my paper presented in ERA 2012 at velammal Engg college chennai
STUDY
ON INTERFERENCE
OF SOLID
OXIDE FUEL
CELL (SOFC) IN
GAS TURBINE
POWER PLANT
Immanuvel.M
ANNA UNIVERSITY OF
TECHNOLOGY,
DEPARTMENT OF MECHANICAL ENGINEERING,
MAIN CAMPUS – TIRUCHIRAPALLI.
mosesimmanuvel@gmail.com
ABSTRACT
Based on load requirement we have to modify the
requirement of the gas turbine power plant. In order to increase pressure
developed in combustion chamber we have to replace the combustion chamber into
solid oxide fuel cell in a gas turbine power plant. Air and Fuel are added to
the combustion chamber and Solid oxide fuel cells follow the electrochemical
conversion reaction in a combustion chamber .In this reaction temperature will
increase and it will reduce the ohmic resistance and also increase the ionic
conductivity. This Fuel cell consist of
a anode like Cermat made up of nickel mixed with ceramic material and YSZ
skeleton . Yttria Sstabilized Zirconia(YSZ) is used as a electrolyte in this
SOFC. Lanthanum Strontium Manganite(LSM) act as a cathode. The hot gases from
fuel cells are at high pressure (around 0.6Mpa) Reason for this reducing the
polarization resistance and increase the operating voltage of a fuel cell. Also
we decrease the resistance by reducing the electrolyte thickness. This increase
in open circuit voltage can be obtain by utilization of exhaust gas from the
turbine. The turbine rotates the generator it is used to produce high voltage .It
can also rotates compressor to produce better suction in an initial condition. The
efficiency of the power plant is raised to approximately 60%. Solid oxide fuel
cell has high efficiency ,long term stability, flexibility of fuel, Low
emission.
KEYWORDS:
Fuel cell, Turbine,
Compressor, Cermart, YSZ, LSM
INTRODUCTION:
In this modern society people want to live
rich life but they don’t worry about the environmental pollutions. Sir William Grove (1811-96), a British lawyer and amateur
scientist developed the first fuel cell in 1839. The principle was discovered
by accident during an electrolysis experiment. Solid oxide fuel cells offers clean and pollution free method to electrochemically generate electricity at high efficiencies .
These fuel cells provide many advantages over traditional energy conversion
systems including high efficiency, reliability, modularity, fuel flexibility
and low emission of Nox and Sox. Normally SOFC is operating between 900 to
1000°C.This efficiency also depends upon the source and sink temperature of the
system.
SOFC configuration provides a simple
manufacturing process and high current densities, but requires hermetic sealing
to prevent fuel-oxidant mixing and to electrically-insulate the stack In addition to the capability of reforming of
hydrocarbon fuel. As a standalone power station its power output should be
adjust quickly to follow the load demand.
SOFC provide high pressure exhaust gas for cogeneration and it is
supplied to the gas turbine to increase the overall efficiency of the system.
At lower temperatures electrolyte conductivity and electrode kinetics decrease
significantly.
To overcome these drawbacks alternate cell materials and designs
are being extensively investigated. In addition to the above materials selection
criteria the fabrication processes have been chosen so that every sequential
component fabrication process does not affect those components already
fabricated on to the cathode tube and to minimize the cell fabrication cast. The
weakness of the of this approach include (1) speed matching between the turbine
and the air compressor was ignored (2)
the mass flow rate which links the two sub-systems, was not directly
involved in the performance map of the turbine and the fuel cell stack. Speed
and inlet temperature of the gas turbine could be adjusted to achieve the
maximum power output at different mass flow rate and it is sufficiently
powerful.
WHAT IS SOLID OXIDE FUEL CELL:
Solid oxide fuel is a complete solid-state
device that uses an oxidie-conducting ceramics materials. It consist of two
essentially two porous electrodes separated by a dense oxide in conducting
electrolyte. Only two phases involved in this cell. They are gas and solid. In
this both hydrogen and as well as carbon monoxide can act as a fuel. light
hydrocarbon fuels such as methane, propane and butane can be internally
reformed within the anode.
Power systems can increase the efficiency by using
the heat given off by the electrochemical oxidation within the fuel cell for
endothermic stream reforming process. A solid oxide fuel cell is generally made
up of four layers. A single cell consisting of these four layers stacked
together is typically only a few millimeters thick. Hundreds of these cells are
then connected in series to form what most people refer to as on SOFC stack.
