Biofuels (DP IB Chemistry)

Biofuels

What is the difference between renewable and non-renewable resources?

Renewable resources

• Renewable resources will not run out in the foreseeable future because they can be replaced over a relatively short period of time

• They can also be called infinite 

• They can be considered sustainable as they can be produced at the same rate, or faster, than they are being used

• Biofuels are an example of a renewable resource

Non-renewable resources

• Non-renewable resources will run out in the foreseeable future as there are limited supplies

  • They cannot be replaced within a short time period

• They can also be called finite 

• They are not a sustainable resource

• Fossil fuels are an example of a non-renewable resource

Reasons for the development of biofuels and other renewable resources:

• Reduce pollution from the combustion of fossil fuels 

• Have positive impacts on global warming and climate change

• The finite supply / limited amount and depletion of fossil fuels, which are non-renewable resources  

What is a biofuel?

• The three main biofuels are:

  • Bioethanol

  • Biodiesel

  • Biogas

• Biofuels are made from organic compounds 

• These organic compounds are produced by biological carbon fixation

Bioethanol

• Green plants absorb atmospheric carbon dioxide and convert it into glucose via photosynthesis

  • Word equation for photosynthesis:

carbon dioxide + water → glucose + oxygen

• Symbol equation for photosynthesis:

6CO₂ (g) + 6H₂O (l) → C₆H₁₂O₆ (aq) + 6O₂ (g)

• The glucose is then further converted into ethanol by fermentation

  • The name bioethanol is simply identifying how the ethanol has been produced

• When biofuels are produced this way, they can be considered carbon neutral

  • This is because the carbon dioxide absorbed during photosynthesis equals the carbon dioxide produced by the combustion of the biofuel

Biodiesel

• Biodiesel is made from renewable vegetable oils rather than non-sustainable petrochemicals

• Natural triglyceride oils are converted to esters of methanol, which makes them less viscous

• Biodiesel made from rapeseed oil, for example, is produced by transesterification

  • The triglyceride is converted into the less viscous fatty acid methyl ester (FAME) using methanol

    • An acid works by protonating the carbonyl group

    • An alkali works by deprotonating the alcohol / methanol

    • However, it is more common to use an alkaline catalyst such as NaOH / KOHAcids and alkalis can both be used to catalyse the reaction

• The transesterification is reversible, so an excess of methanol is used to drive the equilibrium to the right

• Under optimum conditions, this process can produce a very successful yield of 98%

How to make biodiesel

Transesterification forming methyl esters

Biogas

• Biogas is a renewable fuel that is released when organic matter, such as food or animal waste, is broken down by microorganisms in the absence of oxygen

• It consists mainly of methane and carbon dioxide but can also include small amounts of hydrogen sulphide and other chemicals

• The relative amounts of the components of biogas depend on the type of waste used

  • The bacterial decomposition of carbohydrates such as produces biogas with 50.0% methane content

C₆H₁₂O₆ (s) → 3CO₂ (g) + 3CH₄ (g) 

• Fatty acids such as heptadecanoic acid, commonly called oleic acid, can undergo bacterial decomposition in a more moisture-rich environment to produce biogas with a 69.4% methane content

4C₁₇H₃₃COOH (l) + 34H₂O (l) → 21CO₂ (g) + 51CH₄ (g) 

Advantages of biofuel

• Carbon neutral, renewable and sustainable if crops / trees are replanted 

• Reduce greenhouse emissions / pollution

• Biodiesel and biogas can reduce the amount of waste going to landfill sites 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

• Can provide jobs in the agriculture and energy sectors

Disadvantages of biofuel

• High costs

  • Conversion of engines and machinery to run on biofuels instead of petrol / diesel

  • Harvesting and transportation

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

  • This can lead to deforestation in an effort to meet the demand 

• Removes nutrients from the ground / uses large amounts of fertilisers and pesticides

• Biofuels typically have lower specific energy than fossil fuels