7.7 Steam Engine Journal

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7.7MW Steam power plant

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  • peyUni

    Keywords:Thermodynamic analysisExergySecond lawOptimizationPower plantFluidized bed

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    coal combustor (FBCC) model, the amount of irreversibilities occurring in the system is calculated at each

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    ysis must be applied [13]. For second-law analysis, the keyconcept is exergy (or availability). The concept of exergy is a di-rect outcome of second law of thermodynamics. The exergy of asystem is dened to be its work potential with reference to a pre-scribed environment known as exergy reference environment.The term work potential implies physically the maximum theo-retical work obtainable if the system of interest and the prescribedenvironment interact with each other and reach the equilibrium.

    based on the rst and second laws of thermodynamics. The useof exergy principles enhances understanding of thermal and chem-ical processes and allows sources of inefciency to be quantied.Lower exergy efciency leads in general to higher environmentalimpact [6,7]. Applications of exergy analysis for the performanceevaluation of power-producing cycles have increased in the recentyears. A lot of works are now available in the literature where thesecond-law-based analyses have been applied for optimizingperformance on coal-based power generation using conventional[810], uidized bed and combined cycle technology [1113]applications.

    * Corresponding author. Tel.: +90 532 397 30 88; fax: +90 388 225 01 12.

    Energy Conversion and Management 50 (2009) 24282438

    Contents lists availab

    n

    seE-mail address: afsingungor@hotmail.com (A. Gungor).essential. Optimum designs are obtained by detailed analysis of en-ergy systems where thermodynamics achieve its utmost impor-tance. On the other hand, from the thermodynamics point ofview, it has long been understood that traditional rst-law analy-sis, which is needed for modeling the engine processes, often failsto give the engineer the best insight into the engines operation. Inorder to analyze engine performance that is, evaluate the inef-ciencies associated with the various processes second-law anal-

    cesses). Rather exergy is consumed or destroyed, due toirreversibilities in any real process. The exergy consumption duringa process is proportional to the entropy created due to irreversibil-ities associated with the process. Summaries of the evolution ofexergy analysis through the late 1980s are provided by Kotas [1],Moran and Sciubba [2], Bejan et al. [3], Rosen [4], and Dincer [5].Reviews of literature reveal that the exergy analysis method over-comes the limitation of the rst law of thermodynamics and it is1. Introduction

    The world has nite natural resoconcerning especially energy demansign techniques for an energy syst0196-8904/$ - see front matter 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.05.035location. Analysis results are compared with the test results obtained from the measurements at severallocations in the system and good agreement is observed. These measured values are the temperatures atthree levels in the FBCC and boiler, economizer exit temperatures as well as ue gas composition at theboiler exit and steam ow rate. The maximum error observed in temperature values and steam ow rateis about 3.03% and 4.03%, respectively. Through the developed and validated model, effects of excess airand ambient temperature on rst and second law efciency of the subsystems and overall system areinvestigated. The second-law analysis reveals that the FBCC has the largest irreversibility, with about80.4% of the total system exergy loss. The FBCC temperature, rst and second law efciencies decrease19.8%, 5.1% and 5.2%, respectively, as the excess air increases from 10% to 70%. Also steam ow ratedecreases 5.1%. As the ambient temperature increases from 25 to 45 C, the FBCC temperature, systemrst and second law efciencies increase 0.8%, 1.3%, and 1.3%, respectively.

    2009 Elsevier Ltd. All rights reserved.

    and innite necessitiesthe development of de-ith minimized costs is

    The term exergy is sometimes referred by thermodynamically syn-onymous term availability and is a composite property of the sys-tem and the reference environment. The destruction of availabilityis usually termed irreversibility. Unlike energy, exergy is not sub-ject to a conservation law (except for ideal, or reversible, pro-as auxiliary systems in the thermal plant. The analysis is performed for the whole system and subsystemsby considering the available energy balance. In this analysis which consists of a detailed uidized bedThermodynamic analysis of a FBCC steam

    Nurdil Eskin a, Afsin Gungor b,*, Koray zdemir a

    a Faculty of Mechanical Engineering, Istanbul Technical University, 34437 Istanbul, TurkbDepartment of Mechanical Engineering, Faculty of Engineering and Architecture, Nigde

    a r t i c l e i n f o

    Article history:Received 21 March 2008Received in revised form 24 October 2008Accepted 30 May 2009

    a b s t r a c t

    This article presents the anplant located in Torbali (Izeconomizer as subsystems

    Energy Conversio

    journal homepage: www.elll rights reserved.ower plant

    versity, 51100 Nigde, Turkey

    sis of rst and second laws of thermodynamics in a 7.7 MW steam powerr, Turkey). It involves a uidized bed, a waste heat boiler (WHB) and anns, pumps, cyclone and chimney are also considered through the analysis

