Exergy Analysis of a 600 MW Oxy-combustion Pulverized-Coal ...cpc.energy.hust.edu.cn/__local/3/52/3D/71477110C97...Exergy Analysis of a 600 MW e Oxy-combustion Pulverized-Coal-Fired

Published: July 08, 2011

r 2011 American Chemical Society 3854 dx.doi.org/10.1021/ef200702k | Energy Fuels 2011, 25, 3854–3864

ARTICLE

pubs.acs.org/EF

Exergy Analysis of a 600 MWe Oxy-combustion Pulverized-Coal-FiredPower PlantJie Xiong, Haibo Zhao,* and Chuguang Zheng

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074 Hubei,People’s Republic of China

ABSTRACT: In an oxy-combustion pulverized-coal-fired power plant (PC), a high CO2 content flue gas could be obtained, and thisallows CO2 sequestration in an efficient and energy-saving way. To better understand the thermodynamic characteristics of the oxy-combustion process, a detailed exergy analysis of a 600 MWe oxy-combustion PC was carried out. The whole oxy-combustion PCsystem was divided into four models: boiler, turbines and feedwater heaters (FWHs), air separation unit (ASU), and flue gastreatment unit (FGU). The exergy (including physical exergy and chemical exergy) of each model was obtained, showing that theoxy-combustion boiler could reach higher exergy efficiency than the conventional combustion boiler. There is a significant differencebetween the exergy efficiencies of furnaces in the two boiler models, and the combustion exergy efficiency in the oxy-combustionfurnace is about 4% higher. In each boiler model, the combustion process contributes nearly 60% exergy destruction. The exergyefficiency of the air heater in the oxy-combustion boiler is 10.3% higher than that in the conventional combustion boiler because ofthe flue gas recycle in the oxy-combustion boiler. The exergy destruction in the turbine and FWHmodel is just 9.01% of the total fuelexergy, and the turbines contribute about half. The exergy efficiencies of the ASU and FGU processes are 15.84 and 73.45%,respectively. The exergy efficiency of the oxy-combustion system is 37.13%, which is 4.08% lower than that of the conventionalsystem.

1. INTRODUCTION

The emission of CO2 in China has reached about 6.55 gigatons(22.3% of the world’s CO2 emission)1 in 2008. Coal-fired powerplants contributemost CO2 emission in China, because over 60%of China’s total energy is supplied by coal-fired power plants.CO2 capture and sequestration (CCS) from power plants is afeasible and effective choice, perhaps the only choice, to controlthe CO2 emission at the present stage,

2 especially for China. Oxy-combustion (or oxy-fuel) technology is considered a feasiblechoice to reduce the CO2 emission from the coal-fired powerplant through combining a conventional pulverized-coal-firedpower plant (PC) with a cryogenic air separation unit (ASU) anda flue gas treatment unit (FGU) (as shown in Figure 1).

High-purity oxygen (greater than 95% by volume3) from theASU, instead of air, is used as the oxidizer in the oxy-combustiontechnology. About 70�80% of the flue gas4,5 is recycled back tothe furnace with the oxygen stream, which could keep thecombustion temperature inside the furnace within the conven-tional range by appropriately adjusting the recycle ratio and otherparameters, such as the arrangement of the heat-exchange surfacein the furnace. The resulting flue gases from the furnace consistprimarily of CO2 and water vapor5 because there is no N2

dilution during the fuel combustion. The flue gas from the boileris then cleaned, dried, and compressed, followed by separation ofnoncondensable gases (Ar, O2, and N2) from CO2. Then, a 99mol % CO2 product could be obtained and finally boosted topipeline pressure.5,6 In comparison to conventional air-firedcombustion flue gases, which contain a high N2 fraction andrelatively low CO2 fraction (13�15% by volume),7,8 the CO2-enriched flue gas from the oxy-combustion process is obviouslyless energy-demanding.

The addition of ASU and FGU processes in oxy-combustionplants leads to a net power output decrease (25�30%)3�5,9

and an electricity generation cost increase (30�50%).3,9�11

However, the economic feasibility of the oxy-combustiontechnology still holds if the CO2 tax and CO2 sale can beconsidered11 or policy rewards can be conducted. Althoughthere are presently some economic barriers for the oxy-combustion technology, even for any CO2 emission controltechnology, the oxy-combustion technology demonstrates ahigh adaptability for existing coal-fired power plants andactually better economic properties than some other CO2

emission control technologies.4,9,10

System process simulation is an effective tool to help usunderstand the thermodynamic properties and adjust operationconditions of oxy-combustion systems. Moreover, the simulationresults are an indispensable basis to conduct a more sophisticatedthermodynamic analysis, such as exergy analysis, and evenoptimization as well as thermoeconomic cost accounting.12

Aspen Plus is considered to be a proper tool to study the oxy-combustion technology, and some works about that have beenpublished.5,13,14 The authors have previously finished a processsimulation work of an 800 MWe supercritical oxy-combustionPC.5 In that paper, Aspen Plus was used to simulate the wholesystem. The results of the previous work show that the CO2

concentration in the flue gas from the boiler can be more than80%, and the purity of the CO2 product from FGU can reach99%. In addition, some critical parameters (such as recycle ratio,oxygen concentration from ASU, etc.) and different cases were

Received: May 12, 2011Revised: July 7, 2011

3855 dx.doi.org/10.1021/ef200702k |Energy Fuels 2011, 25, 3854–3864

Energy & Fuels ARTICLE

discussed. However, to better understand the thermodynamiccharacteristics of the oxy-combustion technology and find out itssource of inefficiency for further optimization, an exergy analysison the oxy-combustion system is necessary, which activatesthis work.

