fuels

VCE 3/4 CHEMISTRY 2023

K.B Tutoring Notes

Lesson 1: Fuels

Learning Objectives

• How fossil fuels are obtained

• Use of fossil fuels

• Production of biofuels

• Distinction between fossil fuels and renewable fuels

• Distinction between biodiesel and petrodiesel

• Environmental implications of fuels

Units of Energy:

• The SI unit for energy is known as a joule (J)

• By extension:

  • 1 kilojoule (kJ) = 103 Joules

  • 1 megajoule (MJ) = 106 Joules

  • 1 gigajoule (GJ) = 109 Joules

  • 1 terajoule (TJ) = 1012 Joules

Conservation of Energy:

• During a chemical reaction, atoms are neither created nor destroyed. They are merely rearranged to form new substances.

• During a chemical reaction, mass is neither gained nor lost. In other words, the mass of the reactants is equal to the mass of the products. This is the Law of Conservation of Mass.

• Likewise, during such a reaction, energy is neither created nor destroyed, it is merely converted to another form of energy. The total amount of energy remains unchanged. This is the Law of Conservation of Energy.

Fossil Fuels

• Fossil Fuels are fuels that have been formed from the decomposition of organic matter over a long period of time in anoxic (without oxygen), pressurised and hot conditions.

• These fuels are currently the prevalent sources of energy in society, with key examples being coal, oil and natural gas.

• Societies are now looking to move away from these sources of fuel because:

  • Concern over the particulates being released into the atmosphere due to their combustion and usage, i.e. CO2 (contributes to greenhouse effect and global warming), SO2 (can cause acid rain)

  • The reserves of fossil fuels are finite and will eventually run out

Key Question: Why are fossil fuels considered to be non-renewable despite being formed through natural processes?

Answer:

• They are not an energy resource that can be replaced by natural processes in a relatively short period of time (they are consumed at a rate that is far greater than the rate they are produced).

Coal

• As plant material such as wood starts to decay, some gradual chemical changes occur

• This leads to the carbon content of this material increasing, and the hydrogen and oxygen content decreasing with water evaporating during this process.

• This process of decay involves this matter turning into peat , then brown coal and finally black coal.

• As this material decays further and further into black coal, the energy content (energy released per mass of fuel) increases, making black coal a better fuel than the other two.

• This is due to two factors:

  • The carbon content per gram increases, the substance which releases the energy when combusted

  • The water content per gram is reduced, meaning that energy is not consumed to vaporise this water and more energy is available for the desired usage.

  • The carbon dioxide released per unit of energy is less than that of peat and brown coal.

Worst Fuel Best Fuel

Peat Brown Coal Black Coal

• Significant quantities of ash and other pollutants are released in the combustion of coal, making natural gas and oil cleaner fuels by comparison.

Oil

• Refined oils such as petrol are initially obtained from crude oil

• Crude oil is a mixture of hydrocarbons, predominantly alkanes

• This crude oil is then put through fractional distillation to obtain fractions of hydrocarbons with similar molar masses (for example, crude oil can be separated into diesel which contains larger hydrocarbons and petrol, which has shorter hydrocarbons)

• Each fraction would have similar boiling points due to the similar molar mass and similar strength of dispersion forces

Natural Gas

• Natural gas consists of predominantly methane (CH4), with traces of other hydrocarbon gases

• Methane is available in two forms: as a biofuel and as a fossil fuel

• Methane is considered to be a non-renewable fossil fuel when it is obtained from gas reservoirs trapped between rocks, in coal seam gas and shale gas.

• As these are the ways methane is obtained naturally, natural gas is considered non-renewable.

• Extracting these forms of natural gas involves fracking, where the rock or coal is fractured to release the gas.

Energy Transformations

• Combusting fuels releases the stored energy, which can be converted to other forms of energy such as kinetic or electric energy to use in various situations

• Energy efficiency is the proportion of stored chemical energy that is converted to the desired form of energy. Usually, the more transformation steps, the lesser the efficiency.

Energy Transformations in Coal-fired power station

• Coal is burnt in the furnace: chemical energy to thermal energy

• Heat from the coal is used to boil water: thermal energy (coal) to thermal energy (steam). Usually don’t have to mention this, good to know.

