Spectroscopic analysis of Jet A-1 heteroatomic components
Introduction
Jet fuel is thermally stressed when routed through fuel oil heat exchanger and fuel handling equipment before entering the combustor (Hazlett, 1991, Totten et al., 2003). Thermal oxidative instabilities may degrade jet fuel when it is used as a coolant and heated to an average temperature of 190 °C (Totten et al., 2003). As a result, large mass species can be generated in the thermally stressed fuel and alter its composition. Additionally, routing of jet fuel through injection nozzles and valves increases the flow residence time and supports deposition. The assessment of thermal stability of jet fuel thus is vital for its use in the gas turbine engine. Thermal stressing of jet fuel produces soluble macromolecular oxidatively reactive species (SMORS) and polar heteroatomic fuel components (Hardy and Wetcher, 1990). SMORS are deposit precursors with elementary heteroatomic units, unsaturated and aromatic hydrocarbons. Jet fuel thermal stressing in the autoxidation regime produced solid deposits containing nanostructures and micron size carbonaceous particles (Sharma et al., 2019). In a very recent study, thermal stressing of Jet A-1 by flask tests in the low temperature autoxidation regime generated nanoparticles and carbon spheres (Sharma et al., 2019). Fuel additives and antioxidants can inhibit SMORS and carbonaceous solid deposit formation within limited heating residence time and temperature range. In this study, unstressed and thermally stressed Jet A-1 samples were analyzed by spectroscopic methods to investigate the polar heteroatomic fuel components and large mass species formation.
Jet A-1 is a complex organic mixture of several hydrocarbons with many additives added in trace amounts to improve properties such as thermal stability, lubricity, anti-static, anti-icing, and corrosion inhibition. Jet fuel additives can be categorized as hydrocarbon diluents and active ingredients (Saleh, 2015, Saleh, 2017, Saleh, 2018). Static dissipater additives and lubricity improvers are types of hydrocarbon diluents. Active ingredients include (i) antioxidants, (ii) metal deactivators (MDA), and (iii) icing inhibitors or fuel system icing inhibitors and leak detection additives. Fuel antioxidants contain phenols, thiols, and arylamines with weak OH, SH, and NH bond strengths. Heteroatomic species including phenols, indoles, pyrroles (2, 5 dimethylpyrrole) and carbazoles are deposit precursors. Although undesirable for thermal stability, heteroatomic species enhance the lubricity of jet fuel; thus, the presence of trace levels of N, O, and S containing species is useful. However, refining processes such as hydrotreatment removes compounds which contain sulfur. Few examples of mono and disubstituted heteroatomic deposit precursors are quinones, hydroquinones, 2, 5 dimethylpyrrole (2, 5 DMP), thiophene, benzothiophene, dibenzofuran, indoles, and quinoline. Dissolved oxygen supports hydroperoxide oxidation reactions; hence, oxygen removal can improve thermal stability (Hazlett, 1991). Trace components impact the deposit formation and thermal stability of jet fuel (Hazlett, 1991). Heteroatomic species and trace metals in the jet fuel catalyze the carbonaceous solid deposit formation (Hazlett, 1991, Sharma et al., 2019). Thermal degradation of fuel hence can depend on compositional, process and operational factors including longer heating residence time, dissolved oxygen, additives, crude oil source, refinery treatment and distribution or routing of fuel in compact zones.
Lichen secondary metabolites are useful natural products with antioxidant, antimicrobial, anti-inflammatory, and anticancer properties. Lichens are plant-like organisms similar to lower plants primarily composed of fungi combined with algae and cyanobacteria with a similar nutritional and symbiotic pattern. Lichen metabolites or lichen acids are produced extracellularly and deposited as crystals over fungi hyphae varying from 0.1 to 10 total weight percentage and contain weak phenolic acids, e.g., depsides and depsidones (Ranković and Kosanic, 2015, Hawksworth and Francis, 1976). Lichens grow in the extreme environments such as Antarctica and old growth forests of North America, and their survival in such complex climate is supported by the presence of various compounds, e.g., antioxidants, which reduce oxidative stress. In biological systems, reactive oxygen species along with similar reactive nitrogen and sulfur species can harm other molecules such as proteins, RNA, and DNA by creating oxidative stress. Similarly, thermal stressing of hydrocarbon fuel generates fuel-free radicals (FFRs) which can include R, H, ROO, and OH (where R indicates alkyl or hydrocarbon chain) which can initiate multiple reactions resulting in solids formation in the fuel. Deposition as a result of thermal and storage instabilities accelerated by FFRs can change jet fuel composition and properties, which is undesirable for the operation of aircraft gas turbine engine. Suppression of oxidation reactions, however, can be accomplished by antioxidants which scavenge free radicals and control oxidation.
