CFD simulation of biofuel and coal co-combustion in a pulverized coal fired furnace

Torrefied (thermally treated) biomass and pyrolysis oil are examples of processed, energy dense biofuels that might be utilized in energy production instead of fossil sources. Computational fluid dynamics code Fluent equipped with relevant user-defined sub-models was applied to simulate co-combustion of these supplementary fuels and coal in a full scale pulverized coal fired furnace with a capacity of 275 MWfuel. Biofuel shares up to 40 % by energy were considered. Changes in combustion process (heat transfer, burnout, NOx emissions) compared to normal coal operation were investigated and their influence in boiler performance analysed. According to the result there are in general no drastic changes in the furnace process, hence utilization of the supplementary biofuels considered seems feasible from the combustion point of view. Evaporator heat transfer is predicted to decrease slightly especially in pyrolysis oil co-firing. Solid combustion efficiency remains at acceptable level in all cases. Share of unburned carbon fly ash is estimated to increase in torrefied biomass co-firing as a consequence of degrading particle fineness assuming integrated milling of biomass and coal. However total ash flow is remarkably lower. NOx emission reduction up to 20 % can be expected in co-firing cases with investigated biofuel shares.