7/27/2019 a. cano ref 1

1/6

IEEE Afr icon 2002 797

MODELLING OF POWER PLANTS BASED ON GASIFIEWGAS TURBINETECHNOLOGIES

A Cano, F JuradoUniversity of Jadn . Spa inABSTRACTThe olive tree in Spain can generate large quantities ofby-product biomass suitable for gasification.Gasification teehnologies under development wouldenab le these fuels to be used in gas turbines. Biomassconversion to a clean essentially ash-free form, usuallyby gasification and purification, is necessary to obtainhigh efilciency. This paper reports results of detailedfull-load perfo rman ce modelling of cogeneration systemsbased on gas ifid gas turbine technologies.

1. INTRODUCTIONThere have been significant changes in the generation ofelectric power over the last few years, with changes inownership and dispatch patterns, addition of newgeneration, and retirement or repowering of oldergeneration. One of the significant trends is the widespreadapplication of combined-cycle technology for new powerplants.The traditional approach in electric power generation isto have centralised plants distributing electricity throughan extensive transmission and distribution network.Distributed generation provides electric power at a sitecloser to the customer, eliminating the unnecessarytransmission and distribution costs [l] .It is widely recognised that distributed generation permitsan improved flexibility and allows to delay the upgradeand construction of transmission system facilities. Thecharacteristics of combined cycle power plants are indeedquite different from conventional power plants for whatconcerns the process, the regulating capabilities undernormal conditions and the possibility of facingemergencies through islanding transition and, eventually,through load-shedding facilities [2]-[6].The history of biomass fuelled power systems is as oldas the steam engine. In the early days, and for decadesafterwards,wood w as a com mon fue l, and was used infairly simple combustion systems with littlepreparation other than su e reduct ion and some a i r -drying. As steam technology developed andcompetit ion from other fuels increased, thecharacteristics of wood and other biomass fuelsbecame bet te r understood, a nd these characteristicsincreasingly came t o control the design of the powersystem.

J CarpioUNED, Madrid, Spain2. PRODUCING ELECTRICITY FROMBIOMASSMany older wood-buming steam power plants use steamtemperatures below 40O0 C (75O0F ) and sometimesbelow 300" C (570 C) so ash behaviour is a minorconcern a s long as sulphur-bearing secondary fuels (oil orcoal) are not used. When backpressure turbines are usedto provide process steam as well as power, goad overallenergy utilisation is possible, typically 70 percent ormore, but the electrical output is only 10-15percent. Withcondensing turbines (no process steam supply) theelectrical efficiency may be 15-20 percent. Io moremodern plants, especially those of larger size (over 50MW) steam temperatures up to 480 C ( W O F) havebeen used and electrical efficiencies around 25 percent canbe reached, but with increasing concern about theformation of glassy ash deposits and superheatercorrosion.The latest generation of electric power plant utilises gasturbines combined with steam turbines to utilise exhaustheat. Thermal efficiencies of 60 percent are being targeted[7] while 58 percent has been attained [SI in large utilityscale systems. In the small sizes (5-20 MW ) combinedcycle efficiencies are over 40 percent [9]. Theseefficiency levels are unattainable by the direct use ofbiomass fuels because of the high sensitivity of gasturbines to erosion by solid particles, deposit formation b ydust, and corrosion by m olten ash or salts. Attempts hav ebeen made to operate modified gas turbines by directcombustion of wood [ IO ] , but even with turbine inlettemperatures (TIT) as low as 750" (1380'F) seriousproblems have been experienced with fuel ash [IO]. ATI T of 750C represents a large departure from normalmodem gas turbine practice in which a TIT of 980-1200C is more usual. Biomass conversion to a cleanessentially ash-free form, usually by gasification andpurification, is necessary to obtain high efficiency [ll].

3. BIOMASS GASIFICATIONGum z is the earliest reference found describing the conceptof combining a pressurised gasifier with a gas turbineengine, although Gumz himself references an earlier workproposing this concept [12]. He also states that thecombination could certainly benefit from f uturedevelopment of pressurised hot gas cleaning to avoidexcessive turbine blade wear. G u m was speakiig of coal-fuelled plants but the concept is similar when using biomassas fuel.

