Energy Conversion and Management 50 (2009) 2428–2438

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Energy Conversion and Management

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Thermodynamic analysis of a FBCC steam power plant

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

a Faculty of Mechanical Engineering, Istanbul Technical University, 34437 Istanbul, Turkeyb Department of Mechanical Engineering, Faculty of Engineering and Architecture, Nigde University, 51100 Nigde, Turkey

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

Keywords:Thermodynamic analysisExergySecond lawOptimizationPower plantFluidized bed

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.05.035

* Corresponding author. Tel.: +90 532 397 30 88; fE-mail address: afsingungor@hotmail.com (A. Gun

a b s t r a c t

This article presents the analysis of first and second laws of thermodynamics in a 7.7 MW steam powerplant located in Torbali (Izmir, Turkey). It involves a fluidized bed, a waste heat boiler (WHB) and aneconomizer as subsystems. Fans, pumps, cyclone and chimney are also considered through the analysisas 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 fluidized bedcoal combustor (FBCC) model, the amount of irreversibilities occurring in the system is calculated at eachlocation. 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 flue gas composition at theboiler exit and steam flow rate. The maximum error observed in temperature values and steam flow rateis about 3.03% and 4.03%, respectively. Through the developed and validated model, effects of excess airand ambient temperature on first and second law efficiency 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, first and second law efficiencies decrease19.8%, 5.1% and 5.2%, respectively, as the excess air increases from 10% to 70%. Also steam flow ratedecreases 5.1%. As the ambient temperature increases from 25 to 45 �C, the FBCC temperature, systemfirst and second law efficiencies increase 0.8%, 1.3%, and 1.3%, respectively.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The world has finite natural resources and infinite necessitiesconcerning especially energy demand thus the development of de-sign techniques for an energy system with minimized costs isessential. 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 first-law analy-sis, which is needed for modeling the engine processes, often failsto give the engineer the best insight into the engine’s operation. Inorder to analyze engine performance – that is, evaluate the ineffi-ciencies associated with the various processes – second-law anal-ysis must be applied [1–3]. 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 defined 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.

ll rights reserved.

ax: +90 388 225 01 12.gor).

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-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 first law of thermodynamics and it isbased on the first and second laws of thermodynamics. The useof exergy principles enhances understanding of thermal and chem-ical processes and allows sources of inefficiency to be quantified.Lower exergy efficiency 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[8–10], fluidized bed and combined cycle technology [11–13]applications.

Nomenclature

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

NC cell number of FBCCNCA cell number in the bottom zone of FBCCNtube number of heat exchanger tubes_n gas flow 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 specific entropy (kJ/kg K)T temperature (K)U overall heat transfer coefficient (W/m2 K)U0 superficial velocity (m/s)Umf minimum fluidization 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 ash

asp exhaustb bubblebot bottomc carbonchem chemicalchim chimneycomb combustioncons rev consumed reversibilitycyc cyclonedestr destructione emissioneco economizerent entranceFB fluidized bedf fluidhor horizontalo 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 flow rate consumed from physical/chemi-

cal process (kg/s)D _n gas flow 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 first law efficiencygII second law efficiencyk excess airl gas viscosity (kg/ms)

N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438 2429

The 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 inefficiency[9,14,15]. The available literature concerning studies on fluidizedbed applications [11–13], 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 flow 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 first 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

and 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 specifications (insulation thickness and materials, etc.) of sub-systems, auxiliary systems’ characteristics (power, flow rate, etc.),coal feed rate and particle size, coal properties, inlet air pressureand temperature, ambient temperature, the superficial velocityand the steam pressure. The simulation model calculates the gasemissions, pressure drop, water inlet–outlet temperatures, amountof heat transferred and the heat losses to the ambient of all compo-nents and the steam flow rate of the plant. Moreover, the pressurelosses in each device and connector equipments and their fittingsare 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 flue gas composi-tion at the boiler exit and steam flow rate. Through the developedand validated model, effects of excess air and ambient temperatureon first and second law efficiency of the subsystems and overallsystem are investigated.

Fig. 1. Schematic diagram of the analyzed plant.

2430 N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438

2. Power plant description

The steam power plant is a 7.7 MW which involves a fluidizedbed, 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

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

Operating parameters FBCC

Coal feed rate (range) 1.45–1.55 t/hOperation velocity (range) 1.60–1.70 m/sBed temperature 840–860 �CBed area 7.2192 m2

Size of coal feed (range) 0.03–9 mmMean size of sorbent feed 0.3 cm

feeder. 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 coefficients inside the tubes are considered as two-phaseflow conditions in both horizontal and vertical heat exchangers[17,18]. The insulation used in the bottom zone is fire 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 first passes through the deaerator,then into the economizer and finally into the WHB. The steam gen-eration in the plant takes places in the WHB via horizontal and ver-tical heat exchanger tubes.

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

N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438 2431

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 influence 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 efficiency ofpower plant are described. These are energy efficiency based onthe first law of thermodynamics and exergy efficiency based onthe second law of thermodynamics.

