Asymmetric Remediation of Soot Particles in a Bipolar Electrostatic Precipitator using Transient Plasma Discharge

Recently, a nanosecond pulse transient plasma has been used to enhance the performance of electrostatic precipitation as a strategy to overcome the size limitation of traditional ESPs. Here, the transient plasma supplements the conventional DC corona-based remediation by increasing the ion density inside the ESP, thereby increasing particle collection efficiency for a given size of ESP. The work reported here investigates the amount of charge per particle that is imparted by the transient plasma on the nanoparticles, the fraction of particles with non-zero charge (i.e., charged particle fraction, f_c), and particle size distributions in a transient plasma-enhanced electrostatic precipitator (PE-ESP) using a tandem differential mobility analyzer (TDMA) and a scanning mobility particle sizer (SMPS). The transient plasma produces a bipolar charge distribution, in contrast to a DC corona, which is unipolar. In the conventional DC-only ESP (coaxial configuration), all particles are swept to the outer wall, regardless of the polarity of the applied DC voltage. For the PE-ESP, the highest remediation is achieved for particles swept towards the center wire electrode, representing highly anisotropic remediation (i.e., positive vs. negatively charged particles). In fact, a 40-fold asymmetry was observed in the remediation of negatively charged particles compared to positively charged particles remediated under a DC bias of V_DC = +1kV. This asymmetry arises due to the wire-cylinder geometry of the ESP, which produces an electric field near the center electrode that is 40X stronger than the electric field near the outer wall due to local field enhancement produced by the sharp curvature of the center wire. In addition, the DC bias provides an 80% increase in the charge particle fraction by sweeping out the high mobility free electrons, leaving a net positive plasma remaining for enhanced ESP remediation, ultimately providing a unipolar environment from the bipolar transient plasma, which results in enhanced particle charging.