Sulfur poisoning has been widely observed and the sulfur must be removed before
entering the cell through the use of adsorbent beds or other means.
COMPONENTS OF A SOLID OXIDE FUEL CELL:
ANODE:
Cermat made up of metallic nickel and a
skeleton of YSZ. Zirconia serves to inhibit sintering of the metal particles.
The anode has a porosity(20-40%) so that mass transport of reactants and
product gases is not inhibited.
CATHODE:
The cathode is also porous structure which
must allow the rapid transport of reactant and product gases. Strontium doped
lanthanum manganite (La0.84Sr0.26) MnO3.This is most commonly used
material for anode of a SOFC.
ELECTROLYTE:
Zirconia dipped with 8 to 10%
yttria(yttria-stabilized zirconia) is still the most effective electrolyte for
the high temperature SOFC. Zirconia is highly stable in the both the reducing
and oxidizing environment that are experienced at the anode and cathode
respectively. The ionic conductivity of YSZ is comparable with that of liquid
electrolytes and it can be made very thin ensuring the ohmic losses in the SOFC
is comparable with the other fuel cell types.
WORKING PRINCIPLE OF A SOLID OXIDE
FUEL CELL IN A GAS TURBINE PLANT:
At anode : O2-(s) + H2(g)
= H2O(g)+2e-
At cathode : 1̸2 O2(g)+2e-=o2-
At electrolye : Y2O3
→2Yʹzr + 3O0X + VʹʹO (in the presence of Zro2)
Over all cell reaction: H2 +
1̸2 O2 → H2O
At initillay the large amount air
is pressurized in the compressor upto 3atm.Then compressed air passes through
the recupator where it is preheated and then enter into the solid oxide fuel
cell. And also fuel is added to the combustion chamber in the region of anode
area. The compressed air is passes through the region of cathode area. At
initially the the O2 is depleted by the electron when the previous
exhaust of a combustion. Using that electron the O2 is converted
into O2-.These O2- ions passes through the electrolyte and then
it is added with the H2 from the fuel inlet. Then it will produce the
steam. And also reforming process also happens in this time. Then the outlet of
the fuel cell approximately at 0.6Mpa.Then it will allowed to expand in
turbine. The turbine rotates the compressor as well as heat recovery steam
generator to produce electricity.
COMPONENETS OF A SOLID OXIDE FUEL
CELL-GAS TURBINE POWER PLANT:
A typical SOFC-GT power plant
system is shown in figure. The system consist of one SOFC stack, compressor,
Heat recovery steam generator and Gas turbine to produce a network output. The
mechanical work produced by GT is used to drive the compressor and the sole
mechanical power available for electrical power generation.
COMPRESSOR:
The compressor is a device which
is used to increase the pressure of the inlet air and also little amount of
temperature is increased. The compressor is made up of the fan and alternating
stages of rotating blades and static vanes. The pressure rise is created as air
flows through the stages of rotating blades and static vanes. The blade
accelerate the air increasing its dynamic pressure, and then the vanes
decelerate the air transferring kinetic energy into static pressure rises.
Mainly there are two types of compressor are generally used in gas turbine
power plant. They are Centrifugal compressors & Axial flow compressors. For
low pressure ratios the Centrifugal compressors are used. In case high pressure
ratio required we have to use Axial-flow compressors are used. In generally
compressor material is aluminium alloys are used.
FUEL CELL AS A COMBUTION CHAMBER:
In this solid oxide fuel cell gas
turbine power plan combustion chamber is replaced by a fuel cell. In this fuel
cell the fuel is given to the anode side. And also compressed air from the
compressor is fed to the cathode side. Solid oxide fuel cells are a class of
fuel cell characterized by the use of a solid oxide material as the
electrolyte. The largest disadvantage is the high operating temperature which
results in longer startup times and mechanical and chemical compatibility
issues. SOFC use a solid oxide electrolyte to conduct negative oxygen ions from
the cathode to anode.
The electrochemical oxidation of the oxygen ions with hydrogen or
carbon monoxide thus occurs on the anode side. More recently, proton conducting
SOFCs are being developed which transport protons instead of oxygen ions
through the electrolyte with the advantage of being able to be run at lower
temperatures than traditional SOFCs. Because of
high temperature, light hydrocarbons fuels like methane, propane, and
butane can be internally reformed with in the anode. SOFC power systems can
increase efficiency by using the heat given off by the exothermic
electrochemical oxidation within the fuel cell for endothermic steam reforming
process. Thermal expansion demands a
uniform and well-regulated heating process at setup.
