📘 Definition
Biofuel = any fuel derived from recent biological carbon fixation (biomass). Renewable because crops regrow on a human timescale.
Bioethanol Production
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (endothermic)
- Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (catalysed by yeast enzymes, ~37 °C, anaerobic)
- Fractional distillation: purifies ethanol (yeast dies at ~15% concentration)
Carbon Neutrality. Theory vs Reality
| Theory | Reality | |
|---|---|---|
| CO₂ balance | CO₂ absorbed in photosynthesis = CO₂ released in combustion + fermentation | Fossil fuels are used for farming, transport, and distillation → net positive CO₂ |
| Verdict | Perfectly carbon neutral | Rarely truly carbon neutral (Life Cycle Assessment needed) |
Advantages vs Disadvantages
| Advantages ✅ | Disadvantages ❌ |
|---|---|
| Theoretically CO₂-recycling | Deforestation for farmland |
| Reduces dependence on fossil fuels | "Food vs fuel" competition → food prices ↑ |
| Compatible with existing engines (gasohol blends) | Lower specific energy than petrol |
| Grows domestically → energy security | Huge water & fertiliser demands |