33 is used to reflect the laser light emitted by the laser light source 21 so that the laser light can reach the dichroic mirror 31; the second total reflection mirror 34 is used to reflect the visible light emitted by the visible light source 22 so that the visible light can reach the dichroic mirror 31; the third total reflection mirror 35 is used to reflect the partial light beam after the splitting so that each light beam can reach the corresponding measuring cell; the fourth total reflection mirror 36 is used to reflect the light beam coming out of the measuring cell, and the beam combining mirror 37 is used to reflect and transmit the light beam coming out of the measuring cell so that the light beam can reach the computing control system 4.

[0044] When the measuring cell group 1 includes two groups of measuring cells, the beam splitter 31 splits the light beam into two beams, and the third total reflection mirror 35, the fourth total reflection mirror 36 and the beam combining mirror 37 each use one piece; when the measuring cell group 1 includes more groups of measuring cells, the beam splitter 31 splits the light beam into more beams, and the third total reflection mirror 35 and the beam combining mirror 37 also use more pieces accordingly.

[0045] The switching device 38 is used to switch and block between the light beams split by the beam splitter 31. The switching device 38 includes a rotation drive device and a light baffle. The light baffle is installed at the rotating end of the rotation drive device. The rotation drive device can adopt an electromagnetic device such as a DC motor and a steering gear.

[0046] The computing control system 4 is used to complete the calculation of polluted gas concentration, the switching control of the detection light path and the control of the laser light source 21. The computing control system 4 includes a photodetector 41, a preamplifier 42, an analog-to-digital converter 43 and a computer 44; the photodetector 41 uses a detector to detect the absorption spectrum signals of different detection light paths in time-sharing; the preamplifier 42 is used to perform transimpedance amplification and filtering on the current signal output by the photodetector 41, and output an AC spectrum absorption signal with DC removed and a spectrum absorption signal containing DC (including light intensity information). The light intensity of the absorption signal of different optical paths will be different. In order to prevent the signal collected at the later stage from being distorted or cutoff, the AC spectrum absorption signal with DC removed is used to keep the signal amplitude stable through the gain automatic control circuit; the analog-to-digital converter 43 is used to complete the data collection of the spectrum absorption signal; the computer 44 includes a memory, a microcontroller, a digital-to-analog converter and peripheral equipment; the microcontroller and the digital-to-analog converter generate a modulation signal for laser wavelength tuning to complete the control of the laser light source 21; the microcontroller and the peripheral equipment complete the calculation of the polluted gas concentration and the switching control of the detection light path by executing the algorithm program in the memory.

[0047] like Figure 2 As shown, the embodiment of the present invention also discloses a range division method for an online exhaust emission measurement system, which is used to divide the range of the measurement cell group 3. The core of the range division is to select a measurement cell with a suitable optical path, and then determine it by calibration with a standard substance. The range division includes the following steps:

[0048] S1. Select a measuring cell that can reach the minimum detection limit optical path length as the first measuring cell, and introduce a variety of concentrations of standard substances gradually increasing from the minimum detection limit concentration to the full range SPAN. The intermediate concentration points can be increased by 10 times, and the calibration curve of the first measuring cell is fitted by a polynomial. The more intermediate concentration points, the more realistic the fitting of the calibration curve;

[0049] S2, finding the nonlinear turning point in the calibration curve of the first measuring cell, and recording it as the full scale value SPAN1 of the first measuring cell;

[0050] S3, select a measuring cell that can reach the optical path length of SPAN1 as the second measuring cell, and introduce various concentration standard substances gradually increasing from SPAN1 to the full scale SPAN according to step S1, the intermediate concentration points can be increased by 10 times, and the calibration curve of the second measuring cell is fitted by a polynomial;

[0051] S4, finding the nonlinear turning point in the calibration curve of the second measuring cell, and recording it as the full scale value SPAN2 of the second measuring cell;

[0052] S5. If the value of SPAN2 is greater than or equal to the full-scale value SPAN, the full-scale value SPAN is used to replace SPAN2. At this time, the measurement range is divided into a low concentration measurement range [detection limit, SPAN1] and a high concentration measurement range [SPAN1, SPAN]; if the value of SPAN2 is less than the full-scale value SPAN, continue to repeat steps S3 and S4, select the third measurement cell and find out the full-scale value SPAN3 of the third measurement cell, and so on until the linear range of the calibration curve includes the full-scale value SPAN.

[0053] like Figures 3 to 5 As shown, taking three concentrations of standard substances as an example, the method of fitting the calibration curve is as follows: the calculation control system 4 obtains the spectral absorption signal of the target gas, that is, the AC spectral absorption signal and the spectral absorption signal containing DC (including light intensity information), accumulates and averages the spectral signal to eliminate random noise, uses polynomial fitting to achieve baseline fitting of the spectral signal, and then uses the Levenberg-Marquardt nonlinear fitting algorithm to perform linear fitting to obtain the integrated absorbance. Since the light intensity fluctuations caused by the windows and airflow disturbances of different measuring cells are different, the spectral absorption signal containing DC can represent the current light intensity detection information. By eliminating the light intensity fluctuation of the absorbance and normalizing the signal amplitude, the light intensity error of different measured values ​​can be eliminated, that is, the light intensity influence of different measuring cells can be eliminated.

