The mass balance of the ashes and their potassium component was investigated when different biomass fuels were incinerated using a fluidized bed furnace and stoker furnace, respectively. The properties of the ashes were analyzed, and the dependence of the potassium content in the ash upon its particle size was also examined. Very little K is exhausted in the flue gas, and nearly all is included in the ashes. The stoker furnace is thought to be more appropriate than the fluidized bed furnace to obtain bag filter ash, which has the highest potassium concentration. Moreover, the potassium content in the ash decreased with an increase in its mass median diameter. To enrich the potassium concentration in the ash, a method was proposed for collecting the finer ash selectively using a particle-size classification technique. Using it, ash having >35% of K concentration could be successfully obtained at factory scale. It was also confirmed that the enriched potassium bag filter ash, and the chemical fertilizer manufactured from it, completely satisfy the official specifications fixed by the Fertilizers-Regulation Act in Japan. (C) 2016 Elsevier B.V. All rights reserved.