Introduction:
Around 35% of
total energy is obtained by coal in world. Present use of coal is inefficient
and polluting. Hence there is need for technologies for utilization of coals
efficiently and cleanly, substitution of lesser reserves of oil and gas with
abundantly available coals and prolonging the reserves of all the fossil fuels
for use of future generations. These requirements can be met through
application of coal gasification technology.
Rather than
burning coal directly, coal gasification reacts coal with steam and controlled
amounts of air or oxygen under high temperatures and pressures to produce a
gaseous mixture, typically hydrogen and carbon monoxide. These hot, coal gases
exiting the gasifier are used to power a gas turbine (in the same manner as
natural gas). Hot exhaust from the gas turbine is then fed to a heat recovery
steam generator (HRSG). The steam from the HRSG is then fed to a conventional
steam turbine, producing a second source of power (just as in a combined cycle
plant).
Coal
gasification is not only used for power generation but also it can become
source of synthetic oil, coal chemicals, natural gas substitution. Also it can
be used for development of the fuel cell. Thus coal gasification become member
of clean coal technology with other advantages.
This report gives
principle and implementation of coal gasification with development of coal
gasification for fuel cell. Report describes how coal gasification is
beneficial to India
GASIFICATION:
The gasification process converts any
carbon-containing material into a synthesis gas composed primarily of carbon
monoxide and hydrogen, which can be used as a fuel to generate electricity or
steam or used as a basic chemical building block for a large number of uses in
the petrochemical and refining industries. Gasification adds value to low- or
negative-value feedstock by converting them to marketable fuels and products.
Gasification technologies
differ in many aspects but share certain general production characteristics.
Typical raw materials used in gasification are coal, petroleum based materials
(crude oil, high sulfur fuel oil, petroleum coke, and other refinery
residuals), gases, or materials that would otherwise be disposed of as waste.
The feedstock is prepared and fed to the gasifier in either dry or slurried
form. The feedstock reacts in the gasifier with steam and oxygen at high
temperature and pressure in a reducing (oxygen starved) atmosphere. This
produces the synthesis gas, or syngas, made up primarily of carbon monoxide and
hydrogen (more than 85% by volume) and smaller quantities of carbon dioxide and
methane.
The high temperature in the
gasifier converts the inorganic materials in the feedstock (such as ash and
metals) into a vitrified material resembling coarse sand. With some feedstocks,
valuable metals are concentrated and recovered for reuse. The vitrified
material, generally referred to as slag, is inert and has a variety of uses in
the construction and building industries.
Gas treatment facilities
refine the raw gas using proven commercial technologies that are an integral
part of the gasification plant. Trace elements or other impurities are removed
from the syngas and are either recirculated to the gasifier or recovered.
Sulfur is recovered either in its elemental form or as sulfuric acid, both
marketable commodities.
If the syngas is to be used to
produce electricity, it is typically used as a fuel in an integrated
gasification combined cycle (IGCC) power generation configuration.
IGCC is the cleanest, most efficient means
of producing electricity from coal, petroleum residues and other low- or
negative-value feedstocks. The combined cycle system has two basic components.
A high efficiency gas turbine, widely used in power generation today,
burns the clean syngas to produce electricity. Exhaust heat from the gas
turbine is recovered to produce steam to power traditional high efficiency steam
turbines.
The syngas can also be
processed using commercially available technologies to produce a wide range of
products, fuels, chemicals, fertilizer or industrial gases. Some facilities
have the capability to produce both power and products from the syngas,
depending on the plant’s configuration as well as site specific technical and
market conditions.
COAL
GASIFICATION PRINCIPLE:
Coal
gasification is an old and well-known technology, which was commercially used
in many countries. But due to
availability of abundant natural gas and crude oil, this technique was
neglected by mid 50’s. Due to rapid depletion of crude oil and natural gas
resources the coal gasification attracted renewed interest and again large numbers
of countries are using this technique.
The principle of
coal gasification is, when coal is brought into contact with steam and
controlled amount of oxygen under high temperature and pressure, thermo
chemical reaction occur and produce a fuel gas, which consists of carbon
monoxide and hydrogen.
The heat and pressure
break the chemical bond in coals complex molecular structure, which then reacts
with steam and oxygen to form coal gas, which is a mixture of carbon monoxide
and hydrogen.
INTEGRATED
COAL GASIFICATION COMBINED CYCLE (IGCC):
Integrated coal gasification
combined cycle power generating systems are presently being developed and
operated in Europe and USA
WORKING:
In an integrated, coal
gasification combined cycle plant, both a high temperature gas turbine using a
Bray ton cycle and a lower temperature steam turbine using a Rankine cycle are
used to generate electricity more efficiently. Crushed coal is first gasified
by reacting it with a mixture of air or oxygen and high temperature steam to
form a "syngas" fuel consisting mostly of CO, H2, N2,
and CO2. This gas is then cooled, cleaned, and reheated using hot
gas exiting the gasifier in a gas-to-gas heat exchanger. Next the gaseous fuel
is combusted to turn a compressor and a gas turbine to generate electricity.
