Very
special circumstances were required for the formation of fossil fuels that are
so important to our modern life. Coal, oil, and gas are organic materials, that
is, they are plant and animal remain composed mostly of reduced (i.e., hydrogen
rich) carbon as is all life. As plant life evolved some 3 billion years ago a
great deal of organic material was formed, most of which was oxidized
relatively soon and turned back to carbon dioxide in the atmosphere, which is
not an energy source but is available for new plant growth. Some very small
part of this organic material found its way to anaerobic (meaning without
oxygen) basins, such as deep lakes or marine areas for oil and gas, and hence
accumulated as various deposits.
Chemically
natural gas (methane) has four molecules of hydrogen per molecule of carbon,
oil has about equal amounts, and coal is mostly carbon, although with small
amounts of hydrogen and sulfur and trace amounts of many elements, including
troublesome mercury and uranium. Thus, there is a progressive increase in CO2
per unit of energy delivered from natural gas to oil to coal, with coal
releasing almost double the CO2 per heat unit relative to gas.
The
creation of exploitable oil and gas fields has been quite rare in the geologic
past. It happened mostly some 90 and 150 million years ago when the Earth was
very warm, and in very special and limited environments. The time required to
turn the organic source material into oil and gas is extremely long and
requires the organic material being buried at just the right depth (about 3000
m or two miles) and temperature (about 100 °C) to “pressure cook” the organic material
into oil. As a consequence, significant quantities of commercially exploitable
oil and gas are found in only a relatively few regions of the Earth’s surface.
Coal, formed in great fresh water swamps, required far less stringent
conditions for its production and is more common. Gas too is widely dispersed,
but large reservoirs are relatively rare. On the other hand, gas is found
widely at low concentrations associated with coal and in “tight” shales and
sandstones. Exploitation of these diffuse resources is becoming increasingly
important as the large gas fields found earlier face serious depletion. Whether
or not these newer “unconventional” fields can maintain production at the
present level for very long is unknown at this time.
In
this developing world, where the coal reserves are still far greater when
compared to oil and gas and hence, on an average 2 coal-based power stations
are being constructed in a week, fossil fuels will continue to serve as a
primary energy source for major parts of the world for this century.
As
demonstrated in Fig. by International Energy Agency (IEA), fossil fuels fulfill
more than 80 % of the primary energy needs; remaining portions are made up of
atomic- and hydro-electricity, and renewable energy (like wind and solar
energy, geothermal energy and biomass). Countries like United Kingdom which are
having nuclear power and renewable energy sources also have large coal
deposits. These deposits might serve as a suitable security in the future as an
energy source.
Fossil
fuels originate from terrigenous and aquatic plants and animals deposited and
preserved in oxygen-limited and/or acidic environments then buried so that the
organic matter is not oxidized and returned to the biosphere as CO2. Coals
usually come from woody plants deposited in swamps and bogs, while petroleum
comes primarily from phytoplankton deposited in lakes and on continental
shelves.
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