The Fate and Behavior of Mercury in Coal-Fired Power Plants

Abstract

For the past 22 years in the Netherlands, the behavior of Hg in coal-fired power plants has been studied extensively. Coal from all over the world is fired in Dutch power stations. First, the Hg concentrations in these coals were measured. Second, the fate of the Hg during combustion was established by performing mass balance studies. On average, 43 +/- 30% of the Hg was present in the flue gases downstream of the electrostatic precipitator (ESP; dust collector). In individual cases, this figure can vary between 1 and 100%. Important parameters are the Cl content of the fuel and the flue gas temperature in the ESP. On average, 54 +/- 24% of the gaseous Hg was removed in the wet flue-gas desulfurization (FGD) systems, which are present at all Dutch coal-power stations. In individual cases, this removal can vary between 8% (outlier) and 72%. On average, the fate of Hg entering the power station in the coal was as follows: <1% in the bottom ash, 49% in the pulverized fuel ash (ash collected in the ESP), 16.6% in the FGD gypsum, 9% in the sludge of the wastewater treatment plant, 0.04% in the effluent of the wastewater treatment plant, 0.07% in fly dust (leaving the stack), and 25% as gaseous Hg in the flue gases and emitted into the air. The distribution of Hg over the streams leaving the FGD depends strongly on the installation. On average, 75% of the Hg was removed, and the final concentration of Hg in the emitted flue gases of the Dutch power stations was only -3 microg/m3(STP) at 6% O2. During co-combustion with biomass, the removal of Hg was similar to that during 100% coal firing. Speciation of Hg is a very important factor. An oxidized form (HgCl2) favors a high degree of removal. The conversion from Hg0 to HgCl2 is positively correlated with the Cl content of the fuel. A catalytic DENOX (SCR) favors the formation of oxidized Hg, and, in combination with a wet FGD, the total removal can be as high as 90%.

The influence of carbon particle type in fly ashes on mercury adsorption

“…It has been observed that the sub-products originated in FGD (flue gas desulphurization) systems and fly ashes capture mercury in different proportions depending on their characteristics and on the process conditions. In fact, power stations equipped with SCR DeNOx and FGD systems are able to capture significant quantities of mercury as a consequence of Hg oxidation in SCR DeNOx that may be subsequently captured in FGD systems [6] The retention of hazardous elements by fly ashes produced in combustion plants has been extensively studied in recent years. In the case of mercury it has been observed that some fly ashes may capture this element which would otherwise be emitted to the atmosphere.…”

Characteristics of Mercury Emissions from a Coal-fired Power Plant

“…After the burning of raw coal at the high temperature in combustion zone of boilers, the Hg in coal releases into the exhaust gas as elemental mercury (Hg 0 ) vapor. With the existence of Cl, Br,SO 2 and particles in flu gas, part of the Hg 0 is oxidized into Hg 2+ either by gas phase oxidation or catalytic oxidation due to thermo-chemical processes [16][17]. Less than 1% Hg was removed by the bottom ash, and about 41.82-53.4% Hg was adsorbed by the fly ash and removed by the dust collector, and about 17.26-39.96% Hg was removed by Wet-FGD, the remaining mercury emitted to the atmosphere.…”

Study of boron behaviour in two spanish coal combustion power plants

“…in coal) (% 131 ash content in coal / 100). Based on this RE factor, Meij grouped trace elements into three 132 classes denoted as: Class I, II and III (Meij et al, 2002;Meij et al, 2007). Class I elements 133 are defined as elements that do not vaporise during combustion and their RE factor is around 134 1.0.…”

“…Boron partitioning among ashes was comparatively evaluated by using its relative enrichment 128 (REs) factors as defined by Meij (Meij et al, 1983(Meij et al, , 2002Meij, 1994). The term "relative 129 enrichment" was adopted in order to describe the behaviour of a particular trace element 130 during coal combustion.…”