Chemistry of Fuels - Study Notes

Definition and Scope

Fuels are substances which undergo combustion in the presence of air to produce a large amount of heat that can be used economically for domestic and industrial purposes.
- This definition does not include nuclear fuel.
Common fuels include: wood, coal, kerosene, petrol, diesel (gasoline), coal gas, producer gas, water gas, natural gas (LPG), etc.

History of Fuels

The first known use of fuel: combustion of wood or sticks by Homo erectus about $2 \times 10^6$ years ago.
For most of human history, fuels derived from plants or animal fat were used.
Charcoal, a wood derivative, has been used since about $6{,}000\ \text{BCE}$ for melting metals.
Charcoal was supplanted by coke as forests in Europe became depleted around the 18^{\text{th}}\,$\text{century}.
Charcoal briquettes are now commonly used as a fuel for barbecue cooking.

Charcoal

Charcoal is a lightweight black carbon residue produced by removing water and other volatile constituents from animal and plant materials.
Produced by slow pyrolysis — heating of wood or other organic materials in the absence of oxygen (charcoal burning).
The finished charcoal consists largely of carbon.
Advantages over burning wood:
- Absence of water and other components
- Burns at higher temperatures
- Produces very little smoke; wood can release steam, organic volatiles, and unburnt carbon particles (soot) in smoke if not burned completely.

Coke

Coke is a grey, hard, porous fuel with a high carbon content and few impurities, made by heating coal or oil in the absence of air (destructive distillation).
Used mainly in iron ore smelting; also used as a fuel in stoves and forges when air pollution is a concern.
The unqualified term "coke" usually refers to the product derived from low-ash and low-sulphur bituminous coal by coking.
A similar product, petroleum coke (pet coke), is obtained from crude oil in oil refineries.
Coke may also be formed naturally by geologic processes.

Crude Oil and Petroleum

Crude oil was distilled by Persian chemists; descriptions appear in Arabic handbooks such as those of Muhammad ibn Zakarīya Rāzi.
Rāzi described distilling crude oil/petroleum into kerosene and other hydrocarbons in his Kitab al-Asrar (Book of Secrets).
Kerosene was also produced from oil shale and bitumen by heating the rock to extract the oil, which was then distilled.
Rāzi gave the first description of a kerosene lamp using crude mineral oil, referred to as the "naffatah".

Kerosene and Lamps

Kerosene lamp (also known as a paraffin lamp in some countries) is a lighting device using kerosene as fuel.
Kerosene is a combustible hydrocarbon liquid derived from petroleum; used in aviation and households.
The name kerosene derives from the Greek κηρός (keros), meaning "wax".
Gesner registered kerosene as a trademark in 1854 before it evolved into a generic term.

Tar and Peat

Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from destructive distillation of various organic materials (coal, wood, petroleum, peat).
Tar can be produced from coal, wood, petroleum, or peat; coal tar is a byproduct of coke production.
Mineral products resembling tar can be produced from fossil hydrocarbons, such as petroleum.
Peat (also called turf) is an accumulation of partially decayed vegetation or organic matter.

Coal and the Industrial Revolution

With energy stored as chemical energy from combustion, the steam engine (developed in the United Kingdom in 1769) made coal a dominant power source.
Coal later powered ships and locomotives.
By the 19th century, gas extracted from coal was used for street lighting in London.
In the 20th and 21st centuries, the primary use of coal has been electricity generation; it provided about $40\%$ of the world's electrical power supply in 2005.

Fossil Fuels and Renewable Trends

Fossil fuels were rapidly adopted during the Industrial Revolution because they were more concentrated and flexible than traditional energy sources (e.g., water power).
They have become a pivotal part of modern society.
Currently, there is a trend toward renewable fuels, such as biofuels (e.g., alcohols).

