This document discusses optimizing the operation of binary cogenerative thermal power plants with solid oxide fuel cells (SOFC) that run on natural gas. It introduces SOFC technology and describes a model power plant that uses tubular SOFC modules from Siemens-Westinghouse to generate electricity. The power plant also includes a gas turbine and steam turbine to maximize energy efficiency. The document establishes a complex optimization function involving numerous variables that impact the plant's overall exergy efficiency. These variables relate to parameters of the SOFC, gas turbine, steam turbine and plant operation. The goal of optimization is to determine the optimum values for these variables to achieve maximum exergy efficiency from the integrated power plant system.

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 5, Issue 1, January (2014), © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 1, January (2014), pp. 122-131
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IJMET
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OPTIMISATION OF BINARY COGENERATIVE THERMAL POWER
PLANTS WITH SOLID OXIDE FUEL CELLS ON NATURAL GAS
Done J.Tashevski1,
Risto V. Filkoski2,
Igor K. Shesho3
Faculty of Mechanical Engineering, University “Ss Cyril and Methodius”,
Karpos II bb, 1000 Skopje, Republic of Macedonia
ABSTRACT
The present paper deals with the issue of large capacity binary co-generative thermal power
plants with high temperature solid oxide fuel cells (SOFC) on natural gas.A complex function with a
number of variables is employed in order to optimize the operation cycle of this type of power plant
from the energy and environmental standpoint. Characteristic parameters of such plant are calculated
on a basis of the optimization results: basic dimensions; optimal number of modules; electrical power
and unit efficiency of the fuel cell; electrical power and unit efficiency of the gas turbine facility; as
well as electrical and thermal power of the steam turbine facility. A software package is created on a
basis of the developed optimization procedure, consisting of a number of subprograms and functions,
which is flexible for further development. For verification of the optimization procedure, the
obtained calculation results are compared with operation parameters of a power plant of well-known
manufacturer. Also, a comparison is performed between a binary cogeneration TPP with SOFC and a
cogeneration plant without fuel cell, in terms of energy efficiency and environmental benefits.
Key words: Binary Cogeneration Power Plant, Natural Gas, Solid Oxide Fuel Cell.
1. INTRODUCTION
The power sector trends in the world are generally directed towards the development of
thermal power plants (TPP) that should meet several main goals: high performance, reliability and
flexible operation, reduced fuel consumption, reduced emission of harmful substances and
profitability. Increasing energy demand causes growing permanent consumption of fossil fuels and
gradual and continuous eradication of stocks of fuels. Therefore, it is necessary to find new sources
of energy that can adequately replace the existing fuels, as well as new technical solutions for power
generation that can operate on that kind of fuels. One of the prospective alternative fuels for this
century is hydrogen.
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Fuel cells are electrochemical devices that convert directly the energy contained in the fuel
matter into electricity with high efficiency and low environmental impact. In order to generate
significant power, many single cells should be brought together by connecting them in series. There
are various types of fuel cells, which differ from each other according to the type of electrolyte and
fuel used. Solid oxide fuel cells (SOFC) are known as high temperature energy conversion devices,
considerably higher than the other fuel cell types, making them suitable for application in stationary
power and heat generation systems [1].
A detailed explanation of the function principles, materials used, performance efficiency and
chemical reactions mechanism, including a unit cell chemistry and fuel conversion modeling
approaches, is given in [2]. The development of the high-temperature SOFC, capable of operating at
1000 °C, has opened up intriguing possibilities for integrating it within the integrated gasification
combined cycle [3]. Cycle performance calculations have already been undertaken [4,5].
The main problem with stationary fuel cells is that they are either not reliable enough or too
expensive at the present time [6]. The SOFC appears ideal for the application in binary cogenerative
power plant, because it can easily use fossil fuels (solid, liquid and gaseous), but also derived fuels
rich with hydrogen, as well as pure hydrogen. High-temperature fuel cells are permanently
developed and hybrid cycles with SOFC can offer overall system efficiencies of over 70% [2].
2. HIGH-TEMPERATURE SOFC IMPLEMENTED IN A THERMAL POWER PLANT
In the model binary cogenerative power plant with fuel cells (BCFC) high-temperature solid
oxide fuel cell (SOFC) is applied, which operates at a temperature of 1000 °C and pressure up to 15
bar. This type of fuel cells is a product of Siemens-Westinghouse, a company which is specialized in
manufacturing of tubular (cylindrical) fuel cells. Their model of individual fuel cells consists of
standard components and with the following characteristics [6-9]: Diameter 0.4 m; Height 2 m;
Power of individual cell unit 315 W; Power of a module 1.7 MW; Number of individual cells in one
module 5600. For the modeling and calculation purpose, the modular solid oxide fuel cell is selected,
based on an internal reforming of natural gas (up to 99 % methane) into hydrogen, which is a
primary fuel for combustion in the fuel cells.
