1 Chapter 23Fossil Fuels 23.1Fossil fuels as a major energy source 23.2Coal 23.3Petroleum and natural gas 23.4Refining petroleum CONTENTS OF CHAPTER 23.

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Chapter 23 Fossil Fuels

23.1 Fossil fuels as a major energy source

23.2 Coal

23.3 Petroleum and natural gas

23.4 Refining petroleum

CONTENTS OF CHAPTER 23

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23.1 FOSSIL FUELS AS A MAJOR ENERGY

SOURCE

ENERGY AND FUEL

An important way of obtaining energy is to burn fuels.

23.1 FOSSIL FUELS AS A MAJOR ENERGY SOURCE

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Figure 23.1 Burning fuels to provide energy.


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A FUEL is a substance which is burnt to produce heat.

At present, the commonest fuels are fossil fuels.

WHAT FOSSIL FUELS ARE

Coal, petroleum and natural gas are collectively called fossil fuels.

They are so called because they were formed from the remains of

plants and animals that lived hundreds of million years ago.

Different fossil fuels have different properties. Yet they have

one thing in common — they all contain hydrocarbons.

(Hydrocarbons are compounds containing hydrogen and carbon

only.)


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23.2 COAL

23.2 COAL

The most abundant fossil fuel is coal. Coal is a black solid, usually

quite hard. It consists of 65% – 95% carbon, together with hydroc

arbons and some other compounds.

Figure 23.3

Coal is a black solid consisting mainly of

carbon. Its composition varies with the

type of coal.

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ORIGIN OF COAL

Coal was formed from the remains of plants that grew in swamps

250 million years ago. Due to movements of the Earth’s crust, the

plant remains were deeply buried under layers of mud and sand.

Under high pressure and temperature, the decaying plants

gradually turned into coal.

23.2 COAL

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Figure 23.4

Plants that lived 250 million years ago were buried and turned into coal that we burn today.

23.2 COAL

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Figure 23.5 The formation of coal.

23.2 COAL

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USING COAL

About two-thirds of the coal mined today is burnt in power stations

to generate electricity.

Coal can be changed to a number of useful substances by a

special process.

23.2 COAL

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Figure 23.8

The Lamma power station in

Hong Kong burns coal to

generate electricity. The

picture shows coal being

unloaded from a ship.

23.2 COAL

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23.3 PETROLEUM AND NATURAL GAS


Petroleum is a smelly oily liquid, usually quite thick; its colour rang

es from greenish brown to black, depending on where it is obtaine

d.

Petroleum is a complex mixture consisting mainly of hydrocar

bons.

Natural gas is also a mixture mainly of hydrocarbons. It consis

ts chiefly of methane CH4, with small amounts of ethane C2H6, pro

pane C3H8 and butane C4H10.

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Figure 23.9 A sample of crude oil.


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ORIGIN OF PETROLEUM AND NATURAL GAS

Petroleum and natural gas were formed from very small sea

animals and plants (e.g. planktons) that lived hundreds of million

years ago.


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Figure 23.10 Planktons are very small sea organisms.


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Figure 23.11 The oil story.


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A23.1

This is due to the movements of the Earth’s crust.

USING PETROLEUM AND NATURAL GAS

Petroleum is not burnt directly to supply energy. In fact, petroleum

must be treated or refined first. The process of separating

hydrocarbons in petroleum into several parts (fractions) is called

the refining of petroleum.

Unlike petroleum, most natural gas is burnt directly as a fuel. It

burns with a clean blue flame, causing little pollution.


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Figure 23.16 Natural gas burns with a clean blue flame.


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Uses of petroleum

At present, petroleum supplies about 40% of the world’s energy

needs.

natural gas25% petroleum

38%

others 16%coal 21%

Figure 23.17

Contribution of various

energy sources to the

world’s energy needs.


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Figure 23.18 Uses of petroleum.


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A23.2

Drugs (e.g. Panadol), soapless detergents, plastic items (e.g.

electrical socket), insecticides etc.

(Many answers are possible.)

Petroleum resource is running out

Petroleum resource is limited and non-renewable.


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23.4 REFINING PETROLEUM


WHAT IS OIL REFINING?