3.1. First law of thermodynamics

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

Table 3Details of system components.

Inner diameter (mm) Outer diameter (mm

Horizontal heat exchanger tubes 53 60Vertical heat exchanger tubes 42.4 50Waste heat boiler 1938 1948Waste heat boiler tubes 70 76Economizer 1000 1010Economizer Tubes 70 76Chimney 1000 1010

Table 4Energy balance equations for FBCC.

Fluidized bed

Bottom zone

_Qgas;i�1 þ _Qsolids;i�1 þ _Qwater;hor;ent;i þ _Qwater;ver;i�1 � _Qgas;i � _Qsolids;i

� _Qwater;hor;i � _Qwater;ver;i � _Qamb;i þ _Qrelease;i ¼ 0

csolids;iþ1 _me;iþ1Tiþ1 � ½csolids;ið _mw;i þ _me;iÞ þ cp gas;i _ngas;i�Ti

þðcsolids;i�1 _mw;i�1 þ cp gas;i�1 _ngas;i�1ÞTi�1 þ _mcomb;iqc þ _nb;WM;iqVM

þ _ne;VM;iqVM þ _nCO;iqCO ¼ D _Qwater;hor;i þ D _Qwater;ver;i þ _Qamb;i

D _Qwater;hor;i ¼ Ntube;hor;iAhor;inUhor;iðTi � Tst:satÞD _Qwater;ver;i ¼ Ntube;verAd1;iUver;iðTi � Tst:satÞ_Qamb;i ¼ Ntube;verAref ;tube;iUamb;iðTi � TambÞ

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 ¼ Energy Accumulation ð1Þ

The steady state simulation of the plant has been performedwith 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 fired 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 efficiencyand within acceptable levels of gaseous emissions. To simulate andoptimize the behavior of a FBCC, firstly 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 gas–solid flow 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 flowdomain. 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 fluidized beds exhibit very complex hydrodynamics due tothe non-linear interactions between the two independent media

) Length (mm) Insulation thickness (mm) Number of tubes

3760 567500 886000 1506000 2227000 150

300 40 (in flow direction)30,000

Freeboard

_Qgas;i�1 þ _Qsolids;i�1 þ _Qwater;ver;i�1 � _Qgas;i � _Qsolids;i

� _Qwater;ver;i � _Qamb;i þ _Qrelease;i ¼ 0

ðcsolids;i�1 _msolids;i�1 þ cp gas;i�1 _ngas;i�1ÞTi�1 � ðcsolids;i _msolids;i þ cp gas;i _ngas;iÞTi

þ _mcomb;iqc þ _nVM;iqVM;CO þ _nCO;iqCO ¼ D _Qwater;ver;i þ _Qamb;i

D _Qwater;ver;i ¼ Ntube;verAver;out;iUver;iðTi � Tst:satÞ_Qamb;i ¼ Ntube;verADtube ;iUamb;iðTi � TambÞ

2432 N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438

with their own individual movement tendencies – the particlesand the fluid [20]. In the model, the combustor is characterizedby two flow regimes: a dense phase at the bottom (bottom zone),and a dilute phase above the solid entry or secondary air inlet(freeboard). The primary loop of the FBCC is broken down intocells. These cells are homogenous, fully mixed sections. The cellsin the bottom zone and the freeboard are also subdivided intotwo phases: the emulsion phase and the bubble phase.

The single-phase back-flow cell model is used for solid mixingcalculation. The overall material balance for the solids in the ithcell, in terms of the backmix flow in emulsion and bubble phases,_me;i and _mb;i is given by the following equation.

dmdt

� �i

¼ _mb;i�1 � _mb;i þ _me;iþ1 � _me;i � _mburn;i þ _mash;i ð2Þ

Two-phase bubble assemblage model is used for gas phase materialbalances. The material balances are made for gases, O2, CO, CO2, SO2,NO, and for water vapor in the bubble and emulsion phases. Thematerial balance for the gas phase in the ith cell for emulsion andbubble phases, are given below, respectively.

dnk

dt

� �e;i¼ _ne;k;i�1 � _ne;k;i � kbeDVieb;iðCe;k;i � Cb;k;iÞ þ D _ne;k;i ð3Þ

dnk

dt

� �b;i¼ _nb;k;i�1 � _nb;k;i þ kbeDVieb;iðCe;k;i � Cb;k;iÞ þ D _nb;k;i ð4Þ

Table 5Energy balance equations for WHB, economizer and chimney.

