Solid deposit formation from the pyrolytic and oxidative degradation of jet fuel and diesel fuel

by Venkataraman, Ramya.

Deposit formation from fuel degradation is a widespread problem in jet and diesel engines that shortens engine life and decreases engine efficiency. The nature and mechanism of formation of these deposits depend on the fuel composition, metal surface properties, and operating conditions of the engine. The objective of this study was to characterize the deposits obtained from the pyrolytic and oxidative degradation of jet fuel, and deposits collected from commercial high-pressure diesel injectors (hpdi) to better understand the mechanisms of deposit formation. Short duration stressing experiments were conducted to observe the initial steps that lead to deposit formation under both pyrolytic and autoxidative conditions. This is the first study that attempts a systematic examination of solid deposits produced from thermal oxidative stressing of fuels and capture initial stages of pyrolytic deposition processes. A commercial aviation Jet A fuel sample was stressed in a flow reactor under pyrolytic and oxidative conditions on different metals (SS316 and Inconel 600) and inert (alumina coated SS316) surfaces. The deposits formed were examined by a combination of microscopic, chemical, and spectroscopic techniques. The same techniques were also used to characterize the deposits obtained from hdpi and deposits formed from the thermal oxidative stressing of a diesel range model compound, n-hexadecane. There are no earlier reports of a systematic examination of the solid degradation products from the thermal oxidative stressing of fuels. Pyrolytic degradation of jet fuel for short durations produced metal sulfides first on alloy surfaces, followed by the formation of carbonaceous solids. The solid carbon deposits consisted of amorphous films produced by a heterogeneous process similar to chemical vapor deposition (CVD) and of spheroidal particles formed by homogeneous nucleation and growth in the fluid iii phase. Although the formation of film-type deposits was suggested in earlier studies, this is the first time their presence has been confirmed. The deposits formed from the oxidative degradation of Jet A and from the hpdi consisted of condensed polyaromatic hydrocarbons showing varying degrees of structural order in solid particles. The spherical morphology of the deposits indicated formation and growth in the fluid phase. Formation in an oxidative environment led to the incorporation of oxygen complexes both on the external and internal surface of these deposits. The heteroatom compounds in the fuel did not seem to affect the nature and amount of solid deposits formed under autoxidative conditions. The formation of PAH from Jet A, a predominantly aliphatic fuel and n-hexadecane showed that aromatization of straight chain alkanes and polycondensation of aromatic rings was possible at temperatures as low as 160 ºC in the presence of oxygen. Alkylperoxy and alkoxy radicals which are reactive intermediates formed during the oxidative degradation of fuels are proposed to aid in aromatization reactions by facilitating hydrogen abstraction. The results from this study indicate the sulfur content of the fuel and the substrate composition control the heterogeneous carbon deposition, but do not affect the nucleation and growth of deposit particles in the fluid phase. Since pyrolytic degradation of jet fuel forms deposits by heterogeneous mechanisms, lowering the sulfur content of the fuel and surface stabilization by physical or chemical treatments can inhibit the solid formation under these conditions. Surface treatment may slow down the accumulation of solid deposit under autoxidative conditions by decreasing the adherence of deposits to the substrates. iv