Biogas production
Biogas is a mixture of gases, mainly methane (CH4) and carbon dioxide, produced by the anaerobic digestion of organic waste by bacteria. Sources of organic waste include animal manure, crop residues, sewage and food waste.
The waste is placed in a sealed container called a biogas generator (or digester). Anaerobic bacteria break down the organic matter in the absence of oxygen, producing methane. The methane can be burned as a fuel for heating, cooking or generating electricity. The remaining digestate is a nutrient-rich material that can be used as fertiliser.
Bioethanol production
Bioethanol is ethanol produced by yeast fermentation of sugars from crops. Sugar cane and maize (corn) are the most common feedstocks. The sugars are extracted from the plant material, and yeast ferments them anaerobically:
glucose → ethanol + carbon dioxide
The ethanol is distilled to increase its concentration and can then be mixed with petrol to make a blended fuel (e.g. E10 contains 10% ethanol). Brazil is a major producer of bioethanol from sugar cane, using it to power vehicles.
Advantages of biofuels
Renewable: Biofuels can be produced continuously from new crops or waste, unlike fossil fuels which are finite. Carbon neutral (in theory): The CO2 released when biofuels are burned equals the CO2 absorbed by the plants during photosynthesis. Waste management: Biogas production disposes of organic waste that would otherwise produce methane in landfill. Reduces dependence on fossil fuels: Biofuels diversify the energy supply.
Disadvantages and limitations
Land use: Growing crops for biofuel competes with food production for arable land, potentially raising food prices. Deforestation: In some regions, forests are cleared to grow biofuel crops, releasing stored carbon and destroying habitats, which can mean the biofuel is not truly carbon neutral.
Energy input: Growing, harvesting, transporting and processing biofuel crops requires energy, often from fossil fuels. Lower energy density: Biofuels generally contain less energy per unit volume than fossil fuels, so more is needed for the same output. Large areas required: Growing enough biofuel crops to replace fossil fuels would require vast areas of land.