预提升气对循环流态化气力输送的影响

Influence of pre-lifting gas on circulating fluidized pneumatic transportation

在循环流化床冷态模拟实验装置上,对预提升气在气力输送过程中的作用进行了研究,详细分析了气力输送中的压力平衡,考察了底部气体返混及操作流型对颗粒循环速率的影响,同时描绘了提升管预提升段内微观气固流动结构。实验结果表明,提升管底部通入预提升气可避免颗粒在床层底部的积累,增大颗粒向前输送的推动力,从而提高系统循环量。其中,喷嘴进料气体积流量的增加将加剧气体返混,底部气体内循环和局部涡流导致气流夹带增多,颗粒循环速率逐渐提高,一定程度上可削弱预提升气对循环量的影响;随预提升气体积流量的增加,底部气固流动结构由密相流态化为主向气力输送为主转变,瞬时颗粒浓度信号波动减弱,颗粒浓度显著降低。概率密度分析结果表明,预提升气体积流量的增大致使预提升段内气固两相分布相对更均匀,有利于气固相的混合及接触。

The effect of pre-lifting gas in pneumatic transportation process was studied in a cold model experimental installation with circulating fluidized bed. Pressure balance in the pneumatic transportation was analyzed, the influences of gas back-mixing and gas-solid flow pattern at the bottom of the riser on solids circulation rate were investigated, and the flow structure inside the pre- lifting section was described. It was showed that, the injection of the pre-lifting gas into the riser was favorable to avoiding solids depositing on the bottom, pushing the solids upwards and increasing the solids circulation flux. Increasing the volume flow rate of nozzle-feeding gas at the inlet could intensify the gas back-mixing, the internal recycle and local vortex led to air entrainment increasing, so the solids circulation rate increased and the impacts of the pre-lifting gas on the solids flux reduced. With increasing the pre-lifting gas flow rate, the gas-solid flow regime in the bottom region was transformed from dense-phase fluidization to the pneumatic transportation, meanwhile, transient solids holdup fluctuation weakened and solids concentration decreased significantly. The analysis of probability density distribution indicated that, increasing the pre-lifting gas flow-rate could lead to more uniform solids distribution, which was beneficial to gas-solid mixing.