SOFC with planer geometry
require on the order of an hour to be heated to light-off temperature. Micro
tubular fuel cell design geometries promise much faster startup times,
typically on the order of minutes
Most of the downtime of a SOFC stems from the
mechanical balance of plant, the air preheater, prereformer, afterburner, water
heat exchangers, anode tail gas oxidizer, and electrical balance of plant,
power electronics, hydrogen sulphide sensor and fans. Internal reforming leads
to a large decrease in the balance of plant costs in designing a full system.
In this power plant initially the fuel is fed
to the anode area of the cell. The anode is commonly the thickest and strongest
layer in each individual cell, Because it has a smallest polarization losses,
and is often the layer that provides the mechanical support. Electrochemically
the anode job is to use the oxygen ions
that diffuse through the electrolyte to oxidize the hydrogen fuel. The
oxidation reaction between the oxygen ions and the hydrogen produces heat as
well as water and electricity. Reforming
reaction is endothermic which cools the stack internally.
The electrolyte is a dense layer of ceramic
that conducts oxygen ions. Its electronic conductivity must be kept as low as
possible to prevent losses from leakage currents.
The high operating temperatures
of SOFCs allow the kinetics of oxygen transport to be sufficient for good
performance. However as the operating temperature approaches the lower limit
for SOFCs at around 600°C, The electrolyte begins to have large ionic transport
resistances and affect the performance.) Popular electrolyte materials yttria
stabilized zirconia(YSZ),Scandia stabilized zirconia(SCZ), gadolinium doped
ceria(GDC). The electrolyte material has crucial influence on cell performance.
If the conductivity for oxygen ions in SOFC can remain high even at lower
temperature .Material choice for SOFC will broaden and many existing problems
can potentially be solved. Certain processing technique such as thin film
deposition can help solve problem with existing material .
Cathode materials must be minimum,
electronically conductive. Currently Lanthanum Strontium Manganite is the
cathode material of choice for commercial use because of its compatibility with
doped zirconia electrolytes. LSM is poor ionic conductor and so electrochemically
active reaction is limited to triple phase boundary where the electrolyte, air
and electrode meet. LSM work well as a cathode at high temperatures but it
performs quickly falls as the operating temperature is fall into below 800°C.
(1) Producing grain structures that are less
resistive such as columnar grain structure
(2) Controlling the micro-structural
nano-crystalline fine grains to achieve fine tunning of electrical properties
(3)
Reducing the travelling distance of oxygen ions and electrolyte
resistance as resistance is inversely proportional to conductor length
The cathode, or air electrode, is a thin
porous layer on the electrolyte where oxygen reduction takes place. The overall
reaction is written in Kröger-Vink Notation as follows
1̸2O2(g)+2e-+Vₒ••→O0x
TURBINE:
Like steam turbines gas turbines are also of
the axial flow type. The basic requirement of a the turbine are light weight,
high efficiency, reliability in operation and long working life. Large output
can be obtained per stage with high blade speeds when the blades are designed
to sustain higher stresses. Its harvesting the power output of this plant. The
turbine is an assembly of discs with blades that are attached to the turbine
shaft. Nozzle guide vanes, Casting and structures. The turbine extracts energy
from the hot gas stream received from the fuel cell. Turbine blades convert the energy stored within the
gas into kinetic energy. Like the compressor, the turbine blade compresses of a
rotating disc with static blades and guide vanes called nozzle guide vanes. The
temperature and gas pressure both fall as it passes through the turbine.
Process carried in this turbine in consideration is isentrophic
process(entrophy=c). More stages are always preferred in gas turbine power
plants, Because it helps to reduce the
stresses in the blades and increase the over all life of the turbine. In
generally the turbine id made up of
Austenitic steels with 12 to 18%
chromium, 8 to 12% of the nickel and small % of tungsten ,molybdenum and
titanium.
PROCESS CARRIED OUT IN THIS POWERPLANT:
(1)
Initially the atmospheric air is compressed in a compressor upto
3bar.The blade accelerate the air increasing
its dynamic pressure, and then the vanes decelerate the air transferring
kinetic energy into static pressure raises.
