水汽相变促进烟气中细颗粒物成核长大特性研究

摘要

针对燃煤烟气中细颗粒物粒径过小不易被传统除尘设备捕集的问题,提出利用水汽相变技术以提高湿法烟气脱硫(WFGD)系统中细颗粒物的脱除效率.首先对燃煤飞灰及无机盐气溶胶颗粒物在过饱和水汽环境下异质核化凝结长大特性进行了数值预测,计算结果显示,烟气温度约为323 K条件下,水蒸汽在细颗粒物表面异相凝结可使其粒径在极短时间内增大几十到几百倍,并且较高的水汽过饱和度和较大的细颗粒物初始粒径有利于细颗粒物异质核化凝结长大.为验证数值计算结果并考察水汽相变促进细颗粒物凝结长大并脱除的实际效果,基于湿法烟气脱硫系统利用PDA和ELPI两种测试仪器,对燃煤细颗粒物及无机盐气溶胶颗粒在过饱和水汽环境下核化凝结长大特性及脱除性能进行实验研究,脱硫后净烟气温度约为40～50℃.结果表明,燃煤飞灰和无机盐气溶胶颗粒粒径均可在极短时间内增大3～5倍,由亚微米级颗粒长大成微米级的含尘液滴,其中亲水性的无机盐气溶胶颗粒更易潮解逐渐形成溶液滴,在饱和度接近1时就能凝结长大;长大后的含尘液滴可被丝网除雾器有效捕集,因此水汽相变结合丝网除雾器可有效促进细颗粒物凝结长大并脱除,使细颗粒物数量浓度由9.5×106/cm3显著降至5×106/cm3左右,质量浓度也可由80 mg/m3明显降至20 mg/m3左右,相应的脱除效率分别高达50％和75%.%Traditional dust collecting equipments cannot capture fine particles from coal-fired flue gas efficiently due to their small sizes.A heterogeneous condensation technology was proposed for a wet flue gas desulfurization (WFGD) system to improve its efficiency of removing fine particles.Numerical study was first carried out to present the growth of coal-fired fly ash and inorganic salt particles as nucleation for heterogeneous condensation under supersaturated water vapor environment.The results indicated that the diameter of fine particles increases from tenfold to hundreds of times in a very short time with the presence of heterogeneous condensation of water vapor on the surface of fine particles under a flue gas temperature of 323 K.In addition,higher vapor supersaturation and larger initial particle diameter are beneficial to the grown up of fine particles.Experiments were then conducted to verify the numerical results and evaluate the practical effect of the technology on growth and removal of coal-fired fine particles and inorganic salt particles in a wet flue gas desulfurization system containing supersaturated water vapor with the help of PDA and ELPI.The temperature of desulfurated flue gas was in the range of 40 ～ 50 ℃.The results indicated that fine particles grow larger by 3-5 times in a very short time in the presence of heterogeneous condensation,changing from submicron particles to micron-sized dusty droplets.Hydrophilic inorganic salt particles deliquesce much more easily,forming solution droplets,which could be nucleated and grew up under the condition of saturation degree close to 1.Due to its good ability of capturing these grown dusty droplets,wire-mesh demister was employed to promote the growth and removal of fine particles combined with heterogeneous condensation.The number concentration of fine particle decreases significantly from 9.5 × 106/cm3 to 5 × 106/cm3 and the mass concentration is reduced sharply from 80 mg/m3 to 20 mg/m3.The corresponding removal efficiencies of fine particles are 50% and 75%,respectively.