Energy & Fuels 2008, 22, 1115–1120
1115
Potential Additives to Promote Seal Swell in Synthetic Fuels and Their Effect on Thermal Stability Dirk D. Link,*,† Robert J. Gormley,† John P. Baltrus,† Richard R. Anderson,† and Paul H. Zandhuis‡ National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236-0940, and RDS-Parsons, P.O. Box 618, South Park, PennsylVania 15129 ReceiVed September 21, 2007. ReVised Manuscript ReceiVed December 21, 2007
Synthetic fuels derived from the Fischer–Tropsch (F-T) process using natural gas or coal-derived synthesis gas as feedstocks can be used for powering ground vehicles, aircraft, and ships. Because of their chemical and physical properties, F-T fuels will probably require additives in order to meet specifications with respect to lubricity and seal swell capability for use in ground and air vehicles. Using both experimental and computational studies, the propensity of certain species to enhance the seal swell characteristics of synthetic fuels and surrogates has been determined, and promising additives have been identified. Important structural characteristics for potential additives, namely an aromatic ring along with a polar constituent, are described. The thermal stability of synthetic and surrogate fuels containing the single-component additive benzyl alcohol, which is representative of this structural class, has been determined by batch stressing of the mixtures at 350 °C for up to 12 h. Synthetic fuels spiked with benzyl alcohol at concentrations (vol %) of 1.0, 0.75, and 0.5 have demonstrated the ability to swell nitrile rubber o-rings to a comparable degree as petroleum jet fuel. Further, batch reactor studies have shown that addition of benzyl alcohol does not degrade the thermal oxidative stability of the fuel based on gravimetric analysis of the solid deposits after stressing. GC-MS was used to characterize the products from thermal stressing of neat and additized surrogate jet fuel, and their compositions were compared with respect to the creation of certain species and their potential effect on deposition.
Introduction Interest by the United States Department of Defense (DoD) in kerosene-based fuels produced from nontraditional sources has grown for a number of reasons. Energy security, supply chain vulnerability, and the potential for on-demand availability are all drivers.1,2 Recently, there has been increased interest in liquid fuels derived from the Fischer–Tropsch process, which, depending on the upgrading processes used, can produce a highly paraffinic fuel with little or no heteroatomic species and exceedingly high thermal oxidative stability. These paraffinic mixtures do not contain aromatic species or heteroatomics, which contribute to their very low emissions characteristics.3,4 However, these same species are believed to be responsible for the lubricity of the fuel, as well as the ability of the fuel to adequately swell the various o-rings, bladders, and other seals contained within fuel delivery and circulation systems. The issue of seal swell is important because the components of legacy fleets have been exposed exclusively to conventional petroleumbased fuel, and switching the composition of the fuel to one with no swelling species may lead to elastomer shrinkage and failure. Recent studies have examined the propensity for * Author to whom correspondence should be addressed. Phone: (412) 386-5765. E-mail:
[email protected]. † U.S. Department of Energy. ‡ RDS-Parsons. (1) Forest, C. A.; Muzzell, P. A. Fischer–Tropsch Fuels: Why Are They of Interest to the United States Military. SAE World Congress and Exhibition, Detroit, MI, Apr 11–14, 2005. (2) Lamprecht, D. Energy Fuels 2007, 21 (3), 1448–1453. (3) Corporan, E.; DeWitt, M. J.; Belovich, V.; Pawlik, R.; Lynch, A. C.; Gord, J. R.; Meyer, T. R. Energy Fuels 2007, 21 (5), 2615–2626. (4) Song, C. Catal. Today 2003, 86 (1–4), 211–263.
different aromatic compounds to swell nitrile rubber o-rings when blended into a synthetic jet fuel and their effect on emissions characteristics.5,6 Addition of aromatic species to the synthetic fuel may indeed be a solution to solve the swelling problem. One potential approach is to increase aromatic levels by blending a conventional petroleum fuel into the synthetic fuel at different ratios. Recent tests have shown that a 50:50 blended fuel, referred to as “semi-synthetic” fuel, retains many of the performance characteristics of the conventional fuel.7–10 The importance of maintaining a threshold concentration of aromatic species is evidenced by the minimum aromatics requirement of 8% in DefStan 9191 for a blended jet fuel.11 However, this minimum threshold does not take into account the different structure-swelling relationships among aromatic species, which, as has been shown, (5) DeWitt, M. J.; Corporan, E.; Graham, J. L.; Minus, D. M. Effects of Aromatic Type and Concentration in Fischer–Tropsch Fuel on Emissions Production and Material Compatibility. International Conference on Stability, Handling, and Use of Liquid Fuels, Tucson, AZ, Oct 7–11, 2007. (6) Graham, J. L.; Striebich, R. C.; Myers, K. J.; Minus, D. M.; Harrison, W. E. Energy Fuels 2006, 20 (2), 759–765. (7) Corporan, E. Air Force Research Laboratory’s Research Activities on Alternative Fuels. AlternatiVe Energy Now Conference, Orlando, FL, February 21, 2007. (8) Fletcher, A. Material Compatibility of F-T Fuels. Fischer–Tropsch (FT) Workshop for Turbine Engine Applications, Midwest City, OK, Aug 17, 2006. (9) Gumbel, A. B-52 Bomber Takes Test Flight Using Synthetic Fuel. The Independent; Independent News and Media, Ltd.: London, U.K., September 23, 2006. (10) Woodbury, M. B-52 Tests Alternative Jet Engine Fuel. Air Force Print News Today; Air Force e-Publishing: Edwards AFB, CA, September 19, 2006. (11) Turbine Fuel, AViation Kerosine Type, Jet A-1; Defence Standard 91-91/Issue 5 (DERD 2494), Ministry of Defence Dorset: U.K., 2005.
10.1021/ef700569k CCC: $40.75 2008 American Chemical Society Published on Web 02/27/2008
1116 Energy & Fuels, Vol. 22, No. 2, 2008
Link et al.
Table 1. Selected Properties and Specifications of the Fuels Used in the Current Study property/spec
Jet A
JP-5
Fischer–Tropsch S-514
aromatics (vol %) density (kg/L) distillation 10% rec (°C) distillation 50% rec (°C) distillation 90% rec (°C) distillation end point (°C) flash point (°C) freeze point (°C) sulfur (wt %)
19.9 0.792 176 199 231 250 48 -48 0.16
20.2 0.8035 188 206 235 252 62 -50 0.117