Exergy is an important concept in the thermodynamics. It isdefined as “the maximum theoretical useful work (shaft work orelectrical work) obtainable as the systems interact to equilibrium,heat transfer occurring with the environment only”.15,16 Exergyanalysis focuses on the quality of energy rather than the quantityof energy. Therefore, the exergy analysis is usually carried out todetermine the magnitudes, locations, and types of exergy lossesoccurring in the system,17 and in this situation, there will be aguideline for reducing the inefficiencies, saving energy consump-tions, and optimizing the system. Not surprisingly, the exergyanalysis has been widely used to study different thermodynamicsystems. Lots of processes, such as coal-fired steam powerplants,18 combustors and combustion processes,19 combinedcycle power plants,20 fluidized beds,21 and some CO2 emissioncontrol processes, such as integrated coal gasification combinedcycle (IGCC),22 chemical looping,23 CO2 capture by amineor chilled ammonia scrubbing,24,25 as well as oxy-fuel, have beenstudied from the viewpoint of the exergy analysis methodology.With respect to the oxy-fuel systems analyzed, there aretwo different types: firing gaseous fuels, such as natural gas,26

methane, and propane,27 as well as syngas,28 and firing coal.29

Actually, the exergy analysis of an oxy-combustion PC is still verylimited. Seepana and Jayanti proposed a “flue gas conditioner”addition to the “conventional oxy-combustion” for high ash coalcombustion.29 An energy analysis of these two oxy-combustionsystems was performed, and the results were compared. Never-theless, the exergy analysis was processed in a low disaggregationlevel [such as ASU, CO2 compression, and high pressure (HP)].Furthermore, only physical exergies (almost all are powerproductions and consumptions) were calculated. As known, fluegas from oxy-combustion PC and conventional PC differs greatlyin gas composition and flow rate, and chemical components offlue gas are obviously different from these in an atmosphericenvironment. It is thus necessary to calculate definitely chemicalexergy, which takes into consideration the composition differ-ence between the system analyzed and the environment.

Therefore, a detailed exergy analysis of a complete oxy-com-bustion system, especially the boiler model, which was notanalyzed in detail before, is required. The simulation study onan 800 MWe supercritical oxy-combustion PC carried outbefore5 was based on a U.S. Department of Energy (DOE)report,30 which also provides some benchmark results forvalidating our simulation results from Aspen Plus. In this paper,we focused on a 600 MWe supercritical PC, which is typicalin China. First, Aspen Plus, version 7.1, was used to simulate a

Figure 1. Schematic diagram of the oxy-combustion PC system.



600 MWe supercritical oxy-combustion PC fueled by Shenhuacoal (shown in section 2). Then, a detailed exergy analysis on thesystem (shown in section 3) was carried out based on thesimulation results and exergy calculation methods. At the sametime, an exergy analysis of a conventional PC with the same grosspower output was also conducted as a comparison case. In theexergy calculation process, physical exergy and chemical exergyof each stream in both systems were calculated and, moreover,material streams were divided to different phases for moreaccurate results.

2. SYSTEM SIMULATION

In this paper, a 600 MWe supercritical oxy-combustion PC fueled byShenhua coal was simulated using Aspen Plus. This simulation work issimilar to that presented in ref 5, and the simulation results are the basisto perform the exergy analysis, which is the objective of this paper; thus, abrief introduction of the system simulation is given as follows. Thissystem also has four sections: boiler, turbines and feedwater heaters(FWHs), ASU, and FGU. The schematic diagram of this oxy-combus-tion system is shown in Figure 1. Some initial inputs required in thissimulation work can be found in ref 5. The turbine and FWH model inthis simulation work is in proportion to that in the 800 MWe systemsimulated in ref 5. It should be pointed out that the preferred flue gasrecycle ratio5 in this boiler model was adjusted to be 0.695, which is alittle different to the value given in ref 5 (0.705) because of the differentcoal samples. With the preferred flue gas recycle ratio, the combustiontemperature inside the furnace could be within the conventional rangeand the coal combustion properties in the oxy-combustion case could befine.4,31 A detailed sensitivity analysis and discussion of the recycle ratiocan also be found in ref 5.The proximate and ultimate analyses of the Shenhua coal are listed in

Table 1. In the table, all data are on the as-received (ar) basis, C, H, O, N,and S mean carbon, hydrogen, oxygen, nitrogen, and sulfur in the coal,respectively, and LHV is the lower heating value of the raw coal.Some important simulation results of the oxy-combustion system are

listed in Table 2, and more detailed simulation results and discussionscan be found in ref 5. Process simulation on a 600 MWe supercriticalconventional PC firing Shenhua coal was also carried out (also shown inTable 2).

