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1 Copyright 2008 by ASME

Proceedings of the ASME International Mechanical Engineering Congress and ExpositionIMECE2008

October 31 - November 6, 2008, Boston Massachusetts, USA

IMECE2008-68339

EXERGY ANALYSIS OF A SOLID OXIDE FUEL CELL-GAS TURBINEHYBRID POWER PLANT

Valentina AmatiUniversity of Roma1 La SapienzaDept. of Mechanical Engineering

Via Eudossiana 18, 00184, Rome, ItalyTel: +390644585272; E-mail:[email protected]

Enrico SciubbaUniversity of Roma1 La SapienzaDept. of Mechanical Engineering

Via Eudossiana 18, 00184, Rome, ItalyTel: +390644585244 ; E-mail:[email protected]

Claudia ToroUniversity of Roma1 La SapienzaDept. of Mechanical Engineering

Via Eudossiana 18, 00184, Rome, ItalyTel: +390644585272; E-mail:

[email protected]

ABSTRACT

The paper presents the exergy analysis of a natural gas fuelled

energy conversion process consisting of a hybrid solid oxide

fuel cell coupled with a gas turbine.

The fuel is partly processed in a reformer and then undergoes

complete reforming in an internal reforming planar SOFC

stack (IRSOFC). The syngas fuels in turn a standard gas

turbine cycle that drives the fuel compressor and generates

excess shaft power. Extensive heat recovery is enforced both

in the Gas Turbine and between the topping SOFC and the

bottoming GT.

Two different configurations have been simulated and

compared on an exergy basis: in the first one, the steamneeded to support the external and the internal reforming

reactions is completely supplied by an external Heat Recovery

Steam Generator (HRSG), while in the second one that steam

is mainly obtained by recirculating part of the steam-rich

anode outlet stream.

The thermodynamic model of the fuel cell system has been

developed and implemented into the library of a modular

object-oriented Process Simulator, Camel-Pro; then, by

means of this simulator, the exergetic performance of the two

alternative configurations has been analyzed. A detailed

analysis of the exergy destruction at component level is

presented, to better assess the distribution of irreversibilitiesalong the process and to gain useful design insight.

INTRODUCTION

Solid oxide fuel cells (SOFCs) are direct energy

conversion devices with great potential. The technology has

distinct features which makes it suitable for electric utility

power generation in both large central station power plants

and distributed generation units [11].

SOFCs are solid state, ceramic cells, characterized by the

highest operating temperature (600-1000C) among all types

of fuel cells under development. Their high operatingtemperature not only dispenses of the need for expensive

catalysts but also produces high quality heat which can be fed

to cogeneration or bottoming cycles for additional electricity

generation [16].

Because of the feasibility of integration of solid-oxide fuel cell

(SOFC) and gas-turbine (GT) technologies in power

generation, preliminary results predict that an overall system

efficiency of 70% (net ac/LHV) or higher is possible with a

more complex thermodynamic cycle [15],[20]. These studies

were mainly based on an energy analysis, combining the first

Law of Thermodynamics with a techno-economical

evaluation.


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The models of the SOFC sub-system (SOFC+Reformer) used

in this work have been developed and implemented by the

authors in the Camel Pro simulator, as described in a previous

work [2]. The fuel cell model developed in this study is of a

planar design: its geometric and material data are taken from

[1] .

The SOFC electrochemical performance is modeledconsidering all forms of overpotential: the zero-dimensional

model implemented herein calculates stack power and outlet

stream parameters at the stack temperature.

The hybrid system was modeled by means of macroscopic

energy and mass balances performed on each component.

1.1 SOFC/GT cycle

Figure 1 shows the plant layout of the simple SOFC/GT

hybrid system simulated with Camel Pro.

External pre-filtered input air (Stream 24) is first compressed

in the main process compressor (C1) and then preheated bythe turbine exhaust gas in a gas/gas heat exchanger (AHR2).

The fuel (natural gas, Stream 22) is compressed in the fuel

compressor (C2) and then preheated in the heat exchanger

(AHR1). The required steam (Stream 18) is generated in a heat

recovery Boiler (HRB) fed by the gas turbine hot exhaust.

The compressed and pre-heated fuel is partially reformed in

the reforming unit and then completely converted into a

hydrogen-rich gas within the internally reforming SOFC unit.

