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CHAPTER 4

MODELING OF MOLTEN CARBONATE FUEL CELL

(MCFC) – GAS TURBINE COMBINED CYCLE POWER

PLANT

4.1 Introduction

Molten Carbonate Fuel Cells (MCFCs) are mainly considered for

stationary power generation and a better understanding of the

transport processes in the electrodes and cells are needed to improve

viability. MCFCs were initially developed with the intention of

operating directly on coal. The primary fuel currently in use is either

more commonly natural gas or coal gases [17]. MCFCs are still under

development and have not reached market acceptance as a possible

source of energy. The concept of the Molten Carbonate Fuel Cell is

almost a century old with the first patent awarded in 1916 to W.D.

Treadwell. It was first developed in Europe in the 1940s as an attempt

to convert coal to electricity in carbonate media. An initial

demonstration of MCFC was successfully completed by Broers and

Ketelaar in the 1950s, with the first pressurized stack appearing in

the 1980s. Current development concentrates on base-load utility

applications as well as distributed electric-power generation with heat

co-generation. Due to long startup times and low power densities,

there is limited potential for mobile applications, although it is

suitable as a power-train for trains and large surface ships.

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4.2 Physical and Chemical Processes

Molten Carbonate Fuel Cells are strictly electro-chemical conversion

devices, not heat engines which are subject to a Carnot cycle efficiency

limit. In order to keep the electrolyte liquid, the stack or cell must be

maintained at a temperature around 650◦C, which is primarily

provided by the heat released through exothermic reactions. To

maintain these high temperatures consistently, the system must

include a precise temperature control for stable conditions of the fuel

cell operation. At this temperature, the cell is able to tolerate higher

levels of CO and CO2, eliminating any problems arising from catalyst

poisoning due to the presence of these molecules. The fuel cell also

produces high quality waste heat that can be fed to a gas turbine

system as a bottoming cycle leading to higher electric power

generation efficiency. The high temperature of the MCFC provides the

advantage of removing the need for precious metals such as Pt,

commonly used as a catalyst for low-temperature fuel cells, because of

enhanced chemical reaction kinetics at high temperatures. The

reaction inside low-temperature fuel cells takes place in a very thin

catalyst layer. Whereas the reaction inside the Molten Carbonate Fuel

Cell, takes place in a thicker electrode and near the three-phase

boundary between the solid, liquid and gas phases since the liquid

electrolyte penetrates the porous electrodes.

The carbonaceous fuels can be fed directly to the system due to

the high operating temperatures. Reforming is a process used to

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convert hydrocarbons to hydrogen. In the Molten Carbonate Fuel Cell,

there exists three types (two of these are shown in Figure 4.1. of

reforming processes; Indirect Internal Reforming, Direct Internal

Reforming and External Reforming. Indirect Internal Reforming

involves supplying the fuel to a chamber connected to

Figure 4.1: Schematic of a MCFC system depicting both internal reforming

methods (IIR and DIR) from the Fuel Cell Handbook [19].

the electrode assembly of the cell where the fuel is reformed and then

supplied to the electrode itself. Direct Internal Reforming involves

directly supplying the fuel to the cell electrode assembly where it is

reformed while it is flowing down the gas channel and entering the cell

electrodes. External Reforming involves a chamber not directly

connected to the system where the fuel is reformed and passed to the

fuel cell for the reaction to take place. The Molten Carbonate Fuel Cell

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temperature matches the one needed for the steam reformation[19] of

natural gas, which consists mainly of methane,

CH4 + H20C0+ 3H2,

which is the largest source of H2 globally. There exists a possibility for

Molten Carbonate Fuel Cell energy efficiency greater than 60% when

using internal reforming.

The basic operational principle is shown in the schematic of

Figure 4.2. The reforming process is eliminated for simplification. At

the anode, hydrogen is supplied into the electrode where it reacts with

the carbonate ions to produce the electrons that will move through the

external electric circuit. At the cathode, carbon dioxide and oxygen are

supplied where they will react with the electrons moving through the

electric circuit to produce the carbonate ions that complete the circuit

by migrating to the anode side. The exhaust from the anode reaction

contains carbon dioxide and water vapour.

