Effects of overfire air ratio on the aerodynamic flow fields of a 350-megawatt supercritical boiler incorporating multi-injection and multistage combustion technology

Cold airflow experiments were performed using a 1:10 scale model of a down-fired, pulverized coal 350-megawatt (MW) supercritical utility boiler incorporating multiple-injection and multiple-staging combustion technology. The effects of varying the overfire air (OFA) ratio on the aerodynamic flow fields in the furnace were investigated using ratio settings of 1.9, 9.6, 19.1, and 28.7%. The results showed that, at ratios of 9.6, 19.1, and 28.7%, symmetrical flow fields were present in the lower furnace, whereas a deflected flow field was observed at a ratio of 1.9%. At OFA ratios of 19.1% and above, suitable horizontal airflow penetration depth was observed, and the flow fields were symmetrical. The velocities in the region H/H0=0-0.4 were fit by linear functions, which showed that the vertical velocity of the overall airflow along the arches was linear in this section of the furnace, and that the slope of the linear function was reduced from 3.63 to 2.70 as the OFA ratio was increased, indicating that the rate of decay was reduced. Based on an overall consideration of the symmetrical flow field and appropriate airflow penetration depth, the optimal OFA ratio required for operation of this furnace is greater than 19.1%.