SCR脱硝过程中PM2.5物性变化机制及对燃煤电厂排放特征的影响研究

The emission of PM2.5 is an important environmental issue in China，and coal-fired power plant is one of the main sources of PM2.5 in the atmosphere. To meet stringent emission standards of NOx，coal-fired power plants have been equipped with selective catalytic reduction (SCR) systems. As a result, the secondary PM2.5 generated from NOx in the atmosphere would be reduced, but the emission of primary PM2.5 might be increased at the same time. Meanwhile the removal, chemical composition of PM2.5 in flue gas and the emission characteristics of coal-fired power plant would be affected. However，related investigates have been still confined to macroscopic phenomena so far, and the quantitative studies on the effects of SCR on emission characteristics of coal-fired power plants are of great necessity. This project mainly focus on the following two key issues: the heterogeneous interaction and conversion between PM2.5 and catalysts, gaseous components (e.g. SO3, NH3, etc) during SCR process, and the removal and conversion mechanism of ammonium sulfate salt, SO3 and escaping NH3 in the electrostatic precipitator (ESP) and wet flue gas desulfurization (WFGD) system. Therefore, the relationship between the formation-decomposition-volatilization-condensation of the ammonium sulfate salt and the changes of physical and chemical characteristics of PM2.5 in SCR system and air pre-heater, as well as the removal process of ammonium sulfate salt, SO3 and escaping ammonia in ESP and WFGD system will be investigated in this project. The aims are to disclose the inherent relationship between the variation of PM2.5 properties and SCR process, and quantitatively identify the effects of SCR installation on emission characteristics of coal-fired power plants, and provide new insights for enhancing co-removal PM2.5 and NO, SO3, escaping NH3 and other pollutants.

燃煤产生的PM2.5在最终排放到大气环境中之前需要经过脱硝、除尘、脱硫等污染物控制设施，其中SCR脱硝对PM2.5的生成和脱除以及化学组成都可能产生影响，并引起燃煤电厂排放特征的改变，但迄今还只局限于宏观现象的考察。本项目围绕“PM2.5与催化剂及烟气中SO3、NH3等气态组分间的相互作用和非均相转化”及“SCR脱硝过程中形成的硫酸铵盐、SO3及逃逸氨在后续烟气系统中的转化、脱除机制”两个关键科学问题，对SCR脱硝反应器及空预器中硫酸铵盐的形成-分解转化-挥发凝结特性及其与PM2.5物性变化的关系，以及硫酸铵盐、SO3、逃逸NH3在后续电除尘、湿法烟气脱硫中与燃煤PM2.5的相互作用及其转化、脱除过程加以系统研究，进而揭示PM2.5物性变化与脱硝过程间的内在关联，定量识别SCR脱硝对燃煤电厂排放特征的影响，并为增强PM2.5与NO、SO3、逃逸NH3等污染物的共同脱除提供新思路。

燃煤产生的PM2.5在最终排放到大气环境中之前需要经过脱硝、除尘、脱硫等污染物控制设施，其中SCR脱硝对PM2.5的生成和脱除以及化学组成都可能产生影响，并引起燃煤电厂排放特征的改变。本项目结合SCR脱硝的基本工艺过程，重点针对商用钒钨钛催化剂，构建了PM2.5物性变化与SCR脱硝催化剂和烟气中SO3、NH3等气态组分之间的关系，同时，试验研究了脱硝生成的硫酸铵盐和逃逸氨在后续电除尘和湿法脱硫中的迁移转化机制，识别了SCR脱硝对电厂排放特征的影响，并在此基础上，通过试验和量子化学计算相结合的方法，对现有催化剂进行改性研究，以达到减少硫酸铵盐生成和促进分解转化的目的。结果表明，SCR脱硝烟气中的硫酸铵盐气溶胶来自于脱硝催化剂上生成的硫酸铵盐向气相主体的迁移，以及NH3、SO3和H2O在气相主体中的反应。在空预器的气相环境中，硫酸氢铵是主要产物，因为硫酸氢铵和硫酸铵的形成取决于反应物的NH3/SO3摩尔比；硫酸氢铵的形成温度在231-260°C之间，受到反应物浓度的影响，与NH3和SO3的浓度积之间有如下关系：P(NH3)P(H2SO4)=7.9×10^13exp(-234968/RT).。在下游的电除尘系统，硫酸铵盐气溶胶由于是亚微米级细颗粒物脱除效率较低，但同时由于粘附在PM2.5表面能够增强黏性并降低比电阻，使得电除尘对细颗粒物的脱除效率有所提高。在湿法脱硫系统中，脱硫浆液的洗涤可以脱除部分硫酸铵盐气溶胶和逃逸氨，而脱硫浆液中累积的NH4+的夹带和蒸发又会产生新的铵盐颗粒物和气态氨。总体来看，电除尘器和湿法脱硫系统都对硫酸铵盐细颗粒物有一定的脱除效果，但是SCR脱硫系统的运行仍然会造成PM2.5、NH4+和NH3排放量的增加。将SiO2和CeO2作为助剂掺杂在TiO2载体中，一方面能够抑制SO2向SO3的氧化，从而减少硫酸铵盐的生成，另一方面能够提高催化剂上硫酸氢铵中NH4+基团的反应活性，从而促进硫酸铵盐的分解和反应。最后，量子化学理论研究表明硫酸氢铵在TiO2表面的沉积过程和NO与NH4+基团反应生成H2O和N2的过程均为放热过程，NO扩散到TiO2表面和H2O、N2脱离TiO2表面几乎不影响系统的能量，类硫酸盐物种能稳定的结合在TiO2表面Ti位点上。