Environmental, economic and business goals are driving interest in the use of alternative fuels in many combustion applications, including gas turbines, boilers and burners. These new fuels present technical and design challenges that require new understanding and tools for the combustor designer, in particular a heavy emphasis on combustion simulation. Unfortunately, traditional combustion and flow simulation strategies oversimplify the combustion process into a single, or reduced, set of chemical steps. Newer, more advanced simulation tools connect Computational Fluid Dynamics (CFD) simulation and detailed chemistry to provide the complexity and accuracy needed to simulate key combustion parameters in alternative fuels.

1. Simulating Fuel Flexibility and
Combustion Performance
By Scott A. Drennan, P.E.
Director of Applications Engineering and Marketing
sdrennan@reactiondesign.com
Reaction Design
6440 Lusk Blvd, Suite D205
San Diego, CA 92121
Phone: 858-550-1920
www.reactiondesign.com

2. Reaction Design White Paper
INTRODUCTION
Environmental, economic and business goals are driving interest in the use of
alternative fuels in many combustion applications, including gas turbines, boilers and
burners. These new fuels present technical and design challenges that require new
understanding and tools for the combustor designer, in particular a heavy emphasis on
combustion simulation. Unfortunately, traditional combustion and flow simulation
strategies oversimplify the combustion process into a single, or reduced, set of chemical
steps. Newer, more advanced simulation tools connect Computational Fluid Dynamics
(CFD) simulation and detailed chemistry to provide the complexity and accuracy needed
to simulate key combustion parameters in alternative fuels.
INCREASING INTEREST IN ALTERNATIVE FUELS
Governments worldwide are implementing regulations aimed at reducing CO2
and other harmful emissions. As a result, more aircraft and power generation
technologies are being developed that burn various combinations of fuels. For
example, boilers and turbines are being modified to run on bio-fuels, as well as
traditional fuels such as natural gas, diesel and kerosene. Alcohol-based bio-fuels are
also being evaluated as possible carbon-neutral alternatives.
There is great interest in the use of bio-fuels for power production in gas turbines
and boilers. Bio-fuels are produced from sustainable plant products making them a
“carbon-neutral” fuel, thus reducing the global warming impact of CO2 emissions. Some
bio-fuels are alcohol-based and originate from crops such as corn or other non-food
sources and have less heating value per gallon than competing alternative fuel options.
Other bio-fuels are produced from crops such as rapeseed (canola), soybeans and
sunflowers. These fuels are high in fat content and allow for higher energy content than
alcohol-based bio-fuels. These fuels are also less stable than petroleum fuels and
suffer from the same oxidation process that makes cooking oils go rancid. But beyond
heating value, pumpability and shelf-life issues, the fact remains that these fuels have
different combustion characteristics than traditional fuels.
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Given the wide palette of alternative fuel options, the traditional method of relying
on experimental testing to evaluate combustion performance is simply unrealistic.
Another option is needed to avoid excessive cost and time delays in adapting systems
to make use of these less polluting fuels.
CFD ENABLED THE DEVELOPMENT OF LOW NOX DESIGNS
To solve the emission problem of NOx – which causes smog or brown haze –
designers forced the combustion flame to a low temperature through combustion
staging with either lean or rich burn approaches. Because fundamental detailed
chemical studies showed NOx formation increased exponentially with temperature, it
followed that lower combustion temperatures would achieve lower NOx emissions. It is
widely accepted that current lean Dry Low NOx (DLN) and rich-lean (RQL) combustor
designs were largely developed by using an understanding of detailed NOx chemistry
and combustion simulation with CFD as a critical step in predicting NOx reductions.
Modern CFD software is capable of resolving complex combustor geometries
and producing simulations of the flows within the combustor, but with only limited
chemistry information. While today’s computers are fast enough to handle combustor
design models with millions of cells, they are still not fast enough to incorporate all of
the detailed combustion chemistry of the fuel.
Figure 1: CFD solution of gas turbine combustor.
