ARTICLE pubs.acs.org/IECR
Effect of Aviation Fuel Type on Pyrolytic Reactivity and Deposition Propensity under Supercritical Conditions Matthew J. DeWitt,*,† Tim Edwards,‡ Linda Shafer,† David Brooks,† Richard Striebich,† Sean P. Bagley,§ and Mary J. Wornat§ †
University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469, United States Air Force Research Laboratory, Fuels Branch AFRL/RZPF, Wright-Patterson AFB, Ohio 45433, United States § Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States ‡
ABSTRACT: Development of reusable liquid-hydrocarbon-fueled hypersonic vehicles requires improved understanding of the effect of chemical composition on the controlling reaction chemistry and deposition propensity as the fuel is used to cool the system. In this effort, supercritical pyrolytic stressing studies were performed using two petroleum-derived fuels and a Synthetic Paraffinic Kerosene (SPK) comprised predominantly of normal and branched paraffins. All fuels decomposed via free radical pathways with high yields of unsaturates and lower molecular weight products consistent with pyrolysis at high pressures and moderate temperatures. However, the SPK was significantly more reactive than the petroleum-derived fuels due to a lack of efficient hydrogen donors that act to terminate chain reactions (higher net propagation rate). High-pressure liquid chromatography was used to identify and quantify polycyclic aromatic hydrocarbons (PAH) in the stressed fuels, conclusively determining that these are produced during thermal stressing. A notable observation was the presence of PAH during SPK stressing, as the neat fuel did not contain cyclic precursors for growth to PAH. During stressing with stainless-steel tubing, the formation of filamentous deposits via metal-catalyzed reactions of stressed fuel components with reactor surfaces was observed for all fuels studied. However, the SPK fuel exhibited a much higher pyrolytic deposition rate, which was attributed to higher lateral growth rates of surface filaments via noncatalytic free radical addition pathways. The PAH formed during SPK stressing are indicators of the highly reactive intermediates prone to participating in the surface coke addition pathways. Studies blending benzene with the SPK indicated that low PAH solubility in the paraffinic fuel is not the dominant cause for the high deposition propensity. Testing with the petroleum-derived fuels showed that metal sulfide filament formation can occur under endothermic conditions, and higher fuel sulfur content can increase carbon deposition propensity. Studies with surface passivated tubing (Silcosteel) suppressed filamentous carbon formation and rendered a substantial reduction in SPK deposition to levels similar to the petroleum-derived fuels. Overall, these studies provided guidance regarding the controlling chemistry during supercritical pyrolysis of current and potential synthetic hydrocarbon fuels and insight into prevalent deposition pathways.
’ INTRODUCTION A major complication to the development and implementation of liquid-hydrocarbon-fueled hypersonic vehicles is achieving sufficient heat sink capability in the fuel, which is used to regeneratively cool the vehicle and engine structure and subsystems. One approach, termed an “Endothermic Fuel,” achieves high levels of heat sink by supplementing the “sensible heating (CpΔT)” via deliberate bulk endothermic (e.g., heat-absorbing) reactions of the fuel, such as thermal (e.g, dehydrogenation) and/or catalytic cracking.1 3 Current demonstrated endothermic capability is approximately 1500 BTU/lb at fuel temperatures of approximately 700 °C for a 15-min duration.2,3 A major limitation to viable implementation of endothermic fuels is the undesirable formation of carbonaceous deposits (also known as “coking”). The deposition can reduce fuel flow, increase resistance to heat transfer, and foul injector nozzles. Ideally, the fuel would be selectively converted to low molecular weight products, such as ethylene and propylene, which are favorable for both overall heat sink and combustion efficiency, without forming high molecular weight deposits.3 However, condensed and supercritical phase pyrolytic reaction pathways produce a wide range of products, with smaller paraffins/olefins and aromatics r 2011 American Chemical Society
