Effects Of Lower Heat Value Fuel On The Operations Of .
Energy and Power Engineering, 2009, 28-37doi:10.4236/epe.2009.11005 Published Online August 2009 (http://www.scirp.org/journal/epe)Effects of Lower Heat Value Fuel on the Operations ofMicro-Gas TurbineAiguo LIU, Yiwu WENGKey Laboratory of Machinery and Power Engineering of Education Ministry, Shanghai Jiao Tong University, Shanghai, ChinaEmail: liuaiguo119@gmail.comAbstract: The characteristics of fuel from biomass, coal and some waste materials are lower heat value anddifferent compositions. The lower heat value fuel (LHVF) can be used on power engine such as boiler, gasengine and gas turbine. Some laboratory and pilot work have been done, but the work done on micro-gas turbine is still limited. The characteristics of LHVF can cause the operations change of micro-gas turbine designed for nature gas. Some possible adjustment and modification methods were mentioned for the use ofLHVF on micro-gas turbine. One kind of representative LHVF was chosen and the operations of micro-gasturbine were analyzed. The temperature field and the non-uniformity scale of temperature distribution ofcombustor were calculated using FLUENT. The feasibility of different adjustment and modification methodswere analyzed according to the efficiency, output power and the non-uniformity scale of temperature distribution.Keywords: lower heat value fuel, micro-gas turbine, operations1 IntroductionThe distribution of LHVF from biomass, coal and wastematerials is wide and the energy reserves are huge [1].Effective use of the LHVF is becoming an attractive project, and much work is being done in this field. Thecharacteristics of LHVF are lower heat value and different combustible compositions compared with nature gas.Heat values of the fuel gases depend on the process, butare typically one-tenth to one-half that of natural gas [2].It will be different for the use of LHVF compared withtraditional fuel according to its characteristics, so somedifferent methods have ever been mentioned for the useof low heat value fuel such as catalytic combustion [3-5].Efficient conversion of LHVF to electrical power can bewas designed for higher heat value fuel, and some problems will appear when using LHVF as fuel. Catalyticcombustion chamber can take the place of traditional onefor the LHVF, but some defects will appear such ashigher pressure and loss, slow reaction rate and so on. Sothe traditional combustion chamber is still important forthe use of LHVF.Primary issue for the gas turbine combustor when using LHVF is its large volumetric flows. The operationsof micro-gas turbine will be changed and even stop work.The gas turbine should be adjusted and even modified tomake the micro-gas turbine work smoothly. In this paperthe effects of LHVF on the micro-gas turbine are firstlydiscussed, and then some possible methods of adjustment and modification are mentioned. The effects ofaccomplished by gas turbines, preferably in combinedmentioned methods to the operations of micro-gas tur-cycle mode, where thermal efficiencies can be greaterbine were presented and discussed. The feasibility wasthan 65%. Simple open cycle, high pressure ratio ma-also discussed. At last the temperature field of combus-chines can achieve efficiencies greater than 40% andtion chamber was presented. The maximum temperature,form the basis for Integrated Gasification Combined Cy-average temperature, and non-uniformity coefficient atcles [6]. Usually the combustion chamber of gas turbinethe outlet of combustor were calculated for the judgmentCopyright 2009 SciResEPE
29A. G. LIU, Y. W. WENGof the feasibility.can be described as functions of pressure ratio C, re-2 Model Descriptionduced flow G T / P , reduced speed n / T and effi-2.1 LHVF Modelmination of the pressure and efficiency as a function ofThe combustible components and heat value of LHVFmass flow and shaft speed for the description of com-are different since they are from different way such aspressor. Figure 1 shows the performance map of com-biomass gasification, blast furnace tar and the coal minepressor. It is assumed that off-design thermodynamic andventilation air [1][7]. The compositions of 3 representa-flow processes are characterized by a continuous pro-tive kinds of LHVF were shown in Table 1. The