Grantee Research Project Results
Final Report: Fundamentals of Mercury Speciation Kinetics: A Theoretical and Experimental Study
EPA Grant Number: R828168Title: Fundamentals of Mercury Speciation Kinetics: A Theoretical and Experimental Study
Investigators: Wendt, Jost O.L. , Blowers, Paul
Institution: University of Arizona
EPA Project Officer: Hahn, Intaek
Project Period: September 1, 2000 through August 31, 2002 (Extended to August 31, 2004)
Project Amount: $225,000
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Water , Land and Waste Management , Air
Objective:
The goal of this research project is to understand the homogeneous speciation of mercury in the flue gases of coal combustion. As the flue gas cools, thermochemical equilibrium calculations indicate that elemental mercury, Hg0, is converted to oxidized mercury, Hg2+, in the form of HgO or HgCl2. Hg0 is insoluble in water, HgO has low solubility in water, and HgCl2 is highly soluble in water. As HgCl2 is water-soluble, it can be captured in wet chemical scrubbers to prevent its release to the atmosphere. Therefore, understanding the mechanisms of mercury’s oxidation in flue gases is paramount when considering mercury capture. This research project attempted to elucidate the mechanisms of oxidation through a detailed kinetic and thermodynamic analysis. The research focused specifically on the oxidation of mercury via chlorine-containing compounds. Quantum chemistry is used to determine accurate transition structures, which are required for the calculation of activation energies and rate constants from theory. Simultaneous to the theoretical work, an experimental apparatus was designed and fabricated with the inclusion of a quadrupole mass spectrometer. The mass spectrometer was used in conjunction with a laminar flow reactor to simulate the oxidation of mercury via chlorine-containing compounds in flue gases. The specific objective of this research project was to obtain a potential mercury oxidation mechanism based on theoretically predicted kinetic parameters, which were validated through concentration profiles obtained from experimental measurements. In addition, results from the experimental work indicate that at ambient conditions, the oxidation of mercury via chlorine may result as a consequence of heterogeneous reactions involving the Pyrex reactor surface. This work not only allows for a more thorough understanding of mercury’s speciation in the flue gas environment, but also questions current sampling devices and their potential interference with reactivity measurements involving mercury-chlorine species.
Summary/Accomplishments (Outputs/Outcomes):
As a result of the many questions that remain regarding the mechanisms by which mercury is oxidized in the flue gases of coal combustion, a smallscale “model” involving the results of rate constant calculations for the series of mercury oxidation reactions of interest will predict mercury speciation in a less complex environment. The best methodology to understanding the entire picture is to examine each complexity in isolation. Examining the rate constants for mercury reactions involving solely chlorine species will foster the understanding of chlorine’s role in the mercury oxidation process.
The experimental results of the current work are in agreement with the experimental results of Hall, et al., and lead to the conclusion that mercury reacts readily with Cl2 at room temperature. It is important to note, however, that the amount of mercury oxidized is dependent upon the reaction vessel. As was exhibited in the final experimental results, the Pyrex surface is highly reactive to mercury-chlorine species, and it has been concluded from this experimental work that the oxidation of mercury via Cl2 at ambient conditions must occur, to a certain extent, heterogeneously. Results of Mamani-Paco, et al. are very different and claim that, at low temperatures, mercury oxidation via Cl2 is not favored. The reactor materials of construction, however, are not addressed in their work. A study was performed by Medhekar, et al., determining that mercury-chlorine species are reactive on the surfaces of Inconel, quartz, stainless steel, and Teflon-coated stainless steel. In addition, there has been a great deal of mercury speciation research involving the addition of HCl. Again, at approximately 500ºC, Hall, et al., obtain high levels of mercury oxidation. On the other hand, work by Sliger, et al., and Mamani-Paco, et al., indicates that the mercury oxidation via HCl is not favored even at elevated temperatures. The current experimental research concludes that mercury does not react with HCl at room temperature.
Examination of the possible reaction mechanisms that result in the combination of mercury with each of the chlorine species will allow for the elucidation of the speciation of mercury that may exist in the coal combustion flue gases. Table 1 lists both the theoretically predicted rate constants and the experimentally derived equilibrium constants for a series of mercury oxidation reactions that may take place in the flue gases.
