This study examines the effect of removing unburned carbon from fly ash depending on the treatment time associated with the cold plasma conditions and the characteristics of the fly ash powder during the process. The optimum conditions for the cold plasma process at the substrate temperature of 150 included the gas type of O2, 200 W power, gas flow of 50 sccm, and the one-time treatment time of 10 minutes. Moreover, under these conditions, the loss on ignition of fly ash decreased from 5.8% to 1.0%. The loss on ignition decreased to 0.1% when the process was implemented eight times (10 minutes × 8 cycles) under optimum conditions. As unburned carbon was removed from the fly ash during the cold plasma process, the BET specific surface area decreased from 5,809 m2/kg to 1,597 m2/kg, whereas the mean particle size decreased from 15.5 μm to 11.7 μm and the particle-size distribution narrowed. The unburned carbon particles became smaller in size as their micropores eliminated in the plasma process, but there were no changes in size or shape to the ash particles.

Keywords: Unburned carbon, Fly ash, Cold plasma, Loss on ignition