3. BINARY COGENERATION POWER PLANTS WITH SOFC
Binary cogeneration thermal power plants with high-temperature SOFC (BCFC) are
combined plants that comprise three thermal power units: high-temperature SOFC, gas turbine unit
and cogeneration steam turbine power plant. In these plants electricity is generated in triple way: at
the fuel cell inverter, as well as at the generators of the gas turbine and steam turbine plants. At the
same time, thermal energy is obtained from the cogeneration steam power plant. The cogeneration
steam power plant may be designed in different ways: as a condensing steam turbine unit with steam
extraction for technological needs, as a unit with back-pressure steam turbine or as a unit with
condensing and counter-pressure turbines. For the purpose of the analysis in the present work a
BCFC unit is selected, with fuel cell power of 100 MW. The technology scheme of the natural gas
BCFC power plant is presented in Fig. 2.
4. OPTIMISATION OF BCFC
Optimisation of BCFC with SOFC is a very complex task due to the number of systems,
components and functions needed for plant operation. These plants should fulfill high demanding
requirements, such as economical operation, efficiency, reliability, durability, availability and
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flexibility in operation. Therefore, the optimization of these plants is necessarily related with many
compromises.
Figure 2. Schematic presentation of cogenerative power plant with SOFC on NG
The analysis of BCFC clearly shows that operation of these plants depends on many factors.
The fuel cell produces most of the electricity and is also the main component, which decisively
affects the parameters that are going to be achieved in the gas-turbine and steam-turbine power
plants. This means that proper optimization of the parameters in the fuel cell has a major influence to
the optimization process of the entire plant. It must be noted that the significance of the parameters
important for the gas-turbine and steam-turbine plants must not be neglected during optimization.
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The (important) parameters of plants with fuel cells vary depending on a number of
influencing factors: the voltage of individual fuel cell, fuel composition, fuel utilization, oxidant
composition, utilization of oxidant, the pressure in the fuel cell (compressor pressure ratio), the
temperature of the fuel at the inlet into the fuel cell, temperature of oxidant at the fuel cell inlet and
others. Therefore, a proper balancing of these variables is essential in order to achieve optimum
operation of the plant.
Apart of the mentioned variables related directly to the fuel cell, the variables that define the
gas-turbine and steam-turbine plants significantly affect the operation of BCFC power plant. Some of
them are: temperature in the gas turbine inlet, pressure and temperature of the superheated steam at
the exit of the heat recovery steam generator (and at the steam turbine inlet), pressure and quantity
(flow) of steam extraction for technological needs, pressure in the deaerator, condenser pressure, etc.
The number of variables that affect the performance of BCFC defines also the number of
variables in a function that is necessary to be optimized. A criteria of optimality is maximum total
exergy efficiency of the BCFC, which indicates the overall efficiency of the BCFC.
In many cases of practical variation calculations it is necessary to optimize a function that
depends on many variables. The function in this case has the following general form [14-16]:
η = η (VFC ,U g ,U o , Tgv , Tov , Π K , ( pI )∗ )
(2)
where: VFC V - voltage of individual fuel cell, Ug % – fuel utilization, Uo % – oxidant
utilization, Tgv K – fuel temperature at the fuel cell inlet, Tov K – temperature of oxidant at the fuel
cell inlet, ΠC – compressor pressure ratio (pressure in the fuel cell), pI MPa – steam pressure at the
outlet of heat recovery steam generator, ηex,BCFC – total exergy efficiency.
The range of expected actual variation of the function η is (0,3-0,999) and, in any case its
vale is less than 1, while the elementary range of the variation of variables is presented in Table 1. In
order to eliminate the optimization errors, the most practical and also the most accurate approach is
to apply the method of replacing each value for a particular variable and determining the value of the
criterion for optimization for that value. It is possible only at optimization of plants for which there is
not a program, created in advance, for calculation of certain parameters of the plant. Changing the
values of the variables is possible by modifying the program for calculation of the plant.
