B107 NUMERICAL SIMULATION OF THE EFFECT OF OVER-FIRE AIR ON FLOW AND COMBUSTION IN FURNACES(Combustion-2)

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The use of over-fire air as a means of reducing the concentration of NOR, emitted by boiler, has gradually got popularized in China. A numerical simulation study on the effect of over-fire air on flow, combustion and heat transfer, during full capacity operation of a pulverized coal fired 600MW boiler with cross firing, has been started. Mixture fracture/probability density functions are used to simulate turbulent combustion; a P-1 radiation model is used for simulating radiation heat transfer, the Langrange/Euler's method is used for dealing with momentum, mass and energy exchange between the solid and the gas phase; the single rate model for devolatilization and the kinetics/diffusion limited combustion model for simulating surface combustion of pulverized coal particles. Study results indicate that over-fire air helps the current to spread wider in the furnace, delays the introduction of oxygen during the combustion process; the reducing atmosphere in the furnace gets boosted, and the maximal flame temperature is reduced, which helps to reduce the concentration of emitted NO_x. But on the other hand, the use of over-fire air reduces the combustion efficiency of pulverized coal, Figs 5, Tables 2 and Refs 8.
2009-11-16