Staged-air ratio optimization for a new down-fired technology within a cold small-scale model of a 350 MW_e utility boiler

In this paper, a new combustion technology based on the concept of multiple injection and multiple staging was developed especially for a down-fired pulverized-coal 350 MW_e utility boiler with particularly high NO _x emissions and severe asymmetric combustion. Compared to the prior technique, the new technology creates a completely unique combustion system necessitating a burner redesign and reconfiguration. To establish efficient furnace operating conditions for the reconfigured furnace, an appropriate range for the staged-air ratio must be ascertained. For this purpose, cold airflow experiments were conducted by recording aerodynamic field measurements within a small-scale model at various staged-air ratio settings (viz., 0%, 10%, 20%, 25%, 30%, and 35%). Aerodynamic fields and distributions of velocities throughout the furnace were measured, in addition to decay and penetration depths of downward airflows and overfire air. At lower staged-air ratios of 0%, 10%, 20%, and 25%, a well-formed symmetric flow field appeared in the lower furnace and the furnace throat region. Velocity distribution, as well as the decays in the downward airflow and OFA jets, also displayed well-defined symmetries along the furnace center in the zones near the front and rear walls. At higher ratios of 30% and 35%, a deflected flow field developed in the lower furnace, as well as in the furnace throat region, although this airflow was redirected higher up the front wall than for the rear wall. To establish a strongly symmetric flow field and appropriate penetration depths, a staged-air ratio of 25% was found optimal for the newly reconfigured furnace.