In case of BCFC change the values of important variables that affect the performance of the
plant are changed, i.e. the total exergy efficiency of BCFC. This means that for each value of the
variable the total exergy efficiency is calculated, and after the operation (multiple variation and
iteration) a variant of the values is selected at which the highest total exergy efficiency is achieved.
The input data for the optimization model given in Table 1, are subsequently changed with certain
previously defined step, or appropriate (maximum, medium or standard) values given, at which it is
known by previous analysis that the plant achieves the optimum efficiency [14, 15].
Optimization is performed for the cases of BCFC without additional combustion (excluding
B1/B2) and with additional combustion (with B1/B2). After completion of power plants optimization
the maximum value of total exergy efficiency is obtained, as well as corresponding optimum values
of the variables (Table 2). It must be noted that in the case of the optimization procedure of the
BCFC without burners B1 and B2, additional variable for the steam pressure at the exit of the heat
recovery steam generator is included. The plant achieves a relatively low temperature of the flue
gases exiting the air recuperator, which causes decrease of the temperature of the steam exiting the
steam generator. Therefore, the pressure of the steam exiting the steam generator needs to be reduced
in order to fulfill the necessary parameters of the steam turbine.
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Table 1. Input data for optimization model – a case of BCFC on natural gas [14,15]:
Input data for modular fuel cell
Power of modular SOFC
PAC
MW
100 (selection)
Voltage of individual fuel cell
VFC
V
0,6 - 0,7 (step 0,1)
Fuel utilization
Ug
%
93 - 98 (step 1)
Oxidant utilization
Uo
%
75 - 85 (step 1)
Fuel temperature at the inlet of SOFC
Tgv
K
773 - 823 (step 10)
Oxidant temperature at the inlet of SOFC
Tov
K
973 - 1023 (step 10)
Fuel composition (natural gas)
Standard analysis
Oxidant composition (air)
79 % N2, 21 % O2
Input data for gas turbine plant
1 - 15 (step 1)
Compressor pressure ratio
ΠC
Air temperature at the compressor inlet
t1
°C
15 (average)
Temperature at the gas turbine inlet
TiGT
K
1770 (max)*
Input data for steam turbine plant
Steam pressure at the outlet of the heat recovery steam
pI
MPa
14 (standard - high)
generator (HRSG)
1 – 10 (step 1)**
Steam temperature at the outlet of the HRSG
tI
°C
560 (max-standard)
Pressure of steam extraction for technology consumers
pHC
MPa
0,5 (selection)
Deaerator pressure
pD
MPa
0.7 (standard-selec.)
Condenser pressure
pCO
MPa
0.006 (standard)
Specific quantity of steam extraction for HC
αOT
kg/kg
0.3 (selection)
*
Gas turbine technology, gas turbine G series, Mitsubishi heavy industries[17]
**
The variable is a subject of optimization only in BCFC without additional burners
Table 2. Optimization results of BCFC
Variable
Voltage of the individual fuel cell
Utilization of gaseous fuel
Utilization of oxidant
Fuel temperature at the inlet of FC
Oxidant temperature at the inlet of FC
Compressor pressure ratio
Steam pressure at the outlet of the HRSG
Maximum value of the criteria
Total exergy efficiency
pI
V
%
%
K
K
MPa
Without B1/B2
0.7
93
85
823
973
3
5
With B1/B2
0.7
93
85
823
973
9
-
ηex,BCFC
-
0.759
0.738
VFC
Ug
Uo
Tgv
Tov
ΠC
5. CALCULATION OF BCFC
For determining the electrical and total efficiencies of the BCFC power plant the exergy
method is applied in the present case, which gives a realistic insight into the operational efficiency.
By applying other (classical) method of determining the efficiency in which electricity and thermal
energy are summarized, unrealistically high efficiency values are obtained [14].
After optimization performed, and on the basis of the determined optimal values of the
variables, the results are estimated for all parameters characteristic for this type of plant, Table 3.
From Table 3 it can be seen that the facility achieves high electrical efficiency and high exergy
efficiency, which is mostly due to the high efficiency of the fuel cell itself. The total electric power
of the plant is mainly generated in the fuel cell and a smaller part in the gas-turbine and steamturbine plant.
