Papes for the Peeps – Fuel Oxidation #1Posted: February 22, 2013
The latest research article to come out of my group is now available online in the American Chemical Society journal Energy and Fuels (paywalled). The title “Oxidation of neat synthetic paraffinic kerosene fuel and fuel surrogates: quantitation of dihydrofuranones” is probably more than enough to put off any non-specialist reader, but I think it is a reasonable summation of the entire paper. So in breaking down the title, I will explain what the paper is all about.
Oxidation: The word ‘oxidation’ can be used to describe lots of different chemical reactions, and will mean different things to people even within different disciplines of chemistry. In the context of this work, oxidation refers to the reaction of the molecules in fuel with oxygen from the atmosphere, or dissolved in solution, to incorporate the oxygen into the structure of the molecules.
Fig.1 An example of one type of oxidised fuel molecule.
Fuels oxidise naturally over time, but generally get used up (burned in an engine) long before it could become a problem. However, oxidation, like most chemical reactions, happens much more quickly at high temperatures. A lot of modern aeroplane and ships are designed in such a way that the fuels are cycled through hot areas of the engine or fuel system before they get burned. This means that the fuels can become oxidised within the fuel system in a matter of minutes or hours. The oxidation reactions lead to the formation of solid deposits and gums which can damage engine parts, making it run less efficiently and require more maintenance.
Neat: ‘Neat’ is one of those weird words that means something completely different in the scientific vernacular to regular conversation. Rather than meaning ‘nifty, good, or tidy’, the scientific ‘neat’ refers to something being pure, unadulterated, or unblended.
It is quite common for fuels to be blended with other fuels or additives before use, for lots of different reasons (I will talk more about this in the next section). But the fuels that we used in this study were used ‘as is’ – unblended and more or less pure. This makes it a little easier for us to investigate, eliminating the introduction of possible unknown quantities into the fuel.
Synthetic Paraffinic Kerosene Fuel (SPK): SPK is a generic term for jet fuel which has been created from a non-crude oil source, usually via one of two processes; synthesis from carbon monoxide and hydrogen gases (Fischer-Tropsch process), or processed biological oils and fats.
- Synthetic: This really just refers to the fact that these fuels are not traditional fossil fuels, made from dead dinos and dug up out of the ground.
- Paraffinic: Paraffin is just another name for hydrocarbon, the molecules which make up fuels.
- Kerosene: Kerosene is a generic name for a mixture of hydrocarbons with characteristics which makes it suitable for use as an aviation fuel.
Fuel Surrogates: This is a term we use for mixtures that resemble a fuel in a particular way, but have been simplified in order to study them in more detail. Fuels can contain over a million different chemicals, so it’s often necessary to create ‘model fuels’ from a reduced selection of chemicals which are far less complex and easier to analyse.
In this study we’ve used two different fuel surrogates. One is a single component surrogate, one pure compound which we studied to determine if the oxidation was dependent on reaction with other molecules in the fuel. Turns out it’s not, all you need is one hydrocarbon, oxygen and heat. The other surrogate has nine components, representing the main classes of compounds found in real fuels. This was able to give us a better idea of the range of chemicals that are formed when a real fuel is oxidised.
Quantitation: Quantitation is just a fancy way of saying ‘measured with a known amount of accuracy and precision’. Generally, chemical analysis can be qualitative (what is it?), quantitative (how much is there?), or both. In this paper we have used some different techniques to try and quantify the compounds of interest
- Fourier Transform Infrared Spectroscopy (FTIR)
An FTIR instrument uses molecular vibrations to look at the different functional groups within molecules. Generally the functional groups that exist in oxidised compounds would be well suited to FTIR analysis, but in this case the complexity of the fuel mixture coupled with the very low concentrations of the compounds of interest makes it quite difficult to get accurate determinations. This is why the 2 separation techniques described below are more useful.
- High Performance Liquid Chromatography (HPLC)
A liquid chromatograph is often used for quantifying lots of different chemicals and has a huge range of applications across many industries. In this case the HPLC was used purely for its separating power, in order to facilitate quantitation with another technique (GC-MS, below). The interactions that occur between the fuel sample and the instrument allow the compounds of interest to be (mostly) separated from the fuel matrix.
- Gas Chromatography-Mass Spectrometry (GC-MS)
GC operates on the same principle as HPLC above – that is, interactions between the instrument and the sample allow for separations of mixtures to occur. Here, GC is really useful because the separations are very high quality and made even better by the rough separation already carried out by the HPLC. The coupling of a GC to another instrument (mass spectrometer, MS) increases the power even more as it allows for fast and simple identification of the molecules in the mixture.
Dihydrofuranones: Now, I’ve saved the most exciting part for last. A dihydrofuranone (or furanone for short), is a molecule which arises when a hydrocarbon becomes oxidised and eats its own tail, forming a cyclic molecule. In the example below, the yellow-coloured section could represent any hydrocarbon chain. These types of molecules have only been seen in fuels before where the oxidation temperature was much higher, or the oxidation time was much longer.
Fig. 2 Generic structure of a furanone molecule.
So what, I can hear you say, so what? There are two ‘so what’ aspects to this.
- These furanones have two oxygen atoms very close to each other, incorporated into the molecule. Normal unoxidised fuel hydrocarbons have no oxygen molecules in them. The presence of oxygen atoms in the molecule like this tends to attract water into the fuel from the atmosphere, particularly in humid environments. When water gets all friendly with the furanone molecules and becomes incorporated into the fuel, this is really bad news. Water in fuel can form ice crystals, which block the fuel system, and has been known to cause crashes. It also increases wear and corrosion, so is generally a very undesirable thing to have in your fuel and you definitely don’t want stuff in your fuel which increases the susceptibility to take up water.
- The oxidation reactions don’t stop once the furanones are formed. The fuel keeps on reacting with itself and creating new molecules which then go on to form insoluble particles and gums in the fuel. This is another way that engine blockages and wear can occur. So if we can figure out the mechanism of how the gums and particles are formed, we can work out ways to stop it happening in the first place.
So there you go, a whole scientific paper explained using only the title. The next paper I’m working on is about trying to figure out faster and more accurate ways of measuring oxidation products in fuels.