(2)
This consist of a fuel cell to increase the temperature than
combustion chamber. The compressed air is supplied to the fuel cell cathode
side. And also fuel supply is at the side of anode side. The process carried
out in this method exothermic and
endothermic reactions. Due to the exothermic reaction large amount heat
will be generated and exothermic reaction the cooling effect attain in this. But
this is not play a major role in this fuel cell.
(3)
The high pressure gas coming out from the turbine is
approximately 6bar.This pressurized hot
gas is allowed to expand in a gas turbine to produce a generation of
electricity. And also used to drive the compressor of the inlet system
(4)
The heat recovery steam generator is used in this power plant for
production of electricity
PARAMETRIC STUDY OF SOLID OXIDE FUEL CELL:
The reference system has the above value for
conducting an experiment on an SOFC-GT. Electrical efficiency nearly 60% a
total efficiency of 86% and specific work around 350KW/Kg/s air. Output from the SOFC and gas
turbine generator were 311KW and 173KW,respectively,corresponding to 36% gas
turbine of total output. Average and maximum solid temperatures of the fuel
cell was 956°C and 1100°C,fuel utilization 0.77,outlet fuel
and air temperatures 1000°C and current densities 3100 A/m2.
(1)
Turbine inlet temperature and cell voltage:
The influence of turbine inlet temperature and
cell voltage on system performance can be analyzed. Increasing the turbine
inlet temperature does not increase the efficiency of specific work. Initially
at high operating temperature more fuel is consumed in gas turbine and also
decreasing the fuel flow to the fuel cell. At initially 728°C,no primary fuel
is added to the gas turbine output is 24% of net output. As a small portion of
the gas turbine work to net output is better for overall performance
The fuel cell voltage shows virtually no
impact on system performance. A voltage range between 0.66 and 0.74 was
studied. Corresponding fuel utilization between 0.94 and 0.40. The reason performance does not change is the same as
described above, that is is the portion of gas turbine to total output is constant(36%).
At high cell voltages relatively more fuel can be directed to the SOFC unit and
less to the gas turbine, thus compensating for the low fuel utilization. The
solid temperature follows the fuel utilization trend, decreasing with
increasing voltage
(2)
Compressor pressure:
The most interesting parameter is compressor
pressure, due to its large impact on system performance. At lower pressure
ratios, primary fuel in the gas turbine must be reduced to meet a constant
exhaust temperature and more fuel can be send to the SOFC. This means less
output from the gas turbine and more from SOFC increasing a pressure ratio 3.5.
Only at pressure ratios below 2, and above 4 is the solid temperature
constraint 1100°C.
(3)
Air flow rate and air inlet temperature:
Increasing the air flow can be necessary for
reducing the solid temperature. When reducing the maximum solid temperature
from 1100°C to 1050°C, 110% air flow is
needed causing an efficiency drop by four % points. Increasing the air inlet
temperature raises the solid temperature of the SOFC. In spite of this
efficiency slightly decreases with air inlet temperature, owing to a higher
fuel utilization when returning less hydrogen in the anode gas feedback loop;
this decreases the Nernst cell potential therefore the current density
Cycle variants :
Air compression intercooling improves
electrical efficiencies by 1.5% points but decreases the total efficiency
nearly 10% points over the base case. Reheat of the gas turbine decrease the
efficiently slightly and increases exhaust temperature rapidly. Both variants
boost the specific work by more than 2% points each nearly five percentage
points jointly
CONCLUSION:
And
also it will pollution free method and hence there is no effect to the
environmental surroundings. SOFC is that hydrogen and carbon monoxide are used
as a fuel in this cell. This means that SOFC can use commonly hydrocarbon fuel
such as natural gas, diesel, gasoline, and alcohol without the need of to
reform the fuel into pure hydrogen. SOFC have potentially lower cast and hence
it will not play major economic payment in the power plant. Normally if a
combustion chamber will work in that same condition its efficiency is
approximately 35% to 40%The TIT and cell voltage did not show a big impact on
system performance Intercooling of air compression and gas turbine reheat would
not be worthwhile as the gain in performance is relatively small, especially
for the reheat case. The fuel utilization of the cell was raised by decreasing
the operating voltage, decreasing the fuel flow rate and by increasing the air
inlet temperature. Efficiency of the plant when increasing the cell voltage and
flow rates and flow rates of fuel and air decreased and when air inlet
temperature increased .
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