Waste heat boiler

ð _Qfluegas;WHB;in � _Qfl

þð _Qwater;hor;in � Q

_Qfluegas;WHB;in � _Qflu

DT ln;WHB ¼ðTfluegas;WH

lnTflueg

Tflueg

�

Economizer Chimney

ð _Qfluegas;eco;in � _Qfluegas;eco;outÞ þ ð _Qwater;eco;in � _Qwater;eco;outÞ þ _Qamb ¼ 0 _Qfluegas;chim;in � _Qflu

_Qamb ¼ Ueco;srf Ain;srf ;ecoDT In;amb_Qamb ¼ Uchim;srf Achi

DT In;amb ¼ðTfluegas;eco;in�Tfluegas;eco;out Þ

InTfluegas;eco;in�Tamb

Tfluegas;eco;out�Tamb

� � DT In;amb ¼ðTfluegas;chim

InTflueg

Tfluega

�

where _nk indicates the gas flow rate of gas components (volatilegases, O2, CO, CO2, SO2, NO, and water vapor in the emulsion phaseand O2, CO2, SO2, and NO in the bubble phase, respectively), Vi is thevolume of the ith cell. The gas exchange, between the bubble andthe emulsion phases is a function of the bubble diameter and variesalong the axis of the riser and it is considered in the model as fol-lows [21]:

kbe;g ¼11Db

ð5Þ

where Db is the bubble diameter predicted by a correlation estab-lished by Mori and Wen [22]. In the model, the minimum fluidiza-tion velocity is obtained according to Wen and Yu [23]:

Umf ¼l

Cdpð33:72 þ 0:0651ArÞ0:8 � 33:7h i

ð6Þ

where C is the gas mixture concentration in the control volume, dp

is the particle diameter, Ar is the Archimedes number. For the charat the bottom zone, complete vertical mixing is assumed. Char isentering the bottom bed with feed coal, but also from the solid re-turn leg with the recycled solids from the cyclone. The combustormodel takes into account, the devolatilization of coal [24], subse-quent combustion of volatiles followed by residual char, SO2 cap-tured by limestone particles [25], and reduction of NO by the charin the combustor [26]. The heat transfer mechanisms of FBCC are

uegas;WHB;outÞ þ ð _Qwater;ver;in � _Qwater;ver;outÞ_

water;hor;outÞ þ _Qwater;feed � _Qsteam � _Qamb ¼ 0

egas;WHB;out ¼ Utot;WtubeAWtube;totDT ln;Wtube

B;in�Tfluegas;WHB;out Þas;WHB;in�Tst:sat

as;WHB;out�Tst:sat

�

egas;chim;out ¼ _Qamb

m;srf DT In;amb

;in�Tfluegas;chim;out Þas;chim;in�Tamb

s;chim;out�Tamb

�

Table 6Comparison of model temperature predictions with power plant data.

Measurement points Data (�C) Model (�C) Error (%)

TFB,in 60.7 60.7a –Tbot,90 857.6 831.6 3.03Tbot,110 852 830.3 2.54TFB,out 597 579.9 2.86TWHB,in 597 579.9 2.86TWHB,out 261.2 255.1 2.34Teco,in 261.2 255.1 2.34Teco,out 185.2 181.2 1.65

a Temperature value in fluidized bed inlet is used as model input.

Table 7Fluidized bed operational conditions for temperature comparisons.

Operational parameters of FBCC

Psteam (bar) 6.3Tamb (�C) 30U0 (cm/s) 169.37k (%) 38_mcoalfeed (kg/h) 1518_mair (N m3/h) 8049.3

dp (cm) 0.6dbed material (cm) 0.3

Table 8Comparison of model emission predictions with power plant data.

Measured values Data Model Error (%)

TWHB,out (�C) 260.7 251.7 3.4O2 (%) 5.7 6.5 14.03CO2 (%) 13.4 14.1 5.2SO2 (mg/m3) 3648 3579.5 1.9k (%) 37 37a –

a Excess air value is used as model input.

Table 9Fluidized bed operational conditions for emission comparisons.

Operational parameters of FBCC

Psteam (bar) 5.3Tamb (�C) 30U0 (cm/s) 165.1k (%) 37_mcoalfeed (kg/h) 1490.6_mair (N m3/h) 7848.8

dp (cm) 0.6dbed material (cm) 0.3

N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438 2433

considered as: (i) the heat transfer from the bed to the heatexchangers, (ii) the heat transfer from the heat exchangers to theambient, (iii) the heat transfer from the bed to the ambient. Thestructure and details of the FBCC model are given in the literature[16].

The control of the feedwater of the system and the steam pro-duction are carried out by the WHB. In the boiler, the saturatedsteam exists in the upper region and the saturated water exits inthe bottom region. The water level stays constant in the boiler.The boiler feedwater mixes suddenly and reaches the fluid satura-tion temperature. The water temperature in the boiler has the fluidsaturation temperature. Since the water from the WHB is fully sat-urated the water temperature stays constant along the FBCC cellwhere the quality of produced steam changes only in the heatexchangers. e-NTU method is used to determine the flue gas andwater exit temperatures for the economizer calculations.

For a thermal system operating normally in a continuous steadystate steady flow process mode, energy balance equations for sys-tem subsystems (fluidized bed, a WHB and an economizer) andauxiliary systems (fans, pumps, cyclone and chimney) are givenin Tables 4 and 5.

