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

Abstract

An extensive examination of combustion gases containing trace amounts of mercury shows unambiguously mercury's propensity for heterogeneous chemistry. Although additional mechanisms for the oxidation chemistry of mercury have been implied by the continuing inadequacy of modeling attempts, details of the specific chemistry have remained unknown. Now it is shown that mercury can efficiently chemi-deposit onto surfaces encountered in practical combustors. If sulfur is present, condensed mercuric sulfate forms momentarily. This is then converted by gaseous HCl to HgCl2 that may sublime into the flow or be retained. This elusive and efficient noncatalytic mechanism most likely explains the observed fractional conversions to the dichloride observed in coal combustors. A receptive surface acts solely as an intermediary, facilitating the conversion while disguising its role. Without sulfur, a corresponding mechanism occurs but via HgO that is similarly converted to the dihalide. Such heterogeneous dynamics have significant repercussions for both full-scale combustors and bench-type experiments, which data have been reassessed and reviewed. Conclusions imply that observations concerning mercury will be system dependent and no two combustors can be exactly alike. The re-examination of prior work provides significant support for these conclusions. This fundamental understanding now lays a foundation for meaningful interpretations and program planning. It has indicated also the extreme care needed in sampling and monitoring the speciation of mercury in such combustion flows for reliable results. It now points to a simple low-cost surface-induced mitigation method for effectively converting the mercury in flue gases to the water-soluble dichloride. It is in essence no more than an optimization of the natural process that is currently occurring in combustors but to only limited degrees.

Mercury emissions of a coal-fired power plant in Germany

“…Mercury and especially methyl mercury, which bioaccumulates in the aquatic nutritional chain, are harmful to humans and animals (e.g., Mergler et al, 2007;Scheuhammer et al, 2007;Selin, 2009;and references therein). Therefore, Hg emissions are on the priority list of several international agreements and conventions dealing with environmental protection and human health, including the United Nations Environment Program (UNEP) Minamata convention on mercury (www.mercuryconvention.org).…”

“…That is because of varying composition of coal burned, complex chemistry in the stack gases (e.g., Lohman et al, 2006;Schofield, 2008;Tatum Ernest et al, 2014), and the large number of different methods used to clean CFPP flue gases, with very different percentages of GOM to total mercury, ranging from less than 10 up to 90 % (Wang et al, 2010;Schütze et al, 2012Schütze et al, , 2015, and references therein). Analytical problems also contribute to the uncertainty: the current emission monitoring systems are not sensitive enough to measure and speciate low mercury concentrations in flue gases of modern CFPPs (Mayer et al, 2014).…”

Oxidation Catalysts for Elemental Mercury in Flue Gases—A Review

“…Platinum Pt [8,37], palladium Pd [29,37], ruthenium oxide RuO 2 [36], and iridium Ir [38] have been tested on laboratory and pilot scale in order to determine their potential as mercury oxidation catalysts.…”

“…Here, the gas phase oxidation is kinetically limited. A selective heterogeneous catalyst, upstream of the scrubber, is needed to increase the speed of mercury oxidation reaction in the cooled flue gases [8].…”

A review of elemental mercury removal processing

“…Mercury emissions have attracted an increasing amount of concern due to the high toxicity, volatility, bioaccumulation in the environment and the neurological effects of mercury. According to the Global Mercury Assessment Report, coal-fired power plants are the primary source of anthropogenic emissions of mercury into the atmosphere [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”