Characteristics of coal powder-gas mixture combustion in rotary kiln

Currently, a large amount of high temperature furnace gas is discharged in the smelting process of electric furnace, which causes energy waste and environmental pollution. The process of recovering combustible components such as CO from electric furnace flue gas as auxiliary fuel for rotary kilns was proposed. A new five-channel burner was designed to realize the mixed combustion of pulverized coal and electric furnace flue gas in rotary kilns. Afterwards, CFD(Computational Fluid Dynamics) theory was used to establish the mathematically model of pulverized coal and electric furnace flue gas co-combustion. The process of gas-solid two-phase fuel co-combustion in a rotary kiln was numerically simulated. The temperature fields, combustion characteristics and NOx changes in rotary kilns were analyzed under different electric furnace flue gas mixing ratios. The results show that the inner and outer circulation zones can be observed in the rotary kiln, which can accelerate the mixing of fuel and air. As a result, the amount of heat transfer in the rotary kiln is increased. The addition of electric furnace flue gas helps to improve the combustion characteristics of pulverized coal. When the electric furnace flue gas ratio increases from 20% to 40%, the temperature distribution inside the rotary kiln exhibit axisymmetry, and the length of the black flame gradually decreased from 4.51 m to 2.32 m. Then the peak flame temperature gradually decreases, the combustion rate of volatile components gradually increases from 4.21 mol/(m3·s) to 7.33 mol/(m3·s). Besides, the concentration of NOx at the rotary kiln outlet gradually decreases from 1 013 mg/m3 to 677 mg/m3. With increasing the electric furnace flue gas ratio from 20% to 30%, the flame shape gradually elongates and narrows from a “pellet”, the combustion position of pulverized coal moves forward from X=4.68 m to X=2.26 m, and the length of rotary kiln roasting zone increases from 18.50 m to 20.50 m. However, with increasing the electric furnace flue gas ratio from 30% to 40%, the flame gradually shift to a “wooden-club shaped”, there is no significant change of the combustion position of pulverized coal. Besides, the length of roasting zone correspondingly decreases from 20.50 m to 16.43 m. By comprehensively analyzing the length of the roasting zone and the characteristics of fuel co-combustion, it is determined that the optimal electric furnace flue gas ratio is 30%.