C.2FOSSIL FUELS

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FOSSIL FUELS C.2

OPTION C: ENERGY

By: Merinda Sautel Alameda Int’l Jr/Sr High School Lakewood, CO [email protected]

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The energy of fossil fuels originates from solar energy which has been

stored by chemical processes over time.

These abundant resources are nonrenewable but provide large

amounts of energy due to the nature of chemical bonds in hydrocarbons.

NATURE OF SCIENCE (4.1)

Scientific community and collaboration—the use of fossil fuels has had a key role in the development

of science and technology.

ESSENTIAL IDEA

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The choice of fossil fuel used by different countries depends on availability, and economic, societal, environmental and

technological factors.

Different fuel rating systems (RON, MON or PON) are used in different countries.

Ocean drilling, oil pipelines and oil spills are issues that demand

international

cooperation and agreement.

INTERNATIONAL-MINDEDNESS

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Fossil fuels were formed by the reduction of biological

compounds that contain

carbon, hydrogen, nitrogen, sulfur and oxygen.

UNDERSTANDING/KEY

IDEA C.2.A

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• The energy from fossil fuels comes from sunlight which was trapped by green

plants millions of years ago.

• Fossil fuels are produced by the slow and partial decomposition of plant and animal matter that is trapped in the absence of air.

• Oxygen is lost from the biological

molecules containing carbon, hydrogen,

nitrogen, sulfur and oxygen at a faster rate than other elements which results in

reduced biological compounds which are often hydrocarbons.

• There are 3 types: coal, gas and crude oil(petroleum)

FOSSIL FUEL VIDEO

FOSSIL FUELS

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• Coal is the most abundant fossil fuel.

• It is a combustible sedimentary rock formed from the remains of plant life which has been subjected to geological heat and pressure.

• Coal has gone through many stages and at each stage the percentage of carbon increases.

• Anthracite is almost pure carbon.

• Coal usually contains between 80 and 90% carbon by mass.

COAL

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• Crude oil or petroleum is one of the most important raw materials in the world today.

• It is a complex mixture of straight and

branched-chain alkanes, cycloalkanes and aromatic compounds.

• It supplies us with the fuel we need for transport and energy generation.

• It is a very important chemical feedstock for the production of important organic

compounds like polymers, pharmaceuticals, dyes and solvents.

• It was formed over millions of years from the remains of marine animals and plants trapped under layers of rock.

CRUDE OIL

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• In the last 50 years, crude oil has overtaken coal as the world’s most important source of energy.

• Oil and gas are easier to extract than coal since they can be pumped instead of mined.

• We still use 90% of the refined product as fuel. However, as supplies decrease, this proportion will fall.

• Crude oil must be refined before it is used.

CRUDE OIL

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Petroleum (crude oil) is a complex mixture of

hydrocarbons that can be

split into different component parts called fractions by

fractional distillation.

UNDERSTANDING/KEY

IDEA C.2.B

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Crude oil (petroleum) needs to be refined before use. The

different fractions are separated by a physical

process in fractional distillation.

UNDERSTANDING/KEY

IDEA C.2.C

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Be able to identify the various fractions of

petroleum, their relative volatility and their uses.

APPLICATION/SKILLS

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• Crude oil (petroleum) is of no use before it is refined.

• It contains a vast mixture of hydrocarbons of varying chain lengths.

• Long-chain hydrocarbons have stronger van der Waal’s forces between them than do the shorter chains, so their differing boiling

points can be used to separate the crude oil into “fractions” of various chain lengths.

• At oil refineries the various fractions are separated by fractional distillation.

• Note – sulfur impurities must be removed first.

FRACTIONAL DISTILLATION

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• The crude oil is heated to a temperature of about 400

^◦

C. • At this temperature all the different

components of the mixture are vaporized and passed up a distillation column.

• The level at which the molecules condense depends upon their size.

• The smaller molecules between one and four carbons collect at the top as the refinery gas fraction.

• Molecules of successively longer chains

condense at lower levels corresponding to their higher boiling points.

• Finally, there is a residue at the bottom with higher BP under normal atmospheric

conditions.

FRACTIONAL DISTILLATION

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FRACTIONAL DISTILLATION OF CRUDE OIL

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UNDERSTANDING FRACTIONAL DISTILLATION

FRACTIONAL DISTILLATION

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The fraction between 5 and 10 carbons is the most in demand (used in cars).

