As coal-fired boilers in The Netherlands are considered to be converted to high biomass co-firing or pure biomass boilers, issues with fouling by alkali salts are expected. The behaviour of gaseous alkalis is different in converted coal boilers than in dedicated biomass boilers due to the higher firing temperature. In coal boilers, a glass phase can form that allows for alkali dissolution, preventing formation of alkali aerosols. This was investigated by both SEM on fly ashes from full-scale co-firing tests as well as thermodynamic modelling. Alkali dissolution decreases with increasing (CaO + MgO) to SiO2 ratio in the glass which, in its turn, increases with biomass co-firing ratio. Addition of 1-10 g/MJ coal fly ash can mitigate fouling with firing only wood, MBM-mix or sheanut-mix. Formation of molten alkali salts depends on both the S to Cl ratio and the K to Na ratio: in equilibrium, a molten salt forms with S:Cl <= 1.2 if K:Na > 10, with S:Cl <= 2.1 if 2.4 < K:Na < 4.7 and with S:Cl <= 3.5 if K:Na = 1. The S:Cl ratio decreases with increasing biomass co-firing ratio. Addition of 1 g/MJ ammonium sulphate can fully mitigate fouling with firing sheanut-mix that contains relatively little Na, and can lower fouling with firing wood up to a flue gas temperature of 700 degrees C; however, it will increase fouling with wood above 700 degrees C. Fouling with firing MBM-mix does not change with adding ammonium sulphate since it is already a sulphur-rich fuel mix.