A system includes a model-based control unit configured to receive a lower heating value for a liquid fuel having a composition. The processor is configured to determine a specific gravity, a hydrocarbon ratio, or both, for the liquid fuel based, at least in part, on the lower heating value of the liquid fuel. The processor is configured to compare the hydrocarbon ratio, the specific gravity, and/or the lower heating value of the liquid fuel to a collection of hydrocarbon ratios, specific gravities, and/or lower heating values for a plurality of mixtures of methane and ethylene. The processor is configured to identify a mixture of methane and ethylene from the collection that best matches the hydrocarbon ratio, the specific gravity, and/or the lower heating value of the liquid fuel and configured to use the identified mixture of methane and ethylene as a model for the composition of the liquid fuel.