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  Bio-fuels for the gas turbine: A review K.K. Gupta a, *, A. Rehman b , R.M. Sarviya b a Mechanical Engineering Department, Medi-Caps Institute of Technology and Management, Pigdamber, Rau, Indore (M.P.), India b Department of Mechanical Engineering, MANIT, Bhopal (M.P.), India Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29462. Straight vegetable oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29473. Bio-diesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29484. Bio-ethanol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29495. Bio-methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29506. Lean, premixed, pre-vaporization of liquid bio-fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29507. Biogas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29508. Syngas derived from biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29509. Pyrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295110. Dimethyl ether (DME) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295111. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295212. Comparison of liquid fuel characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295313. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2954References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2954 1. Introduction World energy consumption has increased 17-fold in the lastcentury and emissions of CO 2 , CO, SO 2  and NO  x  from fossil-fuel Renewable and Sustainable Energy Reviews 14 (2010) 2946–2955 A R T I C L E I N F O  Article history: Received 24 May 2010Accepted 15 July 2010 Keywords: Gas turbineBio-fuelAlternative fuelsEmissionBio-dieselBiomassSyngas A B S T R A C T Duetodepletionoffossilfuel,bio-fuelshavegeneratedasignificantinterestasanalternativefuelforthefuture. The use of bio-fuels to fuel gas turbine seems a viable solution for the problems of decreasingfossil-fuel reserves and environmental concerns. Bio-fuels are alternative fuels, made from renewablesourcesand having environmentalbenefit.In recent years, thedesireforenergyindependence,foreseendepletion of nonrenewable fuel resources, fluctuating petroleum fuel costs, the necessity of stimulatingagriculture based economy, and the reality of climate change have created an interest in thedevelopment of bio-fuels. The application of bio-fuels in automobiles and heating applications isincreasing day by day. Therefore the use of these fuels in gas turbines would extend this application toaviation field. The impact of costly petroleum-based aviation fuel on the environment is harmful. So thedevelopment of alternative fuels in aviation is important and useful.The use of liquid and gaseous fuels from biomass will help to fulfill the Kyoto targets concerningglobalwarmingemissions.Inaddition,toreduceexhaustemissionwastegasesandsyngas,etc.,couldbeused as a potential gas turbine fuel. The term bio-fuel is referred to alternative fuel which is producedfrom biomass. Such fuels include bio-diesel, bio-ethanol, bio-methanol, pyrolysis oil, biogas, syntheticgas (dimethyl ether), hydrogen, etc. The bio-ethanol and bio-methanol are petrol additive/substitute.Bio-diesel is an environment friendly alternative liquid fuel for the diesel/aviation fuel.The gas turbine develops steady flame during its combustion; this feature gives a flexibility to usealternativefuels.Thereforesotheuseofdifferentbio-fuelsingasturbinehasbeeninvestigatedbyagoodnumber of researchers.The suitability and modifications intheexisting systems arealso recommended.   2010 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +91 9827406077; fax: +91 7312856263. E-mail addresses:  guptakks@yahoo.com (K.K. Gupta), manit1963@yahoo.com(A. Rehman), rmsarviya@rediffmail.com (R.M. Sarviya). Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser 1364-0321/$ – see front matter    2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.rser.2010.07.025  combustion are the main causes of atmospheric pollution.Worldwide petroleum reserves are expected to be depleted inless than 50 years at the present rate of consumption. In thisscenario, bio-fuels have emerged as alternative sources of energyand offering many other benefits including sustainability, reduc-tion of greenhouse gas emissions, rural development and securityof supply [1].At present bio-energy use covers 9–14% of the global demand,most of which as traditional, low tech and inefficient using incooking and heating in developing countries. Different globalenergy scenario studies indicate that in India biomass