“Virtual” Smoke Point Determination of Alternative Aviation Kerosenes by Threshold Sooting Index TSI) Methods

Abstract

One benefit attributable to blending of alternative jet fuels into conventional petroderived kerosenes is reduced sooting tendency relative to the conventional unblended kerosenes. This benefit is largely due to the lower aromatics content of the alternative fuels/blends, and it is desirable with respect to sooting tendency limits embedded in aviation turbine fuel specifications (e.g., ASTM D1655 and D7566). However, the relatively high smoke points of many alternative jet fuels are not directly measurable by the prevailing ASTM D1322 smoke point (SP) method and its international variants. This frustrates characterization of these alternative fuels and prediction of their blending properties. The present work addresses an extrapolative "virtual" smoke point (VSP) technique for determination of smoke points compatible with the ASTM D1322 standard. Importantly, this compatibility removes the significant ambiguity historically associated with measurements of non-standard smoke points, herein categorically designated SP*. The VSP approach invokes the linear-by-mole blending functional basis of the Threshold Sooting Index (TSI), which has been empirically demonstrated in the literature for both defined molecular species and complex hydrocarbon fluids. If the average molecular weights (MWs) of the blending component fuels are known, then the VSPs of low-sooting tendency fuels can be forecast using D1322 SP measurements. This is demonstrated here for iso-octane and ndodecane as illustrative pure-component test fuels, as well as the full boiling range POSF 7720, a camelina sativa-derived hydrotreated jet fuel (HRJ). In part, the approach is facilitated by the ability to easily determine the average molecular weight of a fuel using a method recently developed by this laboratory. Nomenclature a = device-dependent parameter for calculation of TSI (molmm/g) b = device-dependent parameter for calculation of TSI (-) m i-j = slope of linear-by-mole TSI blending rule for fuels i and j (-) MW = (mixture-average) molecular weight (g/mol) r = repeatability limit of the ASTM D1322 smoke point method (mm) SP = ASTM D1322 smoke point (mm) SP* = non-standard, device-dependent smoke point used in general definition of TSI (mm) TSI = threshold sooting index (-) VSP = "virtual" smoke point (mm) X i = (molar) blend fraction of fuel i (-) γ i-j = slope of linear-by-mole MW/SP blending rule for fuels i and j (g/mol/mm) δ = fuel i intercept of linear-by-mole MW/SP blending rule for fuels i and j (g/mol/mm) 1

A minimalist functional group (MFG) approach for surrogate fuel formulation

“…For these fuels, a "Virtual" Smoke Point (VSP) compatible with the ASTM D1322 standard was extracted using the extrapolation method described by Haas et al [106]. This method exploits the lower smoke points acquired by the mixing of a "poorly" sooting fuel, with a "well" sooting fuel.…”

“…Knowledge of this functional relationship is therefore required in order to extrapolate the blend mole fraction to the value corresponding to the full absence of the "well" sooting fuel. To resolve this difficulty Haas et al [106] cleverly suggest exploiting the empirically demonstrated molar linearity of the TSI to the blend fraction of binary mixtures defined by Eq. (9):…”

Sooting Propensity Estimation of Jet Aviation Fuel Surrogates and Their n-Alkane Components by the Virtual Smoke Point Method

“…(a) Smoke point and (b) TSI data of n -alkanes from virtual smoke point and normalized smoke point methods as a function of the carbon number (for VSPs in this study, manually measured smoke points of seven binary mixtures of n -alkane with m -xylene are used to extrapolate VSPs according to the method of Haas et al, and VSP error bars indicate the standard deviation of the extrapolations).…”

“…Unlike conventional jet fuels (e.g., JP-8, JP-5, and Jet A) with smoke points below 50 mm, these demonstration fuels contain little or no aromatics, resulting in smoke point lengths exceeding the limits of the standard measurement device. For example, the smoke point of HRJ-Camelina (POSF 7720) is around 59 mm, which exceeds 50 mm. Some alternative biojet fuels, such as coconut-based hydroprocessed esters and fatty acids (HEFA) were reported by Yang et al to have smoke points of 92.7 and 79.5 mm, although their method of smoke point determination is unclear …”

Characterization of Global Combustion Properties with Simple Fuel Property Measurements for Alternative Jet Fuels

“…Furthermore, it is noteworthy that the current ASTM standards for DCN and TSI determination have been developed based on historical experiences from petroleum-derived fuels, thus exhibiting the limitations to fruitfully incorporate the fuel properties of emerging alternative fuels. Currently, a novel approach to measure the SP of alternative jet fuels, which SP cannot be determined by existing ASTM standards, is under the development by using a concept of "virtual" smoke point from the blends of petroleum-derived and alternative jet fuels 48 . It In summary, three future works can be suggested in order to improve quantitative predictability of CPT Index method.…”