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Energy Technology & Innovation Initiative

FACULTY OF ENGINEERING

Mercury oxidation

Alastair Clements

13/06/2012

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Outline

Introduction to mercury

Mercury oxidation

Oxidation modelling

Summary

Further work

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Mercury

ToxicityMethyl mercury ([HgCH3]

+) forms in the environment.Potent neurotoxin; RfD = 0.1-0.3g/kg per day.Bioaccumulates through the food web into lethal doses.

Transport6-18 months atmospheric lifetime. Global distribution fromsource.

CorrosionMercury forms amalgams with many metals, includingaluminium.

Alastair Clements Mercury oxidation 3/18

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Mercury oxidation

Elemental Hg thermodynamically favoured at hightemperatures.

Hg is volatile, relatively inert and has low solubility.

Oxidised at moderate temperatures by halides.

Oxidised to HgCl2 or HgBr2. Mercuric halides are soluble and so can be captured.


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Relative abundance of Cl

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Bulgaria

Ukraine

Australia

USA

Canada

Japan

China

SouthAfrica

UK

Coalcontent(ppmw

)

ChlorineBromine

[Vassilev et al. 2000][Spears and Zheng 1999]Alastair Clements Mercury oxidation 5/18
http://www.sciencedirect.com/science/article/pii/S0016236199002367http://www.sciencedirect.com/science/article/pii/S0166516298000123http://www.sciencedirect.com/science/article/pii/S0166516298000123http://www.sciencedirect.com/science/article/pii/S0016236199002367

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Correlation of Hg2+ and HCl

0

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500 3000 3500 4000

HgE

(g/dscm)/H

gT

(ppmw

)

Chlorine (ppmw)

[EPA 2011]

http://www.epa.gov/ttn/atw/utility/utilitypg.htmlhttp://www.epa.gov/ttn/atw/utility/utilitypg.html

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Oxidation pathways

Gas-phase

Focus of early work. Unable to model in all situations.

Paricle-bound

Activated carbon and fly ash. Influenced by O/S/halogen species.

Surface-catalysed

Uncoated surface effects observed. Speculative mechanisms.


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Gas phase oxidation

Widmer et al. mechanism:

Hg + Cl + M HgCl + M (R1)

Hg + Cl2 HgCl + Cl (R2)

Hg + HCl HgCl + H (R3)

Hg + HOCl

HgCl + OH (R4)

HgCl + Cl+ M HgCl2+ M (R5)

HgCl + Cl2 HgCl2+ Cl (R6)

HgCl + HCl HgCl2+ H (R7)

HgCl + HOCl

HgCl2+ OH (R8)

ReactionR 1-R 4produce the unstable HgCl intermediate. OnlyR 1can proceed at an appreciable rate.

ReactionsR 5-R 8complete mercury oxidation and occur muchfaster.

[Widmer et al. 2000]


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Oxidation levels

0

20

40

60

80

100

HCl(1

00ppm)

HCl(4

00ppm)

HBr(2

5ppm)

HBr(2

5ppm)

+HCl(100ppm

)

HBr(2

5ppm)

+HCl(400ppm

)

HBr(5

0ppm)

HBr(5

0ppm)

+HCl(100ppm

)

HBr(5

0ppm)

+HCl(400ppm

)

HCl(1

00ppm)

HCl(4

00ppm)

HBr(2

5ppm)

HBr(5

0ppm)

Percentag

e

Hg

oxidation

Oxy-firingAir firing

[Buitrago 2011]

http://content.lib.utah.edu/cgi-bin/showfile.exe?CISOROOT=/us-etd3&CISOPTR=465&CISOMODE=printhttp://content.lib.utah.edu/cgi-bin/showfile.exe?CISOROOT=/us-etd3&CISOPTR=465&CISOMODE=print

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Chlorine modelling

Leeds mercury-chlorine model:

Reaction Ref. MethodHg + Cl + M HgCl + M Donohoue et al. PLP/PLIFHg + Cl2 HgCl + Cl Widmer et al. TST/ab initio

Hg + HCl

HgCl + H Wilcox TST/ab initioHg + HOCl HgCl + OH Wilcox TST/ab initioHgCl + Cl + M HgCl2+ M Niksa et al. Collision limitedHgCl + Cl2 HgCl2+ Cl Wilcox TST/ab initioHgCl + HCl HgCl2+ H Wilcox TST/ab initioHgCl + HOCl

HgCl

2+ OH Wilcox TST/ab initio

[Gharabaghi et al. 2011]

http://www.sciencedirect.com/science/article/pii/S1540748910003408http://www.sciencedirect.com/science/article/pii/S1540748910003408

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Chlorine modelling results

0

2

4

6

8

10

0 100 200 300 400 500

Percentage

Hgo

xidation

HCl concentration (ppmv)

Experimental 440K/s

Experimental 220 K/sModel 440 K/sModel 220 K/s

Experimental data from[Cauch 2008]

http://content.lib.utah.edu/cgi-bin/showfile.exe?CISOROOT=/us-etd2&CISOPTR=90425&CISOMODE=printhttp://content.lib.utah.edu/cgi-bin/showfile.exe?CISOROOT=/us-etd2&CISOPTR=90425&CISOMODE=print

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Bromine modelling

Leeds mercury-bromine model:

Reaction Ref. MethodHg + Br + M HgBr + M Donohoue et al. PLP/PLIFHg + Br2 HgBr + Br Okano TST/ab initio

Hg + HBr

HgBr + H Okano TST/ab initioHg + HOBr HgBr + OH Niksa et al. Collision limitedHgBr + Br + M HgBr2+ M Goodsite et al. TST/ab initioHgBr + Br2 HgBr2+ Br Niksa et al. Collision limitedHgBr + HBr HgBr2+ H Okano TST/ab initioHgBr + HOBr HgBr

2+ OH Niksa et al. Collision limited

[Hughes et al. 2011]

http://www.sciencedirect.com/science/article/pii/B9780444537119500353http://www.sciencedirect.com/science/article/pii/B9780444537119500353

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Bromine modelling results

0

20

40

60

80

100

0 10 20 30 40 50

Percentage

Hgo

xidation

HBr concentration (ppmv)

Experimental 440K/s

Experimental 220 K/sModel 440 K/sModel 220 K/s

Experimental data from[van Otten et al. 2011]

http://pubs.acs.org/doi/abs/10.1021/ef200840chttp://pubs.acs.org/doi/abs/10.1021/ef200840c

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Summary

Mercury oxidation chemistry has proven challenging formodellers.

Existing gas-phase oxidation models have failed to bevalidated across datasets.

Experimental data only covers the atomic recombinationreactions.

Not all potential gas-phase pathways have been fullyinvestigated.

Work has also started on identifying surface-catalysed andparticle-bound mechanisms.


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Further work

Continue to improve and validate current model.

Measure reaction rates under flue gas temperatures. Using flash photolysis and adsorption spectroscopy.

Analyse and integrate models for heterogeneous reactions.


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References

Buitrago, P. A. Gas-phase mercury oxidation: effects of bromine, chlorine and SO2under air firing and oxy-fuel conditions, experimental and modeling study TheUniversity of Utah, 2011Cauch, B. Experimental Investigation And Kinetic Modeling Of HomogeneousMercury Oxidation By Halogens The University of Utah, 2008Donohoue, D. L.; Bauer, D. & Hynes, A. J. Temperature and Pressure DependentRate Coefficients for the Reaction of Hg with Cl and the Reaction of Cl with Cl: A

Pulsed Laser Photolysis-Pulsed Laser Induced Fluorescence Study The Journal of Physical Chemistry A, 2005, 109, 7732-7741Donohoue, D. L.; Bauer, D.; Cossairt, B. & Hynes, A. J. Temperature and PressureDependent Rate Coefficients for the Reaction of Hg with Br and the Reaction of Brwith Br: A Pulsed Laser Photolysis-Pulsed Laser Induced Fluorescence StudyTheJournal of Physical Chemistry A, 2006, 110, 6623-6632Donohoue, D. L. Kinetic Studies of the Oxidation Pathways of Gaseous ElementalMercury University of Miami, 2008Environmental Protection Agency,http://www.epa.gov/accessed 11/06/2012

http://www.epa.gov/http://www.epa.gov/

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References

Gharebaghi, M.; Hughes, K.; Porter, R.; Pourkashanian, M. & Williams, A. Mercuryspeciation in air-coal and oxy-coal combustion: A modelling approach Proceedings of the Combustion Institute, 2011, 33, 1779 - 1786Goodsite, M. E.; Plane, J. M. C. & Skov, H. A Theoretical Study of the Oxidation ofHg0 to HgBr2 in the Troposphere Environmental Science & Technology, 2004, 38,1772-1776Hughes, K. J.; Ma, L.; Porter, R. T. & Pourkashanian, M. E.N. Pistikopoulos, M. G.

& Kokossis, A. (Eds.) Mercury Transformation Modelling with Bromine Addition inCoal Derived Flue Gases 21st European Symposium on Computer Aided ProcessEngineering, Elsevier, 2011, 29, 171 - 175Niksa, S.; Helble, J. & Fujiwara, N. Kinetic modeling of homogeneousmercury/oxidation: The importance of NO and H2O in predicting oxidation incoal-derived systems Environmental Science and Technology, 2001, 35, 3701-3706Niksa, S.; Padak, B.; Krishnakumar, B. & Naik, C. V. Process Chemistry of BrAddition to Utility Flue Gas for Hg Emissions Control Energy & Fuels, 2010, 24,1020-1029Okano, T. High Temperature Mercury Oxidation Kinetics via Bromine MechanismsWorcester Polytechnic Institute, 2009van Otten, B.; Buitrago, P. A.; Senior, C. L. & Silcox, G. D. Gas-Phase Oxidation ofMercury by Bromine and Chlorine in Flue Gas Energy & Fuels, 2011, 25, 3530-3536


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References

Spears, D. & Zheng, Y. Geochemistry and origin of elements in some UK coalsInternational Journal of Coal Geology, 1999, 38, 161 - 179Vassilev, S.; Eskenazy, G. & Vassileva, C. Contents, modes of occurrence and origin ofchlorine and bromine in coal Fuel, 2000, 79, 903 - 921Widmer, N. C.; West, J. & Cole, J. A. Proceedings of the Air & Waste ManagementAssociation Annual Conference, 2000Wilcox, J. A kinetic investigation of high-temperature mercury oxidation by chlorine

Journal of Physical Chemistry A, 2009, 113, 6633-6639


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9. Mercury LU