Effect of Aviation Fuel Type on Pyrolytic Reactivity and Deposition Propensity under Supercritical Conditions

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.

Thermal stability and decomposition of diesel fuel under subcritical and supercritical conditions

“…10F). The morphology of solid deposits formed at 440 o C is consistent with that previously reported for the pyrolytic regime at higher temperatures [32]. The different morphologies of solid deposits observed under different thermal stressing conditions may imply different solid deposit formation mechanisms.…”

Evaluation of heat sink capability and deposition propensity of supercritical endothermic fuels in a minichannel

“…GH3128 tubing was selected for its tolerance in working temperatures up to 1000°C. The catalytic activity of nickel could affect the growth, shape and formation of coke deposition [2], but the main focus of this experiment was to evaluate the amounts of coke deposited by each kind of fuel (rather than to examine the thermal cracking and coke formation mechanism). The experimental system has been described in detail in our previous reports [24][25][26].…”

“…The HRM is based on the principle that as coke deposition constricts the flow area, the flow resistance increases significantly in the minichannel. According to Poiseuille's law, the pressure drop DP in the test channel is inversely proportional to the fourth power of the channel's internal diameter D at a given mass flow rate and a constant fluid thermal condition (constant density and viscosity), as shown in equation (2). DP ¼ 128LMl…”

“…Endothermic fuels [1][2][3] have been used as coolants in regeneratively cooled hypersonic vehicles, as they help to remove the http://dx.doi.org/10.1016/j.fuel.2015.05.068 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.…”

Understanding the Initial Decomposition Pathways of the n‐Alkane/Nitroalkane Binary Mixture

“…[1,2] However, these chemical reactions require very high temperatures to occur at the useful reaction rates, which hence reduce the allowable stress of heat exchanger material and cause a remarkable increase in coking deposition during the thermal decomposition process. [3][4][5] Small amounts of nitroalkanes (R-NO 2 ) serving as soluble initiators have been incorporated into n-alkanes (R'H) which are the major components of endothermic hydrocarbon fuels, [6] and then significant promotion on the overall thermal decomposition was observed. [7][8][9] The thermal decomposition mechanism of hydrocarbons is generally accepted as the free-radical chain reaction.…”