Release of potassium (K) during biomass combustion, may cause significant operating problems in terms of ash deposition and high-temperature corrosion of superheater tubes. Other ash-forming elements, such as calcium (Ca), silicon (Si), and phosphorus (P), may to a certain degree control the K release. The aim of this work was to study the release of K front simple systems, to obtain information on the retaining effects of the elements Ca, Si, and P. Further objectives were to investigate the effects of temperature. the presence of water vapor, the speciation of K and Ca. and the sample size on the release rate of K, from the simple ternary systems K-Ca-Si and K-Ca-P. Well-defined mixtures of K, Ca, and Si (or P) species were heat-treated in a reactor. at constant temperature (900 or 1000 degrees C). in a gas flow of 4 nL/min N-2 containing 2%: (v/v) H2O. Average C release rates were calculated from weight measurements of the samples after every 15 min of the heat treatment (and subsequent cooling to room temperature). The presence of water in the gas low was found to significantly enhance the K-release rate, from both the K-Ca-Si system and the K-Ca-P system. For the K-Ca-si system, a significantly higher release rate was observed at 1000 degrees C compared to 900 degrees C. Furthermore. doubling the Ca/Si molar ratio K2CO3-CaO-SiO2 mixture strongly enhanced the K-release rate (by about 2 times) at 1000 degrees C. This suggests that SiO2 preferentially reacts with CaO, so that more K is being released to the gas phase instead of being incorporated into the silicate structure. For the K-Ca-P system. with K2CO3 as the K source, the Ca/P molar ratio had a strong effect oil the K-release rate: a decrease in the Ca/P molar ratio (or increase in the P content) significantly decreased the K-release rate from the K-Ca-P mixtures. As opposed to the K-Ca-Si system, it thus seems that K is preferentially incorporated in (nonvolatile) (K2O)(k)center dot(CaO)(l)center dot(P2O5)(m) Structures. The effects of temperature and Ca source oil the K-release rate front this system were limited but most pronounced for the mixtures with the highest Ca/P ratio (lowest P content). Furthermore, the sample size had a strong influence on the K-release rate. In the case of K-Ca-P mixtures containing KCl as the K source, the K-release rate was significantly higher at 1000 degrees C compared to 900 degrees C in the first 15 min of the heat treatment, whereas the Ca/P ratio had no effect oil the K-release rate. Selected samples of the K-Ca-Si and K-Ca-P mixtures. before and after the heat treatment, were Studied by scanning electron microscopy (SEM) in combination with energy-dispersive X-ray (EDX), to investigate the morphological and compositional changes. Moreover, selected samples of the K-Ca-P mixtures were heated from room temperature to 1400 degrees C, in a simultaneous thermal analyzer (STA), to investigate the melting and gas-phase release behavior. Both methods confirmed the effects of the Ca/Si and Ca/P ratios and the speciation of K and Ca on the release behavior observed in the heating experiments.