Biofuels (SL IB Chemistry)

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