An external heat supply is required by the reformer to

maintain the desired operating temperature (Stream 2).

The air and the pre-reformed fuel enter the SOFC that

consumes hydrogen to produce power. The gas (cathode side

and anode side) at the cell outlet passes through a combustor

(CB), where the residual hydrogen (and carbon monoxide) in

the gaseous mixture is burned and expands in the gas

turbine(GT) to produce additional power.

The turbine exhaust gases are used to preheat fuel (AHR1),air (AHR2) and also provide the heat needed in the heat

recovery steam generator (HRB). The temperatures of both the

air and fuel exiting the SOFC are controlled by throttling the

inlet air flow.

1.2 SOFC/GT cycle with anodic gas recirculation

The second layout considered in this work is based on the

anode recirculation arrangement: the reforming reaction must

be driven by the use of steam, so in this case part of this steam

is obtained from the anode outlet stream. The latter must be

high enough to avoid problems of carbon deposition thatwould irreversibly damage the cell [6].

Figure 2 presents the scheme of the SOFC/GT hybrid system

with anodic gas recirculation.

Also in this case external pre-filtered input air (Stream 23) is

first compressed in the main process compressor (C1) and

then preheated by the turbine exhaust gas in a gas/gas heat

exchanger (AHR2).

The fuel, (natural gas, Stream 21) is compressed in the fuel

compressor (C2) and then preheated in the heat exchanger

(AHR1).

Figure 2 Camel Pro flow sheet of the SOFC/GT hybrid system with anode-recirculation flow sheet in

Air

Fuel

Water

Power

Power

Heat CC

RB

AHR1

AHR2GT

CP1

CP2

PUM

REFSOFC


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The compressed and pre-heated fuel stream is then mixed with

a part of the steam-rich high- temperature stream coming from

the cell anode outlet.

Since the adopted model prescribes the SC Ratio to be an

input parameter, the remaining steam required is calculated

taking into account also the steam content of the recirculated

flow.Methane is partially reformed in the reformer and the residual

CH4 is internally reformed at the SOFC anode.

An external heat supply is required by the reformer to

maintain the desired operating temperature (Stream 2).

Part of the steam-rich anode exhaust gas is recirculated to the

reformer inlet (Stream 29) and the remaining (Stream 13) is

mixed with the cathode exhaust gas (Stream 7) and channeled

into the combustion chamber (CC).

To reach the desired turbine inlet temperature, part of the fuel

at compressor outlet bypasses the SOFC group and enters

directly into the combustion chamber (Stream 14). After

combustion, the high temperature outlet gas (Stream 10) is

directed to the gas turbine. The turbine exhaust gas is used topreheat air (AHR2), fuel (AHR1) and the feed water (HRB).

2. EXERGY ANALYSIS

2.1 Exergy concepts

Exergy can be defined as the maximum amount of work which

can be obtained from a system or a flow of matter when it is

brought reversibly to equilibrium with the reference

environment.

Exergy analysis is based on the Second Law of

Thermodynamics and on the concept of irreversible entropyproduction. The exergy consumption during a process is

proportional to the entropy production due to irreversibilities.

In an exergy analysis, three forms of exergy transfer are

usually considered, i.e., exergy transfer by work-, heat- and

mass interaction.

For each sub-unit, the outlet exergy is always less than the

inlet exergy because of irreversible processes. When

calculating the exergy of a process component, the difference

between the exergy losses and exergy destruction are

recorded. Exergy losses include the exergy flowing to the

surroundings, whereas exergy destruction indicates the loss of

exergy within the system boundary due to irreversibility.

The exergy of a stream of matter can be divided into differentcomponent exergies. In the absence of nuclear, magnetic,

electrical and surface tension effects, exergy is calculated as

the sum of :

K P Ph ChEx Ex Ex Ex Ex= + + + (1)

where ExK, ExP , Exth and Exch are the kinetic, potential,

physical and chemical exergy respectively.

The changes in the kinetic and gravitational potential energies

are neglected in the present study.

Physical exergy is defined as the maximum amount of work

which can be obtained when a stream of matter is brought

from its initial state to the environmental state while only

exchanging heat with the thermal reservoir of the

environment, whereas chemical exergy is defined as the

maximum amount of work which can be obtained when the

stream of matter is brought from the environment state to thetotal dead (unrestricted) state as a result of heat transfer and

exchange of substances only with the environment.

