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 April 2007
OREN HADALLER is a fuels research engineer for Boeing Commercial Airplanes and a member of ASTM Subcommittee D02.J0 on Aviation Fuels. Since 1976, he has been investigating jet fuel production, properties, and delivery, and he has conducted research on alternative aircraft fuels. Hadaller has authored many technical papers on aviation fuel characteristics and alternatives for both subsonic and high-speed commercial transports.

Image: Sasol's Oryx gas-to-liquid facility in Qatar

Alternative Aircraft Fuels

Original equipment manufacturers and regulatory agencies require a great deal of testing before any alternative fuel can be approved for use. The outcome of such testing is a new fuel specification or the revision of an existing fuel specification.

ASTM International, through Committee D02 on Petroleum Products and Lubricants, is in a unique position to bring users, producers, government agencies and regulators together to make alternative fuel use in cars, trucks and airplanes a reality. ASTM is developing new and revised fuel test methods and specifications. These standards are the culmination of debate on the properties and quality of alternative fuels offered to the marketplace versus minimum acceptable vehicle requirements. ASTM is ideally suited and uniquely positioned to bring all parties to the table for this debate and has the structure to provide consensus and closure so that alternative fuel products can be expeditiously approved for use.

Alternative Aircraft Fuels Fall into Three Categories

Synthetic fuels — Synthetic kerosene can be made from coal, natural gas, or other hydrocarbon resources and can be produced by first turning the resource into gases, which are then recombined to form hydrocarbon liquids. Synthetic kerosene can be tailored to have similar properties to petroleum kerosene and can thus be thought of as a “drop-in” replacement. Synthetic kerosene from coal is presently used at one airport as a supplement to petroleum supplies and in the future this fuel will be approved as a total replacement to petroleum jet fuel. “Drop-in” replacements are receiving a lot of attention. These “drop-in” replacements include:

• FT (Fischer-Tropsch) using natural gas, coal or any carbonbased material including plant crops (discussed in a related article on page 40);
• Direct coal liquefaction; and
• Biofuels with refinery-based upgrading to jet fuel.

Biofuels — Biofuels start with oil squeezed out of crops/feedstock, such as soybeans and palm nuts. This oil is additionally processed to transform it into a fuel. An additional processing step is then required to transform these biofuels into biojet fuel. Research into other feedstocks and other processes, such as the production of biomass from the carbon dioxide generated by fossil fuel power plants, is ongoing.

Other Alternative Fuels — Hydrogen, methane, ethanol, methanol, and liquefied petroleum gases have all been considered as alternative aircraft fuels. All are very challenging to use. These fuels would require new aircraft and fuel delivery systems. Hydrogen and methane must be used in their liquid form. These liquids must be maintained at extremely cold, cryogenic temperatures. Hydrogen burns cleanly, but produces more water vapor emissions, resulting in more contrails.

Biofuels Are Being Worked to Address Environmental Concerns

Biofuels are combustible liquids that are manufactured from renewable resources such as plant crops or animal fats. Crops with high oil content such as soybeans, rapeseed (canola), and sunflowers are the starting materials used to produce bio-oils or bio-oil blending components.

The oil is obtained by first cleaning, cracking and conditioning the beans. The beans are subsequently compressed into flakes. The oil is then extracted from the flakes. The primary components of bio-oils are triglycerides. These triglycerides are converted into fatty acid methyl esters by a process known as transesterification. The soybean fuel is called soy methyl ester and the rapeseed fuel is rapeseed methyl ester. These esters can be used directly or can be modified into a variety of products.

One of the challenges of using some of the current biofuels in commercial aircraft is its propensity to freeze at normal operating cruise fuel temperatures (see Figure 1). By selecting specific triglycerides, it is possible to obtain different properties, such as a lower freezing point. Another option is to use a separation process to extract the lower freezing point ester material.

Another challenge of methyl ester biofuels is their lack of long-term fuel storage and thermal stability. Currently, it is advised that the product be used within six months of manufacture.

Ester-based biofuels have lower energy content than petroleum jet fuel because, like the alcohols, these fuels also contain oxygen in their molecular structure. The use of ester biofuels would reduce aircraft range and payload. Using methyl ester biofuels in aircraft may require blending these fuels with petroleum-based fuels to mitigate some of the biofuel property deficiencies.

