The conversion of biomass is significantly influenced by the presence and chemical behaviour of catalytically active minerals within the solid fuel. This chapter discusses the role of impregnated minerals during biomass conversion using extensive characterisation techniques. Iron oxides, alkali and alkaline earth metal sulphates undergo complex transformations during heating that significantly affect their catalytic activity. Kinetic analysis showed reduced activation energies for doped fuels in gasification and oxidation following an overall reactivity sequence of K > Na > Fe > Ca > Mg. Fast-heating pyrolysis experiments revealed mineral effects on the formation of all three pyrolysis products influencing tar distribution, light gas formation, and char evolution. Additionally, combustion studies in a flat flame burner demonstrated earlier ignition and varied impacts on the combustion temperature depending on the type of doping. Overall, the results demonstrated a significant influence of minerals on biomass conversion and highlight the importance of mineral characterisation in optimising biomass-based energy production.