Figure 4.2: Schematic of a Molten Carbonate Fuel Cell displaying the

basic operating principles and electro-chemical reactions at both

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electrodes[19].

The carbon dioxide can be recirculated to the cathode side to

complete the chemical reaction with the electrons and oxygen. Heat is

released at the anode and cathode electrodes of the cell, which can be

used for co-generation purposes.

The overall electro-chemical reaction occurring at the cathode

electrode is,

2

322 22

1COeCOO

and at the anode electrode,

2

2 3 2 2 2H CO H CO e

Therefore, the net cell reaction is,

2 2 2

1Heat + Electric Energy

2H O H O

The anode and cathode electrodes are three-phase media (electronic

conductor, electrolyte and gas pore). The porous structure of the cell

allows the gas and liquid electrolyte to move through the cell electrode

while the electrons migrate towards the solid surface, providing the

three-phase boundary where the chemical reaction takes place.

4.2.1 Cathode

Inside the cathode, carbon dioxide and oxygen flow in the same

direction across the cell electrode at different rates. The three-phase

boundary allows for chemical reactions to occur along the length of

the domain of the electrode. Across the flow channel, only carbon

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dioxide and oxygen are able to flow while the electrolyte is kept from

flowing into the channel and causing corrosion. The liquid electrolyte

distribution is only controlled by capillary pressure. Therefore, the

pore size must be maintained very carefully during manufacture. At

the electrode/electrolyte boundary, the electrolyte concentration is

much larger and fills the pores, which prevents to avoid any gas

leakage into the electrolyte assembly.

The inlet gas at the cathode is mainly composed of N2, 02 and

C02 but contains traces of other molecules since the main source of

oxygen comes from air. The effects of nitrogen are neglected in this

work since it does not flow within the cathode. Only a few metals (like

nickel oxide) are stable as the cathode material due to the extremely

corrosive nature of the molten carbonate electrolyte. Only

semiconducting oxides are feasible from a economical point of view [8].

The mean pore size of NiO electrodes is about 10µm. The smaller

pores are filled with the electrolyte to form the three-phase boundary

needed for the chemical reaction, while the larger pores remain open

for gas flow. Nickel oxide is also slightly soluble in the electrolyte

which limits the lifetime of the cell,

2

2 3NiO CO Ni CO

The optimal cathode performance of the cell depends upon the gas

composition where there exists a 2:1 ratio of CO2 to 02 consumed in

the overall electro-chemical reaction. In order to increase lifetime and

to reduce the NiO solubility, the carbon dioxide concentration should

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be reduced, although if it is too low, the dissociation of carbonate ions

becomes significant

2 2

3 2CO CO O

There by limiting the fuel cell lifetime due to electrolyte losses. The

balance between dissociation of C0- and NiO solubility can become

very difficult to control, and to predict cell lifetime is generally

challenging.

The optimal thickness of the cell electrode, which depends upon

the current density and gas composition as well as other operating

conditions, ranges from 0.4 − 0.8 mm [152].

4.2.2 Anode

The hydrogen oxidation process is not the only chemical reaction

occurring in the anode. The primary fuel is natural gas, which is made

up of 99% CH4 and other hydrocarbons, and the anode inlet gas

composition will also contain carbon monoxide, which is oxidized as

well by conversion to hydrogen. The oxidation of carbon monoxide,

which is also produced during the methanation reaction, occurs

mainly via the water-gas shift reaction,

2 2 2CO H O CO H O

since direct electro-chemical oxidation in the cell occurs much more

slowly. In the presence of catalysts such as nickel, the water-gas shift

occurs very rapidly at the high temperatures of a Molten Carbonate

Fuel Cell. In the presence of water vapour, CO is no longer a poison

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for this fuel cell.

The direct electro-chemical reaction of methane in MCFC also

appears to be negligible and, along with other hydrocarbons, must be

steam-reformed through the methanation reaction,

4 2 23CH H O CO H

which involves the process of external or internal reforming.