In order to make the case the most general possible, the primaryair fan flow is taken as reference. It is assumed that all gases areideal gases, preserving the variation of the specific heat with tem-perature. This hypothesis is fully acceptable for the pressure andtemperature ranges of the cycle. The thermodynamic data are ob-tained from thermodynamic tables for water, steam and all gases[18]. The thermodynamic properties of reference environmentare the ambient external conditions (T0 = 25 �C, P0 = 101.3 kPaand relative humidity of the air 70%). The pressure losses in eachdevice and connector equipments and their fittings are consideredin the model. The cyclone is considered to have 98% collection effi-ciency [27].

The inputs for the model are the dimensions and the construc-tion specifications (insulation thickness and materials, etc.) of sub-systems, auxiliary systems’ characteristics (power, flow rate, etc.),coal feed rate and particle size, coal properties, inlet air pressureand temperature, ambient temperature, the superficial velocityand the steam pressure. The simulation model calculates the gasemissions, pressure drop, water inlet–outlet temperatures, amountof heat transferred and the heat losses to the ambient of all compo-nents, and steam flow rate of the plant [17].

3.1.2. The model validationThe steady state simulation of the plant has been performed

with a simulator at design and off design conditions. The simula-tion results are compared with the test results obtained from themeasurements at several locations in the system as given in Table6. These measured values are the temperatures at three levels inthe FBCC (at the inlet of bed and at the heights of 90 cm and110 cm above the distributor plate) and WHB, economizer exittemperatures and steam flow rate. TESTO 454 measurement unitwhich uses NiCr–Ni, K-type thermocouple is used for the measure-ment of air temperature on the bed inlet (measurement ranges�30. . .+140 �C, sensitivity ± 0.5 �C) and TESTO 300 M-I measure-ment unit which uses NiCr–Ni, K-type thermocouple is used forthe measurement of the other temperature values (measurementranges �40. . .+1200 �C, sensitivity ± 0.5 �C). The comparisons areperformed using the actual power plant operational data. The FBCCoperating conditions for this comparison are given in Table 7. Themaximum error observed in temperature values is about 3.03%.The predicted and measured steam flow rates in the power plantare 7085.7 and 6799.8 kg/h, respectively, the maximum error ob-served in steam flow rate is about 4.03%.

The simulation results are also compared with test results interms of gas compositions (CO2 and SO2) based on 6% O2 in flue

gas at the WHB exit which has the maximum error of about 5.2%(Table 8). The analysis of the flue gas is carried out on the gasstream exiting from the WHB. Composition of the flue gas is mon-itored by using TESTO 300 M-I (for O2, CO2 and SO2) analysis equip-ment which are the portable flue gas measurement devicesworking on electrochemical principles. CO2 is calculated from thecomposition of the O2. The sensitivities are as follows: ±0.8%(0. . .+25% O2) for O2, and ±10 ppm (0. . .±200 ppm) and ±0.5 ppm(200.1. . .±5000 ppm) for SO2. The FBCC operating conditions forthis comparison are given in Table 9.

3.2. The second law of thermodynamics

The second-law analysis is useful to identify the componentshaving maximum irreversibility thus enables proper selection ofthe process for maintaining high quality of energy.

2434 N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438

Exergy is defined as the maximum amount of work which canbe produced by a system or a flow of matter or energy as it comesto equilibrium with a reference environment. Exergy is consumedduring a process due to irreversibilities and is, therefore, subject toa non-conservation law. General balance equations for exergy canbe written as follows:

Exergy Input� Exergy Output

� Exergy Consumed Reversibility� Exergy Destruction

¼ Exergy Accumulation ð7Þ

Exergy balance equations for system subsystems (fluidized bed,a WHB and an economizer) and auxiliary systems (fans, pumps, cy-clone and chimney) are given in Tables 10 and 11.

3.3. Thermodynamic analysis

Studies of engineering designs and thermodynamic analyses forpower generation systems are of scientific interest and also essen-tial for the efficient utilization of energy resources. For this reason,the thermodynamic analysis has drawn much attention by scien-tists and system designers in recent years.

Table 10Exergy balance equations for FBCC.

Fluidized bed

Bottom zone

_Egas;i�1 þ _Esolids;i�1 þ _Ewater;ver;i�1 þ _Ewater;hor;in;i � _Egas;i � _Esolids;i � _Ewater;ver;i � _Ewater;hor;i � _Eamb_Ewater;hor;in;i ¼ _mwater;hor;iðephy;f þ echem;f Þ_Ewater;hor;i ¼ _mwater;hor;ið1� xhor;iÞðephy;f þ echem;f Þ þ _mwater;hor;ixhor;iðephy;steam þ echem;f Þ_Ewater;ver;i�1 ¼ _mwater;verð1� xver;i�1Þðephy;f þ echem;f Þ þ _mwater;verxver;i�1ðephy;steam þ echem;f Þ_Ewater;ver;i ¼ _mwater;verð1� xver;iÞðephy;f þ echem;f Þ þ _mwater;verxver;iðephy;steam þ echem;f Þ_Egas;i�1 ¼ _ngas;i�1