[0054] In the measuring cell group 1, the optical path of each measuring cell is an ideal design parameter. According to the molecular absorption spectrum theory, the incident light intensity I 0 and the transmitted light intensity I t Satisfying the Beer-Lambert law, the absorbance can be expressed as The calculation formula is:

[0055]

[0056] In the formula, k v (cm -1 ) is the spectral absorption coefficient, L (cm -1 ) is the absorption path length;

[0057] Therefore, in the actual measurement process, there will be deviations in the optical path of the measuring cell, and errors will occur in the calculation of absorbance. By correcting the calibration curve of the next measuring cell through the threshold point of the previous measuring cell, the influence of the optical path difference of different measuring cells on the absorbance calculation can be eliminated, thus realizing the full-range linear calibration of the measurement system.

[0058] like Figure 6 , Figure 7As shown, taking the optical path correction of the measuring cell group of two measuring cells as an example, if two measuring cells are used to meet the range of the full scale SPAN, that is, only one nonlinear turning point SPAN1 needs to be found, and the absorbance value corresponding to this point is called the absorbance threshold; the absorbance threshold TH1 is determined by the nonlinear turning point of the first measuring cell, and the standard substance with the concentration corresponding to the threshold point TH1 is introduced into the second measuring cell, and the absorbance value TH2 is calculated to obtain the optical path correction coefficient k=TH1 / TH2, which is stored in the memory to realize the correction of the absorbance of the second measuring cell, thereby realizing the linearization of the calibration curves of different measuring ranges.

[0059] like Figure 8 As shown, the embodiment of the present invention also discloses an online measurement method for exhaust gas emission, which adopts the online measurement system for exhaust gas emission, including the following steps: the exhaust gas is sequentially introduced into each measuring cell of the measuring cell group 1 through the sampling gas path, and finally discharged from the exhaust port; during initialization, the optical path switching module 2 is in the detection optical path corresponding to the lowest range measuring cell, and the laser enters the calculation control system 4 after passing through the lowest range measuring cell, and the calculation control system 4 calculates the absorbance of the absorption spectrum in real time; each time a new absorbance spectrum absorbance is calculated, it is compared with the range of the current measuring cell, when it is judged that the absorbance value is within the range of the current measuring cell, the detection optical path of the optical path switching module 3 remains unchanged, when it is judged that the absorbance value is greater than the maximum value of the range of the previous measuring cell, the calculation control system 4 controls the optical path switching module 2 to switch to the detection optical path corresponding to the next measuring cell, and when it is judged that the absorbance value is less than the minimum value of the range of the previous measuring cell, the calculation control system 4 controls the optical path switching module 2 to switch to the detection optical path corresponding to the previous measuring cell.

[0060] Taking two measuring cells as an example, when the optical path switching module 2 is in the detection optical path corresponding to the first measuring cell and judges that the absorbance value is less than or equal to the absorbance threshold, the optical path mode is not changed; when the optical path switching module 2 is in the detection optical path corresponding to the first measuring cell and judges that the absorbance value is greater than the absorbance threshold, the computing and control system 4 controls the optical path switching module 2 to switch to the detection optical path corresponding to the second measuring cell; when the optical path switching module 2 is in the detection optical path corresponding to the second measuring cell and judges that the absorbance value is greater than the absorbance threshold, the optical path mode is not changed; when the optical path switching module 2 is in the detection optical path corresponding to the second measuring cell and judges that the absorbance value is less than or equal to the absorbance threshold, the computing and control system 4 controls the optical path switching module 2 to switch to the detection optical path corresponding to the first measuring cell.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it should be understood by those skilled in the art that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An online exhaust emission measurement system, characterized in that: It comprises a light source, an optical path switching module, a measuring cell group and a computing control system; the measuring cell group comprises a plurality of measuring cells with different effective detection optical paths, and the gas paths of the measuring cells are connected in series; the light source comprises a laser light source; the optical path switching module comprises a plurality of switchable detection optical paths, and the detection optical paths correspond to the measuring cells one by one. The laser light emitted by the laser light source can be emitted into the corresponding measuring cell through one of the detection optical paths for each measurement, and then reaches the computing control system; the computing control system is used to complete the calculation of polluted gas concentration, the switching control of the detection optical path and the control of the laser light source.

2. The exhaust emission online measurement system according to claim 1, characterized in that: The optical path switching module includes a dichroic mirror, a beam splitter and a switching device. The laser emitted by the laser light source passes through the dichroic mirror and the beam splitter in sequence. The beam splitter splits the light beam into multiple beams. The switching device is used to switch and block between the light beams separated by the beam splitter.

