It is well known that dust resistivity and particle size play a major role in electrostatic precipitation processes. The electrostatic precipitator design also has to be adapted for carbon rich fly ashes such as those found in heavy fuel oil combustion plants. In this work, an ash sample from an electrostatic precipitator downstream an industrial biomass boiler operated under poor combustion conditions was air classified into 5 particle size fractions. Those fractions were then analyzed regarding chemical composition, loss on ignition, particle size distribution and dust resistivity. Furthermore scanning electron as well as optical microscope images were taken to investigate particle structure and shape. Diverse types of particles were found in the different fractions, varying from unburned coal particles to inorganic sand-like particles. It was also found that chemical composition as well as resistivity varied significantly with different size fractions. During resistivity measurements, fluctuating voltage and current values were recognized, similar to what can be seen during the back-corona phenomenon, but found at medium resistivity values. Furthermore, suggestions for an electrostatic precipitator design will be given based on findings in this study.