Experiment and numerical simulation investigations of the combustion and NOx emissions characteristics of an over-fire air system in a 600 MWe boiler

Several numerical simulations are conducted to investigate the influence of pulverized-coal combustion characteristics and NO_x emissions with different configurations of nozzle and arrangements of an over-fire air device for a 600 MWe boiler unit. It is found by a series of comparisons that the numerical simulation results are almost in agreement with the in-situ experimental results, including the flue gas temperature of the burner outlet, the flue gas temperature along the furnace height, the NO_x concentration, and combustible content in the fly ash of the air preheater outlet, which indicates that the numerical model and the grid are reasonable. Numerical simulation results show that setting the over-fire device in which the inner is straight flow and the outer is swirl flow and the staggered arrangement of two layers of over-fire air (OFA) in the boiler are both conducive to the pulverized-coal combustion in the furnace and to the reduction of NO_x emissions. The results also show the values of 241.64?mg/m³ @ 6% O₂ for the lowest NO_x concentration at the furnace outlet. Compared with the boiler without OFA, the NO_x concentration decreased by 60.4%. Using the staggered arrangement of two layers of OFA in the practical 600 MW boiler unit, the gas temperature can reach 1100?K at the height of 100?mm away from the burner outlet, leading to coal particle ignition immediately; moreover, the NO_x emission concentration reduced to 284?mg/m³ @ 6% O₂ and heat loss due to unburned carbon in refuse of the air preheater outlet is 3.17%.