The study was conducted to obtain the scale-up factor and the optimum design criteria for the development of a commercial scale electrostatic separator using a continuous, bench-scale electroseparator composed of two vertical electrode plates and an ejector-tribocharger. Tests of the charge density and separation efficiency to study the removal possibility of unburned carbon from coal fly ash were evaluated under various operating conditions. It was found that the experimental conditions for obtaining the maximum charge density were an air flow rate of 1.75 m(3)/min and a feed rate of less than 50 kg/hr when operated at lower than 30% relative humidity using a SUS304 ejector tribocharger. Also, the optimum instrument conditions for recovering the clean ash with less than LOI 3% at yield over 65% when operated at above experimental conditions were found to be a diffuser slit gap of 4 mm, and a distance between diffuser slit and splitter of 15 cm. Overall, the feed rate per unit electrode surface area was about 0.074 kg/cm(2) · hr, which can be used as a scale-up factor of the electroseparator.