Influence of the parallel oil-secondary air and F-layer secondary air distribution on the flow, combustion, and NOx generation characteristics of FW down-fired boilers retrofitted with a stable combustion technology

To further reduce the NOx emissions of the down-fired boiler while improving its low-load stable combustion capacity, a new low NOx combustion organization mode that utilizes the high velocity parallel oil-secondary air to improve the pulverized coal combustion process was proposed based on a novel stable combustion technology. We carried out cold airflow experiments under different oil-secondary air and F-layer secondary air distributions (mass flow rate ratios of 1:9, 1:3, 2:3) based on a 300 MWe Foster Wheeler down-fired boiler to study its influence on the flow and mixing characteristics of the coal/airflow. Results show that the mixing between the oil-secondary air and the fuel-rich coal/airflow is gradually delayed with the increase of the secondary air distribution. When the secondary air distribution reaches more than 2:3, the oil-secondary air starts to become the dominant airflow that influences the whole flow field in the furnace. Based on the result of the cold airflow experiment, we carried out industrial measurements under different parallel oil-secondary air and F-layer secondary air distributions (4:6, 5:5, 6:4) on a 600 MWe FW down-fired boiler under the condition of using the high velocity oil-secondary air. The ignition distance of the fuel-rich coal/airflow, the flue gas temperature, the O₂, CO, and NOx concentrations in the furnace were measured. At the secondary air distribution of 4:6, the ignition distance of the fuel-rich coal/airflow was about 1.31 m. When the secondary air distribution was increased to 5:5 and 6:4, the ignition was put off, but the pulverized coal/airflow could still catch on fire in time. With the increase of the parallel oil-secondary air proportion, the flame center in the furnace did not move downward substantially. In three cases, the lowest O₂ concentrations in the inspection port area under the burner were all below 1% (peak CO concentration values were all above 28,000 ppm). The pulverized coal all burned in a strong reducing atmosphere. At secondary air distributions of 4:6, 5:5, and 6:4, NOx emissions of the boiler were, respectively, 825.83, 819.48, and 853.86 mg/Nm³ (O₂ = 6%). The thermal efficiency was, respectively, 90.09%, 89.16%, and 90.98%. In actual operation, the recommended secondary air distribution is 6:4.