being predominant products, all of which can form carbonaceous deposits.2,4 10 There are varying types of complex deposit mechanisms which have been observed during gas, condensed, and supercritical pyrolysis of hydrocarbons. It is generally accepted that there are three basic mechanisms relevant to deposit formation under these reaction regimes.3,10 23 These include the formation of filamentous coke via catalytic reactions with surface metals, free radical growth reactions between bulk phase components and surface coke (noncatalytic mechanism), and the production of condensation (amorphous) coke via formation of polycyclic aromatic hydrocarbons (PAH) in the bulk phase which subsequently condense on tubing surfaces. The initial filament formation has been shown to involve metal-catalyzed reactions of fuel components with the reactor surfaces, primarily by nickel and iron, resulting in the formation of metal carbides as intermediates.12 The metal atoms are extracted from the surfaces Received: February 9, 2011 Accepted: August 12, 2011 Revised: July 15, 2011 Published: August 12, 2011 10434
dx.doi.org/10.1021/ie200257b | Ind. Eng. Chem. Res. 2011, 50, 10434–10451
Industrial & Engineering Chemistry Research
Figure 1. Comparison of linear and lateral growth mechanisms of carbon filaments.13
and catalyze linear filament growth while remaining at the tips of filaments. Sulfur-containing compounds can also promote production of metal sulfide filaments and can potentially have a significant effect on coke formation rates.18,21 24 Once formed, the filaments can undergo lateral growth (thickening) via free radical reactions between bulk phase components and surface coke; this mechanism has been likened to chemical vapor deposition (CVD) processes.22 This noncatalytic mechanism can result in significant growth of the filamentous deposits and blockage of the extracted metal atoms. A schematic of the formation of filamentous coke formation via catalytically assisted decomposition (linear growth) and free radical growth reactions (lateral growth) proposed by Kopinke and colleagues,13 which is consistent with the mechanism discussed by Albright and Marek,12 is shown in Figure 1. In general, it is believed that formation of PAH is a key precursor to amorphous coke formation, with PAH concentrations increasing rapidly with pressure and extent of fuel conversion.25,26 The impact of this mechanism on overall deposition levels becomes more prevalent at higher reaction temperatures and extents of reaction.3 The formation of aromatics and PAH have been hypothesized to potentially have a dual role, as actual amorphous coke precursors and as an indicator for short living, highly reactive intermediates which participate in coke formation (via free radical growth).15 Detailed product analyses have confirmed extensive PAH formation during supercritical fuel pyrolysis,25,27 33 but the PAH distribution is substantially different from that observed at the higher temperatures and lower pressures associated with combustion.34 38 Thus, deposit formation in supercritical fuel pyrolysis superficially resembles soot formation during combustion in that PAH are believed to be primary intermediates or indicators in the formation of high molecular weight carbonaceous products, but the controlling reactions and growth mechanisms appear to be different in the two reaction environments. The use of alternative (nonpetroleum derived) aviation fuels derived from coal, natural gas, and biological feedstocks has received significant attention recently.39 Alternative fuels provide a potential domestic fuel source which can increase energy and economic security. The majority of the initial efforts have focused on Synthetic Paraffinic Kerosene (SPK) derived from the Fischer Tropsch (FT) process.40,41 These have resulted in the approval of SPK as a blend feedstock of up to 50% by volume with both commercial Jet A/Jet A-1 (per ASTM D756642) and military JP-8 (MIL-DTL-83133G43) fuels, provided the final fuel specification properties are satisfied. Although the potential
ARTICLE
feedstock sources (including biological) for the production of alternative fuels are rapidly increasing, near-term drop-in blend candidates will have chemical compositions very similar to currently approved SPKs. As synthesized, neat SPK exhibits excellent thermal-oxidative stability with minimal oxidative deposition propensity,39,40 however, the behavior under pyrolytic conditions is unknown. Previous studies performed with single-component model compounds and blends have shown differences in relative reactivity and deposition propensity depending on fuel structure. For conditions representative of steam cracking coils (>800 °C, gas phase), studies were performed to investigate the effect of fuel structure on coke formation rates.3,13,15 17 These were performed by adding small (