0-7803-7570-X/02/$17.000 002 IEEE

7/27/2019 a. cano ref 1

2/6

IEEE Africon 2002

Carbon o gas *HHV, M J k g

798

Low-pressure Low- pressure High-pressunindirect-heat ai r -blown air -blown70.1 96.9 91.418.1 6.47 5.48

Biomass gasification is a technology that transforms solidbiomass into syngas (hydrogen and carbon monoxidemixtures produced from carbonaceous fuel). Current use ofbiomass, which stands at about 12% of the total energysupply to the world, is primarily used in combustion forimmediate use. Small-scale gasification for CHP indistributed generation (m Europe sometimes calledembedded generation), and village power applications is afield that has expanded very rapidly. Many villages andmini-grids can be served by biomass power generation inthe size ange of I kWe to 5 MWe.Biomass fuels are characterised by high and variablemoisture content, low ash content, low density, andfibrous smcture. In comparison with other fuels, they areregarded as of low quality despite low ash content andvery low sulphur content.The residual biomass of the olive-grove in Spain with apotential energy use is classified into two groups. Thefirst group is constituted by residual biomass of olive inthe extraction process of the olive oil. Depending on theextraction system, traditional, decanter in three phases ordecanter in two phases, the available energy from the by-products is different. In case of a traditional or a decanterin three phases system, the by-product is the foot cake(4.600 kcalikg heating value), and the olive paste ofsecond centrifugation (3.500 kcalkg heating value) forthe last extraction system.The second group of this biomass is constituted byresidual biomass from the olive tree, wood, small parts ofthe olive tree and the forest resources due to forestryworks (bushes cleanliness, etc). The products of bothabove groups present, from an energy point of view,favourable aspects in their use, e.g., the ensured annualproduction, its relative concentration in a place, the properhumidity conditions, the low sulphur content and otherharmful emissions, and finally, its high thermal value. No tusing those resources originates environmental problemsdue to foot cake and olive paste storage, plaguespropagation and forest fires.A variety of relatively large-scale biomass gasificationtechnologies are at various advanced stages ofdevelopment. Three gasifier@ cleanup designs areconsidered here: (i) atmospheric-pressure air-blownfluidised-bed gasification with wet scrubbing, e.g., thetechnology under development by Waldheim et al. [13]its higher heating value (HHV) is 1500 kcalikg; (ii)pressurised air-blown fluidised-bed gasification with hot-gas cleanup, e.g., the techno logy under developm ent bySa10 et al. [I41 the HHY is 1300 kcallkg; and ( i i i )atmospheric-pressure indirectly-heated gasification withwet scrubbing by Paisley et al. [15] the HHV is 4300kcalikg. Several scenarios point to the potential marketfor gasifier power systems at about IO 000 MW by 2010.Table 1gives modelled perform ance of alternative gasifiers.The feedstock in all cases is biomass with 20 percentmoisture content with the following composition (dry massbasis): 50.2 percent carbon, 5.4 percent hydrogen, 34.4

percent oxygen, 0.2 percent nitrogen, and 4 percent ash. I t sHHYis 20.47 MJ/dry kg.

Table 1. Modelled performanceof alternative gasifiers

* Percent carbon in fuel divided by carbon into gasifier.

GasifierFigure I : Gasifier and gar turbine.

This power plant generates electric power using biomassfrom the olive tree. The gasifier is capable of convertingtons of wood chips per day into a gaseous fuel that is fedinto a gas turbine as shown in Figure 1. The gasifierimproves significantly electrical generating efficiency in avariety of applications. The biomass gasifier enables theuse of advanced power systems that will nearly double theefficiency of today's biopower indusby. The gasifier heatsthe wood in a chamber filled with hot sand until the woodbreaks into basic chemical components. The solidssandand char- are separated &om the gases, which then flowthrough a scrubber. The final result is a very clean-buminggas fuel suitable for direct use in modern power systemssuch as gas turbines.4. MODELLING OF A COMBINED CYCLEPOWERPLANTA combined-cycle plant can be seen as the coupling of agas turbine and a steam turbine through a heat recoverysteam generator (HRSG) 16]. Overall system efficiencycan be greatly improved by linking together these twodifferent thermal cycles. Figure 2 represents a simplifiedcombined cycle model.