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Table 3. Output data (results) for BC with modular high-temperature SOFC:
Output data (results)
Without B1/B2
With condensing steam turbine (ST) with regulated steam extraction
Fuel consumption in FC
B
kg/s
Fuel consumption in B1
B1
kg/s
Fuel consumption in B2
B2
kg/s
Air flow in FC
mv
kg/s
Gas flow at the FC outlet
mg
kg/s
Gas flow at the GT inlet
mg1
kg/s
Gas flow at the inlet of the HRSG
mg2
kg/s
With B1/B2
3.29
62.03
65.32
-
3.29
0.978
0.012
62.03
65.322
66.301
66.313
Temperature at the outlet of the FC
tgKD
°C
1041.123
949,698
Power of the FC (E)*
Efficiency of the FC (E)
Number of individual FC
PAC
ηEFC
nfc
kW
cells
100,000.00
0.635
327,278.670
100,000.00
0.635
327,278.670
Number of modules
Area of the modular FC
Length/Width
nmfc
Afcm
afcm /bfcm
mod
m2
m
59
143.769
9.790/14.685
59
143.769
9.790/14.685
Power of the GTP (Е)
Efficiency (Е) of GTP (+FC)
Steam flow trough STP
Steam flow for HC
Power (Е) of STP
Efficiency (Е) of STP
Thermal power of STP
Power (Е) of BCFC
PEGTP
ηEGTP
mo
mOT
PESTP
ηESTP
QTSTP
PE,BCFC
kW
kg/s
kg/s
kW
kW
kW
12,064.258
0.711
7.34
2.20
5,783.64
0.273
5,095.15
117,847.898
34,344.203
0.658
11.24
3.37
14,417.57
0.366
8,299.88
148,337.047
Efficiency (Е) of BCFC
Total exergy efficiency
With back-pressure steam turbine
Power (Е) of BCFC with FC
Efficiency (Е) of BCFC with FC
Total exergy efficiency
*(E) electrical
ηE,BCFC
ηex,BCFC
-
0.748
0.759
0.724
0.738
PE,BCFC
ηE,BCFC
ηex,BCFC
kW
-
115,069.375
0.731
0.769
144,312.797
0.704
0.755
When applying a condensing steam turbine the electrical efficiency of the plant grows, but
thermal utilization of available energy. On the other hand, applying counter-pressure steam turbine
reduces electrical efficiency, but increments the total exergy. That means, certain reduction of the
plant’s electrical power and increasing of the thermal power. Optimization procedure can be repeated
for any power level, by defining the appropriate power of a modular fuel cell with suitable
composition of the fuel and oxidant.
The mathematical model with appropriate software package is composed of numerous
complex calculation procedures, but there are also standard calculations for determination of
properties of gas, steam, water etc. Selected and most important section of the results is presented in
this paper. The model has ability to calculate other combinations of combined power plant and other
variations of the BCFC.
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5. VERIFICATION OF THE MATHEMATICAL MODEL RESULTS
Assessment of the validity of the results obtained with the mathematical model is done using
the data for a plant on gaseous fuel from literature sources [9, 10].
For comparison of the results obtained with the BCFC model for gaseous fuels, utilized data
are relating to an existing plant with a power of 4 MW that runs on natural gas and is the product of
one of the leading companies in this field Siemens Westinghouse, with assistance from the U.S.
Company Heron [10]. The plant is a combination of a modular SOFC with high-temperature fuel cell
and a gas-turbine plant. The results from the calculation are presented in Table 4. Comparison shows
that the obtained model calculations are very close to the data of the selected plant. It must be noted
that the plant under consideration is a combination of modular fuel cell and gas-turbine plant.
However it does not diminish the reliability of the obtained results.
