United States
Environmental Protection
Agency
Air and Energy
Engineering Research Laboratory <
Research Triangle Park, NC 27711
Research and Development
EPA/600/S7-88/021 Feb. 1989
&EPA Project Summary
Kinetic Modeling of NOx
Formation and
Destruction and
Combustibles Burnout
J. A. Cole, J. C. Kramlich, and W. S. Lanier
A model of the gas-phase
chemistry involved in the combustion
of simple hydrocarbon fuels and the
interconversion of fixed nitrogen
species has been developed. One
focus of the work was on modeling
the chemistry involved in reburning
and other advanced NOX control
strategies. A second focus was on
the decay rate of various hydro-
carbon species under high-tem-
perature conditions. This provided an
initial step towards the chemistry
needed to model fuel burnout In
small combustors, e.g., wood stoves.
The approach was to compare
rates for elementary reactions as
represented in the different
compilations available in the
literature. Where inconsistencies
appeared, the original literature on
the rates was consulted. The
mechanism was tested against
benchmark data so that the key
reactions that controlled any
inconsistencies could be identified.
Adjustments to the rates were
allowed if they fell within the error
bounds of the original determination.
This Project Summary was
developed by EPA's Air and Energy
Engineering Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
The control of nitrogen oxides (NOX)
from practical sources has become a
relatively mature science. However, the
basis for many of the technologies are
understood only qualitatively. Thus, a
major research thrust is to develop the
quantitative understanding necessary to
optimize NOX control. Ultimately, this will
aid in the design of scaled-up units.
A good example of such an NOX
control technology is reburning. Over the
last several years, a diagram showing the
key reactions that govern the reburning
process would have changed only in its
minor details. However, calculations of
reburning would have changed
considerably because the kinetic rates
associated with many of the key
reactions have evolved over this time
frame.
This report describes the
development of a chemical kinetic
mechanism for NOX formation and
destruction, and fuel fragment burnout.
The goal of this development is to
provide a state-of-the-art tool for the
analysis, optimization, and generalization
of NOX control technologies. In other
words, the reported work is intended to
help shift a qualitative understanding of
these processes toward a quantitative
understanding.
This program focused on the NOX
chemistry of the reburning process.
Experimental studies of reburning have
raised many questions. Fuel type,
reburning temperature, and mixing
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configuration all impact the final NOX
concentration. Also important are the
primary and secondary (reburning)
stoichiometnes. Because of the many
parametric studies of reburning and NOX
reduction, the overall understanding is
fair. However, all of the key mechanisms
are still not fully understood.
In order to better understand and
ultimately predict this process, a
homogeneous kinetic mechanism was
assembled. Currently, the mechanism
consists of the modified Arrhenius rate
constants for 192 chemical reactions
obtained from the open literature, plus
two "phenomenological" reactions which
were added to improve the model
predictions. Forty chemical species are
considered in the mechanism. It is
solved with a model for one-
dimensional (non-diffusional) chemical
kinetics. The model contains options for
temperature and pressure control, in
addition to adiabatic operation. It also
permits sidestream addition (fuel or air
staging) and can simulate either well-
stirred- or plug-flow-reactor
chemistry.
The hydrocarbon combustion portion
of the mechanism was first validated
against shock-tube data. The nitrogen
chemistry and modeling format was then
tested against reliable bench-scale data
on the reburning process.
Development of Kinetics
Rate Selection
A thorough search of the primary
literature for kinetic rate data was beyond
the scope of this program. Many highly
regarded researchers are specialists in
the evaluation of chemical kinetic data.
Therefore, their work was used as
references in order to assemble a
specialized mechanism. Eight existing
compilations and critical evaluations of
kinetic rates were used as the principal
sources for this study.
An important tool developed for the
selection of rate constants was a
collection of utility programs, written in-
house, linked to a commercial graphics
software package. This tool was used to
produce Arrhenius plots of all of the rate
data for each reaction.
For many reactions the differences
between cited expressions were
negligible and so the choice became
immaterial. This prevailed in the H2 -
O2 subset where rate data are relatively
accurate for most reactions. A complete
set of 254 Arrhenius plots is contained in
the Appendix to the subject report.
Validation
Validation of a kinetic model involves
a comparison of the model's predictions
with reliable data. An important
consideration here is whether the
experimental conditions can be
accurately represented. Heat losses, wall
effects, mixing, and inhomogeneities all
constitute modeling issues which go
beyond chemical kinetics.
The present mechanism was
compared with shock-tube data. The
correlation parameter was a function
containing CH4 and 02 concentrations
(mole/cm3) and an induction time defined
as that time (seconds) required for 10%
of the CH4 to be destroyed. The model
predictions fell well within the range of
the data.
Reburning Modeling
Modeling the reburning process
requires attention to the details of the
physical, as well as chemical processes
occurring in combustion systems. The
model was applied against bench-scale
data obtained in the EER Controlled
Temperature Tower (CTT) in order to
address some of these needs.
