Study On The Characteristics Of Laser Absorption Spectrum And Simultaneous Measurement Of CO And CO₂ With Large Concentration Difference

Global warming caused by greenhouse effect has become one of the main environmental pollution problems facing human beings.Carbon peaking and carbon neutralization have been incorporated into the overall layout of ecological civilization construction in China.At present,CO₂ emissions from coal-fired power plants are more than 40%of the national carbon emissions.Controlling coal-fired power and improving energy efficiency are important ways to reduce carbon emissions.CO₂ is the main greenhouse gas in boiler flue gas,and CO,as the main incomplete combustion product,can reflect the boiler combustion efficiency.Therefore,accurate real-time monitoring of CO and CO₂ is great significance for carbon emission reduction policy-making and boiler low-carbon operation.Tunable diode laser absorption spectroscopy（TDLAS）with high resolution,high selectivity and fast response compared with traditional CO and CO₂ gas detection technology,can realize on-line,fast and high-precision gas detection.Therefore,TDLAS is suitable for the measurement of flue gas emitted from boilers in harsh environment such as complex gas composition and high temperature.In this paper,aiming at the research background of high concentration CO₂ and low concentration CO in boiler flue gas,the simultaneous measurement of CO and CO₂ with large concentration difference by using a single laser and digital filtering technology is explored.Firstly,the basic principle of TDLAS technology for gas detection,including the detection process of direct absorption technology（DAS）and wavelength modulation technology（WMS）are introduced.An experimental system for CO and CO₂ spectrum measurement based on TDLAS is designed and built.Then,high concentration CO₂ gas is measured by direct absorption spectroscopy with calibration free.In order to simplify the system,the present paper proposes a new gas-concentration retrieving method based on calibration of the direct absorption peak value.To verify the feasibility of the method,concentration measurements were collected in a CO₂detection system based on TDLAS according to the flue gas environment at the end of flue,and the measurement error is compared with the traditional absorbance integration method.However,the low concentration CO measurement can’t be measured by direct absorption method because of DAS high detection limit,so the second harmonic detection technology is used to measure CO and CO₂ simultaneously with a single 1580nm DFB laser.11 groups of multicomponent gases with different concentrations are proportioned in the range of 0.01%-5%CO and 0.5-10%CO₂,of which the highest concentration difference is up to 200 times.The noise reduction effect of different digital filtering techniques on multi-component gas simultaneous measurement under large concentration difference is studied.The results show that the two level decompose with db3 wavelet function is the best to improve the spectral signal of multi-component gas（is validated to be more accurate than the other filtering methods in the noise reduction）.which can decrease the limit of system for 0.004%to0.0027%.The mean squared relative error of CO inversion concentration decrease from7.11%to 5.18%by using the wavelet filter,and the CO₂ decreased from 1.28%to 1.2%.The amplitude fluctuation of the same concentration measurement gas is also reduced under different concentrations of background gas,which reduces the noise interference and the measurement stability of the system was improved.