大型煤粉锅炉汞的排放特性和迁移规律研究

采用EPA 30B方法,对某大型火电厂四台典型煤粉锅炉进行了烟气Hg排放测试,并选取其中两台代表性锅炉对各输入／输出物料进行了Hg量对比分析.通过系统的汞质量平衡核算,得出各物料中汞所占的比例,并据此分析了选择性催化还原脱硝装置(SCR)、静电除尘器(ESP)和湿法脱硫装置(WFGD)等主要烟气净化设施对汞排放的影响,在此基础上研究得到系统汞的迁移规律.结果表明,四台锅炉的烟气Hg排放浓度都不高,普遍在3 μg／m³以下,明显低于新国标规定的排放限值,其中,配备SCR时Hg排放浓度明显更低.其原因在于,设置有SCR时,烟气中相当部分的Hg⁰会被催化氧化成Hg²⁺,Hg²⁺易于被飞灰吸附而脱除.WFGD对Hg²⁺的吸收比Hg⁰强得多,因此,排放烟气中汞的形态以Hg⁰为主,吸收的Hg绝大部分转移至脱硫石膏中.脱硫废水和炉渣对Hg的富集能力都非常有限.

Study on mercury emission and migration from large-scale pulverized coal-fired boiler

In order to study the characteristics of mercury emission and migration, EPA 30B method was used to determine the Hg concentrations in flue gases emitted from four typical pulverized coal-fired boilers in a large-scale power plant, and the Hg contents of related input／output materials in two of these boilers were also analyzed and compared. According to mass balance, the Hg distribution in different materials was obtained, and then the effect of major flue gas purification systems including selective catalytic reduction (SCR), electrostatic precipitation (ESP) and wet flue gas desulfurization (WFGD) on Hg emission was clarified. Based on these, the migration pattern of Hg in pulverized coal-fired boilers was summarized systematically. Experimental results indicated that Hg emissions of all the four boilers were below 3 μg／m³, which was much lower than the limit specified by the new Chinese national standard. The emission was even significantly lowered when SCR system was located upstream. The reason was in the fact that, when there was SCR system located, lots of Hg⁰ in flue gas could be catalyzed to Hg²⁺ and then adsorbed by fly ash. The solubility of Hg²⁺ was much higher than that of Hg⁰ in WFGD system, and the major Hg speciation in exhaust flue gas was Hg⁰. Most of the Hg absorbed in WFGD was transferred into desulfurization gypsum, while the enrichment in both desulfurization waste water and bottom ash was limited.