Today’s market opportunities for combustion systems require focus on high-efficiency, low emissions and fuel-flexibility. In three previous white papers , we have discussed how use of detailed chemical kinetics in combustion simulation can provide accurate emissions predictions, simulate fuel effects and help gain insight into instability phenomena like Lean Blow Off (LBO). All of these topics focus on the use of high-fidelity chemistry simulation models for advanced combustion simulation by using highly accurate and detailed kinetics mechanisms. This white paper describes what a detailed kinetics mechanism is, how it is developed and validated and how it can be used in high-fidelity combustion simulation models to accelerate advanced combustion technology development.

1. Using a Detailed Chemical-Kinetics Mechanism to
Ensure Accurate Combustion Simulation
By Reaction Design
6440 Lusk Blvd, Suite D205
San Diego, CA 92121
Phone: 858-550-1920
www.reactiondesign.com

2. Reaction Design White Paper
INTRODUCTION
Today’s market opportunities for combustion systems require focus on high-
efficiency, low emissions and fuel-flexibility. In three previous white papers1 , we have
discussed how use of detailed chemical kinetics in combustion simulation can provide
accurate emissions predictions, simulate fuel effects and help gain insight into instability
phenomena like Lean Blow Off (LBO). All of these topics focus on the use of high-
fidelity chemistry simulation models for advanced combustion simulation by using highly
accurate and detailed kinetics mechanisms.
However, to use these high-fidelity models to your advantage, you need to find
an appropriate mechanism for your application – just as you needed DVD’s to enjoy
your first DVD player. Identifying the right detailed kinetics mechanism has great
influence over the accuracy of your combustion simulation results. Selection of the right
mechanism requires an understanding of kinetics, the goals of the simulation and some
practical combustion design issues. The multitude of detailed kinetics mechanisms
available in the literature makes it too easy to choose the wrong one. Using the wrong
mechanism can hurt you both by wasting simulation resources and potentially missing
valuable collection of data points.
This white paper describes what a detailed kinetics mechanism is, how it is
developed and validated and how it can be used in high-fidelity combustion simulation
models to accelerate advanced combustion technology development.
COMBUSTION REQUIRES A DETAILED KINETICS MECHANISM
When we think of the chemical reaction for combustion of hydrocarbon fuels, we
typically think of burning fuel with oxygen and producing combustion products in a
single step, as shown below for methane (CH4):
1 See http://www.reactiondesign.com/products/open/energico_resources.html
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We can adjust this simple, global reaction mechanism for methane in oxygen for
the combustion of methane in air by adding the nitrogen species, as shown below:
This single-step reaction mechanism contains five species and represents, in an
ideal sense, combustion chemistry in its simplest form for one of the simplest
hydrocarbon fuels.
We understand that this is only a global representation of the actual reaction
mechanism and that real life is much more complex. In reality, the decomposition of the
fuel and the formation of reaction products occur in hundreds or thousands of steps that
can involve more than a thousand short-lived chemical species and radicals – each of
which may affect the combustion behavior and the formation of pollutant species. The
set of elementary reactions and species that are involved in the global reaction process
is typically called a “detailed kinetics mechanism.” The additional reactions and species
represented in a detailed mechanism are required to predict important combustion
phenomena, including ignition, flame propagation and formation of pollutants such as
NOx, CO, unburned hydrocarbons and soot. In other words, to gain a thorough
understanding of how real fuels burn in real applications, you will need a detailed
kinetics mechanism for your combustion simulation.
DEVELOPMENT AND REDUCTION OF A DETAILED KINETICS
MECHANISM
Just like the mapping of the human genome, developing a fully detailed chemical
mechanism is a task best suited for experts. In-depth understanding and broad
experience in chemistry, kinetics, fuel analysis and combustion application conditions
are required to successfully develop an accurate chemical kinetics mechanism from
scratch. Understanding of the state-of-the-art research results in the fields of complex
kinetics and validation results is necessary to piece together the building blocks of
reaction kinetics that embody a comprehensive detailed chemical mechanism – a
“Master Mechanism” that represents a comprehensive set of chemical reactions (or
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steps) and species that are known to be involved in the combustion of that fuel for a
broad set of operating conditions.
The first step in developing a Master Mechanism is to carefully define the
application conditions and fuel components involved. Then, an exhaustive investigation
is carried out to identify all the reactions that could be important in the decomposition of
the fuel during combustion under those conditions. Thermodynamic data for the
species involved in the reactions must be determined. Reaction rates for each reaction,
with temperature and (in some cases) pressure dependency, will also need to be
determined. For many applications, transport properties are also required for each
species in the reaction system. Assembling all of these data for a new fuel-combustion
system is a significant effort and involves poring over volumes of research results and
peer-reviewed technical articles. Assembly of a Master Mechanism employs an
understanding of complex organic and physical chemistry to fill in the gaps that exist in
the available literature.
