The release of alkali metals is investigated by means of ash element measurement and chemical equilibrium calculation during co-firing biomass (cornstalk derived fuel) and coal. The experiments are operated under different mass blending fraction and temperature conditions. With increasing cornstalk fraction, the normalized ash content decreases according to a quadratic curve because the formation of alkali aluminosilicate may result in more elements being retained in ash. Similarly, the release ratios of K, Na, and S do not change linearly with the variation of cornstalk fraction either, which are suppressed significantly by the interaction of biomass and coal ashes. For the effect of CaO additive, the release ratios of K and Na increase with its enhancement. The equilibrium analysis is used to predict and evaluate the release of alkali metals. The release of alkali metals enhances with the increase of temperature and cornstalk fraction. For silicon-lean blending fuels, adding Ca will decrease HCl(g) and release more KCl(g) and KOH(g) in both fuel-rich and air-rich conditions. More KCl(g) and NaCl(g) will be formed with increasing Ca/(S + 0.5Cl) ratio due to the formation of CaSiO3(s) and the reduction of alkali aluminosilicate. (C) 2016 Elsevier Ltd. All rights reserved.