0-7803-7570-X/02/$17.008 2002 IEEE

7/27/2019 a. cano ref 1

3/6

IEEE Africon 2002 799

7/27/2019 a. cano ref 1

4/6

IEEE Afr icon 2002

Therefore, the output of an isochronous governor willintegrate in a corrective direction until the speed error iszero. For isochronous control, the digital setpoint remainsat zero deviation from the frequency reference, and thegas turbine matches the system load up to its ratedcapability. The speed governor is the primary means ofgas turbine control under part-load conditions. The digitalsetpoint is the normal means for controlling gas turbineoutput when operating in parallel and using a droopgovernor [17]-[19].Temperature control is the normal means of limiting gasturbine output at a predetermined firing temperature, independent of variation in ambient temperature or fuelcharacteristics. Since exhaust temperature is measuredusing a series of thermocouples incorporating radiationshields, there is a small transient error due to the timeconstants associated with the measuring system. Undernormal system conditions, where gas turbine output isdetermined by the slow rate o f digital setpoint, these timecoostants are of no significance to the load limitingfunction. However, w here increasing gas turbine output isthe result of a reduction of system frequency andtherefore may occur quite rapidly, exhaust temperaturemeasurement system time constants will result insome transient overshoot in load pickup. The design ofthe temperature controller is intended to compensate forthis transient characteristic.These two control functions-speed governing under part--load conditions, and temperature control acting as anupper limit, are input to a low value selector. The outputof the low value selector, which is called VCE, is thelowest of the two inputs, whichever requires the leastfuel. Transfer from one control to another is bumpless andwithout any time lags. The output of the low valueselector is compared with maximum and minimum limits.Of the two, the maximum limit acts as a backup totemperature control and is not encountered in normaloperation; the minimum limit is the more importantdynamically. This is because the minimum limit is chosento maintain adequate fuel flow to insure that flame ismaintained within the gas turbine.Gas urbine fuel systems are designed to provide energyinput to the gas turbinein proportion to the product o f the command signal(VCE) imes the unit speed. This is analogous to theactual mode of operation of the fuel system, since liquidfuel pumps are driven at a speed proportional to turbinerotor speed.4.2 HEAT RECOVERY STEAM GENERATORThe modelling of this part of the combined cycle plantmay greatly change from case to case. This is because,while the gas section of the plant is usually the largest onein terms of power output and its operation is rather well-founded and known, the heat recovery and steam powerproduction are often tailored on the producers specificneeds which, in turn, depend on the industrial process.

0-7803-7570-X/02/$17.00 002 EEE

800

Anyhow, some practical approaches may be suggested toapproximately model the steam power production stage.The HRSG considered in this paper is a heat exchangerwith no post-combustion. Tw o appropriate time constantsT, and Tb account for the dynamic of the steamproduction stage and allow to simulate the behaviour ofthe considered heat exchanger 1201.

4.3 STEAM TURBI NEThe dynamic behaviour of the steam turbine in combinedcycle power plant modelling barely w eighs on the overallmodel performance. In fact, the HRSG large timeconstants filter out the quick changes of the variablesthat interface the gas turbine model with the heat-recoveryapparatus. Thus, the steam turbine dynamics arepractically negligible when considering the overall plantdynamic behaviour. As a result, only the static aspect ofthe steam power production has been considered andimplemented in the combined-cycle dynamic model.Since the steam turbine contribution to the overall powerproduction in the plant considered fo r this paper is small,compared to that of the gas turbines, it was decided tobase the calculation of the power produced by the steamturbine only on steam flow, not considering the pressurevariations [21]. This rough approximation can beconsidered acceptable as steam pressures at the turbineinlet and outlet can be considered not so criticalparameters. Consequently, the block adopted to model thesteam turbine simply divides the actual steam flow by thesteam flow needed to have a power output from theturbine.

5. RESULTSA load is fed fiom the combined-cycle plant. The selectedsystem comprises a gas turbine and a steam turbine (20MW). The load and the combined-cycle plant are modelledusing M ATLAB [22]. The system is shown in Figure 3 anddescribed by the data in Tables 11-IV [23],[24].