Building on accurate thermodynamic data, the next step is to make sure that the
mechanism is complete so that all applicable elementary reactions for the conditions of
interest are included. For example, mechanisms for alkanes can be divided into several
reaction classes containing similar types of reactions, for example, H-atom abstraction
reactions. These classes of reaction tend to follow rate rules, based on observed trends
related to several properties such as bond strength, number of abstractable hydrogen
atoms, etc. The mechanism should follow such rules for known classes of reactions to
maintain internal consistency. That way, a developer can merge different fuel
components into a comprehensive fuel-surrogate mechanism that yields consistent
predictions with different fuel components. Using this approach, the Master Mechanism
developer can leverage knowledge of other molecules in the determination of which
mechanism is best suited for a new fuel system.
Many research groups all over the world work in development of detailed kinetics
mechanisms. As a result, the combustion literature includes several detailed kinetics
mechanisms for the same fuel components. This makes it crucial for the combustion
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designer to evaluate the mechanisms carefully before applying them to specific
conditions and also before merging different components from different sources.
VALIDATION AGAINST FUNDAMENTAL KINETICS EXPERIMENTS
Once a Master Mechanism has been assembled for a new fuel component, the
developer should rigorously validate it against theoretical tests and fundamental
experimental results. The most relevant experiments for this exercise can be
represented well with a reduced-order model (for example, 1-D laminar flame or 0-D
reactor models), so the developer can include the full Master Mechanism in the
validation simulation without compromise. Such experiments include measurements in
shock-tubes, laboratory flame experiments, stirred reactors or flow reactors. The
selection of validation data will depend on the application, where the focus may be on
prediction of ignition delay, laminar flame speed or trace species concentrations. To be
reliable, a Master Mechanism is validated for the performance results of interest. In
general, the more fundamental the underpinnings of the Master Mechanism, the more
likely it will extrapolate well to conditions not explicitly validated in the development
work. So, while we can map the full human genome based on what we know today, the
Master mechanism must always be compared to and evolve with new information and
understanding as it becomes available.
If you think this approach to developing a Master Mechanism sounds
complicated, involved and time-consuming, then you are right. That is why the
development of a Master Mechanism is usually a costly process conducted by experts
in the field. Fortunately, there are many experts in the field that have been developing
foundation data for many families of fuel components over several decades. The
benefit of identifying a Master Mechanism is that, once all the hard work by experts is
complete, the resulting mechanism can be used by non-experts to achieve a high level
of simulation accuracy.
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MAINTAINING ACCURACY DURING MECHANISM REDUCTION
A Master Mechanism typically represents a comprehensive set of chemical
reactions (or steps) and species that are known to be involved in the combustion of that
fuel for a broad set of operating conditions. These mechanisms can be quite large. For
example, the latest diesel fuel mechanism developed by the Model Fuels Consortium, a
group of companies that are working to enable the design of cleaner burning, more
efficient engines and fuels, now contains over 3,500 species and 12,000 reactions or
steps. Luckily, you often do not need all of that detail for your specific application
results of interest. You can reduce the Master Mechanism in varying degrees to suit
your simulation model while maintaining accuracy and optimizing your simulation run
times.
Mechanism reduction is a process that eliminates “excess baggage” in the
Master Mechanism that is not needed for a specific result of interest (for example, major
vs. trace species) and the application conditions of interest (for example, high-
temperature conditions or near-stoichiometric fuel-air mixtures). You may choose from
many different techniques to reduce a mechanism, but they all employ the same
strategy: remove the specific reactions and species that are not of interest for the
desired results and simplify groups of reactions using similar species into a smaller
group of reactions. It is important to understand that error is introduced every time you
employ a reduced mechanism as you are removing information from the fully most
accurate form of the mechanism. You must balance the trade-offs between the size of
the mechanism and the acceptable error.
For example, Reaction Design has developed a suite of advanced mechanism-
reduction methods, including both skeletal-reduction and more severe reduction
methods. Skeletal methods systematically remove elementary reactions and species
from the system, without changing any properties of the remaining species or rates of
the remaining reactions. Our implementations of mechanism-reduction techniques are
useful over a broad range of conditions and testing in any of CHEMKIN’s reactor
models. Skeletal methods can typically achieve 50-80 percent reduction, depending on
the application, with very good accuracy over a wide range of conditions. We can
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strictly control the target accuracy and achieve the optimal size to meet the target
application of the mechanism.