The compressor has a dual role. It pressurizes air for combustion in the gas
turbine and provides heated compressed air to use in the gasification process.
The exhaust gas from the gas turbine is still very hot and is subsequently used
to produce steam in another heat exchanger. This steam turns a steam turbine to
produce additional electricity.
Integrated Gasification
Combined Cycle (IGCC) systems meet the challenge of supplying energy cleanly
and efficiently. The systems are successful at removing sulfur dioxide (SO2),
reducing nitrogen oxide (NOx) emissions, and removing particulate
matter. At the same time, IGCC efficient levels top 42% and continue to
increase along with advances in gas turbine technology.
One
of the gasification technology developed in the U.S.A.
TECHNOLOGY:
IGCC systems are
extremely clean, and more efficient than traditional coal-fired systems. The
systems replace the traditional coal combustor with a coal gasifier that is
coupled with an advanced gas turbine. The result is an integrated gasification
combined-cycle configuration that provides ultra-low pollution levels &
system efficiency.
In an
IGCC system (as shown in figure), coal is converted into a gaseous fuel that
after cleaning is comparable to natural gas. The process eliminates 99% of the
coal's sulfur. An additional process further cleans the hot gas, and then it is
forwarded to the gas turbine. Also, exhaust heat from the gas turbine is used
to produce steam for a conventional steam turbine. This results in two cycles
of electric power generation.
The IGCC
systems that are available commercially have demonstrated exceptional
environmental performance. SO2 and NOx emissions are less
than one-tenth of that allowed by New Source Performance Standards limits.
Moreover, IGCC
efficiency levels top 40%. Modern coal-burning plants that
employ flue gas desulphurization processes reach a maximum of 34% efficiency.
PERFORMANCE:
IGCC technology brings many
advantages to an energy-hungry but cost-conscious world:
1. HIGH EFFICIENCY:
The efficiency level of a power
plant is the ratio of the quantity of electrical energy produced per quantity
of coal energy used to create it. Over the last fifty years, conventional
coal-fired plants have been able to increase efficiency by only about 9% to achieve
a maximum level of 34%. With the advent of IGCC systems, coal-fired plants can
realistically expect to attain efficiency levels of up to 45% by the turn of
the century. Industry analysts anticipate that IGCC systems based on advances
in gas turbine technology will experience net system efficiencies of 52% as
early as the year 2010. In less than half a decade, IGCC technology promises to
raise efficiency levels by more than twice the amount achieved over the last
half century!
2.
A CLEAN ENVIORNMENT:
- IGCC systems meet all EPA constraints
concerning power generation. In an IGCC system 99% of the coal's sulfur is
removed before combustion, NOx is reduced by over 90%, and CO2is
cut by 35%. IGCC systems offer significant reductions in CO2
emissions per unit of power produced, because their higher efficiency
means that less coal must be burned to produce each unit of power. When
combined with the fuel cell systems of the future, IGCC technology will
further reduce CO2 emissions. This environmental performance
matches or exceeds that of alternate energy sources.
- Reusable process media remove sulfur from the
coal-derived gas prior to combustion in the gas turbine. By contrast,
plants that employ flue gas desulfurization techniques as well as
fluidized-bed power plants use limestone, dolomite, or other sulfur
sorbents. These substances require disposal.
- The water required to operate an IGCC plant is
only 50 to 70% of the quantity required to run a pulverized coal plant
with a flue gas desulfurization system.
BY-PRODUCTS:
- The IGCC process generates a minimum of waste.
Moreover, the by-products produced by the process have marketability.
Sulfuric acid and elemental sulfur are two primary by-products for which
there is market demand. Ash and any trace elements that have melted become
an environmentally safe, glass-like slag once they are cooled. That slag
is useful to the construction and cement industries.
- In addition to producing electricity, the coal
gasification process is easily diverted to co-produce such products as
methanol, gasoline, urea for fertilizer, hot metal for steel making, and
assorted chemicals.
ADDITIONAL ADVANTAGES:
- IGCC offers low cost electricity. Currently,
the cost of IGCC-generated electricity is comparable to that produced by
conventional coal plants. By 2010 IGCC plants will produce power at a rate
of 75% of the cost incurred by conventional plants.
- The components of the IGCC system are modular.
This permits a user to integrate the technology into an existing system.
Installation can proceed in blocks ranging in size from 100 to 450
megawatts. The staged additions can be designed to match one or more steam
generators and the resulting system will have two-and-a-half times its
previous generating capacity. As advanced turbine systems evolve, the
capacity of single units will increase, and the trend will be to add
large-capacity modules.