maycontribute much more: up to 30% of the energy supply by 2100[2]. Bio-fuels produced from biomass such as plants or organicwastecouldhelptoreducedependenceonoilandGHGproduction.These bio-fuels have the potential to reduce CO 2  emission becausethe plants they are made from use CO 2  as they grow [3,4].The gas turbine is a continuous-flow engine which developssteady flame during its combustion. This favorable feature allowsthe use of various fuels and also provides clean combustion in thegas turbine. Moderate compression ratios, robust mechanicaldesigns and versatile combustion systems helps to utilize a wideseries of bio-fuels such as alcohols, bio-diesel, LCV gasifiedbiomass, synthetic gas, hydrogen, etc., in addition to conventionalnatural gas. These fuel properties influence on gas turbineefficiency, NO  x  emission and combustion design [5]. The cleancombustion of biomass-derived biogas (LHV < 25% of natural gas)and hydrogen enriched fuels is due to lean, premixed combustion,resulting low-NO  x  emissions [6].The gas turbine engine has number of attractive features suchas compact size, short delivery, high flexibility, reliability, faststarting, fast loading, lower manpower operating needs and betterenvironmental performance, as compared to steam turbine plant.However,itsuffersfromlowefficiency,especiallyatpartload.Thisshortcoming of gas turbine is overcome by using cogeneration,which is a simultaneous production of power and thermal energywiththeuseofenergyoftheexhaustgases[7].Thecombinedcycletechnology is now well established and superior to any of thecompeting gas turbine based systems. Mixed air steam turbines(MAST) are among the possible ways to improve the performanceof gas turbine based power plants. Water or steam injection in tothe combustion chamber for NO  x  emissions control is alsocommonly used. Depending on the amount of water or steaminjection needed to achieve the desired NO  x  level, output will alsoincrease because of the additional mass flow [8].Gas turbine driven cogeneration plants using bio-fuels can belocated close to energy consumption centres, especially forremote areas of developing countries where grid power is notavailable. They can produce their own fuel such as biomass-derived fuels. Gas turbines are having large power range, andthey are suited with certain modifications in fuel supplysystems. Current gas turbine systems though, are capable of burning such fuels, are normally developed for a single specificfuel (such as natural gas or diesel fuel). In these remote areas,different types of vegetable oils are grown/produced locally butit may not be possible to chemically process them due tologistics problems in rural settings. Hence using heated orblended vegetable oils as petroleum fuel substitutes/additives isalso an attractive option [9].Sunflower and rapeseed oil are the generally used raw materialfortheproductionofbio-dieselinEuropewhereasSoybeanisusedin USA. Thailand uses palm oil, Ireland uses frying oil and animalfats. In India it is proposed to use non-edible oil for producing bio-diesel. Presently many species are being grown which yield seedcontaining non-edible oils. The present production is being usedand much surplus is not available. It is required to initiate a majorplantation program of different oil born plants to provide the oilneeded. This plantation can be easily under taken in the farmers’fields and their boundaries, deserted lands, public lands alongrailway tracks, highways, canals and community and governmentlands in villages, Its plantation, seed collection, oil extraction, etc.,will create employment opportunities for a large number of people, particularly the tribal and the poor, and it will help torehabilitate unproductive and wastelands and save preciousforeign exchange by substituting imported crude.The fibrous, grassy materials, agricultural residues, municipalwastes and industrial wastes are also the feed stocks for theproduction of the synthetic fuels such as bio-methanol, bio-ethanol, DME (dimethyl ether), SNG (synthetic natural gas) andhydrogen via gasification and synthesis. Production of biogas canbe produced either from organic wastes or recovered frommunicipalsolidwastelandfills.Therecoveryofbiogasisimportantnot only as a resource, but also for avoiding the discharge of greenhousegasintheatmosphere.Upgradedbiogascompressedata pressure around 200 atm and filled in gas cylinders can also beused as a transport fuel [10].Compatibility of different bio-fuels with turbine fuel deliverysystems is primarily a function offuel viscosityand, inthecase