Analysis of mercury species present during coal combustion by thermal desorption

Abstract

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg 0) and oxidized (Hg 2+ , primarily as HgCl 2). Because Hg 2+ is more condensable and far more water-soluble than Hg 0 , the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl 2 <HgS<HgO<HgSO 4 .Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.

Speciation of mercury in fly ashes by temperature programmed decomposition

“…The Hg compounds present in the sample were carried through the oven tube in a stream of argon, at a flow rate of 250 ml min -1 . The heating rate from room temperature to 650°C was nominally 10 o C min -1 [22,[29][30]. A number of pure Hg compounds (HgCl 2 , HgS, HgSO 4 , HgO) were first tested in order to determine their specific profiles so as to have a set of "fingerprints" which could serve as a standard of comparison for the profiles obtained from the fly ashes.…”

“…In line with previous publications on temperature programmed decomposition [22,[29][30], the aim of the present work is to develop an experimental method for studying the thermally induced behaviour of the main Hg species in fly ashes. The fly ashes were used as Hg sorbents in different flue gas compositions.…”

“…As in previous studies carried out by Lopez-Anton et al [22], a series of Hg standards was used to determine the specific thermograms of each species. In this study, in order to simulate the real behaviour of Hg in fly ashes, the pure Hg compounds were mixed in a matrix of mercury-free fly ash.…”

“…In this study, in order to simulate the real behaviour of Hg in fly ashes, the pure Hg compounds were mixed in a matrix of mercury-free fly ash. Since certain constituents of the fly ash might affect the temperature of decomposition of the Hg species, the Hg standards were mixed with fly ash instead of with silica flour [22].…”

“…The thermal desorption method for Hg species takes advantage of the fact that the species can be arranged in increasing order (e.g. HgCl 2 <HgS<HgO<HgSO 4 ) according to the temperature of desorption [22]. Thermal decomposition has been used to identify Hg compounds in soil contaminates, sediment samples, iron-based sorbents [23][24][25], and even in mercury lamp wastes [26].…”