FOSSIL FUELS
The three main fossil fuels are oil, coal and natural gas. They are called fossil fuels
because they were made over millions of years from the remains of plants and animals.
They release energy when they are burned. In the U.K. about 85% of the energy we use
comes from fossil fuels, and our way of life depends on them.
Gas
23%
Oil
36%
Nuclear
4%
Hydroelectric
1%
Coal
36%
The Earth’s supply of fossil fuels is limited. Once used up, they cannot be replaced
because they form so slowly. They are, therefore, non-renewable energy sources. The
more economical people are with these fuels, the longer they will last.
CRUDE OIL
FORMATION OF OIL
Crude oil is obtained from the Earth's crust. It is often found together with gas. It was
formed from the remains of marine organisms which lived millions of years ago. When
these creatures died, they sank to the bottom of the sea and were slowly covered by
mud and sand. The mud and sand gradually hardened to form sedimentary rock. When
subjected to heat and pressure over millions of years, in the absence of air, the
organic remains gradually changed to oil.
IMPERMEABLE ROCK
GAS
OIL
WATER
POROUS ROCKS
TOPIC 10.1.4: CRUDE OIL & FUELS 1
Over time, oil percolates upwards through
porous rocks and would eventually rise to
the surface unless it was stopped by an
impermeable layer of rock. When the
impermeable layer is dome-shaped (an
anticline) oil and gas collect beneath it,
allowing a reservoir to form. Such
reservoirs are often hundreds or even
thousands of metres below the surface,
and the oil and gas are obtained from them
by drilling deep wells. These wells can be
on land or under the sea.
Like coal, gas and wood, crude oil consists of organic compounds; organic compounds
are compounds of carbon. Crude oil is a mixture of a very large number of compounds,
most of which are hydrocarbons.
A mixture contains two or more substances, which may be elements or
compounds, which are not chemically joined together.
A hydrocarbon is a substance which is made up of
carbon and hydrogen atoms ONLY.
ALKANES
The alkanes are a group of hydrocarbons which all have the same general formula
(CnH2n+2). Because they are hydrocarbons, they contain ONLY carbon and hydrogen
atoms. The atoms are joined by covalent bonds.
One covalent bond is formed by the sharing of two electrons between two atoms.
Carbon atoms form the spine of hydrocarbon molecules. Each carbon atom forms four
covalent bonds; each hydrogen forms one covalent bond.
When the carbon atoms are joined only by single covalent bonds, the molecule contains
the maximum possible number of hydrogen atoms for its particular number of carbon
atoms. The molecule is said to be saturated.
Alkanes are SATURATED hydrocarbons.
The first five alkanes are:
H
methane (CH4)
H
C
H
ethane (C2H6)
H
propane (C3H8)
H
H
H
H
C
C
C
H
H
H
TOPIC 10.1.4: CRUDE OIL & FUELS 2
H
H
H
H
C
C
H
H
H
Note that each carbon
atom has formed four
single bonds and is joined
to four other atoms. Each
hydrogen atom has formed
one bond.
butane (C4H10)
H
pentane (C5H12) H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
FRACTIONAL DISTILLATION
The properties of each substance in a mixture are unchanged. This makes it possible to
separate substances in a mixture by physical methods including distillation.
The many hydrocarbons in crude oil can be separated into fractions with a narrow range
of boiling points, each of which contains molecules with a similar number of carbon
atoms. This is done by evaporating the oil and allowing it to condense at a number of
different temperatures. This process is called fractional distillation.
The crude oil is heated to about 400oC and is then pumped into a tall tower called a
fractionating column. Most of the hydrocarbons have been converted to gases by the
heating and start to rise up the column. The lower the boiling point of a hydrocarbon, the
further it will rise up the column before it cools enough to condense. In this way, the
different fractions are collected at different points up the column. The number of
different fractions which are collected and the amount of each which is produced
depends on the source of the crude oil.
Most of the fractions from crude oil are burned as fuels.
The hydrocarbon molecules in crude oil vary in size. The larger the molecules (the
greater the number of carbon atoms) in a hydrocarbon:
. the higher its boiling point
. the less volatile it is
. the less easily it flows (the more viscous it is)
. the less easily it ignites (the less flammable it is)
This limits the usefulness of hydrocarbons with large molecules as fuels.
The quantities of the different fractions produced by fractional distillation do not usually
match up with the market requirements for each fraction. There is a shortage of light
TOPIC 10.1.4: CRUDE OIL & FUELS 3
fractions, especially gasoline and an excess of the heavier fractions. To resolve this
problem, some of the heavier fractions (larger molecules) are converted into lighter
fractions (smaller molecules) by cracking.
TOPIC 10.1.4: CRUDE OIL & FUELS 4
FRACTIONAL
DISTILLATION
PETROLEUM (bottled gases)
GASES
100oC