You may further reduce mechanisms from their skeletal forms using lumping
methods for species or reactions. For example, you may lump several isomers together
and treat them as a single species. Rate constants are then adjusted to account for the
combination of the various pathways and product branching represented by the original
isomers. For high-temperature combustion, lumping techniques usually provide
reasonably accurate results. Other severe-reduction methods use quasi-equilibrium
assumptions and the process for achieving these can be automated to various degrees.
You can combine these methods to get the smallest mechanism possible, while still
achieving a targeted level of accuracy relative to the Master Mechanism. This approach
of determining error using direct comparisons in reduced-order CHEMKIN simulations
provides quantitative uncertainty information that is key to managing the speed vs.
accuracy trade-off.
DIFFERENT REACTION MECHANISMS ARE APPROPRIATE FOR
DIFFERENT SIMULATIONS
The type of combustion simulation models that you employ will also influence
your choice of detailed kinetics mechanism. You must consider the practicality of the
mechanism size when determining what degree of reduction, if any, is necessary from
an appropriate Master Mechanism. Early on, in the concept definition phase of
combustion equipment design, it may make sense to employ reduced-order models,
such as the reactor models available in CHEMKIN, to scope out a general approach to
achieve the overall system goals (see Figure 1). For example, you may decide to use
0-D Perfectly Stirred Reactors (PSR) to identify the range of fuel-air ratios and overall
system residence times that are needed to achieve a certain level of NOx. Additionally,
you may use a 1-D Plug Flow Reactor (PFR) model to determine the length of the
dilution section of a gas turbine combustor that is required to oxidize CO down to an
acceptable level. One benefit of using these reduced-order models is that you can use
the fully detailed kinetics mechanism (i.e., a Master Mechanism) within a reasonable
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computational time. If you need to explore many conditions, or the mechanism is
extremely large, then you can employ some mechanism reduction techniques to
optimize the Master Mechanism for your application.
Combustion system designers typically move to more detailed system geometry
as the design matures from a concept to hardware definition. Computational Fluid
Dynamics (CFD) is commonly employed to provide 3-D simulation of the combustion
process in the realistic complex geometry of the hardware. While CFD provides
excellent accuracy of the complex geometry, it is typically unable to accommodate the
use of a complex, detailed kinetics mechanism beyond a handful of reactions and
species. In general there is always a tradeoff in the degree to which detailed chemistry
can be included and the degree to which geometry details can be captured (as
suggested in Figure 1). While CFD simulations may be limited to using Global, Single-
Step or severely reduced reaction mechanisms, the value of the Master Mechanism is
that you can at least test these global mechanisms against the full chemistry to
understand what errors are being introduced under what conditions. The reduced-order
models, such as those found in CHEMKIN, can be very helpful to make these
comparisons and assessments. In most cases, there is a significant amount of error
associated with these severely reduced mechanisms within the CFD simulation that
prevents the accurate prediction of chemical phenomena, such as pollutant species
production.
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Geometric Complexity
Full 3-D CFD Simulation
Reduced-Order Kinetics Model
(0-D or 1-D)
Chemistry Complexity
Figure 1: Spectrum of Combustion Simulation Models with Reasonable Simulation Times
IDENTIFYING A DETAILED KINETICS MECHANISM FOR YOUR
SIMULATION
As described above, the development and validation of a detailed chemical
mechanism for real fuels in real applications typically requires the undertaking of a
complicated and costly process that is predicated by way of an expert understanding of
combustion and fuel chemistry. Once complete, the mechanism provides the recipe or
description of the fuel chemistry that you can use for competitive advantage. If the
development and validation of the detailed mechanism is funded by public sources such
as the US DOE, NASA, or other agencies in the business of providing technology for
the public good, then these mechanisms will be made publicly available.
On the other hand, if the funding source for the detailed chemical mechanism
development and validation is from private, commercial entities, then it is likely that the
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mechanism will be treated as a proprietary trade secret. This is similar to the
proprietary recipes for Coca-Cola and Heinz Ketchup that are carefully protected from
the competition.
Fortunately, a wealth of key mechanisms is available in the public domain for
typical fuels and applications. If your application and the fuel are both fairly common, it
is likely you will find a mechanism that is suitable for your simulation needs. Reaction
Design2 and other organizations provide online resources to help you find publicly
available models for you to evaluate.
However, if you are developing a novel or advanced combustion technology
using non-standard fuels, then a suitable mechanism may not be publicly available. It is
important to note that there might not be appropriate public mechanisms for standard
applications using non-standard fuels – or standard fuels being used in non-standard
applications.
