science of jet fuel

Guest Author: Tim Nelson, Fuels & Chemicals Global Marketing Manager

During the holidays it is a tradition for many of us to travel by airplane to visit friends and family.  Flying is often harried, but we all appreciate the convenience that the airplanes provide.  Certainly no one likes to be crammed into tiny seats, and we begrudgingly endure the waiting, bumps, and squeezing during the loading and unloading process.  You might be wondering what you are really paying for as your space on a plane begins to get smaller and smaller. One of the largest parts in airplane operational cost is the jet fuel, which averages about 20% of the cost to fly. Jet fuel is a complex substance that with the wrong formula, things could go seriously wrong – that’s where chromatography comes in.

The fuel industry has made great strides in improving the global availability of jet fuel which has allowed us to access more destinations for adventures around the globe.  An extra bonus is that this has allowed access to farther out locations, with the added benefit in the financial improvement of the rural poor regional economies along with the health of their citizens.  There is a growing list of global manufacturers making jet fuel as travel expands to developing regions, and it is important to have consistent fuel quality for controlling the airplane’s mileage efficiency and more importantly the safety of jet engine operation. 

Aviation has evolved over time, especially during times of war. World War II had an enormous impact on the airplane industry as there were fewer than 300 air transport aircraft in the United States at the beginning of the war, and  by its end the U.S. aircraft manufacturers were making 50,000 planes a year. After World War II the airplane gasoline piston engine was fully developed and from that time going forward it was undergoing only incremental improvements.

The introduction of the jet turbine engine paved the way for quantum leaps in aircraft performance. It enabled man to break the sound barrier, and for us to now routinely fly to explore the world and to visit family in a matter of hours.  An aircraft turbine engine generates power by converting chemical energy stored in the fuel into mechanical energy and heat.  Because the energy contents of individual hydrocarbons can differ, jet fuel composition has a major effect on the performance of the jet turbine engine. 

Kerosene, not gasoline, was used as the fuel source during the development of the jet engine since in addition to being more readily available, it is less volatile than the lower boiling point gasoline while being an excellent source of energy. Gasoline must combust readily since the frequent combustion is what drives the pistons, but in a jet engine which has no pistons the fuel has to combust only once before it is burned continuously, which favors a lower volatile fuel such as kerosene. 

When a fuel’s hydrocarbons combust, it produces carbon dioxide and water. However, when actual jet fuel is burned, other emissions including sulfur oxides, nitrogen oxides, unburned hydrocarbons, and particulates (soot) are created. This is because there are both trace amounts of contaminants in the fuel, plus there are efficiency factors from the engine design along with the variable operating conditions. Kerosene jet fuel has a carbon number distribution between about 8 and 16 carbon numbers in length.  For compounds of the same carbon number, the order of increasing energy content per unit weight by class is aromatic, naphthene, and then the best is paraffin.  So, paraffin components provide the most desirable energy characteristics for jet fuel, and aromatics the least desirable.  In addition, the more aromatic rings on a molecule then the sootier the flame, plus there is a higher level of thermal radiation. 

Fuels with high aromatic content, in addition to high naphthalene content, can form carbonaceous particles. In a jet engine, small carbonaceous particles are formed early in the combustion process which start emitting light at high temperatures. When the combustor walls are exposed to this infrared radiation it creates hot spots that can lead to cracks and component failure. Also, if these carbonaceous particles are not completely consumed by the jet flame then they can form carbon deposits that will plug the system and reduce air flow resulting in performance failures. 

Another potential problem is that water is very slightly soluble in jet fuel, and the amount of water that jet fuel can dissolve increases with the aromatics content.  When the water is in solution there is not a problem, however if the temperature of the water-saturated fuel decreases, then some of the water dissolved in the fuel will separate as free water.  Free water at low temperatures can freeze and adversely effect the flow of fuel which could impart engine failure during flight. 

For all these reasons among others, there are specifications for the composition of jet fuel, and for the commercial grades the U.S. EPA mandates meeting ASTM D1655 testing requirements.  ASTM D1655 contains a long list of physical and analytical test methods which includes ASTM D6379 for measuring the aromatic content.  ASTM D6379 (and the equivalent IP 436) is a high-performance liquid chromatography (HPLC) test method for the determination of monoaromatic and di-aromatic hydrocarbon contents in aviation kerosene with a set maximum limit of 26.5%.  Again, it is important to meet this aromatic component level requirements to ensure that the fuel performs properly in the engine during operation.  This protects the precious cargo of commercial flight, and yet not an easy task in the complex refining processes.  Test method D6379 is used in the refinery as a guide to determine the necessary process adjustments to meet these quality specifications, and to qualify the different fuel blending source streams. 

Complex is an understatement, since there are tens of thousands of different components in jet fuel, making Phenomenex’s Luna® CN HPLC column is a great choice for breaking apart the multifarious jet fuel matrices to map out the aromatic components per ASTM D6379.  Phenomenex’s Luna® CN is a uniquely designed stable cyano chemistry which has great reproducibility from column-to-column and batch-to-batch, so that the refiners can make jet fuel to the exact specification to ensure the most consistent and reliable jet engine performance.  So, the next time we are sitting in our cramped airline seat competing for an elbow resting spot, relax and take a deep breath, rest easy and dream about the joyful time soon to be spent with family and friends. 

For further details on Luna® CN and its high-flying separation performance please see the technical note on our website or by clicking the images below:

Determination of Aromatic Hydrocarbon Types in Jet Fuel and Petroleum Middle Distillates by HPLC

Determination of Aromatic Hydrocarbon Types in Jet Fuel and Petroleum Middle Distillates by HPLC

Determination of Aromatic Hydrocarbon Types in Jet Fuel and Petroleum Middle Distillates by HPLC

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