Mercury chemistry on brominated activated carbon

doi:10.1016/j.fuel.2012.04.036

Fuel

Volume 99, September 2012, Pages 188-196

https://doi.org/10.1016/j.fuel.2012.04.036 Get rights and content

Abstract

Activated carbon-based sorbents are the most widely tested sorbents for mercury removal in coal-fired power plants. A major problem in mercury removal is the limited understanding of the mechanism associated with elemental mercury (Hg⁰) oxidation and its subsequent adsorption. This work investigates the possible binding mechanism of Hg⁰ onto brominated fiber and powder activated carbon sorbents through packed-bed experiments in a stream of air. To better understand the mechanisms involved, a combination of spectroscopy and quantum mechanical modeling were used to characterize the sorption process. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to analyze the surface and bulk chemical compositions of brominated activated carbon sorbents reacted with Hg⁰. It was found that Hg⁰ is oxidized at the brominated carbon surfaces at both 30 °C and 140 °C. The oxidation state of adsorbed Hg is found to be Hg²⁺, and coordinated to two Br atoms with no detectable bonding between Hg and O. Though plane-wave density functional theory (DFT) and density of states (DOSs) calculations indicate that Hg is more stable when it is bound to the edge C atom interacting with a single Br bound atop of Hg, a model that includes an interaction between the Hg and an additional Br atom matches best with experimental data obtained from EXAFS spectroscopy. Because the most stable structures optimized in the DFT simulations were not found on the samples analyzed using EXAFS spectroscopy, Hg surface reactions on the carbon surface are thought to be kinetically controlled.

Highlights

► Hg is adsorbed on brominated AC as Hg²⁺ at both 30 °C and 140 °C. ► Chemisorption is the likely adsorption mechanism of Hg. ► Hg interacts with two Br atoms at a distance of 2.55 ± 0.01 Å inside the C matrix. ► Hg s- and Hg p-states hybridize with Br and C p-states to form stable complexes.

Introduction

The US is putting significant effort into decreasing harmful emissions into the environment through regulations such as the EPA’s Clean Air Interstate Rule (CAIR) and the Clean Air Mercury Rule (CAMR), which place caps on NO_x, SO_x, and Hg emissions [1]. Recently, on March 2011 the EPA stated that all hazardous air pollutants must have emission standards and proposed that for existing sources in the category, that the standards are at least as stringent as the emission reductions achieved by the average of the top 12% best controlled sources for source categories with 30 or more sources. With this new rule, a reduction of mercury from coal emissions of approximately 90% is anticipated [1]. Recent studies indicate that mercury content in coal varies between 0.01 and 1.5 g per ton of coal, with world coal consumption in 2006 estimated at 6118 million tons per year [2]. In 1999, the EPA estimated that US coal combustion emits approximately 50 tons Hg/year into the air [1], while global emissions are approximately 810 tons Hg/year [2]. The need for effective sorbent materials to capture harmful pollutants of flue gases continues to increase as coal consumption increases worldwide.

