A Theoretical Investigation of Arsenic and Selenium Reaction Kinetics in Coal Combustion Flue Gases

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A Theoretical Investigation of Arsenic and
Selenium Reaction Kinetics in Coal Combustion
Flue Gases Department of Chemical Engineering,
Worcester Polytechnic Institute David Urban,
Jennifer Wilcox
17
Goal
Kinetics/Thermodynamics
To determine kinetic parameters using
computational chemistry for various reactions
involving As and Se to be used in developing a
kinetic model.
Kinetics - necessary for determining rate at
which reaction will proceed - require knowledge
of reaction pathway (Potential Energy Surface) -
two reaction types, those with a transition
structure and those without
Background

• Coal-derived energy is the most prevalent form in
  US, comprising roughly half (51) of the total
  energy produced.
• The combustion of coal releases, among other
  things, trace metals such as Hg, As, and Se into
  the flue gas. Without removal they enter the
  atmosphere where the can pose a significant
  health and environmental risk
• To effectively remove these metals, knowledge of
  the specific compounds present is required
  therefore, speciation is a necessary precursor.
• Given that speciation is dependent upon the
  kinetics of the reaction system, a comprehensive
  kinetic model of the flue gas environment is
  essential for developing efficient control
  strategies.
• Because experimental data is lacking for a
  number of As/Se containing compounds, a
  computational chemistry approach is a more
  desirable method of developing an overall kinetic
  model.

Bimolecular Reaction w/ True Transition State -
Transition State Theory (w/ tunneling correction
factor)
- Collision Theory (effective upper bound, all
collisions result in reaction)
Unimolecular Reaction w/ True Transition State -
Rice-Ramsperger-Kassel-Marcus (RRKM) Theory
where

• Uni/Bimolecular Reaction w/o Trans. State
  (Barrierless)
• Variational Transition State Theory TST or RRKM
  used (for bimolecular and unimolecular reactions,
  respectively) to calculate values of k
• Several k values calculated along the asymptote,
  minimum k value taken to be the reaction rate
  constant

Thermodynamics - necessary for determining
likelihood of reaction proceeding - require
knowledge of only reactants and products
Theoretical Computations
Ab initio methodology used. It is a means of
calculating electronic properties without any
empirical data by solving approximations to the
Schrodinger Wave Equation.
Ab initio is used in cases where experimentation
is too expensive and/or dangerous or when
analytical equipment is not sophisticated enough
to detect everything. (i.e. species present for
lengths of time too small to measure) To perform,
a level of theory (comprised of a method and
basis set) is required. Method Mathematical
approach to approximating SWE -Density
Functional Theory (DFT) -Variational
(Hartree-Fock, QCI, CC, etc.) Basis Set
Mathematical description of the orbital space
occupied by electrons -Complete (all electrons
accounted for) -Relativistic Effective Core
Potential (inner electrons frozen) Theoretical
computations are performed on all species present
in a given reaction at several levels of theory.
The level which best predicts the experimental
reaction enthalpy is selected for investigation
into the kinetic and thermodynamic parameters.
Kinetic Rate Constants (cm3/mol/s)
Results
Using the various reaction theory models, the
kinetic parameters were determined for the As/Se
reactions shown. Notice that in instances where
the collision model was calculated, the
pre-exponential factor came out to be between 1-4
orders of magnitude greater than that found using
TST. As the collision model is expected to be an
upper bound value, the fact that all TST
pre-exponential factors are below this suggests
that the calculations are reasonable.
Conclusions
The greater attention given to trace metal
release during coal combustion requires that more
comprehensive flue gas models be developed for
purposes of speciation. Because of the lack of
experimental data for many compounds containing
arsenic and selenium, the determination of
kinetic and thermodynamic parameters, upon which
speciation is based, is best accomplished through
the use of computational chemistry. To that end,
the current study has fully ascertained the
parameters for three reactions involving such
compounds with additional reactions underway. The
agreement between the theoretical and
experimental thermodynamics is excellent as is
the comparison of the theoretical kinetics to
thermodynamics. At this point, there is not
enough information to generate a truly
comprehensive model, but the incorporation of the
existing data along with that of the reactions
currently under investigation, should create a
model capable making a reasonably accurate
prediction of the As/Se-oxide speciation.
The data included in the table presents a
comparison of the forward and reverse rate
constant ratio and the equilibrium constant. In
an ideal situation, these values would be exactly
equal, but given the assumptions made in the
calculations, values within the same order of
magnitude are considered accurate. It can be seen
quite clearly that for each bimolecular reaction,
the quantities are quite similar, again serving
to validate the calculations. The unimolecular
reaction, however, is not as good which may be
the result of using TST to determine the forward
rate constant.