Mercury and selenium retention in fly ashes: Influence of unburned particle content

Mercury and selenium are present as trace elements in coal and may be emitted to the environment in gas phase during coal conversion processes or be partially retained on the fly ashes. The present work explores the possibility that selenium may contribute to mercury capture in fly ashes in two different situations: firstly the power station itself, in order to evaluate the influence of typical working conditions, and secondly in a fixed bed of fly ashes enriched with Se, in order to study the capture of mercury in more severe conditions. It was found that the presence of selenium in fly ashes may improve their capacity to capture mercury. However, in the four fly ashes of different origin studied, selenium is not the most important component for mercury retention. In fact, the presence of selenium in fly ash samples enriched in unburned carbon does not have any significant effect on mercury retention.     

Analysis of mercury species present during coal combustion by thermal desorption

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 (Hg⁰) and oxidized (Hg²⁺, primarily as HgCl₂). Because Hg²⁺ is more condensable and far more water soluble than Hg⁰, 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 HgCl₂ < HgS < HgO < HgSO₄. 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.     

Provenance and Technology Investigation of Agia Sophia Bricks, Istanbul, Turkey

Bricks from the Agia Sophia in Istanbul, Turkey, were investigated to better formulate a plan for restoration. According to an ancient text, these bricks are extremely light and were manufactured on the island of Rhodes. Results of neutron activation analysis, grouped by multivariate statistics, show that the brick samples do not correlate with clays thought to be local and used in contemporary churches in Istanbul. The bricks are similar to those used in the Great Basilica of Rhodes. However, the Agia Sophia bricks are lighter (45% porosity) than the bricks from the other churches (35% porosity). The Agia Sophia bricks are made of a noncalcareous, fine paste with quartz temper, fired at low temperature (∼750°C). The exceptional characteristic is their homogeneity and small pore-size distribution (0.3–0.8 μm). The tensile strength of the dome bricks is unexpectedly high (up to 1.3 MPa), even though the bricks have high porosity.