Evaluation of staged air and overfire air in regulating air-staging conditions within a large-scale down-fired furnace

• A deep-air-staging combustion technology consisting of staged air and overfire air.
• Evaluating effects of staged air and overfire air in deepening air-staging conditions.
• Staged air actually acting as the combustion air in the primary combustion zone.
• Deep-air-staging conditions for great NOx reduction relying on opening OFA.

To understand the deep-air-staging combustion performance and evaluate effects of staged air and overfire air (OFA) in regulating deep-air-staging conditions within a 600-MWe down-fired supercritical boiler, industrial-size measurements were performed in turn at three settings (i.e., damper opening partners of 30%:15%, 50%:15%, and 30%:40% for staged air and OFA, respectively). It was found that the staged-air effect on combustion and NOx emissions was opposite to that of OFA. At a shallow OFA opening of 15%, the furnace attained low carbon in fly ash of 4.47–5.24% and high NOx emissions of 1234–1360 mg/m3 at 6% O2. Under these circumstances, deepening air-staging conditions by increasing the staged-air damper opening from 30% to 50% essentially favored the NOx formation and improved burnout rate. With the staged-air opening fixed at 30%, opening OFA from 15% to 40%, on the contrary, acted as a positive role in strengthening air-staging conditions, i.e., reducing NOx emissions to levels of about 1000 mg/m3 at 6% O2 and raising combustible loss. The results suggested in the down-fired furnace, regulating deep-air-staging conditions to sharply reduce NOx emissions relied on opening OFA rather than staged air.