LITERATURE view, coal reserves are available up

LITERATURE
REVIEW

Coal is one of the main
resources for world energy supply. It remains central to the world energy
system. Coal is the world’s largest source of electricity, accounting for 40%
of global electricity production. 1
It is currently the world second largest source of primary energy, and is
widely expected to replace oil as the world’s largest source of primary within
a few years. 1 According to BP
Statistical Review of World Energy 2016, there was 891,531 million tonnes of
coal reserves in the world in 2015. 2 Keeping the annual
consumption of coal in view, coal reserves are available up to 2112 and will be
the only fuel remaining after 2042. 3

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Pakistan is amongst the
countries with the most potential for coal generation. Pakistan’s coal
potential is estimated at 186 billion tons, more than known oil and gas
figures. 4
Currently, the coal in Pakistan is being utilized by coal power plants. On
average, the efficiency of existing global coal-fired powerplants is low, at
about 33%. 5

An alternative to
coal-fired power plants is the carbon fuel cell. The carbon fuel cell is a
special type of high temperature fuel cell that directly uses carbon as anode
and fuel. As an electrical power generator for power plant it has greater
efficiency and less emissions than conventional coal-burning power plants. 6 A DCFC converts the chemical energy in solid carbon
directly into electricity through its direct electrochemical oxidation. The
fuel utilization can be almost 100% as the fuel feed and product gases are
distinct phases and thus can be easily separated. This is not the case with
other fuel cell types for which the fuel utilization within the cell is
typically limited to below 85%. The theoretical efficiency is also high, around
100%. The combination of these two factors, lead to the projected electric
efficiency of DCFC approaching 80% – approximately twice the efficiency of
current generation coal fired power plants, thus leading to a 50% reduction in
greenhouse gas emissions. 7
The amount of CO2 for storage/sequestration is also halved. Moreover, the exit
gas is an almost pure CO2 stream, requiring little or no gas separation before
compression for sequestration. Therefore, the energy and cost penalties to capture
the CO2 will also be significantly less than for other technologies.
Furthermore, a variety of abundant fuels such as coal, coke, tar, biomass and
organic waste can be used.

Major types of Direct
Carbon Fuel Cells (DCFC) exist as follows. 1.Polymer Electrolyte Membrane Fuel
Cell (PEMFC) 2. Direct Methanol Fuel Cells 3. Alkaline Fuel Cell (AFC) 4.
Phosphoric Acid Fuel Cells 5. Molten Carbonate Fuel Cells 6. Solid Oxide Fuel
Cells (SOFC)

The polymer
electrolyte membrane fuel cell (PEMFC) is also known as proton exchange membrane
fuel cell, polymer electrolyte fuel cell (PEFC) and solid polymer fuel cell
(SPFC).
The electrolyte is an ion conducting polymer membrane. Anode and cathode are
bonded
to either side of the membrane. This assembly is normally called membrane
electrode assembly (MEA) or EMA which is placed between the two flow field
plates (bipolar plates) to form what is known as “stack”. The basic
operation of the PEMFC is the same as that of an acid electrolyte cell as the
mobile ions in the polymer are H+ or proton. 8

At
Anode:                    H2
? 2H+ + 2e-
At Cathode:                 O2 + 2H+ +
2e- ? 2H2O
Overall Reaction:       H2
+ O2 ? H2O
The high cost of PEM fuel cells remains a major
barrier that prohibits their widespread applications. A cost of $61 has been
achieved in 2009 whereas a lifetime of around 2500 h was reported for transportation
PEM fuel cells. PEM fuel cells can be superior to ICEs in several aspects
except the initial cost. 9
The disadvantage is that the low quality thermal output cannot be used
effectively in all places. Another disadvantage associated with PEMFC is that
platinum catalysts & required to promote the electrochemical reaction. The
low temperature of operation also means that little, if any, heat is available
from the fuel cell for any endothermic reforming process. 8

 

The
direct methanol fuel cell (DMFC) has been considered as the ideal fuel cell
system since it produces electric power by the direct conversion of the
methanol fuel at the fuel cell anode. This is more attractive than the
conventional hydrogen fuel cells, particularly for transportation applications,
which rely on bulky and responsive reformer systems to convert methanol, or other
hydrocarbon fuels, to hydrogen.
At Anode:                    CH3OH
+ H2O ? CO2 + 6H+
At Cathode:                O2 + 6H+
+ 6e- ? 3H2O
Overall Reaction:       CH3OH
+ O2 + H2O
? CO2 + 3H2O 10
However, commercialization of DMFC has been impeded by its poor performance
compared with hydrogen/air systems, the major limitation being the anode
performance which requires highly efficient methanol oxidation catalysts. Such
catalyst materials have been sought, and it appears that only platinum-based
materials show reasonable activity and the required stability. 10 But platinum-based
materials and platinum itself are very costly and require a hefty initial
expense. 11

