Thermodynamic equilibrium assessment of D-limonene/n-heptane blends for energy performance and nitrogen dioxide formation limits

Developing renewable bio-additives that enhance energy density without exceeding environmental limits is a critical challenge for sustainable transportation. Addressing the ‘density gap’ inherent in light paraffinic diesel surrogates like n-heptane, this study numerically investigates the thermodynamic equilibrium of D-limonene/n-heptane blends (0–25 wt.%). Using the Gibbs free energy minimisation method in the DWSIM simulator with the Peng–Robinson equation of state, we established the absolute theoretical limits for oxidation and energy performance. Unlike previous studies focusing primarily on combustion kinetics or empirical engine testing, the novelty of this work lies in identifying a ‘thermodynamic parity point’ that balances volumetric energy gains against toxic emission thresholds. Results show that D-limonene increases blend density from 681.04 to 722.20 kg/m3, driving a 6.56% increase in volumetric energy density at 25 wt % addition. While the adiabatic flame temperature rises from 2333 K to 2345.7 K, a strategic parity point was discovered at 13.33 wt.% D-limonene. At this threshold, the system achieves a 3.4% net volumetric energy gain while maintaining NO2 emissions exactly at the reference fuel level (6.25 ppm). This study goes beyond existing literature by providing a foundational ‘theoretical ceiling’ for citrus-based terpenes, offering a strategic optimisation baseline for high-density biofuel formulations.HighlightsCombustion of D-Limonene/n-Heptane blends analyzed using DWSIM.D-Limonene acts as a density modifier for diesel surrogate fuels.25% blend increased volumetric energy density by 6.5% vs n-Heptane.13.33% blend identified as a theoretical parity point for NO2.Study proposes balancing energy gain with emission constraints.