Table I I . Gas urbine. Charactcristicsand consmts .

Turbine inlet prcrrurc I IO barTurbine outlet pressure 1.015 bar* (is) isentropic, (m) mechanic

7/27/2019 a. cano ref 1

5/6

IEEE Afr icon 2002

Table 111. Steam turbine. Characteristicsand constants.

Steam turbineI[nuy emp. In OP

Table IV.Values for block diagram shown in Figure 3.

1 0.05 1 0.4 0 0. 1 0.2 5 20Initially, the combined-cycle plant develops a

mechanical power. Mechanical power increases kom itsinitial value to the final value required by the load.A stepload is applied and the resu lts with t$e derived model aresummarised. Figure 4 presents the mechanical powerdelivered by gas turbine Pmpwhere the simulation time is 2s. Figure 5 displays the turbine speed N where thesimulation time is I .5 s. Pressurised air-blown fluidised-bedgasification with hot-gas cleanup is the technologv underdevelopment in Spain and the XHY is about 12 00 kca lkg .

0-7803-7570-x102/$17.00 002 EEE

Figure 4: Mechanical power delivered by gas turbine.G a s L . m . l p . d N

Trn.,.)Figure 5: Gas turbine speed.

801

6. CONCLUSIONProcess and performance information of biomass gasifier-based power station was simulated using MATLAB. Adetailed model for the regulator of a gas turbine has beendeveloped, as well as a simplified model for a heatrecovery steam generator and for its downstream steamturbine.7 . REFERENCES[ I ] Begovic, M., Pregelj, A., Rohatgi, A. and Novosel,D.: Impact of Renewable Distributed Generation onPower Systems Proceedings of the 34th AnnualHawaii International on System Sciences, 2001.[2] Adibi, M.M., Borkoski, J.N., Kaka, R.J. andVolkmann, T.L.: Frequency Response of PrimeMovers During Restoration IEEE Trans. on PowerSystems, Vo1.14, No. 2, May 1999,pp. 751 -756[3] Matsumoto, H. and Takahasi, S.: Improvement ofThermal Efficiency for Combined Cycle PowerPlant During Load Following OperationIEEE Trans. on Energy Conversion, Vo1.14, No. 3 ,Sept. 1999, pp.787-794.[4] Hannett, L.N . and Feltes, J.W.: Testing and ModelValidation for Combined-Cycle Power PlantsPower Engineering Sociery W M er Meeting IEEE,Vol. 2 , January 2001, pp. 664-670.[ 5] Roy, S . : Optimal Efficiency as a Design Criterionfor Closed Loop Combined Cycle IndustrialCogeneration IEEE Trans. on Energy Conversion,Vo1.16, No. 2, June 2001, pp. 155-164.Lasseter, R.: Dynamic Models for Micro-Turbinesand Fuel Cells Pow er Engineering Sociery Summer

Meeting IEEE, Vol. 2, 15-19 July 2001, pp.761-766.[7] Webb, H.A., Parsons, E.L. and Bojuror, R.A.:Advanced Turbine Systems Program and CoalApplications ASME Paper No. 93-GT-356, 1993.[SI Dodman, K.W., ed., Gas Turbine DevelopmentEmphasizes Improved Efficiency PowerEngineering Inlernalional, May-Apr. 1996.[9] DeMoss, T.B., ed , Theyre Here (almost): Th e 60%EEcient Combined Cycle, Power Engineering,July 1996.[IO] Ragland, K.W., Misra, M. K., Aerts, D.J. andPalmer, CA.: Ash Deposition in a Wood Fired GasTurbine, ASME Journal of Engineering for GasTurbines and Power, Vol. 117, 1995, pp. 509.Craig, J.D. and Purvis, C. R.: A Small ScaleBiomass Fueled Gas Turbine Engine Journal ofEngineering For Gar Turbines And Power, Vol.121, No. 1 , 1999, pp. 64-67.[I21 Gumz, W.: Ga s Producers andBIost Furnaces, JohnWiley and Sons, New York, 1950, pp. 166-167.[I31 Waldheim, L. and Carpentieri, E.: Update on theProgress of the Brazilian Wood BIG-GTDemonstration Project ASME Journal ofEngineering fo r Gas Turbines And Power, Vol. 123,N0.3, July 2001 , pp. 525-536.

