Investigation of the Behavior of Mercury Compounds in Coal Combustion Products

Abstract

The main mechanisms of transformation of mercury compounds in coal combustion products in the region of high temperatures have been analyzed. A kinetic model of the process of gas-phase oxidation of metal mercury vapors is proposed. The features of the behavior of the investigated compounds in systems of cleaning combustion products from harmful impurities have been considered.Mercury compounds are among the most toxic and long-lived microimpurities in the atmospheric air. Native sources of mercury emitted into the atmosphere are water surfaces, soils, plants, forest fires, volcanos, etc. and, according to the estimates of different authors, the total emission of mercury is about 3000 tons/year [1]. Among the anthropogenic sources of mercury compounds are industries, heat power engineering, and incineration of rubbish. The anthropogenic emission of mercury in Europe is about 600 tons/year. In the countries of Eastern Europe (including the European part of Russia), the release of mercury compounds into the atmosphere is mainly due to the operation of solid-fuel thermal power plants [2].Unlike the majority of metals forming small impurities in solid fuel, mercury is an extremely volatile element. Therefore, mercury compounds are present in solid-fuel combustion products before their release into the atmosphere practically completely in the gas phase. The further behavior of these toxic components of flue gases is determined by the processes of their photochemical oxidation in cloud drops and precipitation on the earth's surface near the emission source [3].The dominant forms of mercury in solid-fuel combustion products are vapors of metal mercury Hg 0 , as well as compounds of bivalent mercury Hg(II): HgCl 2 (g) and HgO(g, s) [4]. As the results of thermodynamic calculations show, the redistribution of mercury between different compounds varies over wide ranges and depends on the regime parameters of the process (initial fuel composition, temperature, pressure, etc.) [5,6].Because of the poor solubility of metal mercury vapors in water, removal of Hg 0 from combustion products is only possible upon its chemical conversion into readily soluble compounds of Hg(II). In the present paper, the main mechanisms of Hg 0 -Hg(II) conversion have been analyzed, a kinetic model of the process is proposed, and the features of the behavior of mercury compounds in systems of removing harmful impurities from solid-fuel combustion products have been considered.Thermodynamic Analysis of the Process. The content of mercury in power-generating coals varies from 0.01 to 1 mg/kg [7]. In the process of fuel combustion, mercury goes to the gas phase in the form Hg 0 . At an average content of this impurity in the initial fuel of 0.1 mg/kg, the Hg 0 concentration in combustion products is about 15 µg ⁄ nm 3 .The processes of further conversion of Hg 0 are rather complicated and consist of homogeneous and heterogeneous reactions of the gaseous components containing Hg and Cl atoms [8]. Among the methods of mathematical modeling of such proc...

Survey of Catalysts for Oxidation of Mercury in Flue Gas

“…Regardless of the dominant mechanism for mercury oxidation, it is well-known that the transformation from Hg 0 to Hg 2+ in flue gas is kinetically limited. For temperatures below ∼450 °C, at equilibrium, nearly all mercury should exist as Hg 2+ ( , ). Due to the excess of chlorine-containing species (HCl and Cl 2 ), HgCl 2 is assumed to be the dominant form of Hg 2+ .…”

“…As the flue gas cools, some of the Hg 0 is oxidized, presumably to HgCl 2 because of the large excess of chlorine present in coal. The extent of mercury oxidation depends upon a number of factors, including combustion characteristics, coal composition (including chlorine content) ( − ), concentrations of other species (i.e., NO x and SO 2 ) ( , ) in the flue gas, and the time−temperature history ( , ). Both Hg 0 and Hg 2+ can enter the particulate phase by adsorption onto fly ash particles ( − ).…”

Fuel-Mercury Combustion Emissions: An Important Heterogeneous Mechanism and an Overall Review of its Implications

“…In the same study, the presence of oxygen is shown to modify this by interacting with the chlorine and reducing the dominance of the dichloride to 1200 K. Such calculations also are very dependent on an accurate and complete database for any species that may be relevant. For example, previous calculations that still incorporate a strong HgO(g) bond strength value based on the JANAF source (268 kJ mol -1 ) (14) or have elements present that have a significant affinity for chlorine or bromine, will erode the thermodynamically expected dominant role for the gaseous dihalides (36)(37)(38). However, as will be seen, this is quite irrelevant and very misleading as the formation in the gas phase of such dihalides is kinetically constrained.…”

Mercury in Bituminous Coal Used in Polish Power Plants

“…Industrial research results confirm that mercury emission to the atmosphere depend on several factors (Hall et al, 1991;Galbreath & Zygarlicke, 2000;Gerasimov, 2005;Zhang et al, 2008). These include: mercury content and chemical composition of combusted coal, type of boiler, mercury speciation of flue gases leaving boiler, types and efficiency of exhaust gas purification processes and presence of specified components in fly ashes and flue gases.…”