Research On The On-line Measurement Of Particulate Matter Mass Concentration By Laser Scattering Method

At present,the particulate matter(PM)emissions from coal-fired power plants are characterized by small particle size and low concentration.Under real-time and accurate emission monitoring requirements,the on-line measurement of particulate matter concentration faces significant challenges.The light scattering method has a fast response,simple structure,and excellent response characteristics for low-concentration PM.It has been widely used in the on-line measurement of low-concentration PM mass concentration in coalfired power plants.The principle of the light scattering method is that when the particle characteristics are stable,the scattered light intensity is proportional to the PM mass concentration.Changes in the measurement environment and particle characteristics will cause the measured results to be distorted.In coal-fired power plants,the flue gas’ s relative humidity is high,and the size of the emitted particles fluctuates with the operating conditions.The simple comparison and calibration of the light scattering method are challenging to meet the effective long-term measurement of the PM mass concentration.In view of the above problems,an on-line measurement platform for the PM mass concentration had been built,which can achieve real-time measurement of the mass concentration of particulate matter(minimum interval: 1 s);the influence mechanism of relative humidity on the light scattering method and the influence law of the average particle size of polydisperse particles on the light scattering method was Explored.Based on research,on-line measurement methods of PM mass concentration based on particle size correction were formed.Finally,according to actual measurement requirements,the on-line measurement model of light scattering coupled β-ray method for PM mass concentration measurement was constructed,and laboratory measurement research was carried out.Firstly,the influence law of flue gas humidity on the measurement by light scattering method was explored.An on-line test platform for the mass concentration of moisturecontaining aerosol particles had been built.The results show that the scattered light intensity per unit PM mass concentration(PM mass concentration sensitivity,PMCS)of different aerosols has apparent differences in response characteristics to relative humidity.Further experiments and theoretical calculations reveal the influence mechanism of relative humidity on the PM mass concentration measurement by the light scattering method.The agglomeration and hygroscopic growth of moisture-containing aerosol particles are the fundamental reasons relative humidity affects the light scattering method,and the two have opposite effects on light scattering measurement.The effect of relative humidity on light scattering measurement depends on the strength of agglomeration and moisture absorption,which is related to the moisture absorption characteristics of particles and the relative humidity of aerosols.Furthermore,under various incident wavelengths and relative humidity conditions,the sensitivity of the PM measurement with different types via light scattering to changes in aerosol relative humidity was explored,which provides data support for the PM mass concentration on-line measurement in industrial applications.Secondly,an on-line measurement method of PM mass concentration based on asymmetry factor was proposed.After clarifying that the influence of relative humidity on the light scattering measurement is still the change of particle size,the angular scattering characteristics of particles with different particle sizes were measured.For the same scattering angle,the results show that the PMCS decreases gradually with the particle size increase,and the PMCSs of different scattering angles have other responses to the change of particle size.The ratio of scattered light intensity between 45° and 135°symmetrical scattering angles was defined as the scattering asymmetry factor.The results show that the asymmetry factor is linearly related to the average particle size.Furthermore,the relationship between the PMCS and the asymmetry factor was established using the average particle size,and an on-line measurement method based on the correction of the scattering asymmetry factor was proposed.The measurement results show that for Si O2 particles and the fly ashes of coal-fired and biomass-fired power plants,compared with the traditional light scattering method,when the particle size changes,the measured accuracy of the light scattering method based on the asymmetry factor correction is significantly improved.With a 95 % confidence level,the average deviations are reduced from 191.11 % ± 9.12 %,26.76 % ± 2.77 %,and 54.14 % ±8.28 % to 8.90 % ± 3.20 %,4.93 % ± 1.35 % and 9.16 % ± 1.82 %,respectively.In addition,the spectral scattering characteristics and spatial angle scattering characteristics of particles are different manifestations of the light scattering characteristics of particles.Analogous to multi-angle measurement(scattering asymmetry factor),an on-line measurement method based on multi-wavelength light scattering was proposed.The ratio of the scattered light intensity of different wavelengths is defined as the wavelength scattering ratio factor.The correlation between the PMCS and the wavelength scattering ratio factor and the particle size was derived from the theory.An on-line measurement method of PM mass concentration based on wavelength scattering ratio factor was formed,using the average particle size of the particles as a medium.The results of Si O2,Arizona dust,and fly ash show that the measurement accuracy of the light scattering method based on wavelength scattering ratio factor is significantly improved compared with the traditional light scattering method.With a 95 % confidence level,the average deviations were reduced from 49.75 % ± 1.11 %,48.62 % ± 2.62 %,and 46.48 % ± 0.83 % to 8.11 % ± 1.99 %,4.34 % ± 2.26 %,and 2.99 %± 0.55 %,respectively.Finally,in the actual measurement,the particle composition and other factors may change slowly,causing the correction function to fail.And the β-ray method measurement is almost unaffected by the particle characteristics.Based on this,a laser scattering coupled β-ray method of PM mass concentration on-line measurement technology was formed.Furthermore,the light scattering measurement module was designed,processed,assembled,and connected in series with the β-ray measurement module to form an on-line measurement system for PM mass concentration measurement.Under laboratory conditions,the measurement system was calibrated,and the variable particle size measurement test was carried out.The measurement results of fly ash show that when the particle size changes,the average measurement deviation of the traditional light scattering method is 28.6 %,the average measurement deviation of the coupling measurement method is 6.6 %.The average measurement deviation of the coupling method is only one-fourth of the traditional light scattering method,which significantly improves the measurement accuracy.