The aim of the present project was the development of an impulsive micro-electrostatic precipitation (m-ESP) technology which combines DC high voltage (HV) and sub-microsecond high voltage impulses for energisation of the precipitator electrodes.
The main objectives of this project were: to investigate the behaviour of sub-microsecond impulse corona discharges in air, including the hydrodynamics of the corona wind in the impulsive micro-electrostatic precipitation electrode systems; to investigate the dynamics of fine particles under the influence of DC bias and impulse electric fields; to develop impulsive micro-electrostatic precipitation systems for effective capture of PM2.5 particulate matter (airborne particles smaller than 2500nm); to obtain the parameters to develop a phenomenological model which will allow the evaluation and optimisation of miniature precipitation systems.
Several precipitation reactors have been developed including reactors with a cylindrical topology and a multi-electrode reactor. An energisation system comprising of the Blumlein generator which produces HV impulses of 250ns duration and DC HV charging circuit has been constructed. The precipitation efficiency has been measured for coarse particles (half of particles are larger than 45000nm by weight) and for fine particles (average particle size is 5500nm). Three different energisation modes (positive HV DC voltage, positive HV impulses and their combination) have been used in the precipitation tests. Size monitoring of the fine airborne particles has been conducted using Grimm 1.109 aerosol spectrometer. The precipitation efficiency of coarse particles was obtained by measuring the ratio of the mass of precipitated particles and the total mass of particles delivered into the reactor. The precipitation efficiency of fine particles was obtained by measuring the ratio of the number of particles in the energized mode of the precipitator to the number of particles with no high voltage. These measurements were performed using the aerosol spectrometer in 19 size ranges, from 250 nm to 5000nm.
An analytical analysis of the electric field distribution in the precipitation reactor has been conducted and parameters of corona discharges in this coaxial system have been obtained. These characteristics are important for optimisation of the energisation parameters of the m-ESP systems.
As a result of this project 1 paper has been published in a peer reviewed journal and 3 posters have been presented at national and international conferences.
The following key results have been obtained.
The electric field distribution have been obtained using electrostatic field simulation package "ELECTRO". An analytical solution to the space-charge saturated current in the cylindrical topology have been obtained using the Poisson and the continuity equations. The DC current-voltage characteristics of the corona discharges have been measured and the developed analytical model has been used to obtain space-charge influenced electric fields. A critical corona ignition field has been calculated using Peek's phenomenological approach and the depth of the ionisation zone in the reactor has been obtained. The behaviour of sub-microsecond impulsive corona discharges in air has been investigated. The impulse wave-forms have been measured using a high-speed HV probe and current transformers. The magnitude and time delay of current impulses, charge delivered into the reactor, and resistance of the corona discharges have been obtained for voltages up to 32kV. Hydrodynamic parameters of the corona discharges in air have been studied and it has been shown that these discharges are capable of generation of the air flow with velocity up to 3.5m/s which can be used to control the gas flow in the precipitation systems.
The precipitation experiments were conducted using single and double-stage cylindrical reactors. The precipitation efficiency has been obtained for positive HV DC voltage, positive HV impulses and combination of DC and HV impulses. It has been found that 16kV DC energisation results in 77% efficiency for a coarse powder; combination of DC voltage and 26-30kV impulses increases the precipitation efficiency up to 85%. In the case of fine powder the combined 15kV DC and 26kV impulses energisation mode significantly improves the precipitation efficiency, up to 81-99.6% in size ranges from 250nm to 5000nm. A reduction in the precipitation efficiency has been observed for 400-650nm particles, which can be attributed to a lower efficiency of their charging. The analytical Cochet approach which takes into account field and diffusion charging mechanisms has been used for calculation of the particle migration velocities. It has been shown that particles in this size range have minimum velocities. Experimental migration velocities obtained in the present study are in the range from 2.1 to 8.4m/s and they depend on the size of particles. These parameters are used in modelling and optimization of the performance of the m-ESP systems.
As a result of this project, it has been established that the combination of DC voltage and short impulses improves the precipitation efficiency of fine (less than 2500nm) and coarse airborne particles; efficiencies of ~100% have been achieved for particles larger than 2500nm.