- IGCC technology provides flexibility to power
producers because the combined-cycle portion of the process can be fueled
by natural gas, oil, or coal. A plant can switch to coal from natural gas
as gas becomes unavailable or unacceptably expensive. In addition, most
gasifier systems are easily adapted to different coals.
·
As much as 98% of sulfur and other
pollutants can be removed
And processed into commercial products
such as chemicals and
Fertilizers. Unreacted solids can be
collected and marketed as a
Co-product such as slag (used, for
example, in road building).
COSTS
The current capital cost of building
an IGCC power plant is as low as $1,500 per kilowatt. Improvements in hot gas
desulfurization and hot gas particulate removal will lower this figure to
$1,200 per kilowatt within the next five years. Continued technological
advances will cause the capital cost to drop as low as $1,050 per kilowatt by
2010. This is 75% of the cost of electricity produced from current conventional
power plants.
IGCC systems are unique in the manner
by which they meet the demands of utility growth patterns. A first-phase
installation might include only a gas turbine, operating as a simple
natural-gas-fired cycle that will yield about two-thirds of the plant's
ultimate capacity. Addition of a steam turbine would create a combined cycle
with full capacity. A third phase would integrate the gasifier and gas cleanup
systems.
EXPERIENCE IN
TECHNOLOGY:
Advanced power generation
technologies such as IGCC will be responsible for the continuing preference of
electric power producers for coal. There are several IGCC demonstration projects
that show conclusively how successfully the system operates and how effectively
it can be maintained. Industry experts predict that coal will remain the most
economical, long-term fuel of choice for electricity production throughout the
world. Already the prices of coal and electricity have, in real terms, declined
throughout the 1980s. When one compares environmental benefits, efficiency
levels and costs, IGCC technology scores higher than many other advanced
technologies that will be available to energy producers for the next several
decades.
DEMONSTRATION PROJECTS
PSI Energy and Destec Energy
have joined forces in Indiana Wabash
River
PINON PINE IGCC POWER PROJECT.
In Reno , Nevada Nevada
TAMPA ELECTRIC COMPANY IGCC PROJECT.
Tampa Electric in Lakeland , Florida
Pioneering
Gasification Plants
As early as the 1890s,
lamplighters once made their rounds down the streets of many of America China
In the 1970s, interest in coal
gasification revived, due largely to concerns that the U.S.
The massive Great Plains Coal
Gasification Plant in Beulah, North Dakota, was built with federal government
support to use coal gasification to create methane, the chief constituent of
natural gas that could be fed into nearby commercial gas pipelines. When
government price controls on natural gas were lifted, however, large quantities
of natural gas became available, and no other coal-to-methane gasification
plants have been built to date in the United States
Coal gasification, however,
likely found its most important market application in the 1980s and 90s. Driven
primarily by environmental concerns over the traditional burning of coal,
gasification emerged as an extremely clean way to generate electric power. By
turning coal into a combustible gas that could be cleansed of virtually all of
its pollutant-forming impurities and burned in a gas turbine, coal could rival
natural gas in terms of environmental performance.
The federal government had
investigated a coal gasification-based power plant in the 1970s, but a project
to build the Powerton pilot plant near Pekin ,
Illinois United States Barstow ,
California
Coal gasification-based power
concepts got their biggest boost in the 1990s when the U.S. Department of
Energy's Clean Coal Technology Program provided federal cost-sharing for the
first true commercial-scale IGCC plants in the United States
How Gasification Power Plants Work
The heart of gasification-based
systems is the gasifier. A gasifier converts hydrocarbon feedstock into gaseous
components by applying heat under pressure in the presence of steam.
A gasifier differs from a
combustor in that the amount of air or oxygen available inside the gasifier is
carefully controlled so that only a relatively small portion of the fuel burns
completely. This "partial oxidation" process provides the heat.
Rather than burning, most of the carbon-containing feedstock is chemically
broken apart by the gasifier's heat and pressure, setting into motion chemical
reactions that produce "syngas." Syngas is primarily hydrogen, carbon
monoxide and other gaseous constituents, the proportions of which can vary
depending upon the conditions in the gasifier and the type of feedstock.
Minerals in the fuel (i.e., the
rocks, dirt and other impurities which don't gasify like carbon-based
constituents) separate and leave the bottom of the gasifier either as an inert
glass-like slag or other marketable solid products. Only a small fracture of
the mineral matter is blown out of the gasifier as fly ash and requires removal
downstream.
Sulfur impurities in the
feedstock form hydrogen sulfide, from which sulfur is easily extracted,
typically as elemental sulfur or sulfuric acid, both valuable byproducts.
Nitrogen oxides, another potential pollutant, are not formed in the
oxygen-deficient (reducing) environment of the gasifier; instead, ammonia is
created by nitrogen-hydrogen reactions. The ammonia can be easily stripped out
of the gas stream.
