What, you might ask, is ’synfuel’? As noted previously, ’synfuel’ is a synthetic fuel most commonly made from coal or natural gas. Ok, master of the obvious I know, but let me provide a little more detail: coal, natural gas, or in some cases, biomass, can be converted into a mixture of gases through a process known as gasification. Gasification is basically burning something (at >400 C) in the presence of a limited amount of oxygen to produce a specific mixture of gases, namely carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2).

Ok, stay with me here, and don’t forget about the carbon dioxide that’s produced during gasification - that’s important.

This gaseous mixture of CO, CO2, and H2 is the precursor to making synthetic liquid diesel fuel (synfuel), via another production method known as the ‘Fischer Tropsch‘ process. The reaction uses a catalyst to convert carbon monoxide and hydrogen into hydrocarbon chains, which composes the basic structure of diesel fuel. This is a historically important process: German researchers Franz Fischer and Hans Tropsch developed the reaction in 1923, and it provided Nazi Germany with as much as 124,000 barrels of synthetic diesel per day during WWII (1).

To recap:

Coal =(gasification)=> CO + H2 + CO2
CO + H2 =(Fischer Tropsch)=> synthetic diesel + CO2

Now, keep that in mind as we jump back to the Air Force, which plans on testing synthetic diesel in a 50/50 blend with regular jet fuel:

“The acquisition of these 315,000 gallons of synthetic fuel this year is one more step toward meeting the Air Force goal of testing and certifying the entire fleet for use of the fuel by 2010. Additional acquisitions of synthetic fuel will be made for testing and certification over the next three years. The ultimate goal of the Air Force is to acquire 50 percent of its [Continental United States] fuel by 2016 from domestic sources producing a synthetic fuel-blend and using carbon capture and sequestration technology,” said William C. Anderson, Assistant Secretary of the Air Force for Installations, Environment & Logistics.”

Boeing’s prediction seems right on the money: The short-term fuel replacement, at least for the Air Force, will be synthetic diesel. Whether or not that’s a good idea is hazier. Synfuel actually burns a bit cleaner than regular fuel, because it doesn’t contain the sulfur and aromatics contained in diesel. But there’s one major problem, if you remember the chemical equation above. The standard conversion of coal to synthetic fuel nearly doubles life-cycle emissions of the fuel it replaces. If synthetic diesel from coal was widely implemented for air travel, it would double the greenhouse gas emissions for that form of travel.

Fortunately, according the the National Renewable Energy Laboratory (NREL), this problem could be mitigated or even overcome by the use of biomass as a feedstock, instead of coal. NREL states in one report that synfuel from biomass can be ‘largely carbon neutral’. (3)

While a transition to synthetic aviation fuel seems inevitable, it must once again be highlighted that the sustainability of alternative fuels depends entirely on their source materials and production methods. Nevertheless, welcome to the future of aviation…

Southwest Nebraska News: Synfuel Contract Awarded by Defense Department (June 11, 2007)
(1) U.S. DOE: The Early Days of Coal Research
(2) U.S. DOE Energy Efficiency and Renewable Energy: Catalytic Conversion
(3) NREL: Improving the technical, environmental and social performance of wind energy systems using biomass-based energy storage
(4) Clean Diesel from Coal A novel catalytic method could let you fill up your tank with coal-derived diesel, cutting U.S. dependence on foreign oil.