Biofuels can be produced from various types of biomass via numerous technologies and pathways which are often separated into 'generations'. The main commercialised or developing biofuel production pathways, are outlined below.
First generation biofuel technologies are in commercial production and are made from sugar, starch, animal fats and vegetable oil. Biomethane, produced through anaerobic digestion, can be upgraded for transport fuel use and is also regarded as a first generation biofuel.
Biodiesel and renewable diesel
Biodiesel can be produced from waste fats and oils, tallow and oilseed energy crops such as mustard, canola and palm seed through various processes. Biodiesel can be mixed with diesel, or used unblended in many modern diesel engines.
Ethanol is produced through fermentation of sugars and starches extracted from agricultural wastes or crops including sugar cane, wheat and corn. Brazil produces around 80% of the world's ethanol from sugar cane. Ethanol is typically blended with petrol at rates of 5-10%. E85 engines, specifically designed to run on 85% ethanol, 15% petrol have been in use for decades.
Biomethane is biogas which can be upgraded to natural gas standard and exported to the natural gas grid or used in gas-powered vehicles.
Second generation biofuels generally refer to a broad range of liquid fuels derived from new and non-edible feedstocks produced via a range of novel, and often complex, technologies and pathways.
Bioethanol from woody biomass
The lignocellulosic, (woody) components of plants, i.e. lignin, hemicellulose and cellulose from a wide range of non-edible feedstocks such as wood or straw, can be converted to biofuels via different technologies that are in varying stages of development and commercialisation.
Biochemical 2nd generation biofuel production involves pre-treatment to separate out the biomass into lignin, cellulose and hemicellulose. This process is typically followed by either chemical hydrolysis of cellulose, then fermentation or, via biochemical pathways, using enzymes and micro-organisms to hydrolyse sugars prior to fermentation to produce ethanol, methanol, butanol and other biofuels.
Canola - flowering, close up
Mustard oil, just extracted from seed
Thermochemical 2nd generation biofuel pathways incorporate pyrolysis or gasification technologies. Fast pyrolysis processes produce bio-oil which is further refined into biofuels or high value chemicals. Various industries and researchers are investigating upgrading of pyrolysis oil into biofuels. Gasification methods produce syngas which can be purified and used in vehicles as an LPG substitute, or further processed using Fischer-Tropsch, (FT) synthesis or other processes to produce syndiesel and aviation biofuels. FT was originally developed in the 1920s to produce transport fuels from coal. Alternatively, syngas from high temperature gasification can also be converted into biofuels via biological processes or chemical catalysts.
Novel starch, oil and sugar from crops including Jatropha and Miscanthus can produce, what the International Energy Agency also regard as, 2nd generation bioethanol and biodiesel.
Greater efficiencies in some of the processes are needed for 2nd generation biofuels to match costs of 1st generation biofuel and fossil fuel production.
3rd generation biofuel technologies include hydrogen production from biomass, (which can be used in fuel cells or special internal combustion engines) and algae production for biofuels.
Algae for biofuel production systems can be sorted into two categories:
Microalgae are microscopic, photosynthetic organisms that produce substances, such as lipids, that can be harvested and converted to a range of products, including biodiesel. Many technical challenges need to be overcome to reduce the high cost of algae biofuel production Other potentially high-value by products can be extracted from some algae species and thereby reduce overall production costs.
Macroalgae such as seaweed, can potentially be grown and converted to heat and power, eg. via biodigestion to biomethane, or fermentation to ethanol. Macroalgae is still at an early stage of development.
A number of other biofuels can be produced from biomass. Examples include: DiMethyl Ether, produced via gasification, is recognised as a viable additive to transport fuels. Biobutanol, produced via bacterial fermentation, can be used in petrol-fuelled cars without changes to the engine in blends up to 85%.