Vol. 49 No. 1January/February 2008 - $7.50

The Global Journal of Energy Equipment

INTERNATIONAL

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GE Oil & Gas Meeting— page 20

Burning alternative fuels — page 26

Siemens’ flexible products — page 30

Aero vs Industrial — page 34

The Adaptive HRSGs— page 16

Buying a gas turbineNo quick pick— page 16

SCOTT A. DRENNANREACTION DESIGN

There is a growing interest in the use ofalternative fuels in gas turbines. Thedrivers are economic, environmentaland strategic — energy indepen-

dence. But the combustion of these fuelsposes technical and design challenges.

Combustion and flow-simulation tech-niques are widely employed to develop gasturbine combustors. However, currentmethods often employ an overly simplisticapproach to the combustion process. Theyuse a single, or reduced set of chemicalsteps that cannot provide the level of detailrequired to accurately simulate key com-bustion parameters, such as pollutant emis-sions and combustion stability. Therefore,advanced combustion simulation tech-niques need to be developed if the highcost of experimental testing is to be avoid-ed while developing low emission combus-tors that burn alternative fuels.

Ongoing development of detailed fuelchemistry mechanisms has shown thatvarious compounds within the fuel canhave significant impacts on combustionperformance that must be taken intoaccount by combustor designers. Thereneeds to be an effective method of merg-ing the benefits of Computational FluidDynamics (CFD) modeling and detailedchemistry simulation on complex com-bustor geometries so that key parametersof combustion stability, such as LeanBlow Out (LBO), can be evaluated.

The creation of a seamless manner toapply detailed chemistry to realistic com-bustor geometries will provide a powerfultool, giving combustor designers freedomto adopt low emissions designs for alterna-tive fuels. Stakeholders, such as fuel sup-pliers, gas turbine manufacturers and soft-ware vendors should cooperate in further-ing the combustion of alternative fuels.

Modeling combustionPower providers are increasingly interestedin gas turbines with fuel flexible designsthat meet both regulatory and performancestandards to take advantage of lower-costfuels. They also have a strong desire todecrease their dependence on experimentaltesting in developing new engine designs.This is because it is becoming cost-prohib-itive to emulate all combustion conditions.Increasingly, designers of gas turbine com-

bustors are incorporating more simulationinto their design processes to minimizeengine testing, development time, andmaximize performance.

The process of combustion converts ahydrocarbon fuel to carbon dioxide andwater vapor through a series of detailedchemical steps that are highly dependentupon the environmental conditions, tur-bulence levels and chemical concentra-tions present in the combustor. It is pos-sible to reduce all the detailed chemicalsteps in the mechanism to a single, or aset, of “reduced” reactions, but informa-tion on the path of key steps in the reac-tion are lost in the process. These reac-tions include steps that control the forma-tion of pollutant species, such as NOx,CO, soot and SOx. Research over the last30 years has significantly increased ourunderstanding of the detailed stepsinvolved in the reaction of fuel to formcombustion products and the impacts ofvarious fuel compounds on combustion.

Development of a detailed fuel mech-anism involves creating a complete list ofchemical reactions and chemical speciespresent during combustion. It is impor-tant that the mechanism is validatedagainst experimental data under condi-tions representative of the combustionenvironment in order for the mechanismto be able to accurately predict combus-tion behavior.

The automotive industry has been ableto accurately predict complex, chemicallydriven phenomena, such as knock, withdetailed chemistry information. It followsthat the same level of understanding of thechemical mechanisms in a gas turbinewould have an impact on the ability to accu-rately predict combustion stability parame-ters, such as LBO, ignition and flashback ingas turbines. The challenge has been in pro-

viding the combination of fuel mechanismand simulation software tools that canquickly compute the result that can be effi-ciently used in the design flow.

Both the understanding of detailedchemistry and the processing power ofcomputers have greatly increased in thelast decade, enabling accurate simulationof combustion for enhanced, clean-tech-nology design. Petroleum fuels, such askerosene, contain hundreds of differenthydrocarbon species that all contribute inspecific ways toward ignition, flamepropagation and pollutant formation.

The traditional technique of simulat-ing these fuels using empirically derivedchemistry parameters does not provideaccurate emission prediction of tracespecies or the necessary detail requiredfor use in design and optimization. Thus,the development of accurate surrogatefuel models for use in chemical kineticsimulations is a critical step towardenabling computer-aided engine and fueldesign for petroleum and alternative fuels.

NOx challengesHistorically, gas turbine combustion hasallowed highly compact, efficient andstable designs that encouraged its wide-spread adoption as a power generationdevice. At the onset of NOx regulations,techniques such as steam injection, waterinjection and backend cleanup withSelective Catalytic Combustion (SCR)were used to meet single-digit NOx regu-lations, but with undesired cost and per-formance penalties.

