Combustible waste is very heterogeneous and the variation in chemical composition is of great significance for the performance of the combustors in terms of boiler availability and power efficiency. For example, the content of alkali, Chlorine and sulfur affect agglomeration, fouling and corrosion mechanisms, which often limits the steam data and requires counteracts such as soot blowing and outages. An increased knowledge on favorable levels and ratios of fuel components are therefore highly important when developing waste combustors, both existing and future. However, to be able to make good predictions of reactions, reliable fuel analyses are a necessity and they are difficult to perform because of the heterogeneity of waste. As a consequence, it is also difficult to complete pro-active measure to reduce unwanted reactions. This work has investigated the composition of the fuel during one year in a 40 MW commercial BFB waste plant. Twelve samples have been performed in order to estimate the variation of key components. The fuel samples were analyzed chemically for alkali and several other components of interest, such as Cl and S. Moreover, thermo-chemical multi-phase equilibrium calculations were performed for prediction of the chemical composition of the furnace in the temperature range 400-1000 ºC. In this temperature range NaCl and PbClx are formed as well as gas phase HCl and solid silicates. Furthermore, the calculations show that the chemistry is very sensitive to the input chemical composition, suggesting that already a minor shift in fuel mineral matter may change the behavior of the fuel radically in terms of its fouling and corrosion tendency.