In integrated gasification
combined-cycle (IGCC) systems, the syngas is cleaned of its hydrogen sulfide,
ammonia and particulate matter and is burned as fuel in a combustion turbine
(much like natural gas is burned in a turbine). The combustion turbine drives
an electric generator. Hot air from the combustion turbine is channeled back to
the gasifier or the air separation unit, while exhaust heat from the combustion
turbine is recovered and used to boil water, creating steam for a steam
turbine-generator.
The use of these two types of
turbines - a combustion turbine and a steam turbine - in combination, known as
a "combined cycle," is one reason why gasification-based power
systems can achieve unprecedented power generation efficiencies. Currently,
gasification-based systems can operate at around 45% efficiencies; in the
future, these systems may be able to achieve efficiencies approaching 60%. (A
conventional coal-based boiler plant, by contrast, employs only a steam
turbine-generator and is typically limited to 33-38% efficiencies.)
Higher efficiencies mean that less
fuel is used to generate the rated power, resulting in better economics (which
can mean lower costs to ratepayers) and the formation of fewer greenhouse gases
(a 60%-efficient gasification power plant can cut the formation of carbon
dioxide by 40% compared to a typical coal combustion plant).
All
or part of the clean syngas can also be used in other ways:
·
As chemical "building blocks"
to produce a broad range of liquid or gaseous fuels and chemicals (using
processes well established in today's chemical industry);
·
As a fuel producer for highly efficient
fuel cells (which run off the hydrogen made in a gasifier) or perhaps in the
future, fuel cell-turbine hybrid systems;
·
As a source of hydrogen that can be
separated from the gas stream and used as a fuel (for example, in President
Bush's hydrogen-powered Freedom Car initiative) or as a feedstock for
refineries (which use the hydrogen to upgrade petroleum products).
Another advantage of
gasification-based energy systems is that when oxygen is used in the gasifier
(rather than air), the carbon dioxide produced by the process is in a
concentrated gas stream, making it much easier and less expensive to separate
and capture. Once the carbon dioxide is captured, it can be sequestered - that
is, prevented from escaping to the atmosphere and potentially contributing to
the "greenhouse effect."
COAL GASIFICATION FOR
SUSTAINABLE DEVELOPMENT OF ENERGY SECTOR IN INDIA
Though coal
available in our country is in large amount; there are certain problems associated with its use which are
listed as follows-
A.
Indian coals are of inferior quality
having ash content 45 to 50 %
B.
The average calorific value of Indian
coal is low about 3500 kcal/kg.
Thus to overcome
above problems present method of direct combustion of coal (either in lump or
pulverized form) has to be changed. As
energy sector requires clean, efficient and dependable energy sources the use
of coal has to be made to obtain higher efficiency, less pollution.
Therefore coal
gasification technique seems to be best option to fulfill above requirements.
Though coal is
relatively a long lasting fuel, the quality in general is inferior with mineral
content as high as 50%. Since reserves of oil and natural gas are meager, they
need to be substituted with coal to the extent feasible. At the same time all
the three fuels, especially coal needs to be conserved for the future
generations. The energy sector requires efficient, clean and dependable energy
supplies. Hence coal has to be utilized with multi pronged strategy i.e. higher
efficiency, environmental acceptance, prolonging its availability and as
replacement for oil etc. that is possible only through sustainable development
and gasification is the best option to achieve it.
State wise
and depth-wise reserve of Indian coal
(AS ON 01-01-1996)
(Reserves in million tonnes)
(Reserves in million tonnes)
Depth and range
(in meters) |
Proved reserve
|
Indicated reserve
|
Inferred
reserve |
Total reserve
|
1
|
2
|
3
|
4
|
5
|
Gondawana Coal
|
||||
0-300
|
9725.25
|
3176.18
|
537.53
|
13438.96
|
300-600
|
1620.78
|
5689.64
|
2129.87
|
9440.29
|
600-1200
|
36.43
|
2793.97
|
1648.51
|
4478.91
|
0-1200
|
11382.46
|
11659.79
|
4315.91
|
27358.16
|
0-300
|
15537.10
|
14231.32
|
2187.95
|
31956.37
|
0-600*
|
13114.14
|
1093.86
|
0.00
|
14208.00*
|
300-600
|
816.67
|
7643.58
|
3176.96
|
11637.21
|
600-1200
|
1504.26
|
5383.62
|
515.91
|
7403.79
|
0-1200
|
30972.17
|
28352.38
|
5880.82
|
65205.37
|
Uttar Pradesh
|
||||
0-300
|
662.21
|
400.00
|
0.00
|
1062.21
|
Madhya Pradesh
|
||||
0-300
|
9836.40
|
17472.02
|
5854.13
|
33162.55
|
300-600
|
261.13
|
4496.90
|
3124.57
|
7882.60
|
600-1200
|
0.00
|
10.30
|
4.14
|