Water and steam injection add systemcomplexity and increase maintenance costs.While SCR is effective at reducing NOx, theadditional costs and space requirements canbe prohibitive. Achieving Dry Low NOx(DLN) combustion has focused on reducing

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BURNING ALTERNATIVE FUELS

Figure 1: A reactor network representation of a gas turbine combustor. Detailed simulation ofchemical reactions is required to develop liquid-fuel combustors with low emissions

COMBUSTOR DESIGN CAN BE ADVANCED BY INCORPORATING CHEMICALREACTION MODELING MECHANISMS

the flame temperatures through altering thestoichiometry of the fuel-air mixture with-out steam or water injection.

The last 20 years of gas turbine com-bustion design have focused on develop-ment of low NOx emission designs whilemaintaining operational flexibility onbase fuels such as natural gas and dieseloil. In previous generations of combustordesigns, the combustion was limited bythe degree of mixing occurring withinthe combustor as chemistry was typical-ly much faster than mixing. Ultra lowNOx emission combustors primarilyfocused on reducing the flame tempera-tures within the combustor to limit the

formation of thermal NOx through LeanPremixed combustion.

Low NOx combustor designers knowthat the low NOx limit is bounded by theonset of combustion instability in the formof LBO. LBO occurs when the thermalenergy generated by the burning fuel-airmixture is no longer sufficient to heat theincoming fuel to the ignition point. Ignitionand combustion flashback are also keyissues with ultra low NOx combustion dueto the lean premixed design.

Flow simulationGas turbine combustor designers recog-nized the need to accurately simulatecombustion in order to effectively devel-op new DLN combustors. The use ofCFD in combustion has now becomestandard in the industry. Faster computersand better knowledge of fluid dynamicsand combustion chemistry have allowedCFD to become integrated into the com-bustor design process and to provide avaluable design tool.

One of the primary benefits of CFD isthe ability to test various combustordesign options quickly on the computerrather than relying on expensive and timeconsuming experimental testing. CurrentDLN combustor designs were developed

using combustion simulation with CFDand detailed chemical simulation, albeitseparately, as a critical step in producingeffective NOx reductions.

As combustion temperatures decrease,several other undesirable combustion phe-nomena become more prevalent and mustbe addressed. The primary limiting factorin low NOx combustion in premixed sys-tems is LBO. Flashback, in which theflame front propagates upstream poten-tially causing equipment and personneldamage, should be prevented.

Low NOx combustion requires lower-ing the flame temperature, and the nor-mally fast chemical reaction time isslowed down to the point where it beginsto impact the health of the flame. Theresult is a flame that is struggling to staylit and fights extinction with the onset ofcombustion instabilities that can lead tosignificant system vibrations, LBO,flashback or ignition issues. It is wellknown that chemical reaction rates andshort lived chemical species are indica-tors of LBO and combustion instability,so it is important to develop an effectivemanner to simulate full combustionchemistry at low NOx levels.

Modern CFD codes are capable ofresolving complex combustor geome-

Figure 2: A CFD solution of a gas turbinecombustor

UUSSIINNGG BBIIOO FFUUEELLSS AANNDD CCOOAALL--TTOO--LLIIQQUUIIDDSSTTraditionally, liquid fuel use in gas turbines has been limited to kerosene or diesel fuel. New liquid fuels are now being developed fromsources, such as coal, oil and biomass. These fuels are being qualified for combustion in aircraft gas turbines. Results from these efforts mayhelp to extend their use in power generation turbines, as well.

The Fischer-Tropsch (F-T) process converts coal or heavy oil into a liquid fuel with advantageous combustion characteristics through a puri-fying process that eliminates many of the undesirable chemical components that are precursers to soot and other pollutant compounds. Bio-diesel and alcohol-based bio-fuels are also being evaluated as carbon-neutral alternatives.

The National Aeronautics and Space Administration (NASA) is currently working with the U.S. Air Force and aircraft manufacturers to inves-tigate the potential of using alternative liquid fuels in fighter, transport and other airframes. The Air Force has committed to a program tocertify all airframes for use with an alternative fuel blend by 2011, and standardize the fleet by 2016. The new standard fuel will be 50% F-T fuel blended with 50% JP-8 (the U.S. military equivalent of Jet-A fuel).

The primary benefits of using alternative fuel blends is the reduced amount of increasingly more expensive oil required for expanding jetpropulsion needs and the improved particulate and NOx emissions performance of these fuels. Since Fischer-Tropsch jet fuel has fewer sootforming aromatics than JP-8, the blend yields fewer particulates, which are undesirable for both environmental and military reasons.

