The Second Coming of Biofuels

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Plant-based fuels have been a big disappointment to date, but new "green biofuels" might fulfill their promise.

by Chris Dannen

Somewhere between fifth and sixth gear in this roaring, turbo-charged Audi A5, I realize I'm not that excited about electric cars. As an eco-conscious citizen, I should be a little ashamed of my attachment to pistons and gears. But in this test drive, I don't have to be. While the Audi I'm test-driving is a regular production car, the fuel inside it is a new, ultra-potent kind of biofuel that has only a quarter of the per-gallon carbon footprint of petroleum-derived fuel. Sure, that doesn't sound that exciting compared to a zero-emission electric car—until you consider that this new fuel doesn't require new car factories, new fueling infrastructure, or the turnover of our existing fleet of vehicles, all of which are expensive, massively polluting ventures. Could it be that the greenest car on the market is already in your garage—it just needs a new kind of juice?

The New Plant Fuel

"Green diesel," as it's being called, isn't the first effort to use plants to power cars; your gas tank probably has a blend of gas and plant-derived ethanol inside it right now. But it's hard to get excited about biofuels when they already have such a bad rep. The use of corn and sugar in fuels has driven up the cost of food by a whopping 75 percent worldwide, according to a recent report by the World Bank. And a Nobel-prize winning chemist has publicized his findings that biofuels made from nitrogen-thirsty plants (like corn and canola) actually produce a net increase in greenhouse gas emissions, because they release nitrous oxide during their production. As if all that wasn't bad enough, ethanol blends hurt your car's fuel economy. Not exactly the makings of a green energy panacea.

But high-performance green diesel and gas aren't like other biofuels; they're perfect substitutes for petroleum-derived fuels, except they're made from plant matter like forest detritus, also known as cellulosic waste, or from algae. Green fuels can also be refined on the exact same equipment that refines petroleum. By contrast, traditional ethanol requires new equipment and uses edible plants like corn and sugar that need rich farmland to grow.

Also unlike traditional ethanol, green fuels can be also be used in gas tanks in 100 percent concentrations; their cetane numbers, or combustion quality, are actually higher than those of petroleum fuel. That means they could also be used to power jet airplanes, another prime polluter. To boot, green fuels are also more stable and have a lower cloud point than other biofuels. Regular ethanol breaks down over time when it sits in your tank, and turns viscous and sludgy in cold weather.

The secret to this high-performance green stuff is in the production. The refinement of green diesel uses a process called catalytic hydroprocessing. Put simply, cellulosic oils and greases are fed into a "diesel hydrotreater" with a small amount of petrodiesel, and hydrogen is forced into the mixture. That starts a catalytic reaction, the product of which is a potent petroleum substitute. What differentiates this process from tradition biofuel production is the infusion of hydrogen—it removes the dilutive oxygen that most biofuels contain, leaving only the combustible isoparaffins and paraffins, which are indistinguishable from the molecules in refined petroleum. The only byproduct of the process is propane, which can be reintroduced into the production loop as a source of hydrogen.

But Will It Scale?

When you crunch the numbers, the implications of using the most advanced of these feedstocks, algae oil, are tremendous. Replacing just 20 percent of America's current thirst for gasoline with current biofuels made from conventional feedstocks like soy or sugar would require a mass of farmland the size of California, Indiana, Nevada and Michigan combined—260 million acres—devoted only to growing fuel. That's because plants like soy make for a terribly low oil yield: about 100 gallons of oil per acre. Compare that to algae, which can generate 3,500 gallons of oil per acre, and green diesel starts looking feasible on a large scale. Plus, algae can grow well in arid land, so it wouldn't take up valuable farmland in the Midwest. Build a shallow, algae-growing pool of saltwater about the size of Rhode Island under the hot New Mexico sun and you could replace that same 20 percent of our gasoline demand. The saline pools wouldn't use up potable water and could perhaps double as cooling channels for nuclear power plants.

Then there are the fringe benefits: Because we could grow, harvest, and produce algae-based green fuels here in the US, there would be no expensive and polluting transportation from the Middle East, and no energy-intensive drilling or prospecting. That would mean production could come with a 75 percent smaller carbon cost as compared to petro-fuel—not to mention reduced reliance on OPEC. Suddenly biofuels are looking like a hot prospect again.

http://discovermagazine.com/2008/aug/02-th...t:int=1&-C=

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