The combustion of biomass for energy generation is practiced in an increasing scale in Indonesia as the country heads towards the long-term national energy mix targeted by 2025. However, biomass combustion is prone to operational problems caused by the generally low-melting nature of biomass ashes. This work discusses the effects of co-combusting coal with POEFB (palm oil empty fruit bunch) and bamboo with respect to the thermomechanical behavior of the produced ashes. Coal is observed to increase the ash fusion temperatures (AFT) of neat and combined POEFB and bamboo ashes by as much as 300 °C. Aluminosilicate minerals in the coal combine with potassium in the biomass during co-combustion, producing high-melting K-aluminosilicates. A linear correlation is identified between measured AFT and ash liquidus temperatures estimated by FactSage thermochemistry calculation software, enabling the prediction of AFT of coal-biomass co-combustion systems.