This NASA program has already certified the B-52 bomber for use with the 50/50 F-T fuel blend. The process of certifying an existingengine on new fuels involves careful use of engine combustion simulation, full engine tests and flight testing (“Modeling Clean Burning Fuelsto Cut Emissions,” p. 8, Nov./Dec. 2007).

The program involves developing a comprehensive set of fundamental data on the combustion of alternative jet fuels, using a sur-rogate fuel approach. The surrogate, or model, fuels provide guidance to the planning and design of optimal fuel-production process-es. An additional focus of the NASA project emphasizes the development of detailed combustion mechanisms with corresponding val-idation data for bio-kerosene fuels.

There is also great interest in the use of bio-jet-fuels for both power generation and aircraft gas turbine applications. Bio-jet-fuels areproduced from sustainable plant products making them a “carbon-neutral” fuel, which could contribute to reducing the global warm-ing impact of CO2 emissions. Some bio-jet fuels are alcohol-based and originate from crops, such as corn that have less heating valueper gallon than other alternative fuel options. These are not effective for use in aircraft fuel because of the additional weight require-ment it places on the airframe.

However, alcohol-based fuels may be applicable in ground-based gas turbine systems that allow the lower heating value of the bio-fuel.Other bio-fuels are produced from crops, such as rapeseed (canola), soybeans and sunflowers. These non-alcohol-based fuels are high infat content, thereby providing a higher heating value for use in jet fuel.

Bio-diesel is also a viable alternative for liquid-fueled turbines for power generation. Recent tests at a Dynegy facility validated theapplicability and performance of a bio-fuel. Testing indicated lower NOx emissions when firing the bio-diesel as compared with distil-late oil (“Biofuel Passes Test in Gas Turbine Engines,” Nov./Dec.2007, p. 8) The tests were performed on Pratt & Whitney FT4 gas tur-bines in Oakland, California.

tries producing simulations of the flowswithin the combustor, but with limitedchemistry information (Figure 2).While today’s computers are fastenough to handle combustor designmodels with several million cells, theyare still not fast enough to incorporateall of the detailed combustion chem-istry of the fuel. Applying detailed fuelchemistry in combustion simulationtoday requires simplification of thegeometry into a series of idealizedreactors, so detailed chemistry can beapplied in a reasonable amount of com-putational time.

Using fuel chemistryFigure 1 shows an example of how thecomplex combustor geometry is convert-ed into an Equivalent Reactor Network(ERN). Once the ERN is created througha careful devolution of the combustorflow field, the detailed chemical mecha-nism can be used to provide an under-standing of chemical behavior and per-formance using chemistry simulationsoftware packages.

It is critical that the ERN is a true rep-resentation of the actual combustor flowfield in order for the simulation to be accu-rate and valuable in the design process.This requires detailed evaluation by expertpersonnel to create the ERN which takes

so long that it is not practical in the designprocess. So, a trade-off exists todaybetween accuracy in the geometry andaccuracy in the fuel chemistry.

The growing interest in alternative fuelhas enhanced the interaction betweenengine and fuel manufacturers. With fixedfuel options, the engine manufacturershave needed little interaction with fuelsuppliers over how fuel production optionsaffected engine performance. However,new blends of alternative fuels have drivenboth engine and fuel suppliers to recog-nize the value of understanding the effi-ciency impacts of various fuel designsfrom the wellhead through the engine.

The gas turbine industry’s ability toeffectively use alternative fuels will havea large impact on future growth. In orderto help manufacturers realize this poten-tial, combustion simulation must provideseamless, highly integrated methods oflinking CFD and detailed chemistry.

The simulation of combustion systemdesigns requires resolution of the complexgeometry, turbulent flow patterns, heattransfer and complex chemistry. WhileCFD provides a platform to simulate thereacting flow in complex geometries, itcannot accept fuel chemistry mechanismsof more than several steps with reasonablesolution time and model stability.

What is lost with this reduced chem-

istry is representation of the impact oftrace species, such as NOx, CO andUnburned Hydrocarbons (UHC) thatrequire the use of the full chemical reac-tion mechanism with hundreds of speciesand thousands of reactions. A new soft-ware simulation technique is required toallow detailed chemistry to be employedwith CFD in order to produce accuratemodeling of flows and detailed chem-istry enabling accurate combustion sta-bility simulation. Vendors need tools thatseamlessly link CFD and the ERNs toprovide rapid application of detailedchemistry in modern gas turbine com-bustion design.

AuthorScott Drennan is Director of marketing andapplication engineering at Reaction Design, theprovider of CHEMKIN — a software that solveschemical kinetics. His experience includes con-ducting CFD analysis atAllison Gas Turbines.Drennan has a Master ofScience in MechanicalEngineering with anemphasis on low-NOxcombustion and liquidfuel atomization for gasturbine combustors.Reach him at sdren-nan@reactiondesign.com

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