Coagulation contributing to electrostatic precipitation of ultrafine fly ash from small-scale biomass combustions

Abstract

An electrostatic precipitator (ESP) is an efficient device for removing fly ash from small-scale solid-fuel combustion. Under corona discharge, a gaseous medium becomes ionised, and particles become charged by colliding with ions. An electric field forces the movement of charged particles to remove gases and induces the movement of generated ions, causing secondary electrohydrodynamic flow. Thus, particle charge, the presence of neighbouring particles, an ESP electric field, and gas flow turbulence can enhance particle coagulation. Therefore, the ultrafine particle concentration in an ESP is expected to evolve under electrostatic removal and enhanced coagulation. In the present study, the contribution of coagulation is explored and confirmed using experimental measurements performed on an ESP used to control emissions from a 160-kW boiler with biomass combustion. The ESP was operated under several modes to obtain different coagulation conditions. Changes in particle concentrations were measured simultaneously using two techniques. It was found that coagulation could be responsible for up to 5% of the total removal efficiency, depending on the ESP operation parameters. The present study can promote a deeper understanding of the processes involved in electrostatic precipitation, and the obtained results can be useful for practical ESP engineering.