of ethanol, it is material corrosion. In low temperature areas bio-dieselcanalsobepreheatedasdieseloil.Themaxallowablerangeof dynamic viscosity in the gas turbine is about 12 cSt withmodificationsinitsfuelnozzle.Allofthefuelsexceptpyrolysisoilfall within this range. However, blending of pyrolysis oil withethanol would reduce the viscosity of mixture to the desiredrange. Blending of the other fuels with diesel fuel also has afavorable effect on both viscosity and cost [11]. Now finallyselection of the liquid and gaseous fuels will be based upon theatomization and spray characteristics of liquid fuels, thecombustion and emission characteristics of the selected alterna-tive fuel, flames, the sooting tendency and NO  x  emissionproperties of the fuels under various operation conditions. Theadaptabilityofexistingconventionallyfuelledcombustorsforusewith the selected alternative fuels will depend on theseparameters.Theselectionmaybasedonavailability,composition,physical properties, and costs of the fuel [6]. 2. Straight vegetable oils Purevegetable oil cannot be used directly in gas turbine [1]. So,they have to be modified to bring their combustion relatedproperties closer to those of diesel. Such a fuel modification ismainly aimed to reduce the viscosity to eliminate flow/atomiza-tion related problems. Four techniques can be used to reduce theviscosity of vegetable oils; namely heating, dilution/blending,micro-emulsion, and trans-esterification [9].Vegetable oils mainly contain triglycerides (90–98%) and smallamounts of mono- and di-glycerides. Triglycerides contain threefatty acid molecules and a glycerol molecule. They containsignificant amounts of oxygen. The fatty acids vary in their carbonchain length and number of double bonds present in theirmolecular structure. Vegetable oils contain free fatty acids(generally 1–5%), phospholipids, phosphatides, carotenes, toco-pherols, sulfur compounds and traces of water. Commonly foundfatty acids in vegetable oils are stearic, palmitic, oleic, linoleic andlinolenic acid [9].The problems attributed to high viscosity and poor volatility of straight vegetable oils is due to large molecular weight and bulkymolecularstructure.Highviscosityofvegetableoils(30–200 cStata temperature of 40  8 C) as compared to mineral diesel (2 cSt at thesame temperature) lead to unsuitable pumping and fuel spraycharacteristics. Larger size fuel droplets are injected from injectornozzle instead of a spray of fine droplets leads to inadequate air-fuel mixing, poor atomization, lower volatility, and inefficient K.K. Gupta et al./Renewable and Sustainable Energy Reviews 14 (2010) 2946–2955  2947  mixing of fuel with air contribute to incomplete combustion. Thisresults an increment in higher particulate emissions, combustionchamber deposits and gum formations [9].The viscosity of vegetable oils can be reduced by increasing itstemperature (using waste heat of the exhaust gases) and therebyeliminating its effect on combustion and emission characteristics.Vegetable oils have energy density, cetane number, and heat of vaporization comparable with mineral diesel. In addition, they arebiodegradable, non-toxic, and have a potential to significantlyreduce pollution. Vegetable oils and their derivatives lead tosubstantial reductions in emissions of sulfur oxides, carbonmonoxide (CO), poly aromatic hydrocarbons (PAH), smoke andparticulate matter (PM). Moreover the effect of this emission isinsignificant, since carbon dioxide (CO 2 ) emitted during combus-tion is recycled in the photosynthesis process in the plants. [9]Thefuelvolatilityandsprayparametersofliquidfuelinfluencesthe combustion and emission characteristics in diffusion con-trolled gas turbine combustor. The combustion efficiency reducesdrastically with a decrease in fuel volatilities at lower spray coneangle. There is also an influence of some important operatingparameters like inlet swirl, air-fuel ratio, inlet pressure andtemperature on flow and combustion characteristics in thecombustor using both gaseous and liquid fuels [12]. Lefebvrestated that the combustion efficiency should increase with finerinitial droplets, as it increases the evaporation rate of the fuel [13].Trans-esterification is well accepted and best suited methodwithout significant long-term operational and durability issues.However,thisincreasesthecostofprocessingbecauseofthetrans-esterification reaction involving chemical and process heat inputs[14]. 