Fuel Flexible Biomass Reburn Technology

Biomass reburn is a low NO_x alternative to cofiring that effectively uses the high volatility and high char reactivity of biomass for NO_x reduction. In this paper, computational fluid dynamics (CFD) and thermal modeling, and a NO_x prediction model were used to evaluate the impacts of sawdust/coal reburn on the performance of a 250 MW opposed-fired boiler burning bituminous coal as the primary fuel. The results showed that the reburn system maintained overall boiler performance with a 50 – 70 °F reduction in the furnace exit gas temperature. Predicted losses in thermal efficiency were caused by the lower biomass fuel heating value (similar to biomass cofiring) and increase in unburned carbon. The higher unburned carbon emissions were attributed to an order of magnitude larger biomass mean particle size relative to bituminous coal. Thus, LOI emissions can be improved significantly by reducing the biomass mean particle size. The NO_x predictions showed that for reburn rates above about 19%, adding dry sawdust biomass to a coal reburn system can improve NO_x reduction; i.e., using pure dry sawdust as reburn fuel at 30% of the total heat input can lead to NO_x levels about 30% less than those for pure coal reburn under for similar firing conditions.