A numerical investigation was conducted on co-firing biomass and coal under oxy-fuel condition with the refined combustion model. The effects of gas-phase reactions caused by various volatiles of coal and biomass were considered. The influences of biomass fraction and combustion atmosphere on the reaction rate of volatile, the distribution of wall heat flux, and the burnout behavior were analyzed. The results show that the reaction is more active for the volatile of Paulownia wood chips than that of coal. The average wall heat flux suffers a decline when the combustion atmosphere is switched from air to oxy-fuel condition, while the O-2 fraction is 24%. The wall heat flux of hopper rises by 29.97% to 162.7% as the biomass fraction increases from 10% to 30% under various atmospheres. Due to the variation in trajectories of fuel particles and the change of temperature within the furnace, the combustion atmosphere and the biomass share fraction affect the burnout performance to some extent. The working performances of all cases were evaluated, and the optimal operation conditions for co-firing different co-firing scenarios were recommended.