3. EXERGY ANALYSIS

Exergy is a measure of the departure of the system state fromthe environment state and also the potential of a stream to causechange.16 Exergy is also themeasure that coordinates quality withquantity of energy. The total exergy (E) can be divided into fourparts: physical exergy (EPH), kinetic exergy (EKN), potentialexergy (EPT), and chemical exergy (ECH), which can be de-scribed as15,32

E ¼ EPH þ EKN þ EPT þ ECH ð1ÞEKN and EPT are relating to velocity and elevation, respectively;therefore, in a thermodynamic system, such as the coal-fired

power plants analyzed in this paper, they could not be taken intoconsideration because the velocity and elevation have negligiblechanges.32 Just EPH and ECH are left for calculation. EPH is definedas the maximum theoretical useful work obtained as a systempasses from its initial state (T and P) to the environment state(T0 and P0);

15,32 therefore, it arises from the temperature andpressure differences between the system analyzed and the envir-onment. ECH arises from the departure of the chemical composi-tion of a system from the environment.32 The calculationmethodsand equations about EPH and ECH are introduced as follows.3.1. Exergy Calculation Methodology. First of all, an en-

vironment state should be defined for the exergy calculation. Thestate contains not only the temperature and the pressure but alsothe chemical components of the environment. An appropriateenvironment model33 is given in Table 3.With the definition of the environment model, each kind of

exergy could be calculated by the equations presented below.For the unit physical exergy15 (ePH, kJ/kmol)

ePH ¼ Δh� T0Δs ¼ ðh� h0Þ � T0ðs� s0Þ ð2ÞHere, h and smean unit enthalpy (kJ/kmol) and unit entropy (kJkmol�1 K�1), respectively. The subscript “0” means thereference state.For the ECH calculation, there are different cases depending

upon stream materials. First, stream materials can be differen-tiated between components in the reference environment modeland components not in the reference environment. For a gaseous

Table 1. Proximate and Ultimate Analyses of Coal (ar Basis)

proximate analysis (wt %) ultimate analysis (wt %)

moisture 13.8 C 60.51

volatile matter 26.2 H 3.62

ash 11 O 9.94

fixed carbon 49 N 0.7

LHV (kJ/kg) 22768 S 0.43

Table 2. Simulation Results of Two Plants

item oxy-combustion

conventional

combustion

Power Generated (MW)

gross system 599.43 599.43

HP (22) 184.49 184.49

IP (23) 175.66 175.66

LP (24) 239.27 239.27

Power Consumed (MW)

ASU (1) 102.24

FGU (15) 47.13

circulating pump (27) 4.26 4.26

condenser pump (28) 0.75 0.75

total 154.33 5.01

coal feed rate (kg/s) 60.52 61.58

oxygen stoichiometric amount

(kmol/s)

3.42 3.48

raw air flowing into the ASU

(kmol/s)

16.62

net fuel input (MW) 1377.92 (LHV) 1402.05 (LHV)

gross efficiency (%) 43.50 (LHV) 42.75 (LHV)

net efficiency (%) 32.30 (LHV) 42.40 (LHV)

unit power consumption

for oxygen production

(kWh/kg of O2)

0.247

unit power consumption for CO2

emission control

(kWh/kg of CO2)

0.319

CO2 emission control efficiency (%) 96.12



component in the reference environment model, its unit chemi-cal exergy15 (eCH, kJ/kmol) can be calculated by

eCHk ¼ �RT0 ln x0k ð3Þ

in which R is the gas constant, 8.314, and xk0 means the mole

fraction of the gaseous component in the environment model.For example, for O2, xk

0 is 0.2035. It should be declared that all ofthe gaseous components are treated as ideal gases in this paper.For the condensed phase in the environment model, its unitchemical exergy equals 0.On the other hand, for a component not in the environment

model, there are also two different cases: elementary substanceand compound. In this situation, to calculate the chemical exergyof an elementary substance Y, the most stable compound thatcontains this element, YyAaBbCc, should be found and used.Because eCH of this most stable compound equals 0, eCH of thiselementary substance can be calculated by15

eCHðYÞ ¼ �1yðg0ðYyAaBbCcÞ þ aeCHðAÞ

þ beCHðBÞ þ ceCHðCÞÞ ð4Þin which g is unit Gibbs free energy (kJ/kmol). Furthermore, forthe chemical exergy calculation of a compound, the unit chemicalexergies of all of the elements contained in the compound shouldbe already known. The chemical exergy of the compoundAaBbCcDd is then calculated using eqs 3 and 4 as follows:

eCHðAaBbCcDdÞ ¼ g0ðAaBbCcDdÞ þ aeCHðAÞ þ beCHðBÞ

þ ceCHðCÞ þ deCHðDÞ ð5Þ

Therefore, eCH of any component can be obtained using themethods described above. Table 4 shows the calculated unitchemical exergies of some components relating to the systemsanalyzed in this paper. In addition, some basic data about h0, s0,and g0 from the Aspen Plus database are also given in Table 4.However, in an exergy calculation process, streams in the

system often contain many different components; that is to say,they are mixtures. For the chemical exergy calculation of amixture material stream, the gaseous mixture and non-gaseousmixture are treated differently. In detail, for a gaseous mixture, itseCH can be calculated by15

eCH ¼ ∑xkeCHk þ RT0∑xk ln xk ð6Þ

in which xk means the mole fraction of the gaseous component inthe mixture stream. For a non-gaseous mixture, its eCH is theweighted sum of unit chemical exergies of all components in themixture.There is another special material, coal. The chemical exergy

calculation of coal is quite complex, and some empirical equa-tions exist. The detailed chemical exergy calculation process ofcoal can be found in refs 15 and 34, which was adopted in thispaper. An introduction of the calculation method is given below.The coal combustion process could be expressed as

