Ion density-enhanced electrostatic precipitation using high voltage nanosecond pulses

Abstract

This study evaluates the beneficial effects of discharging nanosecond pulse transient plasma (NPTP) in a coaxial electrostatic precipitator for capturing nanoscale soot particles (∼50 nm) produced by an ethylene flame. Here, the nanoscale soot particles are collected using two different reactor geometries: a 3′′ diameter reactor with a mean flow velocity of 1.2 m s⁻¹ and a 1.5′′ diameter reactor with a mean flow velocity 1.5 m s⁻¹, corresponding to volumetric flow rates of 11.5 CFM and 3.6 CFM, respectively. The nanosecond high voltage pulses (+20 kV, 20 ns, 800 Hz) are applied in conjunction with DC bias voltages. While nearly 100% collection efficiency can be achieved without NPTP at sufficiently high DC voltages (|V_DC| > 14 kV), this drops below 50% for lower DC voltages (|V_DC| < 10 kV). With NPTP, we observe substantially enhanced remediation (up to 23×) at lower DC voltages (|V_DC| < 10 kV) due to the enhanced ion density produced by the plasma. For DC-only electrostatic precipitation, the charging of soot particles takes place via a DC corona, whose ion density is several orders of magnitude lower than that of the NPTP, which produces a streamer discharge due to the fast rise times of the nanosecond pulses (i.e., dV/dt ∼ 10¹² V s⁻¹). High speed imaging of the plasma emission profile indicates that ion densities 10⁶ times higher are achieved with the nanosecond pulsed plasma, as compared to that of the DC corona. At lower DC voltages (i.e., |V_DC| < 10 kV), the charging of soot particles is a key factor limiting the DC-only remediation efficiencies, and NPTP provides a way to mitigate this limitation.