    le at ScienceDirect

    and Management

    vier .com/locate /enconman

  • nd MNomenclature

    A area (m2)Ar Archimedes numberC gas concentration (kmol/m3)csolids specic heat of solids (kJ/kg K)cp gas specic heat of gas (kJ/kg K)Db bubble diameter (m)dp particle diameter (m)_E rate of exergy ow (W)e specic exergy (kJ/kg)g gibbs function (kJ/kmol)h specic entalpy (kJ/kg)kbe mass transfer coefcient (1/s)LHVchar lower heating value of fuel (kJ/kg)_m mass ow rate (kg/s)_mburn burnt char mass ow rate (kg/s)NC cell number of FBCC

    N. Eskin et al. / Energy Conversion aThe development of analysis techniques based on the secondlaw of thermodynamics allows us to allocate and quantify irrever-sibilities in the production process and to identify which parts ofthe system and for what reasons they affect the overall inefciency[9,14,15]. The available literature concerning studies on uidizedbed applications [1113], generally consider the thermodynamicanalysis for the whole system and subsystems as a calculation ofmeasured data. Whereas the present study analyzes the whole sys-tem and subsystems using a detailed approach of step by step cal-culations for each system component. The system analyzed in thisstudy is a thermal system operating normally in a continuous stea-dy state, steady ow process mode. The simulation of the plant hasbeen performed with a simulator at design and off design condi-tions. From this point of view this article presents a very detailedanalysis of rst and second laws of thermodynamics in a 7.7 MWsteam power plant located in Torbali (Izmir, Turkey). The analysisis performed for the whole system and subsystems by consideringthe available energy balance. In this analysis which consists of adetailed FBCC model, the amount of irreversibilities occurring inthe system is calculated at each location [16]. It involves a FBCC,a WHB and an economizer as subsystems. Fans, pumps, cyclone

    NCA cell number in the bottom zone of FBCCNtube number of heat exchanger tubes_n gas ow rate (kmol/s)P pressure (Pa)_Q rate of heat transfer (W)qc reaction enthalpy of coal (kJ/kg)qCO reaction enthalpy of CO (kJ/kmol)qVM reaction enthalpy of volatile matter (kJ/kmol)R Universal gas constant (kJ/mol K)s specic entropy (kJ/kg K)T temperature (K)U overall heat transfer coefcient (W/m2 K)U0 supercial velocity (m/s)Umf minimum uidization velocity (m/s)v velocity (m/s)_W rate of work (W)Xc weight fraction of the carbon in the coal (kg-carbon/kg-

    coal)y mass fraction of gas species (kmol-gas species/kmol-

    gas)

    Subscriptsamb ambientash ashasp exhaustb bubblebot bottomc carbonchem chemicalchim chimneycomb combustioncons rev consumed reversibilitycyc cyclonedestr destructione emissioneco economizerent entranceFB uidized bedf uidhor horizontal

    anagement 50 (2009) 24282438 2429and chimney are also considered through the analysis as auxiliarysystems in the thermal plant.

    The inputs for the model are the dimensions and the construc-tion specications (insulation thickness and materials, etc.) of sub-systems, auxiliary systems characteristics (power, ow rate, etc.),coal feed rate and particle size, coal properties, inlet air pressureand temperature, ambient temperature, the supercial velocityand the steam pressure. The simulation model calculates the gasemissions, pressure drop, water inletoutlet temperatures, amountof heat transferred and the heat losses to the ambient of all compo-nents and the steam ow rate of the plant. Moreover, the pressurelosses in each device and connector equipments and their ttingsare considered in the model. Analysis results are compared withthe test results obtained from the measurements at several loca-tions in the system and good agreement is observed. These mea-sured values are the temperatures at three levels in the FBCC andboiler, economizer exit temperatures as well as ue gas composi-tion at the boiler exit and steam ow rate. Through the developedand validated model, effects of excess air and ambient temperatureon rst and second law efciency of the subsystems and overallsystem are investigated.

    o reference statePP power plantphy physicalref refractorysrf surfacest.sat steam saturationstoker,mot stoker motorVM volatile matterver verticalWHB waste heat boilerWtube waste heat boiler tube

    Greek symbolsD _mC carbon mass ow rate consumed from physical/chemi-

    cal process (kg/s)D _n gas ow rate consumed from chemical processes (kmol/

    s)DV volume of the cell/control volume (m3)DT ln log-mean temperature difference (C)eb bubble void fractiongI rst law efciencygII second law efciencyk excess airl gas viscosity (kg/ms)

  • 2430 N. Eskin et al. / Energy Conversion and M2. Power plant description

    The steam power plant is a 7.7 MW which involves a uidizedbed, a WHB and an economizer. The auxiliary components are fans,pumps, cyclone and chimney in the thermal plant. It is located inthe city of Izmir located in western Turkey. The schematic diagramof the analyzed plant is shown in Fig. 1.

