Potassium in straw is relatively high and exists mainly as readily available potassium (K-r). The release and transformation of the K-r during biomass combustion mean a decrease or loss of potash fertiliser value. The purpose of this work is to quantify the transformation and the distribution of the K-r under conditions that can obviously reduce the loss of it, and then to try to offer some suggestions for new biomass projects. A pilot-scale circulating fluidized bed (CFB) combustor (0.5 MWth) has been applied to combust rice straw. The combustion temperature was kept at a low level (lower than 700 degrees C) to reduce the transformation of potassium. The K-r and the total potassium (K-t) in the rice straw, the bottom ash, and the flyash were determined after chemical fractionation and wet acid digestion, respectively. The mass balance of the biomass ash, the K-r and the K-t in stable operation were quantified. The experimental results showed that the flyash accounted for more than 95% of the total straw ash. The flyash contained around 92% of the K-t and 86% of the K-r of the straw. In the flyash about 3% of the K-r of straw was transformed to unavailable potassium (K-u) during combustion. On the basis of the obtained results, four suggestions that reduce the loss of the K-r are given for the better development of biomass-fired power plants.