What are biofuels? Biofuels are renewable substitutes for fossil fuels that are mainly produced from crop plants such as corn, soybeans, wheat, and sugarcane. But animal fats and other byproducts, along with household food waste, can also be used to make biofuels.
Flexibility and variety is what gives biofuels their renewable energy appeal--even municipal and agricultural waste can be turned into liquid, gas, or solid fuels. One of the most common uses is in motor transport, as a replacement for traditional gasoline or diesel, but also air transport and heating systems.
But how is it made? Sugarcane and corn are used to make ethanol, which is often used as an additive to gasoline, while soybeans and rapeseed (canola) are used to create biodiesel. These plant-derived fuels can be used on their own to drive combustion engines but more often they are blended with traditional gasoline or diesel.
As much as 10 or 15 percent ethanol mixture fuels (labelled E10 and E15) can be added safely to many car engines, but higher percentages require engine modification. Ethanol burns cleaner and produces less harmful greenhouse gas emissions (such as carbon dioxide) than petroleum-based fuels, so it is considered as a possible replacement.
Biofuels are carefully assessed for their environmental impacts by working out how much energy is used in their production, including fertilisers, farming resource, transport, and process manufacturing. Another consideration is the amount of land given to biofuels that might require deforestation or could have been used to produce food instead.
The US and Brazil are the world's largest producers of ethanol fuel, with Brazil leading the way in sugarcane-derived ethanol and the US in corn. Sugarcane ethanol is acknowledged as an advanced biofuel by the Environmental Protection Agency because of its total life cycle reduction in greenhouse gas emissions compared to oil-based fuel. The Brazilian automotive industry has adapted so that car engines use up to 27 percent ethanol.
In 2017 the US turned 5.4 billion bushels of corn (38 per cent of the US supply) into ethanol feedstock. Corn ethanol – created from corn starch – is currently more expensive to produce, requires more energy to make and is less efficient in miles per gallon than gasoline. But it is hoped that processing technologies will improve to make it comparable. By adding cellulosic ethanol – derived from crop or wood residues that grow on land unsuitable for food production, and other waste materials – much less energy is required to grow, collect and convert plant sources to ethanol.
What's the difference between biofuel and biodisel? Biodiesel is made from plant or animal fats including soybean oil and tallow. It can even be made with recycled restaurant fryer oil. Just as ethanol is blended with gasoline, so biodiesel comes in different blends – such as B20 or B5. Engines with no, or few, changes can use B20 but higher blends and B100 may require modification.
Renewable diesel is another alternative 'drop-in' fuel to petrochemical diesel – ie, it can be used as a direct replacement. It is similar in composition to diesel, but is made from fats and cellulosic biomass. This renewable fuel produces fewer emissions and is highly compatible with current engine and generation technology.
Alternatives to the current crop of biofuel sources could boost efficiency and reduce emissions even further. Crops such as hemp produce nearly four times as much oil per acre as soybeans, but are currently illegal to grow in many parts of the US. Native plants such as prairie switchgrass and exotic pests such as giant reed (infesting thousands of acres of the southern US) offer better ethanol production potential than corn.
The US could sustainably grow enough biomass for fuel to provide more than 25 percent of its current energy consumption by 2030. The Department of Energy is now developing systems for the efficient harvesting, collection, preprocessing and transport of fuel feedstocks to biorefineries.
Algae that grow in water are also promising for their biofuel potential. The US Department for Energy and large oil companies have looked to exploit these siimple aquatic organisms for oil production for a decade. They could produce 200 times more oil per acre than soy and they grow in salt water, waste water lagoons, or in man-made basins in the desert.
One clear advantage is the lack of arable land needed to grow them. Scientists at the University of Manchester in the UK are exploring synthetic biology methods to turn Halomonas bacteria into bio-based jet fuel. By using methods similar to those in the brewing industry they hope to turn this abundant seawater microbe into fuel using water and sugar.
Another project is turning mustard seeds into aviation fuel for Australian airline Qantas. In a world first, the company used a blended fuel that was part-derived from the brassica carinata mustard seed plant – a fallow crop used by farmers in between regular planting. These types of breakthroughs could revolutionise biofuel production worldwide.
Suddenly, fuels and their precursors can be found in sources as modest as everyday trash. At the Berkeley Lab in California a process has been discovered to turn municipal solid waste into chemicals to make biodiesel. London's Stansted Airport is turning its coffee waste into solid fuel logs that are used in mult-fuel stoves and burners.