14.44
|
0-1200
|
100097.53
|
21979.22
|
8982.84
|
41059.59
|
0-300
|
3487.32
|
1025.62
|
518.89
|
5031.83
|
300-600
|
37.21
|
422.61
|
1144.75
|
1604.57
|
0-600
|
3524.53
|
1448.23
|
1663.64
|
6636.40
|
Andra Pradesh
|
||||
0-300
|
4670.29
|
583.92
|
122.20
|
5276.41
|
300-600
|
1912.18
|
1972.93
|
831.74
|
4716.85
|
600-1200
|
0.00
|
945.51
|
1981.73
|
2927.74
|
0-1200
|
6582.47
|
3503.36
|
2935.67
|
13020.50
|
Orissa
|
||||
0-300
300-600
|
6869.74
0.00
|
17589.95
4672.21
|
10829.15
6723.94
|
35288.84
11396.15
|
600-1200
|
0.00
|
36.67
|
0.00
|
36.67
|
0-1200
|
6869.74
|
22298.83
|
17553.09
|
46721.66
|
0-300
300-600 0-600 |
98.81
129.56 228.37 |
14.19
9.85 24.04 |
32.82
32.19 65.01 |
145.82
171.60 317.41 |
Arunachal Pradesh
|
||||
0-300
|
31.23
|
11-04
|
47.96
|
90.23
|
Meghalaya
|
||||
0-300
|
88.99
|
69.73
|
300.71
|
459.43
|
Nagaland
|
||||
0-300
|
3.43
|
1.35
|
15.16
|
19.94
|
Grand Total
|
||||
0-300
0-600* 300-600 600-1200 0-1200 |
51010.77
13114.14 4777.53 1540.69 70443.13 |
54578.11
1093.86 24907.72 9170.07 89749.76 |
20446.50
0.00 17164.02 4150.29 41760.81 |
126035.38
14208.00* 46849.27 14861.05 201953.70 |
¨Sustainable
Development through Coal Gasification:
According to the World
Commission on Environment and Development, Sustainable Development is the
exploitation of resources (i.e. coal resources), the orientation of
technological developments (i.e. coal utilization technologies) and the
direction of investments must be in harmony to enhance both current and future
potential to meet human needs (i.e. energy). Sustainable development aims to
promote economic growth, efficient use of natural resources and their secured
long term supply and protection of environment to ensure survival of the future
generations.
1. Power Generation
About 70% of the electricity
generated in India India
The high mineral matter content in Indian
coals posses several problems such as low calorific value, delayed combustion,
corrosion, erosion, deposits, fouling and slagging. The fly ash in Indian coals
has significant amounts of mullite (5 to 30%) and quartz (up to 30%), which is
mainly responsible for causing erosion especially of, pulverizes, boiler tubes
and I.D. fans etc. . . Due to poor quality of coal, Indian power plants are
achieving only 63% PLF while it is above 80% in advanced countries. Similarly
the specific coal consumption i.e. kg of coal consumed per kWh of power
generated is very high in India India
3.3 Integrated
Gasification Combined Cycle (IGCC) Electric Power Generation
Coal based electric power generation
(i.e., direct combustion of coal in stoker fired and pulverized coal fired
boilers) has historically been the backbone of the electric utility industry
and this technology is well proven. But the technology has reached a plateau of
maximum efficiency with only marginal potential for further improvements due to
technical limitations.
In addition to this limitation on
efficiencies, tightening of environmental control requirements have resulted in
substantial increase in both capital and operating costs to reduce emissions
from conventional coal fired power plants and also in lowering plant efficiency
and reliability, on the other hand coal gasification technology has emerged as
the most environmentally benign and competitive way of coal utilization. Thus
it would be of enormous benefit to the electric utility industry to find some
practical means for combining the high efficiency of combined cycle system with
the clean coal gasification-process for utilizing coal, which is a low cost and
abundantly available fossil fuel. This has lead to the development of IGCC Power
system.
IGCC is the technology designed
to meet the higher efficiency and stringent environmental regulations required
in the 21st century. IGCC systems have the potential to compete economically
with conventional coal fired steam plants and have lowest possible level of
pollution. As environmental control requirements increase, the economic
advantages of IGCC would correspondingly increase. Similarly with further
developments in coal gasification and gas turbine technologies taking place,
the economic and performance benefits of IGCC would increase significantly. The
efficiency of IGCC, which is now around 40-45%, is likely to increase to
55-60%. The capital cost of large and mature technology IGCC plants and PC
plants with FGD are projected to be nearly same. IGCC is the most economical
system when compared to the conventional pulverized coal fired plant for
removal of sulfur and nitrogen. With high sulfur coals the efficiency
difference between the two plants is higher since the auxiliary power
consumption for the sulfur removal is up to 3% in the flue gas desulfurisation
(FGD) unit of coal fired plant and negligible in the IGCC plant.
