Synthetic Fuels for Aviation
Jet fuel made from coal by the Fischer-Tropsch synthesis is currently being supplied by Sasol, its producer, to the Johannesburg, South Africa, airport and is being used by every commercial airplane that passes through that airport. Subcommittee D02.J0 on Aviation Fuels, part of Committee D02 on Petroleum Products and Lubricants, approved this fuel by revising standard D 1655, Specification for Aviation Turbine Fuels, to allow up to a 50 percent blend with petroleum-derived jet fuel.
The FT synthesis has been known for decades. It was developed in the 1920s and 1930s and was used by Germany to produce fuel during World War II. More recently, Sasol has used the FT process to produce synthetic fuels in South Africa.
When jet fuel supply constraints at Johannesburg International Airport became a concern, Sasol approached aviation authorities for approval to use FT synthetic fuel. After a thorough review, approval was given in 1999 for blends of up to 50 percent FT synthetic fuel with conventional jet fuel. Since that time, FT synthetic fuel has been used routinely at Johannesburg, although most batches of fuel contain much less than the 50 percent synthetic fuel allowed by specification.
The approval process was truly an international aviation industry effort, led by the United Kingdom Ministry of Defense. Approval for a 100 percent synthetic jet fuel produced by Sasol is nearing completion.
With the exception of Sasol, the FT process has not been widely used to produce synthetic fuels because the process is more expensive than refining petroleum. However, recent increases in the price of crude oil, improvements in the FT process, and a desire to utilize large quantities of “stranded” natural gas have led to a resurgence of interest in the FT process to produce synthetic fuels.
The Coordinating Research Council is developing a synthetic fuel approval protocol, which will be adopted by D02.J0. The goal is to define a generic process so that producers of synthetic fuel will know exactly what it takes to get their synthetic fuel approved for aviation use. Hopefully, adoption of this approval protocol will stimulate the widespread use of synthetic jet fuel.
In order to avoid producer-by-producer approval, Subcommittee D02.J0 is also developing a list of jet fuel properties that can be added to the jet fuel specification; this will ensure that synthetic jet fuels will be acceptable independent of the producer and nuance of production methods.
Fischer-Tropsch synthesis converts a mixture of carbon monoxide and hydrogen, known as synthesis gas, into higher molecular weight hydrocarbons. It can be thought of as a catalytic polymerization of carbon monoxide accompanied by reaction with hydrogen to make the CH2 methylene units of paraffins.
CO + H2 -(CH2)n- + H2O
Since the FT synthesis starts with carbon monoxide, potentially any source of carbon can be used. The first FT plants used coal as the starting material; this conversion is called coal-to-liquids. CTL is experiencing renewed interest today both in China and the United States, although no new CTL plants have been built in those countries.
The current generation FT plants will use natural gas as the starting material; this is called gas-to-liquids conversion, or GTL. Biomass can also be used as the starting material after going through a gasification step to produce the synthesis gas; this process is called biomass-to-liquids conversion, or BTL. FT liquids produced from any starting material are essentially the same, since the connection to the starting material is lost during the conversion to synthesis gas.
The FT process produces mainly straight chain hydrocarbons. The product composition will vary somewhat depending on the hydrogen-to-carbon monoxide ratio as well as the catalyst and process conditions. This raw product of FT synthesis must be further processed to make an acceptable fuel. This processing includes cracking the long chains into smaller units and rearranging some of the atoms (isomerizing) to provide the desired properties. This upgrading process produces a wide boiling range material, which is then distilled into final products.
The FT process results in a product virtually free of aromatic compounds. There are two disadvantages of not having aromatics in the fuel. First, FT kerosene that meets all other jet fuel specification properties will be below the minimum density requirement. Second, the aromatics in conventional fuel cause some types of elastomers used in aircraft fuel systems to swell. There is concern in the industry that switching from conventional jet fuel to aromatic-free FT synthetic jet fuel will cause some of these elastomers to shrink, which may lead to fuel leaks. The effect of aromatics on elastomers is an area of active research in the industry.
These two disadvantages are eliminated when FT synthetic fuel is blended with conventional jet fuel. Conventional jet fuel provides the aromatics that cause elastomer swell and also increase the fuel density to meet the minimum requirement. The industry is using 8 percent aromatics content as a guiding minimum. This minimum is based mainly on experience and could be revised up or down in the future, based on new data.
The FT industry appears to be on the verge of a period of expansion. Several major companies have announced plans to build large plants. If completed, these projects could yield about 1 million barrels per day of total product by 2020, some of which could potentially be used as aviation fuel.
Why Do Synthetic Fuels Need Special Approval?
Jet fuel specifications are not true material specifications, but are based on experience with conventional petroleum-derived jet fuel; as a result, the specifications include implicit assumptions that are met by conventional jet fuel. For example, since the specifications limit the maximum aromatics concentration, but do not specify a minimum, a fuel with zero aromatics would meet this part of the specification.
Because jet fuels with high aromatics content do not burn as cleanly as fuels with a lower aromatic content, the specifications include a maximum aromatics concentration to limit this effect. Historically, there has not been a need to define a minimum aromatics concentration because conventionally refined petroleum-derived kerosene has a significant aromatics concentration, typically between 8 and 22 percent by volume.
Boiling range is another example of implied assumptions in jet fuel specifications. The specifications include a maximum limit of 205 °C on the 10 percent boiling point and a maximum limit of 300 °C on the final boiling point. A single component with a boiling point of 200 °C or less meets the distillation requirement, although this is not the intent or expectation of the specification.
Typical refinery processing will produce fuels with a smooth boiling range distribution. Historically, all fuels used in the testing and development of aircraft engines have had this property. Therefore, the specifications include only minimal limits on distillation properties, yet the industry has confidence based on knowledge and experience that conventionally processed petroleum-derived jet fuel will have the desired smooth boiling range distribution.
When synthetic fuels are being considered, these implicit assumptions must be recognized. The candidate fuel must meet the ultimate requirement a fit-for-purpose aviation fuel in addition to meeting the letter of the specification requirements, which were developed for petroleum-derived fuel use.
Currently there is only one non-conventional fuel approved for aviation use. The synthetic FT kerosene produced from coal by Sasol in South Africa can be blended with conventional jet fuel up to 50 percent by volume (semi-synthetic jet fuel). This approval, which is written into both the U.K. Ministry of Defense standard and ASTM specifications, was granted only after extensive testing was conducted to determine that the blended fuel met the fit-for-purpose criteria.
Some of the fuel properties tested include dielectric constant, thermal conductivity, specific heat, bulk modulus, air solubility, surface tension, thermal and storage stability, additive solubility and effectiveness, and elastomer compatibility. This is just a partial list of the testing done to ensure that none of the assumptions implicit in the specification were violated and that the semi-synthetic fuel was fit-for-purpose.
Commercial Aviation Is Working Closely with the U.S. Military on FT Fuels
The U.S. Department of Defense is in the forefront of activities on FT synthetic fuels, driven mainly by concerns for energy security. In 2006, the Air Force conducted a test flight of a B-52 using a 50 percent blend of FT synthetic fuel with conventional jet fuel. The DoD is planning to purchase 200 million gallons of synthetic fuel for additional field testing. Their goal is to establish the requirements for operational use of FT fuels. The DoD has set an ambitious goal of using 50 percent synthetic fuel by 2016.
Commercial aviation is working closely with the military to resolve common synthetic fuel issues. ASTM International’s Subcommittee D02.J0 will be contributing to that work by developing the specification and test methods that must be in place before synthetic fuels can find their way into the marketplace. //