Abstract
Gypsum from the wet flue gas desulfurization system of the lignite fired thermal power plant Sostanj, Slovenia, can efficiently retain mercury (Hg), of which most is contained in finer gypsum fractions, with concentrations above 10 kg-1. The aim of this work was to identify and study the temperature stability of Hg species in gypsum by a temperature fractionation (TF) method. A self-constructed apparatus was used that consisted of an electrical furnace for controlled heating up to 700°C, with a heating rate of 2.2°C·min-1, and an AAS detector with Zeeman background correction. The pattern of Hg release during temperature increase depends highly on the matrix/substrate in which it is contained. Based on spiking gypsum with known Hg compounds we concluded that the largest proportion of Hg in gypsum belongs to Hg-Cl and Hg-Br compounds appearing at 160°C to 200°C, followed by smaller amounts of HgO, HgS and Hg sulfates appearing at 300°C and 450°C. Further development of methodology for identifying Hg species would require identification of the decomposition fragments of Hg and other compounds, complemented by a better understanding of Hg reactivity at higher temperatures.
Highlights
Mercury (Hg) is a global pollutant, the main source in the atmosphere being the combustion of fossil fuels in particulate coal burning and other high temperature industrial processes [1]
This section is organized in two parts, in which the first discusses the results of multi-element analysis in gypsum samples from the Thermal Power Plant (TPP) Šoštanj and the second part the results of thermal fractionation analysis
The results of the latter are divided into two sub-sections the first of which contains results presented as thermograms of different TPP gypsum samples from TPP Šoštanj (FF, FF1, FF2, CF) (Figure 3) and the second, results in thermograms of TPP gypsum samples spiked with pure Hg compounds mixed with CaSO4∙2H2O (Figures 4-7)
Summary
Mercury (Hg) is a global pollutant, the main source in the atmosphere being the combustion of fossil fuels in particulate coal burning and other high temperature industrial processes [1]. Flue gas released from coal combustion contains elemental Hg (Hg(0)), oxidized Hg(II) and Hg bound to particles, Hg(P) [3] [4]. Hg(P) could be retained in electrostatic precipitators (ESP) or baghouses, while Hg(0) and Hg(II) remain predominantly in the gaseous phase of the flue gas [4]. Hg could be oxidized in the gas phase on particles (e.g. active carbon) over a selective catalytic reduction unit (SCR) [5], in interaction with different gases or in a wet flue gas desulfurization (WFGD) system. Oxidized Hg is much more soluble in water than Hg(0) and could be effectively removed in wet scrubbers
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