Alternative Fuels (Edexcel International A Level Chemistry): Revision Note

Reducing Emissions

• To reduce the amount of pollutants released in car exhaust fumes, many cars are now fitted with catalytic converters

• Precious metals (such as platinum, rhodium and palladium) are coated on a honeycomb to provide a large surface area

• The reactions that take place in the catalytic converter include:

  • Oxidation of unburnt hydrocarbons:

C_nH_2n+2 + (3n+1)[O] → nCO₂ + (n+1)H₂O

•  Oxidation of CO to CO₂:

2CO + O₂ → 2CO₂

or

2CO + 2NO → 2CO₂ + N₂

Catalytic removal of NO_x and CO

• Carbon monoxide and the nitrogen oxides released through cars’ exhaust fumes pollute the atmosphere

• The nitrogen oxides are reduced on the surface of the hot catalyst to form the unreactive and harmless nitrogen gas which is then released from the vehicle’s exhaust pipe into the atmosphere

• The chemical reaction for the reduction of nitrogen oxide to nitrogen gas by the catalyst is as follows:

2CO (g) + 2NO (g) → 2CO₂ (g) + N₂ (g)

A catalytic converter helps reduce the pollutants from motor vehicles

Alternative fuels

• There are a number of reasons for finding and developing alternative fuels:

  • Reducing pollution from the combustion of fossil fuels, including issues associated with global warming and climate change

  • The limited amount and depletion of non-renewable fossil fuels are two reasons for finding alternative fuels

• Biofuels are renewable fuels 

  • Renewable meaning that they can be replaced over a short period of time

  • The bio part of biofuel meaning that it comes from living matter

• The three main biofuels are biodiesel, bioethanol and biogas 

  • Biodiesel - made by refining renewable fats and oils

  • Bioethanol - made by fermentation

  • Biogas - made / released when organic waste breaks down

Benefits of biofuels

• Biofuels are often considered as carbon neutral

• Biodiesel and biogas can reduce the amount of waste going to landfill as the waste can be used to produce them

• Biofuel production could provide money for less developed countries as they have the space to grow the crops required

Drawbacks of biofuels

• The cost of converting engines and machinery to run on biofuels instead of petrol / diesel

• Many developed countries don't have the space to be able to produce enough plants to make the biofuels because the land is needed for food production

Greenhouse Gases

• When shortwave radiation from the sun strikes the Earth’s surface it is absorbed and re-emitted from the surface of the Earth as infrared radiation

• Much of the radiation, however, is trapped inside the Earth’s atmosphere by greenhouse gases which can absorb and store the energy

• Carbon dioxide, methane and water vapour are gases that have this effect

• Increasing levels of carbon dioxide, although present in only a small amount, is causing significant upset to the Earth’s natural conditions by trapping extra heat energy

• This process is called the enhanced greenhouse effect

Carbon Neutrality

• Carbon neutral is defined as a net-zero effect on the amount of carbon dioxide in the atmosphere

• Hydrogen gas is an important carbon neutral fuel that produces only water when it combusts

H₂ (g) + O₂ (g)  → 2H₂O (l) 

• Although it produces a large amount of energy, the low density of hydrogen means you need large amounts compared to fossil fuels to give the same amount of energy per gram

  • Along with its explosive nature, this results in storage problems when using hydrogen as a fuel

  • Producing hydrogen is not cheap, so although it qualifies as a carbon neutral fuel its widespread use remains limited

• Biofuels are commonly thought of as being plant-based 

  • As a plant grows, it absorbs carbon dioxide from the atmosphere during photosynthesis

  • When the plant is then burnt as a fuel it releases the carbon that is stored as carbon dioxide

  • If the amount of carbon dioxide during growth is the same as the amount of carbon dioxide released during combustion, then this is considered to be carbon neutral

As a plant grows, it absorbs carbon dioxide from the atmosphere during photosynthesis

• There is an argument for fossil fuels being carbon neutral because they are releasing the carbon dioxide that they absorbed millions of years ago

  • However, they are not carbon neutral because they absorbed carbon dioxide millions of years ago when the levels of carbon dioxide in the atmosphere were higher than the present day

• Biofuels are considered to be carbon neutral, although this is not necessarily true

  • There is no consideration for the carbon dioxide used in their production, harvesting, manufacture and any transport involved

• Biofuel is a generic term applied to any fuel that is considered to have a biological (often plant-based) origin and is an indication of the way that the fuel was produced

• One of the most common biofuels is bioethanol, which is the same as ethanol

• It is produced by the fermentation of sugars using yeast enzymes

• There is a limit to the concentration of ethanol in solution that can be produced using this method, although more efficient and higher yield processes are being developed

• The separation of the ethanol from the solution is one of the reasons that bioethanol can be argued not to be carbon neutral

• The following four points are the major considerations for biofuels:

  1. Land - should the land used for biofuel production be used for food production instead?

  2. Yield - what volume of biofuel can be produced in the land provided? What percentage of the crop becomes useful fuel?

  3. Manufacture and transport - what are the energy costs involved in growing, harvesting, processing and transporting the biofuel from crop to finished product?

  4. Carbon neutrality - how close is the finished product (including manufacture and transport) to being carbon neutral?