Experimental Study On The Mechanism Of Bipolar Charging Agglomeration Of Fine Particles In Turbulent Flow

Removal of fine particles smaller than 2.5 μm from industrial flue gases is one of the most important problems in air pollution control. These particles constitute a major health hazard because of their capability to act as a vehicle for noxious agents and go a long way in the air. For promoting the collect efficiency of the fine particles, the technique of bipolar charging agglomeration was presented in this paper. Charged in positive and negative charging sections, particles with different charges agglomerate to enough sizes to precipitate in agglomeration section. This paper firstly elucidates the feasibility of the bipolar charging agglomeration. The effects of the gas flow, dust concentration, and the positive and negative discharge intensities on the agglomeration efficiency were studied. Talcum powder and boiler fly ash were used as the test powder. The results show that the agglomeration efficiency is raised by 10% through the bipolar charging. Bipolar charging was also found to be more effective than unipolar charging by studying the applied voltage and the square root of the corona current characterization because the charge on the particles is proportional to the square root of the corona current. So the bipolar charged particles are easier to agglomerate than the unipolar ones.Based on the technique of the image segmentation, the agglomerates were studied. A method based on a combination of image segmentation technology and statistics was developed to analyze the particle agglomeration in different sections of the experimental system, the agglomeration efficiency, and the strength of the agglomerates. The coverage ratio of fine particles on large particles was used to evaluate and compute these factors. The results show that the mean value of the coverage ratio represents the efficiency of particle agglomeration while the standard deviation of the coverage ratio represents the strength of the agglomerates. A formula was developed to relate the coverage ratio and the mass collection efficiency. The agglomeration mainly takes place in the agglomeration section. Bipolar charging agglomeration of particles was observed and measured in a model electrostatic precipitator (ESP) with simple geometry consisting of an agglomeration section with blades. A new particle supply system was designed based on the principle of fluid bed. The influence of the blades in the agglomeration section on the particle agglomeration inside the ESP and on the turbulence condition of the flow was elucidated through experimental and numerical analysis. The profiles of streamwise particle velocities and sizes were measured in the agglomeration section with a phase Doppler anemometer (PDA). As the positive and negative corona voltages were maintained and the particles were injected into the ESP continuously and uniformly, the velocities of the particles flowing inside the agglomeration section decreased due to the corona wind and the deviation from the flow by the particle agglomeration. When the particles were bipolar charged for agglomeration, the mean diameter of the particles increased 9.86% compared to the diameter of without charging. Based on the spectrum estimation of the fluctuating velocities, the micro scale and the dissipation rate about the gas flow with and without charging were also studied. It was found that the change of the spectrum can reflect the kinetic energy in each frequency and the agglomeration among the particles affects the parameters of the gas flow.