CFD simulations can only accept greatly simplified representations of the fuel
composition. Figure 2 shows how real fuels are a blend of hundreds of various
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chemical compounds. These compounds are averaged to give a single, oversimplified
fuel composition to allow them to be simulated with CFD. It is easy to see how this
oversimplification of the fuel required by CFD cripples the ability of the combustion
designer to vary the impact of real fuel compositions in their favorite simulation tool.
The process of combustion involves a series of detailed chemical steps that are
highly dependent upon the environmental conditions, turbulence levels and chemical
concentrations. It is possible to reduce all the detailed chemical steps in the
mechanism to a single set of “reduced” reactions, but information on the path of key
steps within the reaction is lost. These reactions include steps that control the formation
of pollutant species and combustion stability. Research over the last 30 years has
significantly increased our understanding of how fuel decomposes into combustion
products and the impacts of various fuel compounds on combustion. However,
designers simply can’t model real fuels using traditional CFD. Applying detailed fuel
chemistry today in combustion simulation requires simplification of the geometry into a
series of idealized reactors, so detailed chemistry can be applied in a reasonable
amount of computational time.
Real fuels are a complex Currently modeled as a single
mixture of hundreds of species component fuel in CFD
Representative examples of Major
Classes of molecules in real fuels
*
n-heptane
Figure 2: Real fuel composition vs. CFD fuel representation.
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LOW-EMISSION, FUEL-FLEXIBLE COMBUSTION CHALLENGES ARE
KINETICALLY DRIVEN
A well understood result of low-NOx designs in turbines and burners is that they
are inherently less stable than older technologies. Lower temperature combustion
makes turbines and burners less amenable to alternative fuels. A typical modern
natural-gas-fired turbine, for example, is less tolerant of the variation in composition of
LNG or low-carbon fuels from different sources. That makes detailed chemistry
simulation even more important for next generation combustors.
As combustion temperatures decrease, several other undesirable, kinetically
driven combustion phenomena become more prevalent and must be addressed. In a
lean low-NOx flame, the normally, very fast chemical reaction time slows down to the
point that it becomes limiting over fluid mixing at lower flame temperatures. The result
is a flame that is struggling to stay lit and thus fights extinction. The primary limiting
factor in lean low-NOx combustion in premixed systems is lean blow-out (LBO). LBO
occurs when the heat generated by the burning fuel/air mixture is no longer sufficient to
heat the incoming fuel to the ignition point. Two key combustion stability challenges in
low NOx combustion are flashback, in which the flame front propagates upstream, and
ignition resulting from the lower flammability of the fuel mixture. Chemical reaction rates
and short-lived chemical species are indicators of LBO and combustion stability.
EFFECTIVELY USING DETAILED CHEMISTRY SIMULATION IN REAL
COMBUSTION SYSTEMS
CFD simulation does an adequate job of predicting temperature globally, so it
served well to solve the NOx problem. However, combustion challenges discussed
above, that designers need to simulate today, are kinetically driven and require detailed
chemical simulation. This simulation has been accomplished for over 30 years through
the use of idealized chemical reactor modeling using chemistry simulation software
packages such as CHEMKIN®. The challenge has always been how to convert the
complex 3-D flow field and geometry into a simplified chemical reactor simulation.
Recently, researchers have proven the capability of building networks of these idealized
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reactors that represent a reduced-order model of the flow field and geometry with
excellent combustion results.
Figure 3 shows an example of how the complex combustor geometry is
converted into an Equivalent Reactor Network (ERN). Once the ERN is created through
a careful devolution of the combustor flow field, the full detailed chemical mechanism
can be used to provide an understanding of chemical behavior and performance.
Figure 3: Reactor network representation of a gas turbine combustor.
For the simulation to be accurate, it is critical that the ERN be a true
representation of the actual combustor flow field. Traditionally, expert personnel would
be required to create the ERN manually: a process so time consuming that it is not
practical in the design process.
SIMULATING FUEL FLEXIBILITY AND ALTERNATIVE FUELS
Using new simulation tools and techniques such as ENERGICO™ can extend
the ERN approach to the simulation of both fuel flexibility and the use of alternative
fuels. For example, an ERN can now be used to answer key real-world fuel flexibility
challenges:
• How do you determine the impact of IGCC or opportunity fuel composition
variations in CO, HC, CH4, etc., concentrations?
