Biofuel is any fuel in which the energy is derived from biological carbon fixation that has occurred relatively recently. Biofuels include biomass derivatives, biogases, and liquid fuels. Biofuels can be widely divided into bioalcohols, biodiesel, green diesel, biogas, syngas, solid biofuels, and even vegetable oil. The previous all constitute what are known as first generation biofuels because they are made from sugar, starch, and vegetable oil. In other words, they are made from things that are part of the food supply.
Second generation biofuels are made from “sustainable” organic material. In general, this refers to any biological carbon that is not a part of the food supply. Often this takes the form of cellulose or algae.
Alcohols like ethanol and butanol can only be considered “biofuels” if they were produced through the process of fermentation, which involves either bacteria or yeast. Ethanol is the most commonly used form of bioalcohol, but biobutanol is gaining in favor because it can be used directly in gasoline engines without their needing to be modified.
Ethanol can be used in engines so long as the percentage of fuel containing ethanol remains at or below 15%. Above 15%, ethanol causes damage to rubber and plastic components of cars such as fuel lines. In order to run higher percentages of ethanol, vehicles must be modified.
Butanol has high net gains in terms of energy when compared to ethanol and is less corrosive to engine components. DuPont and BP are currently working to develop butanol as a viable alternative to gasoline.
Biodiesel is more common in Europe than are bioalcohols. This is in part due to the fact that Europe has historically had a higher adoption rate for diesel power vehicles than has the U.S.
In general, biodiesel is less corrosive than the bioalcohols. Many manufactures support the use of B100 (100% biodiesel) in their vehicles. In many countries, 5% biodiesel is quite common.
This includes any diesel fuel derived from renewable resources such as canola oil or algae. The real difference between green diesel and biodiesel is how they are produced. Biodiesel is produce through fermentation or, more accurately, transesterification. Green diesel is produced by fractional distillation, which is the same process used to produce crude oils.
Biogas is methane produced from fermentation. It is attractive in some respects because it is produced as a byproduct of mechanical waste treatment and from landfills. Thus, it is already available in some locales. Farmers often produce it from manure.
Things such as wood, sawdust, grass cuttings, and garbage, which can be directly burned, are included in this category. These fuels, while releasing particulates, have less environmental impact than fossil fuels.
Second Generation Biofuels
Any biofuel produced from sustainable feedstock will fit into this category. The basic point is that food is not diverted from the food chain (animal or human) to produce these fuels. Currently, bacteria and fungi are used to produce these fuels. The recent discovery of a fungus in Patagonia that can produce diesel fuel from cellulose has raised hopes that biodiesel could be produced without any reliance of foodstocks and at relatively low cost.