Pkykð�gk � �g0;kÞ þ

Pkyk�echem;k þ RT0

Pkyklnyk

� �_Egas;i ¼ _ngas;i

Pkykð�gk � �g0;kÞ þ

Pkyk�echem;k þ RT0

Pkyklnyk

� �k : gas species ðO2;CO;NO; SO2 and H2OÞ_Esolids;i�1 ¼ _mw;i�1Xc;i�1fðhc;i�1 � h0;cÞ � T0ðsc;i�1 � s0;cÞ þ echem;cg þ _mw;i�1ð1� Xc;i�1Þfðhash;i

þ _me;iþ1Xc;iþ1fðhc;iþ1 � h0;cÞ � T0ðsc;iþ1 � s0;cÞ þ echem;cg þ _me;iþ1ð1� Xc;iþ1Þfðhash_Esolids;i ¼ ð _mw;iXc;i þ _me;iXc;iÞfðhc;i � h0;cÞ � T0ðsc;i � s0;cÞ þ echem;cg þ ð _mw;i þ _me;iÞð1� Xc;iÞfð_Ebot;in ¼ _Echar þ _Eair;bot;in_Echar ¼ _mcharechem;char

_Eair;bot;in ¼ _nairP

kykð�gk � �g0;kÞ þ 12 U2

air;in

� �

Freeboard

_Egas;i�1 þ _Esolids;i�1 þ _Ewater;ver;i�1 � _Egas;i � _Esolids;i � _Ewater;ver;i � _Eamb;i � _Eloss;consrev;i � _Edestr;i ¼_Esolids;i�1 ¼ _msolids;i�1Xc;i�1fðhc;i�1 � h0;cÞ � T0ðsc;i�1 � s0;cÞ þ echem;cg þ _msolids;i�1ð1� Xc;i�1Þf_Esolids;i ¼ _msolids;iXc;ifðhc;i � h0;cÞ � T0ðsc;i � s0;cÞ þ echem;cg þ _msolids;ið1� Xc;iÞfðhash;i � h0;ashÞ

_Eamb;i ¼ 1� T0Tamb

� �_Qamb;i

The energy efficiency may be defined as the ratio of the energyoutput to the energy input. The general efficiency equations for en-ergy can be written as follows:

gI ¼energy outputenergy input

¼ 1� energy lossenergy input

ð8Þ

The energy efficiency of the FBCC based on the first law of ther-modynamics can be derived by considering the energy transferredto the coolant inside the heat exchangers as the energy output andthe chemical exergy of the fuel as the energy input. The first lawefficiency for overall system and system subsystems (fluidizedbed, a WHB and an economizer) are given in Table 12.

The parameter that gauges the effectiveness of a system in pre-serving its exergy in performing a physical process is known asexergetic efficiency. This is also called as second law efficiency.Lower is the irreversibility, higher is the exergetic efficiency andvice versa. The importance of the exergetic analysis is to diagnosehow much of the theoretical maximum work the system is able toperform. The method of exergetic analysis used consists of evaluat-ing each bearer of energy along the system identifying its chemicalcomposition, physical state and flow rate. In determining theexergetic efficiency of a process performed by a system, one has

;i � _Eloss;consrev;i � _Edestr;i ¼ 0

�1 � h0;ashÞ � T0ðsash;i�1 � s0;ashÞg;iþ1 � h0;ashÞ � T0ðsash;i�1 � s0;ashÞghash;i � h0;ashÞ � T0ðsash;i � s0;ashÞg

0

ðhash;i�1 � h0;ashÞ � T0ðsash;i�1 � s0;ashÞg� T0ðsash;i � s0;ashÞg

Table 11Exergy balance equations for WHB, cyclone, economizer and chimney.

Waste heat boiler Cyclone

_Efluegas;WHB;in þ _Ewater;ver;in þ _Ewater;hor;in þ _Ewater;feed � _Efluegas;WHB;out � _Ewater;ver;out

� _Ewater;hor;out � _Esteam � _Eamb � _Eloss;consrev;i � _Edestr;i ¼ 0

_Efluegas;cyc;in � _Efluegas;cyc;out � _Ecyc;solids;out � _Eloss;consrev;i � _Edestr;i ¼ 0

_Ewater;ver;in ¼ _mwater;verð1� xver;inÞðephy;f þ echem;f Þþ _mwater;verxver;in ðephy;steam þ echem;f Þ

_Ewater;ver;out ¼ _mwater;verðephy;f þ echem;f Þ_Ewater;hor;in ¼

PNCBi¼1

_mwater;hor;ið1� xhor;in;iÞðephy;f þ echem;f Þþ _mwater;horxhor;in;iðephy;steam þ echem;f Þ

� �

_Ewater;hor;out ¼ _mwater;horðephy;f þ echem;f Þ_Efluegas;WHB;in ¼ _Ebot;NC_Efluegas;WHB;out ¼ _ngas;NCðephy;WHB;out þ echem;WHB;outÞ

þ _msolids;NC Xc;NCðephy;c þ echem;cÞþ _msolids;NCð1� Xc;NCÞephy;ash

_Ewater;feed ¼ _mwater;feedðephy;f þ echem;f Þ_Esteam ¼ _msteamðephy;steam þ echem;f Þ