• How do you determine the impact of alternative low-carbon bio-fuels in
existing and new combustion system designs?
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An ERN can provide an excellent platform to accurately simulate combustion
performance on real fuels. The ability to use accurate fuel chemical mechanisms in an
efficient model allows for greater accuracy in combustion predictions.
Obtaining accurate combustion simulation with alternative fuels with detailed
chemistry can be summed up as follows:
1. Use the best global or severely reduced mechanism possible in the CFD
analysis to get the best starting point for the ERN analysis, then…
2. Use the appropriate accurate detailed chemical mechanism in the ERN
analysis for accurate results, then…
3. Use the ERN to conduct parameter variations on fuel composition to
understand their impact on combustion results.
THE NEED FOR BETTER MECHANISMS
Remember the day when you purchased your first CD player or DVD player?
The first thing you wanted was to upgrade your record or videotape collections to the
higher fidelity media. You even replaced records and videotapes that you already had.
The parallel is true with ERN analysis. Using a high-fidelity tool such as an ERN will
require a user to upgrade their collection of fuel mechanisms from the global or severely
reduced mechanisms that they currently use in CFD. The good news is that, to a large
extent, detailed chemical mechanisms are available or being developed for many
common fuels. The mechanism must be developed and validated for the conditions that
are specific to the combustion system. It is exceedingly important that the mechanism
you use has been developed and validated for the operating conditions of interest (e.g.,
pressure, temperature, equivalence ratio, etc.).
Consider the well-known GRI-Mech 3.0 mechanism that was developed and
validated on conventional boiler burners. While this mechanism is tailored to fit these
devices, it lacks the pressure-dependent NOx reactions to make it adequate for gas
turbines and also does not include some recently understood medium- and low-
temperature NOx reactions that are important in highly staged, ultra-low NOx flames. In
the area of liquid fuels, where the real fuel has over 500 different chemical species, it is
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important to use a fuel surrogate approach where the simulation fuel is built from a
blend of pure compounds such as toluene and isooctane to create a “model” fuel.
Employing a new generation of simulation tools, the combustor designer can
apply detailed chemical mechanisms for each of these pure compounds in the surrogate
fuel to create an accurate detailed chemical mechanism for the model fuel. The
designer can then use mechanism reduction techniques to create the best global, or
severely reduced, mechanism with a small number of species for the CFD analysis.
Once the ERN is created from the CFD case, mechanisms are then used to add
detailed chemistry into the simulation process.
EVOLVING NEEDS FOR IMPROVED COMBUSTION SIMULATION
The industry’s ability to take advantage of the opportunity to use alternative fuels
will secure future markets for gas turbines, boilers, automobile engines and other
combustor applications. Combustor designers are seeking to use larger, more refined
grid systems to resolve the highly detailed geometry of a modern combustor with its
fuel/air injectors, swirler flows and cooling air. The use of reduced-order turbulence
models, such as k- , has been targeted as a key factor in the inability of today’s CFD
codes to predict combustion stability. Advanced turbulence models such as Large Eddy
Simulation have shown promise in predicting the unsteady flow regimes in the gas
turbine combustor, but at the cost of 20 times longer computational times. So, even
with faster computers, it is likely that comprehensive turbulence and chemistry modeling
will take more time to emerge.
For today, new simulation techniques are helping designers employ detailed
chemistry with CFD to produce reasonably accurate modeling of flows and detailed
chemistry for fast, accurate combustion stability simulation. The ongoing development
of detailed fuel chemistry mechanisms has shown that various compounds within the
fuel can have significant impacts on combustion performance. Because of these new
challenges, designers are adopting effective methods of merging the benefits of CFD
modeling and detailed chemistry simulation on complex combustor geometries, so key
parameters of combustion stability, such as LBO, can be evaluated without significant
amounts of costly experimental testing. These methods apply detailed chemistry to
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realistic combustor geometries to give combustor designers the freedom to adopt low-
emissions designs for alternative fuels in the future.
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