An alternate approach to esterification is the hydroprocessing of the raw oil. In this process, hydrogen is added to the fuel to remove oxygen atoms and to improve product stability. The resulting biofuel is a carbon/hydrogen fuel that looks very similar to petroleum jet fuel. The unique advantage of this approach is that the hydrogenation of the biofuel can be done in existing refineries. This opens up a whole new avenue for production without building new processing plants.

The driving force for using biofuels in aviation is environmental. Bio jet fuels are expected to be approximately carbon neutral over their life cycle, offsetting about the same amount of carbon as is produced when the fuel is burned in the jet engine. OEMs and regulatory agencies will require a great deal of testing before biofuels can be approved for unlimited use.

Hydrogen and Other Alternative Fuels

Currently, alternative fuels (hydrogen, methane, liquefied petroleum gas, ethanol and methanol) present some challenges when compared with conventional kerosene jet fuel. These alternative fuels require switching aviation to a totally new fuel.

As shown in Figure 2, fundamental requirements for a commercial jet fuel are that it have 1) a low weight per unit heat of combustion to allow the transport of revenue-producing payload, and 2) a low volume per unit heat of combustion to allow fuel storage without compromising aircraft size, weight or performance.

Hydrogen, publicized as the most environmentally benign alternative to petroleum, has its own drawbacks as an aircraft fuel. Hydrogen burns cleanly, but produces significantly more water vapor, so its effect on cloud formation and the atmosphere is uncertain. Hydrogen production needs an abundantly available source of energy, such as electrical power, produced from nuclear fusion or solar and a large source of clean water. Although the combustion of hydrogen emits no carbon dioxide emissions and is lightweight, its production, handling, infrastructure, and storage offer significant challenges. The volumetric heat of combustion for liquid hydrogen is so poor that it would force airplane design compromises and is only a superior fuel for long range flights.

Methane is the primary component of natural gas. Natural gas will need to be stripped of its non-methane content to be suitable for aircraft. Both hydrogen and methane must be used in their liquid form, which are at extreme cold, cryogenic temperatures. Both liquid hydrogen and methane will require all new aircraft. In addition, the use of liquid hydrogen and methane will require an entirely new and more complex ground transportation, storage, distribution and vent capture system.

Liquefied petroleum gas is not a cryogen but has many of the same storage and transfer problems associated with a cryogen. In-depth studies of these fuels have not been conducted because the natural supply is not sufficient to support a worldwide aviation fleet and these fuels offer no availability, cost or environmental advantage as replacements for conventional jet fuel.

Alcohol fuels such as methanol and ethanol have very poor mass and volumetric heats of combustion and are not satisfactory for use as a commercial aircraft fuel. Their low energy content results from the oxygen that is present in their molecular structure. Even though they are not useful for commercial aviation, their widespread production and use could influence the supply and cost of conventional jet fuel by freeing up additional petroleum resources for aircraft. Their production might have merit in that context.

What Is Next?

All sectors of the aviation industry — airlines (through the Air Transport Association), aerospace manufacturers (through the Aerospace Industries Association), safety, environmental, and regulations (the Federal Aviation Administration), airport operators (Airports Council International-North America) and fuel suppliers have joined forces to work alternative fuels for aviation. They have formed an organization called the Commercial Aviation Alternative Fuels Initiative with the FAA as the lead organization. These commercial aviation sponsors and stakeholders agreed to work together with the U.S. Departments of Defense and Energy and the National Aeronautics and Space Administration to pursue alternative fuels with the objectives of:

• Securing a more stable fuel supply;
• Improving energy security while reducing costs;
• Exploring the feasibility of synthetic and biofuels;
• Furthering research and development; and
• Potentially reducing the environmental impacts of aviation.

Four sub-panels have been formed on research and development; environmental; economics, business and policy; and certification/qualification. The last panel meets with ASTM Committee D02 to ensure maximum synergy and commonality of efforts going on within ASTM International and efforts worldwide to the greatest extent possible. This panel understood from the beginning the impact that fuel specifications and test methods, and in particular the role of ASTM, would have on the development of alternative fuels.

The aviation community is committed to alternative fuels research and to working together with the other energy users to be part of a common set of solutions. It is the fuel supply sector that will make synthetic and alternative fuels a reality. ASTM International and Committee D02 provide a forum for working together on this global priority. //