_Efluegas;cyc;in ¼ _ngas;NC ð�ephy;cyc;in þ �echem;cyc;inÞþ _msolids;NC Xc;NC ðephy;c þ echem;cÞ þ _msolids;NC ð1� Xc;NC Þephy;ash

_Efluegas;cyc;out ¼ _ngas;NCð�ephy;cyc;out þ �echem;cyc;outÞ

þð1� gcycÞ_msolids;NC Xc;NCðephy;c þ echem;cÞþ _msolids;NCð1� Xc;NCÞephy;ash

�

_Ecyc;solids;out ¼ gcyc_msolids;NC Xc;NK ðephy;c þ echem;cÞþ _msolids;NC ð1� Xc;NC Þephy;ash

�

Economizer Chimney

_Efluegas;eco;in þ _Ewater;eco;in � _Efluegas;eco;out � _Ewater;eco;out � _Eamb � _Eloss;consrev ;i � _Edestr;i ¼ 0 _Efluegas;chim;in � _Efluegas;chim;out � _Eamb � _Eloss;consrev;i � _Edestr;i ¼ 0

_Efluegas;eco;in ¼ _ngas;NCð�ephy;eco;in þ �echem;eco;inÞ

þð1� gcycÞ_msolids;NC Xc;NC ðephy;c þ echem;cÞþ _msolids;NC ð1� Xc;NC Þephy;ash

�

_Efluegas;eco;out ¼ _ngas;NC ð�ephy;eco;out þ �echem;eco;outÞ

þð1� gcycÞ_msolids;NC Xc;NK ðephy;c þ echem;cÞþ _msolids;NC ð1� Xc;NC Þephy;ash

�

_Ewater;eco;in ¼ _mwater;feedðephy;f þ echem;f Þ_Ewater;eco;out ¼ _mwater;feedðephy;f þ echem;f Þ

_Efluegas;chim;in ¼ _ngas;NCð�ephy;chim;in þ �echem;chim;inÞ

þð1� gcycÞ_msolids;NC Xc;NK ðephy;c þ echem;cÞþ _msolids;NCð1� Xc;NCÞephy;ash

�

_Efluegas;chim;out ¼ _ngas;NC ð�ephy;chim;out þ �echem;chim;outÞ

þð1� gcycÞ_msolids;NC Xc;NCðephy;c þ echem;cÞþ _msolids;NCð1� Xc;NCÞephy;ash

�

_Eamb ¼ 1� T0Tamb

� �_Qamb

N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438 2435

to consider the exergy quantities that cross the system boundarieseither as flow exergy associated with mass flux, or as exergy trans-fer built in with the transfer of energy quantities (heat and work).Identifying the main sites of exergy destruction shows the direc-

tion for potential improvements. Exergy analysis of a complex sys-tem can be performed by analyzing the components of the systemseparately. The exergy analysis includes the calculation of the exer-gy destruction and the exergetic efficiency of the plant.

Table 12The first and the second law efficiencies of the system.

Fluidized bed

gI;FB ¼_QFB;out_QFB;in¼ ð

_Qwater;hor;in� _Qwater;hor;out Þþð _Qwater;ver;in� _Qwater;ver;out Þ_mchar LHVchar

gII;FB ¼_EFB;out_EFB;in¼ ð

_Ewater;ver;in� _Ewater;ver;out Þþð _Ewater;hor;in� _Ewater;hor;out Þþð _Egas;NCþ _Esolids;NC Þ_Echarþ _Eair

Waste heat boiler

gI;WHB ¼_QWHB;out_QWHB;in

¼ ð _Qsteam� _Qwater;feed Þð _Qwater;ver;in� _Qwater;ver;out Þþð _Qwater;hor;in� _Qwater;hor;out Þþð _Qfluegas;WHB;in� _Qfluegas;WHB;out Þ

gII;WHB ¼_EWHB;out_EWHB;in

¼_Esteam� _Ewater;feed

ð _Ewater;ver;in� _Ewater;ver;out Þþð _Ewater;hor;in� _Ewater;hor;out Þþð _Efluegas;WHB;in� _Efluegas;WHB;out Þ

Economizer

gI;eco ¼_Qeco;out_Qeco;in

¼ ð _Qwater;eco;out� _Qwater;eco;inÞð _Qfluegas;eco;in� _Qfluegas;eco;out Þ

gII;eco ¼_Eeco;out_Eeco;in

¼_Ewater;eco;out� _Ewater;eco;in

_Efluegas;eco;in� _Efluegas;eco;out

Overall system

gI;PP ¼_QPP;out_QPP;in

¼ ð _Qsteam� _Qwater;inÞ_mchar LHVcharþ _Wfan1þ _Waspþ _Wpumpþ _Wstoker;mot

gII;PP ¼_EPP;out_EPP;in¼

_Esteam� _Ewater;in_Echarþ _Eair;outþ _Wfanþ _Waspþ _Wpumpþ _Wstoker;mot

Table 14The effects of excess air on the first law efficiencies of the power plant.

