| With the consumption of natural resources and the aggravation of environmental pollution,energy conservation and emission reduction has become the focus of global attention.At present,the energy structure of power plants in our country is still dominated by traditional fossil energy.In order to achieve the purpose of energy saving and pollution reducing,it is necessary to conduct optimization control of the boiler combustion in thermal power plants.Carbon monoxide(CO)produced by incomplete combustion of fuel in thermal power plants can be used as the basis for combustion optimization.At the same time,as a flammable,explosive and toxic gas,real-time on-line monitoring of CO is imperative.Therefore,this paper has developed the research on carbon monoxide gas monitoring method in thermal power plants.The tunable diode laser absorption spectroscopy(TDLAS)is a technology that utilizes the selective absorption of light by gas molecules to measure gas parameters.Compared with other gas detection technologies,TDLAS has significant advantages:non-contact measurement,good selectivity,fast response,high sensitivity,simultaneous measurement of multi-component and multi-parameter,and simple system.In recent years,TDLAS has been gradually applied to various fields such as combustion diagnosis,industrial process monitoring,pollution gas measurement,medical diagnosis,and food safety detection.In actual measurement process of TDLAS technology,the change of environmental conditions will have a great influence on the concentration detection results.However,the research on the accuracy of TDLAS gas detection system thermal power plants under special application environment(specially,the existence of dust particles,modulation depth fluctuation and other factors)is still not in-depth,which limits the further application of this technology.Aiming at the problem of how to improve the detection accuracy of CO concentration in thermal power plants under complex environment,this paper first analyzes the sensitivity of various errors and conducts error distribution for each error from the overall point of view,and obtains the error terms that need to be mainly controlled;then discusses the influence mechanism of two kinds of key error factors on the detection accuracy,and makes corresponding correction.The details are as follows:(1)Aiming at the problem of overall error distribution of TDLAS gas concentration detection system under the influence of various error factors in the measurement environment of thermal power plants,this paper firstly analyzes and summarizes the error terms in the measurement based on the concentration measurement principle of wavelength modulation spectroscopy(WMS),and establishes the error model of the concentration measurement system.Secondly,the influence form of each error on the concentration error is studied by using the randomization method,the sensitivity of each error to the concentration measurement error is calculated,and the error terms that need to be strictly controlled are obtained.Considering the measurement requirements,the relationship of sensitivity,and the level of available devices,each error of the system is distributed reasonably.Finally,an experimental platform is built for CO concentration measurement.The results of simulations and experiments show that the relative error of measurement is less than 1%for the CO gas with a concentration of 2.5%.(2)Aiming at the problem that the transmitted laser intensity attenuation caused by particles in the measurement environment of thermal power plants affects the accuracy of gas concentration detection,this paper firstly analyzes the absorption and scattering of laser by particles,and gives the influence forms based on the theory of Mie scattering.Secondly,according to the principle of WMS gas concentration detection,the influence of the transmittance of particles on the output laser intensity and second harmonic signal of the detection system is studied.The conclusion is that the slope of the output laser intensity and the peak value of the second harmonic are both in linear relationship with the transmittance of the particles.Based on this relationship,a method of correcting the second harmonic signal by using the linear fitting slope of the output laser intensity signal is proposed to eliminate the influence of the particle transmittance,and the relationship between the gas concentration and the corrected second harmonic peak value is established.Finally,the above method is verified by simulation.The simulation results show that the gas concentration is in linear relationship with the corrected peak value of the second harmonic by use of the output laser intensity.For the gas with the concentration range of 1.5%?5.5%,the measured relative error is less than 1%,and the root mean square error is in the order of 10-4,which effectively reduces the influence of the particles.(3)Aiming at the problem that the modulation depth change caused by the fluctuation of temperature,pressure and modulation current in the measurement environment of thermal power plants affects the accuracy of gas concentration detection,this paper firstly analyzes the variation of the peak value and valley spacing of the second harmonic signal under different modulation depths based on the principle of WMS concentration inversion.Secondly,a method of calibrating the modulation depth by using the linear relationship between the valley spacing and the modulation depth is proposed,and an improved concentration inversion model is established by polynomial fitting of peak value and modulation depth,where the calibrated modulation depth is taken as a variable to reduce the influence of its variation on the measurement.Finally,the above method is verified by simulations and experiments.The results show that the root mean square error of modulation depth calibration is in the order of 10-2,compared with the traditional model,the root mean square error of the concentration measurement using the improved model is reduced from 10-4 to 10-5,and the maximum relative error is reduced from 1.68%to 0.36%,which verifies the effectiveness of the proposed method.The error distribution method of TDLAS concentration detection system,and the correction method for the influence of two kinds of key error factors,particles and modulation depth,are proposed in this paper,which are helpful to improve the accuracy of CO concentration detection in thermal power plants,and are of great significance to promote the engineering application of TDLAS technology. |
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