Research On Multi-wavelength Based Optical Sensing Method For Particulate Matter And Its Application

Environmental pollution is becoming more and more serious with social development,as an important pollutant in emission,particulate matter（PM）,which is commonly known as PM₁₀ and PM_2.5,affects human health in a way related not only to its concentration but also closely to its particle size.However,existing emission monitoring technology only measures the total mass concentration of PM,and cannot give the mass concentration with particle size distribution（PSD）.That is,due to the lack of accurate particle size and mass concentration monitoring methods,it is currently difficult to accurately and comprehensively evaluate the hazard of PM emitted from stationary sources.Therefore,it is necessary to develop new monitoring technology related to particle size,and provide solid support for environmental protection and improvement.Aerosol photoelectric sensing technology is the mainstream of the existing PM emission monitoring technology.This method has the advantages of fast response and high sensitivity,but the measurements are easily affected by the change of particle size,resulting in large errors.Tapered element oscillating microbalance（TEOM）and beta ray technology has high accuracy in measuring the mass concentration of PM,but its sampling time is too long and needs manual maintenance.Therefore,these technologies are not suitable for high-intensity continuous monitoring in stationary sources.At the same time,the common PSD measurement technologies are only suitable for laboratory research,which are difficult to apply to the measurement environment in stationary sources.For example,the differential mobility analyzer（DMA）method is difficult to measure the particle size greater than 1μm due to the limit of voltage intensity;The aerodynamic method cannot measure particles less than 0.5μm because the response threshold limit,and the measurement range of quantity concentration is low.The existing PSD measurement technology is limited in the monitoring of particles emitted from stationary sources.The key scientific problem is that the particle size segmentation method is difficult to measure continuously.Through the further study of light scattering theory,the measurement of PSD without physical segmentation of particles is achieved.This thesis focuses on the technical challenges of continuous emission monitoring of high temperature and high humidity flue gas emitted from stationary sources and the standard requirements of in-situ measurement,a multi-channel particle mass concentration measurement technology based on PSD inversion is proposed.Firstly,the Sauter mean diameter（SMD）sensing technology based on dual-wavelength and the PSD sensing technology based on three-wavelength are obtained through simulation.At the same time,the key optical structure is developed based on the anti optical interference of multiple incident/receiving light paths,and the particle continuous monitoring system is completed.Finally realize the multi-channel mass concentration measurement of PM emitted from stationary source.The main research contents include:Firstly,the method of extracting SMD of aerosol by light scattering signal is studied.By analyzing the relationship between the light scattering intensity of particles,particle size and incident light wavelength,a method of measuring SMD with only two wavelengths is proposed.The appropriate wavelength of incident light is selected according to the size of measured particle,and the information of particle size is obtained.Then,a low-cost and high-precision SMD sensor is designed with structure easy to implement,and the optical parameters are optimized by adding a third light path to improve the measurement accuracy of SMD.The relative standard deviation（RSD）of the measurement result is reduced from39.1%to 7.61%.Through practical measurement of aerosols with different particle sizes,the measurement error of the SMD sensor is not more than 6.83%.Secondly,the method of obtaining the key parameters of PSD by three-wavelength light scattering signals was studied.To use as few wavelengths as possible to obtain the key parameters of PSD,the relationship between the light scattering intensity of incident lights with different wavelengths,the mean particle size and standard deviation in the lognormal distribution function representing the PSD is studied.Therefore,a method to retrieve PSD by using only three wavelengths is proposed.By measuring the PSD and adjusting the conversion coefficient of mass concentration,the measurement accuracy of particle mass concentration is improved,and the mass concentration measurement in response to the change of PSD is realized innovatively.At the same time,the interference problem of three scattered light paths is solved,a PSD sensor with integrated optical structure is designed,and the performance of the prototype is tested with different standard dusts.The error of mass concentration measurement is between 11.01%and 14.13%;the error of measuring the mean particle size is less than 23.45%,and the error of the distribution standard deviation is less than 20.75%.Thirdly,the measurement of flue gas emitted from stationary sources needs to overcome the problem of high temperature and high humidity.Based on the three-wavelength inversion method of PSD,a multi-channel mass concentration continuous monitoring instrument is achieved by designing a stable and high-fidelity sampling system and a steady optical structure that can protect photoelectric units.The project demonstration was completed in a thermal power plant,and the performance of the monitoring system was verified by field test under different load conditions of the generator set.The average errors of PM₁₀,PM_2.5,and PM₁ measured by the engineering prototype are 5.25%,3.61%and13.67%,respectively.It creatively achieves the continuous monitoring of PM mass concentration with multi-channel in stationary sources.The innovations of this thesis include:based on the research of Mie scattering theory,a method for extracting aerosol SMD with scattering signals with only two wavelengths is proposed.By adding incident light of the appropriate wavelength,the information on the key parameter of the PSD was obtained,and the sensing method for PSD based only on three wavelengths is proposed.Based on the methods above,a set of continuous monitoring equipment for PM was developed.Measurement of multi-channel mass concentration is realized at the demonstration site,and pollutant monitoring technology in line with actual working conditions was developed.