( ) ( )0 0 0PhEx h h T s s= (2)

0

0 , lnch i i i ch iEx R T x x x ex= + (3)

where xi is the mole fraction of the species i in the flow and

exch,i0

is the molar chemical exergy of the ith

species.

In order to identify and quantify irreversibilities and losses in

energy quality, an exergy analysis of the two SOFC/GT plants

was performed. The modular process simulator calculates theexergy associated to each energy and material stream in the

plant and provides the values of the exergy destruction,ExD

and of the exergy efficiency,ExEff, of each component.

2.2 Streams

Electricity

The exergy content of the electricity is equal to its energy

content:

[ ]Elec ElecEx W kW= (4)

Gas Mixture and Steam

The specific exergy of gas mixture, exgas is the sum of its

physical (exph) and chemical exergy (exch), calculated as

follows:

[ / ]gas ph chex ex ex kJ kg= + (5)

0 0 0( ) ( ) [ / ]phex h h T s s kJ kg= (6)0

, ln[ / ]

i i i ch ni

ch

gas

R T n n n exex kJ kg

MW

+= (7)

Where the subindex 0 denotes reference conditions and the

exch0 for each species are tabulated in [19].

2.3 Components

The equations used to calculate ExD and ExEff, for each

component of the SOFC/GT plant are reported below.

Reformer

0

Ref

1 [ ]i i i i o oF F W W i w w

TExD m ex m ex Q m ex kW

T

= + +

(8)


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Sys

TotI

Fuel Fuel in

W

m LHV Q =

+(24)

The fuel utilization factor of SOFC is defined as:

( )2 44f i i iz

UH CO CH

=+ + (25)

where each CH4 mole generates 4 H2 moles (3 by the internal

reforming and 1 by the water-gas shift reaction). The variable

z is the number of H2 moles which are oxidized within the

SOFC. The power output of systems investigated are reported

in Table 4 and Table 5 together with the power needed by the

air and fuel compressors, the pump and the thermal power

required by the reformer. The two systems have been analyzed

for the same total SOFC electric current. From Tables 4 and 5

it is possible to calculate the efficiency: values of 65.4 % are

obtained for the standard SOFC/GT system and of 66.4 % for

the anode-recirculation one (as reported in Table VI).

Table 2 SOFC/GT: Power flows within the system

Sofc Electric Power

Id P Current Voltage

- kW A V

8 284.84 410060 0.728

Shaft Power

Id P n

- kW rpm

13 151.81 300017 0.0075 3000

23 2.2401 3000

25 55.094 3000

26 149.57 3000

27 94.478 3000

Reformer Heat

Id P

- kW

2 78.986

Thus the introduction of the anode-recirculation actuallyincreases the electrical efficiency because it allows to reduce

both the amount of heat required by the reformer and the

amount of external water needed.

On the basis of the results of the simulations, the First Law

efficiency of the two alternatives have been compared (Table

6): the cycle with anode-recirculation has a higher efficiency,

due to a lower heat request by the reformer even if the SOFC

electrical efficiency is lower.

In fact in this cycle the fuel entering the SOFC, obtained by

mixing a 50% of anode outlet gas with CH4 extracted from the

compressor, is more diluted and contains 37% of CO2.

In the SOFC/GT with anode-recirculation the system outlet

gas (Stream 14) consists of 13% O2, 73% N2, 6% CO2, 7%

H2O at 508 K while in the fundamental one has almost the

same composition at 435 K.

3.2 Second law analysis of results

The purpose of this work was to perform an explicit exergy

analysis of the plant, and to identify -for each process

configuration- the components affected by the highest

irreversible losses, so that a more accurate analysis could be

made in the design phase. In fact, an exergy analysis provides

the designer with better insight as to where a design

modification is necessary, both at the single component and at

a process level, to better exploit the available resources.

Table 3 SOFC/GT with anode recirculation : Power flows

within the system

The exergetic efficiency of the system is defined as:

,, ,

Sys

Ex SysFuel in Q in

W

Ex Ex=

+ (26)

The exergy analysis of the plants (Table 7 and Table 8 ) shows

a net exergy efficiency of 63.2% and 64.4% for the first

configuration and for the one with anode-recirculation

respectively.