The Boudouard equilibrium reaction,

22CO CO C

produces carbon particles that can deposit in the gas-flow channels of

the cell. Therefore, supplying the anode channels with water vapour

will not only push the water-gas shift and methanation chemical

reactions forward, but also it prevents carbon deposition. Since the

hydrogen oxidation reaction also produces water vapour, these

reactions will occur at the anode at much faster rates and produce

more hydrogen. The gas composition is driven by CH4 and H2O partial

pressures and the current density which affects the hydrogen

concentration.

4.2.3 Electrolyte

The electrolyte is made from a mixture of lithium carbonate (Li2 CO3),

potassium carbonate (K2CO3), and smaller amounts of sodium

carbonate (Na2CO3). The optimization of the cell operating temperature

and composition is extremely important as they affect ohmic

resistance, cell polarization (gas solubility, and oxygen reduction

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kinetics) and nickel oxide solubility, limiting the cell performance and

cell lifetime. The liquid molten carbonate is very corrosive and

penetrates through the electrode pores.

The cell performance depends on the d activation polarization

and ohmic resistance of the cell at the electrodes. The Li-rich

electrolytes have higher ionic conductivi- ties and therefore decrease

ohmic losses. Although, the solubility and diffusivity of the reactant

gases are lower in Li2CO3 -rich melts. There exists better fuel cell

performance. In terms of NiO dissolution in Li-Na than Li-K melts but

the electrolyte matrix is the most stable in a Li2CO3 – K2CO3

electrolyte [152].

A porous electrolyte matrix is used to hold the liquid in place

through capillary effects, thus preventing the reactant gases from

crossover to opposite electrodes. The electrolyte matrix is ionically

conductive but electronically insulating to prevent the electrons from

crossover. It is mainly made up of ceramic powder, such as lithium

aluminate (LiAlO2).

The Molten Carbonate Fuel Cell has one of the thickest

electrolytes at 0.5−1.5 mm among all hydrogen-oxygen fuel cells.

However, the ohmic resistance remains acceptable even with a thick

matrix [153].

4.2.4 Cell Performance

The MCFC output voltage is equal to the potential difference between

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the cell electrodes. The actual cell potential of the cell, E is given by

the reversible potential, Er, and all over potentials (activation, ηa, and

concentration, ηc,) and ohmic losses, ∆Eohmic, (mainly in the

electrolyte) as

r a c ohmicE E E

The reversible potential, Er, represents the best possible performance

(open-circuit voltage).

As the current is supplied from the cell, the degree of

irreversibility increases. The processes that lead over potential,

physical or chemical, are referred to as polarization.

There are three types of polarization; activation, concentration

and ohmic. These are due to resistance to electro-chemical reactions,

ions in compounds, resistance to limitations in mass transfer and

transport of electrons [152].

The performance of the MCFC may be improved by reducing the

cathode

Figure 4.3: Depiction of the three-phase boundary in the cathode of a

MCFC[19].

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polarization in the fuel cell, while the anode polarization has a smaller

effect on the output voltage of the cell. The voltage versus current

density graph for a Molten Carbonate Fuel Cell has a linear profile,

which is usually attributed to the ohmic drop but this remains

unclear. The polarization curve of the MCFC is the steepest among the

hydrogen-oxygen fuel cells. The Molten Carbonate Fuel Cell has the

lowest current densities, up to 150mA/cm2, and high electric

efficiency of the cell [153].

The factors that influence the operating condition of Molten

Carbonate Fuel Cells are stack size, heat transfer rate, energy

conversion efficiency, operating voltage and load requirement. The

MCFC operates at a current density of 100−200 mA/cm2 with a cell

potential of 0.75−0.90V at atmospheric pressure. The performance is

improved at higher pressure but doing this causes the build-up of

carbon through the Boudouard reaction in the anode [152].

