高湿烟气降温冷凝过程中细颗粒物凝并长大及脱除特性研究

The pollution of fine particles has become a prominent environmental problem in our country. Improving and optimizing the existing production process of coal-fired power plant is an extremely important technical method to enhance the removal of fine particles. Based on the usual technical means for smoke plume removal in coal-fired power plant, it is proposed the method to improve the fine particle removal by heterogeneous vapor condensation. This project mainly focuses on the following two vital scientific issues: the condensation and coalescence characteristics of fine particles in the flue gas cooling and condensing process, motion behaviors and deposition performance of fine particles in the near-wall region of a condensing heat exchanger. Therefore, the formation rules of supersaturated vapor environment in condensing heat exchanger, the nucleation and enlargement characteristics of different fine particles in supersaturated vapor environment, the motion and deposition behaviors of fine particles in near-wall region of condensing heat exchanger, and the critical parameters of fine particle removal in flue gas cooling and condensing process will be investigated. Furthermore, the general kinetic equation of aerosol for fine particle enlargement in supersaturated vapor environment, and the motion and deposition numerical model of fine particles in near-wall region of condensing heat exchanger will be obtained, which will contribute to fundamentals for the removal of fine particles from desulfurized flue gas.

细颗粒物污染已成为我国突出的大气环境问题，结合燃煤电站现有工艺过程进行改进优化是实现经济高效控制细颗粒物排放的重要技术手段；本项目针对目前燃煤电厂消除有色烟羽过程中常见的烟气降温冷凝手段，提出利用高湿烟气降温冷凝过程中形成的过饱和水汽环境促进细颗粒物的凝并长大与脱除。围绕“高湿烟气降温冷凝过程中气相主体内细颗粒物的凝并长大特性”与“冷凝换热器近壁区细颗粒物向冷凝壁面运动沉积规律”两个关键科学问题，系统研究冷凝换热器中过饱和水汽环境的形成规律、不同物性细颗粒物共存时的核化凝并长大特性、细颗粒物向不同性质冷凝壁面的运动沉积规律、以及细颗粒物脱除的关键影响参数，并建立描述气相主体中细颗粒物凝并长大的气溶胶通用动力学方程、构建冷凝换热器近壁区细颗粒物向冷凝壁面运动沉积的数值模型，为利用高湿烟气降温冷凝过程协同促进细颗粒物脱除提供理论依据与技术基础。

对燃煤电站现有工艺过程改进优化是实现经济高效控制细颗粒物排放的重要技术手段，基于烟气消白中常见的降温冷凝手段，借助高湿烟气降温冷凝过程中形成的过饱和水汽环境促进细颗粒物的凝并长大与脱除是有效控制细颗粒物排放的方法之一；因此，了解掌握高湿烟气降温冷凝过程中气相主体内细颗粒物的凝并长大特性与冷凝换热器近壁区细颗粒物向冷凝壁面运动沉积规律，是迫切需要解决的关键科学问题。本项目针对湿法脱硫过程中常见的单塔单循环、单塔双循环以及双塔双循环等技术，自主搭建模拟试验平台，探究了可过滤颗粒物（FPM）和可凝结颗粒物（CPM）在高湿烟气降温冷凝过程中的脱除特性；并对烟气降温冷凝过程中过饱和水汽环境的建立、不同物性细颗粒物在过饱和水汽环境中的核化凝结长大特性进行了系统深入研究。结果表明，脱硫净烟气出口温度一般在45~55℃，相对湿度一般为90~100%，烟气经换热器降温5~8℃后可建立过饱和水汽环境，平均过饱和度可达1.2~1.4，平均可凝结水量可达25~35g/Nm3，且过饱和度与可凝结水量随换热器沿程增加；水汽会以细颗粒物为凝结核通过非均相凝结至细颗粒物表面，气相主体内细颗粒物粒径分布总体由亚微米级向微米级移动，0.1~1.0μm粒径范围内细颗粒物数量浓度显著降低；同时，细颗粒物在热泳力、扩散泳力等作用下会迁移至冷凝壁面，沉积的难溶性细颗粒物主要集中于0.02~0.1μm，水溶性细颗粒物主要含Ca2+与SO42-离子；高湿烟气冷凝降温过程中，可过滤细颗粒物数量脱除效率约为30~60%，可凝结颗粒物脱除效率约为25~35%，脱除效率受烟气降温幅度、烟气温湿度、脱硫方式等参数的影响。此外，研究还发现，烟气降温冷凝还可促进多种污染物的协同脱除，可凝结颗粒物、逃逸氨、Hg等的排放浓度均在此过程中出现明显降低。通过本项目研究，为突破高湿烟气降温冷凝过程中细颗粒物脱除技术发展的瓶颈，实现细颗粒物的经济高效脱除，以及烟气降温冷凝系统的设计与运行奠定理论依据与技术基础。