Many biofuels raise a gnawing question: Is it better to make a given grain, legume or other edible plant into fuel, or should we protect it as a source of food?
Corn is a staple of the Western diet, so you're not likely to see more of it in your fuel than on your plate. But with increasingly tight emissions standards around the world, lower-emission ethanol blends are common at many gas pumps.
While rapeseed is often grown for livestock forage and its oil, it's not a common food in many Western diets. We're more likely to see this one in the tank rather than in the pantry.
Soybeans have a seemingly endless list of uses, from tofu to plastics to biodiesel. With many fuel processors developing biodiesel plants focused on extracting oil from soy, there's a good chance that you will one day see just as much soy on the road as in the grocery store.
This one's a bit of a trick: While plenty of restaurants cook with oil, it's a cooking fluid, not a food. With the exception of the grease soaking through the container of your fast-food fries, the fry oil you encounter outside a restaurant in the near future may possibly be in a greasecar's gas tank.
Given the massive deforestation of palm oil plantations, supporters of rainforest preservation argue loudly that coconuts, and the palm trees that produce them, should remain fare for the grocery store, not the gas station.
Believe it or not, saltwater can ignite when excited by concentrated radio frequency waves. But the technology that could make salt -- and salt water -- a viable fuel is still in its infancy. It currently takes far more energy to cause the reaction than is produced by the reaction. While powering your car with a cup of seawater makes for great science fiction, it's going to be a while before that technology becomes viable.
This one's actually closer than you think. Between work on hydrogen fuel cells, which use hydrogen to produce electricity and water, and undeniable power potential of pure hydrogen for internal combustion, it's possible that hydrogen, and the water we drink, could help power our future transportation needs.
The current market demand for peanuts and peanut oil makes them too expensive to use as a biofuel on a large scale. But remember: George Washington Carver identified more than 300 uses for the little legumes. If their diverse track record is any indication, peanuts might one day add "viable fuel source" to their list of uses.
The oil from cotton seeds is indeed edible, and it has been used as livestock feed for centuries. But its low nutritional density, combined with unfamiliarity with the plant's food qualities in the West, mean many of us will only see cottonseed oil when it becomes a source of plant-based fuel.
Sunflower oil is one of the more promising sources for biodiesel. Its composition lends itself to producing potent fuel. But like the peanut, the sunflower's popularity as a food product may keep its value too high for it to be an economical choice for near-future biofuel use.
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