Classifications of Fuels

By origin (natural vs derived):
- Primary or Natural Fuels: coal, wood, etc.
- Secondary or Artificial or Derived Fuels: petrol, diesel.
By physical state: Solid Fuels, Liquid Fuels, Gaseous Fuels.

Primary & Secondary Fuels

Primary fuels (primary energy sources) are dense sources of energy found in nature and can be extracted, captured, cleaned, or graded without any energy conversion.
Secondary fuels are derived from primary fuels through chemical or physical processes; the energy for these initially comes from primary energy sources.
Example: Gasoline is a secondary fuel, made from oil through distillation.

Basis of Origin, Physical State, and Source

Natural or Primary / Solid Fuels: Wood, Peat, Lignite, Coal; Semi-coke, Charcoal.
Liquid Fuels: Crude Oil, Vegetable Oils; Petrol, Kerosene, Gas Oil, Coal Tar, Alcohol.
Gaseous Fuels: Natural Gas; Producer Gas, Coke-Oven Gas, Water Gas, Blast Furnace Gas, Compressed Butane gas, LPG.

Solid Fuels

Solid fuels are solid materials that can be burnt to release energy via combustion.
Examples: wood, charcoal, peat, coal, hexamine fuel tablets, wood pellets, corn, wheat, rye, and other grains.
Solid fuels are extensively used in rocketry as solid propellants.

Liquid Fuels

Liquid fuels are combustible molecules that can be harnessed to generate mechanical energy.
They must take the shape of their container; the fumes of liquid fuels are flammable rather than the liquid itself.
Most liquid fuels in widespread use are derived from fossil fuels.
Other types include hydrogen fuel (for automotive uses), ethanol, and biodiesel.
Liquid fuels play a primary role in transportation and the economy.

Gaseous Fuels

Gaseous fuels are hydrocarbons, hydrogen, and carbon monoxide mixtures in the gaseous state that form the basis of potential heat or light energy and can be delivered via pipes to consumption sites.
Gaseous fuels may be divided into four classes: natural gas, producer gas, water gas, and coal gas.

Characteristics of Fuels

Physical properties (parameters typically tested):
- Calorific value or specific heat of combustion; fuel efficiency (how much heat is produced).
- Ignition temperature.
- Flame temperature.
- Flash point and fire point.
- Aniline point.
- Knocking.
- Specific gravity.
- Cloud point and pour point.
- Viscosity.
- Coke number.
Chemical properties (composition):
- For solids and liquids: %C, %H, %O, %N, %S, etc.; moisture; volatile matter.
- For gaseous fuels: % of combustible gases (e.g., CO, H2, CH4, C2H4, C2H6, C4H10, H2S, etc.).

Combustion

Combustion is a process in which oxygen from the air reacts with elements or compounds to release heat.
General representation (illustrative):
- $\text{Fuel} + O<em>2 \rightarrow CO</em>2 + H_2O + \text{heat}$
Combustion efficiency and emissions depend on fuel type, impurities, and combustion conditions.

Connections, Implications, and Real-World Relevance

The evolution of fuels shows a shift from biomass and charcoal to coal and petroleum, enabling industrial growth and modern energy systems.
Environmental and health considerations drive policy changes, such as reducing coal use and promoting cleaner fuels and renewables.
The trend toward renewable fuels (biofuels, alcohols) reflects concerns about sustainability and climate change.
Understanding classifications (primary/secondary; solid/liquid/gaseous) helps in energy planning, storage, transport, and end-use applications.

Notable Dates and Facts (for quick recall)

Early human use of fuels: $2 \times 10^6$ years ago.
Charcoal used for metalworking since about $6{,}000\ \text{BCE}$ .
Coke development around the 18^{\text{th}}\,$\text{century}.
Kerosene lamp described by Rāzi and later by others; kerosene trademark registered in $1854$ by Gesner.
Industrial Revolution and coal: steam engine development around $1769$ ; gas lighting in London in the 19th century.
Coal contributed about $40\%$ of global electricity in 2005.