Perspective on Fuel Property Blending Rules for Design and Qualification of Aviation Fuels: A Review

The push toward sustainable aviation fuels (SAFs) is intensifying in response to global decarbonization efforts. This review discusses the development and assessment of probabilistic fuel property prediction methods in use today and vital for the formulation of these next-generation SAFs. We discuss the rigorous quality assurance specifications necessary for aviation fuels before they can be integrated into the supply network, with a particular emphasis on the stringent requirements for non-petroleum-based fuels or synthetic aviation turbine fuels. In this review, the relative merit of predictions based on component data and predictions derived from other modeling approaches is discussed. A total of 16 scalar products between a concentration vector (mass, mole, or volume fraction) and a property vector can be used to estimate 31 properties, counting the whole distillation curve as one property. The accuracy of these blending rules, as applied to mixtures of aliphatic and aromatic hydrocarbons in the jet fuel volatility range, is documented here. The relative contribution of blending rule accuracy to the fuel property prediction uncertainty is discussed. In most (if not all) cases, the predictive uncertainty is small relative to uncertainties arising from incomplete characterization of the component concentration or property data and/or the accuracy of reported component property data. The data used to populate the property vector comes from authoritative sources, such as the National Institute of Standards and Technology (NIST), the Design Institute for Physical Properties (DIPPR), and the periodic table of elements, or internal measurements. Additionally, the freeze point can be estimated from the crucial components' concentration and property data from a simple equation-of-state model (enthalpy, entropy, and temperature) applied to each of the crucial components. A total of 75 additional "properties" with specified criteria stated in ASTM D4054 follow directly from the measurement of trace or bulk materials or can be determined exactly from blending rules that invoke the conservation of mass. A total of 20 properties listed in ASTM D4054 are not discussed in this review. A total of 16 of the 20 properties not listed here are believed to depend upon trace level impurities (such as the jet fuel thermal oxidation test or JFTOT), lacking identification or composition data, while simple blending rules are not advised for the remainder.