Table 1. Comparison of Rate and Equilibrium Constants at Various Temperatures
Reaction | Temperature | Rate Constant | Equilibrium Constant | Activation Energy | Basis Set/ Method |
(4.1) Hg + Cl + M HgCl + M | 298 | 1.57*1012 | 1.2*1014 | -9.138 | 1992/QCISD |
| 600 | 1.74*109 | 6.7*104 | ||
| 1,000 | 9.53*107 | 1.2*101 | ||
| 1,200 | 2.76*106 | 8.3*10-2 | ||
(4.2) Hg + Cl2 HgCl + Cl | 298 | 3.68*10-7 | 1.21*10-1 | 88.95 | 1997/B3LYP |
| 600 | 4.11*101 | 7.38*102 | ||
| 1,000 | 6.17*104 | 5.00*10-4 | ||
| 1,200 | 3.84*105 | 1.43*10-5 | ||
(4.3) Hg + HCl HgCl + H | 298 | 6.22*10-65 | 1.9*10-57 | 98.29 | 1992/QCISD |
| 600 | 1.20*10-28 | 1.1*10-28 | ||
| 1,000 | 2.52*10-14 | 2.7*10-17 | ||
| 1,200 | 9.59*10-11 | 2.0*10-14 | ||
(4.4) Hg + Cl2 + M HgCl2 + M | 298 | 2.26*10-16 | 9.7*1030 | 34.46 | 1997/B3LYP |
| 600 | 8.32*10-2 | 5.2*1012 | ||
| 1,000 | 5.11*104 | 3.5*105 | ||
| 1,200 | 1.50*106 | 5.9*103 | ||
(4.5) HgCl + HCl HgCl2 + H | 298 | 3.93*10-15 | 1.1*10-17 | 31.22 | 1992/QCISD |
| 600 | 1.32*10-3 | 3.3*10-10 | ||
| 1,000 | 4.67*101 | 4.2*10-7 | ||
| 1,200 | 6.40*102 | 2.6*10-6 | ||
(4.6) HgCl + Cl2 HgCl2 + Cl | 298 | 1.48*106 | 1.21*1014 | 4.01 | 1997/B3LYP |
| 600 | 4.47*107 | 7.47*101 | ||
| 1,000 | 1.71*108 | 4.04*10-6 | ||
| 1,200 | 2.40*108 | 4.16*10-8 | ||
(4.7) HgCl + Cl + M HgCl2 + M | 298 | 2.02*1014 | 7.04*1053 | -1.28 | 1997/B3LYP |
| 600 | 8.04*109 | 2.19*1022 | ||
| 1,000 | 3.17*104 | 1.73*106 | ||
| 1,200 | 1.11*103 | 1.29*102 |
Conclusions
The purpose of the thermodynamic and kinetic data presented in Table 1 is to predict the speciation of mercury among HCl and Cl2 in an attempt to elucidate the mechanism by which mercury is oxidized. This prediction will answer the question of whether mercury can be oxidized by one, both, or neither of these species at ambient conditions.
The flue gases from coal combustion processes tend to contain higher concentrations of HCl than Cl2.) For this reason, possible oxidation of mercury via hydrogen chloride will be considered first. The data in Table 1 indicate, from both a thermodynamic and kinetic viewpoint, that it would be impossible for mercury’s first stage of oxidation to take place via HCl. From reaction (4.5) in Table 1, mercury’s second stage of oxidation via HCl to the water-soluble form, HgCl2, is neither thermodynamically nor kinetically favorable at low temperatures. At very high HCl concentrations and at temperatures above 1,000K, however, this second stage of oxidation is possible. In general, the elemental form of mercury is dominant at high temperatures such as those present during the combustion process, and the oxidized forms of mercury are dominant at the lower temperatures such as those of the flue gases during the post combustion processes. The flue gases are cooled prior to entering the pollution control devices so that both stages of mercury’s oxidation play key roles within the possible oxidation mechanism. Because reaction (4.5) is only favorable at high temperatures, it will not be considered as a dominant reaction in the prediction for mercury’s oxidation. As a result of the high concentration of HCl in the flue gases of coal combustion, however, its participation should not be neglected completely. The work of Sliger, et al., proposed that chlorine atoms were formed via the following bimolecular reaction:
HCl + OH Cl + H2O. (1)
A theoretical rate expression calculated from transition state theory. is given by:
(2)
from 138 to 1,060K. The question is if reaction (1) is favored as the flue gases are quenched. Reaction (1) is thermodynamically favored from 298 to 1,200K. This reaction also is kinetically favored from 298 to 1,200K. Using equation (2), the reaction rate at 298K is 4.35*1011 cm3/mol*s and is 1.89*1012 cm3/mol*s at 1,200K. Because HCl is present in the flue gases at much higher temperatures compared to Cl2, it is likely that the reaction that produces chlorine atoms involves HCl.