maingression along the steady-state performance curves.combustible components from biomass gasification andblast furnace tar are hydrogen and carbon monoxide andciency ηC. Performance maps are introduced for deter-The turbine model can be processed using a similarmethod.in the coal mine ventilation is methane. But the effects ofIn the combustion chamber, the fuels is burned awayLHVF on the operations of micro-gas turbine are thewhich increases the temperature of the gas. The follow-same, so we choose a representative low heat value fueling reactions are considered in calculating the exit gasfrom biomass gasification as an example to analyze. Thetemperature of the combustor:CO 1/ 2O2 CO2 QCOLHVF includes different combustible components and(1)the fuel is deal as composite variables. The thermodynamic property of gas is calculated according to the calculation manner of mixed fuel with incombustible component [8].Table 1. Components in different low heat value fuelComposition / %Biomass gasBlast furnace tarCoal mineventilation airCO213.0180O21.65020.79All of the calculations are based on the modeling of theCO21.4260micro-gas turbine C30 from CAPSTONE using theH212.240software of Matlab/Simulink [9]. The gas turbine is aCH41.8721N249.886078.212.2 Micro-Gas Turbine Modelsingle-shaft micro-gas turbine equipped with centrifugalcompressor, radial turbine, combustion chamber andrecuperator. The design compressor pressure ratio is 3.2and the turbine inlet temperature (TIT) is 1 173K. Thedesign mass flow of 0.31 kg s-1 is assumed to produceroughly 30 kW power with an efficiency of 26% at ISOconditions. This single shaft gas turbine is capable ofrunning at different shaft speed which generates higherflexibility.The modeling of micro-gas turbine is possible by utilizing real steady state engine performance data [10]. Ingas turbine cycles, the changed relationship betweenmass flow and pressure will cause a change in the operation point and efficiency. The features of a compressorCopyright 2009 SciResFigure 1. Equivalence circulates curve of compressorEPE
30A. G. LIU, Y. W. WENGH 2 1 / 2O2 H 2 O QH 2(2)CH 4 2O2 CO2 2 H 2 O QCH 4Tm is the average wall temperature between the hotand cold side gas.(3)dTm1(qh qc ) dtC pm M mAssuming that the process is adiabatic, the enthalpy ofthe reactants with combustion efficiency taken into account would be equal to the enthalpy of the products.(11)The power output from the gas turbine is obtained byusing the following equation:Knowing the temperature of the reactants, the productWGT gen ( T Wt Wc ) Wauxtemperature T2 can be calculated by iteration as the(12)properties of each product gas are temperature- depend-where ηgen is the generator efficiency; ηT is the turbineent:mechanical efficiency; Wt ,Wc and Waux are the turbine( h QCO QH 2 QCH 4 ) comb ni c pm dTT2iTstd(4)where the h is the enthalpy change of reactions fromthe original status to the standard status, the combustionefficiency comb was set conservatively at 98%, though itcan be as high as 99.5%.i represents each gas compo-sition of the product.The schematic figure of the recuperator is shown inFigure 2 Setting p2、h2、p4、h4 as the state variables，we can get the following equations based on the massand energy balance [11][12].dp2 (m2 h2 m1h1 qh ) dtVht (1 c p 2 / R2 )power, compressor power and auxiliary power respectively.2.3 CFD Model of Combustion ChamberThe temperature field in the combustor will be changed;especially the temperature field of combustor outlet willhave direct effect on the safety of turbine. The mainanalysis method of temperature field in combustor isCFD. Firstly the model was built by the software ofPRO/E and the plot of gridding is completed by Gambitusing non-structure gridding, the number of gridding is223271. The plot of gridding is shown in Figure 3. And(5)dh2 (m1h1 m2 h2 qh ) (h2 h2 R2 / c p 2 )(m2 m1 ) (6)dtVht ( 2 R2 2 / c p 2 )dp4 (m4 h4 m3 h3 qc ) dtVcl (1 c p 4 / R4 )(7)dh4 (m3 h3 m4 h4 qh ) (h4 h4 R4 / c p 4 )(m4 m3 )(8) dtVcl ( 4 R4 4 / c p 4 )Figure 2. Schematic figure of heat transfer in the recuperatorwhere qh,, qc are the heat transfer between fluid (hot andcold) and the wall of the recuperator.qh h Ah (T12 Tm )(9)qc c Ac (Tm T34 )(10)T TT1 T2, T34 3 4 are the average22value of inlet and outlet temperature respectively.And the T12 Copyright 2009 SciResFigure 3. Gridding of annular combustorEPE
