Abstract: The present study presents a three-dimensional numerical model for co-firing lignite and biomass in large-scale utility boilers. The model takes into account the non-spherical form of the biomass particle, which influences the drag coefficient and its devolatilization and combustion mechanisms. Simulations under different co-firing rations and biomass particle sizes are performed for a 300 MWe pulverized-fuel, tangentially fired boiler located in Northern Greece, operating with low quality lignite. Validation of the simulations is performed using plant data for the reference case of lignite combustion. The results obtained, which include temperature and species fields, particle tracks and burnout per burner level and NOx calculations, provide useful conclusions regarding the maximum allowable biomass particle size and substitution ratio for an efficient boiler operation. In particular, the combustion behavior of particles from different burner levels suggests the optimum positions for dedicated biomass burners.