In-line Measurement Of Particle Size And Mass Flow Rate Of Pulverized Coal Using Electrostatic Sensors

Inline measurement of size and mass flow rate of pulverized coal provides useful data for the efficiency improvement of pulverizing system,optimized fuel combustion,and emission reduction in coal-fired power plants.Electrostatic sensors have been widely used for the measurement of velocity and concentration of pneumatically conveyed particles due to its advantages of reliability,convenient installation,high sensitivity,low cost for maintenance and intrinsic safety.The feasibility of the particle size measurement of pulverized coal using electrostatic sensors was evaluated both theoretically and experimentally,firstly,in this study.Finite element simulation software COMSOL was used to analyze the effects of particle size on magnitude and main frequency of electrostatic signals.Measuring methods based on transit time and velocity variation are,then,presented for measuring the size of a single particle and the variation of averaged size of particles,respectively.Two types of electrostatic electrodes with axial widths of 2 mm and 10 mm are adopted to measure the electrostatic signals of nonporous glass beads and porous activated carbon powder on a 74 mm bore vertical pipeline of a gas–solid two-phase flow test rig.The effects of moisture content on mass flow rate measurement was also studied through experiments and a calibration method for the improvement of measurement accuracy is presented.The finite element simulations results show that the magnitude of electrostatic signals is an indication of particle size for particles with the same charge density.The radial distance between a particle and pipe wall plays a more important part than particle size in the frequency characteristics of measured electrostatic signals.Because of the velocity of particles decreases with particle size,the variation of velocities can be used for estimating particle size.The particle size affects both magnitude and frequency characteristics of electrostatic signals according to experiments.The deviation of mass flow measurement that caused by the variation of moisture content is analysed,and a recalibration process is demonstrated to be effective for the improvement of measurement accuracy.