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The ash behaviour during combustion of a wheat straw sample was assessed using chemical fractionation and thermodynamic equilibrium calculations. Chemical fractionation has been used to distinguish the inorganics into reactive and non-reactive fractions. The reactive part of fuel comprises mainly of alkalis (K and Na), chlorine, sulphur and a part of alkaline earth metals (Ca, Mg), while the non-reactive fraction is dominated by silicon, aluminium and iron. The reactive fraction of inorganics is expected to reach equilibrium during combustion, while the non-reactive fraction simply passes through the combustion process unaffected. Literature on interaction of silica with sodium has indicated that, due to the high temperatures involved, a part of non-reactive fraction may react. The use of chemical fractionation alone does not take these reactions into account. In order to take these reactions into account a second …