General Overview of Fuels:

Production of Fuels:

Fuels are produced both naturally and through controlled reactions in laboratories. Natural fuels include fossil fuels (created by the burial of organisms under sediment, then applied pressure and heat), the main type of which is petroleum. Petroleum deposits are drilled on land or under sea, and contain a viscous crude oil, and natural gas.

Natural gas consists of four different alkanes which are in a gaseous state at room temperature, but is 88-95% methane. The remaining 5-12% of the gas consists of the paraffin series (most which is removed and compressed into liquid petroleum gas), and does not contain carbon monoxide. Natural gas may be extracted straight from wells, and piped to households, or it may also be created through cracking. Hydrocracking of the methane in natural gas creates methanol, which in turn care be converted by the Mobil Process of catalytic dehydration into a high quality synthetic gasoline.

The rest of the petroleum is refined by fractional distillation, which is the separation of the material into useful groups by utilising the boiling points of the various substances. The distilling tower is heated from the bottom, and works on vapor rising, then condensing under bubble caps as it reaches the level of temperature where it condenses back to liquid, so that gas is piped off the top of the tower, and thick residue remains at the base.

Petrol (gasoline) consists of mainly hydrocarbon compounds C5-C10 with boiling points ranging between 40 and 180ºC. As petrol (the main fuel of human transport) is in great demand, and that a lot of unneeded petroleum remains in the higher fractions of the tower, these higher fractions of hydrocarbons are often cracked into smaller hydrocarbons that are useful in petrol production.

Cracking normally occurs at very high temperatures, so the presence of a catalyst aids in cracking the hydrocarbons at a lower temperature. An example (500ºC with Al2O3-SiO2 catalyst) is a 12-Carbon hydrocarbon cracked into two smaller hydrocarbons: C11H24 —> C9H20 + C2H4 (undecane —> nonane + ethene) This process creates more liquid (at room temperature) hydrocarbons which can be used in the manufacturing of petrol, which is mainly octane. The nonane is incorporated into petrol (which is of higher quality, due to a cracked hydrocarbon's tendency to create more branches, therefore making it more suitable for use in engines), and the other product of the reaction, ethene can be hydrated into ethanol.

Ethanol is also produced both naturally and artificially. Ethanol is produced by fermentation of sugars by enzyme action (from yeast cells), a form of anaerobic respiration. As ethanol is toxic to the enzymes when over a concentration of 10%, the alcohol must be distilled. This is not efficient for industrial purposes, so ethanol is created by hydrating the hydrocarbon ethene in the presence of an acid catalyst (often phosphoric acid), an addition reaction at 330ºC with steam and a pressure of 60 atmospheres. For example: C2H4 + H2O(g) —catalyst—> C2H5OH (ethene + water —————> ethanol)

Liquid Petroleum Gas is simply the creation (via cracking) or extraction (from petroleum and natural gas by liquefying these heavier gases based on boiling points) of the paraffin series (alkanes C2-C4). These alkanes are bottled under pressure in steel cylinders as liquids to be used for fuel when a centralised gas supply is unavailable.

Properties of Fuels:

Petroleum based fuels are, in general, immiscible with water, and therefore insoluble, as they are virtually non-polar. As they are less dense than water, they float on the surface of water. These properties relate to their common use as fuels, as they can be stored in underground units without a risk of leaking and contaminating the surrounding soil, as they are stored on top of a layer of water, and also help to water-proof the container so that substances do not leak into the container.

They generally have low melting points and boiling points, so they can be stored under pressure then changed to a gas (or atomised), which is useful for heating (eg cooking) and in car engines, which are internal-combustion units that rely on atomised petrol being ignited.

When burnt in air, light hydrocarbon mixtures (eg petrol, natural gas and LPG) burn readily in oxygen and can reach high temperatures (eg natural gas can burn at 1930°C), making them highly suitable as fuels. The flames produced through combustion are relatively clean (becoming less so as the hydrocarbon molecule chains increase in length), which indicates near-complete combustion. This characteristic of these fuels is a factor that sees them used often, as less harmful by-products are produced.

Natural Gas and Liquid Petroleum Gas have low boiling points, meaning that they generally stay in a gaseous state when released from their pressurised (or normal in the case of Natural Gas) containers except in extreme cold. This property makes them suitable as fuels for portable heating. LPG is used as fuel in paraffin stoves, as it can be taken into relatively extreme climates and still become a jet of fuel to cook food.

Through cracking and hydrocracking, it is possible to change the products of petroleum to different fuels with relative ease, with the products of these reactions being higher quality fuels. This property, the ability to branch, relates to their use as fuels, as fuels with more branches off the main chain release more energy in combustion.

One of ethanol's distinguishing properties that relates to its use as a fuel is its ability to be created in different ways. It can be hydrated ethene, or produced through metabolic processes, making it both versatile and renewable as a fuel resource. Ethanol, like its hydrocarbon counterparts, combusts relatively cleanly, and when in a plentiful supply of oxygen, will produce little carbon monoxide (a toxic gas). It is also miscible with other substances, and therefore is useful when used in conjunction with other fuels.

