Initial Post: 05_25_2008
The coal
energy source in its purest form is simply fossilized carbon, which
are formed in excess of 100 million years ago from once living
plants. Generally these plants die, or otherwise, and are buried
underground. Such usually happens in cataclysms and as a result the
plant trunks do not rot naturally, instead the trunks get submerged
under the ground and are rapidly covered up with earth. The high
intense pressure squeezes the water of the plant sap out of the
trunks living only the non-liquid matter. Intense heat underground
then begin the process of vaporizing the volatile materials out of
the trunk. After the volatiles, all other materials with low latent
heat of vaporization and low temperature, then become gasified and
effuse out of the plant remains. Ultimately mostly only the carbon
components of the plant matter and some remnants of the volatiles
and low-temperature boiling materials remain. The high pressure and
high temperature then continue to force a structural change of the
carbon matter resulting the material known as coal in this evolution
of the earth.
Besides the innate materials
of the plants, as of the time of submergence underground,
which define the components of the residue of the geochemical
transformation products, the earth materials of the region suffering
the geological activities also occasionally get thrust into the
plant trunk leaving the residue traces of other chemicals that are
ordinarily not innate materials of the plant. As a result, the form
in which the coal obtains in the earth, is a composite of carbon,
and metallic and non-metallic elements.
The length of time during
which the coal had been subjected to the intense temperature and
pressure also affects the nature of the coal. As a result coal is
classified or categorized into four types or rank: Lignite,
Sub-bituminous, Bituminous, and Anthracite; and have varying coal
content as well as heat energy content but the heat energy content
does not necessarily correlate with the coal contents, although for
the most part, the higher ranks of coal contain more heat-energy. The impact of the high pressure and temperature on the coal
content and heat energy content has been
well documented.
On the other hand the trace
chemicals found in coal are independent of the time of exposure of
the coal to the extreme temperature and pressure. The specific
concentrations of these elements depend on the area of the world and
the mine from where the coal is mined, as determined through extensive studies and
analyses. The most common impurities are mercury and sulphur. One of
the issues of coal processing therefore is the removal of these
impurities as to avoid health hazards on people living downstream of
the flue gases as well as prevention of atmospheric pollution.
In general when coal is
deployed for energy generation, the process occurs at such a high
temperature that these impurities are discharged as either vapours
or as oxides of the element. Moreover, Nitrogen oxides also are
formed because of the high temperature of operation.
The coal in the earth is
obtained by mining, which entails the digging up and out of the
coal. Modern mining methods allow us to easily reach most of
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our coal
reserves. Two methods: surface Mining and Underground Mining; are
used to mine the coal from the earth.
- Surface mining
is used to produce coal wherever and whenever
the coal is buried less than 200 feet underground. With this mode of
mining, giant mining machines remove the top-soil and layers of rock to
expose the large beds of coal beneath. Once the mining is finished, the dirt and
rock are returned to the pit, the topsoil is replaced, and the area
is replanted, and often, the
land is reused for other human needs.
- Underground
mining, also known as deep mining,
is used when the coal is buried several hundred feet below the
surface. Some underground mines are 1,000 feet deep. To remove coal
in these underground mines, miners ride elevators down deep mine
shafts where they run machines that dig out the coal.
Generally surface mining is
less expensive than underground mining, and due to growth in surface
mining and improved mining technology, the productivity of miners
has tripled over the years.
Energy Type:
The energy form of coal is chemical and as such
obtains through the breakage of chemical bonds. Specifically this is
accomplished through a chemical reaction between the carbon - the
chemical make-up of coal - and
oxygen. The reaction gives off heat energy and is of the class of chemical
reactions generally termed exothermic reaction, of a fairly
extensive reaction chemistry. During the reaction, one
part of carbon combines with two parts of oxygen and forms carbon
dioxide.
Energy Adoption
Carbon, the primary chemical
of coal, partakes in myriads of reactions, as such though carbon
reacts with oxygen and generates/gives off heat the process of
adopting the coal energy is just as varied.
The Energy Adoption-Technology Analysis, presents for consideration
two situations: The
preferred
approach of converting the coal energy is to produce steam
in some form of boiler or steam generator and to then drive a
turbine with the steam to produce electricity, The conversion of the
coal energy to Carbon Monoxide with subsequent reaction with oxygen
to produce heat energy. The specifics and engineering designs
incorporating these two approaches have been many.
Irrespective of the equipment
design adopted for the conversion of the coal in the
process, the process has a fairly standard concept structure. By
this layout, the process begins with the reaction involving the
coal, the flue gas is then fed in sequence into equipment to be
processed for sulphur dioxide, mercury, nitrogen oxides and other
mineral oxides as may be necessary. Except for minor variations this
is the basic structure; and it is within and from this structure
that all innovations have been and are being made.
Recently the downstream
section of the process has suffered changes in accommodation for the
mitigation of the global warming
climatic changes.
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The apparent
development is that most
equipment or mini-processes engineered to
mitigate greenhouse gases
discharge by existing coal conversion processes will be retrofitted
to this end section of the coal burner process.
The coal burner proper
however, has been developed over the years in many different
equipment . Yet all the burner equipment can be categorized under
three main design: Coal Fired Burner in which the coal is pulverized
and sprayed into the furnace that is jacketed by an annular water
shell; Gasification Burner in which the coal is reacted with steam
to convert it into carbon monoxide, methane, and other trace gases;
Co-Generation Burner in which the Gasification Burner is also
jacketed with an annular water shell.
The Coal Fired Burners
generally have the standard design as described above. This is the
preferred approach for most large scale power generation operations.
Power station burners pulverise the coal and blow it into the
combustion chamber as a fine dust. Obviously, the mixing of fuel and
air is much better with pulverised coal and the combustion reaction
is both quicker and cleaner. However in the flue gas, sulfur dioxide
is always present to some degree; the concentration of nitrogen
dioxide may also be elevated due to nitrogen-bearing compounds in
the coal; and with lignite coal which may contain free water, water
vapour that is present, presents problem, boosting the level of condensate
greatly
Gasification Burners is
preferred, however, for distribution of the heat energy of coal.
Again the coal is pulverised and blow into the combustion chamber as
a fine dust together with oxygen as with the coal Fired Burner,
however, but in contrast in this process the quantity of oxygen is
tightly controlled to prevent complete combustion of the coal
particles. Again, obviously, the mixing of fuel and oxygen is much
better with pulverised coal and the combustion reaction is both
quicker and cleaner. As a result the effluent gas, called Syngas, is
a mixture of carbon monoxide and hydrogen and traces of gases due to
the minerals in the coal. Steam instead of oxygen is also used
in this type of Coal Burner in some cases. This mode of operation
produces a lot more hydrogen than the use of the oxygen as
co-reactant, amongst other benefits. In this case both the coal
energy and steam energy for the gasification reaction are stored in
the syngas and hydrogen which are distributed in cylinders for uses
in remote operations by both consumers and industrial.
Cogeneration Coal Gasification
Burner is a hybrid of the Coal Fired Burner and the Gasification
Burner. In essence this Burner uses purely steam for the
gasification of the coal, using the heat energy in the steam to
ignite and support the combustion reaction. However to make the
entire process self-sustaining, some of the syngas is burnt to
generate the steam that is used to convert the coal into the syngas.
In this circumstance then, only the start-up energy needed to
generate the initial reactant steam is required to kick-start the
processes.
Global warming and other
environmental issues that need to be mitigated then must be attacked
within this context of the existing designs
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