31A. G. LIU, Y. W. WENGthen the model was solved by the software of FLUENT.2.3.1 Flux Control EquationThe flux control equation is N-S equation, the turbulencein combustor using double equation model. The3D flux N-S equation in pole coordinate is as follows:1 [ (r u ) (r v ) ( w )] r x r (13) 1 1 1 [ ( ) ( ) ( )] S x r r xr rr Figure 4. Relation of output power and efficiency to fuel flux2.3.2 Combustion Model3.1 Effects of LHVF on the Micro-Gas TurbineThe actual combustion process is the interaction of tur-The energy provided by LHVF is less than the traditionalbulence and chemistry reaction, the chemistry reactionfuel when the same fuel/air ratio was used. The fuel fluxvelocity is strong nonlinear and strong stiff. Usual chem-should be increased to improve the turbine inlet tem-istry reaction mechanism includes tens of compositionperature. The relation of output power and efficiencyand hundreds of base reaction and the difference of reac-with the fuel flux was shown in Figure 4 with the as-tion time is large. So the quantity of calculation andsumption of constant air flow.storage is very large in the solution of actual problem.The turbine inlet temperature (TIT) will increase withThe different chemistry dynamics solution methods havethe increase of fuel flux which causes the increase ofbeen used aims at the different combustion phenomenaoutput power and efficiency. The TIT will not be thein FLUENT. The model used in this calculation is thesame with design value when the output power is theSpecies Transport model. This model is usually used insame with design value, so two special conditions havethe premixed combustion, part premixed combustion andbeen presented according to the calculation. The first onediffusion combustion. The chemistry reaction is usuallywas the output power of micro-gas turbine attained thesimplified as single-step reaction. The solution of thedesign value (case2), the second one was the turbinecompositioninlet temperature attained the design value (case3). Thetransportequationandgettingthetime-averaged mass fraction of each composition is asfollows: ( Yi ) ( vYi ) J i Ri Si tcalculation results were shown in Figure 5 and werecompared with the design value (case1).(14)The reaction source item of composition j is the production rate of composition j in all the reactions:Ri R jk(15)kIn the formula, the reaction velocity of composition jin reaction k can be solved with the Arrhenius formula.3 Results and DiscussionCopyright 2009 SciResFigure 5. 3 different conditionsEPE
32A. G. LIU, Y. W. WENGFrom the calculation results in Figure 5 we can see theused in heavy gas turbine do not all fit to the micro-gasmass flow of fuel will be more than 14 times for case 2turbine because the operations of micro-gas turbine areand 16 times for case 3 compared with case1. The effi-different from heavy gas turbine [18]. Some possibleciency of case 2 and case 3 are both lower than case1andadjustment and modification methods were mentioned inthe output power is higher in case 3. The decrease ofthis chapter and the feasibility was discussed.efficiency is influenced by both the increase of the fuelflux entering combustor without being preheated and the3.2.1 Adjustment of Micro-Gas Turbinedecrease of turbine efficiency. About the increase of1) Pressure ratio and TIToutput power in case 3 is due to the increase of gas pass-The adjustment of operation parameters was viewed asing through turbine.the simplest method to match the compressor and turbineThe mass flow of low heat value fuel will be more 10according to the experience of heavy gas turbine. Thetimes than the design value. There is flux difference be-pressure ratio of compressor should be increased as hightween compressor and turbine, so the compressor andas possible because which can not only increase the flowturbine can not match together. Some work should bedone on the micro-gas turbine to make the compressorand turbine match again.ability of turbine but also decrease the flow ability ofcompressor. But the compression ratio can not be increased very high, so the turbine inlet temperature should3.2 Adjustment and Modification of Micro-GasTurbineSome experience has been gained and adopted in heavygas turbine for LHVF [13-16]. The heavy gas turbine canbe decreased at the same time for the purpose of matching. The adjustment process should be: firstly the compression ratio was increased as high as possible and thendecreasing the TIT until the matching was achieved. Thebe adjusted in large scale to fit the operations of LHVF.operation parameters were shown as case4 in Table 2 forFor example, the flow ability of turbine can be increasedthe adjustment of pressure ratio and TIT. From the re-10%, the compression ratio can be increased 12% andsults it was found that the efficiency and output powerthe output power can be increase 20% for the 9F modelboth decreased and the compressor has been near theheavy gas turbine from GE [17]. At the same time theresurge boundary. So this adjustment method is inapplica-exist variable-area nozzles which can be adjusted to de-ble to the micro-gas turbine in the view of efficiency andcrease the flux of compressor. The adjustment methodsoutput power.Table 2. Four operation cases of micro-gas turbine in different asturbineCopyright 2009 SciResRotation speedCompression ratioOutlet flux/ kg/sOutlet temperature / KPower /kWOutlet pressure/ kPaEfficiencyFuel flux / kg /sInlet temperature / KOutlet pressure / kPaOutlet temperature / KOutlet temperature / KOutlet flux / kg/sPower /kWEfficiencyOutput power / kWEfficiency / PE
33A. G. LIU, Y. W. WENG2) Rotation speedThe matching between compressor and turbine canalso be realized by adjusting the compressor itself toreduce the compressor air flux. There are several ways toadjust the compressor [19]:① Outlet throttle；② Inlet throttle；③ Variable rotation speed.Whichever method will result in the increase of powerconsumed by compressor, and the variable rotation speedadjustment is the best way in the view of power consumption.Figure 7. Efficiency and power at 0.9NThe matching between compressor and turbine can beThe variable rotation speed adjustment was chosenachieved by the adjustment of the operation parametersand the relationship of efficiency and output power withof micro-gas turbine. But the adjustment is not goodcompression ratio was shown in Figure 6 and Figure 7enough as it will cause the lower efficiency, exceedingwhen the rotation speed is 0.95N and 0.9N (N is the de-temperature and even the danger of compressor surge.sign rotation speed value) respectively. The operationparameters are listed in Table 3 as case5 and case6 whilekeeping the output power as design value.The power generated by turbine will decrease whenthe rotation speed decreases, but the power consumed by3.2.2 Modification of Compressor and TurbineIn general the micro-gas turbine does not adopt thetechnology of variable-area nozzle or stationary blade[20], the vane of compressor is very thin and the modifi-compressor also decreases at the same time. So the out-cation of variable-area nozzle or stationary blade will beput of micro-gas turbine also can achieve the designvery difficult. But the advantages will be great in thevalue, but the efficiency will decrease due to the de-view of the operations if the compressor and turbine cancrease of turbine and compressor efficiency. When thebe modified.rotation speed is 0.9N the turbine inlet temperature wasSome theoretical calculations have been done on thehigher than the design value for the design output powersupposing that the modification of compressor and tur-which can result exceeding temperature.bine can be realized. The calculation started from theoperation point in case4. Firstly was the modification ofcompressor. Starting from the operation point in case4and then decreasing the flux of air while keeping thecompression ratio unchanged. The turbine inlet temperature will increase due to the increase of fuel flux. Theinlet temperature of turbine attained the design value andthe output of micro-gas turbine was 35.02kW when themass flow of air is 0.965 design value. For the purposeof protecting turbine the turbine inlet temperature shouldbe kept constant after this point. This time the turbineinlet temperature was kept unchanged and the decreaseFigure 6. Efficiency and power at 0.95NCopyright 2009 SciResof the compressor flux would result the decrease ofEPE
