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
Mercury (Hg) is a toxic trace element which is emitted mostly in gas phase during coal combustion, although some Hg compounds may be retained in the fly ashes depending on the characteristics of the ashes and process conditions. To improve the retention of Hg in the fly ashes a good knowledge of the capture mechanism and Hg species present in the fly ashes is essential. The temperature programmed decomposition technique was chosen
Temperature programmed decomposition was used to identify mercury (Hg) species in gypsum samples produced from flue gas desulfurization in two Spanish power stations (A and B). As stricter emission control/reduction policies, particularly those focusing on Hg, are being implemented, wet flue gas desulfurization (FGD) technologies used for the removal of SO 2 can result in the co-removal of highly-soluble oxidized Hg. The amount of Hg retained in FGD products may increase in the future if these units are optimized for co-capture. For this reason, it is important to identify the mercury species in FGD products not only to determine the potential risk when the wastes are finally disposed of, but also to understand the behaviour of mercury during combustion and therefore to improve the technologies for mercury removal. Different mercury species were identified in the gypsum samples. In power station A, Hg-S were the most probable Hg species, whereas in power station B the main compound was Hg halogenated compounds.
Fly ashes and gypsum are one of the main wastes produced in coal-fired power stations which may be sent to landfills for their disposal. In this work, leaching and speciation of mercury in fly ashes and gypsum from a modern co-combustion power plant equipped with a selective catalytic reduction (SCR) unit in the Netherlands were studied. The mercury leachable contents were checked against different regulations, including Dutch, German and the Council Directive 2003/33/EC. The speciation of mercury in coal combustion products is essential not only to determine the risk when the wastes are finally disposed but also to understand the behaviour of mercury during combustion and therefore to select the appropriate mercury removal technology. A temperature programmed decomposition technique was used in order to identify and quantify which mercury species are associated with coal combustion products. The main mercury species identified in fly ash samples was mercury sulphate, whereas in the gypsum sample the mercury present was mercury chloride. The quantitative mercury results carried out using the thermal desorption method may be considered accurate. The results obtained show that fly ash and gypsum samples from this power plant can be acceptable at landfills as a non-hazardous waste.
An unusual and different speciation of Hg in the outgoing gaseous stream of the flue gas desulfurization (OUT-FGD) system was revealed at two Spanish power plants (PP1 and PP2) equipped with a forced oxidation wet FGD system with water recirculation to the scrubber. At PP1 and PP2, a high proportion of Hg escapes from the electrostatic precipitator in gaseous form, Hg(2+) (75-86%) being the species that enters the FGD. At PP1 Hg(0) (71%) was the prevalent Hg OUT-FGD species, whereas at PP2 Hg(2+) was the prevalent Hg OUT-FGD species in 2007 (66%) and 2008 (87%). The unusual speciation of gaseous Hg OUT-FGD and the different Hg retentions between 2007 and 2008 at PP2 were attributable to the evaporation of HgCl(2) particles from the aqueous phase of gypsum slurry in the OUT-FGD gas and the Al additive used at PP2, respectively. The Al additive induced the retention of Hg as HgS in the 2007 FGD gypsum, thus reducing gaseous emissions of Hg in the OUT-FGD gas.
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