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烟气经湿法脱硫后会产生大量含可溶性物质的白色水雾[1],导致环境污染[2]。除雾方法主要是加装除雾器[3],如折流板除雾器[4]、丝网除雾器[5]、静电除雾器等[6]。为提高除雾器的气液分离作用,NARIMANI 等[7]运用CFD的方法优化了带倒钩的折流板除雾器结构;EL-DESSOUKY 等[8]研究了不同网层厚度下丝网除雾器的最佳过滤风速;袁惠新等[9]研究了旋风除雾器在引入静电场后对细水雾的脱除效果。然而,目前水雾排放浓度依然难以达到相关标准规定的排放限值[10]。因此,白色烟羽排放的有效控制已成为湿烟气深度净化的一个研究热点[11-12]。
烟气“脱白”技术鲜有突破,其原因是除雾工程应用限制了2个高效技术路径:要求成本低、能耗少、占地小,不宜采用湿电;要求压损小、无堵塞,不能采用过滤。因此,近年来利用空气动力分离的技术方法倍受关注[13-15]。然而,采用空气动力分离除雾,需要在细而长的螺旋管中带动整个气流高速旋转,导致能耗过高,而且细水雾易被高速气流带出除雾器。
为实现高湿烟气的高效除雾,笔者提出了自由旋线除雾方法[16-17],并开展了单层自由旋线除雾实验研究[18]。结果表明,自由旋线除雾器的压损小、效率高,并且发现在控制能耗情况下增加旋线根数比增加旋线转速的提效作用更显著。然而,关于自由旋线除雾器的除雾性能还缺乏基础理论研究。本研究将基于经典的单根柱状纤维捕集和旋流离心分离机理,建立自由旋线除雾器的除雾效率理论,并通过实验加以验证,进而揭示旋线根数和转速对自由旋线除雾作用的影响规律,以期为自由旋线除雾技术的发展及应用提供参考。
旋线除雾技术的理论建立与应用检验
Demisting theory development and its validation of rotary thread demister
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摘要: 基于孤立圆柱状纤维对颗粒捕集机理和颗粒运动方程,分别建立了拦截、惯性碰撞以及离心分离除雾效率及其复合机理除雾效率理论表达式。为验证其合理性,实验中采用筒径500 mm双层旋线除雾器,旋线采用直径3 mm聚酯纤维,每层分别布置100根和200根旋线,旋线转速250~800 r·min−1,在水雾中位径20.1 μm、水雾入口浓度约4 100 mg·m−3、除雾器断面风速2.5 m·s−1的条件下,当转速500 r·min−1时,每层100根旋线的除雾器总效率约95%,每层200根旋线的除雾器总效率超过98%,表明增加旋线层根数的提效作用显著。根据总捕集效率与分级效率的经验关系式,得出旋线除雾器的分级效率经验预估值,并与所建立的旋线除雾器复合机理分级效率理论值比较,结果表明,旋线除雾器复合机理分级效率理论值与分级效率经验预估值相当吻合,在雾径0~40 μm范围内,双层旋线除雾器分级效率理论值和实验预估值的平均误差小于1.4%。对不同机理的除雾作用进行比较,发现旋线的惯性碰撞效应起主导作用,旋流分离次之,拦截效率极低,当每层200根旋线时,对20 μm雾滴的拦截效率仅2.5%左右,旋流离心分离除雾效率服从指数规律,且是雾径的平方和转速的平方的函数,对较大雾滴的提效作用明显。Abstract: Based on the particle capture mechanisms of an isolated cylinder normal to the gas flow and the particle motion equations in vortex flow, the theoretical demisting efficiencies of the interception, the inertial impaction, the centrifugal separation, and the combination demisting efficiency of the independent mechanisms of a rotary thread demister were developed. To test the theoretical demisting efficiencies, a rotary thread demister with diameter of 500 mm was used in this experiment. Two rotary thread layers were arranged in the rotary thread demister. The threads were made from polyester with diameter of 3 mm. The rotary speed of the threads ranges from 250 to 800 r·min−1. The medium diameter of the dorplets is 20.1 μm. The inlet concentration of water sprary is about 4100 mg·m−3. The gas velocity in the rotary thread demister is 2.5 m·s−1. Experimental results indicated that the overall separation efficiency could be greatly enhanced by increasing the thread number. For instance, when the rotary speed was 500 r·min−1, the overall efficiencies of the double rotary thread layer demister with 100 threads per layer and with 200 threads per layer were 95% and 98% respectively. According to the empirical equation of the relation between the overall collection efficiency and the fractional efficiency, the approximate prediction of the fractional efficiency of the rotary thread demister can be found from the experimental value of overall collection efficiency. The comparison results show that the theoretical fractional efficiency of the combination mechanism fitted well with the empirical prediction values. In the range of droplet diameters from zero to 40 μm, the mean deviation was less than 1.4%. From the comparison of the independent mechanisms, it was found that the inertial impaction was predominant, the centrifugal separation was lesser, while the interception was negligible since the interception efficiency was only about 2.5% for 20 μm droplet collection at 200 threads per layer. The centrifugal separation efficiency of the vortex flow created by the fast spinning threads followed the exponential mode which was the function of the square of drop diameter and rotary speed. Thus, the centrifugal separation effect is distinct for larger water droplets.
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表 1 k0值计算表
Table 1. Calculation value of k0
转速n/
(r·min−1)角速度ω/
(rad·s−1)每层旋
线根数总效率
实验值/%k0 500 16.7π 100 95.0 0.149 200 98.1 0.197 -
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