Characterization Of Pulverized Fuel Flow In A Square-Shaped Pipeline Based On An Electrostatic Sensor Array

Nowadays,pneumatically conveying pipelines are widely used in many industries.Successful on-line mass flow rate measurement of pneumatically conveyed particles can improve combustion efficiency and reduce atmospheric pollution for the power generation industry.Sensors based on a variety of sensing principles have been proposed for the measurement of mass flow rate of particulate solids,such as capacitance,optics,modulation/attenuation of ultrasonic waves or radiation and so on.However,the complicated characteristics of pneumatically conveyed particles in pneumatic pipes cause great difficulties for these sensors in gas-solid two-phase flow parameters measurement.In comparison with methods mentioned above,electrostatic sensors have the advantages of structural simplicity,robustness and low cost,thus they are extensively applied in gas-solid two-phase flow measurement.Since the solid particles in pneumatic pipelines carry a certain amount of electrostatic charge due to interaction among particles,pipe wall and air,the charge carried by the particles can be detected by electrostatic sensors.The signals of the electrostatic sensor derives from the fluctuations in electric filed caused by the passage of the flowing particles,thus the electrostatic signal is supposed to be suitable in representing mass flow rate of the particles.Nowadays,many theoretical progresses have been made in analyzing the properties of electrostatic sensors with ring-shaped electrodes as well as the applications of electrostatic sensor to gas-solid two-phase flow parameter measurement in circular pneumatic conveyance.However,very little research has been undertaken on electrostatic sensors for square-shaped pipe section.In order to optimize production processes for industries using square-shaped pipes such as circulating fluidized beds and thermal power plants in certain countries(e.g.Germany),it is important to make in-depth investigations into the characterization of pulverized fuel flow in square-shaped pipelines.In this paper,an induced charge method and a conducted charge method for mass flow rate measurement of pneumatically conveyed particles in square-shaped pipelines base on an electrostatic sensor are introduced.The main achievement and contributions of the research are as follows.First,the electrification mechanism of solid particles in the pneumatic conveying pipe and the measuring principle of the electrostatic sensor are introduced in this paper.And also,the possibility of using charge amplifier in the signal conditioning circuit to optimize the electrostatic methods is analyzed.Besides,the principle of trans-resistance amplifiers and charge amplifiers are introduced.The comparison and analyze of these two devices is completed.In order to compare the measuring results of electrode with different widths,by using SolidWorks software,the structure and the component material of the sensor array are defined in detail.Each of the electrode in the array are exposed to the fluid in the pipe.Besides,the axial width of the electrodes are determined according to the inner diameter of the pipeline.A square-shaped pipeline test rig which can continuously adjust air speed and mass flow rate of the particles is designed and built.The test rig is consisted of a square-shaped pipeline,a screw feeder,a voltage regulator and a vacuum cleaner.Relying on these parts,mass flow rate measurement of gas-solid two-phase flow under different working condition can be realized.During the experiment process,flour is selected as experimental materials.The induced charge method and the conducted charge method are used in a 54 mm*54 mm square-shaped pneumatically conveying pipeline.The difference between induced charge method and conducted charge method are in the signal processing stage.Induced charge method uses a trans-resistance amplifier to condition the electrostatic signal,and the output signal is applied to represent volumetric concentration.The conducted charge method uses a charge amplifier to condition the electrostatic signal,and the output signal is applied to represent mass flow rate.As theoretical analysis and experiment study show,the transferred charge accumulates with time on the electrodes,thus,slope of the electrostatic signal is regarded as suitable variable to characterize the mass flow rate of the particles.As the experimental result shows,the slope of electrostatic signal versus time curve linearly increases with the mass flow rate;as the electrode width increases,Increase in the particle velocity can give a slightly rise to the transferred charge,and the cumulative charge increases in the same growth pattern of the electrode width.