Higher Chemistry Unit 2(a) Fuels

Petrol The gasoline fraction of crude oil is the main source of petrol. The gasoline fraction is obtained by the fractional distillation of crude oil. The fractions are separated according to their boiling point ranges. The petrol fraction contains straight chain hydrocarbon compounds which do not perform well as a fuel in a petrol-burning engine.

Petrol engines versus diesel engines  In a petrol engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs.  In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites.

Reforming Because petrol from the gasoline fraction contains only straight chain hydrocarbon compounds it auto- ignites and so does not burn efficiently. To prevent auto-ignition (also known as ‘knocking’) aromatic, branched and cyclic hydrocarbons are added. Reforming of the molecules in the naphtha fraction is used to produce aromatic, cyclic and branched chain hydrocarbons. These compounds are then added to the compounds from the gasoline fraction to produce petrol for use in petrol engines.

Reforming alters the arrangement of atoms in molecules. It may also alter the number of carbon and/or hydrogen atoms in the molecule. e.g. Straight chain – branched chain CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 ↓ CH 3 C(CH 3 ) 2 CH 2 CH(CH 3 )CH 3 Straight chain – aromatic [see Unit 2(b)] C 6 H 14 → C 6 H 6 + 4H 2

Straight chain – cyclic C 6 H 14 → C 6 H 12 + H 2 The removal of hydrogen from a molecule is also known as dehydrogenation.

Volatility The volatility of a substance is the measure of how quickly it evaporates. If a substance has a high volatility it evaporates easily. The hydrocarbon blend in petrol is adjusted in summer and winter to ensure that it is volatile enough for the temperatures. In low temperatures more high-volatility compounds are included like butane. In higher temperatures more low volatility compounds are included.

Octane rating Petrol's octane rating is a measurement of the fuel's ability to resist engine knocking. Knock occurs when the fuel-air mix in the cylinder explodes instead of burning in a controlled way. This shockwave moves within the combustion chamber, and creates a metallic 'pinging' sound. Knocking is inefficient and can cause damage to the engine. An octane rating is often referred to as an 'anti- knock index'. If fuel has a high octane number, it will have a higher resistance to engine knocking.

Lead and unleaded petrol Leaded petrol was banned from use in 2001 due to the harmful effects of the lead compounds to the environment. Leaded petrol contained the lead compound, lead tetra-ethyl, which acted as an anti- knocking agent. Unleaded petrol contains aromatic, cyclic and branched hydrocarbon compounds to prevent auto-ignition.

Alternative fuels Currently, the majority of our fuels are derived from fossil fuels which are finite and so are going to run out. Fuels from fossil fuels also contain carbon which forms carbon dioxide when the fuel is burned. Carbon dioxide cause global warming. Alternative renewable fuels include ethanol, methanol, biogas and hydrogen.

Ethanol Ethanol, C 2 H 5 OH, is produced by fermentation of sucrose from sugar cane. Gasohol is a lead-free petrol containing between 10 – 20% ethanol. Both ethanol and gasohol have a high octane rating. Although ethanol contains carbon and so produces carbon dioxide on burning, the sugar cane it is made from takes in carbon dioxide during its growth. Ethanol is therefore neutral in its contribution to global warming.

Methanol Methanol is currently used as a fuel in racing cars. Advantages: - burns more efficiently than petrol so less CO produced. - contains no carcinogens. - less volatile and explosive than petrol. - car engine requires little modification.

Disadvantages - methanol absorbs water forming immiscible layers which become corrosive to car engines. -methanol is toxic. - less energy is produced than from the same volume of petrol so bigger fuel tanks are required. - methanol is made from synthesis gas which is made from methane – a fossil fuel. This increases greenhouse gases.

Biogas Biogas is produced when animal and vegetable matter decay in the absence of oxygen. Biogas is a mixture of methane (approx. 60%) and carbon dioxide. Sewage works can now be powered by the biogas produced in the plant and the biogas produced from decaying matter in landfill sites can be used in local heating schemes.

The burning of biogas does not contribute to global warming since it is produced by the decay of plant material which used up CO 2 during photosynthesis.

Hydrogen Hydrogen is being seen as the new fuel of the future when fossil fuels run out. Hydrogen burns efficiently to produce only water and no CO 2 which contributes to global warming. Hydrogen is made by the electrolysis of water. Renewable energy sources such as solar power are being investigated as the best way to provide the energy required for electrolysis.

The major problem is the difficulty for a small car to carry enough of it in liquid form (it must be kept below -250 o C) in well-insulated tanks. This adds considerable weight to the vehicle. Fuel cells are used on board spacecraft. The fuel cell uses hydrogen and air in the reverse of electrolysis to form water and electrical energy. T:\Departments\Chemistry\Higher\Unit 2 pp presentations\cleanenergy-dsl2.wmv T:\Departments\Chemistry\Higher\Unit 2 pp presentations\cleanenergy-dsl2.wmv