Effects of OFA Ratio on Coal Combustion and NOx Generation of a 600-MW Downfired Boiler after Changing Air Distribution around Fuel-Rich Flow

Abstract

Both experimentation and numerical simulations were conducted to study the effects of the overfire air (OFA) ratio on the coal combustion and NOx generation characteristics of a 600-MW downfired boiler after changing the air distribution around the fuel-rich flow. In these experiments, the arrangement of the secondary air ports close to the fuel-rich flow was adjusted to modify the air distribution around the fuel-rich flow. The air distributions in the total air duct at OFA damper openings of 30%, 40%, 50%, 70%, 80%, and 100% were calculated by numerical simulation to determine the theoretical air ratios, and these values were in good agreement with experimental results. The calculated air ratios were used as the inlet boundary conditions in subsequent simulations of the boiler thermal states. The calculated NOx emissions, carbon in fly ash, and gas temperatures in the lower furnace were also in good agreement with experimental data. Increasing the OFA damper opening from 30% to 80% generated both good flow and temperature symmetry while gradually reducing the airflow declination angles in the tertiary air regions, indicating reduced downward airflow depth. The flame kernel was also found to move upward. The OFA was readily drawn into the lower furnace at OFA damper openings below 50%, whereas larger openings gradually enhanced the OFA penetration. The corresponding experimental work demonstrated that increasing the opening from 30% to 80% increased the O2 concentration at the furnace exit from 3.02% to 3.51%, and NOx emissions were decreased from 1,337 to 735 mg/m3 at 6% O2. Reducing the outer secondary air while simultaneously increasing the secondary air ratio between the fuel-rich flow nozzles increased the carbon in fly ash from 4.93% to 5.75%. However, values of 5.94%–15.1% where obtained upon increasing the OFA damper opening from 0% to 70% in conjunction with prior secondary air distribution around the fuel-rich flow. Under these conditions, the coal burnout was greatly enhanced and the lower furnace temperature and NOx emissions were slightly increased.