Energy efficiency Excess air (%) Total changes (%)

10 30 50 70

gI;FB 0.531 0.496 0.459 0.426 19.8gI;WHB 0.987 0.987 0.986 0.985 0.2gI;eco 0.931 0.941 0.944 0.946 1.6gI;PP 0.703 0.694 0.680 0.667 5.1

2436 N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438

The exergy efficiency may be defined as the ratio of the exergyoutput to the exergy input. The general efficiency equations forexergy can be written as follows:

gII ¼exergy outputexergy input

¼ 1� exergy lossexergy input

ð9Þ

The exergy efficiency of the FBCC based on the second law ofthermodynamics can be derived by considering the exergy trans-ferred to the coolant inside the heat exchangers, the flue gas exergyand the exergy of the drifted waste solids as exergy output and thechemical exergy of the fuel and the exergy of the air feed as theexergy input. The second law efficiency for overall system and sys-tem subsystems (fluidized bed, a WHB and an economizer) are gi-ven in Table 12.

The exergy of the solids in the FBCC riser is considered as theexergy of the coal. The exergy of the ash is neglected because ofits minor contribution when compared with the exergy of the coal[9]. The exergy of the coal is calculated with both its physical andchemical exergies. The fuel specific exergy value is considered inthe model as the fuel lower heating value (LHV).

Table 15The effects of excess air on the second law efficiencies of the power plant.

Exergy efficiency Excess air (%) Total changes (%)

10 30 50 70

gII;FB 0.367 0.363 0.366 0.369 0.5gII;WHB 0.819 0.800 0.781 0.764 6.7gII;eco 0.577 0.575 0.570 0.565 2.1gII;PP 0.230 0.227 0.223 0.218 5.2

4. Results and discussion

The thermodynamic analysis has been performed to show howdifferent operational parameters affect the first and second lawefficiency of the thermal power plant. In the present study, the ef-fects of the excess air and the ambient temperature on first andsecond law efficiency of the subsystems and overall system areinvestigated through the developed and validated model.

Table 13The effects of excess air on the system temperature values.

Calculation points (�C) Excess air (%)

10 30

Tamb 30.00 30.00TFB,in 66.98 62.25Tbot,90 875.56 844.90Tbot,110 874.74 843.71TFB,out 581.61 580.91TWHB,in 581.61 580.91TWHB,out 248.60 252.40Teco,in 248.60 252.40Teco,out 172.34 178.25Tchim,out 165.57 169.42

The effects of excess air on the system temperature values(FBCC – at the inlet of bed and at the heights of 90 cm and110 cm above the distributor plate – and WHB and economizertemperatures) and on the first and second law efficiency of thethermal power plant subsystems and overall system in modelingresults are given in Tables 13–15, respectively. The tables showthe predicted model results for four excess air ratios (of about10%, 30%, 50% and 70%). For this assumption coal feed rate is1515.6 kg/h, the steam pressure is 6 bar, the ambient temperatureis 30 �C.

It can be seen that the total changes in the energy efficiency arefound to be higher than the exergy efficiency for the FBCC which isdirectly derived from the very nature of their descriptions. It is ob-served that the excess air has the negative effect on the tempera-ture values of the FBCC, especially in the bottom zone. As theexcess air value increases, the mean bed temperature decreasesdue to higher heat losses with increasing flue gas flow rates tothe WHB. It is seen from the Table 13 that the energy efficiency de-creases as the combustion losses increase with increasing excessair value. Excess air affects combustion efficiency in two ways:one is due to higher heat losses with increasing flue gas flow ratesto the WHB. As expected, decreasing the temperature decreasesthe carbon combustion efficiency due to the decrease in the reac-tion rates [28]. The other effect is that the bed temperature de-creases as the excess air ratio increases and it affects thecombustion efficiency due to decrease in the reaction rates and re-sults in higher carbon content in the mass discharged from thecombustor. The decrease of the reaction rate of char combustion

Total changes (%)

50 70

30.00 30.00 –56.99 54.72 18.3

809.86 775.60 11.4808.38 773.93 11.5575.36 566.21 2.6575.36 566.21 2.6255.10 256.85 3.3255.10 256.85 3.3183.57 188.10 9.1173.27 176.79 6.8

0 20 40 60 80 100Excess air

6500

6600

6700

6800

6900

7000

Stea

m fl

ow ra

te (k

g/h)

Fig. 2. Effects of excess air on steam generation in the plant.

Table 17The effects of the ambient temperature on the system temperature values.

Calculation points (�C) Tamb Total changes (%)

25 �C 45 �C

Tamb 25.00 45.00 80.0TFB,in 56.17 76.05 35.4Tbot,90 843.13 850.27 0.8Tbot,110 841.93 849.09 0.9TFB,out 579.78 583.87 0.7TWHB,in 579.78 583.87 0.7TWHB,out 252.06 252.99 0.4Teco,in 252.06 252.99 0.4Teco,out 178.04 178.44 0.2Tchim,out 168.42 172.03 2.1

Table 18The effects of the ambient temperature on the first law efficiencies of the power plant.

