Alkali compound emission during biomass thermochemical conversion gives rise to a number of problems such as agglomeration, slag, fouling and metal corrosion in the conversion process system. Understanding the behaviour of alkali emission from biomass fuels is important to solve these problems. In this work, a study focused on the potassium compound release in biomass pyrolysis is carried out. The transformation of the element, K, is interpreted based on literature and the present study. A mathematic model of the potassium compounds emitting from biomass during pyrolysis is proposed. The different existing forms of potassium in biomass, the chemical equilibrium of the compounds at the pyrolytic environment and the potassium compound release due to vaporization at high temperature are taken into account in the model. The result of the mathematic model provides an understanding of the alkali compound release in biomass pyrolysis and the effects of the pyrolytic temperature, the fuel composition and residence time on the release process.