📘 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 |
Photosynthesis: The Carbon Cycle Link
Biofuels are produced from biomass grown via photosynthesis:
6CO2 + 6H2O → C6H12O6 + 6O2
In theory, biofuels are carbon neutral because the CO2 released during combustion equals the CO2 absorbed during photosynthesis. In practice, energy is required for farming, harvesting, processing, and transport, so biofuels are not truly carbon neutral.
Advantages vs Disadvantages
| Advantages | Disadvantages |
|---|---|
| Renewable: crops can be regrown | Lower specific energy than fossil fuels |
| Theoretically carbon neutral | Competes with food production for land |
| Can be blended with petrol | Large-scale farming causes deforestation |
| Biodegradable, less persistent pollution | Production requires significant energy input |
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