Systematic variations of reburning
parameters were applied to modeling the
reburning process in order to improve
the model predictions and to provide
insight into the processes occurring
during reburning in practical combustion
systems. By examining the influence of
physical parameters, such as
temperature profile and staging rate, the
chemistry of the reburning system was
found to be very sensitive. However, the
uncertainty in the physical situation was
not sufficient to allow the model
predictions to match the data. It was
necessary to assume two phenome-
nological chemical reactions.
Modeling Parameters
Significant parameters which were
investigated were:
Reburning fuel addition rate
Burnout air addition rate
Temperature change during re-
burning fuel addition
Temperature change during burn-
out air addition
The impacts of staging rate and heat
release staging on model predictions of
NOX destruction during reburning were
investigated. The heat of combustion of
the remaining fuel was found to be
important in determining reburning
behavior during fuel addition, although
less so during burnout (air staging). Th(
rate at which mixing and reaction arj
assumed to occur during staging i!
important to the model predictions ii
both cases. This, of course, also implie:
that mixing is an important parameter ii
determining reburning effectiveness.
Phenomenological Modeling
Even accounting for the physica
parameters, the model did no
adequately predict the concentrations c
the reduced nitrogen species NH$ am
HCN. In order to predict the observe!
HCN concentration, it was necessary ti
include a mechanism for fixation of N2.
Two reactions which are proposei
and their a priori rate expressions are:
C2H2 + NO = CHO + HCN
k = 8.133E5*exp(+14140/RT).
C2H +N2 = CN + HCN
k = 1.26E12"exp(-21000/RT).
These reactions were appended to th
reaction mechanism and evaluate
against the CTT reburning data. The
rate expressions were then adjuste
parametrically to best fit the data over th
range of SR2 = 1.0 to SR2 = 0.7. Th
resulting "best fit" rate expressions are:
C2H2 + NO = CHO +HCN
k = 2.2E5*exp (+14140/RT).
C2H + N2 = CN -i-HCN
k = 1.26E12*exp(-25000/RT).
HCN concentrations predicted wit
the two phenomenological reaction
included in the reaction mechanism wer
very good over the range of SR2 = 1.(
0.7. The trend of NHs concentration wa
quite good, although the predicted value
were generally low. Predictions of NC
concentrations were excellent, except <
SR = 1.0.
Burnout JVOX Predictions
The chemistry occurring durin
burnout air addition is complicated.
large volume of air relative to the tot;
system volume is needed to effec
burnout. This results in a quench of th
radical species concentrations followe
by reignition and reaction at lean*
stoichiometry. Initially the radical specie
reignite the reburning reaction
responsible for NOX reduction. Howeve
the excess oxygen and increasing radic
levels soon begin to oxidize th
remaining fixed nitrogen species.
The model predictions for NC
concentration after burnout show th
qualitative features of the data. Howeve
the predicted concentrations of NOX \
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are too high. This indicates that the
predicted oxidation of the remaining fixed
nitrogen species is too efficient. An
analysis of the reactions taking place
indicates that about 80% of the HCN at
the end of the reburning zone is oxidized
to NO, using the current mechanisms.
Summary and Conclusions
The developed kinetic mechanism
represents the current state of
knowledge in homogeneous combustion
modeling. The rate data are a
compilation of the most reliable and up-
to-date reviews available in the field of
gas-phase chemical kinetics. By
accounting for environmental factors and
with the addition of two
phenomenological reactions the present
mechanism is able to adequately model
NOX destruction during the reburning
process.
In summary, several things were
required to develop the mechanism and
carry out the modeling efforts described
in the report:
• Reliable rate data and references.
• Reliable fundamental and bench-
scale data against which to
compare the model and
mechanism.
• Computer programs to assist in the
organization and selection of rate
data.
• Understanding of the complexities
involved in the reburning process.
Several points can be made based
upon the experience gained during this
program:
1. Real processes result from complex
coupling of fundamental processes;
i.e. mixing, heat transfer, diffusion,
etc. These can be effectively
modeled one dimensionally if certain
key parameters can be correctly
identified. The key parameters will
vary between different experimental
or full-scale configurations.
2. Physical processes (mixing, heat
transfer, etc.) set the environment for
the chemistry.
3. Chemistry is the means of NOX
removal, but chemistry can modify
the physical environment; e.g.,
through heat release during
combustion.
4. Development of the kinetic
mechanism was only one step
toward developing a valid model of
the reburning process.
Understanding the physical
processes and accommodating them
in the modeling format were equally
important in not only modeling
reburning, but also in deciding where
changes could be made in the
mechanism itself.
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J. A. Cole, J. C. Kramlich, and W. S. Lanier are with Energy and Environmental
Research Corp., Irvine, CA 92719-2798.
William P. Linak is the EPA Project Officer (see below).
The complete report, entitled "Kinetic Modeling of NOX Formation and
Destruction and Combustibles Burnout," (Order No. PB 89-124 358/AS;
Cost: $42.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-88/021
OOUG529 PS
CHCGO
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