EXAMPLE MECHANISMS FOR NATURAL G AS TURBINES
To illustrate, let us consider natural gas combustion in industrial, power-
producing gas turbines. In the 1990s, the Gas Research Institute developed and
validated a widely used mechanism called GRI-Mech 3.0 for use with natural-gas-fired,
atmospheric-pressure burners for boilers and furnaces at conventional NOx levels. The
mechanism is excellent for use with those applications, and is also the most commonly
used natural gas mechanism in gas turbine combustion. However, there are two recent
industrial gas turbine applications in which this mechanism has been shown to be
lacking.
Ultra-Low NOx Gas Turbine Combustion: The first application3 involves the
use of natural gas in an ultra-low NOx (i.e., single-digit level) gas turbine. GRI-Mech 3.0
2 http://www.reactiondesign.com/support/open/datalinks.html
3 Drennan, S.D, et al. “Flow field Derived Equivalent Reactor Networks for Accurate Chemistry
Simulation in Gas Turbine Combustors” GT2009-59861, Proceedings of the ASME 2009 Turbo Expo,
2009.
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was found to consistently under-predict the NOx emissions from these applications
because the mechanism did not include some of the pressure-dependent, middle-to-
low-temperature reactions that are present in highly staged, ultra-low NO x gas turbine
combustors (note the dashed line with triangle symbols in Figure 2). When a new
mechanism was developed that incorporated the modern understanding of these
additional NOx formation reactions, the resulting mechanism produced excellent
agreement with ultra-low NO x experimental results.
Figure 2: Comparison of NOx and CO simulation accuracy against experimental data
using GRI-Mech 3.0 and an improved C2-NOx mechanism
High CO2 Gas Turbine Combustion for Efficient Carbon Capture: There is
great interest in reducing the greenhouse gas emissions from fossil fuel sources. One
approach for low CO2 emissions from power-producing combustion turbines that fire
natural gas is Carbon Capture and Sequestration (CCS). The CCS approach captures
the CO2 from the exhaust and then sequesters it in a manner to prevent its release into
the atmosphere. Carbon Capture technologies are most effective at extremely low
oxygen levels in the exhaust, but conventional gas turbines are unable to produce such
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low oxygen levels due to materials temperature limitations in the hot section of the
turbine (i.e., the combustor, transition piece and high pressure turbine blades).
In a new approach, the design method of Exhaust Gas Recirculation (EGR) can
supply CO2 into the combustion air upstream of the turbine. Through this method, the
increased CO2 in the combustion air acts to inhibit the temperatures of the combustion
gases, allowing full consumption of the available oxygen without overheating section
parts. There are a couple of key challenges to combustion stability and emissions when
you employ such a dramatic shift in the composition of the combustion air and the
conditions in the combustor. The first challenge is that the combustion is occurring at
lower, or vitiated, oxygen levels than it does with air at 21 percent oxygen by volume.
The second challenge is that both NOx and CO are also recirculated with the CO2 in the
EGR gases and they can affect the NOx and CO formation processes that determine the
exit NOx and CO emissions. Even though this is an application of natural gas in a
power generation combustion turbine, the conditions of the application are substantially
different than the GRI-Mech 3.0 validation conditions, raising questions on mechanism’s
applicability.
A recent investigation by Alstom4 determined that the GRI-Mech 3.0 mechanism
was not appropriate for the reburn of NOx that was carried along with the EGR into the
combustion air. Effort was taken to develop a modified, or improved, NOx mechanism
specifically to address the high CO2 , low combustion temperature conditions in this
application.
SUMMARY
Accuracy in combustion simulation can provide significant strategic advantages
for combustion system designers by reducing expensive experimental testing, speeding
new technologies to market and reducing costly field problems associated with poor
combustion performance predictions. Understanding how fuel burns through the use of
4 Guethe, F., M. de la Cruz Garcia, et al. (2009). Flue Gas Recirculation in Gas Turbine: Investigation of
Combustion Reactivity and NOx Emission. ASME Turbo Expo 2009. Orlando, FL, ASME.
GT2009-50221.
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detailed kinetics mechanisms can help designers use simulation models, such as
reduced-order kinetic reactor and comprehensive 3-D CFD, to better emulate real world
systems. The development of these mechanisms is a costly, time-consuming process
that requires expert knowledge of detailed combustion and fuel chemistry. However,
once these accurate Master Mechanisms have been developed, they can be used by
designers who are not experts in detailed chemistry for accurate simulation results.
Mechanism reduction techniques allow the model to use a smaller version of the Master
Mechanism for increased simulation speed, however careful attention must be paid to
the amount of error that is introduced through the mechanism reduction process.
Detailed kinetics mechanisms are available for many common fuels and applications,
but the use of these mechanisms must be carefully evaluated for use under your
specific conditions.
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