Biodiesel is the second most popular liquid biofuel. “Diesel” fuel got its name from the type of internal combustion engine in which it is used. The engine was first demonstrated by Rudolf Diesel in 1893, and uses high compression heating instead of spark plugs to ignite the fuel. That first prototype was run on a biofuel – peanut oil – and not on a petroleum distillate.
Whereas ethanol is ethanol regardless of how it was produced, biodiesel is chemically quite different from petroleum diesel, and the two should not be confused. Petroleum-derived diesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H23, ranging approximately from C10H20 to C15H28. Biodiesel refers to a vegetable oil- or animal fat-based fuel consisting of long-chain alkyl (methyl, propyl or ethyl) esters made by chemically reacting lipids (e.g., vegetable oil, animal fat (tallow)) with an alcohol. As with alcohol biofuels vs. gasoline, biodiesels degrade more polymers than petroleum diesel, so one has to be careful in the choice of fittings. However, biodiesel has the advantage that it is less prone to fouling fuel injectors. At 37 MJ/litre, biodiesel has a 9% lower energy density than petroleum diesel, but a higher flash point. The typical density is marginally higher, at 0.88 kg/litre for biodiesel and 0.85 kg/litre for diesel. It also has a high “gelling temperature” at which some of the ingredients begin to crystalize. This temperature depends on the feedstock, but it is too high to use the fuel in pure form in cold countries, particularly if made from tallow. Additives can help keep the fuel liquid.
Production of biodiesel has been increasing but it puts a strain on agricultural resources. Currently biodiesel is more popular in Europe than in the Americas. Globally palm oil and soybean oil are the most commonly used oil crops for making biodiesel with about 1/3 market share each, but of those grown within Europe, rapeseed oil and sunflower oil are the most popular. The end product is usually used as a 5% blend in petrodiesel. In 2006, global production of biodiesel was slightly more than 6 gigalitres, but by 2008 production in Europe alone reached 8.9 gigalitres. Total global production of vegetable oil for all purposes (in 2005-2006) was about 125 gigalitres.
Whereas sugar cane yields ethanol at approximately 7000 litres/hectare/year, common vegetable oil crops from temperate climatic zones such as soy, rapeseed, peanet, and sunflower all yield below 1000 litres/ha/yr. Palm has the highest yield at 4700 litres/ha/yr, followed by coconut at 2100 litres/ha/yr. Admittedly the energy content per litre is better with biodiesel than bioalcohol, but not enough to overcome the difference in crop yields. Also, these are gross yields, not net yields after subtracting the energy consumed in production. In other words, they do not take the energy balance into account. The energy balance of biodiesel varies with the crop, but some estimates for rapeseed range from 1.8 to 3.7, which is worse than for cane ethanol but better than corn alcohol. According to the EPA, biodiesel from soy oil results, on average, in a 57% reduction in greenhouse gases compared to fossil diesel, and biodiesel produced from waste grease results in an 86% reduction.