In oil refining, the complex mixture of hydrocarbons is separated

(by fractional distillation) into less complex mixtures which are use

ful. Fractional distillation can be used because the hydrocarbons h

ave different boiling points. In general, a hydrocarbon with larger

molecules has a higher boiling point. This is because van der Wa

als’ forces are greater between larger molecules.

These parts (simpler mixtures) are called fractions.

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Figure 23.19

Before and after refining of petroleum:

(a) Petroleum is a complex mixture, consisting of hundreds of hydrocarbons.

(b) A petroleum fraction is still a mixture, but it is a mixture simpler than petroleum.


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An OIL FRACTION is a mixture of hydrocarbons of similar boiling

points and other properties.

THE REFINING PROCESS

In an oil refinery, petroleum is first heated in a furnace to about

400oC. The hot oil, now partly liquid and partly vapour, is pumped

into the bottom of a tall tower called fractionating tower.


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Figure 23.20

Fractionating towers in an oil refinery.

(Each tower is about 30 meters high.)


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A typical fractional distillation of petroleum.

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The heavy fractions (with high boiling point ranges) condense in

the lower (hotter) compartments. The lighter fractions (with lower

boiling point ranges) condense in the higher (cooler)

compartments.

RELATIONSHIP BETWEEN BOILING POINT RANGE

AND NUMBER OF CARBON ATOMS IN

HYDROCARBONS


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Figure 23.23 The petrol fraction consists of C5 – C10 hydrocarbon molecules.

An oil fraction consisting of hydrocarbon molecules with more

carbon atoms has a higher boiling point range.

FRACTIONAL DISTILLATION OF CRUDE OIL IN

THE LABORATORY


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0 – 360oC thermometer

clamp

short rubber connecting tubing

bent delivery tube

small test tube

water oil fraction

heat

rocksil soaked with crude oil

Figure 23.24

Fractional distillation of crude

oil in the laboratory.


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Fractional distillation of crude oil in the laboratory.

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An oil fraction with a higher boiling point range has a darker colour

and is more viscous; it is less volatile, less flammable and burns

less completely.

A23.3

There is physical separation but no chemical decomposition.

A23.4

Industrial refining, different, same, fractions, laboratory, different,

in turn


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Activity 4

Some examples are:

(1) Plastics

There are many useful plastics. The commonest plastic is

polythene, made from ethene. Ethene is obtained by cracking

naphtha or gas oil.

(2) Alcohol

Ethanol can be manufactured from ethene.

(3) Detergents

Synthetic detergents are made from petroleum products.

(4) Medicines and insecticides

Many of these are made from substances obtained from

petroleum.


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SUMMARY

1. Coal, petroleum and natural gas are fossil fuels.

2. Coal was formed from the remains of plants that lived 250 mil

lion years ago.

Petroleum and natural gas were formed from the remains of v

ery small sea animals and plants that lived hundreds of millio

n years ago.

SUMMARY

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SUMMARY

3. Petroleum has great economic importance. Petroleum-based

fuels are burnt to provide heat and electricity for homes and

industries. Petroleum also provides the starting chemical

materials for the manufacture of a variety of important

products. However, petroleum resource is limited and non-

renewable.

4. Petroleum is a complex mixture mainly of hydrocarbons.

Hydrocarbons are compounds containing hydrogen and

carbon only.

5. In refining, petroleum is separated into several useful parts

(fractions) by fractional distillation. This works because

different hydrocarbons have different boiling points.

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SUMMARY

6. Each petroleum fraction has its particular uses. Refer to

Figure 23.21 for the major uses of the various fractions.

7. A small-scale refining of crude oil can also be performed in

the laboratory by fractional distillation.

8. As the number of carbon atoms in the molecules of an oil

fraction increases, the properties of the fraction change as

follows:

Boiling point range increases

Volatility decreases

Colour darkens

Viscosity increases

Flammability decreases

Flame (on burning) becomes darker and sootier

1 Chapter 23Fossil Fuels 23.1Fossil fuels as a major energy source 23.2Coal 23.3Petroleum and natural gas 23.4Refining petroleum CONTENTS OF CHAPTER 23.

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