Co-firing biomass is the principal means of mitigating the future energy crisis by expanding the use of renewable energy. Oxy-fuel combustion is the most capable technologies for carbon capture and storage (CCS) system. This paper presents a 3D numerical study considering co-firing concepts in a 550 MW tangentially fired furnace using a commercial CFD code AVL Fire ver. 2009.2. Necessary subroutines were written and coupled with the code to account for chemical reactions, heat transfer, fluid and particle flow fields and turbulence. Due to irregularities of the biomass particle shape, a special drag effect was considered. Three different co-firing cases (20% biomass with 80% coal, 40% biomass with 60% coal and 60% biomass with 40% coal) were considered. All the co-firing cases were simulated under air-firing and three different oxy-firing cases (25% O-2/75% CO2, 27% O-2/73% CO2 and 29% O-2/71% CO2). Level of confidence has been achieved by conducting a study on co-firing of biomass with coal in a 0.5 MW small scale furnace under air and oxy-fuel conditions. Similar findings have been observed in the present study which indicates the model can be used to aid in design and optimization of large-scale biomass co-firing under oxy-fuel conditions. This study enables the calculation of species transport and mixing phenomena and the simulation of ignition, combustion and emission formation in industrial furnace. Results were presented by the aerodynamics of burner flow, temperature distributions, gaseous emissions such as O-2 and CO2 distributions. With the increase of biomass sharing, peak flame temperature reduced significantly. The dominant effect of the lower calorific value of biomass dampens the effect of volatile content contributing to lower temperature. Comparatively, improved burnout is observed for the improved oxy-fuel cases. But, the CFD model predicted a significant increase in unburned carbon in fly ash for the increase of biomass co-firing sharing. Overall, this study highlights the possible impact of changing the fuel ratio and combustion atmosphere on the boiler performance, underlining that minor redesign may be necessary when converting to biomass co-firing under air and oxy-fuel conditions. (C) 2015 Elsevier Ltd. All rights reserved.