Conventional sources of Energy

1. CHAPTER 7
Coal
Introduction
Coal is a combustible black or brownish-black sedimentary rock normally
occurring in layers or veins referred to as coal beds or coal seams. It is com-
posed primarily of elemental carbon along with variable quantities of other
elements—chiefly hydrogen, with smaller quantities of sulfur, and even smaller
quantities of oxygen and nitrogen.
Coal was formed from the energy that plants absorbed from the sun millions of
years ago. These living plants converted solar energy to living plant materials
through a process known as photosynthesis.

2. Introduction
Under both heat and the increased pressure, the plant matter underwent
chemical reactions and physical changes. This resulted in leaving rich
hydrocarbon deposits; in effect, the plants gradually turned into coal.
As the preceding physical and chemical processes continued, the plant decay
matter was successively transformed into:
1. Peat, the first product in the formation process of coal, is a heterogeneous
material consisting of partially decomposed plant and mineral matter. Its
color ranges from yellow to brownish black, depending on its geologic age.
Peat has a moisture content up to 70 percent and a heating value as low as
3,000 Btu/lb (7,000 kJ/kg).

3. Introduction
2. Lignite is the lowest ranking coal. Lignites are relatively soft and brown to
black in color with heating values less than 8,300 Btu/lb (19,000 kJ/kg).
The moisture content of lignites is as high as 30 percent, but the volatile
content is also high; consequently, they ignite easily.
3. Sub-bituminous coals are black, having little of the plant-like texture and
none of the brown color associated with the lower rank lignite coal. Sub-
bituminous coals are noncoking (undergo little swelling upon heating) and have
a relatively high moisture content, which averages from 15 to 30 percent.
Because they have reason- ably high heating values (8,300 to 11,500 Btu/lb
[19,000 to 27,000 kJ/ kg]) and low sulfur content, switching to sub-bituminous
coal has become an attractive option for many power plants to limit SO2
emission (which can, however, adversely affect some air pollution control
devices (e.g., electrostatic precipitators)).

4. Introduction
4- Bituminous coal is the rank most burned in electric utility boilers. In general,
it appears black with banded layers of glossy and dull black. Typical
bituminous coals have heating values of 10,500 to 14,000 Btu/lb (24,500 to
32,500 kJ/kg) and a fixed carbon content of 70 to 85 percent.
5- Anthracite. Anthracite, the highest rank of coal, is shiny, black, hard, and
brittle, with little appearance of layers. It has the highest content of fixed
carbon, 86 to 98 percent. However, its low volatile content makes it a slow
burning fuel. Most anthracites have a very low moisture content of
approximately 3 percent; heating values of 15,000 Btu/lb (35,000 kJ/kg) are
slightly lower than the best quality bituminous coals.

5. Introduction
The chemical and physical process referred to previously that transformed
plant debris to coal is referred to as coalification.
The degree of coalification was determined by:
1. Varying types of vegetation from which the coal originated
2. Depths of burial
3. Temperatures and pressures at those depths
4. Length of time the coal has been forming in the deposit

6. History
Coal in many ways was the fuel of choice during the Industrial Revolution as
the steam engine replaced the water wheel. Coal was used during the
Industrial Revolution for metallurgical processes, glassmaking, fuel for
railroads and, in general, for the steam engine.
Today, approximately 95 percent of coal use in the United States is to produce
electrical energy; however, coal use is currently in decline in the U.S. due to
regulatory constraints.

7. Availability/Distribution
It has been estimated that there are nearly one billion tons of proven coal
reserves worldwide. This level of availability could last over a century at current
rates of use.
Resources are the amount of coal that may be present in a deposit or
coalfield.
Reserves can be defined in terms of proven (or measured) reserves and
probable reserves.
Peak coal is defined as the point in time at which the maximum global coal
production rate is reached after which the rate of production will enter
irreversible decline.

