Grantee Research Project Results
2002 Progress 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. , Wilcox, Jennifer , Blowers, Paul
Current 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 Period Covered by this Report: September 1, 2001 through August 31, 2002
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 objective of this research project was to develop a fundamental understanding of the kinetics, which determine mercury speciation in combustion flue gases. This research has two novel attributes: (1) it attempts to measure the temporal profile of various mercury species directly, using mass spectrometry rather than traditional methods of inferring them by difference; and (2) it involves theoretical estimation of gas phase mercury reaction rate constants and subsequent modeling of detailed kinetics of the Hg, Cl, S, C, H, O, and N system.
The occurrence of elemental mercury in flue gases from coal combustion is a problem of current environmental concern. Oxidized mercury species can be removed effectively from the flue gases by chemical scrubbers, however, the detailed mechanism by which oxidation occurs remains unclear. It is our specific objective within this research to determine the mechanism(s) by which this oxidation takes place.
Progress Summary:
The experimental and theoretical aspects of this research project are as follows:
Experimental. Currently, a quadruple mass spectrometer system is being developed to measure concentrations of species in reactions containing mercury.
With the help of a literature reference, calculations were performed to determine the orifice diameters required for the two separate stages of pumping (given the pumping speed of each mechanical pump). The first orifice is located in the tip of the quartz sample probe, which was designed by the University of Arizona glass shop. The diameter for the first orifice is approximately .5 mm. The second orifice is made of sapphire and inlaid in a blank copper gasket. This second orifice was made by Bird Precision and is approximately 0.002 inches.
Currently, the system is able to reach a pressure of 10-6 Torr. This is accomplished
with two mechanical pumps in combination with a 760 L/s diffusion pump. This
low pressure will ensure good detection with the mass spectrometer and a large
mean free path for the products from the reactions of interest.
Data acquisition software and the Merlin Automation Data System (MADS) have
been acquired from ABB/Extrel. A representative from the company visited our
department on August 6 to go through the software package and to answer general
questions.
A full literature search by researchers studying mercury speciation was performed. This allowed insight into the initial reactions that should be a focus of this research. Within mercury speciation research, the major question is which pathway is responsible for the bulk of the mercury oxidation in the flue gases of coal combustion or waste incineration. Each process has a different chemical environment that make up flue gases. It is important to focus on one or the other, to have the experimental setup model, and to have the correct concentrations for one of the two processes. However, other researchers are combining the processes, which makes interpreting the results difficult. Within our research, we are interested in studying the speciation of mercury in the flue gases of coal combustion. Therefore, the concentration of HCl should be in the 1 - 100 ppm (1 - 100 µL/L) range, and the concentration of mercury should be in the 0.291 - 1.45 ppb (1 - 5 µg/m3) range. Another possibility is to have an initial reaction in which chlorine atoms are allowed to react with the elemental mercury. The reactions of initial interest are as follows:
(2) HgC1 + C1 HgC12 + M
(3) HgCl + HCl HgCl2 + H
Currently, there is experimental data for reaction (1) only. The data is from 1968 and is not representative of the chemical environment of the flue gases in coal combustion and lies outside of the combustion flue gas temperatures.
Theoretical. At this point, the experimental and theoretical aspects of this research are unrelated. Once the quadrupole mass spectrometer is completely developed and the samples are run, there will be a direct comparison between the two attributes.
The study of basis sets has been essential for theoretical progress. The basis sets used for the theoretical calculations are composed of recent pseudopotentials developed in 1992, and 1997, both including relative effects. This is an improvement from other similar research, which used older pseudopotentials developed in 1985, and included a fewer number of basis functions, decreasing the accuracy of the calculations (Equation 1). To gain confidence in the theoretical calculations, rate constant data were calculated for reactions, in which experimental data is currently available. The following reaction has been well-studied experimentally:
Transition state theory is traditionally used to calculate rate constants for bimolecular reactions. However, this reaction is unimolecular, making Rice-Ramsberger-Kassel-Marcus (RRKM) theory a necessary tool. All of the parameters required to calculate the unimolecular rate constant can be obtained from theory, with the exception of the collisional efficiency, c. This collisional efficiency is an empirical value obtained through experimental knowledge of the reaction (Equation 2). Many models have been determined for calculating the collision efficiency, but experimental data is necessary (Equations 3 and 4). In many cases, because of the lack of experimental data available, c is used as a fitting parameter that ranges between 0 and 1. The following figure exhibits the agreement between our theoretical approach and the available experimental data (Equation 5).
Figure 1. Theoretical Universal Rate Constant Using 6-311++G(3df, 3pd), Compared to Experimental Rate Constant, Varying 1,000/T.
To gain confidence with our theoretical calculations applied to mercury, the following reaction also was studied and compared to our experiment calculations. Unfortunately, there is only one experimental value to compare with the 1,968 results and is available at only one temperature.
Again, RRKM theory was employed due to the unimolecularity of the reaction. Figure 2 compares theory to experiment.
