Characterization of gas/particle flows with respect to staged-air ratio for a down-fired 600 MWe supercritical utility boiler with multiple injection and multiple staging: A lab-scale study

•The first application of a deep-air-staging down-fired combustion technology in a 600 MWe supercritical boiler.•Severely asymmetric gas/particle flow-filed formation at a staged-air ratio of 35%.•An optimized staged-air ratio range of 20–30%.•Symmetric combustion, good burnout, and low NOx emissions appearing in the real furnace.

To achieve significant reductions in particularly high NOx emissions and to eliminate severely asymmetric combustion found in down-fired boilers, a multiple-injection and multiple-staging combustion technology was developed in our previous study. Representing great progress in this area, one of two newly-designed down-fired 600 MWe supercritical utility boilers using this technology had a recent trial run before its 168-h test. By phase-Doppler anemometry measurements within a cold 1:40-scaled model of the furnace, gas/particle flow characteristics were compared among various staged-air ratio settings (i.e., 20%, 25%, 30%, and 35%) to establish an appropriate staged-air ratio range for boiler operation. Meanwhile, to validate this first industrial application of the newly developed technology, industrial-size measurements were made at a 550 MWe load during the trial run. Experimental gas/particle flow results uncovered that at the highest setting, a severely asymmetric gas/particle flow field developed in the furnace. For the two lower settings, an essentially symmetric gas/particle flow field appeared. Moreover, gas/particle flow-field symmetries at the 30% setting were generally acceptable. Consequently, a staged-air ratio range of 20–30% is recommended for the boiler. As expected, industrial-size data revealed that excellent furnace performance, characterized by symmetric combustion and low levels of carbon in fly ash (5.1%) and NOx emissions (822 mg/m3 at 6% O2), appeared within the boiler. This was despite combustion optimization and a present lack of industrial-size data at full load.