| With the reform process of Chinese efficient,clean,and safe energy pushing forward,air pollutants emissions from coal combustion,including particulate matter,sulfur dioxide,and nitrogen oxide,has drawn constant attentions.Particulate emission monitoring plays an important role in particulate emission controlling for fixed emission sources;the accuracy of particulate emission monitoring technologies also needs to be improved to fulfill the requirements of ultra-low particulate emission monitoring.As one of the most widely-used particulate emission monitoring technologies,light extinction method has advantages of non-invasive,fast response,and simple structure.However,since the sensitivity of traditional light extinction system is severely limited by the efficient light path length,its applicability for low-concentration particulate emissions is quite poor.Secondly,the influence of the relationship between particulate extinction properties and particle properties,including size distribution,morphology,and composition,on measuring particle concentration by light extinction are still ambiguous,hindering the development of approaches for solving the problem.Finally,particle size variation will introduce a significant systematic error into measuring particle concentration by light extinction;for traditional light extinction method,this systematic error due to particle size variation is usually controlled by calibrating the extinction signal with particle concentration measured by manual weighting method periodically,which has poor timeliness and high labor cost.To solve these problems,this work conducted a series of researches on improving the optical structure of traditional extinction system and the relevant measuring methodology.Firstly,to improve the sensitivity of traditional extinction system,a special optical structure,White cell,was adopted in this work to increase the light path length of traditional extinction system;based on the design and establish of the White cell-coupled long light path extinction system,the long-light-path extinction properties of standard spherical silica particles and coal ash collected from a coal-fired power plants were investigated.According to the results,when particle concentration varied in 0～10 mg/m~3,the long-light-path extinction measured by the White-cell coupled extinction system presented excellent sensitivity to particle concentration,and there was a reliable linear correlation between the measured extinction signal and particle concentration.Based on the linear correlation between the extinction signal and particle concentration,the real-time particle concentration was calculated from the measured extinction signal and was compared to the experimentally measured result.When particle concentration varied in 0～10 mg/m~3,the relative error of the extinction-calculated concertation from the experimentally measured value varied in±30%,which totally fulfills the requirement of the industrial standard(HJ 76-2017)on the accuracy of particulate emission monitoring technologies for static emission sources.Secondly,to investigate the influence of particle properties on measuring particle concentration,the long-light-path extinction properties of silica aerosols,alumina aerosols,and ash aerosols with different sizes and morphologies were characterized using the long-light-path extinction system;the correlation between the experimentally derived extinction coefficients and average size for the investigated aerosols was analyzed.According to the experimental results,there were a reliable linear relationship between the experimentally derived extinction coefficient and the mass median diameter for silica aerosols;the sensitivity of particulate extinction coefficient to size variation increased with particle shape deformation.Despite the variation of the long-light-path extinction properties among alumina,ash,and silica aerosols with similar size distributions and particle morphology,the linear correlation between the extinction coefficient and particle size remained among the above aerosols.For particles with fixed composition and morphology,a constant linear correlation existed between the extinction coefficient and average size of the aerosol,and the systematic error of measuring particle concentration by long-light-path extinction method introduced by particle size variation mainly depended on the range of size variation;based on the linear correlation between the extinction coefficient and particle size,the extinction coefficient of the investigated aerosol could be predicted by online measurement of its average size.Thirdly,to decrease the influence of particle size variation on measuring particle concentration by long-light-path extinction method,a method for online measuring particle size and concentration by analyzing the response characteristics between the particulate extinction and forward light scattering signal,the combined light extinction and scattering method,was proposed in this study;the possibility of performing the combined measurements with the long-light-path extinction system coupled with light sources with the wavelength of 638 nm and 520 nm was also investigated.According to the experimental results for spherical silica aerosol,the measured optical signals at the wavelength of 638 nm and 520 nm presented typical characteristics of transmitted light and scattered light,respectively.With a constant form of particle size distribution and a mass median diameter varying in the range 0.7～1.1μm,the ratio between the extinction to mass sensitivity at 638nm and the scattering to mass sensitivity at 520 nm,defined as the extinction to scattering ratio,increased linearly with particle size.The linearity between the particle size inversed from the extinction to scattering ratio and the experimentally measured value was 0.98.When the empirical equation was calibrated by particles with the average particle size of 0.83μm,the relative error of the extinction-calculated concentration was 8～18%for the investigated particles with an average size of 1.02μm;with particle size correction introduced,the relative error of extinction-calculated concentration was reduced to within±3%,which validated the effectiveness of the extinction and scattering combined measurements for controlling the influence of particle size variation on particle concentration measurement.Furthermore,to investigate the influence of particle size dispersity and morphology on measuring particle size and concentration by the combined light extinction and scattering method,the extinction and scattering properties of polydispersed aerosols with different size dispersities and particle morphology were compared based on theoretical calculation and experimental measurements.According to the results of theoretical calculation,the correlations between the extinction coefficient,the extinction to scattering ratio,and the volume equivalent size generally have a constant form;an increase in particle size dispersity or particle shape deformation tend to alter the sensitivity of the optical parameter to particle size variation and the location of the peaks as well as peak-value sizes.According to the extinction and scattering properties of polydispersed spherical silica aerosol and the other irregularly-shaped one,an increase in particle size dispersity decreased the sensitivity of the extinction to scattering ratio to particle size,which also decreased the reliability of particle size inversion based on the combined extinction and scattering measurements;particle shape deformation tended to increase the sensitivity of particulate extinction coefficient to particle size variation,which put forward a higher demand for the accuracy of particle size inversion. |
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