3. Bio-diesel Bio-diesel is an environmental friendly fuel that can be used inany gas turbine without modification. Bio-diesel is better thandiesel fuel in terms of sulfur content, flash point, aromatic contentandbiodegradability.Therehasbeenrenewedinterestintheuseof vegetable oils for making bio-diesel due to its less polluting andrenewable nature as against the conventional petroleum dieselfuel. Due to its environmental benefits, the share of bio-diesel inthe automotive fuel market is growing fast [1].India’s demand for diesel is almost six times that of gasolinehence finding alternative to mineral diesel is important. Diesel islargelyutilizedinthetransport,agriculture,commercial,domestic,and industrial sectors for the generation of power/mechanicalenergy, and the substitution of even a small fraction of totalconsumption by alternative fuels will have a significant impact onthe economy and the environment. Out of the alternative fuels,bio-diesel obtained from vegetable oils holds good promises as aneco-friendly alternative to diesel fuel [15]. Bio-diesel is fatty acidethylormethylestermadefromvirginorusedvegetableoils(bothedibleandnon-edible).Themaincommoditysourcesforbio-dieselin India can be non-edible oils obtained from plant species whichbearseedsrichinoilsuchasJatrophaCurcas(Ratanjyot),PongamiaPinnata (Karanj), etc.It is derived from vegetable oils by modifying their molecularstructure through a trans-esterification process. Trans-esterifica-tion involves a reaction in a triglyceride and alcohol in presence of a catalyst to produce glycerol and ester. Yield of bio-diesel isaffected by molar ratio, moisture and water content, reactiontemperature, stirring, specific gravity, etc. [16].The characteristics of bio-diesel are close to diesel fuels, andtherefore bio-diesel becomes a strong alternative to replace thediesel fuels. The conversion of triglycerides into methyl or ethylesters through the trans-esterification process reduces themolecular weight to one-third that of the triglyceride reducesthe viscosity by a factor of about eight and increases the volatilitymarginally. Bio-diesel has viscosity close to diesel fuels. Theseesters contain 10–11% oxygen by weight, which may encouragemore combustion than hydrocarbon-based diesel fuels. Bio-dieselhas lower volumetric heating values (about 12%) than diesel fuelsbut has a high cetane number and flash point. The high flash pointattributes to its lower volatility characteristics [17].This diesel substitute requires very little or no gas turbinemodifications up to 20% blend and minor modification for higherpercentage blends. It can be blended at any level with petroleumdieseltocreateabio-dieselblendorcanbeusedinitspureform.Itcan be stored just like the diesel and hence does not requireseparate infrastructure. The use of bio-diesel in gas turbine resultsin substantial reduction of un-burnt hydrocarbons (UHC), carbonmonoxide and particulate matters without reducing the outputpower significantly [18]. The reduction in emissions by using 20%blend of bio-diesel in diesel fuel was about 12% for CO andparticulate matter (PM) emission and 20% for UHC emissions. Thereductionin emissions byusing 100% bio-diesel was about48% forCOandPMand68%forUHCforthegasturbine.However,thereisamarginal increase in NO  x  (1–6%) [17]. Uncontrolled emissions likepolyaromatichydrocarbons(PAH),etc., werealsofound tobe less.Bio-diesel is considered clean fuel since it has almost no sulfurcontent (typically it is less than 15 ppm), no aromatics and hasabout 10% built-in oxygen, which helps it to burn fully. Bio-dieselalsohassuperiorlubricity.Itcanalsobeusedasalubricatingagentfor conventional diesel, so the diesel can be free from sulfur.Flash point of bio-diesel is high ( > 100  8 C). Its blending withdiesel fuel can be utilized to increase the flash point of dieselparticularlyinIndiawhereflashpointis44  8 Cwellbelowtheworldaverage of 55  8 C. This is important from the safety point of view.Cetane number (CN) of the bio-diesel is in the range of 48–60, thishigher cetane number improves the ignition quality even whenblended in the petroleum diesel. The viscosity of bio-diesel ishigher(1.9–6.0 cSt)andisreportedtoresultintogumformationoninjector. However, blends of up to 20% do not give any problem.While a gas turbine can be run by 100% bio-diesel use, the existinggasturbinescanuse20%bio-dieselblendwithoutanymodification[1].The presence of oxygen in bio-fuel molecules was expected toresult in leaner combustion, and hence increase in the thermalefficiency. Higherthermalefficiencies with pure bio-diesel maybeattributed to lower equivalence ratios (leaner fuel/air mixtures)and more complete combustion due to the