Flamelet LES of a 40 kW_th pulverized torrefied biomass furnace in air and oxy-fuel atmospheres

For the first time, large-eddy simulations are conducted for a 40 kW_th self-sustained pulverized torrefied biomass furnace in air and oxy-fuel atmospheres using an extended flamelet model. A recently developed chemical reaction mechanism specifically for oxy-fuel combustion (117 species and 840 elementary reactions) is employed. The simulation results are compared to the experimental data, including the axial and tangential velocities and selected species mass fractions. In addition to this, certain characteristics are investigated: the flame structure, thermo-chemical variable distributions and the NO_x emissions in different atmospheres. The flame in the oxy-fuel atmosphere is predicted to be longer and broader than that in the air atmosphere. The main reason for this is the significantly different flow dynamics in different atmospheres that are specifically designed to achieve the same oxidizer-to-fuel ratio near the burner. Two external recirculation zones (ERZs) and a central internal recirculation zone (IRZ) exist in the air atmosphere due to the designed boundary condition with a strongly swirled flow. In contrast, only a single ERZ and a weak IRZ can be observed in the oxy-fuel atmosphere. These different flow dynamics significantly influence the torrefied biomass combustion characteristics for the two atmospheres. Overall, the combustion characteristics in the air atmosphere exhibit features which are expected for a strongly swirling flow, while the thermo-chemical variable distributions in the oxy-fuel atmosphere are similar to those of a jet flame. It is also found that the atmospheres have significant effects on the NO_x emissions, particularly for the furnace studied, which features staged oxidizer combustion.