Biomass co-firing into the existing pulverized coal boilers has economic and environmental advantages, such as reduced slagging inside the furnace and lower GHG emissions. Compared to other pre-treatments, hydrothermal treatment has high conversion efficiency, capability to eliminate the energy-intensive drying process, and relatively low operating temperatures. In this study, a simulation study using computational fluid dynamics (CFD) has been performed to explore the potential of coal co-firing with hydrothermally treated empty fruit bunch (HT-EFB) into an existing coal power plant. In the co-firing process, it is important to predict the combustion performance for all stages of the combustion. Furthermore, an empirical investigation is performed to clarify the result of simulations. In the findings, an HT-EFB mass fraction in the range of 10–25 % seems to be the most preferable co-firing condition.