THE INFLUENCE OF BURNER INJECTION MODE ON PULVERIZED COAL AND BIOMASSCOFIRED FLAMES

A new dual fuel burner designed for the co-firing of waste-derived sol id fuels (e.g., biomass, refused-derived fuel, sewage sludge) with pul verized coal in practical combustors was evaluated through trials unde rtaken in a 0.5 MW down-fired furnace. A new mathematical procedure wa s also constructed that accounts for multimode combustion of these fue ls. It includes the influence of the heating and devolatilization rate s of each fuel on the effective stoichiometry of the volatiles in the combustion domain depending on their respective particle trajectories. Results included for sawdust-coal flames, show the significant effect of co-firing ratio and fuel injection mode on flame ignition, combust ion aerodynamics, and nitric oxide emissions. Predicted indices of the coal devolatilization rate along the particle trajectories emphasize the influence of the faster devolatilization and ignition of the sawdu st on coal combustion in the near burner region. When the sawdust part icles are injected through the center of the burner, surrounded by an annular coal jet, they immediately ignite thereby enhancing the combus tion intensity of the coal within the internal recirculation zone. Thi s injection mode leads to a subsequent reduction in the nitric oxide f ormation along with a higher combustion efficiency as compared with a flame where the sawdust and coal injection positions are reversed. An optimum co-firing ratio in which the sawdust provided 30% of the total heat input was found to exhibit the maximum particle burnout and mini mum nitric oxide emissions. Co-firing results obtained for a lower rea ctivity and higher nitrogen content fuel (pulverized sewage sludge) as compared with sawdust, show that the fuel injection mode had a margin al effect on burnout and NO emissions. The sawdust and sewage sludge c o-firing results emphasize the need to consider both the reactivity an d nitrogen content of the fuel prior to selecting an injection mode.