[a]

[ l l ]

7/27/2019 a. cano ref 1

6/6

IEEE Africon 2002 802

[14] Salo, K. , Horvath, A. and Patel, J.: Pressurizedgasification of biomass, ASME paper GT-349,1998.[IS] Paisley M. and Anson. D.: Gasification for GasTurbine Based Power G eneration ASME Journai ofEneineerinz For Gas Turbines And Power. Vol. 8. *THO=

[24] Kim, J.H., Song, T.W., Kim, T.S. and Ro, S.T.:Model Development and Simulation of TransientBehavior of Heavy Duty Gas Turbines Trans. of theASME, Vol. 123, July 2001, pp. 589-594.

120, 1998,upp.284-288.Working Group on Prime Mover and EnergySupply Models for System Dynamic PerformanceStudies: Dynamic Models for Combined CyclePlants in Power System Studies IEEETransactions on Power Systems, Vol. 9, No. 3,Rowen, W.J.: Simplified MathematicalRepresentations of Heavy Duty Gas TurbinesASME Journal of Engineering fo r Power. 83-GT-63.August 1994, pp. 1698-1708.

.Vol. 105, Oct. 1983, pp. 865-869.Rowen, W.J.: Simplified MathematicalRepresentation of Single Shafi Gas Turbines inMechanical Drive Service TurbomachineryInternational, July-August 1992, pp. 26-32.Hannett, L.N. and Khan, A.H.: CombustionTurbine Dynamic Model Validation from TestsIEEE Trans. on Power Systems, Vol.8, No. 1, Feb.1993, pp. 152-158.Bagnasco, A., Delfino, B., Denegri, G.B. andMassucco, S.: Management and DynamicPerformances of Combined Cycle Power PlantsDuring Parallel and Islanding Operation IEEETrans. on Energv Conversion, Vo1.13, No. 2, June1998, pp. 194-201.Working Group on Prime Mover and EnergySupply Models for System Dynamic PerformanceStudies: Dynamic Models for Fossil Fueled SteamUnits in Power System Studies IEEE Truns. onPower Systems, Vol. 6, No . 2, May 1991, pp. 753-761.MATLAB, Math Works, Inc., Natick, MA, USA,2000.Eidensten, L. , Yan, J. and Svedberg, G.: BiomassExternelly Fired Gas Turbine Cogeneration Trans.of the ASME, Vol. 1 18, July 1996, pp. 604-609. Presenter:The paper is presentc

-Antonio CMO was born inJ &n, Spain. He received his BSc fmm theUniversity of Granada in 1992. He obtainedthe MS c degree fmm the UNED, Madrid,Spain, in2000. Since 1996he is Professor atthe Department of Electrical Engineering ofthe University of Jakn, Spain. He i s currentlyworking toward the Ph. D. degree at UNED,Madrid, Spain. His research interest focusesin power systems, modelling and renewableenergy.

Co-author: Francisco J u d o was bornin Linares, Spain. He received his BSc fromthe University of Granada in 1982. Heobtained the MSc and Ph.D. d e w s from theW E D , Madrid, Spain, in 1995 and 1999,respectively. Since 1985 he is Professor attheDepartment of Electrical Engineeringof theUniversity of l&n, Spain. His researchactivities have been devoted to several topics:power systems, modelling and renewableenergy He is a member of the IEEE andEEUG.

Co-author: J ose Carpi0 received the MSc.Electrical Engineering in 1985 and the P h.D.in 1988, both from Technical University ofMadrid. He joined the Electrical andComputer Engineering Department at WEDin 1989 asAssociate Professor and now is fullProfessor of Electrical Engineering. In 1993,he was vis iting researcher in the Depamncntof Operation Research at StanfordUniversity. He i s currently Director o f theElectrical and Computer EngineeringDepartment a1 W E D . His research in tewtfocuses in power systems and technology fordistance learning.

:d by Antonio Can0

0-7803-7570-x102/$17.002002 EE E