ðcC þ hH þ oO þ nN þ sSÞ þ vO2O2

f vCO2CO2 þ vH2OH2OðlÞ þ vSO2SO2 þ vN2N2 ð7Þin which c, h, o, n, and s (kmol/kg) mean the mole amount ofelements C, H, O, N, and S in unit mass coal under the dry andash-free (DAF) basis, respectively. On the basis of the equationequilibrium rule, we can obtain

vCO2 ¼ c, vH2O ¼ 12h, vSO2 ¼ s, vN2 ¼ 1

2n,

vO2 ¼ c þ 14h þ s� 1

2o ð8Þ

Thus, the unit chemical exergy of coal [kJ/kg (DAF)] can becalculated by

eCHDAF ¼ HHVDAF � T0ðsDAF þ vO2 sO2 � vCO2 sCO2

� vH2OsH2O � vSO2 sSO2 � vN2 sN2Þ þ ðvCO2eCHCO2

þ vH2OeCHH2O þ vSO2e

CHSO2

þ vN2eCHN2

� vO2eCHO2

Þ ð9Þin which HHVDAF [kJ/kg (DAF)] is the higher heating value ofcoal under the DAF basis and sDAF is the unit standard entropy ofcoal [kJ kg�1 (DAF) K�1] under the DAF basis, which can becalculated by15,35

sDAF ¼ c 37:1653� 31:4767 exp �0:564682h

c þ n

� ��

þ 20:1145o

c þ nþ 54:3111

nc þ n

þ 44:6712s

c þ n

�

ð10Þ

Table 3. Definition of the Environment Model

temperature, T0 298.15 K

pressure, P0 1 atm

component

gaseous phase mole fraction condensed phases (T0 and P0) state

N2 0.7567 H2O liquid

O2 0.2035 CaCO3 solid

H2O 0.0303 CaSO4 3 2H2O solid

Ar 0.0091

CO2 0.0003

Table 4. Unit Enthalpies, Unit Entropies, Unit Gibbs FreeEnergies, and Unit Chemical Exergies of Some Components

component

h0(kJ/kmol)

s0(kJ kmol�1 K�1)

g0(kJ/kmol)

eCH

(kJ/kmol)

N2 0 0 0 691.07

O2 0 0 0 3946.50

Ar 0 0 0 11649.16

CO2 �393510 2.88 �394370 20107.51

H2O �241810 �44.35 �228590 8667.46

H2 0 0 0 235284.21

NO 90250 12.34 86570 88888.78

CO �110530 89.28 �137150 275354.26

NO2 33180 �60.87 51328 55620.03

SO2 �296840 11 �300120 306269.34

SO3 �395720 �83.08 �370950 237412.59

C(s) 0 0 0 410531.01

S(s) 0 0 0 602442.84



If the HHVDAF value in eq 9 is not given, it can be estimated bythe following empirical equation:15,35

HHVDAF ¼ ð152:19H þ 98:767ÞðC=3 þ H � ðO� SÞ=8Þð11Þ

In which C, H, O, and S are the mass fractions of elements C, H,O, and S in the coal under the DAF basis.Therefore, eCH of the Shenhua coal can be calculated to be

24 686.6 kJ/kg (ar). It is worth noting that eCH of a coal samplenearly equals its HHV.15,34

3.2. Exergy Analysis Results. To obtain detailed exergyanalysis results, the whole oxy-combustion system was dividedinto four different models: boiler, turbines and FWHs, ASU, andFGU. The oxy-combustion system includes all four models,whereas the conventional combustion system just includes theformer two. However, there is almost no difference between theturbine and FWHmodels for the oxy-combustion system and theconventional combustion system; therefore, a comparison willonly be given to the two boiler models. After the exergy analysisof each model was conducted, an exergy analysis was also carriedout for the two whole systems. The exergy analysis results foreach model are shown as follows.3.2.1. Boiler Model. In the oxy-combustion system, high-purity

oxygen, instead of air, is used in the coal combustion process.Also, about 70% flue gas is recycled back to the furnace. Theschematic diagrams of the two boiler models are shown inFigure 2. Because coal is combusted with oxygen in the furnace(FUR) and the ECH of coal is converted to be EPH as radiationheat and convection heat, then the heat is absorbed by thefeedwater or the reheat steam in several heat exchangers (HEXs).They are two different processes; thus, each boiler model in thissection was divided into FUR and HEXs. Moreover, the HEXsinclude a convective superheater (CSH), a radiation superheater(RSH), a reheater (RH), an economizer (ECO), a water wall(WW), and an air heater (AH).On the basis of the simulation results and exergy calculation

methods introduced above, a detailed exergy analysis work wasperformed on the two boiler models and the results are all listedin Table 5. The items in the table correspond to names defined inFigure 2.The results in Table 5 provide a basis to calculate exergy

destruction (ED) and exergy loss (EL). Exergy destruction arisesfrom irreversibility occurring in a system or units.15,16 The exergyloss for a boiler includes three parts: exergy in the flue gas flowing

Figure 2. Schematic diagrams of boiler models.