Typically, activated carbon injection is used to capture oxidized mercury [3]. A major limitation with using activated carbon is that in flue gases with low halogen concentrations a large amount of activated carbon needs to be added to the system to effectively control Hg⁰. Depending on the system conditions, an activated carbon-to-mercury mass ratio of at least 3000–20,000 (C/Hg) can be necessary to achieve 90% Hg removal [4]. Huggins et al. [5] tested various activated carbon (AC) sorbents in a simulated flue gas containing gaseous Hg⁰ and characterized them using X-ray absorption fine structure (XAFS) spectroscopy. They observed that Hg sorption is dominated by anionic species, such as sulfides, chlorides, oxides and iodides present within the matrix of the sorbent or bound to the sorbent surface. The results of the XAFS spectra obtained from mercury adsorbed on these tested AC samples indicate that little or no Hg⁰ was present and that Hg-anion chemical bonds are formed on the sorbent materials, suggesting chemisorption to be the primary adsorption mechanism. From these results, they infer that an oxidation process is involved in the ultimate capture of Hg⁰ by bonding to iodine (I), chlorine (Cl), sulfur (S) or oxygen (O) anionic species on the carbon surface. Hutson et al. [6] characterized chlorinated AC and brominated AC using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) after exposing the carbons to Hg⁰-laden flue gas. Due to the low coverage of Hg on the carbon, they could not determine the speciation of Hg. Since homogenous oxidation was not observed in their experiments, they proposed that Hg capture by chlorinated and brominated carbons takes place by surface oxidation of Hg⁰ with subsequent adsorption on the surface [6]. They concluded that Hg was expected to bind on the chlorinated and brominated sites of carbon and even in some cases mercury-sulfate species were expected to form due to the presence of sulfur dioxide (SO₂) in the flue gas. Olson et al. [7] suggested that acidic sites on the surface are responsible for Hg⁰ capture on AC. They propose that Hg⁰ has the propensity to be oxidized by donating its electrons to a surface or another gas-phase molecule; therefore, in its elemental state, Hg⁰ acts as a Lewis base with the desire to interact with an acidic site forming a strong C–Hg covalent bond on the carbon surface. However, in their work they were not able to determine the oxidation states of the various surface-bound Hg due to the interference with silicon (Si), which was inherently present in the form of ash in their coal-derived AC sorbents [8].

The speciation of Hg adsorbed on carbon surfaces has yet to be elucidated, and the adsorption mechanism and the effects of flue gas components on Hg adsorption are not well understood. This current work represents a first step in determining the surface chemistry of Hg interacting with powdered AC in the presence of air at 140 °C through surface and bulk characterization using XPS and EXAFS. Additional Hg adsorption experiments have been performed at 30 °C to determine the surface-bound Hg species on powdered AC using XPS. The plane-wave density functional theory calculations (DFT) are carried out to identify the interaction of Hg with Br bound graphene edge sites. This work has been carried out in an attempt to simplify the chemistry of Hg⁰ on AC surfaces and thereby provide a foundation for understanding more complex systems.

Section snippets

Sorbent preparation

Mercury-adsorbed to brominated AC samples were prepared to identify the Hg oxidation state, bond distance, binding ligands, and number of ligands (coordination) using both XPS and EXAFS spectroscopy. To understand Hg adsorption in an air environment, a packed-bed reactor system was constructed (Fig. 1) to prepare samples for spectroscopic analysis. Sorbent particles were placed in the 1.27 cm-diameter quartz packed-bed reactor. The reactor was covered with a 50 cm-long ceramic fiber heater to

Results and discussion

Preliminary tests were conducted to assess the performance of the packed-bed reactor system as well as to determine the experimental conditions necessary for preparing sorbent samples for characterization to determine Hg⁰ oxidation and binding mechanisms. The most efficient configuration for Hg capture involved mixing the carbon sorbent with sand (e.g. 30 mg AC/6 g sand) and supporting this mixture with quartz wool. However, both sand and quartz wool contain Si, an element whose main XPS lines

Conclusion

The speciation of Hg adsorbed on brominated AC sorbents has been investigated in the presence of air. Both XPS and EXAFS experimental characterization results reveal that Hg is adsorbed on brominated AC as Hg²⁺ at both 30 °C and 140 °C, indicating that chemisorption is the likely adsorption mechanism of Hg. Fitting of the EXAFS spectra shows that Hg interacts with two Br atoms at a distance of 2.55 and 2.58 ± 0.01 Å inside the C matrix. Plane-wave DFT calculations reveal that Hg s- and Hg p-states

Acknowledgments

The authors acknowledge the financial support provided by Electric Power Research Institute (EPRI), along with valuable discussions with Ramsay Chang. The computations were carried out on the Center for Computational Earth & Environmental Science (CEES) cluster at Stanford University. We wish to thank Matthew Latimer of the Stanford Synchrotron Radiation Lightsource for his assistance with data collection on beamline 7-3. D.A.B. gratefully acknowledges support by the Stanford Graduate

