Evaluation of overfire air behavior for a down-fired 350 MWe utility boiler with multiple injection and multiple staging

In situ measurements on a down-fired pulverized-coal 350 MWe utility boiler revealed that, aside from particularly high NOx emissions, an asymmetric combustion phenomenon characterized by gas temperatures near the front wall being clearly higher than those near the rear wall, had developed in the furnace. A deflected flow field, obtained by cold airflow experiments within a small-scale model of the furnace, accounts for this asymmetric combustion. With focus on these problems, a new combustion technology based on a concept of multiple-injection and multiple-staging has been developed. To establish optimal overfire air (OFA) ratio settings with this technology, cold airflow experiments were conducted recording aerodynamic field measurements within a small-scale model of the boiler at five different OFA ratio settings, i.e., 0%, 10%, 15%, 20%, and 25%. For all except the highest setting, well-formed symmetric flow fields appeared in the lower furnace and the furnace throat. Increasing the OFA ratio in 0–20% raised the reach of the OFA flow in the furnace throat without incurring adverse effects in the overall flow field. At the highest setting, a deflected flow field developed not only in the lower furnace but also in the furnace throat. To establish a symmetric flow field, along with an appropriate penetration depth of the OFA flow, an optimal setting of 20% was found for the OFA ratio.

► Asymmetric combustion and high NOx emissions are universal in MBEL down-fired boilers. ► With focus on this subject, a new combustion technology has been developed. ► Cold airflow experiments were conducted to evaluate overfire air behavior with this technology. ► An optimal setting of overfire air ratio was found at 20%.