Energy & Fuels 2003, 17, 577-586
577
Surface Deposition within Treated and Untreated Stainless Steel Tubes Resulting from Thermal-Oxidative and Pyrolytic Degradation of Jet Fuel Jamie S. Ervin,* Thomas A. Ward, Theodore F. Williams, and Jarrod Bento University of Dayton Research Institute, Dayton, Ohio 45469-0210 Received August 12, 2002
Flow experiments using heated Jet-A fuel and additives were performed to study the effects of treated surfaces on surface deposition. The experimental apparatus was designed to view deposition due to both thermal oxidative and pyrolytic degradation of the fuel. Carbon burnoff and scanning electron microscopy were used to examine the deposits. To understand the effect of fuel temperature on surface deposition, computational fluid dynamics was used to calculate the two-dimensional temperature profile within the tube. Three kinds of experiments were performed. In the first kind, the dissolved O2 consumption of heated fuel is measured on different surface types over a range of temperatures. It is found that use of treated tubes significantly delays oxidation of the fuel. In the second kind, the treated length of tubing is progressively increased which varies the characteristics of the thermal-oxidative deposits formed. In the third type of experiment, pyrolytic surface deposition in either fully treated or untreated tubes is studied. It is found that the treated surface significantly reduced the formation of surface deposits for both thermal oxidative and pyrolytic degradation mechanisms. Moreover, it was found that the chemical reactions resulting in pyrolytic deposition on the untreated surface are more sensitive to pressure level than those causing pyrolytic deposition on the treated surface.
Introduction Jet fuel is used in military aircraft for purposes of cooling before it is burned in the combustor. As fuel flows through the fuel system, trace species within the heated fuel react with dissolved O2 to form surface deposits. At relatively higher fuel temperatures, the dissolved O2 is depleted, and pyrolytic chemistry becomes dominant (at temperatures greater than ∼450 °C). Pyrolytic reactions change the composition of the major fuel species and, like thermal-oxidative reactions, produce surface deposits and other undesirable products. Under conditions of either thermal-oxidative or pyrolytic reactions, surface deposits collect on surfaces within valves and impede flow through metal passages. Since surface deposits have the potential to cause catastrophic failure of an aircraft fuel system, it is important to study the influence of the surface on deposition occurring under conditions of both thermaloxidative and pyrolytic fuel degradation. It has been observed that the surface material over which heated fuel flows can strongly influence thermaloxidative chemistry.1 For example, in flowing experiments with Jet-A fuels, Jones2 found that the rate of fuel oxidation within treated stainless steel tubes was slower than that within untreated stainless steel tubes. The surface treatment involved the chemical vapor * Corresponding author. E-mail:
[email protected]. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; American Society for Testing and Materials: Philadelphia, 1991; pp 116-120. (2) Jones, E. G.; Balster, W. J.; Pickard J. M. Trans. ASME, J. Eng. Gas Turbines Power 1996, 118, 286-291.
deposition of a proprietary silica-based layer (Silcosteel3). A passivating layer, which prevents active sites on a stainless steel surface from contacting fuel, would be expected to result in a slower rate of dissolved O2 consumption. Moreover, reduction of the consumption of dissolved O2 decreases the potential for surface deposition. In a high-temperature study (pyrolytic conditions) involving the flow of heated jet fuel over a range of surface materials (nickel, SS 316, SS 304, Silcosteel, and glass), it was found, using temperature-programmed oxidation analysis and scanning electron microscopy on the deposits, that different surface materials resulted in deposits of varying character.4 Silcosteel-treated surfaces, which are presumed to be less active than stainless steel surfaces containing nickel, were observed to reduce carbon deposition. Although past research has suggested that surface treatment may be used to reduce surface deposition, the influence of the surface material on deposition in thermal-oxidative and pyrolytic fuel degradation is not well understood. One theory for thermal oxidative degradation is that particles form in the bulk of the fuel, migrate to heated walls, and then adhere there to form surface deposits. Other possibilities for surface deposit formation are that deposits are initiated at the heated surface with little contribution from particles in the bulk flow or that surface deposition results from contributions from both adhering bulk particles and deposits formed only at the wall. A further complication in (3) Silcosteel tubing, Restek Corporation, Bellefonte, PA. (4) Altin, O.; Eser, S. Ind. Eng. Chem. Res. 2001, 40, 596-603.
10.1021/ef020180t CCC: $25.00 © 2003 American Chemical Society Published on Web 03/25/2003
578
Energy & Fuels, Vol. 17, No. 3, 2003
Ervin et al.
Table 1. Description of Experiments experiment
interior tube surface duration of experiment (h) flow rate (mL/min) pressure (MPa) Reynolds number range residence time (s)
1
2
3
dissolved O2 consumption for different surfaces fully treated or untreated 24 20 4.5 120-3,730 11-1st furnace 7-2nd furnace
thermal-oxidative surface deposition varying starting lengths of treated tube 24 20 4.5 120-3,730 11-1st furnace 7-2nd furnace
pyrolytic surface deposition fully treated or untreated 4&8 32 3.89, 5.27 or 6.31 193-27,200 5-1st furnace 2-2nd furnace
Table 2. Characteristics of Neat Jet-A Fuel F3219 characteristic
value
JFTOT breakpoint, °C (°F) sulfur, total, wt % aromatics, vol % hydrogen content, mass % copper iron zinc specific gravity @ 60 °F
285 (545) 0.0321 16.6 13.6