In the standard SOFC/GT plant the exergy entering the system

(fuel and heat) amounts to 600 kW, the overall destroyed

exergy rate to 164 kW and the exergy loss to 56.75 kW thus

the net generation is 380 kW. In the alternative configuration

the exergy input is 531 kW, the overall destroyed exergy rate

Sofc Electric Power

Id P Current Voltage

- kW A V

8 264.561 410060 0.672

Shaft Power

Id P n

- kW rpm

12 135.074 3000

16 0.00114 3000

22 2.081 3000

24 55.094 300025 132.99 3000

26 77.89 3000

Reformer Heat

Id P

- kW

2 9.978


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0% 5% 10% 15% 20% 25% 30% 35% 40%

REF

SOFC

CC

GT

HRB

PUM

AHR1

AHR2

Cfuel

Cair

ExD%

ExFD%

Figure 5 Exergy destruction distribution in the SOFC/GT

cycle.

0% 5% 10% 15% 20% 25% 30% 35% 40% 45%

REF

SOFC

CC

GT

HRB

PM

AHR1

AHR2

CAir

CFuel

FM1

FM2

ExFD%

ExD%

Figure 6 Exergy destruction distribution in the SOFC/GT

cycle with anode recirculation.

Figure 7 Grassmann diagram of SOFC/GT system

Figure 8 Grassmann diagram of SOFC/GT system with anode-recirculation


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Figures 3 and 4 show the exergy efficiency of each component

for both processes. The SOFC has an exergy efficency of

83.4% in the standard plant and of 81.55% in the alternative

one.

Figures 5 and 6 clearly show that in either case the SOFC is

the component affected by the highest degree of irreversibility.Within the SOFC, in fact, most of the chemical reactions

occur, like steam reforming and electrochemical oxidation of

hydrogen. It is well known that these processes are the most

important source of irreversibilities. The distribution of

irreversibilities between the SOFC and the CC can be

modified by changing the fuel utilization factor. When the

value of Uf is very high almost all chemical processes take

place in the SOFC increasing its contribution to the global

irreversibilities. The opposite occurs for lower values of Uf(Figure 9).

Figure 7 and Figure 8 show the Grassmann diagrams for both

plant configurations. The diagrams show very clearly theprogressive build-up of exergy destruction from the input to

the output of the plant and the different contributions of each

component to the total exergy destruction.

0

10

20

3040

50

60

70

80

90

100

0.55 0.65 0.75 0.85 0.95

Uf

ExD

kW

ExD SOFC

ExD CC

ExD SOFC (Rec)

ExD CC (Rec)

CONCLUSIONS

This paper presented an exergy analysis of a hybrid SOFC/GT

power plant. A standard plant configuration was compared to

an alternative one provided with anode-outlet gas re-

circulation. Although the fuel cell per se is a rather efficient

device, it is the most important source of exergy destruction

in the cycle. The other main source of irreversibility is -as

expected- the combustion chamber. The results show that

anode gas recirculation leads to an increase in both the energy

and exergy efficiency of the system, from 65.6 % to 66.4 %

(First Law) and 63.6% to 64.4% (Second Law) respectively.

In addition, the recirculating option reduces by 85% the

feedwater needed and by 88% the high-T heat required by the

reformer.

.

NOMENCLATURE

ex specific exergy [kJ/kg]

ExD destroyed exergy [kW]

ExEff exergetic efficiency

Fi input syngas stream

Fo output syngas stream

Gi input air stream

Go output air stream

h specific entalphy [kJ/kg]

ITOT SOFC electric current [A]

LHV lower heating value [kJ/kg]

m& mass flowrate [kg/s]

ni molar fraction of ith specie

Q heat flow [kW]

R universal gas constant [J/molK]

s specific entropy [kJ/kg/K]

Si input steam or water stream

So output steam or water stream

SC steam-to-carbon ratio

T temperature [K]

TIT turbine inlet temperature [K]

Ua SOFC oxygen utilization factor

Uf SOFC fuel utilization factor

V potential [V]

W Power [kW]

SOFC SOFC electrical efficiency

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Figure 9 Influence of the fuel utilization factor on the exergy

destruction in the CC and SOFC for the two configurations


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