A comprehensive system model of fluidized bed combustor, previously developed and tested for the prediction of combustion behavior of fluidized bed combustors fired with lignite was extended for the modeling of cofiring of lignite with biomass by incorporating volatile release, char combustion, and population balance for biomass. The model predictions were validated against measurements taken on a Middle East Technical University 0.3 MWt Atmospheric Bubbling Fluidized Bed Combustor fired with lignite only, lignite with limestone addition, and about 50/50 lignite-olive residue mixture with limestone addition. Predicted and measured temperatures and concentrations of gaseous species along the combustor were found to be in good agreement. Introduction of biomass to lignite was found to decrease SO2 emissions but did not affect NO emissions significantly. The system model proposed in this study proves to be a useful tool in qualitatively and quantitatively simulating the processes taking place in a bubbling fluidized bed combustor burning lignite with biomass.