|Fig. 1: Rise in cost of jet fuel in the last ten years.  (Courtesy of the U.S. Department of Transportation )|
The airline industry faced a reality check in 2008 as fuel prices skyrocketed, clearly demonstrated by Fig. 1. After the storm settled, fuel prices are now estimated to make up 30% of an airline's operating costs.  Furthermore, aviation is responsible for 13% of transportation emissions, and 2-3% of the greenhouse gas emissions worldwide.  For long-term focused business plans, these economic and environmental crutches have pushed the airlines to search for a more reliable and controllable source of fuel.
While energy and auto companies are also interested in alternatives to fossil fuels from the ground, aviation has the most stringent requirements for fuel. Primarily, high energy density is a must. Jet A-1 fuel has a minimum energy density of 42.8 MJ/kg, a number that currently can only be achieved by liquid fuels.  Furthermore, the extreme range of operating conditions experienced by an aircraft places additional limitations on alternative fuels. Operation at extremely low temperatures requires a low freezing point; proper swelling reactions in seals require a minimum aromatic content; viscosity requirements are defined by use of fuel as a lubricant. Lastly, jet fuel infrastructure is highly developed and the prohibitive cost of rebuilding the infrastructure and retrofitting the existing aircraft fleet rule out any fuel that cannot be mixed indistinguishably with Jet A-1.  The remaining options, know as "drop-in" fuels, are Synthetic Paraffinic Kerosenes (SPK). Hydroprocessed Renewable Jet (HRJ) and Fischer-Tropsch (FT) are the most promising versions, and they are the only alternative fuels under serious development for the aviation industry. In fact, both types have been approved for use (as a blend) and tested commercially in the last two years.
The current favorite, mostly because some of the infrastructure has already been developed and the technology is extremely simple, is FT-SPK. Producing an alternative to kerosene is a two-step process. First a feedstock, typically coal, natural gas, or biomass, is converted to a gaseous form call syngas. Next, the syngas is run through the Fischer-Tropsch process, a simple, old reaction that builds liquid hydrocarbon chains from the syngas that are chemically identical to kerosene. The fact that coal and gas feedstock is being considered at all reflects how strong the economic motivation is relative to the environmental reasoning. Although claims by FT fuel producers say that coal to liquid fuel is cleaner that standard Jet A-1, it does nothing to curb fossil fuel use.
Nonetheless, the production processes using coal or biomass are virtually identical, so developing one often contributes to the other. However, production goals don’t expect alternative fuels to competitively enter the market until 2020, and much of that will probably be from cheaper, nonrenewable feedstock. 
In terms of GHG emissions, HRJ is the competing favorite for drop-in alternative fuels. The process is based on harvesting naturally grown oils, most popularly from jatropha, algae, camelina, or halophytes, and refining the oil through hydrocracking into the desired hydrocarbon chains. Like FT, the result is a chemically identical SPK with energy contents of about 44 MJ/kg.  While production level plants are still under development, the process is popular, because the plant sources are easy to grow and do not compete with food crops. Honeywell, Solena and Solazyme are growing, major developers of HRJ fuel just in the US, but the technology faces a larger hurdle than the FT process in the scaling up stage. Even with hundreds of billions invested, projections expect Jet A-1 to be less expensive than HRJ for another 30 years. 
|Fig. 2: Typical jet fuel inspection during pre-flight check. (Source: Wikimedia Commons)|
Airlines are seeking alternatives to oil based jet fuel, but do not mistake this for interest in environmental sustainability. The airlines are perfectly happy using liquid fuel derived from coal (CTL) or natural gas (GTL). Many of the promising alternatives do have the potential to reduce greenhouse gas emissions, and some even come from renewable sources such as biomass. However, "alternative" still does not mean "renewable." The cleaner fuels are years, possibly decades, behind CTL and GTL production. While the airlines are investing in both, even the industry estimates put cost competitive biofuel at 30 years away.  This weak forecast makes it clear that cost is in fact the ruling factor. The CEO of Jatro, producer of a jatropha-based fuel for Lufthansa, clearly stated, "At the end of the day, green fuel is only good if it can meet price expectations, because the willingness of the industry to pay a premium for something green and renewable is limited.  With biofuel currently costing 2.5× more than standard Jet A-1, it could be decades before biofuel blends (not even 100%) start to beat out the ever so reliable product of the oil industry. 
Furthermore, there are many concerns surrounding the development of biofuels such as land use conflicts, competition with food, more economic use in automobiles, and even higher GHG emissions over the entire life cycle. One study showed that aviation biofuels have the potential to reduce GHG emissions compared to Jet A-1 by 90% or increase emissions by 800%.  Thus, there remains a lot of work to be done before biofuels become a reliable alternative to fossil based kerosene. Nonetheless, there is a key difference between development for aviation biofuels and other uses for biofuels. For an airplane, liquid fuel is the only option. While HRJ or FT synthetic kerosene may not look like silver bullets, there will huge investments and growth in them over the next two decades. If airlines want to guide their businesses away from volatile oil prices, these alternative fuels are their only shot. Unfortunately, clean alternative fuels are losing out to the dirty ones.
© William Greenbaum. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Novelli, "Sustainable Way for Alternative Fuels and Energy in Aviation," Directorate General for Mobility and Transport, European Commission, April 2011.
 "Aviation and Climate Change," International Civil Aviation Organization, 2010.
 S. Kolesnikov-Jessop, "Flying With Biofuels Gets One Step Closer," New York Times, 25 Oct 11.
 T. Firestine and J. Guarino, "A Decade of Change in Fuel Prices and U.S. Domestic Passenger Aviation Operations," U.S. Research and Innovative Technology Administration, SR-033, March 2012.