Carbon monoxide formation during electrostatic precipitation in small-scale biomass combustion systems

• High carbon content (38.2 %) found in fly ash from small-scale boilers. • Ozone from corona discharge oxidizes carbon deposits on ESP electrodes. • ESP operation generated CO up to 30 ppm during biomass combustion. • Results guide ESP optimisation for carbon-rich particle treatment. This study investigates the unexpected formation of carbon monoxide (CO) during in electrostatic precipitator (ESP) operation in small-scale biomass combustion systems. Using a 15-kW pellet-fired boiler coupled with an ESP (total collecting area: 3 m²), the CO generation was observed up to 30 ppm directly correlated with corona discharge intensity. With maximal energisation, The ESP significantly reduced particle concentrations from an initial 45 mg/Nm³ (equivalent to 1.5 × 10⁷ #/cm³) to 12×10³ #/cm³; NOx concentrations, initially at 178 mg/Nm³, were reduced by 77 %. Chemical analysis of the collected fly ash demonstrated a significant carbon content (38.2 ± 0.5 %), attributed to incomplete combustion characteristics of small-scale boilers. Through systematic mechanistic investigation, the study evidences that ozone molecules, produced in corona discharge, interact with carbon-rich surface, triggering a heterogeneous oxidation process that generates CO. The findings provide critical insights into secondary emissions with significant implications for ESP maintenance and design in applications processing carbon-rich particulate matter. The research is particularly significant for small-scale combustion units, where such secondary emissions have not been previously investigated, and suggests potential strategies for mitigating unintended CO formation.