As a liquid, it is convenient to handle and deliver and has a relatively low bp so it is easy to vaporize, which assists

combustion.

FRACTIONS AND THEIR USES

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The performance of hydrocarbons as fuels is improved by the

cracking and

catalytic reforming reactions.

UNDERSTANDING/KEY

IDEA C.2.D

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• The demand for the different fractions does not necessarily match the amounts present in the crude oil supplied so the hydrocarbon molecules in the crude oil need to be

chemically changed.

• Hydrocarbons with up to 12 carbon atoms are in the most demand because they are easily vaporized and make the best fuels.

• The supply of these can be increased by breaking down or cracking the larger

molecules.

• Thermal cracking involves heating the starting materials.

CRACKING CRACKING VIDEO

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• When a catalyst is used, it is called catalytic cracking.

• This allows the reaction to occur at a lower temperature of around 500 degrees Celsius.

• It also helps give the required product by controlling the mechanism.

• The reactions are generally complicated but do involve carbocations which are produced and rearranged on the catalytic surface.

• The lower temperature requires less energy and reduces the cost of the process.

CATALYTIC CRACKING

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• The catalysts involved are:

• Alumina and silica and zeolites(minerals of Al, Si, and Oxygen)

• Some carbon is formed during the process which can coat the catalyst and stop it from working

• The catalyst must be separated from the

mixture and then the carbon is removed by heating.

• The heating produced from the combustion of the carbon can be used to sustain the

cracking reaction.

• Catalytic cracking tends to form branched chain alkanes and benzene ring aromatics which burn more evenly in a car engine.

CATALYTIC CRACKING

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• Hydrocracking also produces compounds used in high quality gasoline.

• In this process a heavy hydrocarbon fraction is mixed with hydrogen at a pressure of 80 atm and cracked over palladium on a zeolite surface.

• A high yield of branched-chain alkanes,

cycloalkanes and some aromatic compounds is produced.

HYDROCRACKING

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The tendency of a fuel to auto-

ignite, which leads to “knocking”

in a car engine,

is related to molecular structure and measured by the octane

number.

UNDERSTANDING/KEY

IDEA C.2.E

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Be able to discuss the reforming and cracking reactions of hydrocarbons

and explain how these

processes improve the octane number.

APPLICATION/SKILLS

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Be able to discuss the effect of chain length and chain

branching on the octane number.

APPLICATION/SKILLS

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Be able to deduce the

equations for cracking and reforming reactions, coal gasification and liquefaction.

APPLICATION/SKILLS

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• The tendency of a fuel to auto-ignite is measured by the octane number.

• There are four strokes in a gasoline engine:

• Stroke 1 – piston moves to increase volume of the chamber; this involves air and fuel intake

• Stroke 2 – piston moves to decrease volume of the chamber; this involves a compression stroke and spark ignition

• Stroke 3 – piston moves to increase volume of the

chamber; the gases expand because of production of gases and increase in temperature

• Stroke 4 – piston moves to decrease volume of the chamber – exhaust gases are expelled.

KNOCKING

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• In the compression stroke (stroke 2), the fuel and air are squashed into a smaller volume and ignited with a spark plug.

• Some fuels auto-ignite without the need of a spark plug.

• This premature ignition is known as

“knocking” as it gives rise to knocking sound in the engine.

• Knocking reduces engine efficiency because the exploding and expanding gas is not

applied fully to the piston at the optimum time and can cause engine damage.

• Straight chain molecules have a tendency to knock.

KNOCKING

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• The branched-chain isomer of octane, 2,2,4- trimethylpentane (isooctane) does not suffer from premature ignition and is considered the standard from which other fuels are judged.

• The performance of a fuel is given by its octane number which is based on a scale where

isooctane has a value of 100 and heptane has a value of 0.

• A fuel with a 96% octane rating burns as

efficiently as a mixture of 96% isooctane and 4%

heptane.

• Fuels with high octane numbers can be more

highly compressed, which results in more power per piston stroke.

KNOCKING

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• Octane number decreases with an increase in chain length.

• Cyclic compounds have higher octane numbers than linear structures.

• Alkenes have a higher octane number than the isomeric cycloalkanes.

• Aromatic compounds with the benzene ring have even higher octane numbers.

