Genomics:GTL

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Biological Energy Alternatives

Most biological processes that produce energy require solar energy either directly or indirectly via photosynthesis, a complex biochemical pathway in which solar energy is used to drive the chemical conversion of low-energy inorganic molecules such as water and carbon dioxide into energy-rich organic molecules. The organic products of photosynthesis are used to build biomass (proteins, fats, carbohydrates, and cellulose) and store chemical energy needed to drive cellular processes. The biomass of photosynthetic organisms can be used directly as a burnable fuel or converted to such other high-value energy sources as ethanol, biodiesel, methanol, hydrogen, or methane.

Liquid Fuels

Ethanol: Currently the most widely consumed biofuel in the United States, used as a substitute or octane booster for gasoline. A gallon of this biofuel has about 2/3 the energy content of gasoline. Some 3 billion gallons of ethanol were produced from cornstarch in 2004, equaling about 2% of U.S. gasoline consumption (Homegrown 2005; Mann 2004). Inefficiencies in the conversion of biomass (e.g., agricultural residues, plant stems and leaves, grasses, trees, and municipal wastes) to ethanol prevent yields that could meet a larger portion of gasoline demand.

Methanol: High-octane liquid fuel that has about half the energy density of gasoline. Engine modifications are required to improve cold starts and prevent corrosion. In the United States, about a billion gallons of methanol are produced each year, primarily from methane, but methanol also can be thermochemically derived from biomass gasification. Methanol could be a future source of hydrogen for fuel cell vehicles.

Biodiesel: Diesel fuel substitute or extender obtained from chemically reacting organically derived oils and fats (e.g., excess soybean oil and restaurant greases) with alcohol to form ethyl or methyl esters. In its pure form, biodiesel reduces fuel economy and power by about 10% when compared with diesel. Biodiesel blends perform similarly to diesel and can be used in unmodified engines. Only about 30 million gallons of biodiesel are produced each year in the United States today—a tiny fraction of the billions of gallons of diesel consumed each year (National Biodiesel Board).

Gaseous Fuels

Hydrogen: Potential energy source that can be released from the breakdown of biomass by microorganisms or produced directly from water and sunlight via photobiological processes that do not require biomass as an intermediate. Much research is needed, however, before we can use these systems for clean, renewable hydrogen production. Currently, most hydrogen is derived from steam reformation of nonrenewable natural gas and used primarily for industrial chemicals production. Only a small fraction is used as an energy carrier. Each year in the United States, about 9 million tons of hydrogen are produced, enough to power 20 to 30 million hydrogen cars or 5 to 8 million homes (National Hydrogen Energy Roadmap 2002).

Methane: Main chemical component of the fossil-fuel natural gas, which currently makes up about 20% of the U.S. energy supply. Microorganisms naturally produce methane during biomass degradation. Extensive infrastructure already is in place for widespread distribution and use. Organic materials in agricultural, municipal, and industrial wastes could be used as feedstock for biomethane production; however, high production costs and incomplete biological conversion (as much as 50% of organic matter is not used) are major limitations.

References:

• National Biodiesel Board (www.biodiesel.org)
• National Hydrogen Energy Roadmap. 2002. Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (www.eere.energy.gov/ hydrogenandfuelcells/ PDFs/ national_h2_roadmap. PDF).

Text adapted from Genomics:GTL Roadmap: Systems Biology for Energy and Environment, U.S. Department of Energy Office of Science, August 2005. DOE/SC-0090.