presence of extraoxygeninthebio-diesel.Furthermore,theoperationoftheturbinewith the products of a leaner mixture approaches to the air-standard cycle. The addition of bio-diesel such as soya methylester, canola methyl ester, and recycled rapeseed methyl ester in JET-A fuel for gas turbine have reduced the static thrust andspecific fuel consumption, and increased thermal efficiency [19].The turbine inlet temperature and exhaust gas temperature forallfuelsdonotshowsignificantchangeswiththefueltype.COandNO emission decrease when bio-diesel was used. The bio-diesel– JET-Ablendsappearpromising,whileemittingtheleastamountof pollutant with no significant reduction in static thrust [19].The turbine speed is a function of turbine inlet temperature(TIT). As the speed increased, the turbine inlet temperatureincreases due to increase of the heat release/heat loss ratio fromthecombustor.Thetemperaturefollowingcombustionofpurebio-diesel was slightly lower than that of JET-A at low speeds andapproached that of JET-A at high speeds. The exhaust gastemperature (EGT) is almost same for all fuels [19].Micro-turbinesarethesubsetofcombustiongasturbinesbeingused and improved for stationary power generation. Micro-turbines have evolved from the efforts to make gas turbinessmaller for automotive or aerospace applications. It tends to fall in K.K. Gupta et al./Renewable and Sustainable Energy Reviews 14 (2010) 2946–2955 2948  therangeof5–500 kWsizes.Thebio-dieselsanditsblendsusedasfuel in micro-turbines led to no significant changes in the engineperformance and behavior compared to diesel fuel [20].The exhaust emissions were evaluated for pure soya bio-dieseland its blends and conventional diesel. There were no significantchanges in the engine thermal performance even when comparedtotheuseofsoyabio-dieselwithJET-Afuel[21].Duringendurancetests in the turbine engine using rapeseed oil bio-diesel, theturbine nozzle and rotor presented a significant fouling depositand damage. This turbine was also tested with automotive diesel/castor bio-diesel blends. Castor bio-diesel is the one that presentsthehighestviscosity.Inordertoavoidproblemsregardingthehighviscosity of fuel, a preheating system was placed in the bio-dieselreservoir, to reduce its viscosity [18].Due to preheating, bio-diesel created no problem related toatomization process in the tests, and the thermal efficiency of theturbine reaches the level of around 26%. The minimum heat rateobtained at full load was for the bio-diesel from palm oil, and themaximumwasforcastoroil[19].Inordertodiminishtheviscosity,it is necessary to heat the fuel previously, especially for bio-dieselconcentrations higher than 50%. Such an attempt results in goodperformance, and no problems related to viscosity [22].The combustion of bio-diesel derived from waste cooking oil(WCO) indicates that it requires less air for stoichiometriccombustion due to the presence of oxygen in the fuel and sucha bio-diesel stand as a potential alternative fuel for powergeneration application with the good efficiency at blended ratioof 20% bio-diesel and 80% distillate. This has shown the feasibilityof using bio-diesel and its blends for gas turbine application. Bio-diesel was produced from waste cooking oil, mainly from palm oilsources.Theoilburnerwasabletofiretheseblendsoffuelwithoutany modification or pretreatment [23].Tomeasurethefuelperformancedifferentfactorslikevolatility,droplet formation and ease of ignition have been included and anaccelerationtesthasbeendone.Theengineaccelerationusingbio-diesel from used oil is about 15% less than that of bio-diesel madefrom unused oil [24].During the thrust test of bio-diesel made from unused, usedcooking oil and JET-A fuels, the thrust produced by bio-diesel fuelwas proportionately less than the JET-A for all engine speeds. Thethrust difference amountedto about 8% less. Ignitionin the enginewith either of the bio-diesels was not detectably different fromthat with the JET-A. A visual observation of the exhaust jet colorand size indicated no difference. There was a difference in theodors of the exhausts. Exhaust gas temperature (EGT) is verysimilar with all these fuels [24].Fuel properties affect the gas turbine performance such asinjector operation, droplet sizes and fuel vaporization character-istics. The effect of bio-diesel’s properties such as viscosity andvolatility result in low atomization and longer evaporation timescomparedtodiesel.Theoreticalandexperimentalfindingsindicatethat optimizing the fuel injection process will reduce NO  x emissions for bio-diesel. The minimum NO  x  emission levelsachieved for bio-diesel still exceed the minimum level attainedfor diesel. The atomization and some factors contribute to thehigher NO  x  emissions observed for bio-diesel. The use of lean,premixed, pre-vaporized combustion may be use to achieve lowpollutant emissions. Improving atomizationquality will result in areduction of NO  x  and CO emissions [25].For the use of bio-diesel in commercial and militaryflights, fewstudies have been done.  