Adsorption of Elemental Mercury on the Residual Carbon in Coal Fly Ash

Abstract

The injection of large quantities of pulverized activated carbon is one method used to remove elemental mercury (Hg 0 ) from flue gas streams. The purpose of this project was to determine whether the unburned carbon that remains in coal fly ash could be used as an inexpensive and effective replacement for activated carbon. Bench-scale tests were conducted at conditions representative of those found in the flue gas trains of coal-fired power plants and municipal waste incinerators. The temperatures and concentrations ranged from 121 to 177 °C and from 0.019 to 11.7 mg of Hg/m 3 . Two types of data were obtained: equilibrium data suitable for obtaining adsorption isotherms and breakthrough data suitable for obtaining adsorption kinetics. Adsorbed-phase concentrations were as high as 600 ppm. Forward adsorption rate constants were ≈0.06-2.3 m 3 /g/s for particle sizes and carbon contents ranging from 59 to 206 µm and from 2% to 36%. Mathematical models were developed to simulate the capture of Hg 0 in flue gas ducts and in baghouses. The results of the simulations indicate that a negligible amount of Hg 0 can be adsorbed by a dilute suspension of fly ash. The best option for controlling Hg 0 emissions using fly ash appears to be injection in pulses prior to a baghouse.

Mass Transfer within Electrostatic Precipitators: Trace Gas Adsorption by Sorbent-Covered Plate Electrodes

“…9 A common presumption is that the plate electrodes inside an ESP, when coated with fly ash and possibly PAC injected upstream, provide large surface areas for either adsorbing highly condensable Hg 2ϩ or catalyzing the oxidation of Hg 0 to Hg 2ϩ . Various studies have documented a range of fly ash sorption capacities [11][12][13] and the influence of sulfur and chlorine compounds on Hg uptake by activated carbons and fly ash. 14 -17 Despite the advances made in understanding the reaction mechanisms between Hg, other flue gas components, and fly ash, substantially less progress has been made in understanding the mass transfer that would be necessary within an ESP to achieve the removal efficiencies observed in practice.…”

A review of elemental mercury removal processing

“…Mercury has been found to be concentrated in the C-rich fraction of fly ash [53][54][55]. Bituminous coal-ash produced under CESP conditions exhibits a consistent trend of increasing Hg removal with the unburned C content [56]. This trend suggests that a relationship exists between C in ash and Hg removal for plants burning bituminous coal in pulverized-coal-fired boilers with CESP.…”

“…Heterogeneous reactions involve adsorption and desorption, leading to the adsorption of gas-phase Hg onto solid particles (Hg p ) and the formation of Hg those boilers [56,61]. Data from full-scale combustion systems show that Hg removal for ESPs in pulverized-coal-fired boilers burning bituminous coal appears to be related to the LOI or unburned C in the fly ash.…”

“…These conclusions cannot be extended to fly ash from subbituminous coal and lignite, however. There is evidence in the literature [56,59,61] that the surface area of C from Class C (subbituminous) fly ash samples is significantly higher than that of Class F fly-ash samples on a per gram of C basis.…”

Adsorption of elemental mercury vapors from synthetic exhaust combustion gas onto HGR carbon

“…Among the anthropogenic sources, the coal-fired utility boilers (Uddin et al, 2008;Granite et al, 2000), the combustion of municipal solid wastes (MSW) (Serre and Silcox, 2000;Jurng et al, 2002), and the cement production processes (Zheng et al, 2012a;Zheng et al, 2012b;Prisciandaro et al, 2003) are the main sources of mercury emissions. According to Pacyna et al (2006) and Takaoka et al (2011), mercury emissions in MSW incinerations range from 200 to 1000 μg Hg/N-m 3 , while they vary between 5 and 20 μg Hg/N-m 3 in coal combustion plants (Zheng et al, 2012a).…”