Lecture

Jet fuel additives. Types & grades of jFs. Jet fuels service properties

by Mykola Zakharchuk

Jet fuel additives

Additives are fuel-soluble chemicals added in small amounts to enhance or maintain properties important to fuel performance or fuel handling. Typically, additives are derived from petroleum based raw materials, and their function and chemistry are highly specialized. They produce the desired effect in the parts per million (ppm) concentration ranges. (One ppm is 0.0001 mass percent.)

Additives are used in varying degrees in all petroleum derived fuels, but the situation with aviation fuels is unique in that only those additives specifically approved may be added to jet fuel. All jet fuel specifications list approved additives along with allowed concentrations. Some approved additives are required to be added, some are optional, and others are approved for use only by agreement between buyer and seller.

Before an additive can be approved for use in aviation fuel, it must undergo extensive testing to show both that it is effective and that it does no harm to any other fuel properties. To guard against harmful additive interactions, an additive must be tested at four times its maximum dosage in the presence of other additives before it is approved.

Use of additives is the principal difference between commercial and military jet fuels. Some military jet fuels will contain three or more additives. International Jet A-1 contains a static dissipater additive and may also have an antioxidant. Jet A in the United States usually contains no additives at all or perhaps only an antioxidant.

1. Fuel System Icing Inhibitor. Ice can form in fuel tanks at the very low temperatures encountered at high altitude. Generally, this ice is formed from water that was dissolved in the fuel when it was loaded onto the aircraft but separated from the fuel as the fuel temperature dropped. Most commercial aircraft have heaters on their main fuel filters to melt any ice that is collected. However, many aircraft do not have these heaters and are susceptible to reduced fuel flow and filterability if ice crystals form.

Fuel system icing inhibitors (FSII, pronounced “fizzy”) (укр. противодокристалізаціні (ПВК) присадки) work by combining with any free water that forms and lowering the freezing point of the mixture so that no ice crystals are formed.

If a fuel containing FSII comes into contact with free water, the additive will be extracted out of the fuel and form a viscous phase with the water, an obviously unacceptable situation. To avoid contact with free water, FSII is usually not added to fuel at a refinery but at some point in the fuel distribution system. It may be added at the airport or on delivery to the aircraft itself.

The amount of FSII added to fuel depends on a type of aircraft, ambient temperature at the departure airport, and flight duration; it is in a range of 0.1-0.3 % vol.

2. Antioxidants. Oxygen in the small amount of air dissolved in the fuel attacks reactive compounds in the fuel. The initial attack sets off a chain of oxidation reactions. Antioxidants work by interrupting this chain of reactions, preventing the formation of peroxides, soluble gums, or insoluble particulates. Peroxides can attack elastomeric fuel system parts, gums can lead to engine deposits, and particulates can plug fuel filters.

It is important to note that while antioxidants are effective at improving storage stability of jet fuel, they are not effective at improving its thermal stability.

I n addition, straight-run fuels often contain natural antioxidants, so these fuels usually have good oxidative stability and thus do not need antioxidants. However, hydrotreating, even mild hydrotreating to remove mercaptans, can also remove these natural antioxidants, possibly resulting in a less stable fuel. For this reason, antioxidants are sometimes added to hydrotreated fuel. To be most effective, the chemical should be added to the fuel immediately after it has been hydrotreated.

Ionol

Antioxidants are required in any fuel or fuel-blend component that has been hydrogen treated. Antioxidants are optional in non-hydrogen treated fuel.

The approved antioxidants for aviation fuel are hindered phenols such as ionol. Thermostable hydrotreated jet fuels RT, T-8 and hydrogenated T-6 fuel contain 0.003-0.004 % of ionol additive.

3. Metal Deactivator. Metal deactivators are chelating agents – chemical compounds that form stable complexes with specific metal ions. More active metals, like copper and zinc, are effective catalysts for oxidation reactions, and degrade fuel thermal stability. These metals are not used in most jet fuel distribution systems or turbine engine fuel systems. However, if fuel becomes contaminated with these metals, metal deactivators suppress their catalytic effect on fuel oxidation and thus fuel storage life and stability. When fuel contains metal deactivator and is heated, gum formation tendency decreases.

4. Corrosion Inhibitors. The tanks and pipelines of the jet fuel distribution system are constructed primarily of uncoated steel. Corrosion inhibitors prevent free water and oxygen in the jet fuel from rusting or corroding these structures. They are currently not used in TS-1 and RT fuels, and their use is limited in foreign grades.

5. Lubricity Improvers. Lubricity additives are used to compensate for the poor lubricity of severely hydrotreated jet fuel. They contain a polar group that adheres to metal surfaces, forming a thin surface film of the additive. The film acts as a boundary lubricant when two metal surfaces come in contact. These compounds are usually carboxylic acids, whose compositions are proprietary.

6. Electrical-Conductivity Additives. Static electricity is generated by movement of fuel through a high flow rates fuel transfer system. A static dissipated additive (SDA) is added to jet fuel to increase its conductivity and allow dissipation of charge to the ground. That electrostatic charges are dangerous is a well-known fact. The risk can be reduced by correctly handling the product and by observing the well-established safety procedures of earthing and bonding all equipment near or in contact with fuel. However while handling fuels at high loading rates, it becomes impossible to ensure that electrostatic discharges will not occur merely by handling the fuel carefully. The only sure way to prevent buildup of such charges is to raise the conductivity of the fuel to such levels that charge if formed will leak away rapidly.

Overconducting fuel can compromise instrument calibration, resulting in erroneous volume readings and possible overfilling of tanks. For this reason, most jet fuel specifications also have an upper limit for conductivity.

When the additive is used, the conductivity of the fuel must be between 50 and 450 CU at the point of delivery into the aircraft.

7. Biocides. Biocides are designed to kill microorganisms, which include bacteria and fungi (yeasts and molds). Since biocides are toxic, any water bottoms that contain biocides must be disposed of appropriately.