KNOCKING

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• Catalytic reforming involves the

rearrangement of the atoms in a molecule to form a more branched chain or aromatic

compound.

• The process is referred to as “plat forming”

if platinum is used as the catalyst.

• Palladium, iridium and rhenium can also be used.

• Isomerization can increase the branching in a molecule by heating them in the presence of a catalyst such as AlCl

₃

.

• Alkylation involves reacting lower mass alkenes with alkanes to form higher mass alkanes.

CATALYTIC REFORMING

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• This can be thought of as the reverse of cracking.

• The alkane is effectively adding across the double bond in the alkene making a larger branched chain alkane.

ALKYLATION

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ALKYLATION

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Coal gasification and liquefaction are chemical processes that

convert coal to gaseous and liquid hydrocarbons.

UNDERSTANDING/KEY

IDEA C.2.F

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• Reacting coal with hydrogen under high pressure in the presence of a catalyst

produces liquid hydrocarbon fuels.

• The resulting mixture can be separated by fractional distillation.

• As crude oil prices rise, this will be an increasingly economic option.

• The formula for this process is:

nC + (n+1)H

₂

→ C

_n

H

_2n+2

where n>4

COAL LIQUEFACTION

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• This process involves using a mixture of carbon monoxide and hydrogen as the

feedstock and produces a variety of alkanes along with water.

• The formula for this process is:

(2n+1)H

₂

+ nCO → C

_n

H

_2n+2

+ nH

₂

O

FISCHER-TROPSCH PROCESS

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• Natural gas is the cleanest of the fossil fuels to burn due to its high hydrogen to carbon ratio.

• Impurities can easily be removed so the

combustion of natural gas produces minimal amounts of carbon monoxide, hydrocarbons and particulates.

• Where it is available, it takes little energy to get from ground to the consumer since it

can be piped directly.

• However, setting up a distribution network requires a massive capital investment.

• Some countries use liquefied gas (butane or propane) as an option for domestic heating and cooking.

NATURAL GAS

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• COMPLETE/INCOMPLETE COMBUSTION VID EO

• Methane is the primary component of natural gas.

• Natural gas was formed millions of years ago by the action of heat and pressure and bacteria on buried organic matter.

• The gas is trapped in geological formations capped by impermeable rock.

• It is also formed from the decomposition of crude oil and coal deposits.

• It can occur on its own, dissolved under pressure in oil, or in a layer above oil in a reservoir.

• Natural gas can also be found with coal, where it is a major hazard as it forms an explosive

mixture with air.

NATURAL GAS

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• Coal can be converted to methane by the process of coal gasification.

• The crushed coal is mixed with superheated steam and a mixture of carbon monoxide

and hydrogen known as synthesis gas is produced.

C

_(s)

+ H

₂

O

_(g)

→ CO

_(g)

+ H

_2(g)

• Synthesis gas can be used directly for fuel or processed further to make methane.

CO

_(g)

+ 3H

_2(g)

→ CH

_4(g)

+ H

₂

O

_(g)

COAL GASIFICATION

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• Synthetic natural gas can also be made by heating crushed coal in the presence of

steam with a potassium hydroxide catalyst to produce methane and carbon dioxide.

2C

_(s)

+ 2H

₂

O

_(g)

→ CH

_4(g)

+ CO

_2(g)

COAL GASIFICATION

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A carbon footprint is the total amount of greenhouse gases

produced during human activities. It is generally

expressed in equivalent tons of carbon dioxide.

UNDERSTANDING/KEY

IDEA C.2.G

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• One measure of the impact our activities have on the environment is given by our carbon footprint.

• The carbon footprint is a measurement of all the greenhouse gases (primarily CO

₂

and CH

₄

) we individually produce.

• It has units of mass of carbon dioxide

(sometimes expressed as equivalent tons) and depends on the amount of greenhouse gases we produce in our day to day

activities through the use of fossil fuels such as heating, transport and electricity.

CARBON FOOTPRINT

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Be able to calculate the

carbon dioxide added to the atmosphere, when different fuels burn and determination

of carbon footprints for different activities.

APPLICATION/SKILLS

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• Work out the carbon footprint for a car

journey of 100km. Assume that the car uses 7dm

³

of fuel for the journey and that the

fuel is octane. The density of octane is .703 g/cm

³

.