References (38)

J.C. Hower et al.
Mercury capture by native fly ash carbons in coal-fired power plants
Prog Energy Combust Sci
(2010)
F.E. Huggins et al.
XAFS characterization of mercury captured from combustion gases on sorbents at low temperatures
Fuel Process Technol
(2003)
E.S. Olson et al.
New developments in the theory and modeling of mercury oxidation and binding on activated carbons in flue gas
Fuel Process Technol
(2009)
J.D. Laumb et al.
X-ray photoelectron spectroscopy analysis of mercury sorbent surface chemistry
Fuel Process Technol
(2004)
P.E. Blöchl
Projector augmented-wave method
Phys Rev B
(1994)
G. Kresse et al.
From ultrasoft pseudopotentials to the projector augmented-wave method
Phys Rev B
(1999)
L.R. Radovic et al.
On the chemical nature of graphene edges: origin of stability and potential for magnetism in carbon materials
J Am Chem Soc
(2005)
F.H. Yang et al.
Ab initio molecular orbital study of adsorption of atomic hydrogen on graphite: Insight into hydrogen storage in carbon nanotubes
Carbon
(2002)
A. Montoya et al.
Spin contamination in hartree-fock and density functional theory wavefunctions in modeling of adsorption on graphite
J Phys Chem A
(2000)
E. Papirer et al.
XPS study of the halogenation of carbon black – Part 1. Bromination
Carbon
(1994)
G.S. Duesberg et al.
Modification of single-walled carbon nanotubes by hydrothermal treatment
Chem Mater
(2003)

V.L. Budarin et al.

Chemical reactions of double bonds in activated carbon: microwave and bromination methods

Chem Commun

(2004)

US environmental protection agency. Clean air mercury rule. <http://www.epa.gov/airquality/powerplanttoxics/>; 2011...

N. Pirrone et al.

Global mercury emissions to the atmosphere from anthropogenic and natural sources

Atmos Chem Phys Discuss

(2010)

S. Chen et al.

Mercury removal from combustion flue gas by activated carbon injection: mass transfer

Pap – Am Chem Soc, Div Fuel Chem

(1996)

N.D. Hutson et al.

XAS and XPS characterization of mercury binding on brominated activated carbon

Environ Sci Technol

(2007)

Rostam-Abadi M, Lu Y, Chang R, Shaban C, Botha F. Production of activated carbon for mercury emission control:...

J. Wilcox et al.

Heterogeneous mercury reaction chemistry on activated carbon

J Air Waste Manage Assoc

(2011)

PHI multipak softwareTM. Japan: Ulvac-phi Inc.;...

A.D. Jew et al.

New technique for quantification of elemental hg in mine wastes and its implications for mercury evasion into the atmosphere

Environ Sci Technol

(2011)

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Mercury chemistry on brominated activated carbon

Abstract

Highlights

Introduction

Section snippets

Sorbent preparation

Results and discussion

Conclusion

Acknowledgments

Prog Energy Combust Sci

Fuel Process Technol

Fuel Process Technol

Fuel Process Technol

Phys Rev B

Phys Rev B

J Am Chem Soc

Carbon

J Phys Chem A

Carbon

Chem Mater

Chem Commun

Global mercury emissions to the atmosphere from anthropogenic and natural sources

Atmos Chem Phys Discuss

Mercury removal from combustion flue gas by activated carbon injection: mass transfer

Pap – Am Chem Soc, Div Fuel Chem

XAS and XPS characterization of mercury binding on brominated activated carbon

Environ Sci Technol

Heterogeneous mercury reaction chemistry on activated carbon

J Air Waste Manage Assoc

New technique for quantification of elemental hg in mine wastes and its implications for mercury evasion into the atmosphere

Environ Sci Technol