Figure 2. Canonical Variational Transition State Theory Rate Constant, (kCVT) for Each Combination, Compared to Experimental Rate Constant, Varying Temperature, 1/T. Note: c = 0.2, HgCl + M Hg + Cl + M,
Many calculational methods were employed and compared. The findings from this research have been submitted in a paper to Fuel Processing Technology. Aside from kinetic data, thermodynamic data also was compared and is shown in Table 1.
Table 1. aRef 7, bRef 8, cNIST Chemistry Webbook, dRef 6.
Experimental | LANL2D Z B3LYP |
1985 MP4S Q |
1985 QCIS D |
1992 MP4S Q |
1992 QCIS D |
1997 MP4SD Q |
1997 QCISD |
|
Geometry (Å) | 2.28a 2.34b |
2.612 | 1.689 | 1.689 | 2.406 | 2.412 | 2.404 | 2.407 |
Hrxn(kJ/mol) | 104.23c | 104.38 | - | - | 108.08 | 108.32 | 137.99 | 138.16 |
Frequency (1/cm) | - | 228 | 1583 | 1582 | 292 | 290 | 300 | 292 |
Activation Energy (kJ/mol) at 393K | - | 82.15 | - | - | 71.60 | 67.52 | 87.01 | 84.34 |
Rate Constant (M-1s-1) at 393K | k1=4.309 k-1=1.95*1010 [d] |
k1=2.29*10-1 | - | - | k1=4.25 | k1=14.6 | 2.98*10-2 | 6.11*10-2 |
Our research focused on the following reaction:
Because this is a bimolecular reaction, traditional transition state theory was employed in this case. Comparison of methods and basis sets are listed in Table 2. The rate constant calculated from LANL2DZ/B3LYP was from Sliger, et al., 1997. It is important to note that their rate expression is only valid in the 900-2,000 K temperature range, which may explain the drastic difference in our rate constants. Currently, the rate constants are being calculated at varying temperatures and are not yet complete with that aspect of the research. Also, because of differing results from Sliger et al., 1997 in the other categories listed, the other data under this combination of method and basis set were calculated during our research. Sliger, et al., 1997, reported a negative activation energy for the reverse of the above reaction, which differed from our calculated results using the same method and basis set.
Table 2. aRef 7, bRef 8, cNIST Chemistry Webbook
Experimental | LANL2DZ B3LYP |
1992 QCISD |
1997 QCISD |
|
Geometry (Å) | ||||
HgCl | 2.28a 2.34b |
2.6122 | 2.4121 | 2.4084 |
HCl | 1.2746 | 1.3149 | 1.2833 | 1.2833 |
HgCl2 | 2.252 | 2.4417 | 2.3002 | 2.3116 |
Frequency (1/cm) | ||||
HgCl | - | 228.92 | 290.69 | 297.81 |
HCl | 2709.61 | 2947.12 | 2947.12 | |
HgCl2 | - | 66,66,288,345 | 97,97,340,394 | 89,89,338,388 |
Hrxn(kJ/mol) | 85.55c | 104.11 | 98.46 | 82.05 |
Activation Energy (kJ/mol) | ||||
forward | - | 108.34 | 126.66 | 113.42 |
reverse | - | 4.224 | 24.48 | 27.64 |
Rate Constant (cm3 /mol s) | ||||
Forward (298K) | - | 6.95 * 10-5 | 4.04 * 1010 | 1.45 * 1011 |
References:
Sliger RN, Kramlich JC, Marinov NM. Fuel Processing Technology 2000;67:423.
Troe J. Journal of Chemical Physics 1977;66:4758.
Keck J, Carrier G. Journal of Chemical Physics 1965;43:2284.
Tardy DC, Rabinovitch BS. Journal of Chemical Physics 1966;45:3720.
Carabetta RA, Palmer HB. Journal of Chemical Physics 1967;16:1333.
Horne DG, Gosavi R, Strausz OP. Journal of Chemical Physics 1968;48:4758.
Braune H, Knoke S, Zeits F. Physik Chemie 1933;B23:163.
Gregg AH, Hampson GC, Jenkins GI, Jones PLF, Sutton LE. Trans. Faraday Soc. 1937;33:352.
Future Activities:
The future activities for this research project include:
- · Attach Merlin Data Automation System and install the software.
- Abstract: Science Direct - Abstract
Exit - 2001
- 2003
- Final Report
· Order permeation tubes and finish the reactor design.
· Build a control panel that controls the cooling water for the diffusion pump and the filament on the mass spectrometer.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 24 publications | 4 publications in selected types | All 4 journal articles |
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Type | Citation | ||
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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. |
R828168 (2002) R828168 (Final) |
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Supplemental Keywords:
mercury speciation, quadrupole mass spectrometer, Rice-Ramsberger-Kassel-Marcus, RRKM theory, transition state theory, pseudopotentials., 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 gasesRelevant Websites:
http://www.emsl.pnl.gov:2080/forms/basisform.html Exit
Progress 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.