Fuels

Fuels and Earth Science

Crude Oil

Crude oil is a fossil fuel, and is made up of a complex mixture of hydrocarbons. This mixture contains many useful compounds (in which carbon atoms are in chains or rings) that need to be separated using a process called fractional distillation. Crude oil can be described as a finite resource.

A hydrocarbon is a chemical that contains hydrogen and carbon atoms only. Crude oil mostly contains hydrocarbons called alkanes.

The hydrocarbon molecules in crude oil each have a different number of carbon atoms, and therefore have different chain lengths.

Crude oil is an important source of useful substances. The chemicals it contains can be used as either fuels, or feedstock for the petrochemical industry.

Properties of hydrocarbons

As the chain length of a hydrocarbon increases, the properties of the hydrocarbon change with it.

A longer hydrocarbon chain will:

  • have a higher boiling point
  • be less flammable (harder to ignite)
  • be more viscous

The reason the boiling point and viscosity increases is due to there are more intermolecular forces of attraction holding the chains together (more energy needed to break these apart).

Fractional Distillation

Fractional distillation is the process of taking a complex mixture of liquids (like crude oil) and separating it out into simpler, more useful mixtures.

To separate the different hydrocarbons, the crude oil mixture must be passed through a fractionating column (also called a distillation column), and is first heated to vaporise the substances into a gas. This separating technique works as each of the chemicals has a specific boiling point, and once each gas has cooled to its specific boiling point - they will condense back into a liquid and can be collected.

Each mixture that is collected from fractional distillation, is called a fraction. A fraction is a mixture of similar length hydrocarbon chains, with similar boiling points.

A common exam question asks about the steps involved in separating crude oil into its fractions. Let's take a look:

  1. Crude oil is heated as it enters the fractionating column, and is vaporised
  2. The column is cooler at the top, and hotter at the bottom
  3. The gases will begin to rise and as they do so they will cool, and will condense once they reach their boiling point
  4. The longer the carbon chain, the higher the boiling point - these will condense near the bottom of the column
  5. The shorter the carbon chain, the lower the boiling point - these will condense near the top of the column
  6. The different molecules will condense into fractions (mixtures of hydrocarbons with similar carbon chain lengths, and similar boiling points)

Fractional Distillation of Crude Oil

  • gases, used in domestic heating and cooking
  • petrol (gasoline), used as fuel for cars
  • kerosene, used as fuel for aircraft
  • diesel oil, used as fuel for some cars and trains
  • fuel oil, used as fuel for large ships and power stations
  • bitumen, used to surface roads and roofs

Homologous Series: Alkanes

A homologous series is a group of compounds that:

  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties
  • have similar chemical properties

Alkanes are one example of a homologous series. They:

  • have the general formula of CnH2n+2
    • this means that for every 1 carbon atom in an alkane, there are two times the amount of hydrogens... plus another two
  • differ by CH2 in molecular formulae from neighbouring compounds
    • increasing the carbon chain length by 1 carbon atom, also increases the number of hydrogens by 2
  • show a gradual variation in physical properties
    • boiling points increase with increased carbon chain length
    • viscosity increases with increased carbon chain length
  • have similar chemical properties

Because alkanes only contain single bonds between atoms, we describe them as saturated compounds. Think of when a sponge cannot hold any more water, it is "full". A saturated molecule can't fit any more single bonds into it. Alkanes can be considered "full".

Homologous Series: Alkanes

Petrol, kerosene and diesel oil are non-renewable fossil fuels obtained from crude oil. They are all mixtures of alkanes.

Methane is a non-renewable fossil fuel found in natural gas.

Cracking

Some of the heavier hydrocarbons removed during fractional distillation aren't very useful. They are thick liquids with have very high boiling points, and they don't burn very easily. 

Cracking involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones: an shorter saturated alkane, and an unsaturated alkene.

Turning longer chain alkanes into shorter alkanes is extremely beneficial as the current world demand for short alkanes (such as petrol and diesel) is much higher than the supply of these molecules from fractional distillation.

The process of cracking is a thermal decomposition. Large chain hydrocarbons are heated, and either passed over a hot catalyst or are reacted with steam. This breaks the large chained alkanes into a smaller alkane and alkene molecule.

Adding bromine water to an alkene will turn the solution colourless (from yellow/orange). However, adding bromine water to alkanes will result in no colour change, and so this is a handy test to tell the difference between the two - as both alkanes and alkenes are often colourless liquids.

Cracking and Alkenes

Combustion

When fuels burn they are reacting with the oxygen in the air. If there is an abundance of air, then complete combustion will take place. This means carbon dioxide is produced. If they isn't enough air, then incomplete combustion happens and carbon monoxide is made. In both cases, we can say the carbon (and hydrogen) has been oxidised. Both of these types of combustion are exothermic processes, releasing energy.

Complete combustion

fuel + oxygen (abundant) → carbon dioxide + water

Incomplete combustion

fuel + oxygen (low levels) → carbon monoxide + water (+ carbon particulates)

Incomplete combustion produces carbon monoxide, and carbon particulates, because there is not enough oxygen to fully form carbon dioxide.

Complete and Incomplete Combustion of Fuels

Pollutants From Fuels

The combustion of fuels is a major source of atmospheric pollutants.

When a fuel is burned it releases many gases into the atmosphere. These gases can include:

  • carbon monoxide
    • this is a toxic invisible gas, and it preferentially (and permanently) binds to haemoglobin in red blood cells instead of oxygen
  • carbon particulates
    • unreacted carbon atoms that cause soot that can damage/clog up appliances
    • contributes to smog and global dimming
  • sulfur dioxide
    • sulfur impurities in fossil fuels like coal and crude oil can react with oxygen when the fuels are burned
    • sulfur dioxide can dissolve in rain water to produce acid rain, which can damage structures, and wildlife habitats such as lakes and forests
  • nitrogen oxides
    • when fuels are burned in engines, oxygen and nitrogen from the air can react together at high temperatures to produce oxides of nitrogen
    • these can also dissolve in rain water to produce acid rain

Atmospheric Pollutants

Hydrogen as a Fuel

Hydrogen can be used instead of petrol in cars, in something called a fuel cell. These are electrical cells that use energy released from a reaction between a fuel and oxygen. They do not need replacing or recharging, and instead they have a fuel tank that needs refilling. 

Modern fuel cells use hydrogen as the fuel, and produce water as the product:

hydrogen + oxygen → water
2H2 + O2 → 2H2O

Compared to a 'normal' fuel (such as petrol/diesel), hydrogen fuels cells have many advantages and disadvantages:

Advantages Disadvantages
chemical energy is converted straight to electrical energy, compared to a fossil fuel which converts chemical energy to heat energy first (so fewer transfer stages, and less 'lost' energy)
hydrogen is a gas, so takes up a lot more space to store than a liquid fuel
there are less pollutants - in fact no pollutants, as water is only product
hydrogen is explosive, so has to be stored safely
rechargeable batteries have a limit on how many times they can be recharged, whereas fuel cells can keep going as long as they're refilled
currently hydrogen gas is produced from hydrocarbons, or electrolysis of water (requires use of electricity, generally powered by burning fossil fuels)

Hydrogen Fuel Cells