Table 5. Exergy Analysis Results of Boiler Models

oxy-combustion conventional combustion

EPH (kW) EPH (kW)

item

gaseous

part

solid

part

ECH

(kW)

gaseous

part

solid

part

ECH

(kW)

C1 1494033.03 1520126.77

G1 0 12861.07 0 1398.47

G2 30537.42 130749.82 28601.96 1398.47

G3 398002.71 6501.81 195669.38 431057.67 6532.55 51514.14

G4 304787.51 5141.59 195669.38 311405.84 4886.98 51514.14

G5 130424.25 2593.15 195669.38 139110.64 2543.46 51514.14

G6 60372.72 1484.79 195669.38 69318.79 1527.08 51514.14

G7 17288.39 630.79 195669.38 19054.15 641.96 51514.14

G8 17288.39 195669.38 19054.15 51514.14

G9 539.25 58490.19 3474.69 49174.34

G10 2305.99

G11 4817.03 193353.19

G12 1228.78 133280.94

G13 1433.95 130749.82

Q1 242083.71 231153.90

Q2 378643.76 361548.41

Rad 647940.99 618687.17

ECOA 59707.84 59795.66

WWA 263001.91 250951.20

RSHA 181736.96 171866.61

CSHA 75083.94 97004.99

RHA 149677.76 149676.27

AHA 29103.47 28601.96

ECOS 71159.89 70808.24

WWS 378643.76 361548.41

RSHS 242083.71 231153.90

CSHS 94575.42 121297.40

RHS 176911.70 174638.72

AHS 43938.33 51149.75



into the environment, exergy in the ash, and radiation exergy loss.The calculation results about ED and EL are given in Table 6,which show that the yD,b of FUR in the oxy-combustion system is2.05% lower than that in the conventional system; the sum of theyD,b of all HEXs (WW, RSH, CSH, RH, ECO, and AH) in theoxy-combustion system is 0.09% lower (almost equal) than thosein the conventional system. Moreover, the yD,b,T decreases 1.01%,and the sum of yD,b,T and yL,T decreases 0.46% in the oxy-combustion boiler model in comparison to that in the conven-tional boiler model.Furthermore, the two boiler models have a similar ED distribu-

tion rule. FUR is the main exergy destruction unit, taking about60%, that is because the combustion process is high irreversible.HEXs take about 36%, and others take just a little. It is worthnoting that the yD,b of the MIX unit is about 1% because of themixture process of oxygen and recycled flue gas; therefore,reducing this ED should be kept in mind during the operation.

Exergy efficiency (ηex) of each unit or even the whole systemcan also be calculated. ηex is also called second-law efficiency,effectiveness, or rational efficiency and is usually defined as usedexergy divided by provided exergy.15,36 Because there are manykinds of definitions of the “used exergy” and “provided exergy”,there are lots of different exergy efficiency definitions.36 In thispaper, ηex is defined as the ratio of product (P) to fuel (F), whichis illustrated as eq 1237

ηex ¼ EP=EF ð12Þwhere EP and EF are exergies from product and fuel, respectively.Usually, each device has its own productive purpose, such assteam for the boiler and power for the generator. The productivepurpose of a process device measured in terms of exergy is named“product”, and the consumed exergy flow to create the product is“fuel”.38,39 Therefore, it is important to define the F and P foreach unit in the system, and F�P definitions about someimportant thermodynamic devices can be found in refs 15 and40. For a compressor, pump, or fan, the power consumption isthe fuel, but for a turbine or expander, the power generated is theproduct. For a HEX, there are two cases. If the HEX is used forheating, then the exergy decrease of the hot steam is the fuel andthe exergy increase of the cold steam is the product, whereas if theHEX is used for cooling, the definition is opposite.15 The F�Pdefinition of a boiler is more complicated, which is given inTable 7.15

Some ηex definitions about units in this paper are described asfollows:

FUR: ηex, FUR ¼ ðEPHRad þ EG3Þ=ðEG2 þ ECHC1 Þ ð13Þ

HEX: ηex,HEX ¼ EPHA =EPHS ð14Þ

boiler: ηex, B ¼ EPHA, FW=ðECHC1 þ EG1 � ELÞ ð15Þ

Table 6. Results about Exergy Destruction and Exergy Loss for Boiler Models (kW)


unit ED,i(EL,i) ED,i/ED,T (%)a ED,i(EL,i)/EF,B (%)b ED,i(EL,i) ED,i/ED,T (%) ED,i(EL,i)/EF,B (%)

FUR 407205.38 58.89 27.02 442335.67 61.99 29.07

WW 115641.85 16.72 7.67 110597.21 15.50 7.27

RSH 60346.76 8.73 4.00 59287.30 8.31 3.90

CSH 19491.48 2.82 1.29 24292.41 3.40 1.60

RH 27233.94 3.94 1.81 24962.45 3.50 1.64

ECO 11452.04 1.66 0.76 11012.58 1.54 0.72

AH 14834.86 2.15 0.98 22547.79 3.16 1.48

ESP 630.79 0.09 0.04 641.96 0.09 0.04

FGD 14787.55 2.14 0.98 17919.26 2.51 1.18

DRY 4631.06 0.67 0.31

MIX 15187.02 2.20 1.01

ED,T 691442.74 100.00 45.89 713596.63 100.00 46.90

ED,T,HEX 249000.94 36.01 16.52 252699.73 35.41 16.61

EL,FG 59029.44 3.92 52649.03 3.46

EL,Rad 27213.52 1.81 25984.86 1.71

ED,T + EL,T 777685.71 51.61 792230.51 52.07

EF,B 1506894.10 1521525.24aRatio of ED to ED,T, named as yD,a.

bRatio of ED or EL to EF provided, named as yD,b or yL.

