燃煤流化床大颗粒形貌特征和空间分布特性

An Investigation on Behavior of Coarse Particles in a Fluidized Bed Combustor

研究了燃煤循环流化床锅炉原煤和冷渣的颗粒形貌特性和空间分布特性.结果表明:(1)冷渣颗粒与原煤颗粒相比呈更明显的片状特征,两者都显示了颗粒球形度随着颗粒度变大而下降的趋势;Wadell球形度以0.7~0.8为主的原煤在流化床燃烧后,其值明显下降且更加分散;原煤的SF系数主要在0.4~0.8之间,基本呈带状分布,经过流化床燃烧后则主要分布在下三角形区域;统计分析显示,锅炉冷渣当量尺寸小于1mm时SF系数随颗粒尺寸而增加;当两颗粒尺寸在1~3.5mm之间SF系数下降;颗粒尺寸大于3.5mm后SF系数几乎不再变化.(2)冷态循环流化床实验台上进行的颗粒分层取样表明,鼓泡流化床或循环流化床密相区的中上部,颗粒分布以中小颗粒为主;而在床层下部,则以大中颗粒为主;超过40%的颗粒聚集在布风板上70mm区域,大颗粒的质量分布与中小颗粒的质量分布皆然相反,这种处于床层下部的大颗粒同时也处于流化床运行的低温反应区,根据燃烧学和流化床锅炉运行经验判断,大颗粒应处于动力燃烧控制区,这可能是流化床锅炉冷渣存在黑芯(残碳)的根本原因.

The particle morphology denoted by Wadell sphericity,Krumbein sphericity and SF factor of coarse particles from raw coal and cold ash from a fluidized bed boiler was measured,which showed that cold ash was platier than raw coal particles.(1) Wadell sphericity lied mainly at 0.7~0.8,scattered data obtained from ash particles,especially for those coarser than 2 mm;a ribbon Krumbein sphericity distribution from raw coal and most of the data lied between 0.4~0.8 and a triangle Krumbein sphericity distribution of ash particles was observed from the measurement.SF coefficient,it looked like constant for raw coal but an obvious change was seen at ash particles.An increase was seen when particle size was less than 1 mm and then it decreased before 3.5 mm.It nearly kept constant after that.(2) Sampling from different heights of fluidized bed rig suggested that middle size and coarse particles tended to move within a lower height of dense phase due to dynamic feature,more than 40% bed mass was sampled out under 70 mm height from distributor.So kinetic control reaction should be guessed due to the quenching of cold combustion air from wind box,this might be the real reason of formation of black cores of drained cold ash in the fluidized bed boiler.