The exploitation of non-renewable energy resources certainly leads to their final consumption in the future. This practice will lead humanity into an awkward position, and therefore the use of alternative and renewable energy sources is a hot topic in recent years. Why don’t we let the plants to store solar energy for us, and then convert these plants into fuel, instead of conventional fossil fuel production? Biofuels (such as ethanol, biodiesel, biogas and others) have already found their place in the market.
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Biofuels are fuels that are produced by processing biomass (by living organisms – plants, animals and micro-organisms), and they belong to renewable energy sources as opposed to fossil fuels.
Biofuels are divided into two groups:
- First generation biofuels – they are made from starch, sugar or vegetable oil. Their feedstock is not sustainable and “green” as second generation biofuels.
- Second generation biofuels – they are made from sustainable feedstock, but they are not widely available for use as biofuels from the previous group.
According to the states of matter, all biofuels can be divided into three categories: solid, liquid and gaseous biofuels.
# 1. Solid biofuels
- Wood – in various forms: logs, wood chips, twigs, briquettes, pellets, etc.
- Straw and hay – packed or not
- Other plant residues and energy crops
- Animal dung
- Municipal waste
Solid biofuels are often in a convenient form, so there is no special production in most cases. The exceptions are wood chips, sawdust and pellets. They are mainly put through a special process known as “densifying.” All solid biomass should be in a suitable and usable form. Therefore, the solid materials are compressed or mixed with some bonding agent.
# 2. Liquid biofuels
- Alcoholic biofuels – alcohols produced from biomass (bio-ethanol, bio methanol, propanol and butanol)
- Bio oil – vegetable oils, biodiesel, green diesel
- Waste liquid products
Vegetable oils (olive oil, sunflower oil, nut oil, and others) are used as unmodified or slightly modified. In some areas, sunflower, olive or sugar beets are grown specifically for biofuel production. Also, the pure vegetable oil can be used in modified diesel engine.
Lower-quality vegetable oils and animal fats have been used for the production of the biodiesel. The main advantage of biodiesel is low emissions of carbon monoxide and carbon dioxide.
Green diesel is made from hydrocracking oil and fat feedstock. “Hydrocracking” is a method that requires high temperatures and pressure to break down larger molecules.
# 3. Gaseous biofuels
- Syngas – a mixture of hydrogen, carbon monoxide and other hydrocarbons
- Producer gas – produced by processing biomass (wood gas)
- Some distillation gases produced by processing biomass
- Hydrogen – formed by decomposition of any hydrocarbon
Syngas is a gas produced by partial burning of dried biomass. Syngas can be burned in turbines, internal combustion engines or high-temperature fuel cells.
Wood gas is a kind of syngas, and it can be used as a fuel for stoves, furnaces and vehicles in place of diesel, gasoline or other fuels. During its production, biomass is gasified within the oxygen-limited generator to produce carbon monoxide and hydrogen.
Biogas is gaseous biofuels that can be produced from residues such as residues from paper or sugar production, crop materials, animal dung, and so on. These various waste should be mixed together, and subjected to a natural fermentation to produce methane.
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