Differences in level of demineralization for the used fuels are the result of the nature of inorganic constituents (bound to organic matter or as minerals like carbonates, sulfates, silicates, and so on) and their variable resistance against acid treatment. Compared to brown coals (>91.6%) the biomass samples reach a significantly lower level of demineralization (WS: 34.1%; MS: 47.7%; SWC: 84.3%) because no treatment with HF was done to remove the silicon and aluminum containing species. The relatively high value for wood is related to the fact that calcium carbonate is the dominant inorganic species, while less silicon is present. However, the elements which are supposed to have a catalytic activity, like alkaline, alkaline earth or transition metals, are removed to a satisfying amount.