Alkaline Fuel Cells
(AFCs) use an aqueous solution of potassium hydroxide as the
electrolyte, with typical concentrations of about 30%. 12 The electrodes
consist of a double layer structure: an active electrocatalyst layer, and a
hydrophobic layer. The inherently faster kinetics of the oxygen reduction
reaction in an alkaline cell allows the use of non-noble metal electrocatalysts.
13

At Anode:                    2H2 + 4OH-
? 4H2O + 4e-
At Cathode:    O2 + 2H2O
+ 4e- ? 4OH-
Overall Reaction:       2H2
+ O2 ? 2H2O + energy 13

The
presence of carbon dioxide degrades the cell performance due to which the
stationary application using hydrocarbon fuels is limited. 14 AFCs, like all fuel
cells, have limits to the amount of impurities they can tolerate in their feed
gas streams. The “poisoning” of the fuel cell by impurities can be caused by
any number of different gases like carbon dioxide. Moreover, the removal of the
CO2 from the feed gases would be very expensive and impractical. 13

 

 

Phosphoric
acid fuel cells (PAFC) are a type of fuel cell that uses
liquid phosphoric acid as an electrolyte. The electrolyte, primarily composed
of phosphoric acid (H3PO4), is a proton conductor, thus
the protons migrate from the anode to the cathode, while the electrons migrate
through an external circuit. At the cathode side, air is provided, where oxygen
reacts with the protons and the electrons, coming from the electrolyte and the external
load. 15
At Anode:                    H2
? 2H+ + 2e-
At Cathode:                 O2 + 2H+ +
2e- ? 2H2O
Overall Reaction:       H2
+ O2 ? H2O

As is found for all the
other low temperature fuel cells, the presence of carbon monoxide in the anodic
gas affects the performance of the cell itself. The main reason for this
reduction is the poisoning effect of CO on the Pt electrode catalyst.  Pt electrodes are expensive themselves. Pt can
be supported on carbon and graphite. The use of carbon and graphite, however, imposes
some limitation on the FC operation. In particular, the FC should be run at
potentials of less than 0.8 V, otherwise there is the possibility of corrosion occurring.
16
At high potentials anodic dissolution of Pt takes place, thus no metal is
available to catalyze the corrosion of carbon. Another limitation related to
the use of carbon is the tendency of Pt to migrate to the surface of the
carbon, to agglomerate in large areas, thus reducing the active surface.

Similar to all other fuel cells, the
working principle of the (Molten Carbonate Fuel Cell) MCFC is based on
the indirect combination of hydrogen and oxygen to water via an electron
carrying electrolyte. The hydrogen supply for the anode reaction is generated from
natural gas within the fuel cell block by a process known as steam reforming.
Assisted by a catalyst, methane, the energy carrier in natural gas, combines
with water to combust the carbon and to release all the hydrogen from the
methane as well as from the water. This process absorbs waste heat from the
fuel cell and translates it back into primary energy. After having removed the
sulfur and higher hydrocarbons from the fuel, a molten carbonate fuel cell with
internal reforming can be fed directly with a mixture of natural gas and water—we
call it the direct fuel cell (DFC). 17

Internal Reformer:     CH4 +
H2O ? 3H2 + CO
At Anode:                    H2 +
CO32- ? H2O + CO2 + 2e-
At Cathode:                 O2 + CO2
+ 2e- ? CO32-
Overall Reaction:       H2
+ O2 + CO2
? H2O + CO2 7

In MCFC
technology the dissolution of the state-of-the-art lithiated NiO cathode material
is one of the major lifetime-limiting factors. The NiO material dissolves in
the molten-carbonate electrolyte and is subsequently transported, reduced and precipitated
in the electrolyte matrix. 18

 

The solid oxide fuel cell (SOFC) is characterized by having
a solid ceramic electrolyte (hence the alternative name, ceramic fuel cell),
which is a metallic oxide. A typical unit cell of a planar SOFC
stack is composed of a positive electrode, an electrolyte, a negative electrode
(known as PEN assembly), interconnect plates and seals. 19
In practical applications, multiple cells are assembled in the form of a stack.
At cathode, oxygen is reduced to oxygen ions, which
then pass through the solid electrolyte under electrical load, to the anode,
where they react with the fuel,
generally hydrogen and carbon monoxide, producing water and CO2, as
well as electricity and heat.

The
hydrocarbon fuel is catalytically converted, generally to carbon monoxide and hydrogen
(synthesis gas), within the actual SOFC, and the CO and H2 are then
electrochemically oxidized to CO2 and water at the anode, with production of
electrical power and high-grade heat.

