Electrode geometry effects on the collection efficiency of submicron and ultrafine dust particles in wire-plate electrostatic precipitators

Recent interest in emission control of fine particulate matter has resulted from scientific studies on the effect of fine particulate matter on human health. Hence, many western countries introduced a new emission regulation known as PM2.5, that regulates particles less than 2.5 microns in diameter. The existing particle separation devices such as electrostatic precipitators (ESPs) are of particular interest since they can economically capture particles effectively with a low pressure drop. The present ESPs provide high collection efficiencies of around 99.99% for micron and larger particles. However, the collection efficiency of submicron particles in the range from 0.1 to 1 mum and ultrafine particles, that is with particle diameters less than 0.1 mum, can be less than 50%. In this work, numerical and experimental studies were conducted to examine the effect of electrode geometries on the improvement of collection efficiency of submicron and ultrafine dust particles in electrostatic precipitators. The collection efficiency prediction was based on a modified Deutsche's equation after calculation of the three-dimensional electric potential and ion distribution. The particle charging models for diffusion and field charging methods were considered, based on the Knudsen number (Kn = 2lambdai/dp), where lambdai is the mean free path of negative ions and dp is the dust particle diameter. The constitutive relationship developed from the optical emission experiments was implemented to simulate ion distribution of corona discharge for various discharge electrodes. Experimental validations for total and partial collection efficiencies for particle size from 10-2 to 20 mm were conducted for bench and full scale ESPs.; Results show that the collection efficiency of submicron and ultrafine particles can be predicted with good accuracy for various geometries of discharge and dust collection electrodes. The spike-type discharge electrode with the I-type collecting electrode improves collection efficiency of fine particles when compared to the wire or rod discharge electrode with I-type collecting electrode. In the case of U and C-type collecting electrodes, there is an optimum fin length for which the highest collection efficiency can be reached. Comparison of experimental and predicted results shows that the total collection efficiency predicted by the present model agrees well with experimental results for the bench-scale ESPs. For the large-scale wire-plate type ESP, the present simulation results conducted for various gas temperatures and dust resistivities agree quantitatively and qualitatively with the experimental results.; The model proved to be useful for prototype design of collecting and discharge electrodes, modification and existing ESP's and scale-up of new ESP's in order to meet new emission regulations.