Cars didn't always run on gasoline. Henry Ford envisioned his Model T's puttering along with tanks fully of ethanol. Early diesel engines ran on peanut oil. Of course, the discovery of massive petroleum reserves at the turn of the 20th century quickly put the brakes on that notion—why bother creating biofuel when gasoline and diesel are pennies on the gallon? But now that gas is about $3.64 a gallon in the US, interest in biofuels is on the rise. Here's what we'll use to power 21st century transportation.
What's the Difference?
Gasoline and diesel are "fossil fuels," meaning they're derived from petroleum, fossilized plant and animal matter. These energy-dense products contain flammable hydrocarbons which provide the power to internal combustion engines when burned. Biofuels contain very similar hydrocarbon chains, except that these are refined from freshly grown plant matter, rather than samples that are millions of years old. While current biofuel production rates can't compete with the 75 million-plus barrels (55 gal drums) of crude oil produced daily around the world, the rate of biofuel production is steadily climbing. Twenty-eight billion gallons of biofuel were manufactured in 2010 (23 million of which were ethanol derivatives), and it powered nearly three percent of the world's cars and trucks.
Not all biofuels are created equal, though; or at least, not all come from the same stuff. Here's a look at how the different types break down.
Ethanol is booze for your car, the most common biofuel on the planet. It's produced by fermenting the sugars and starches from a wide variety of sources—wheat, corn, sugar beets, sugar cane, or molasses. Basically if it can be used to make booze for people, it can be used as an ethanol base. The production process is even strikingly similar to normal distilling: The carbohydrates stored in a grain crop's seeds are broken into simple sugars via enzymatic recreations, those sugars are fermented anaerobically by yeast into alcohol, which is then distilled.
The resulting ethanol can be used as either a fuel or a fuel additive. In the US, ethanol is commonly sold as either "gasohol" (90 percent gasoline, 10 percent ethanol) or as E85 (85 percent ethanol, 15 percent gasoline). Brazil, the single-largest ethanol producer and consumer on the planet, uses as 75/25 gas/ethanol mix to power its vehicles while sequestering large amounts of carbon. In fact, most modern gas engines will run just fine on up to 15 percent-ethanol gasohol blends. Ethanol only releases 2/3 the amount of energy as an equal volume of petroleum product, but it does have a higher octane rating (which increases the engine's compression ratio) and produces far fewer greenhouse emissions.
The major issues facing ethanol production in the US, which primarily relies on our biggest food crop—corn—are wide-ranging: Finding a balance between fuel production and food production, figuring out how to grow, process, distill, and deliver the ethanol from corn field to fuel tank using less energy than simply pumping petrol out of the ground, and how to get the final product price (government subsidized or not) as low as current gas prices given that drivers will need to pull over 33 percent more often to refuel, on account of ethanol's lower energy density. In all, ethanol currently doesn't provide enough improvement over prevailing fuel sources to really warrant weaning the American public off of gasoline for it. However, a new generation of ethanol could well change that.
Cellulosic ethanol uses sugars trapped in the woody, non-edible parts of crops (think corn husks and cobs, not kernels). Breaking down these tough cellular walls to release their sugars is difficult, as it requires a slow-working enzyme reaction to take place, similar to how cows break down grasses in their four stomachs. However, 15 strains of synthetic enzymes first derived from fungi in 2009 that remain stable at high temperatures could prove useful, and the process could theoretically be applied to any woody substance, from orange peels to switchgrass to sugarcane stalks to sawdust, not only providing a reliable power source but also a reliable waste management system for input.
The first diesel engines ran on vegetable oil, and many modern diesels still can. These oils, which serve as biodiesel's base, can be collected from any oilseed crop, from cottonseed to sunflower—even hemp and animal fats—though soybean oil is the primary base source in the US. Vegetable oil is comprised of long, fatty acid chains held together with glycerol. Mixing the oil with a bit of methanol and lye, which acts like a catalyst, breaks down these chains into smaller, more easily burnable molecules much like how ethanol's sugars must first be broken out of their carbohydrate cells. This process is known as transesterification or, more commonly, hydrocracking.
The resulting biodiesel has the same chemical properties as petroleum-based diesel, and can be used by existing diesel engines without issue so long as it is mixed with a bit of mineral diesel. And while a diesel engine can technically run on straight vegetable oil, as Rudolf Diesel's early engines did, they don't do so particularly well and often require minor engine modifications for it to work at all.
Biogas is methane produced through the anaerobic digestion by microbes. This process occurs naturally in landfills, where garbage rots in an oxygen-free environment and mechanically in waste recovery operations—consuming anything from sawdust to cow dung to human excrement as the base—to make biogas and digestate, a solid byproduct that makes for excellent fertilizer.