    The FBCC has a 1.92 m 3.76 m square cross-section and 7 mheight. The combustion air is supplied through the distributor (pri-mary air) by a fan of capacity with 12,000 m3/h (90 kW), and thesecondary air inlets are located at 2 m above the distributor. Thefuels are introduced into the bed by means of a screw conveyor

    Fig. 1. Schematic diagram

    Table 1Operating parameters of the plant data referred to in this study.

    Operating parameters FBCC

    Coal feed rate (range) 1.451.55 t/hOperation velocity (range) 1.601.70 m/sBed temperature 840860 CBed area 7.2192 m2

    Size of coal feed (range) 0.039 mmMean size of sorbent feed 0.3 cmanagement 50 (2009) 24282438feeder. The technical parameters of the FBCC are steam capacityof 12 t/h, steam pressure of 6.3 bar. The operating parameters ofFBCC are shown in Table 1. The design fuel for the bed is low gradecoal (Soma lignite) which compositions are given in Table 2.

    The FBCC has horizontal and vertical heat exchangers. The hor-izontal heat exchangers are located along the wider side of the bot-tom zone. The heat exchanger tubes are placed 0.1 m distancedfrom each other and in four lines consecutively. The vertical heatexchangers are located along the bed height peripherally. The de-tails of heat exchangers are given in Table 3. In the model, heattransfer coefcients inside the tubes are considered as two-phaseow conditions in both horizontal and vertical heat exchangers[17,18]. The insulation used in the bottom zone is re bricks andthe whole of the riser wall is insulated with rock wool.

    The power plant has a feedwater pump of a capacity with16 m3/h (10 kW) and an exhaust fan of capacity with 20,000 m3/h (75 kW). The chimney is made of steel and without any insula-tion. The detailed properties of WHB, economizer and chimneyare given in Table 3.

    In the system, the feedwater rst passes through the deaerator,then into the economizer and nally into the WHB. The steam gen-eration in the plant takes places in theWHB via horizontal and ver-tical heat exchanger tubes.

    of the analyzed plant.

  • 3. Model description

    The objective of this study is to conduct an energy and exergyanalysis as a thermodynamic consideration to better understandand to compare the inuence of operational parameters on the pro-cess effectiveness, to develop a thermodynamic modeling of apower plant, and to determine the most effective ways of improv-ing the power plant process.

    Two methods to determine the thermodynamic efciency ofpower plant are described. These are energy efciency based onthe rst law of thermodynamics and exergy efciency based onthe second law of thermodynamics.

    3.1. First law of thermodynamics

    The fundamental thermodynamic relationships are describedby considering balance equations for appropriate quantities.

    Energy, being subject to conservation law (neglecting nuclearreactions), can be neither generated nor consumed. General bal-ance equations for energy can be written as follows:

    Energy Input Energy Output Energy Accumulation 1The steady state simulation of the plant has been performed

    with a simulator at design and off design conditions. It involvesan atmospheric FBCC model with the details given below.

    3.1.1. Fluidized bed modelBurning coal in FBCC has the capability to reduce both NOx and

    SO2 levels from coal red power plants. It also reduces CO2 emis-sions, soil and water pollution [19]. The designing of the FBCC isvery important because it enables burning coal with high efciencyand within acceptable levels of gaseous emissions. To simulate andoptimize the behavior of a FBCC, rstly the mathematical modelingof the hydrodynamic and kinetic characteristics is needed. In thepresent study, previously developed FBCC model is used for powerplant analysis [16]. FBCC model can be divided into three majorparts: a sub-model of the gassolid ow structure, a reaction ki-netic model for local combustion and a convection/dispersionmodel with reaction. The latter formulates the mass balances forthe gaseous species and the char at each control volume in the owdomain. The kinetic information for the reactions is supplied by thereaction kinetic sub-model, which contains description of devola-tilization and char combustion, and emission formation anddestruction, respectively.

    The uidized beds exhibit very complex hydrodynamics due tothe non-linear interactions between the two independent media

    Table 2Coal properties.

    Soma lignite

    Carbon (wt.%, dry) 25.34Hydrogen (wt.%, dry) 4.80Nitrogen (wt.%, dry) 1.12Sulphur (wt.%, dry) 1.60Vol. Mat. (wt.%, dry) 30.88Ash (wt.%, dry) 19.36Moisture (%) 25.42LHV (kJ/kg) 15591.00

    Table 3Details of system components.

    (mm

    N. Eskin et al. / Energy Conversion and Management 50 (2009) 24282438 2431Inner diameter (mm) Outer diameter

    Horizontal h...