Numerical investigation on the flow, combustion, and NO_X emission characteristics in a 660 MWe tangential firing ultra-supercritical boiler

A three-dimensional numerical simulation was carried out to study the pulverized-coal combustion process in a tangentially fired ultra-supercritical boiler. The realizable k-ε model for gas coupled with discrete phase model for coal particles, P-1 radiation model for radiation, two-competing-rates model for devolatilization, and kinetics/diffusion-limited model for combustion process are considered. The characteristics of the flow field, particle motion, temperature distribution, species components, and NO_x emissions were numerically investigated. The good agreement of the measurements and predictions implies that the applied simulation models are appropriate for modeling commercial-scale coal boilers. It is found that an ideal turbulent flow and particle trajectory can be observed in this unconventional pulverized-coal furnace. With the application of over-fire air and additional air, lean-oxygen combustion takes place near the burner sets region and higher temperature at furnace exit is acquired for better heat transfer. Within the limits of secondary air, more steady combustion process is achieved as well as the reduction of NO_x. Furthermore, the influences of the secondary air, over-fire air, and additional air on the NO_x emissions are obtained. The numerical results reveal that NO_x formation attenuates with the decrease in the secondary air ratio (γ_2nd) and the ratio of the additional air to the over-fire air (γ_AA/γ_OFA) was within the limits.