A pneumatic triboelectrostatic beneficiation system of fly ash was being developed and has tested using a continuous, bench-scale electroseparator composed of two vertical 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 relative humidity of the surrounding air was the most influencing factor of the charge density as well as the separation efficiency. The optimum instrument conditions for recovering clean ash with less than 3% at yield over 75% when operated at relative humidity of less than 30% were found to be the diffuser slit gap of 4mm, the diffuser outlet velocity of 16.5~20m/s, and the distance of between diffuser slit and splitter of 15cm. In overall, solid loading per unit electrode surface area was about 0.11 kg/cm(2)ㆍhr, which can be used as a scale-up factor of electroseparator.