IGCC plants require less water
than coal-fired plant as approximately 60% of power is generated from gas
turbine. IGCC plants also require less land. IGCC systems are highly modular
which enable phased construction and higher plants availability up to 85% or
about 7400 hours per year of plant operation and economy at smaller capacities
of the order of 250 MW. Introduction of IGCC technology to utilities can create
new business opportunities in the co-production of electricity with chemicals,
liquid fuels etc.
As the global demand for coal
increases, worldwide carbon emissions will also increase. It is estimated that
if all power producers were to use the most efficient clean coal technologies,
IGCC being one of them, global carbon dioxide emissions could be cut by more
than half, compared with the levels that would be emitted by the existing power
plant technologies, i.e. pulverized coal fired.
The expert group on IGCC
technology appointed by Govt. of India has prepared a Techno-Economic
Feasibility Report (TEFR) in the year 1991 comparing the operational
performance and economics of IGCC and PC based power generation for a 600 MW
capacity plant with 35% ash coal. IGCC is more efficient, pollution is very
less and capital and generation costs are comparable with PC plant.
IGCC technology is now moving from
drawing board to commercial scale. A 250 MW IGCC plant of Tampa Electric Co.
USA has successfully completed one year of commercial operation. The Wabash project in USA of 262 MW IGCC plant began its
commercial operation in November 1995. Sierra pacific pinion pine IGCC project,
USA of 107 MW capacities is undergoing operation trials. A 250 MW IGCC plant at
Buggenum , Netherlands
3.4 Integrated
Gasification Fuel Cell
Fuel cell is the most efficient
and the least polluting system of power generation. Out of the 3 fuel cell
systems based on the type of electrolyte used i.e. Phosphoric Acid Fuel Cell
(PAFC) Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC), the
latter two are suitable to utilize coal gas which resulted in the development
of Integrated Gasification Fuel Cell (IGFC) System. PAFC is nearly commercial
and the other two (MCFC and SOFC) are at development stage.
IGFC can attain efficiencies up
to 60% and are cool enough to prevent NOx formation. Sulfur and
particulate present in the coal are removed during the gasification process
before feeding the fuel gas to the fuel cell. A comparison between the
emissions of a coal fired conventional power plant and IGFC system is given in
the table 4, which shows that fuel cell, generates extremely clean power.
There are two major challenges with
respect to commercialization of fuel cell: initial cost and reliable life. The
two problems have to be solved to improve the economics of fuel cell.
3.5 Other Technologies
In addition to IGCC, two other
relevant technologies for power generation are: Pressurized Fluidized Bed
Combustion (PFBC) and High Concentration Coal Water Slurry (HCCWS). The PFBC
technology is demonstrated in 80-100 MW scale abroad. The PFBC is dependant on
hot gas cleaning for the removal of particulate from the flue gases or on a
heavy-duty gas turbine, which can tolerate particulate matter in the flue
gases. Both hot gas cleanup and heavy-duty gas turbine are under development.
IGCC also incorporates a hot gas cleanup system, which increases the overall
efficiency, but wet scrubbing by water can be employed in place of hot gas
cleanup with some loss in efficiency. Thus PFBC compared to IGCC is constrained
by availability of hot gas cleanup technology. Another major disadvantage with
PFBC is that more power is generated from steam turbine, which is less
efficient compared to gas turbine. Whereas in IGCC, more power is generated
from the gas turbine and hence is more efficient. Continuous developments are
taking place in the gas turbine technology, which could result in higher
efficiencies in IGCC beyond 50%. Such improvements in the steam turbine are
limited.
HCCWS consisting of 70% solids and
30% water is used for power generation either through combustion or gasification
route. High ash content in the coal thermally penalizes the conversion
processes of coal slurry resulting in lower and uneconomical efficiencies.
Therefore the coals have to be necessarily washed to bring down the ash content
to around 15% to improve the efficiency and economics. But the cost of
preparation of slurry itself depends upon the techno- economics of washing,
which are at present unattractive for high ash coals. Thus application of HCCWS
technology to high ash coal mainly depends on techno economics of washing the
coal.
3.6 Synthetic Oil
The commercial application of
liquefaction of coal for production of oil had taken place in unusual
circumstances where price had been a less critical factor than secured or
strategic supply. This was in Germany South Africa US UK Japan
The consumption of petroleum
products in India
The indigenous petroleum
production, which had also grown, correspondingly from 6.8 mt in 1970-71 to
32.9 mt in 1996-97 could not meet the demand. To bridge the gap India India India
As experienced on several
occasions, petroleum supplies and prices are greatly influenced by political
policies. There have been several price hikes in oil since 1971. In all the
cases, India India India
The success of liberalization
policy and economic reforms introduced in the country is largely dependent on
adequate availability of energy resources at affordable prices and oil has a
significant place in it. Therefore any disruptions in oil supplies would hamper
progress of the country. Thus from consideration of national self-reliance,
security and assured energy supply, production of oil in India
There are 3 routes for
liquefaction of coal i.e., directs hydrogenation of coal, hydrogenation of coal
tar and gasification followed by Fisher-Tropsch (F-T) synthesis. The process of
direct hydrogenation of coal requires coal with very low ash content, below 10%
and hence is not suitable to India South Africa India
The option of conversion of coal
into synthetic liquid fuels had been under the consideration of the Government
of India since 1950. Several committees appointed by the Government had
examined the feasibility of producing oil from coal and recommended for setting
up commercial plants of capacity up to 1 million tonnes per year in the
country. The proposals did not materialize mainly due to economic reasons. For
example the estimates made in 1982 had indicated a capital cost of Rs.16, 110
million ($1667 million) for 1 mtpa plant and cost of production of $35 per
barrel.