• Steam then turns a turbine: thermal energy to mechanical energy

• Electricity produced from a generator connected to the turbine: mechanical energy to electrical energy

Energy Transformations in Gas-fired power station

• Gas is burnt in the furnace: chemical energy to thermal energy

• The hot gases then turns a turbine: thermal energy to mechanical energy

• Electricity produced from a generator connected to the turbine: mechanical energy to electrical energy

• This power station is more efficient that the coal-powered station as there are less conversion steps and hence less energy wasted

• This greater efficiency means that gas powered stations emit significantly less carbon dioxide than coal powered station

Energy Efficiency: Useful Energy/Total Energy x 100 (units percentage)

The Need for Renewable Fuels:

• Currently, around the world the prevalent fuels being used are finite fossil fuels.

• The necessity for fuels that have less of an environmental impact are leading to a push for widespread adoption of renewable and other less harmful fuels.

• However, in order to meet this criteria, replacement energy sources need to be reliable, sustainable and cost effective.

• This has led to research into and small-scale implementation of biofuels such as bioethanol, biodiesel and biogas, and other energy sources such as fuel cells (to be explored later).

Biofuels

• Biofuels represent an alternative to fossil fuels in many common daily usages, such as household usages and transport usages

• Biofuels are derived from plant matter. Commonly, these are plant oils, carbohydrate heavy plants such as sugar cane, and plant waste.

• Biofuels get the bio suffix as opposed to their fossil fuel equivalents because they are produced directly from biological sources

• Plant oils are a key ingredient in the production of biodiesel, the carbohydrate heavy plants are commonly used to produce bioethanol, and the plant waste is used to produce biogas.

• These fuels, while renewable, also have less of a carbon footprint, as the carbon content in the biofuel initially came from carbon dioxide in the air and was converted via photosynthesis.

• This leads to a theoretical zero net release, and hence can sometimes be said to be carbon neutral. However, this is not always the case, with fuel being expended to help grow the plants, convert the plant matter and transport it, among other things.

• They are also not necessarily sustainable on a large scale, as the sheer quantity of plant matter necessary to keep up with society would be very large.

• A conflict also arises with the uses of land to grow these biofuel crops as this comes into direct completion with land reserved for food crops, which is already stressed with a growing human population. This means that on a large scale, the land can be used for food or biofuel, but not both.

Bioethanol

• Produced by the anaerobic fermentation of glucose by yeast

• Yeast contains the biological catalysts (enzymes) needed to catalyse this fermentation

• Commonly used as an alternative or supplementary fuel to petrol

Equation: C6H12O6 (aq) -> 2CH3CH2OH (aq) + 2CO2 (g)

• Less energy available per litre opposed to petrol, and causes greater engine wear, but is renewable

• This lower energy content is due to the ethanol molecule being partly oxidised, that is, partially containing oxygen

• Less particulates are released due to the presence of ethanol

Biogas

• A gaseous mixture containing a large proportion of methane gas and carbon dioxide

• Primarily produced by anaerobic digestion of organic waste matter by microorganisms (i.e specific bacteria)

• Not fully efficient, about 60% efficient

• Inputs for the decomposition are widely available, such as from rubbish tips or on farms

• The methane produced is predominantly used for domestic use (cooking, heating, etc.)

Biodiesel

• Biodiesel comprises of a mixture of esters

• These esters are produced by reacting a vegetable fat/oil molecule with an alcohol molecule

• A fat molecule includes a glycerol molecule bonded to 3 different fatty acids (carboxylic acids) via 3 ester links

• An alcohol is added to the presence of this triglyceride and potassium hydroxide catalyst and is lightly heated.

• This results in the formation of 3 ester molecules and a glycerol molecule, the ester molecules of which are separated out for use as a biofuel.

• Used as a transport fuel for larger vehicles

Petrodiesel vs Biodiesel

Definitions

Kinetic Energy: Energy associated with movement

Potential Energy: Energy stored in chemical bonds

Fuel: A substance with stored energy that can be easily released to be converted to another form of energy. e.g. Coal, biodiesel, etc.

Non-Renewable: When a substance is used up at a rate faster than it can be replaced

Renewable: When a substance is continually replenished at a rate faster than its consumption

Shale Gas: Methane trapped in shale rock

Coal Seam Gas: Methane bonded to the surface of the coal

Alkanes: A hydrocarbon with general formula (CnH2n+2)

Transesterification: A process in which the triglyceride reacts with an alcohol to form esters

Triglyceride: An ester molecule comprised of a glycerol molecule bonded to three fatty acids

Viscosity: A measurement of a liquid’s resistance to flow

Cloudpoint: The temperature below which diesel or biodiesel appear cloudy owing to the formation of small crystals. The presence of these crystals thickens the fuel and clogs filters and injectors.