Fine Particle Collection With Electrostatic Precipitation

Fine particles can stay long in atmosphere with large number, and always adsorb lots of harmful compounds. Therefore fine particle emission from the coal fired power plants has become a big problem for the harm to the human health. Coal is the primary energy source in China and electrostatic precipitation (ESP) has become the primary apparatus for particle collection. However, about 90% of the ESPs can not effectively ccapture the fine particles with the mass emission above 50mg/Nm3. For this reason, we focus our studies on the behavior of fine particle collection with ESP, and new technique to improve fine particle collection for industrial power plant.This paper firstly presents the characteristics of the dust layer collected with a laboratory electrostatic precipitator. Experiments are performed with different electrode distances varies from 22 mm to 330 mm, and also with a single discharge electrode. Discharge last for 40 to 50 seconds under 46kV voltage,150 g/m3 inlet dust concentration and 700 m3/h flow rate. Ash layer thickness collected on the plate is about 4 mm in average. We observed that typical shape of the collected dust is rectangle and the dust in the center is much thicker. The density at the center, however, almost keeps constant.In addition, a laboratory ESP together with a bipolar pre-charger has been designed for studying charge-induced agglomeration and fine particle collection. In terms of particle numbers, the ESP collection efficiency drops to its minimum of near 90% for particles with diameters of near 0.2μm and 3μm. For other particles, its value is around 94%-95%. By using the bipolar pre-charger, the grade efficiency can be significantly increased for all particle sizes due to the charge-induced particle agglomeration. The grade collection efficiency rises to about 95%-98%for all size particles.Industrial investigations on fine particle grade collection efficiency of an ESP are performed with a hybrid ESP and fabric filter (FF) on a 30MW coal-fired boiler. Gas flow rates, mass inlet concentration and gaseous temperature are 20000-40000Nm3/h,15g/Nm3 and 110℃, respectively. The ESP specific collection area (SCA) ranges from 10 to 20m2/m3/s. Both single-phase and three-phase transformer-rectifiers (TRs) are used for energizing the ESP. When changing the single-phase TR to the three-phase TR, the maximum average secondary voltage is increased from 55kV to 71kV and average corona current rises from 31mA to 62mA without spark breakdown. As a result, both particle grade collection efficiencyη(r) and migration velocities are significantly improved. With the single-phase TR, the velocity is around 17cm/s for all particles. With three-phase TR, its maximum value is about 35cm/s. For particles within 0.03-0.1μm and 0.1-2.5μm, the efficiencies rise from about 85% to 95% and 92%, respectively. For particles of around 2.5-8.0μm, they rise from about 87% to 98%. Moreover, experiments show that a revised Deutsch equation log(?) gives a good approximation via the average electric field Ea, the specific collection area S and two correction coefficientsαandβ.