Uses:

Liquid Petroleum Gas is sold commercially in pressurised canisters for barbecues, paraffin stoves and patio heaters. As it has been compressed into a liquid, this makes it easier to carry larger amounts of fuel around, as it is in a smaller container, making it a useful fuel for more remote or outside locations.

Petrol is the most common fuel for cars with internal combustion engines. It is atomised before being ignited in the car engine, and cracked fuels with many branches are an efficient fuel (as they have a higher resistance to pre-ignition, or ignition of the fuel before it is supposed to ignite via a spark). Petrol consists of a blend of medium hydrocarbons, and in colder weather, the blend is changed to more volatile hydrocarbons so that vaporisation can be better achieved.

Ethanol, aside from its uses as a steriliser, solvent and intoxicating drink, is used in high- compression internal combustion engines (as a transporting fuel, often mixed with another fuel such as petrol) as the vapour is highly explosive when mixed with air. It is a suitable fuel for cars as it too has a high resistance to pre-ignition.

Natural Gas is a fairly simple fuel, as it is generally extracted from Natural Gas wells and piped to a central station before being piped to households to use in cooking and heating. It is a basic fuel that is used commonly in eastern countries and in places with unreliable electricity supplies.

Interaction with People:

The main interaction of fuels with people is their use for their capabilities to release energy in the form of heat. Fuels are utilised in cars, heating and cooking to the benefit of mankind. Jobs are created by the giant petroleum and petrochemical industries, and the by-products of these industries are numerous - from textiles to detergents to medicines.

Aside from the obvious interaction as a fuel, these organic molecules have serious effects and repercussions on the people who utilise them. The products of combustion (apart from heat energy and water) are:

1. Carbon dioxide - produced in a excess supply of air, a non-toxic gas
   CH4 + 2O2 —> CO2 + 2H2O
2. Carbon monoxide - produced in a limited supply of air, an extremely toxic gas which has an affinity for haemoglobin, and will create a stable compound with this, stopping oxygen from being transported around peoples' bodies. This, if breathed in sufficient amounts, can cause death.
   2CH4 + 3O2 —> 2CO + 4H2O
3. Carbon - produced in very limited air supply, a black soot that can ruin equipment, such as engines.
   CH4 + O2 —> C + 2H2O
Carbon monoxide, in particular, is a product of fuel that has a negative impact on people.

Lead is added to some petrols to reduce the risk of pre-ignition and make the internal combustion a smoother reaction. To prevent the accumulation of lead (a heavy metal) in the engine, 1,2-dibromoethane is added to the petrol, and lead bromide is formed. This is a volatile substance, and exits the car as exhaust. Although this is useful for the engine, lead, which is a neurotoxin, is added to the atmosphere. This lead, when inhaled, can damage both the brain and the nervous system, especially in young children. Generally, in many of today's Western countries, lead is not added to petrol, and has been replaced by methyl tert-buthyl ether. Petrol with MTBE is referred to as 'unleaded petrol'.

Smog produced by car exhausts reacting with sunlight (a photochemical reaction) is also a danger to the health of people, as it increases the risk of both breathing problems (eg asthma) and infections that affect airways (eg pneumonia).

Interaction with the Environment:

The products of the combustion and production of fuels are pollutants to the environment. Crude oil (petroleum) contains amounts of Nitrogen and Sulfur, and these are released from the fractional distillation tower as pollutants. These pollutants are integrated into acid rain, which can cause great damage to ecosystems, water supplies and buildings.

Spills of oil from oil tankers in the ocean are very damaging to the environment because of the density of the hydrocarbons in the oil. As these float on top of the water, beach communities as well as birds and animals that live in or near the ocean are affected.

The carbon dioxide produced by combustion of fuels plays a large part in the accumulation of greenhouse gases. CO2 is 50% of the greenhouse gases currently trapped in the atmosphere of earth, and methane is 15%. These gases which cannot be vented off into space create a blanket around the earth which contributes to the problem of global warming.

Drilling wells for petroleum and natural gas also destroys the habitats of many animals in countries worldwide. These wells (in conjunction with pipe systems that stretch across continents) are aesthetically displeasing, and disrupt wildlife.

Although fuels are of great use to mankind, they also have bad effects, not only on the users of the fuels, but also on communities and ecosystems in the world around us.

Apatrix says: Coal, perhaps exotic stuff like rocket fuel, hydrogen peroxide... where are the rest of the fuels? Or stay with HCs and look for stuff like glucose. :)
Reply: The fuels I have gathered here a merely an example of fuels and their properties. For more detailed information, I would definitely suggest looking at specific writeups on each fuel type - there's much too much information to contain in one node!

Sources:
The Hutchinson Encyclopedia; The Guinness Encyclopedia of Science; 3 Chemistry Text Books; BP - Petroleum Industry