Woody biomass is a growing renewable energy source, and the emitted combustion ash may serve as valuable materials. In this study, the relationship between the potassium concentration in combustion ash and the particle size was investigated for several types of biomass ashes with different morphologies. The form of the potassium components in the ash was also analyzed to examine this relationship. From the results, a method to estimate the degree of potassium enrichment was proposed. The potassium concentration first decreases rapidly with an increasing mass median diameter and then continues to decrease gradually, although this relationship depends upon the properties of the combustion ash. It was also found that almost all of the potassium component in the ash existed as relatively small particles composed of crystallized hydro-soluble potassium compounds, such as KCl, K2SO4, etc., although the particle size and crystalline phases varied according to the type of power plants. The classification process was implemented in an actual biomass power plant and demonstrated to produce combustion ash with enriched potassium. The enrichment factor calculated from the size distributions of the potassium component particles and other components qualitatively agreed with the enrichment factor obtained experimentally for all types of ashes. Ash with a potassium concentration above 300 mg/g could be successfully acquired on an industrial scale by collecting the finer ash selectively, using size classification equipment constructed at the biomass power plant.