THE INFLUENCE OF BURNER INJECTION MODE ON PULVERIZED COAL AND BIOMASS COFIRED FLAMES

A new dual fuel burner designed for the co-firing of waste-derived solid fuels (e.g., biomass, refused-derived fuel, sewage sludge) with pulverized coal in practical combustors was evaluated through trials undertaken in a 0.5 MW down-fired furnace. A new mathematical procedure was also constructed that accounts for multimode combustion of these fuels. It includes the influence of the heating and devolatilization rates 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, combustion 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 sawdust on coal combustion in the near burner region. When the sawdust particles are injected through the center of the burner, surrounded by an annular coal jet, they immediately ignite thereby enhancing the combustion intensity of the coal within the internal recirculation zone. This injection mode leads to a subsequent reduction in the nitric oxide formation 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 minimum nitric oxide emissions. Co-firing results obtained for a lower reactivity and higher nitrogen content fuel (pulverized sewage sludge) as compared with sawdust, show that the fuel injection mode had a marginal effect on burnout and NO emissions. The sawdust and sewage sludge co-firing results emphasize the need to consider both the reactivity and nitrogen content of the fuel prior to selecting an injection mode.