At
Anode:        H2
+ O2- ? H2O + 2e-
                        CO + O2-
? CO2 + 2e-
At Cathode:    O2 + 4e-
? 2O2-
Overall:          H2
+ CO + O2 ? H2O + CO2 + ?E

To build up a useful
voltage, a number of cells are usually electrically connected in series in a
“stack” via interconnects that also separate the fuel at the anode-side of one
cell from the air at the cathode-side of the adjacent cell in planar SOFC
stacks. 20

The theoretical maximum
efficiency is very high, in excess of 80%. 21

Solid oxide fuel cells
(SOFCs) have potential to be the most efficient and cost-effective system for
direct conversion of a wide variety of fuels to electricity. The performance
and durability of SOFCs depend strongly on the microstructure and morphology of
cell components. 22
The SOFC operates at elevated temperatures, conventionally between 800-1000 °C,
though there is considerable interest in lowering the operating temperature of
smaller SOFCs in particular to reduce costs, particularly of interconnect,
manifolding and sealing materials. 21

The elevated
operating temperature of the SOFC also leads to production of high temperature
heat as a by-product in addition to the electrical power. This high quality
heat is not wasted, but can be used in various ways, for example in
cogeneration, in combined heat and power systems, or to drive a gas turbine to
generate more electricity. This significantly increases the overall efficiency
of the SOFC compared to lower temperature variants. SOFCs can be used as
high-temperature water electrolyzers without major changes in the design. The
flexibility in the choice of fuel, the ability to operate SOFCs directly on
practical hydrocarbon fuels, and the higher overall efficiency are three of the
potential advantages of SOFCs over other types of fuel cells. Another
particular advantage is their tolerance to carbon monoxide, which is electrochemically
oxidised to CO2 at the anode, which contrasts markedly with PEM fuel cells,
which are highly susceptible to poisoning by CO, and thus require complex and expensive external
processing of hydrocarbon feeds to convert all the CO to CO2, which is then
removed to leave very pure hydrogen as the fuel, thus emission of CO2 is
considerably lessened. SOFCs also show greater tolerance to impurities in the fuel,
which poison other fuel cells, and to variations in the fuel composition, such
that fuel processing requirements are less demanding and the cell life is
increased. Other advantages include the fact that use of precious metals like
platinum and ruthenium is not necessary, which add significantly to the cost of
the fuel cell, and the fact that the absence of any liquids in the cell
eliminates potential problems due to corrosion and loss of electrolyte. In addition,
SOFC systems can be put together in various ways, some of which are
considerably simpler than PEM fuel cell stacks. 21
23

An SOFC operated between
the temperature range of 700-1000 C. 19 The high temperature
operation gives rise to a significant level of thermal stresses due to mismatch
of coefficient of thermal expansion (CTE) between different components and
temperature gradients in the SOFC system. These kind of thermal stresses can cause
delamination and micro-cracking in different layers of the PEN assembly, each
of which is very critical to the operation of an SOFC. Hence, a comprehensive
thermal analysis of an SOFC stack is required for the successful design and
operation of an SOFC.

A number of studies have
been carried out on the thermal analysis of an SOFC 24 25 26 27 28.
Most of these studies 24 25 26 27 28  used experimental and numerical analysis to
calculate or estimate the stresses in the electrolyte or electrolyte layers at
either room temperature or by applying a uniform temperature gradient within a
simple positive electrode-electrolyte-negative electrode assembly. These
studies 24 25 26 27 28
only focused on the stresses caused by the mismatch of coefficient of thermal
expansion between electrolyte and electrode layers for a given operating
temperature ignoring the effects of temperature gradients within the PEN
assembly and thermal interactions between PEN and the other components of the
cell, like interconnects. A number of studies 29 30 31 use Finite Element
Analysis to calculate the thermal stresses and thermo-chemical models to
establish the required temperature profiles for planar 29 30 and tubular 31 SOFCs. As a
result, the contribution of uneven temperature distribution in the generation
of thermal stresses within the SOFC can be included and simulated.

These models of a single
cell for the planar SOFCs 29 30 included components
like interconnects and PEN such that the effects of coefficient of thermal
expansion mismatch and mechanical constraint between these the components were
also taken into account in the thermal stress analysis. Hence, simulation
approaches like the ones proposed in Refs. 29 30 would provide a very
effective tool for calculation of thermal stress distribution and give very
useful results for design of planar SOFCs. However, in these studies 29 30, for the sake of
simplicity of calculation, only a single stack of cell was considered and the
other parts as gas seals were not included. The planar type of SOFC require
high temperature gas seals to bond the cell components and separate the air and
fuel compartments. Therefore the influence of these gas seals on the thermal
stress distribution also need to be evaluated to obtain highly accurate
results. If a gas seal is damaged, it may cause leaked and degrade the
performance of an SOFC. However this issue lacks sufficient studies in the
literature and hence provides a need for study of the role of gas seals in the
efficient working and durability of an SOFC. To provide an effective tool for
the calculation of thermal stress in a planar SOFC cell, it would be better to
use a simulation model as close as possible to the practical one. As described
above, the models used in prior in these studies 24 25 26 27 28
29 30 have been simplified
to some extent. In this study, we use a simpler approach like the ones used in 24 25 26 27 28
assuming uniform temperature profiles but using an FEA model to approximate the
thermal stresses. 24 25 26 27 28
29 30 31