Heteroatomic species in the middle distillate fuels are present mostly as mono or disubstituted cyclic and aromatic compounds which contribute to the large mass species formation through reactions favored by heating and oxidation (Saleh, 2018, Al-Hammadi et al., 2018). Hydrocarbon species in this class of compounds include quinones, indoles, pyrroles, benzofuran, thiophene, PAHs, and other similar species (Saleh et al., 2017, Saleh et al., 2018, Danmaliki et al., 2017, Danmaliki and Saleh, 2016a, Danmaliki and Saleh, 2016b, Danmaliki and Saleh, 2017). Polar hydrocarbon jet fuel species are extractable in methanol (MeOH) and other organic solvents such as tetrahydrofuran and isopropanol (IPA). Jet A-1 samples were extracted with MeOH and IPA before ESI-MS analysis in this study. Also, as reported by Hardy and Wetcher (Hardy and Wetcher, 1990), SMORS are methanol extractable and possibly contain multiple unsaturated and aromatic molecules with mass larger than 350 Da. MeOH is also a suitable extraction solvent for polyphenolic natural product antioxidants, and a similar lichen acid has been observed in the MeOH extracted Jet A-1 samples analyzed in this study.
Jet A-1 samples analysis by ESI-MS recorded large mass species (300–1000 Da) in the thermally stressed flow and static tests fuel samples in this work. Analysis by FTIR and NMR also confirmed oxygenated compounds as jet fuel components and possibly contributing to the SMORS formation. An antioxidant lichen substance, gyrophoric acid is identified by the jet fuel spectroscopic analysis in this research. Possibility of lichen substance as a fossil fuel component with its natural origin can be due to their growth in the complex climates and long ages of survival. The lichens growth rate is very slow, and species with ages of up to 4500 years are found in Britain and Greenland (Hawksworth and Francis, 1976). Lobaria linita and Lobaraia pulmonaria are lichens found in old growth forests of North America and British Columbia (Canada) which produce gyrophoric acid. Lichen substances are excellent antioxidants, and antimicrobial agents and their potential uses as fuel additives with reference to the jet fuel samples analysis in this study are discussed. In summary, spectroscopic analysis of unstressed and thermally stressed Jet A-1 samples with identification of large mass species and a lichen acid antioxidant as jet fuel component is presented in this paper.
Section snippets
Materials and apparatus
Jet A-1 used for the experiments was purchased from Shell Canada. High Performance Liquid Chromatography (HPLC) grade solvents, methanol (MeOH) and isopropyl alcohol (IPA), formic acid and water were purchased from Sigma Aldrich. A separatory funnel (125 ml) was used for the extraction of Jet A-1 with MeOH. Thermally stressed fuel samples were stored in 50 and 250 ml Borosil sample bottles. A single tube heat exchanger was used for thermal stressing of Jet A-1. Dimensions of heat exchanger tube
Jet A-1 heteroatomic components
Jet A-1 is an aviation turbine fuel analyzed by spectroscopic methods; ESI-MS, FTIR, and NMR in this work. Jet A-1 was thermally stressed by flow and static tests in a single tube heat exchanger. Single tube flow reactors have been used for thermal stressing and thermal oxidative stability study of jet fuel (Corporan, 2011). Flow tests were conducted in the temperature range 200–250 °C and static tests at 250 °C. The pressure inside the experimental apparatus was constant at 600 psig; hence,
Conclusions
In this study, Jet A-1 thermal stressing experiments were conducted, followed by spectroscopic analysis of unstressed and thermally stressed jet fuel. Following important findings summarize the observations and analysis in this work:
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Large mass species (m/z, 300–1000 Da) were recorded in the ESI-MS spectra of thermally stressed Jet A-1 static test samples. ESI-MS spectra show the high relative abundance of Jet A-1 components for static tests which were conducted at a higher temperature (250 °C)
Acknowledgments
Author is thankful to Prof. Omer Gulder, University of Toronto Institute for Aerospace Studies, for guidance and help with experiments.
Declaration of Competing Interests
Author declares no conflicts.
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