Energy efficiency Tamb Total changes (%)

25 �C 45 �C

gI;FB 0.494 0.501 1.4g 0.987 0.987 0.0

N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438 2437

with increasing excess air causes an increase of the loss due to theunburnt carbon contained in the discharged mass. Although theamount of oxygen increases with increasing excess air, decreasingbed temperature causes a negative effect on the combustion effi-ciency [29]. The lower mean bed temperature values lead to a de-crease of heat transfer from the bed to the heat exchanger surfaces.

As Table 14 displays that the energy efficiencies are found to bevery small in other plant devices. The increasing excess air de-creases the FBCC heat exchanger performance and the amount ofsteam generated in the WHB. The lower mean bed temperature de-creases the amount of heat transfer in the WHB. Thus, as the excessair increases it is observed that the availability of the WHB is af-fected negatively, which are further caused by the heat transferbased descriptions of the WHB and economizer (Table 15). The firstlaw efficiency of the plant decreases with the increase of the excessair. The main reason of the efficiency loss is the increase of energyloss from the chimney.

The role of the FBCC becomes less important as the increasingexcess air affects the amount of heat transferred to the waterwhereas the effect of the WHB and economizer increases in viewof the exergy efficiency. The overall second law efficiency of thesystem decreases 5.2% as the excess air increases from 10% to70% (Table 15). It is observed that as a general trend, the results ob-tained for energy efficiency are similar to the results obtained forexergy efficiency for overall system from the thermodynamicviewpoint.

The increase of excess air causes a decrease of energy trans-ferred to the water and results in lower amounts of steam genera-tion in the plant as seen in Fig. 2. The amount of the decrease of thegenerated steam becomes 5.1%, in case the excess air is increasedfrom 10% to 70%.

Table 16System irreversibilities and their rates for excess air value of 30%.

Calculation points Irreversibility (kW) Irreversibility rate (%)

FBCC 3639.4 80.4Waste heat boiler 319.0 7.0Cyclone 3.6 0.1Waste solids 47.7 1.1Economizer 42.3 0.9Flue gases 351.0 7.8Heat transfer to the ambient,

fans and pumps123.2 2.7

Total ırreversibility 4526.1 100.0

The system irreversibilities and their rates for the excess air va-lue of 30% are given in Table 16. The second-law analysis revealsthat the FBCC has the largest irreversibility, with about 80.4% ofthe total system exergy loss. Chemical reaction and physical trans-port processes are the sources of irreversibilities in combustionprocess in FBCC. In many situations, the major part played amongall physical processes is the internal thermal energy exchange.

The effects of the ambient temperature on the system temper-ature values and on the first and second law efficiency of the ther-mal power plant subsystems and overall system in modelingresults are given in Tables 17–19, respectively. Tables show thepredicted model results for two ambient temperature values (ofabout 25 �C and 45 �C). For this assumption coal feed rate is1515.6 kg/h, the steam pressure is 6 bar, the excess air ratio is 30%.

When the power plant operates at varying ambient tempera-tures, the exergy loss at a higher ambient temperature is less thanat a lower ambient temperature. Ambient air density varies inver-sely with its temperature, directly affecting the inlet air mass flowrate and thus directly impacting the FBCC output. For an increase ofabout 20 �C in the ambient temperature, the increasing of energyefficiency is about 1.4% and for exergy efficiency is only about0.8% for the FBCC. On the other hand the results of the exergy effi-ciency show that the ambient temperature does not affect theexergetic efficiency of the WHB and the economizer. The overallefficiency of the system has increased 1.3%. The increase of theambient temperature causes an increase of the steam generationin the plant. The amount of the increase of the generated steam be-comes 1.3%, in case the ambient temperature is increased from25 �C to 45 �C.

I;WHB

gI;eco 0.941 0.942 0.1gI;PP 0.691 0.700 1.3

Table 19The effects of the ambient temperature on the second law efficiencies of the powerplant.

Exergy efficiency Tamb Total changes (%)

25 �C 45 �C

gII;FB 0.362 0.365 0.8gII;WHB 0.799 0.799 0.0gII;eco 0.575 0.575 0.0gII;PP 0.226 0.229 1.3

2438 N. Eskin et al. / Energy Conversion and Management 50 (2009) 2428–2438

5. Conclusions

In the present study, the effects of the excess air and the ambi-ent temperature on first and second law efficiency of the subsys-tems and overall system of a 7.7 MW steam power plant areinvestigated through the developed and validated model. In thisanalysis which consists of a detailed FBCC model the amount ofirreversibility occurring in the system are calculated at each loca-tion and the FBCC has the largest irreversibility, with about 80.4%of the total system exergy loss. The increase of the excess air causesa decrease of the overall energy efficiency in the plant. It is ob-served that as a general trend, the results obtained for energy effi-ciency are similar to the results obtained for exergy efficiency foroverall system from the excess air viewpoint. It is also importantnote that the ambient temperature is an effective parameter it self.A future suggestion is to study the thermoeconomics of thisprocess.

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