4.3 Thermodynamic Analysis of Molten Carbonate Fuel Cell–Gas

Turbine Combined Cycle Power Plant or different Fuels

The main characteristic of the Molten Carbon Fuel Cell(MCFC), is the

necessity to supply it with combustion air with a specific quantity of

carbon dioxide. The high working temperature (650°C) allows the

nickel to be used as a very cheap catalyst and as an electrode. In this

type of Fuel cell, the electrolyte is made up of a fusion of alkaline

carbonates (Li2CO3, K2CO3). The MCFCs are widely being considered

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for stationery power generation systems and because of their high

operating temperature, they are used in industrial plants where a high

temperature is required. Classic MCFC plants can produce a power of

about 300KW, but it is possible to obtain a greater value of about

10MW. Lukas et al [154] presented that a 2-MW MCFC technology has

been demonstrated is Santa Clara Demonstration Project in

California.

Silverial et al [155] proposed that high temp Fuel Cells like

SOFC, MCFC are mainly suited for combined cycle power plants

because they convert energy directly into electrical energy by an

electrochemical reaction while simultaneously producing heat.

Massardo et al [156] have presented that fuel cells can produce 55-

90% of the electricity of the system when they are integrated with the

gas turbines, while turbines produce the remaining power. Fuel cell

systems are considered to be more efficient and environmentally

friendly power generation systems. High temperature Fuel Cells are

capable of using hydrocarbon fuels as long as the system contains a

reformer which can process the fuel to produce a hydrogen / carbon –

monoxide rich mixture. This facility allows fuel cells to use renewable

(biomass) as well as fossil fuels.

The relatively high working temp of MCFC (500-650°C) results in

several benefits no expensive electro-catalysts, are needed as the

nickel electrodes provide sufficient activity, and both CO and certain

hydro carbon fuels for the MCFC. This fuel flexibility is one of the

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features of MCFCs. So MCFC is expected for wide range of

applications i.e., in alternative large – scale combined cycle power

generation and small-scale distributed power generation plants. In

addition high temp waste heat allows the use of a bottoming cycle to

further improve the system efficiency.

Castell et al [157] stated that theoretically, all types of FCs

could be fueled by biogas extracting the hydrogen from it and

removing all poisoning compounds. However, only SOFC, MCFC and

phosphoric acid Fuel cells (PAFCs) can be considered because their

fuel requirements are not as strict as with the Proton exchange

membrane fuel cells (PEMFCs) and the alkaline Fuel Cells (AFCs).

Dennis et al [158], Litzinger et al [159], Torrazza [160] have proposed

that the Fuel Cells may work under pressurized conditions (receiving

high temp and high pressure air from the compressor). Also Ghezel et

al [161]have shown the operation of fuel cell systems at atmospheric

pressure, exchanging heat with the GT cycle through a heat

exchanger. Companari et al [162] have stated that the expected

performance of MCFC/GT hybrid system is around 60-65% net

electric efficiency range, for a MW-scale power plant, while in

perspective up to 70% efficiency is achievable in larger gas-turbine

cycles where efficient components are employed. Such efficiency level

is beyond the maximum presently achieved by the latest, large scale

natural gas fired combined cycles, this is explained by Chiesa et al

[163], justifying the rigorous research and development efforts in the

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industrialization of the MCFC / GT hybrid technology. Because of

their high capital costs due to developing technology, they have not

been widely commercialized. Massardo et al [43] and Kurz et al [164]

have done the enough research on the systems of integrating fuel cells

with other technologies and they can already compete with the costs

of conventional power generation systems.

Robort, et al [165] have proposed that in the FC-GT combined

cycles, it is very essential to suitably match and integrate to FC with

the GT portions of the cycle. Very little mass flow in the cathode could

lead to overheating of the Fuel Cell. At the same time, too much mass

flow could lower the turbine inlet temp (TIT) of the gas turbine leading

to decrease in efficiency, performance degradation and even gas

turbine shutdown. A complete shutdown of the turbine can be

damaging to the Fuel cell and other system components. In the case of

MCFC, the operating temp is generally desired to be around 650°C.