b.p.decreases

Mr decreases

size of molecule
decreases

viscosity
decreases

volatility
increases

easier to ignite
GASOLINE
(fuel for cars)
NAPHTHA
(feedstock for
petrochemicals
)KEROSINE
200oC
(fuel for jet aircraft)
GAS OIL
(diesel: fuel for cars
& large vehicles)
vapour
300oC
LUBRICATING
OIL & WAXES
CRUDE
OIL
VAPOUR
liquids
FUEL OIL
(fuel for ships &
industrial heating)
360oC
BITUMEN
(road surfacing)
TOPIC 10.1.4: CRUDE OIL & FUELS 5
Burning fuels
When a chemical reaction takes place between covalent compounds, existing bonds
have to be broken before new bonds can be formed. However, carbon-carbon and
carbon-hydrogen single bonds are strong bonds, which are not easily broken. This
means that alkanes do not easily undergo chemical reactions: they are UNREACTIVE.
POLLUTION PROBLEMS
When these substances burn, they combine with oxygen in the air and form oxides; they
are oxidised.
When hydrocarbon fuels are burned in the presence of an excess of air, the carbon is
oxidised to carbon dioxide and the hydrogen is oxidised to water (vapour). These
gases are then released into the atmosphere. Carbon dioxide is a major contributor to
the greenhouse effect.
,
methane + oxygen
CH4
+
O2
carbon dioxide + water
CO2
+
H2O
If the air supply is restricted when hydrocarbon fuels are being burned, carbon
monoxide is formed. This is a very toxic gas and can limit the amount of oxygen your
blood is able to carry.
Most fuels derived from crude oil contain some sulphur. When the fuel burns, the
sulphur is oxidised to sulphur dioxide. Sulphur dioxide contributes to acid rain.
At the high temperatures produced by burning, the nitrogen and oxygen in air react
together to form oxides of nitrogen. These are irritant gases which also contribute to
acid rain.
Cleaner fuels
By law new cars must be fitted with catalyic converters. These are expensive since
they contain precious transition metal catalysts, but convert these products of
combustion into less harmful ones. Filters can remove particulates from diesel engines
before they get into the atmosphere.
The coal and oil burned in power stations would release a lot of sulfur imputities which
can be removed by a process called flue gas desulfurisation.
TOPIC 10.1.4: CRUDE OIL & FUELS 6
Biofuels
A biofuel is a fuel that has been made from plant matter that is
recently dead.
Biodiesel
Biodiesel is extracted from plant oils and so are a renewable resource. It is considered
to be carbon neutral since all of the CO2 released during its combustion was removed
during photosynthesis by the growing crop. It can not be considered to be 100% carbon
neutral, however, as making the fertilizer to grow the sugar cane and fuelling agricultural
machinery to harvest and transport it, will invariably involve the burning of fossil fuels.
When burned, biodiesel produces very little sulphur dioxide and also breaks down much
more quickly than normal diesel.
One problem with this approach is that land previously used to grow plants for the
human food chain is now being used to grow plants to make biodiesel instead. This
could result in rising food costs, increased famine and further destruction of habitats.
Ethanol as a biofuel
Ethanol is now being used more and more a fuel. Ethanol produced by the process of
fermentation can be mixed with petrol to produce a biofuel. The ethanol produced in
this way is also considered to be carbon neutral.
TOPIC 10.1.4: CRUDE OIL & FUELS 7