3. The exhaust emission online measurement system according to claim 2, characterized in that: The light source also includes a visible light source. The visible light emitted by the visible light source first passes through the dichroic mirror to combine with the laser beam, and then passes through the beam splitter.

4. The exhaust emission online measurement system according to claim 3, characterized in that: The optical path switching module also includes a first total reflection mirror, a second total reflection mirror, a third total reflection mirror, a fourth total reflection mirror and a beam combining mirror. The first total reflection mirror is used to reflect the laser emitted by the laser light source, the second total reflection mirror is used to reflect the visible light emitted by the visible light source, the third total reflection mirror is used to reflect the partial light beam after splitting, the fourth total reflection mirror is used to reflect the light beam coming out of the measuring cell, and the beam combining mirror is used to reflect and transmit the light beam coming out of the measuring cell.

5. The exhaust emission online measurement system according to claim 2, characterized in that: The switching device comprises a rotation driving device and a light shielding plate, wherein the light shielding plate is installed at the rotation end of the rotation driving device.

6. The exhaust emission online measurement system according to claim 1, characterized in that: The laser light source adopts a single laser or a combination of multiple lasers.

7. The exhaust emission online measurement system according to claim 1, characterized in that: The computing control system comprises a photoelectric detector, a preamplifier, an analog-to-digital converter and a computer; the photoelectric detector uses one detector to detect the absorption spectrum signals of different detection optical paths in a time-sharing manner; the preamplifier is used to perform transimpedance amplification and filtering on the current signal output by the photoelectric detector, and output an AC spectrum absorption signal with DC removed and a spectrum absorption signal containing DC; the analog-to-digital converter is used to complete data collection of the spectrum absorption signal; the computer completes the calculation of the concentration of polluted gas, the switching control of the detection optical path and the control of the laser light source.

8. A range division method for an online exhaust emission measurement system, used for dividing the range of a measurement cell group of an online exhaust emission measurement system according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1, select a measuring cell that can reach the minimum detection limit optical path length as the first measuring cell, pass a variety of concentrations of standard substances gradually increasing from the minimum detection limit concentration to the full range SPAN, and fit the calibration curve of the first measuring cell by polynomial fitting; S2, finding the nonlinear turning point in the calibration curve of the first measuring cell, and recording it as the full scale value SPAN1 of the first measuring cell; S3, selecting a measuring cell that can reach the optical path length of SPAN1 as the second measuring cell, introducing various concentrations of standard substances gradually increasing from SPAN1 to the full scale SPAN according to step S1, and fitting a calibration curve of the second measuring cell by polynomial; S4, finding the nonlinear turning point in the calibration curve of the second measuring cell, and recording it as the full scale value SPAN2 of the second measuring cell; S5. If the value of SPAN2 is greater than or equal to the full-scale value SPAN, the full-scale value SPAN is used to replace SPAN2. At this time, the measurement range is divided into a low concentration measurement range [detection limit, SPAN1] and a high concentration measurement range [SPAN1, SPAN]; if the value of SPAN2 is less than the full-scale value SPAN, continue to repeat steps S3 and S4, select the third measurement cell and find out the full-scale value SPAN3 of the third measurement cell, and so on until the linear range of the calibration curve includes the full-scale value SPAN.

9. The range division method of the exhaust emission online measurement system according to claim 8, characterized in that: When fitting the calibration curve, the absorbance is expressed as The calculation formula is: Where I0 is the incident light intensity, I t is the transmitted light intensity, k v is the spectral absorption coefficient, L is the absorption path length; The absorbance threshold TH1 is determined by the nonlinear turning point of the previous measuring cell, and the standard substance with the concentration corresponding to the threshold point TH1 is introduced into the next measuring cell. The absorbance value TH2 is calculated and the optical path correction coefficient k=TH1 / TH2 is obtained to realize the correction of the absorbance of the next measuring cell.

10. An online measurement method for exhaust gas emissions, using the online measurement system for exhaust gas emissions according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: tail gas is sequentially passed into each measuring cell of the measuring cell group and then discharged; during initialization, an optical path switching module is in the detection optical path corresponding to the lowest range measuring cell, and the laser enters the calculation control system after passing through the lowest range measuring cell, and the calculation control system calculates the absorbance of the absorption spectrum in real time; each time a new absorbance spectrum absorbance is calculated, it is compared with the range of the current measuring cell, when it is judged that the absorbance value is within the range of the current measuring cell, the detection optical path of the optical path switching module remains unchanged, when it is judged that the absorbance value is greater than the maximum range value of the previous measuring cell, the calculation control system controls the optical path switching module to switch to the detection optical path corresponding to the next measuring cell, and when it is judged that the absorbance value is less than the minimum range value of the previous measuring cell, the calculation control system controls the optical path switching module to switch to the detection optical path corresponding to the previous measuring cell.

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