On January 7, 2009, Continental Airlines conducted a test flight of a Boeing 737 where one of the two engines used a 50-50 fuel mix of kerosene and a blend derived from algae and jatropha. The test flight was completed without any problems and the engines performed as expected. This marked the first demonstration of algae as a feedstock for aviation biofuels.  This flight and other tests showed that the use of biofuels as an additive is technically feasible without major adaptation of the aircraft and without a degradation of the performance of the aircraft engines. The move to switch from traditional jet fuel to a biofuel mix is motivated by a combination of a desire to reduce greenhouse gas emissions and a response to rising oil prices. Oil prices skyrocketed in the summer of 2008, leading several carriers to bankruptcy. Although oil prices have receded from those highs, it was a wakeup call to airlines of the perils of relying too much on one source of fuel. Biofuels for aviation have been confirmed as a technically viable option, but there are several challenges to overcome before airplanes will begin using biofuels as a significant fuel source on commercial flights. Perhaps the biggest of these is to cultivate feedstocks on a commercial scale.
An important consideration when developing aviation biofuels is which feedstocks to use. The ideal feedstock will grow quickly, have high energy content, and must be able to grow where it does not compete with food production. Perhaps the most promising feedstock is algae. Algae can be grown in polluted or salt water, deserts, and other places where food cannot be grown and humans do not generally live. Algae can grow much quicker than other feedstocks and can produce up to 15 times more oil per square kilometer than other biofuel crops. Another promising potential feedstock is jatropha. It is a plant that produces seeds containing lipid oil that can be used to produce fuel. Jatropha can be grown in arid and otherwise non-arable areas, leaving arable land available for food crops. Jatropha seeds are toxic to both animals and humans so it is not itself a food source either. Other potential feedstocks include camelina and halophytes. 
In order for aviation biofuels to be practical as a long-term solution to airplane energy usage, it must be possible to grow enough of the required feedstocks to supplant jet fuel. The worldwide aviation industry uses between 1.5 billion and 1.7 billion barrels of jet fuel annually.  One can calculate the number of joules supplied to the aviation industry by jet fuel per year. The required conversion factors are:
Jatropha seed yield varies widely from 0.5 to 12 tons/year/hectare depending on growing conditions. Since this is a feasibility exercise, the most conservative number of 0.5 tons/year/hectare will be used. These seeds contain about 30% oil that can be converted into biodiesel. Also, jatropha biodiesel has an energy density of 39.5 MJ/kg.  The amount of energy produced per unit area of jatropha farmland is thus
The amount of land required to power the airline industry is thus
Note that this is about twice the land area of the United States. Is this amount of suitable land available? It has been estimated that in Africa alone, there are over 1.08 × 109 hectares of prime growing regions for jatropha. An additional 5.8 × 109 hectares of land in Africa is also suitable, bringing the total to about 1.6 × 109 hectares.  This is comparable to the 1.8 × 109 hectares calculated. Since the prime growing regions in Africa would likely generate higher seed yield and there are other places in the world where jatropha can be grown, it is reasonable to conclude that there is enough land to cultivate jatropha on the scale necessary - although it would require an incredibly large area. An analysis conducted by the Boeing Company has estimated that the land required to supply the aviation industry with only jatropha is about 2.7 million square km (0.27 × 109 hectares). Judging by the difference between that value and the one calculated using the most conservative yield (above), Boeing must have assumed a jatropha seed yield of between 3-4 tons/year/hectare. Boeing also conducted the same analysis for algae and camelina, producing lower estimates of land required.  It is unlikely that aviation will rely on only one biofuel, so the amount of estimated land required will be a weighted average of the estimates for all of the useful feedstocks.
Aviation biofuels have been tested and shown to be technically viable. Feedstocks such as jatropha and algae show promise because they do not interfere with food production. The amount of energy supplied each year by jet fuel was calculated and it was shown that it is possible for this to be supplied by jatropha alone given enough land area to grow it. The idea of eventually replacing jet fuel with biofuels grown from sustainable feedstocks is feasible. The use of aviation biofuels has the potential to transform the commercial aviation business by providing a sustainable method of generating the necessary energy.
© Eric Glover. 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. Pae, "Continental Airlines Uses Biofuel on Test Flight," Los Angeles Times, 9 Jan 2009.
 "Beginner's Guide to Aviation Biofuels," Air Transport Action Group, May 2009.
 G. Francis, R. Edinger and K. Becker, "A Concept for Simultaneous Wasteland Reclamation, Fuel Production, and Socio-Economic Development in Degraded Areas in India: Need, Potential and Perspectives of Jatropha Plantations," Natural Resources Forum 29, 12 (2005).
 K. Parsons, "Jatropha in Africa: Fighting the Desert & Creating Wealth," EcoWorld, 21 Aug 2005.