Investigation Of The Effect Of Particle Characteristics On Real-time Measurement Of Particulate Mass Concentration With The Light Scattering Method

The atmospheric particulate pollution has become a serious environmental problem in China or even the world.As the largest coal consuming country,particulate emission from coal-fired power plants is one of the most important sources of atmospheric particulate pollution in China.As the emission control and the standards are more and more rigorous for the particulate matter of coal-fired power plants,the particulate concentration will be lower than 10 mg/m³ as required.In this way,the emitted particulates will have smaller sizes and lower concentration,which poses a new challenge to the online monitoring of industrial particulate emission.The light scattering has gradually become an important method for online monitoring of industrial particulate emission concentration,due to its advantages of wide concentration range,simple structure and nice real-time performance.At present,the measurement accuracy of particulate concentration based on light scattering is usually guaranteed by the calibration of known particle.However,due to the frequently fluctuant particle size,fickle morphology and complex composition of the actual particulates emitted from coal-fired power plants,it is difficult to meet the requirement of long-term continuous monitoring by periodic manual calibration.At present,the theoretical and experimental studies on the measurement of particle mass concentration by light scattering method are not perfect.Based on the above problems,thid paper carried out the research work in the following aspects.Firstly,we perfect the theory of measuring particle mass concentration by light scattering method.Based on the single-particle Mie scattering theory,the relationship model of scattering light intensity and mass concentration of both monodisperse and polydisperse particles are derived,and the theoretical basis of measuring particulate mass concentration by light scattering was established.According to the above theory,an experimental platform for particle concentration measurement with light scattering is built,which is composed of coupling particle generation,multi-wavelength and multi-angle scattering light detection,and real-time particle concentration reference measurement.The standard spherical SiO₂ particles are used to calibrate the experimental system,and it is found that the experimental platform can effectively simulate the emission and monitoring characteristics of coal-generated particles,and which has a good agreement with the theoretical calculation.Secondly,the influence of the spherical SiO₂ particle size on the mass concentration measurement is studied through simulation and experiment,the sensitivity peak particle size is found to be around 1 micrometre.The experimental results indicate that in the forward scattering angles near ?25 degrees for detection,the effects of particle size change on particle concentration measurement can be minimized.In order to solve the problem of particle size effect on concentration measurement,a real-time particle size measurement method based on the scattering ripple width of particles is developed.Combining with the simulation results and experimental measurements,four spherical aerosol samples with different size were tested.The particle size measurement error compared to the reference method is less than 15%.According to the error analysis of the measured data,it is found that the measurement error of the experimental device is lower than 2.00 mg/Nm³,which is lower than the measurement error of 5 mg/Nm³ allowed in the standard HJ 75-2017,and meets the measurement of the mass concentration of fine particles under the condition of ultra-low emission.Coupling with the real-time measurement of particle size,the measurement error caused by particle size variation has been effectively reduced.Thirdly,comparing with the mass sensitivities of one spherical particles and three kinds of non-spherical particles with different morphology under multiple scattering angles,the scattering light intensities of irregular particles are still proportional to the mass concentration,but the measurement error of concentration is larger than that of spherical particles.The measurement error caused by unknown morphology at 25 degrees seems to be lower that at 90 degrees.In addition,the scattering light distribution is more uniform for the higher irregular particles and the polarization degrees of the scattered light is strongly affected by particle shape.Moreover,the effect of inorganic mineral particles with different components on the concentration measurement was studied.The tested SiO₂,Al₂O₃,Fe₂O₃ and so on are the main components in coal-fired fly ash.The mass sensitivity of Al₂O₃ is the highest for its large real part of its refractive index.The sensitivity of SiO₂ particle is modest.Particles contain iron have lower mass sensitivity due to their high magnetic permeability,high light absorption and high particle density.Mosquito-repellent incense smoke,SiO₂ particles and their hybrid particles are compared to study the scattering properties of organic and inorganic particles.It is found that the detection angles of within 25 degrees and long-wavelength red light can effectively reduce the influence of particle composition on the mass sensitivity.In addition,the linear superposition theory of intensity and concentration of multi-component mixed particles is established under the condition of single scattering.Finally,the mass sensitivities of actual ash particles from different emission sources are studied.The results shows that in the scattering angle of 25 degrees,the relative mass sensitivities of different ash particles are as follows:coal ash?1.58?3.64?>MSWI fly ash?1.19?2.58?>biomass ash?1.02?>road dust?0.57?0.68?>carbon black?0.49?.Based on the analysis of the composition,particle size and morphology of the actual ash particles,it is found that the mass sensitivity of the actual ash particles is greatly affected by the composition.Generally,the ash with a high content of aluminum is highly mass sensitive,while the fly ash with a high content of iron and organic components have low mass sensitivity due to the light absorption of the particles.The results of this study can provide an effective guidance for on-line monitoring of particle mass concentrations of different emission sources by light scattering method.