On-line Particle Size Measurement Through Acoustic Emission Sensing

In the modern industrial processes,the on-line sizing of solid particles has increasingly attracted much attention.The size of pulverized coal is an important physical characteristic in the power generation industry,as it closely relates to combustion efficiency,pollutant emissions and milling cost.On-line continuous particle size measurement of pulverized coal is of great importance to improve the operational efficiency of coal-fired power plants.The acoustic emission(AE)method enjoys several advantages,including high sensitivity,on-line measurement,simple structure and low maintenance,which make it as a promising way to realize the on-line size measurement of particles in pneumatic conveying pipelines in recent years.Though the validity of the AE-based method has been demonstrated by some preliminary research,the fundamental mechanism of this approach for particle sizing is still unclear.More work needs to be undertaken in order to improve the measurement accuracy of this method.Therefore,it is desirable to conduct an in-depth investigation on the measurement mechanism of particle sizing by AE methods.The investigations into the characteristics of the AE signals from individual particle impacts is a way to gain an in-depth understanding of the AE-based particle sizing method.With the AE signal from an individual impact event,it is convenient to extract the signal parameters and optimize the particle size inversion model.In this way,the fundamental sensing mechanism of the AE-based technique for particle size measurement is revealed.It also contributes to the design and optimization of the AE sensing system,which is used to realize the on-line particle sizing.Research work has been conducted as follows:(1)The characteristics of the AE signals from individual impact events are analyzed in both time and frequency domains.The wave propagation behaviour of the impact-induced AE signal along a wavguide is also studied.Through the experimental investigations into the influences of different parameters,including particle size,impact velocity,material type and impact location,on the detected AE signals,it is conducive to the understanding of the impact-induced AE signals.(2)The parametric analysis is applied to extract the parameters of the AE signals,including peak amplitude,count,rise time,duration,energy and root mean square(RMS).In this way,the influence of particle size and impact velocity on the AE parameters are quantitatively established and analyzed.The potential of different AE parameters for on-line particle sizing is also assessed,which indicates that the peak amplitude and the energy are the most appropriate AE parameters for particle size measurement.(3)An improved model based on the AE peak amplitude is proposed to achieve particle size inversion.In order to realize more accurate measurement of particle size,the impact between particles and a waveguide is described by the Stronge impact theory,which is a more realistic way to obtain the contact force during the collision process.The quantitative relationship between the AE peak amplitude and the particle size being measured is then established.Glass beads with a mean diameter of 0.4,0.6,0.8,1.0 and 1.2 mm,respectively,are used as the test particles to collide with the waveguide at an impact velocity ranging from 22 m/s to 37 m/s and then the resulting AE signal are collected.It is proven that,in comparison with the particle size inversion model deduced from the Hertz impact theory,the proposed model is advantageous on the particle size inversion in terms of measurement accuracy.For all the tested particles,the measured particle size is in a good agreement with the reference under different experimental conditions.The relative error between the measured and reference particle size is mostly within ± 10%.Besides,the influence of material type on the result of particle size inversion is also investigated.(4)According to the study on the energy dissipation during plastic impact,a theoretical model based on the AE energy is developed to determine the particle size from the AE signal.The proposed method is verified through experimental tests with glass beads on the single-particle test rig.Experimental results obtained indicate that the proposed model is feasible to infer particle size information from the energy of an impact event.The worst relative error of the measured particle size is-13%.(5)In order to realize the on-line size measurement of particles in pneumatic conveying pipelines,the peak amplitudes of the detected AE signals from particle collisions are extracted by a peak detection algorithm based on the local energy.The particle size informention is then deduced using the improved particle size inversion model.Silica sand in three size ranges of 10?246,61?395 and 116?750 ?m,respectively,are used as test particles.The experimental results indicate that the absolute error for most particle size segment is with ±5%,which demonstrates the capability of the proposed particle sizing system.