8. Coal exploration
Coal reserves are discovered through standard exploration activities. The
process usually involves creating a geological map of the area and then
carrying out geochemical and geophysical surveys, followed by some early
exploration drilling. This allows an accurate picture of the area to be
developed.
The area becomes mined if it is large enough and if it contains sufficient coal
deposits that can be economically recovered. Once this has been confirmed,
mining operations begin.
Coal also plays a vital role in electricity generation at the international level.
Coal-fired power plants currently fuel 41 percent of global electricity.

9. Characterization
Coal is heterogeneous and can vary in chemical composition by location. In
addition to the major organic ingredients (carbon, hydrogen, and oxygen), coal
also contains impurities. The impurities that are of major concern are ash and
sulfur.
The system used in the United States for classifying coal by rank was
established by the American Society for Testing and Materials (ASTM).
ASTM classification is a system that uses the volatile matter and fixed carbon
(FC) results from the proximate analysis and the heating values of the coal as
ranking criteria.
There are many other moisture-related terms that arise when characterizing
coal, including equilibrium, free, and air-dry moisture. Their definitions and use
depend on the application.

10. Characterization
Equilibrium moisture is sometimes used as an estimate of bed moisture. The
ASTM standard procedure for moisture determination, D 121, defines the total
coal moisture as the loss in weight of a sample under controlled conditions of
temperature, time, and air flow. Using ASTM D 3302, the total moisture is
calculated from the moisture lost or gained in air drying and the residual
moisture.
Residual moisture is determined by oven drying the air- dried sample.
Because subsequent ASTM analyses (such as proximate and ultimate) are
performed on an air-dried sample, the residual moisture value is required to
convert these results to a dry basis.
Proximate analysis, as described in ASTM D 3172, includes the
determination of volatile matter, fixed carbon, and ash.

11. Characterization
Ultimate analysis, as described in ASTM D 3176, includes measurements of
carbon, hydrogen, nitrogen, and sulfur content plus the calculation of oxygen
content.
Heating value is defined as the gross caloric value of coal, determined using
an adiabatic bomb calorimeter as described in ASTM D 2015, is expressed in
Btu/lb (kJ/kg) on various bases (dry, moisture, ash free, etc.).
Gross (higher) heating value (HHV) is defined as the heat released from
combustion of a unit fuel quantity (mass), with the products in the form of ash,
gaseous CO2, SO2, nitrogen, and liquid water, exclusive of any water added as
vapor.

12. Extraction
There are two traditional methods to remove coal from the Earth:
1- Deep (underground) mining
2- Surface mining
Usually, the most valuable coal must be extracted from deep into the
Earth’s crust.
In underground mining, workers and machinery go down a vertical shaft
or a slanted tunnel called a slope to remove the coal. Mine shafts may
sink as deep as 1,000 feet below the surface.

13. Extraction
Method of underground mining:
1- room-and-pillar mining
2- long-wall mining
Coal is removed by conveyor belts. The roof is held in place by steel supports,
which are moved as the cutting progresses, and the rock is gradually allowed
to fall into the space vacated by the coal. Automation has improved efficiency
and greatly improved safety.

14. Processing
After coal comes out of the ground, it typically goes on a conveyor belt to a
preparation plant that is located at or near the mining site. The plant cleans
and processes the coal to remove dirt, rock, ash, free sulfur, and other
impurities, which increases the heating value of the coal to be transported.
At a typical power plant, the coal is combusted, liberating its heating value.
This occurs in a boiler where the released energy is transferred to steam at an
elevated pressure. The high-pressure steam is passed into a turbine
containing thousands of propeller-like blades. The steam pushes these blades,
causing the turbine shaft to rotate at high speed.

