Influence of different swirl vane angles of over fire air on flow and combustion characteristics and NOx emissions in a 600 MWe utility boiler

•Effect of OFA (over-fire air) swirl angle on flow field, combustion and NOx were determined.•Penetrating distance increases and divergent angle decreases with increasing angle.•NO emissions decrease with increasing angle while thermal efficiency increases.•An OFA swirl angle is recommended considering NOx emissions and thermal efficiency.

Measurements were taken for a 600-MWe wall-fired pulverized-coal utility boiler that was retrofitted with centrally fuel rich swirl coal combustion burners and two levels of OFA (over-fire air) technology. Using various swirl air vane angle settings a three-component particle-dynamics anemometer was used to measure flow characteristics in the near-OFA region in the laboratory together with gas temperature and species concentrations in the OFA and burner region in the utility boiler. The results show that, with decreasing swirl air vane angle, the penetrating depth of the jet stream decreases and the divergent angle increases. Flue gas temperature increases as the OFA swirl air vane angle increases from 25° to 45°. The O2 concentration decreases with decreasing swirl air vane angles. After retrofitting, the thermal efficiency of the boiler shows a slight increase in each case of 0.06–0.23%, with the exception of a swirl vane angle of 45°, where the thermal efficiency of the boiler decreases by 0.38%. NOx emissions are reduced when compared with those prior to the retrofitting, with OFA swirl air vane angles of 25°, 35°, 45° and 90° providing reductions of 240, 273, 294 and 319 mg/m3 (O2 = 6%), respectively.