Coal accounts for 69% of world's
fossil fuel reserve, while it is only 17% for oil [8]. Major consumption of oil
is in the transport sector. Hence many scientists and engineers believe that
coal liquefaction would be required to meet the demand for liquid
transportation fuels. However, world development of coal liquefaction will
depend upon both economics and the reliability of petroleum and natural gas
supplies from the Middle East and other main
exporting areas.
3.7 Natural Gas
Substitution
The emerging role of natural gas
in Indian energy sector is evident from the increase in its share in primary
energy supply from 2% in 1989 to 8% in 1996 and also increase in its production
from 3.1 to 19.4 billion cubic meters during the same period. Natural gas is
used in India Delhi
3.8 Steel Making
A serious concern regarding
coking coal resources in the country is the limited reserves of prime coking
coal and its quality, the gross reserves of which are estimated at 5.3 billion
tonnes (2.6% of total coal reserves). Bulk of this prime coking coal is of high
ash content and requires washing prior to coke making. The reserves of medium
coking and semi coking coals are relatively bettered placed at about 18 billion
tonnes. But in view of the limited reserves of prime coking coal, the medium
and semi coking coal cannot be used in its totality as ternary blend for
manufacture of metallurgical coke. As a result, India India
3.9 Coal chemicals:
Coal was the main source for a
variety of chemicals like benzene, toluene, xylene, naphthalene, anthracene,
phenol etc. till the Second World War. These chemicals present in coal tar
obtained by carbonization of coal were the raw material for the production of
pharmaceuticals, dyes, resins, plastics, and explosives. The first polyethylene
plant of Dupont was based on ethylene from coal gas. There are three important
routes to convert coal to useful chemicals; one of them is gasification
technology to produce synthesis gas as a feed material for chemicals. The wide
spectrum of possible chemicals from synthesis gas includes ethylene, methanol,
formaldehyde, acetic acid, ethyl acetate, etc. The synthesis gas opens up the
field for C1 chemistry. At Sasol in South Africa
Ammonia has also been made from
the hydrogen present in the synthesis gas obtained from gasification of coal. A
number of such coal based fertilizers plants have been setup in many countries
including 3 plants in India
Petroleum and natural gas are
currently the principal sources of basic organic intermediates namely ethylene,
propylene, butadiene, benzene, toluene, xylenes and methanol. The organic
chemical industry, which depends upon petrochemical building block, could face
serious feedstock problems due to any disruptions in oil supplies. In such
situations, synthetic gas from coal can be a suitable feedstock. Technologies
are available in India
Strategy
in Development of Coal Gasification Plant in India
Large amounts of capital are
required to setup commercial plants to produce fuel gas from coal gasification
as a feedstock for gaseous and liquid fuels and chemicals etc. Majority of the
technologies available on commercial scale for productions of the above
products are based on coals with different characteristics than that of Indian
coals specially ash content and ash fusion temperature, which are low.
Therefore these technologies cannot be applied directly to Indian coals but
have to be adapted through indigenous demonstration and R&D before setting
up large commercial scale plants in the country. Thus India India
An option that can be followed
by India India India
CONCLUSION: Coal is relatively a large fossil fuel reserve in India India India India
Coal gasification opens
up several avenues for coal utilization and enables to use coal as a raw
material to improve the economics through cogeneration/co-production of
electricity, chemicals and liquid fuels etc.
Fossil fuels (coal, oil and
gas) are exhaustible, hence they need to be utilized judiciously through the
principle of sustainable development and coal gasification route is the best
option to achieve it.
DEVELOPMENT OF COAL GASIFICATION
TECHNOLOGY
FOR
FUEL CELLS:
Fuel cells are
electrochemical devices that convert the chemical energy of a fuel directly and
very efficiently into electricity (DC) and heat, thus doing away with
combustion. The most suitable fuel for such cells is hydrogen or a mixture of
compounds containing hydrogen. A fuel cell consists of an electrolyte
sandwiched between two electrodes. Oxygen passes over one electrode and
hydrogen over the other, and they react electrochemically to generate
electricity, water, and heat.
1) INTRODUCTION
Development of high-efficiency
direct-power-generation technologies, such as molten carbonate fuel cells
(MCFC) and solid oxide fuel cells (SOFC), is being promoted because they are
expected to become next-generation power generating technologies.
