Aviation and Other Gas Turbine Fuels

Abstract

Because the jet aircraft is a weight‐limited vehicle, a high premium is assigned to hydrocarbon fuels with a maximum gravimetric heat content or hydrogen‐to‐carbon ratio. The global nature of jet aircraft operations has mandated that aviation fuel quality be closely controlled in every part of the world. Specifications tend to be industry standards issued by a consensus organization,Co like ASTM, or by a government body rather than by manufacturer's requirements. Liquid fuels for ground‐based turbines are best defined today by ASTM Specification D2880. Aviation turbine fuels are primarily blended from straight‐run distillates rather than cracked stocks. However, in refineries where heavy gas oil is hydrocracked, aviation fuels can include hydrocracked components since they are free of olefins, are low in sulfur, and are stable. Ground turbine fuels are equivalent to their fuel oil counterparts and are manufactured as dual‐purpose products. Most of the crudes available in greatest quantity today are high in sulfur, and yield products that must be desulfurized to meet specifications or environmental standards. The process most widely used is mild catalytic hydrogenation. Hydrogen treating removes oxygen‐containing species, which tend to perform as natural inhibitors of hydrocarbon oxidation. Recognition of this side effect of hydrogen treatment led refiners to add antioxidants to hydrotreated components; this procedure is now a specification requirement. Preserving the quality of gas turbine fuels between the refinery and the point of use is an important but difficult task. The difficulty arises because of the complicated distribution systems of multiproduct pipelines which move fuel and sometimes introduce contaminants. The extensive processing and cleanup steps carried out on gas turbine fuels produce a purified liquid dielectric of high resistivity which is capable of retaining electrical charges long enough for buildup of large surface voltages. The result is a possible hazard—a tank filled with charged fuel that under some circumstances can discharge its energy to ground in a spark capable of igniting fuel mists or vapors. This aspect of fuel handling has received much attention because of a number of accidents that have resulted in tank explosions. Several approaches have been taken to reduce the risk of static discharge. The most common method requires introduction of an additive to increase the electrical conductivity of the fuel. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. Aviation fuel is exposed to a wide range of thermal environments, and these greatly influence required fuel properties. Fuel stability assumes primary importance since freedom from deposits within the fuel system is essential for both performance and life. Oxidation of hydrocarbons by the air dissolved in fuel is catalyzed by metals and leads to polymer formation, ie, varnish and sludge deposits, by a chain reaction mechanism involving free radicals. Since it is impossible to exclude air dissolved in fuel, oxidation stability is controlled by eliminating species prone to form free radicals and by introducing antioxidants. The volatility of aircraft gas turbine fuel is controlled primarily by the aircraft itself and by its operating environment. Commercial aviation utilizes low volatility kerosene defined by a flash point minimum of 38°C. The flammability temperature has been invoked as a safety factor for handling fuels aboard aircraft carriers. Ground turbine fuels are not subject to the constraints of an aircraft operating at reduced pressures of altitude. Volatile fuels such as naphtha (No. 0‐GT) are normally stored in a ground tank equipped with a vapor recovery system to minimize losses and meet local air quality codes on hydrocarbons. A minimum volatility is frequently specified to assure adequate vaporization under low temperature conditions. The decrease in the temperature of fuel in the tank of an aircraft during a long‐duration flight produces a number of effects which can influence flight performance. Fuel viscosity increases, and fuel becomes saturated with water, and droplets of free water form and settle. Those carried with fuel may form ice on the cold filter which protects the fuel control. For this reason filter heaters are used in civil aircraft to avoid ice blockage and fuel starvation. The effects of water in fuel inside a tank, particularly an aircraft tank, are important because of the demonstrated proclivity of free water to form undrainable pools where microorganisms can flourish. In the aircraft the fungi and yeast growth usually takes place on tank surfaces, forming a fungal mat under which metabolic products such as organic acids penetrate polymeric coatings to attack the aluminum skin itself. It is common to curb growth of organisms by biocidal treatment. In storage tanks a water‐soluble agent is used. Aircraft tanks are opened periodically for hand cleaning and subsequent treatment with a fuel‐soluble boron‐containing biocide. Gas turbine fuels must be compatible with the elastomeric materials and metals used in fuel systems. Elastomers are used for O‐rings, seals, and hoses as well as pump parts and tank coatings. Polymers tend to swell and to improve their sealing ability when in contact with aromatics, but the degree of swell is a function of both elastomer‐type and aromatic molecular weight. Corrosion inhibitors, which tend to form tenacious films on metal surfaces, are generally excellent lubricity agents. The exacting list of specification requirements for aviation gas turbine fuels and the constraints imposed by delivering clean fuel safely from refinery to aircraft are the factors that affect the economics. Compared with other distillates such as diesel and burner fuels, kerosene jet fuels are narrow‐cut specialized products, and usually command a premium price over other distillates. The prices charged for jet fuels tend to escalate with the basic price of crude, a factor which seriously undermined airline profits during the Persian Gulf war as crude prices increased sharply. Demand for aviation gas turbine fuels has been growing more rapidly than demand for other petroleum products since 1960, about 3–5% per year compared with 1% for all oil products. Production of liquid jet fuel from processing of abundant natural gas is a more promising and cheaper source of alternative high quality product than shale or coal. The first commercial supersonic transport, the Concorde, operates on Jet A1 kerosene but produces unacceptable noise and exhaust emissions. NASA is considering a more advanced aircraft, eg, a Mach 5 version, to cut Pacific travel time to about 3 h, but in this case kerosene fuel is no longer acceptable, and liquefied natural gas or liquefied hydrogen would be needed. However, a completely new fueling system would be required at every international airport to handle these cryogenic fluids. Unlike the outlook for aviation fuels, the demand for ground turbine fuels has stabilized because the ground turbine can be replaced by a more fuel‐efficient engine for passenger cars and trucks.

Flammability limits: A review with emphasis on ethanol for aeronautical applications and description of the experimental procedure

“…Current analyses can identify over 200 chemical species in sample fuels [73]. Jet A type fuels consist of the order of 75-85% paraffin, both straight chain and cyclic, with the balance almost entirely aromatic compounds [74].…”