Table 4. Comparison of the results of the optimization model application on BCFC on natural gas
[10, 14]:
Input data
Power of SOFC
PAC
Voltage of individual fuel cells
Vfc
Fuel utilization
Ug
Oxidant (air) utilization
Uo
Temperature of fuel at the SOFC inlet
Tgv
Temperature of oxidant at the SOFC inlet
Tov
Fuel composition
CH4
PCH4v
C2H6
PC2H6v
C3H8
PC3H8v
CO2
PCO2v
N2
PN2v
Oxidant composition (air) O2
PO2ov
N2
PN2ov
Temperature of oxidant (air) at the compressor inlet
t1
Compressor pressure ratio
ΠC
Model results
Comparison power plant data
Fuel consumption
B
Oxidant (air) mass flow rate
mv
Mole flow rate of gases at SOFC outlet
nCO2
nH2O
nO2
nN2
Total gas flow
ng
Mass flow rate of gases at SOFC outlet
mg
Gases composition at SOFC outlet CO2
PCO2KD
H2O
PH2OKD
O2
PO2KD
N2
PN2KD
Power (Е) of the gas-turbine plant
PEGTP
Efficiency (Е) of the gas-turbine plant
ηEGTP
Power (Е) of the GT with SOFC
PEBCPFC
Efficiency (Е) of GT with SOFC
ηEBCFC
Specific consumption of heat (Е)
qEBCFC
128
MW
V
%
%
K
K
%
%
%
%
%
%
%
°C
-
3.5
0.7
94
81.5
767
973
93.9
3.2
1.1
1.0
0.8
21
79
25
3.5
kg/s
kg/s
kmol/h
kmol/h
kmol/h
kmol/h
kmol/h
kg/s
%
%
%
%
kW
kW
kg/kWh
0.13
2.25
265
2.37
8.5
16.5
2.5
72.5
430
0.40
3,930
0.76
4,700
Results
0.129
2.251
22.429
44.384
6.807
192.768
266.39
2.370
8.419
16.661
2.555
72.365
428.84
0.388
3,928.83
0.766
4,696.82

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6. COMPARISON BETWEEN BCFC ON
COGENERATION TPP WITHOUT FUEL CELL
NATURAL
GAS
AND
BINARY
In order to demonstrate the capabilities and good perspective of the BCFC, a comparison is
made between a BCFC (with burners for additional combustion) and binary cogeneration power
plant (BCPP) without fuel cells (classical cogeneration plant), with equal electrical and thermal
power. From the obtained results, shown in Table 5, it can seen that BCFC achieve much higher total
exergy efficiency compared to BCPP without fuel cells [14].
One of the major advantages of BCFC power plants (PP) over the other TPPs are
environmental benefits. In order to assess the environmental benefits, two plants with the same
electrical and thermal power are compared. BCFC plant with electric power of 500 MW and thermal
power of 105 MW is smaller consumer of fuel compared with the BCPP with the same electrical and
thermal power, which is directly reflected in reduction of emissions of harmful substances.
Comparison of specific emissions of CO2 and NOx from BCFC power plant and BCPP is presented
in Table 6, showing that the specific emissions from BCFC are about 20% smaller compared to the
ones from BCPP.
Table 5. Comparison between BCFC and BCPP:
Parameter
Unit
BCFC with B1/B2
BCPP (without
FC)
Bvk
Fuel consumption
kg/s
14.94
19.1
PE
Electrical power
MW
500
500
Thermal power
QT
MW
105
105
ηE
Electrical efficiency
0.68
0.54
ηvk
Total exergy efficiency
0.73
0.58
Table 6. Specific emissions of CO2 and NOx from BCFC PP compared to BCPP:
BCFC with B1/B2
BCPP (without
FC)
eCO2 g/kWh
Specific emission of
294.58
375.00
CO2
eNOx g/kWh
Specific emission of
0.32
0.40
NOx
7. CONCLUSION
From the present work a conclusion can be drawn that the combined thermal power plants
with BCFC achieve high efficiency, i.e. one of the highest efficiencies at the actual combined
thermal power plants. Depending on the choice of cogeneration steam-turbine thermal power plant,
efficiency varies within the limits of 70-75%. Important advantage of these facilities is the possibility
for combined generation of electricity and thermal energy.
Besides the abovementioned, these plants have other advantages:
- They can use any type of fuel, including hydrogen
- They can be built as large power objects and used as the basic source of electricity and
thermal energy
- They can be used as a completely independent source of electricity and thermal energy
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- Fuel cells operate at constant parameters and permanently supply the BCP with constant
amount of thermal energy
- There is a possibility for coupled regulation of the parameters before the gas-turbine and
steam-turbine plants, as well as for an independent operation of one of these plants in the
eventual failure of the other one
- According to the current status, the fuel cells are characterized with a relatively long
operational life, up to 70 000 hours
- The harmful emission of gases is low, that means they are environmentally friendly
- Fuel cell works completely quiet, because there are no rotation parts.
The presented model allows fast and relatively simple optimization and calculation of power
plants with FC. The accompanying program is simple to use, but the user must have prior knowledge
in its use. In the following period, the effort will be put on the model improvement from the
numerical and programming side and not so much by thermal aspect.
When it comes to fuel cells, the most immediate drawback is their high cost, but if they reach
the expectations that in this decade the price of fuel cells power plants will equal the price of other
TPP, these plants will be very competitive on the energy market.
The great interest of many companies taking advantage of fuel cells makes manufacturers of
fuel cells to work on increasing their service life, i.e. reducing the spending of their components.
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