Nitric oxide measurements during air-fired and oxy-fuel combustion of coal and coal/sawdust mixtures

The cofiring of biomass with coal is considered by many to be a feasible near-term approach to mitigate carbon dioxide (CO₂) emissions from coal-fired utility plants. Moreover, when combined with carbon capture and storage (CCS) biomass cofiring can potentially remove CO₂ from the atmosphere. One approach being considered to produce a CO₂ rich capture ready exhaust stream is oxy-fuel combustion, in which the fuel is burned in a stream of oxygen and recycled flue gas. In this work, an experimental investigation was performed on the effects of cofiring sawdust with coal on nitric oxide (NO) emissions under unstaged air-fired and oxy-fuel conditions. The experiments were carried out in an unpiloted 30 kWth nominal combustor. Oxy-fuel conditions were generated using bottled oxygen and carbon dioxide. The sawdust contained 0.15 wt.% nitrogen while the coal contained 0.97 wt.% nitrogen. Results indicated that flame attachment to the burner was critical for maintaining low NO emissions. When sawdust was added to a marginally attached coal-air flame the flame became detached and NO emissions increased. When flame attachment was maintained sawdust cofiring did not influence NO emissions under air-fired or oxy-fuel conditions.