Meanwhile, coal also is expected to become a more important energy
resource because it has abundant reserves that are more evenly distributed
territorially than those of other energy resources. The ash contained in coal
is the greatest obstacle when coal is used in fuel cells. Thus coal must be
supplied to fuel cells after conversion into an ash-free fuel gas. The
objectives of this project are to develop an optimum coal gasifier for fuel
cells and to establish a clean-up system that purifies the gas to a level
acceptable for fuel cells.
Table 1 shows the development schedule. A feasibility study of the
integrated coal gasification fuel cell combined cycle (IGFC) was conducted in
fiscal year 1995 to obtain data necessary for two applications: design of a
coal gasifier in fiscal years 1995–96, and design, both basic and detailed, in
fiscal years 1996–97, of a pilot plant for processing 150 tons of coal per day.
In 1998, construction was started at the test site, the Wakamatsu Operations
& General Management Office, and manufacturing commenced on the gasifier
and other main facilities. This paper describes the performance of IGFC using
MCFC, and gives the status of construction of a pilot plant.
2) PERFORMANCE OF IGMCFC IN 1995
2.1 Outline of the System
The integrated coal gasification
MCFC combined cycle (IGMCFC) is composed of a coal gasification unit, a gas
clean-up unit, an MCFC unit, and a power island as shown in Figure 1.
Coal
gasification
Pulverized coal is transported
by nitrogen to the gasifier, where it reacts with a gasifying agent (95%
oxygen) at 26.5 atm and is converted into a fuel gas. Meanwhile, molten ash is
discharged from the bottom of the gasifier into a water quench. The
high-temperature syngas exits the gasifier and is forwarded to a gas clean-up
unit after heat is recovered by passing it through a syngas cooler, lowering it
to 450°C. Char in the syngas is removed by a cyclone and filter, and recycled
to the gasifier by the syngas.
Gas clean up
Cold gas clean up must be
applied in order to meet the strict tolerance limits of fuel cells. Impurities
such as halogens and sulfur in the syngas are removed by a water scrubber and a
methyldiethanolamine (MDEA) absorber, and the syngas is finely desulfurized by
use of zinc oxide (ZnO). Acid gas removed by the MDEA absorber is burned in air
in a furnace and the sulfur content is recovered as gypsum by use of limestone.
Because the operating pressure of fuel cells is set at approximately 15 atm, to
match that of the gas turbine, the pressure energy of the syngas is recovered
as power through an expansion turbine.
Fuel cell (MCFC)
In coal gas, the CO concentration
is high while the H2 and H2O concentrations are low. Because of this, there is
a possibility that carbon will precipitate at the electrode through the
following reaction and that cell performance will drop. Therefore, steam is
added and the anode exhaust gas is recycled to the anode inlet in order to
increase the H2O and CO2 concentrations at the anode inlet and prevent
precipitation of carbon. The anode exhaust gas is burned in a catalytic burner.
The CO2 produced in the catalytic burner is supplied to the cathode. Meanwhile,
the fuel cell is cooled down by recycling part of the cathode exhaust gas to
the cathode inlet through a heat recovery boiler.
Power island
The cathode exhaust gas (700°C)
is sent to the gas turbine and the gas turbine inlet temperature is increased
to 1,300°C by adding the syngas. Heat is recovered from the gas turbine exhaust
gas through a Heat Recovery Steam Generator (HRSG), and the generated steam is
sent to the steam turbine (150 atm, 538°C) where it combines with steam
generated at the gasification unit and the fuel cell unit.
2.2 Parameter Study
Conditions examined in the
parameter study are shown in Table 2. The anode recycling gas ratio is defined
as the ratio of the anode recycling gas volume to the syngas volume supplied to
the MCFC. The results of examining oxygen concentration as the gasifying agent
are indicated in Figures 2 and 3. Here, the oxygen concentration is adjusted by
mixing 95% pure oxygen with air extracted from the gas turbine. In this study,
the number of fuel cell stacks and the gas turbine inlet temperature are kept
constant.
Therefore, if the oxygen
concentration falls, the calorific value of the syngas will be reduced, thereby
increasing the fuel supply to the gas turbine. The coal supply will then
increase accordingly. As a result, power output from the gas turbine and steam
turbine will increase even though the fuel cell power output is nearly
constant. As has been discussed, because the power output ratio of the fuel
cell is reduced as the oxygen concentration falls, the thermal efficiency is
also reduced. In view of the foregoing, the oxygen concentration was set at 95%
in this feasibility study. Optimization was similarly applied to other
parameters.
Parameter Base case Variation range
O2 conc. in gasifying agent 95 vol% 21 –
95 vol%
Fuel utilization*1 80% 50 – 85%
Oxidant utilization*1 20% 15 – 35%
Anode recycling ratio 6.6 6.0 – 7.4
*1: Utilization with one pass is
indicated