Chapter 14: Energy IB Option C

Part 2: Fossil Fuels (C.2) – pp. 773-787

C.2 – The energy of fossil fuels originates from solar energy that has been stored by chemical processes over time. These abundant resources are non-renewable but provide large amounts of energy due to the nature of chemical bonds in hydrocarbons.

Fossil fuels were formed by the reduction of biological compounds.

COAL: Sedimentary rock formed from the remains of ______life, mostly from the carboniferous period.

Pressure and heat changed plan material in stages from peat to lignite to bituminous soft coal and finally to anthracite, increasing the percentage of ______along each stage.

Coal is the most ______fossil fuel.


Example: A number of carbon fuels are used domestically to provide heating during the winter months.

Fuel / Percentage composition / Specific energy
C / H / O / N / kJ·g-1
wood / 50 / 6 / 43 / 1 / 10-13
peat / 59 / 6 / 33 / 2 / 13
bituminous coal / 88 / 5 / 5-15 / 1 / 30
Anthracite / 95 / 2-3 / 2-3 / trace / 31

a)  Describe the relationship between the oxygen content of a fuel and its specific energy.

b)  Describe how the composition of the fuels changes as they are exposed to high pressure.

c)  Pros and cons of using coal v. oil for fuel

Fuel / Advantages / Disadvantages
Coal
Wood

CRUDE OIL: formed over millions of years from the remains of ______and plants.

Crude oil is no use before it is refined. ______impurities, mainly in the form of hydrogen sulfide, must first be removed, as they would block the active sites of the catalysts used in later chemical processing. The acidic hydrogen sulfide is removed by dissolving it in basic potassium carbonate solution:

The hydrogen sulfide can be recovered from solution by later reversing the reaction. It is burned in the air to form sulfur dioxide:

The sulfur dioxide produced can then react with more hydrogen sulfide to produce elemental sulfur:

The desulfurization step also reduces acid rain pollution, which would otherwise result if the sulfur was burned with the oil. The crude oil is then separated into different ______on the basis of their molecular size. In this fractional distillation process the crude oil is heated to approx. 400 °C. At this temperature, components of the mixture are vaporized and allowed to pass up the distillation column, and they will condense out at different temperatures based on their molecular size. Smaller hydrocarbons exit higher in the column because they remain as gases at lower temperatures.

Fraction / # C atoms / Use
Refinery gas / 1-4 / Fuel (e.g. methane, propane, butane)
Feedstock for petrochemicals
Gasoline/naptha / 5-10 / Gasoline (petrol): fuel for cars
Naptha: chemical feedstock
Kerosene / 10-16 / Fuel for jets
Paraffin for heating
Gas oil / 13-25 / Fuel for Diesel engines, power plants and heating
residue / >25 / Oil-fired power stations, polishing waxes, lubricating oils, bitumen used to surface roads

Figure 2.1: Fractional distillation of crude oil

Since fractions may not be present in desired quantities, ______breaks down (cracks) heavy oil fractions into lighter, more useful oils.

Example:

·  ______cracking uses high temperatures to break down (crack) heavy oil fractions into lighter oils.

·  ______cracking allows the cracking to occur at a lower temperature (______°C) through the use of a catalyst, such as ______minerals which contain a silica-alumina crystalline structure.

·  ______cracking can produce even higher quality branched hydrocarbons that burn more evenly in a car engine. Heavy hydrocarbon fractions are mixed with hydrogen at a pressure of about 80 atm and cracked over palladium on a zeolite surface. A high yield of branched-chain alkanes and cycloalkanes and some aromatic compounds is produced.

In an internal combustion engine, the compression stroke decreases the volume of a cylinder full of fuel and air and ignites it with a spark plug. Some fuels auto-ignite without the need of a spark plug as the fuel-air mixture is compressed by the piston. This premature ignition is known as ______as it gives rise to a knocking sound in the engine. Knocking reduces the efficiency of the engine as the energy of the exploding and expanding gas is not applied fully to the piston at the optimum time and can damage an engine.

·  ______molecules have a higher tendency to knock.

·  The branched-chain isomer of octane, 2,2,4-trimethylpentane (isooctane) is used as the standard by which other fuels are measured, as it does not ignite prematurely (rating of 100).

NATURAL GAS

Mostly ______, but also contains nitrogen and sulfur compounds.

Cleanest fossil fuel to burn due to its high H : C ratio. Easy to extract, but difficult to contain.

Supplies of methane can be increased by cracking larger hydrocarbons or from coal gasification. The crushed coal is mixed with superheated steam and a mixture of carbon monoxide and hydrogen known as ______is produced:

Synthesis gas can be used directly as a fuel. It can be processed further to make methane. It is mixed with additional hydrogen in the presence of a heated catalyst:

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:

Table 2.1: Pros and Cons of different types of fossil fuels

Fossil Fuel / Advantages / Disadvantages
Coal
Crude oil
Natural gas

Carbon footprint: one measure of the impact of our activities on the environment.

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