Performance assessment of staged-air declination in improving asymmetric gas/particle flow characteristics within a down-fired 600 MWe supercritical utility boiler

By using phase-Doppler anemometry (PDA) measurements within a cold small-scale model for a down-fired 600-MWe supercritical utility boiler, gas/particle flow characteristics were acquired at various staged-air declination angle settings (i.e., 0°, 15°, 30°, and 45°) so as to assess the availability of a staged-air declination method in improving the strong asymmetric gas/particle flow field. Detailed comparisons made in the mean velocity, particle volume flux, particle number concentration, gas/particle flow decays, and trajectories of the downward gas/particle flows with respect to different staged-air angle settings, revealed that enlarging the staged-air angle essentially did not change the strong asymmetric patterns in gas/particle flow characteristics. From measurements taken within a real furnace equipped with a designed horizontally-fed staged air (i.e., 0° for the staged-air angle), strong asymmetric combustion characterized by gas temperatures being much higher near the rear wall than near the front wall, low burnout, and particularly high NOx emissions, were found. By considering the appearance of a similar strong asymmetric gas/particle flow field at various staged-air angle settings in this study and at different staged-air ratios in a previous study, staged-air declination and staged-air ratio reduction are very likely to be inapplicable if applied in real furnaces to deal with these problems (i.e., strong asymmetric combustion, low burnout and high NOx emissions). Retrofitting a boiler with deep-air-staging combustion technology developed in our recent investigation is recommended if symmetric combustion, high burnout, and low NOx emissions are to be achieved.

► No essential improvement in the severely asymmetric flow field pattern with enlarging the staged-air angle.
► Asymmetric combustion, low burnout, and high NOx emissions in the real furnace.
► Both the staged-air declination and staged-air ratio reduction being inapplicable in dealing with these problems.
► Recommending a down-fired technology based on the concept of multiple injection and multiple staging for the furnace.