This operating temp is good for reformation and processing of the

original hydrocarbon fuel, but at the same time, this temp is too low

for a typical gas turbine TIT. Therefore MCFC/GT hybrid systems

generally consists of a Fuel cell operating with sufficient excess fuel

followed by oxidation of the excess fuel emitted from the anode so as

to raise the TIT. In the past decade, many articles (Lunghi, et al [44],

Zhang, et al [166], Bargigli, et al [167]) have presented the benefits of

the integration of MCFC with the Gas turbine cycle. Srinivas, et al

[168] have explained the major advantages of using natural gas in a

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combined cycle plant for power generation are the high thermal

efficiency and low emissions. Reddy, et al [169] have stated that

carbon dioxide emission can be reduced from combined cycle with

partial oxidation of natural gas.

Viswanathan et al [150] have explained the principles and

applications of fuel cells. One of the promising aspects of molten

carbonates is the possibility of using, apart from hydrogen, low-cost

fuels, such as methanol, gaseous carbon or CO + H2 (Syngas),

resulting from methane conversion by thermal cracking or reforming

reaction. The oxidant is constituted by a mixture of air and CO2 in the

proportion of 70% and 30% respectively. The different technological

solutions are possible in order to use methane as fuel: external

reforming as well as direct and indirect internal reforming. In the

external reformers operating at about 1073K, used in the conventional

MCFC, the heat required proceeds from the anode exhaust gas via

heat exchangers. Internal reformers operate at low temperature (923K)

and therefore required the use of well adapted catalysts. The heat

required for the reaction, which is endothermic, can be supplied by

the exothermic fuel cell reaction. This system allows the use of other

fuels such as methanol and higher hydrocarbons. In the case of the

indirect process, the reformer is separated, but it is adjacent to the

anodic compartment of the electrochemical cell. The most feasible

solution appears to be the direct internal reforming. The yield of

methane conversion on nickel supported on lithium aluminates is

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relatively high at 923K, also, the vapour forming reaction is suited in

MCFC conditions because the yield increases with the consumption of

hydrogen. Another advantage is that part of the water used in the

vapour forming of methane can be supplied by the anode reaction.

4.4 Thermodynamic System Configuration of GT-MCFC

Combined Cycle

Figure:4.4 Layout of the Gas turbine – MCFC Combined cycle

Power generation system and T-s diagram.

135

Figure 4.4 shows the configuration of a gas turbine-MCFC

combined cycle. The system is fed with the fuel to MCFC and

combustion chamber. The air is pressurized in the compressor and

preheated in the recuperator, is supplied into the cathode of the fuel

cell. The outlet air from cathode is used to burn the residual

hydrogen, carbon oxide and fuel in the anode outlet gas. The products

of chemical reaction are very lean, hence additional amount of fuel is

injected into the combustion chamber in order to stabilize the

combustion. The extra fuel is supplied not for increasing the turbine

inlet temperature. The flue gas from combustion chamber is expanded

in the turbine and preheats the compressor outlet air in the

recuperator. The T-s diagram of the corresponding system is shown in

the Figure 4.4.

For the analysis of the plant a computer program has been

developed which consists of several control loops to calculate fluid

thermodynamic properties and exergy values at various states. The

effects of various parameters, such as compressor pressure ratio,

turbine inlet temperature, air fuel ratio and ambient temperature are

studied on the plant performance.

4.5 Model Assumptions

The assumptions which are made in the analysis GT-SOFC Combined

Cycle Power Plant, are also being used in the analysis of GT-MCFC

Combined Cycle Power Plant

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4.6 System Modeling

Plant layout is shown in Fig.4.4. The thermodynamic performances of

all the components of the system are analyzed in this section.

4.6.1 MCFC

In the operation of an MCFC, electrons are transferred from the

anode, through external circuit to a cathode, where they participate in

reduction reactions. Negative charges are conducted by carbonate

anions (CO32-) from the cathode, through the molten electrolyte, to an

anode. At the anode electrons are produced by oxidation.

