燃尽风对炉内流动和燃烧过程影响的数值模拟

Numerical Simulation of the Effect of Over-Fire Air on Flow and Combustion in Furnaces

燃尽风作为降低锅炉NOx排放浓度的一个措施已在我国得到逐步推广应用。应用数值模拟方法,对1台600 MW对冲燃烧煤粉锅炉,在满负荷下燃尽风对炉内流动、燃烧和传热过程的影响开展了研究工作。应用混合分数/概率密度函数法模拟湍流燃烧,用P-1辐射模型开展辐射传热模拟,利用拉格朗日/欧拉法处理气固两相间的动量、质量和能量交换,对挥发份的析出采用单步反应模型,采用动力/扩散反应速率模型模拟煤粉颗粒的表面燃烧。研究发现:一方面,燃尽风的应用改善了炉内气流的充满情况,延迟了煤粉燃烧过程氧气的供应,加强了炉内的还原性气氛,降低了炉内最高火焰温度,有利于降低NOx排放浓度;但另一方面,燃尽风的应用将导致煤粉燃烧效率下降。

The use of over-fire air as a means of reducing the concentration of NOx, 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 600 MW boiler with cross firing, has been started. Mixture fracture/probability density functions are used to simulate turbulent combustion; a P-I 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 kinitics/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 NOx. But on the other hand, the use of over-fire air reduces the combustion efficiency of pulverized coal.