Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
• 2.6 Synthetic Fuels
There are a number of synthetic fuels in various stages of research and development with
potential for use in the transportation sector. Synthetic fuels typically utilize fossil fuels
such as coal or natural gas as their feedstock, although biomass sources such as vegetable
oils and fats also may be used. Depending on engine type and application, these fuels may
be substituted directly for conventional fuels with little to no modification to existing
engines or fueling infrastructure. Synthetic jet fuel can be used directly or blended with
conventional jet fuel in existing aircraft engines without modification. Synthetic fuels also
contain almost no sulfur species or aromatics. Therefore, sulfur emissions from
combustion are essentially zero, VOC emissions extremely low, and particulate matter can
be reduced 50 to 90 percent. In most cases, however, substantial uncertainty remains
regarding the cost and production potential for these fuels.
The following provides a brief overview of several of the more promising synthetic fuels
under evaluation today.
Fischer-Tropsch (FT) Fuels
The Fischer-Tropsch (FT) process converts gaseous hydrocarbons into liquid fuel. The gas
sources used include natural gas and gasified coal or biomass, each described briefly in the
following sections. Use of the FT process to convert natural gas into liquid fuels has been
done on a commercial scale for decades in South Africa, and more recently in Europe and
Thailand. Most major oil companies have announced plans to investigate gas-to-liquid
production of diesel. In the United States, more than 400,000 gallons of natural gasderived
liquid fuels, including diesel and jet fuel, have been produced by Syntroleum at a
demonstration plant. DOE supports research and demonstration projects for GTL
production and use in vehicles through its Vehicle Technologies Program, which includes
the National Renewable Energy Laboratory’s work on nonpetroleum-based fuel use in
vehicles (U.S. DOE, 2009d).
While natural gas and coal are still fossil fuels, use of these fuels to produce FT diesel may
have some advantages over conventional diesel. However, current production methods
using conventional fossil fuel feedstock appear to increase life-cycle GHG emissions
relative to conventional diesel. For example, one DOE evaluation found that coal-based
feedstocks more than doubled GHG emissions per mile for an SUV relative to
conventional diesel. Natural gas feedstocks had less impact, but still increased emissions
by roughly 20 to 60 percent depending upon the source of gas.
Regardless of the feedstock used, Fischer-Tropsch diesel can be substituted directly for
conventional (petroleum-derived) diesel to fuel diesel-powered vehicles without
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