Courtesy: ISO 16559:2014 Solid biofuels
First-generation biofuels are fuels made from food crops grown on arable land. The crop’s sugar, starch, or oil content is converted into biodiesel or ethanol, using transesterification, or yeast fermentation.
Up to 40% of corn produced in the United States is used to make ethanol, and worldwide 10% of all grain is turned into biofuel. A 50% reduction in grain used for biofuels in the US and Europe would replace all of Ukraine’s grain exports
Second-generation biofuels are fuels made from lignocellulosic or woody biomass, or agricultural residues/waste. The feedstock used to make the fuels either grow on arable land but are byproducts of the main crop, or they are grown on marginal land. Second-generation feedstocks include straw, bagasse, perennial grasses, jatropha, waste vegetable oil, municipal solid waste and so forth. The use of this class of biofuels is thought to increase environmental sustainability, since the non-food part of plants is being used to produce second-generation biofuels, instead of being disposed. But the use of this class of biofuels increases the competition for lignocellulosic biomass, increasing the cost of producing these biofuels. Although the use of this class of biofuels reduces carbon emissions, their use does not yield net zero carbon emission.
Algae can be produced in ponds or tanks on land, and out at sea. Algal fuels have high yields, can be grown with minimal impact on fresh water resources, can be produced using saline water and wastewater, have a high ignition point, and are biodegradable and relatively harmless to the environment if spilled. Production requires large amounts of energy and fertilizer, the produced fuel degrades faster than other biofuels, and it does not flow well in cold temperatures. By 2017, due to economic considerations, most efforts to produce fuel from algae have been abandoned or changed to other applications.
This class of biofuels includes electrofuels and solar fuels. Electrofuels are made by storing electrical energy in the chemical bonds of liquids and gases. The primary targets are butanol, biodiesel, and hydrogen, but include other alcohols and carbon-containing gases such as methane and butane. A solar fuel is a synthetic chemical fuel produced from solar energy. Light is converted to chemical energy, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.
Fourth-generation biofuels also include biofuels that are produced by bioengineered organisms i.e. algae and cyanobacteria. Algae and cyanobacteria will use water, carbon dioxide, and solar energy to produce biofuels. This method of biofuel production is still at the research level. The biofuels that are secreted by the bioengineered organisms are expected to have higher photon-to-fuel conversion efficiency, compared to older generations of biofuels. One of the advantages of this class of biofuels is that the cultivation of the organisms that produce the biofuels does not require the use of arable land. The disadvantages include the cost of cultivating the biofuel-producing organisms being very high.