Table 7. F�P Definition of the Boiler Model

EP (E6 � E5) + (E8 � E7)

EF (E1 + E2) � (E3 + E4 + E9)



In eq 15, EL includes exergy losses of flue gas, ash, and radiationand subscript “FW” means feedwater.On the basis of the data shown in Table 5 and ηex calculation

equations defined above, the ηex calculation results for the twoboiler models are given in Figure 3. The results in Figure 3 showthat ηex,FUR in the oxy-combustion system is about 4% higherthan that in the conventional combustion system, which shouldbe the primary reason why ηex of the oxy-combustion boiler is0.8% higher, because exergy efficiencies of the two FG�FWheat-exchange processes are nearly equivalent.The FG�FW heat-exchange process relates five HEXs: WW,

RSH, CSH, RH, and ECO, and WW has the lowest exergyefficiency among the five HEXs. Moreover, the exergy efficiencyof AH is even lower than that ofWW.The heat-exchange process inAH occurs between flue gas and inlet gas. ηex,AH in the oxy-combustion system is 10.3% higher than that in the conventionalcombustion system. To further understand how the differencesabout the exergy efficiencies happen, a calculation about energyquality coefficients was also performed in this paper, and the resultsare shown in Table 8. The definition of energy quality coefficient(λ) is described as eq 16, in which H means enthalpy (kJ).

λ ¼ ΔE=ΔH ð16ÞThe results in Table 8 show that, for the FG�FW heat-exchangeprocess in each system, λECO is the lowest on the cold side(feedwater) because the cold feedwater enters the ECO first. On

the other hand, λECO is also the lowest on the hot side (flue gas)because the hot flue gas enters the ECO last; thus, the ratio of λECO,cold to λECO,hot is not the lowest. It is worth mentioning that, thelower the ratio of λcold to λhot, the stronger the irreversibility in thatdevice and the lower itsηex. Therefore, althoughλECO is lower thanλWW on the cold side, ηex,ECO is not the lowest because λECO ismuch lower than λWW on the hot side. However, there is asignificant difference between the results of the two AHs. λcold,AHin the oxy-combustion boiler model is 0.044 higher than that in theconventional combustion boiler model. This is because there isabout 70% flue gas recycled in the oxy-combustion system; thus,much heat in the recycled flue gas is used, and λcold,AH is improved.However, in general, the convective heat-exchange properties in thetwo boiler models are similar because the biggest differencebetween the two boiler models is the coal combustion property,the flue gas composition5 (chemical exergy), as well as the flue gasrecycle, but only the flue gas recycle affects the heat exchange inAH.3.2.2. Turbine and FWH Model. An exergy analysis on the

turbine and FWH model was conducted in this section. Thestructure of the turbine and FWHmodel can be found in Figure 1.The turbine and FWH model is a Rankine cycle, and heat isconverted into work in this cycle. The high pressure andtemperature steam expands through the turbines to generatepower, and then the wet steam enters the condenser, where it iscondensed to become a saturated water. The water is pumpedfrom low to high pressure and heated by the steam extracted fromthe turbines. Finally, the feedwater enters the boiler to becomethe dry saturated steam and completes one cycle. Because thereare so many material and energy steams in this model, instead oflisting all of the exergy calculation results, only the results ofexergy destructions and exergy losses are given in Table 9. Itshould be mentioned that water or steam in this model is justrecycled in the pipeline; therefore, only physical exergies aboutthe water/steam were calculated. Furthermore, as mentionedabove, because there is almost no difference between the turbineand FWH models for the oxy-combustion system and theconventional combustion system, the results in Table 9 are forboth systems.The results in Table 9 show that ηex of the turbine and FWH

model could reach up to 81.51%. Because the expansion processoccurring in the turbine is high irreversible, in the turbine andFWH model, turbines contribute most ED, about a half of thetotal ED, and the condenser and FWHs follow (in that order).Exergy destructions of other units are slight.3.2.3. ASUModel.The ASUmodel analyzed in this section and

the FGU model analyzed in the next section are important units

Figure 3. Exergy calculation results for two boiler models.

Table 8. Energy Quality Coefficient Calculation Results forBoiler Models

energy quality coefficient (λ)


HEX unit hot side cold side hot side cold side

ECO 0.588 0.493 0.584 0.493

WW 0.783 0.544 0.783 0.543

RSH 0.783 0.588 0.783 0.582

CSH 0.766 0.608 0.764 0.611

RH 0.704 0.596 0.695 0.596

AH 0.442 0.293 0.445 0.249

Table 9. Results about Exergy Destruction and Exergy Lossfor the Turbine and FWH Model (kW)

unit ED,i ED,i/ED,T (%) ED,i/EF,tur (%)

turbine 64915.03 48.16 8.90

FWH 14897.79 11.05 2.04

condenser 33826.14 25.09 4.64

generator 8634.48 6.41 1.18

deaerator 2009.83 1.49 0.28

BFPT 5435.60 4.03 0.75

Else 5075.52 3.77 0.70

ED,T 134794.40 100.00 18.49

EF,tur 729208.80



in the oxy-combustion system. However, they are not included inthe conventional combustion system. A detailed exergy analysisabout the ASUmodel, which is shown in Figure 4, was performedin this section, and the mole fraction of the O2 product from theASU is designed to be 95%, which is suggested by manyresearchers3 and also discussed in ref 5. In the ASU model, thereare four units: multi-stage compressor (MCOM), heat exchanger(HEX), distillation column (DC), and expansion valve (valve).The exergy analysis results of the ASU model are given inTables 10 and 11.The results in Table 11 show that MCOM and DC have high

