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Factors to consider when choosing a fuel
The factors that should be considered when choosing a fuel for a specific purpose are its:
- Energy Value
- Ignition Temperature
- Ease of Liquefaction (critical temperature)
- Products of Combustion
Energy Value is the heat of combustion of a fuel given per gram of fuel.
The higher the energy value, the more energy is released, the better the fuel.
Heat of combustion of hydrogen is 285 kJ mol-1
1 mole of hydrogen gas (H2) has a mass equal to its relative molecular mass (molecular weight) expressed in grams (known as its molar mass).
molar mass of H2 = 2 × 1.008 = 2.016 g mol-1
The heat produced per gram of hydrogen gas
= 285 kJ
mol-1 ÷ 2.016 g mol-1 = 141.4 kJ g-1
The energy value for hydrogen gas is 141.4 kJ g-1
Ignition Temperature is the minimum temperature to which the fuel-oxidiser mixture (or a portion of it) must be heated in order for the combustion reaction to occur.
High ignition temperature means the fuel is difficult to ignite, low ignition temperature means the fuel ignites easily making the fuel potentially hazardous.
The greater the activation energy of a reaction, the higher the ignition temperature will be.
A match and its striking surface contain a fuel and its oxidiser with a low activation energy and therefore low ignition temperature, so low that the friction of striking the match generates enough heat to raise the temperature sufficiently for ignition to occur.
Petrol and oxygen in a car engine have a higher activation energy and therefore a higher ignition temperature. A spark is needed to raise the temperature of the mixture sufficiently near the spark for the mixture to ignite. The heat of reaction generated heats up more of the mixture so the reaction becomes self-sustaining.
Fuels function by releasing combustible gases (vapours)
Boiling Point is an indicator of volatility: the higher the boiling point, the less volatile the fuel.
Vapour pressure is an indicator of volatility: the higher the vapour pressure, the more volatile the fuel. Vapour pressure increases with temperature, so the volatility of a fuel can be increased by raising the temperature.
A highly volatile fuel is more likely to form a flammable or explosive mixture with air than a non-volatile fuel. By definition, gases are volatile.
Liquid fuels are either sufficiently volatile at room temperature to produce combustible vapour (ethanol, petrol) or produce sufficient combustible vapours when heated (kerosene).
Solid fuels decompose above the vapourisation temperature to produce combustible vapours. Solid fuels will have a higher ignition temperature than liquid or gaseous fuels.
Flashpoint is the minimum temperature to which the pure liquid fuel must be heated so that the vapour pressure is sufficiently high for an explosive mixture to be formed with air when then the liquid is allowed to evaporate and is brought into contact with a flame, spark or hot filament.
Flashpoints are lower than Ignition temperatures.
A fuel with a flashpoint well above room temperature (kerosene) means that it can safely be handled at room temperature since exposure to flames, sparks or hot filaments will not cause an explosion.
A fuel with a flashpoint below room temperature (petrol, alcohol) is a safety hazard since exposure to flames, sparks or hot filaments will cause an explosion. These fuels need to be stored in a cool place to prevent the increased temperature raising the vapour pressure of the fuel and in a well-ventilated place so that any vapours that escape do not accumulate, and preferably in robust metal containers with narrow mouths and tightly sealing lids to prevent vapours escaping.
Ease of liquefaction
Gases occupy large volumes, whereas liquids of the same mass occupy much less volume making them easier to transport.
Critical temperature is the temperature below which a gas can be liquified (condensed) by increasing the pressure.
Liquid Petroleum Gas (LPG) is made up of propane (critical temperature 97°C) and butane (critical temperature 152°C) both of which are gases at room temperature and pressure but can be easily condensed to the liquid at room temperature by increasing the pressure since their critical temperatures are above room temperature.
Products of Combustion
- Complete combustion of carbon-based fuels produce carbon dioxide and water vapour. Carbon dioxide gas is a major contributor to the greenhouse effect.
- Incomplete combustion of carbon-based fuels produces toxic carbon monoxide and solid carbon (soot).
- Sulfur and nitrogen are present in fossil fuels. The sulfur burns to produce oxides which contribute to acid rain, while the nitrogen burns to produce oxides that contribute not only to acid rain but also to photochemical smog.
- Solid fuels such as coal contain incombusitible minerals leading to ash. The ash can damage machinery and can cause lung disease.
- Uncombusted fuel can also be released. Unburnt hydrocarbons from cars contribute to photochemical smog and some are carcinogenic.
- Some fuels contain additives (such as the lead in leaded petrol) which can be harmful.