Model Fuel Design for Technically Relevant Fuels

Fig. 1: Component classes of fuels based on molecular structure

Technical fuels such as natural gas, kerosene, gasoline, or diesel are comprise several different hydrocarbons, the majority of which can be categorized as one of four classes: n-paraffins, iso-paraffins, cyclo-paraffins, and aromatics (see Fig. 1).

In creating models for technical fuels (model fuel design), the appropriate fuel components and their corresponding proportions are designed to accurately represent the target fuel. To this end, the developed model fuel (surrogate) must reflect the target fuel’s respective physical and chemical properties in order for numerical simulations of fuel properties to be valid. Model fuels enable such processes in combustor design, for example, by simulating and predicting the implications of using different type of fuels (see Fig. 2).

Generating a model fuel is an iterative process consisting of two global steps (see Fig. 3). The primary objective of these two steps is to reproduce the physical and chemical properties of the actual fuel through use of a surrogate fuel which, although simpler, remains representative of the original fuel.

Fig. 2: Physical and chemical fuel properties

Step I

Fig. 3: The Model Fuel Design Concept: Surrogate creation based on fundamental properties of real fuels

The goal of this step is to determine the model fuel’s composition. Special attention is given to ensure each of the four component classes are integrated to the proper amount into the model fuel. The exact proportion of each respective hydrocarbon species is optimized through the model fuel’s properties calculated regarding enthalpy of formation, enthalpy of combustion, viscosity at 40 °C, density at 15 °C, H:C-ratio, distillation curve, phase diagram, critical temperature and pressure, smoke point, and cetane number (see Fig. 4). These calculated physical properties are then compared with experimental data collected from the target fuel, for example kerosene, gasoline, or diesel (see Fig. 5).

Fig. 4: Schematic representation of the iterative process in model fuel development

Physical properties are calculated with an in-house numerical code developed specifically for this purpose.

Step II

Fig. 5: Comparison of the calculated properties for a typical model fuel (11 % Propylcyclohexane + 14 % i-Octane + 22 % Dodecane + 28% 1-Methylnaphtalene +24 % Hexadecane) with experimentally obtained values for kerosene

The goal of step two is to model the subsequent surrogate fuel’s chemical properties with the appropriate reaction model regarding chemical composition (C:H ratio) and the following chemical properties: species-concentration profiles, flame speed, ignition delay time, soot formation tendency (TSI Factor), and formation of other pollutants (see Fig. 4). Prior to the verification of the surrogate’s chemical properties, a reaction model must be generated which adequately describes not only each individual hydrocarbon’s chemical properties, but also these of the mixtures as well.

Following this chemical modeling, an additional comparison is performed with respect to experimental data of the technical fuel. The results obtained from this assessment are then applied to scrutinize the model fuel composition.

Steps one and two are subsequently performed iteratively optimizing the model fuel composition until acceptable agreement is reached at which point the model fuel development process is complete (see Figs. 5 and 6).