ED values, each about 40% of the ED,T value. ED,T can reach64.15% of EF,ASU. The power consumption is the major part ofthe ASU EF,ASU value. In addition, yL is about 20%, but this valueis affected by the definition of EL in the ASUmodel. In this paper,EL of the ASU model is defined to be the exergy contained in the“N2 out” stream and the exergy contained in the “O2 out” streamis defined to be P of the ASU. In this case, ηex,ASU is very low, just15.84%.3.2.4. FGU Model. In this section, the exergy analysis was

performed on the FGUmodel, which is shown in Figure 5. Thereare five units in the FGUmodel: clean, compression, and coolingprocess (CCC), HEX, DC, and two valves. In the oxy-combus-tion system, the CO2 content in the flue gas is very high;therefore, a high-purity CO2 product could be easily obtainedby some physical processes. In this FGU model, the flue gas iscleaned, compressed, dried, and distilled and, finally, a 99 mol %purity CO2 product is obtained. The exergy analysis results aboutthe FGU model are given in Tables 12 and 13.During the ED and EL calculation, the clean process, the

compression process, and the cooling process were combined(CCC). The results in Tables 11 and 13 indicate that there is alittle difference between the results of the ASU model and theFGU model. In the FGU model, the CCC process contributesmost ED, about 73%, but ED,DC is much lower. That is because

theCO2content in theflue gas is already very high (above 80mol%)5

and the distillation process of CO2 to other impurities is mucheasier than that in the air separation process. ED of valve2 is alsohigh because of the intense expansion process. In this exergyanalysis, EPH contained in “gas out” is nearly zero; therefore, it isignored. ECH contained in “gas out” is defined to be EL. Also, thepower consumption is the major part of EF,FGU.On the other hand, because there is a high CO2 content of flue

gas flowing into the FGUmodel, although the EF,FGU and the EF,ASU are almost equivalent, the power fraction in EF,FGU is muchlower. Moreover, the power is the primary energy consumptionfor ASU or FGU; therefore, ηex,FGU (73.45%) is much higherthan that of the ASU model. Moreover, this result also indicatesthat this FGU process is proper for flue gas treating.3.2.5. Whole System. In this section, an exergy analysis from

the viewpoint of the whole system was conducted. First of all, onthe basis of the data given above and eqs 17 and 18, ηex of the oxy-combustion system is calculated to be 37.13%, which is 4.08%

Figure 4. Schematic diagram of the ASU model.

Table 10. Exergy Analysis Result of the ASU Model

stream EPH (kW) ECH (kW)

air 0.00 1284.91

C air 75199.71 1284.91

airincol 188643.73 1284.91

N2 79073.45 7784.69

O2 63590.49 12861.07

N2 out 12933.98 7784.76

O2 out 3534.97 12861.07

O2 V 62608.06 12861.07

W 102242.50

Table 11. Results about Exergy Destruction and Exergy Lossfor the ASU Model (kW)

unit ED,i(EL,i) ED,i/ED,T (%) ED,i(EL,i)/EF,ASU (%)

MCOM 27042.79 40.72 26.12

HEX 11768.54 17.72 11.37

DC 26618.95 40.08 25.71

valve 982.43 1.48 0.95

ED,T 66412.71 100.00 64.15

EL 20718.74 20.01

ED,T + EL 87131.45 84.16

EF,ASU 103527.41

Figure 5. Schematic diagram of the FGU model.

Table 12. Exergy Analysis Result of the FGU Model

stream EPH (kW) ECH (kW)

flue gas 539.25 58490.19

dry gas 28783.25 58487.44

airincol 36464.94 58487.44

gas 4869.01 2305.99

gas V 835.62 2305.99

gas out 0.00 2305.99

CO2 28373.91 58789.93

CO2 V 28222.03 58789.93

CO2 out 19183.82 58789.93

W 47127.35



lower than that of the conventional system mainly because of theexergy destructions occurring in the ASU and FGU models. Itshould be emphasized that the exergies contained in the CO2

product from FGU, ECO2 out, is also defined as a part of theproduct of the oxy-combustion plant.

ηex, oxy ¼ ðWnet, oxy þ ECO2 outÞ=ðECHC1, oxy þ Eair � EL, oxyÞð17Þ

ηex, con ¼ Wnet, con=ðECHC1, con þ EG1, con � EL, conÞ ð18Þ

To describe the distributions of exergy destructions and exergylosses in the two systems, two pie pictures are shown in Figure 6.The values in the pictures mean the ratios of exergy destructionsor exergy losses in some important units to the total fuel exergy ofeach system, respectively. Moreover, the “product” in eachpicture means the product of that system, and the “others inboiler” item in the oxy-combustion system does not include theexergy loss of flue gas from the boiler anymore because it is a partof the fuel of the FGU model and does not loss from theviewpoint of the whole system. Thus, in this situation, thepercent of the “others in boiler” item in the oxy-combustionsystem is a little lower than that in the conventional system. ED ofthe turbine and FWHmodel is not very high, just about 9% of thetotal EF. In the oxy-combustion system, ASU and FGU systemscontribute much exergy destructions/losses, which are 7.71% ofthe total EF.It is worth mentioning that the condenser contributes most

energy loss in a coal-fired power plant from the viewpoint of thefirst law of thermodynamics. However, ED of the condenser isjust about 2.22% of the total EF if the energy quality is considered,viz., from the viewpoint of the second law of thermodynamics.This also shows the importance of the exergy analysis.Rosen and Tang41 performed an exergy analysis on a steam

