grassoline

From the July 2009 Scientific American Magazine
Grassoline: Biofuels beyond Corn

Scientists are turning agricultural leftovers, wood and fast-growing grasses into a huge variety of biofuels—even jet fuel. But before these next-generation biofuels go mainstream, they have to compete with oil at $60 a barrel

In general, this process involves first deconstructing the solid biomass into smaller molecules, then refining these products into fuels. Engineers generally classify deconstruction methods by temperature. The low-temperature method (50 to 200 degrees Celsius) produces sugars that can be fermented into ethanol and other fuels in much the same way that corn or sugar crops are now processed. Deconstruction at higher temperatures (300 to 600 degrees C) produces a biocrude, or bio-oil, that can be refined into gasoline or diesel. Extremely high temperature deconstruction (above 700 degrees C) produces gas that can be converted into liquid fuel.

So far no one knows which approach will convert the maximum amount of the stored energy into liquid biofuels at the lowest costs. Perhaps different pathways will be needed for different cellulosic biomass materials. High-temperature processing might be best for wood, say, whereas low temperatures might work better for grasses.

Hot Fuel
The high-temperature syngas approach is the most technically developed way to generate biofuels. Syngas—a mixture of carbon monoxide and hydrogen—can be made from any carbon-containing material. It is typically transformed into diesel fuel, gasoline or ethanol through a process called Fischer-Tropsch synthesis (FTS), developed by German scientists in the 1920s. During World War II the Third Reich used FTS to create liquid fuel out of Germany’s coal reserves. Most of the major oil companies still have a syngas conversion technology that they may introduce if gasoline becomes prohibitively expensive.

The first step in creating a syngas is called gasification. Biomass is fed into a reactor and heated to temperatures above 700 degrees C. It is then mixed with steam or oxygen to produce a gas containing carbon monoxide, hydrogen gas and tars. The tars must be cleaned out and the gas compressed to 20 to 70 atmospheres of pressure. The compressed syngas then flows over a specially designed catalyst—a solid material that holds the individual reactant molecules and preferentially encourages particular chemical reactions. Syngas conversion catalysts have been developed by the petroleum chemistry primarily for converting natural gas and coal-derived syngas into fuels, but they work just as well for biomass.

Although the technology is well understood, the reactors are expensive. An FTS plant built in Qatar in 2006 to convert natural gas into 34,000 barrels a day of liquid fuels cost $1.6 billion. If a biomass plant were to cost this much, it would have to consume around 5,000 tons of biomass a day, every day, for a period of 15 to 30 years to produce enough fuel to repay the investment. Because significant logistic and economic challenges exist with getting this amount of biomass to a single location, research in syngas technology focuses on ways to reduce the capital costs.

Bio-Oil
Eons of subterranean pressure and heat transformed Cambrian zooplankton and algae into present-day petroleum fields. A similar trick—on a much reduced timescale—could convert cellulosic biomass into a biocrude. In this scenario, a refinery heats up biomass to anywhere from 300 to 600 degrees C in an oxygen-free environment. The heat breaks the biomass down into a charcoal-like solid and the bio-oil, giving off some gas in the process. The bio-oil that is produced by this method is the cheapest liquid biofuel on the market today, perhaps $0.50 per gallon of gasoline energy equivalent (in addition to the cost of the raw biomass).

The process can also be carried out in relatively small factories that are close to where biomass is harvested, thus limiting the expense of biomass transport. Unfortunately, this crude is highly acidic, is insoluble with petroleum-based fuels and contains only half the energy content of gasoline. Although you can burn biocrude directly in a diesel engine, you should attempt it only if you no longer have a need for the engine.