A biofuel is a fuel based on a biological source. This typically means that is derived from plant stocks, but it is also possible to make fuels from animal fat. Biofuels have some major advantages over fossil fuels:
- Maintaining the Carbon Balance: the carbon in biofuels comes from atmospheric carbon dioxide (CO2.) Thus, when it is burned, it releases the same amount of CO2 as it consumes - actually less, but only because other nasties like carbon monoxide (CO) are produced.
- Utilizing Solar Energy: the majority of energy that goes into production of plant-based biofuels is solar. This energy would otherwise be "lost". Solar energy is effectively the cleanest source of energy we have, since there's little we can do about it - it's just part of the environment.
- Reduces Emissions - biofuels typically have dramatically less emissions per mile traveled than fossil fuels.
- Reduces Oil Dependence - using less oil means needing less oil, which means importing less oil. It also means that the environmental demands placed by oil production can be reduced - there's plenty of pollution before the crude oil even reaches the refinery.
There's just a handful of highly popular biofuels. I'll hit the high notes:
Ethanol: not just a way to have a good time on a Friday night, ethanol is also probably the world's leading biofuel. Ethanol has about 80% of the energy density of gasoline, which is what it usually replaces, so mileage is correspondingly reduced. Ethanol requires substantial conversion. Ethanol is also used in drag racing, at very high burn rates. Ethanol is more commonly used blended with gasoline, as E85 or E95. One of its most appealing properties is that it is mostly harmless (at least, as compared to other strong alcohols, like Everclear.)
Methanol is pretty much the same thing as ethanol, except instead of an ethyl group, it's got a methyl group. In sharp contrast to Ethanol, it is extremely dangerous and should always be treated with respect. Whether you inhale a strong concentration of vapors, or just get methanol on your skin, it can get into your system and cause blindness or death. In fact, it is much more dangerous than gasoline or petrochemical diesel fuel in this regard. Methanol is also used as a gasoline additive; it specifically replaces MTBE, an even more toxic chemical1 which is used in gasoline for stability and octane adjustment, and which has fallen out of favor and is gradually being outlawed everywhere - California, as usual, comes first in the US. Most Methanol is currently produced from natural gas (as is most hydrogen) but this is not necessary.
E85 is the easiest "biofuel" to find "at the pump." It consists of 85% ethanol, and 15% gasoline, so it's only mostly biofuel. It requires less modification than pure ethanol and it is fairly common to find vehicles that will run on both gasoline and E85, including models of Ford Taurus, the Hummer H2, and others. Obviously, containing gasoline, it is not a pure biofuel.
E95 is not in common use, but it is so similar to E85, I felt it bore mentioning. It's got only 5% gasoline and it is for use not in gasoline engines, but in diesels. It requires an improvement in compression, and changes in fuel delivery and timing. It is definitely, however, not the most popular diesel biofuel.
Biodiesel is a diesel fuel, directly comparable to diesel in terms of energy density, which is made from vegetable oil, typically through a process which uses lye and methanol. It's also possible to make it using ethanol instead, but that's more difficult, albeit safer. As compared to petrodiesel, biodiesel slightly increases nitrous oxides production, but decreases soot, CO, and CO2 emissions.
Vegetable Oil is also a diesel fuel. The same stuff you get at the supermarket can go right into a diesel engine. Because the viscosity of vegetable oil is much higher than that of diesel fuel, it must be heated before injection in order to produce proper atomization, necessary for proper combustion. Any oil can be used, though some are better suited than others. Corn and canola (rapeseed) oils are two of the better options, while soybean oil is among the worst due to its high acidity - a solvable problem, but not generally worth the cost. Veggie oil is referred to as "straight vegetable oil" (SVO) when clean, and "waste vegetable oil" (WVO) when dirty, such as when it is reclaimed from a restaurant. Such oil can be filtered, dewatered, and deacidified, then used in a diesel as-is, or made into biodiesel.
Butanol, or butyl alcohol, is a more complex alcohol that can be utilized as a direct gasoline replacement without modification. It can be burned alone, or mixed with gasoline without ill effects. It has 10% less energy than gasoline, but a 25% higher octane rating, meaning that with higher compression it should be possible to get more energy out of it, making it potentially ideal as a racing fuel. Most butanol is currently made from fossil fuels but it can also be made by a bacterium, Clostridium acetobutylicum - first used to make TNT in 1916.
Methane isn't typically used in cars, but it's a biofuel whose use is extremely widespread. Methane can be collected all over the place, and the easiest way to produce it is through the anaerobic fermentation of organic matter. In particular this means that one fantastic source is feces. Waste "digesters" that process sewage into methane and fairly harmless fertilizer effluent are readily available in a range of capacities, but the largest examples are landfills. A number of housing developments built on landfills were exploding due to methane gas escaping into people's basements, and so began a widespread campaign of tenting landfills and "flare|flaring off" of the gas, which is a fancy term for burning it and letting its stored chemical energy go to waste. As fuel costs have risen, the value of methanol has increased, and now it is often captured, compressed, and sold, or burned onsite for production of electricity. Pig farmers in China get more than enough gas to cook with by pushing all their pig crap into a hole, covering it, and running a hose from hole to kitchen.
Solid organic mass is of course a useful fuel. Most of the world accomplishes heating by burning something solid, like wood, charcoal, coal (not a biofuel as we use the word, although it is from biological sources in the distant past) or "cow chips" (bovine waste.) Wood is renewable and, provided that trees are allowed to grow in the same place again, does not upset the carbon balance.
Finally, a word of caution: Just because something is a biofuel doesn't mean it's good for the environment. Brazil has been stepping up biofuel production all over the nation, but the vast majority of it is topsoil-based, being made from sugarcane or similar feedstocks. This is hopelessly wrongheaded. Agriculture has done more damage to the part of Earth that we're interested in than all the other things man has done combined; for example, it turned Egypt into desert. If we are going to use biofuels, it is absolutely necessary that they be hydroponically grown. Farming depletes topsoil, and uncovers it, allowing it to be blown and washed away. We need soil - even our planet is named Earth.
- Butanol. Wikipedia, 2006. (http://en.wikipedia.org/wiki/Butanol)
- 1. See MTBE: Toxicity and Reality - Fuels and Society C: Toxicity for some salient information.