Biomass co-firing with lignite represents an attractive solution for operating lignite-fired thermal power plants (TPP) with the dual advantage of using local renewable resources and simultaneously reducing emissions. The subject of this study is technical and environmental investigation of the feasibility of the co-firing of Soma lignite with some dried agricultural residues in the Soma thermal power plant from the exergy analysis perspective, using THERMOFLEX simulation software. The use of biomass cofiring with poor quality coal could allow Turkey to comply with Kyoto commitments while benefiting from using indigenous fuel resources and reducing biomass waste disposal problems. Two technologies are considered; (1) direct cofiring, in which biomass is mixed with lignite in the same mill and fed into the boiler furnace and (2) parallel cofiring method, in which biomass is fired in a separate circulating fluidized bed boiler and produced steam is supplemented into the steam network of the power plant. The investigations reveal that both direct and parallel co-firing of the biomass could result in a significant decrease in fuel consumption, emissions and exergy destruction and a slight increase in the exergy efficiency of the Soma TPP. Olive waste, in particular, has a positive effect on general performance and emissions of the TPP, with fuel consumption, CO2, SO2 and dust emissions, in direct co-firing dropping by approximately 20, 4, 19 and 18 percent, respectively, and in parallel co-firing by 26, 3, 20 and 25 percent, respectively.