白色烟羽定量化表征

Quantitative characterization of white plume

湿法脱硫后的饱和湿烟气直接排放形成的白色烟羽不仅会造成视觉污染,还会促进二次气溶胶和雾霾形成。前人大量研究了白色烟羽的产生和控制机理,但对白色烟羽特征的表征缺乏定量化描述手段。为对不同视觉效果的白色烟羽进行定量化表征,研究其中液滴的分布规律,搭建了模拟湿烟气排放试验系统,采用遮光度表征白色烟羽,利用激光粒度仪测定不同气象条件及烟气状态下,白色烟羽扩散的遮光度变化及液滴生长规律。其中,遮光度为颗粒在光束中的遮光截面与光束总面积之比。遮光度越大,白色烟羽视觉遮挡效果越强。定义无量纲相对高度为白色烟羽高度与排烟口直径的比值;液滴平均粒径为按不同液滴粒径体积分数加权的粒径;采用最大遮光度及最大平均粒径来表征白色烟羽,分别为白色烟羽扩散过程中遮光度及液滴平均粒径的最大值。首先,以50℃的饱和湿烟气为研究对象,研究不同环境工况下遮光度和液滴粒径沿白色烟羽竖直及水平方向的变化规律,同时,分析环境温度和含湿量对遮光度及液滴平均粒径的影响。结果表明:沿白色烟羽竖直方向,遮光度及液滴平均粒径均先增大后减小。在任意相对高度处,各粒径的液滴按体积分数均呈正态分布。沿白色烟羽水平方向,遮光度由中心向边缘呈下降趋势,液滴平均粒径几乎不变。环境温度降低、环境含湿量增加均会使白色烟羽的最大遮光度与液滴平均粒径增加。其中,环境含湿量为4.8 g/kg时,随着环境温度由22℃降至10.9℃,白色烟羽最大遮光度由7.4%增至31.9%,液滴最大平均粒径由9.9μm增至26.3μm;在环境温度8.5℃时,随环境含湿量从1.75 g/kg增至4.21 g/kg,白色烟羽最大遮光度由16.89%增至42.50%,液滴最大平均粒径由21.29μm略增至23.47μm。其次,研究了饱和湿烟气初始温度对遮光度及液滴平均粒径随的影响。结果表明,饱和湿烟气初始温度的降低会显著降低白色烟羽的遮光度与粒径。在温度7℃、相对湿度55%的济南冬季大气环境工况下,随饱和湿烟气初始温度由60℃降至40℃,其最大遮光度由39%降至19%,白色烟羽的视觉效果显著消除。研究成果为白色烟羽提供了定量化表征的手段,从而为消白技术的研发与应用奠定了理论基础。

The white plume formed by the direct emission of saturated wet flue gas after wet desulfurization not only causes visual pollution,but also promotes the formation of secondary aerosols and haze.The generation and control mechanisms of white plume have been studied extensively by previous authors,but the characterization of white plume features lacks the means to describe quantitatively.In order to quantitatively characterize the white plume with different visual effects and to study the distribution pattern of droplets in it,a simulated wet smoke emission test system was built in this paper,and the shading degree was used to characterize the white plume,and the change of shading degree and droplet growth pattern of white plume diffusion under different meteorological conditions and smoke states were measured by laser particle size meter.The shading degree is the ratio of the shading cross section of the particles in the beam to the total area of the beam.The greater the shading degree,the stronger the visual shading effect of the white plume.The dimensionless relative height is defined as the ratio of the height of the white plume to the diameter of the smoke vent;the droplet average particle size is the particle size weighted by the volume fraction of different droplet sizes;the maximum shading degree and the maximum average particle size are used to characterize the white plume,which are the maximum values of the shading degree and the droplet average particle size during the diffusion of the white plume,respectively.Firstly,the variation of shading degree and droplet size along the vertical and horizontal directions of the white plume under different environmental conditions with the saturated wet flue gas at 50℃were studied,and analyzed the effects of ambient temperature and moisture content on the shading degree and droplet size were studied.The results showed that the shading degree and the average droplet size showed a peak shape of increasing and then decreasing along the vertical direction of the white plume.At any relative height,the droplets of each size were normally distributed by volume fraction.Along the horizontal direction of the white plume,the shading degree tended to decrease from the center to the edge,and the average droplet size was almost unchanged.The decrease of ambient temperature and the increase of ambient moisture content increase the maximum shading degree and the average droplet size of the white plume.At 4.8 g/kg of ambient moisture content,the maximum shading of white plume increased from 7.4%to 31.9%and the average droplet size increased from 9.9μm to 26.3μm as the ambient temperature decreased from 22℃to 10.9℃.At an ambient temperature of 8.5℃,the maximum shading degree of the white plume increased from 16.89%to 42.50%and the maximum average particle size of the droplets increased slightly from 21.29μm to 23.47μm as the ambient moisture content increased from 1.75 g/kg to 4.21 g/kg.Secondly,the effect of the initial temperature of saturated wet flue gas on the shading degree and the average particle size of droplets was investigated.The results showed that the decrease of the initial temperature of saturated wet flue gas significantly reduced the shading degree and particle size of the white plume.Under the conditions of Jinan winter atmosphere with temperature of 7℃and relative humidity of 55%,as the initial temperature of saturated wet flue gas was reduced from 60℃to 40℃,the maximum shading degree was reduced from 39%to 19%,and the visual effect of white plume was significantly eliminated.The research results of this paper provide a means for quantitative characterization of white plume,thus laying a theoretical foundation for the development and application of whitening technology.