15. Processing
A generator is mounted at one end of the turbine shaft and consists of carefully
wound wire coils. Electricity is generated when these are rapidly rotated in a
strong magnetic field. After passing through the turbine, the steam is
condensed and returned to the boiler to be heated once again. The electricity
generated is transformed into voltages as high as 400,000 V, with transmission
via power line grids. When it nears the point of consumption (e.g., industry or
homes), the electricity is transformed down to the safer 100–250 V systems
used in most markets.

16. Components of Thermal Power Plant
In a thermal power plant, various components are used in the cycle. Here we
have listed, main components of the thermal power plant.
•Boiler
•Turbine
•Super-heater
•Condenser
•Economizer
•Feedwater pump
•Alternator
•Chimney
•Cooling tower

17. Transportation/Transmission
After the coal is mined and processed, it is ready to go to market. It is very
important to consider transportation when comparing coal with other energy
sources since transporting the coal can often cost more than mining it.
Steel mills were built close to the coal mines in the early days of the Industrial
Revolution.
Much of Europe’s coal is transported by small, efficient ships that travel the
various rivers and canals. Most of the coal used in the Midwest and on the Gulf
Coast is transported by barge.
As noted earlier, coal can also be transported as slurry in a pipeline. Water
used for this purpose is separated and recovered at the end of the pipeline.
Many power plants are located near the coal mine so that coal can be
delivered directly from the mine to the coal bunkers by conveyor belt.

18. Environmental Issues
Air pollution generated from coal extraction through electric energy production
is a main concern from an environmental regulatory perspective in the United
States.
Standards are modified from time to time. A standard of performance is a
standard for emissions of air pollutants that reflects the degree of emission
limitation achievable through the application.
The coal industry’s largest specific environmental challenge today is removing
organic sulfur.
Another potential air emission problem is carbon dioxide.
Carbon dioxide combines with other gases, such as those emitted from
automobiles, to form a shield that allows the sun’s light through the
atmosphere but does not let the heat that is produced out of the atmosphere.
This phenomenon is called the greenhouse effect.

19. Future Prospects and Concerns
On the positive side are relatively new processes involving coal:
1- Coking of coal is a solid carbonaceous residue from the burning of
bituminous coal. It is used as a reducing agent in iron ore smelting operations
and as a new fuel.
2. Gasification of coal can be used to produce syngas, a valuable mixture of
both carbon monoxide (CO) and hydrogen (H2) gas.
(Coal) + O2 + H2O → H2 + 3CO
3. Liquefaction occurs when coal is converted into liquid fuels, such as
gasoline or diesel by several different processes. In the direct liquefaction
processes, the coal is either hydrogenated or carbonized.
4. Refined coal is the product of a coal-upgrading technology that removes
moisture and certain pollutants from lower rank coals, such as sub-bituminous
and lignite coals.

20. Future Prospects and Concerns
Five key areas of uncertainty that affect the coal industry today, from an air
pollution perspective, are the following:
1. CO2 emissions
2. SO2 emissions
3. NOx emissions
4. Mercury
5. Other air toxics
Coal is an important raw material for specialized products. The following are
some specific examples:
1-Activated carbon is used in filters for water and air purification and in kidney
dialysis machines.

21. Future Prospects and Concerns
2- Carbon fiber is an extremely strong but lightweight reinforcement material
used in construction, airplanes, mountain bikes, tennis rackets, race cars, etc.
3- Silicon metal is used to produce silicones and silanes, which are in turn
used to make lubricants, water repellents, resins, cosmetics, hair shampoos,
and toothpastes.
4- Carbon nanotubes are seamless cylinders composed of car- bon atoms in a
regular hexagonal arrangement, closed on both ends by hemispherical
endcaps. They can be produced as single- wall nanotubes (SWNTs) or
multiwall nanotubes (MWNTs). While carbon nanotubes can be difficult and
costly to produce and challenges remain when it comes to distributing and
incorporating them homogeneously within the matrices of other materials, they
exhibit unprecedented structural, mechanical, and electronic properties.
Despite these concerns, coal almost certainly will continue to play an important
role in the United States in this decade.