There are other reactions, however, that may result in the formation of chlorine atoms that also should be considered. Chlorine atoms can be produced from the decomposition reactions of HgCl and Cl2. The formation of chlorine atoms via the decomposition of Cl2 is not thermodynamically favored up to 1200K. In addition, the decomposition of Cl2 at ambient conditions occurs slowly, with a predicted rate constant of 0.245 cm3/mol*s. Alternatively, at higher temperatures, this decomposition is not as kinetically limited, with a rate constant of 2.11*105 cm3/mol*s at 500K. Therefore, depending on the final temperatures to which the flue gases are cooled, this pathway of chlorine atom formation may be important.
The decomposition of HgCl to form chlorine atoms also should be considered. At 298K, this decomposition reaction is both thermodynamically and kinetically limited, with an equilibrium constant of 8.25*10-15 and a corresponding rate constant of 13.05 cm3/mol*s. At a higher temperature of 500K, the reaction is not as slow and proceeds with a rate constant of 1.4*106 cm3/mol*s. Even at high temperatures, however, this reaction is thermodynamically limited, with an equilibrium constant of 2.16*10-7 at 500K.
When comparing these three distinct pathways for the formation of chlorine atoms, it appears that reaction (1) is the most kinetically and thermodynamically favorable in the temperature range at which the flue gases are quenched. Therefore, as quenching takes place, the most likely pathway of producing chlorine radicals is via HCl with the addition of OH. At this point, it appears that the formation of chlorine atoms plays an important role in the mercury oxidation process.
Further examination of Table 1 is required to determine if Cl2 or Cl atoms plays the major role in the oxidation of mercury. Reaction (4.2) is neither thermodynamically nor kinetically favorable from 298 to 1200K and will not be considered further. Reaction (4.6) is thermodynamically favorable at low temperatures and also thermodynamically favorable at temperatures above 600K, given high concentrations of Cl2. As a result of the low concentration of Cl2 in the flue gas relative to the HCl concentrations, this reaction may be a secondary pathway but certainly not a dominating one for the oxidation of mercury. Given these arguments, a possible mechanism for mercury’s oxidation in the flue gases is proposed in Figure 1.
Figure 1. Proposed Hg Oxidation Model via Chlorine Species
In total, a mechanism by which mercury oxidation takes place in the flue gases is proposed. Consideration should be taken in the initial step of the mechanism involving the formation of chlorine atoms. Because of the high concentration of HCl in the flue gases relative to that of Cl2, another possible pathway of creating chlorine atoms could involve the heterogeneous reaction, in which HCl reacts with the fly ash to create chlorine atoms, which are then scavenged to create both the mercurous (HgCl) and mercuric (HgCl2) forms of oxidized mercury. This heterogeneous pathway should be examined both theoretically and experimentally in future work involving mercury speciation measurements.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 24 publications | 4 publications in selected types | All 4 journal articles |
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Wilcox J, Robles J, Marsden DCJ, Blowers P. Theoretically predicted rate constants for mercury oxidation by hydrogen chloride in coal combustion flue gases. Environmental Science & Technology 2003;37(18):4199-4204. |
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Wilcox J, Marsden DCJ, Blowers P. Evaluation of basis sets and theoretical methods for estimating rate constants of mercury oxidation reactions involving chlorine. Fuel Processing Technology 2004;85(5):391-400. |
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Wilcox J, Blowers P. Some comments on 'A study on the reaction mechanism and kinetic of mercury oxidation by chlorine species' [J. Mol. Struct. (Theochem) 625 (2003) 277]. Journal of Molecular Structure: THEOCHEM 2004;674(1-3):275-278. |
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Wilcox J, Blowers P. Decomposition of mercuric chloride and application to combustion flue gases. Environmental Chemistry 2004;1(3):166-171. |
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Supplemental Keywords:
Homogeneous speciation, flue gases, coal combustion, oxidation, kinetic analysis, thermodynamic analysis, mercury speciation, HCL, chlorine, quantum chemistry,, RFA, Scientific Discipline, Air, Toxics, Waste, Environmental Chemistry, HAPS, Engineering, Chemistry, & Physics, Incineration/Combustion, Environmental Engineering, mercury , mercury, Chlorine, mass spectrometry, UV, fly ash, chemical transport modeling, mercury speciation, sulfur, chemical kinetics, downflow quartz reactor, Mercury Compounds, combustion, incineration, coal combustion, flue gasesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.