| On-line monitoring of the industrial process gas parameters is the crucial part of a feedback control system.Among many process gas monitoring methods,tunable diode laser absorption spectroscopy(TDLAS)technology has developed rapidly due to its good selectivity,high sensitivity and non-contact measurement.Absorbance is the key parameter of TDLAS,through which the parameters of gas concentration,temperature,pressure and spectral constants can be obtained.Aiming at the problem that wavelength modulation spectroscopy(WMS)with high signal-to-noise couldn't measure absorbance effectively,in this dissertation,four methods of absorbance measurement were proposed based on WMS theory from two aspects:fixed-WMS and scanned-WMS.The proposed methods were applied in the industrial process gas parameters on-line measurement and the optimal control combined with advanced pretreatment modes.The main contents include:1)Two absorbance measurement methods were proposed based on fixed-WMS.Aiming at the problems that the background signal is large in the 2f/1f calibration free method and it needs to be deducted in the measurement but it is difficult to be measured in real time in the industrial field application,the 4f/1f method with small background signal was proposed.The harmonic signals of transmitted light intensity were calculated taking the absorbance by high frequency modulated as the starting point,and then the gas parameters measurement was realized through the 4th harmonic and the 1St harmonic ratio.The method was verified using the CO molecule absorption spectral line located at 4297.705cm-1.Under the weak absorption condition,considering the two features that the even harmonics ratio is a costant and the product of the even harmonics with the linear modulation index of light intensity is a fixed multiple of the absorbance peak when the modulation is fixed to a certain value,an even harmonics ratio method was proposed.The method first computed the fixed modulation index,fixed ratio and fixed multiple corresponding to the two features based on WMS theory,then it was verified using the CO molecule absorption spectral line located at 4297.705cm-1 taking the 1st,2nd,4th harmonic as an example.2)An even harmonics recovering method for absorbance measurement was proposed based on scanned-WMS.Based on the wavelength modulation spectroscopy theory,the method first expanded the Taylor series of laser transmittance modulated by high frequency at the modulated frequency of the absorption line in the time domain,then superimposed the even harmonics to eliminate the high-order terms,and obtained the absorbance.The method was theoretically derived,and the-CO2 molecule absorption spectral line located at 6976.203cm-1 was used for simulation calculation and experimental verification.The results show that the measurement accuracy of the method increases with the increase of harmonic order.3)A WM-DAS method for absorbance measurement was proposed.Different from the traditional scanned-WMS method,which uses the superposition of low-frequency triangle wave or saw tooth wave and high-frequency sine wave to realize wavelength scanning and modulation,the WM-DAS method used high-frequency sine signal to realize wavelength scanning and modulation for laser wavelength.It carried out spectrum analysis for the transmitted light intensity signal after absorption,and used each harmonic to reconstruct the light intensity signal with high signal-to-noise ratio.Then the relationship between the wavelength and the light intensity was established based on the sine scanning and modulation mechanism.Finally,the absorbance could be obtained through the fitting of the transmitted light intensity signal.In this study,the transmitted light intensity signal was reconstructed and fitted from the time domain and frequency domain respectively,and the method was verified using the CO molecule absorption spectral line located at 4297.705cm-1.4)The proposed absorbance measurement methods were applied in the monitoring and control of thermal power plants.For on-line monitoring of CO in flue gas,the 4f/1f method combined with dilution sampling pretreatment mode was used to realize the CO concentration mixed sampling measurement in flue gas of thermal power plants.The device was installed and operated in a coal-fired power plant.TDLAS monitor results were compared with a portable analyzer.The measurement trend of the two devices was consistent,and the error between them was less than 1 ppm.For the ammonia and nitrogen measurement,the WM-DAS method combined with in-situ sampling pretreatment mode was used to realize on-line measurement of gaseous ammonia concentration in thermal power plants.The device had achieved stable operation in both gas power plants and coal-fired power plants,and the measurement results of gas power plants had been calculated by material balance.The method was also applied on the denitrification optimal equilibrium control and the nonuniformity of NOx distribution and ammonia escaping concentration after denitrification were reduced.This dissertation realized high precision on-line measurement of industrial process gas parameters through combining advanced measurement methods and advanced pretreatment modes.It solved the problems that WMS method couldn't measure absorbance effectively and the measuring distortion,system complex and large maintenance in the existing pretreatment.It is expected to provide more accurate feedback parameters for more industrial process control systems. |
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