power plant, and the results show that the boiler is the mostinefficient device and the combustion process and heat-transferprocess in the boiler account for nearly 43 and 40% of the plantexergy consumption, respectively, while the remainder occurs inturbines (about 10%), condenser (about 6%), andFWHs(about 2%).Aijundi42 also carried out an exergy analysis on the Al-Husseinsteam power plant in Jordan, and the results show that the boilersystem accounts for 77% of the total exergy destruction, theturbine accounts for 13%, and the condenser accounts for 9%.Moreover, another exergy analysis work conducted on a steampower plant43 presented a similar result that the exergy loss andexergy destruction occurring in the boiler system account for83% of the total exergy destruction (loss) in the plant and 52% ofthe chemical exergy entering the plant. The results obtained inthis paper coincide with these results.

4. CONCLUSION

A detailed exergy analysis of a 600 MWe oxy-combustion PCand a conventional PC was performed, and the exergy analysisresults for the two boiler models were compared. The results forthe boiler models show that the furnace exergy efficiency in theoxy-combustion system is about 4% higher than that in theconventional combustion system, which should be the primaryreason why the exergy efficiency of the oxy-combustion boiler is0.8% higher. For each boiler model, the combustion processcontributes the most exergy destruction, about 60%, and fol-lowed by the water wall, about 16%. Total exergy destructions ofheat exchangers in twomodels are nearly equivalent to each other(about 36%). Moreover, the water wall and air heater in eachboiler model have very low exergy efficiencies, but the exergyefficiency of the air heater in the oxy-combustion system is 10.3%higher than that in the conventional combustion system.

The results for the turbine and FWH model indicate that thisprocess is not a high-exergy-destructing process. The exergydestruction of this model is just 9.01% of the total fuel exergy ofthe oxy-combustion system, and its exergy efficiency could reachup to 81.51%. Turbines contribute about 50% exergy destruction,followed by condenser and feedwater heaters. The results for the

Table 13. Results about Exergy Destruction and Exergy Lossfor the FGU Model (kW)

unit ED,i(EL,i) ED,i/ED,T (%) ED,i(EL,i)/EF,FGU (%)

CCC 18886.10 72.98 17.79

HEX 2192.15 8.47 2.07

DC 613.55 2.37 0.58

valve1 151.87 0.59 0.14

valve2 4033.39 15.59 3.80

ED,T 25877.06 100 24.38

EL 2305.99 2.17

ED,T + EL 28183.05 26.55

EF,FGU 106156.79

Figure 6. Distributions of exergy destructions and exergy losses in twosystems.



ASU model and FGUmodel reveal that multi-stage compressorsare the highest exergy-destructing units in both models. In theASU model, the distillation column contributes similar exergydestruction (about 40%) as the multi-stage compressor. How-ever, in the FGU distillation column, the exergy destruction ratiois much lower compared to that in the ASU model. Finally, theASU model has very low exergy efficiency (15.84%), but theexergy efficiency of the FGU model is much higher (73.45%)because of the easier distillation process and the higher chemicalexergy contained in the inlet gas.

The exergy efficiency of the oxy-combustion system is 37.13%,which is 4.08% lower than that of the conventional system. Theexergy analysis results obtained in this paper provide a basis tocarry out a thermoeconomic cost account or optimization of acomplete oxy-combustion PC.

’AUTHOR INFORMATION

Corresponding Author*Telephone: +86-27-8754-4779. Fax: +86-27-8754-5526.E-mail: [email protected].

’ACKNOWLEDGMENT

The authors were supported by the National Key BasicResearch and Development Program (Grant 2011CB707300),the National Natural Science Foundation (Grants 50936001 and50721005), and theNewCentury Excellent Talents inUniversity(Grant NECT-10-0395) for funds. Special thanks should begiven to Alstom Power Boiler R&D Execution (Windsor, CT)for providing a 1 year internship and the Aspen Plus software.

’NOMENCLATURE

AbbreviationsAH = air heaterASU = air separation unitBFPT = feed water pump turbineCH = chemicalCCC = clean, compression, and cooling processCCS = CO2 capture and sequestrationCSH = convective superheaterDC = distillation columnDRY = dryerECO = economizerESP = electrostatic precipitatorFG = flue gasFGD = flue gas desulfurizationFGU = flue gas treatment unitFUR = furnaceFW = feedwaterFWH = feedwater heaterHEX = heat exchangerHHV = higher heating valueHP, IP, and LP = high pressure, intermediate pressure, and low

pressureIGCC = integrated coal gasification combined cycleKN = kineticLHV = lower heating valueMCOM = multi-stage compressorMIX = mixerPC = pulverized-coal-fired power plant

PH = physicalPT = potentialRad = total radiationRH = reheaterRSH = radiation superheaterSH = superheaterT = totalWW = water wall

ScalarsH and h = enthalpy (kJ) and unit enthalpy (kJ/kmol)E and e = exergy (kJ) and unit exergy (kJ/kmol)F = fuelg = unit Gibbs free energy (kJ/kmol)P = products = unit entropy (kJ kmol�1 K�1)T = temperature

Greek Lettersη = efficiencyλ = energy quality coefficient

Subscripts0 = reference stateA = absorbedar = as-received basisB = boilercon = conventional combustion plantD = destructionDAF = dry and ash freeex = exergyL = lossoxy = oxy-combustion plantS = supplied

Superscripts0 = reference state

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