34A. G. LIU, Y. W. WENGpower from turbine and micro-gas turbine until the out-compression ratio of compressor. The power and effi-put power reached the design value. The relation of effi-ciency still increased slowly because the mass flow ofciency and output power of micro-gas turbine with thegas keeps increasing at the highest temperature. The re-decrease of mass flow of compressor was shown in Fig-lation of power and efficiency with the increase of fluxure 8.ability of turbine was shown in Figure 9.The operation can also be improved if the flux abilityThe operations of micro-gas turbine were satisfactoryof turbine can be increased with modification. Changingwith the modification of compressor and turbine. But thethe setting angle or height of stationary blade can in-modification of compressor or turbine can cause thecrease the flux area of turbine and improve the opera-output of micro-gas turbine beyond the design valuetions. The calculation point also started from the opera-which maybe has effect on the structure of micro-gastion point in case4. The turbine inlet temperature wouldturbine.increase with the increase of turbine flux ability whichcaused the increase of output power and efficiency until3.3 Temperature Field of Combustion Chamberthe turbine inlet temperature attained the design value.The matching problem can be solved by the methodsAfter this point the increase of flux ability of turbine willmentioned above. But the temperature field in combustorcause the decrease of the turbine inlet pressure and thewill changed as the changed inlet conditions. The temperature fields in four different conditions were calculated which included the design condition, speed adjustment and the modification of compressor and turbine.The main parameters for temperature field weremaximum temperature, average temperature and nonuniformity coefficient which have effects on the safety ofturbine. Figure 10(a) is the temperature distributioncharacteristic of combustor outlet and axis direction atdesign condition and it was also the example comparedby the other conditions. The maximum temperature,minimum temperature, average temperature and non-Figure 8. Effect of compressor flux decrease on efficiency andpoweruniformity coefficient were shown in Tab
micro-gas turbine C30 from CAPSTONE using the software of Matlab/Simulink [9]. The gas turbine is a single-shaft micro-gas turbine equipped with centrifugal compressor, radial turbine, combustion chamber and recuperator. The design compressor pressure ratio is 3.2 and the turbine inlet temperature (TIT) is 1 173K. The design mass flow of 0.31 kg s
2.12 Two-shells pass and two-tubes pass heat exchanger 14 2.13 Spiral tube heat exchanger 15 2.14 Compact heat exchanger (unmixed) 16 2.15 Compact heat exchanger (mixed) 16 2.16 Flat plate heat exchanger 17 2.17 Hairpin heat exchanger 18 2.18 Heat transfer of double pipe heat exchanger 19 3.1 Project Flow 25 3.2 Double pipe heat exchanger .
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A. Heat exchanger A heat exchanger is a device used to transfer of heat from higher temperature to lower temperature. We can be done transfer of heat between two or more fluids, between two solid surfaces and a fluid, or between solid particulates and a fluid, at various temperatures and in thermal contact. B. Types of Heat Exchanger
Both temperature and heat transfer can change with spatial locations, but not with time Steady energy balance (first law of thermodynamics) means that heat in plus heat generated equals heat out 8 Rectangular Steady Conduction Figure 2-63 from Çengel, Heat and Mass Transfer Figure 3-2 from Çengel, Heat and Mass Transfer The heat .
the phase change is normally 10-1000 times larger than typical heat transfer methods such as heat conduction, forced vapor or liquid convection. Even though a heat pipe has a big potential to remove the thermal energy from a high heat flux source, the heat removal performance of heat pipes cannot be predicted well since a first principles of
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microreactor/heat exchanger configuration is shown in Figure 1, where the microreactor is shown on top of the heat exchanger block. Figure 1. Typical Microreactor and Crossflow Heat Exchanger Geometric Configuration 2. Heat Exchanger Theory The study of heat and heat transfer has a long history. The relationship of pressure (P) to volume (V) in .