• First determine the mass of octane burned by using the density.

• 7dm

³

x 1000cm

³

/dm

³

= 7000cm

³

x .703 g/cm

³

= 4921 g

• Convert from grams to moles by dividing by the molar mass of octane.

• 4921 g x mol/114.3 g = 43.07 mol

• Solve for moles of CO

₂

produced. 43.07mol x 8/1 = 345 mol CO

₂

• Solve for grams of CO

₂

. 345 mol x 44.01g/mol = 15200 g or 15.2 kg

SAMPLE PROBLEM 1

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• On a typical winter day 1.33 x 10

⁶

kJ of energy is needed in a home.

• A) Calculate the percentage mass of carbon in the two fuels.

• Coal is CH. % C is 12.01/13.02 x 100 = 92.2 %

• Wood is C

₅

H

₉

O

₄

. % C is (5 x 12.01)/(5x12.01 + 9x1.01 + 4x16) x 100 = 45.1%

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B) Determine the carbon footprint of the two fuels.

Coal: efficiency = useful output energy x 100 = 65%

total input energy

Solve for input energy needed: input = 1.33x10⁶kJ/.65 = 2.05X10⁶ kJ Use specific energy to solve for mass of fuel: 2.05X10⁶kJ x g/31kJ = 66000 g

Use % carbon to solve for mass of carbon burned: . 922(66000g)=60800gC

Solve for mass of CO₂: 60800gC x 44.01gCO₂/12.01gC=22300g = 223kg CO₂

Wood: efficiency = useful output energy x 100 = 70%

total input energy

Solve for input energy needed: input = 1.33x10⁶ kJ/.70 = 1.9X10⁶ kJ Use specific energy to solve for mass of fuel. 1.9X10⁶kJ x g/22kJ = 86400 g

Use % carbon to solve for mass of carbon burned: . 451(86400g)=38900gC

Solve for mass of CO₂: 38900gC x 44.01gCO₂/12.01gC=14200g = 142kg CO₂

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Be able to discuss the advantages and

disadvantages of the different fossil fuels.

APPLICATION/SKILLS

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• COAL

• ADVANTAGES

• Cheap and plentiful throughout the world

• Can be converted to synthetic liquid fuels and gases.

• Safer than nuclear power

• Ash produced can be used in making roads.

• DISADVANTAGES

• Produces many pollutants including CO

₂

and SO

₂

and particulates

• Difficult to transport

• Waste can lead to visual and chemical pollution

• Mining is dangerous.

ADVANTAGES AND

DISADVANTAGES OF FOSSIL FUELS

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• CRUDE OIL

• ADVANTAGES

• Easily transported in pipelines or by tankers.

• Convenient fuel for cars as it is volatile and burns easily.

• Sulfur impurities can easily be removed.

• DISADVANTAGES

• Limited lifespan and uneven world distribution.

• Contributes to acid rain and global warming.

• Transport can lead to pollution.

• CO is a local pollutant produced by incomplete combustion of gasoline.

• Photochemical smog is also produced.

ADVANTAGES AND

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• NATURAL GAS

• ADVANTAGES

• Produces fewer pollutants per unit energy.

• Easily transported in pipelines and pressurized containers.

• Does not contribute to acid rain.

• Higher specific energy.

• DISADVANTAGES

• Limited supplies.

• Contributes to global warming.

• Risk of explosion due to leaks.

ADVANTAGES AND

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The cost of production and availability (reserves) of

fossil fuels and their impact on the environment should be

considered.

GUIDANCE

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• NOTE – ALL FOSSIL FUELS ARE NON- RENEWABLE AND PRODUCE THE

GREENHOUSE GAS CARBON DIOXIDE.

• The choice of fossil fuels used by different countries depends upon historical,

geological and technological factors.

Different societies have different priorities in their energy choices.

• As civilization has advanced, the carbon content of fossil fuels has decreased.

• Coal has been replaced by gasoline and natural gas because

they have higher specific energies and energy densities and are easier to transport. They are also cleaner burning and produce less CO₂ per unit energy.

ADVANTAGES AND

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• The financial costs involved in constructing international pipelines is a main reason

why the international trade in natural gas has been relatively slow.

• At present most natural gas is consumed in the countries in which it is found.

ADVANTAGES AND

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