The electrochemical reactions occurring in MCFC [Williams., et al [19]]

are as following :

At the cathode

2

322 22

1COeCOO (4.1)

At the anode :

1. If hydrogen is the fuel, the products will be water and carbon dioxide.

2

2 3 2 2 2H CO H CO e (4.2)

2. If methane is the fuel, it is first transformed to syngas by steam,

forming

4 2 23CH H O CO H (4.3)

3. If Ethanol is the fuel [171], 6 moles of H2 for 1 mol of ethanol are

produced

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2 5 2 22 6C H OH H O CO H (4.4)

Steam reforming reaction :

2 5 2 22 4C H OH H O CO H (4.5)

The oxidation reaction becomes :

2

2 3 2 22 3 2H CO CO H O e (4.6)

2

3 22 2CO CO CO e (minor) (4.7)

Table 4.1 Standard cycle analysis conditions and input parameters

for the simulation GT-MCFC combined cycle power

generation system.

Ambient conditions Cell voltage=0.752V

Cell area : 834 cm2

DC power output from fuel cell stack=2000kW

Temperature: 298K DC – AC Inverter

Efficiency: : 0.89

Pressure : 101.325KPa Air utilization factor : 25%

Pressure losses [65] Gas Turbine Cycle

Recuperator air side: 4% Turbine efficiency : 0.84

Recuprator gas side: 4% Compressor efficiency : 0.81

Fuel cell stack : 4% Recuperator efficiency : 0.8

Combustor : 5% AC Generator efficiency : 0.95

MCFC Combustor efficiency : 0.98

Fuel utilization factor :85% Steam to carbon ratio: (SCR) : 2

MCFC stack temp : 923K Turbine inlet temp: (TIT) : >10000c

Current density : 0.3A/cm2

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The overall cell reaction is as follows :

2 2 2 2 2

1( ) ( )

2H O CO cathode H O CO anode (4.8)

Shift reaction : 2 2 2CO H O H CO (4.9)

Even though carbonate icons participate in the reactions, the melt has

an invariant composition otherwise, CO2 formed at the anode is

recycled and consumed at the cathode. Besides the reaction involving

H2 and O2 to produce H2O, the equation (8) shows a transfer of CO2

from the cathode gas stream to the anode gas stream. Two faradays of

charge (electricity) or 2 gram moles of electrons are generated by the

use of 1 mole of CO2 at the cathode and the production of 1 mole of

CO2 at the anode. Hence it is desirable to recycle CO2 in order to

maintain the invariant composition of the electrolyte.

The reversible potential for an MCFC, taking into account the transfer

of CO2 is given by the equation [19]

1

20 22 2

2 2

( )( ) ( )ln ln

2 ( ) 2 ( )

c

a

p COp H p ORT RTE E

F p H O F p CO (4.10)

Where E° is the standard value of the cell reaction (when all activities

are 1). When the partial pressure of CO2 are identical at the anode

and cathode, and the electrolyte is invariant, the cell potential

depends on only on the partial pressures of H2, O2 and H2O. Typically,

the CO2 partial pressures are different in the two electrode

compartments and the cell potential is affected accordingly. It is usual

practice in an MCFC system that the CO2 generated at the anode be

transferred to the cathode where it is consumed. This will required

139

some facility that will either (a) transfer the CO2 from the anode exit

gas to the cathode inlet gas (CO2 transfer device), (b) produce CO2 by

combustion of the anode exhaust gas, which is mined direct with the

cathode inlet gas, or (c) supply CO2 from an alternate source.

The DC power produced by the MCFC is given [8] by

cc AjVDCPele , (4.11)

Where, lossc VEV (4.12)

ionconcentratohmicactivationloss VVVV (4.13)

The actual cell voltage ‘Vc’ depends upon the operating parameters like

the current density (j), operating pressure and temperature etc. FC

hand book includes empirical formulae that correlate the performance

of an MCFC to these parameters.

The effect of pressure is given by [19]

1

2ln20)(P

PmvVp (4.14)

The effect of temp is given by

1

24.1)(T

TmvVT (4.15)

The system components i.e. Combution chamber, Compressor,

Recuperator and Gas Turbine are analysed using the equations which

are used in the Chapter 3.