Nitric oxide emissions during coal and coal/biomass combustion under air-fired and oxy-fuel conditions

Nitric oxide (NO) emissions are measured during the combustion of sub-bituminous Powder River Basin coal and coal/sawdust mixtures under air-fired and oxy-fuel conditions in a once-through laboratory-scale (30 kWth) unpiloted combustor. In this system, flame attachment to the burner appeared to be the most important parameter for obtaining the lowest possible NO emissions. Oxy-coal conditions in which air was replaced with a 30% O₂/70% CO₂ mixture in both the primary and secondary oxidizer streams resulted in a 20% reduction in NO. When the O₂ concentration in the primary and secondary streams was varied, an optimum set of conditions resulting in minimal NO formation was found. Varying the adiabatic flame temperature by altering the secondary oxidizer gas composition had little effect on NO emissions, provided that the temperature was sufficient to maintain a burner-attached flame. Sawdust co-firing in one scenario resulted in flame instability and increased NO formation. When stable flame conditions were achieved, the effect of sawdust co-firing on NO emissions was dependent upon the sawdust particle size, with larger particles having little effect on NO emissions and smaller particles reducing NO emissions under air-fired conditions but not under oxy-fuel conditions.