Baylor Institute for Air Science  has testedblends of bio-diesel from waste cooking oil and plant and animalmatter in JET-A (up to 30% by volume) in a modified gas turbineengine. It was found that nitric oxide (NO) concentration in theexhaust gases decreased with bio-diesel content. No difference intheperformanceorfuelconsumptionwasfoundbetweenpureJET-A and JET-A blended with up to 20% bio-fuel for the same poweroutput. There is no significant reduction in efficiency with bio-diesel [19].Palm oil methyl ester derived from palm oil is the mostpromising alternative to diesel because its property is similar toconventional diesel. The tendency of PME is to produce soot is lessthan diesel. NO  x  emission level is a function of fuel atomizingpressure [26]. Several bio-oils and heavy oils tend to be veryviscous and require preheating as well as external compressed airmay be used for satisfactory atomization. The preheating itself hastobecarriedoutinsuchamannerthatthefueldoesnotgothroughharmfulchemicalchangewhileinthefuelline.Forthispurposethealternative fuel tank is equipped with a heater. Bio-oils tend to beacidic and require the entire handling system and the fuel lines tobefabricatedfromcorrosionresistantmaterials.Particulatematterin the fuel needs to be filtered to prevent plugging of fuel nozzlesandforsatisfactorycombustion.Bio-oilproducesnoSO  x emissionsbut NO  x  and CO emissions need to be kept within allowable limit.Thesealternativefuelstendtoproducedepositsonhotsectionthatcanproducecorrosionandmayreduceefficiencysignificantly[27].Fuel nozzle design is extremely important. The fuel nozzle in adualfuelmode,allowsstartupondieselandgradualswitchovertoalternative fuels. In addition, they allow for enhanced atomizationwithexternalcompressedairsupply.Theduelfuelnozzlehasthreechannels for the diesel fuel, for the alternative fuels and for theexternal compressed air to improve fuel atomization. Externalcompressed air is the air taken from the gas turbine compressor.Cooling air supply holes were rearranged in size and location forbetter mixing with the combustion air in the front section of thecombustor and to keep the combustors wall temperature in theallowable range below 800  8 C [27]. 4. Bio-ethanol Ethanol is derived from alcoholic fermentation of sucrose orsugar cane and corn (60–70% starch). It is a well-established andwell-knowntechnology,generallyusedforhumanconsumptioninthe form of beers, wines and spirits [28]. It is possible that wood,straw and even household wastes may be economically convertedto bio-ethanol by hydrolysis process. Starches and cellulosicbiomass usually require expensive pretreatment. Ethanol is themost widely used liquid bio-fuel [1].Sugars are produced during steam explosion, dilute acid pre-hydrolysis process of carbohydrates like hemicelluloses andcelluloseinplantmaterials.Fermentationisananaerobicbiologicalprocess in which sugars are converted to alcohol by the action of microorganisms,usuallyyeast.Theresultingalcoholisethanol.Thechemicalreactionsaretheenzymatichydrolysisofsucrosefollowedby fermentation of sugar. Invertase enzyme in the yeast catalyzesthe hydrolysis of sucrose to convert it into glucose and fructose.Zymase enzyme present in the yeast converts the glucose and thefructose into ethanol.C 12 H 22 O 11Surcose !  C 6 H 12 O 6Glucose þ C 6 H 12 O 6Fructose C 6 H 12 O 6 ! 2C 2 H 5 OH þ 2CO 2 The production process of ethanol from the corn (starch) includesconversion of starch in to  D -glucose in the presence of gluco-amylase.Thenitisfollowedbydistillationanddehydrationtoyieldanhydrous bio-ethanol.Use of ethanol in the pure state or as a blend would probablyrequirereplacementofanywhitemetaloraluminuminthesystemas well as some elastomers. The heat content of ethanol is higherthan methanol. But it is still only 63% of diesel. Emissions fromethanol are about 48% of diesel; it is lowest of any of the fuels. Itsclean burning characteristics extend turbine life, possibly by as K.K. Gupta et al./Renewable and Sustainable Energy Reviews 14 (2010) 2946–2955  2949
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