Development of a Mercury Transformation Model in Coal Combustion Flue Gas

Abstract

A bench-scale entrained-flow reactor was used to extract flue gas produced by burning a subbituminous Belle Ayr coal in a 580-MJ/h combustion system. The reactor was operated at 400 degrees, 275 degrees, and 150 degrees C with a flow rate corresponding to residence times of 0-7 s. Transformations of elemental mercury (Hg0) and total gas mercury (Hg(gas)) in the reactor were evaluated as functions of temperature and residence time. The most significant mercury transformations (Hg0 to Hg(p) and Hg0 to Hg2+) occurred at 150 degrees C, while virtually no obvious mercury transformations were observed at 275 degrees and 400 degrees C. Approximately 30% of total mercury has been oxidized at temperatures higher than 400 degrees C. A mass transfer-capacity limit model was developed to quantify in-flight mercury sorption on fly ash in flue gas at different temperatures. A more sophisticated model was developed to demonstrate not only the temperature and residence time effects but also to consider the effective surface area of fly ash and dependence of mercury vapor concentration on mercury transformations in flue gas. The reaction orders were 0.02 and 0.55 for Hg0 and Hg(gas), respectively. Only a few percent of the total surface area of the fly ash, in the range of 1%-3%, can effectively adsorb mercury vapor.

Transformations of mercury in processes of solid fuel combustion – review

“…Basic parameters which affect chemical and physical speciation of mercury are the process temperature, the content of chlorine, sulphur, iron, and calcium in coals as well as the concentration of HCl and ammonia in fl ue gas (Sliger et al 2000, Pavlish et al 2003, Zhuang et al 2004, Zhao et al 2006. It is estimated that more than 85% of the mercury in fl ue gas from combustion of coal with low-chlorine content (Cl < 150 ppm) represents elemental mercury.…”

“…It is only after the fl ue gas is cooled down to a temperature below 600°C that elemental mercury vapours react with other fl ue gas components, consequently leading to the formation of oxidized forms (Hg +2 ) and Hg p forms bound with fl y ash , Pavlish et al 2003, Zhuang et al 2004. The mechanism of the mercury oxidation in the presence of chlorine has not yet been unambiguously determined, but numerous studies show both homo-and heterogeneous mechanisms of the reaction between mercury and other fl ue gas components (Niksa et al 2001, Zhao et al 2006.…”

“…R828168, 2004), a direct reaction between Hg and HCl, leading to the formation of HgCl, is impossible (Fuente-Cuesta et al 2012). The mechanisms of HgCl 2 formation via homogeneous reactions of Hg 0 and atomic chlorine are suggested to proceed according to the following equations (Niksa et al 2001, Zhuang et al 2004, Zhao et al 2006:…”

Biomarkers of Freshwater Algae Lemna minor as a Model for Urban Pollution with Pesticides and Heavy Metals

“…Fuels in factories is the major source of Hg, As, Cr, and Se (Zhuang et al, 2004;Keegan et al, 2006;Guijian et al, 2007), while combustion of oil is the most important source of Ni and Vanadium (V) (US EPA, 2002c;Dundar, 2006). Therefore, algae play an important role in the disposal, chemical transformation and bioaccumulation of many toxic compounds (Wang et al, 1998;Okay et al, 2000;Todd et al, 2002;Lei et al, 2002Lei et al, , 2007Bopp and Lettieri, 2007).…”

Mercury Capture on Fly Ash and Sorbents: The Effects of Coal Properties and Combustion Conditions

“…No heterogeneous reaction mechanisms were considered in this work. Zhuang et al (2004) have developed a mass transfer-capacity limit model to quantify in-flight mercury sorption on fly ash at various temperatures. This model was further expanded in order to demonstrate not only temperature and residence time effects but also to consider the surface area of fly ash and the concentration of mercury in the flue gas stream.…”