To investigate potassium behaviors during biomass gasification, the potassium migration of wheat straw (WS) or corn stalk (CS) ashes and their modification mechanisms by additives (kaolinite and dolomite) under reducing atmosphere were explored by an X-ray fluorescence spectrometers, X-ray powder diffractometer (XRD), and FactSage software. With increasing temperature, most of potassium in WS or CS ashes transferred into gaseous phase potassium chloride. The kaolinite addition retarded the release of potassium due to dystectic compound kalsilite formation. Kaolinite was suitable for straw circulating fluidized-bed gasification because the amount of potassium captured by kaolinite increased firstly and then decreased with increasing temperature. At high temperature, K+ in silicates gradually replaced by Ca2+ or Mg2+ with dolomite addition, this accelerated potassium release (promote alkali-induced slagging) and increased the ash fusion temperatures of mixtures (decrease silicate melt-induced slagging). Dolomite was effective for low chlorine straw gasification. The combination method of XRD and FactSage software could explain the modification mechanisms of potassium migration behaviors through two additives under reducing atmosphere. (c) 2019 Elsevier Ltd. All rights reserved.