Abstract

Electrofuels produced via Power-to-X (PtX) pathways represent a promising route toward carbon-neutral combustion, yet their deployment requires understanding how their distinct chemical composition affects flame behavior. This work presents a systematic experimental characterization of soot production, maturity, and radiative coupling in flames of renewable synthetic liquefied petroleum gas (e-LG), a PtX-derived drop-in fuel, blended with conventional HD5 propane under varying oxygen index (OI) conditions, contributing benchmark data for the validation of soot formation models in next-generation PtX fuel systems. Six HD5/e-LG mixtures (propane mole fractions XHD5 from 0 to 1) were studied in laminar coflow non-premixed flames at five OI levels (17%-33%) at a constant heat release rate of 90.96 W. Two-wavelength extinction/emission diagnostics combined with radiometer measurements yielded soot volume fraction (fs), flame temperature (T), soot maturity index (beta), and radiative emission ((q) over dot emi, (q) over dots). The isobutane-dominated composition of PtX-derived e-LG monotonically increases soot loading and radiative emission across all OI levels despite a reduction in mean flame temperature, revealing a strong inverse thermal-radiative coupling. The (q) over dotemi response to OI is non-monotonic for e-LG-rich blends, peaking near OI = 25%, reflecting the competing effects of enhanced soot oxidation and elevated flame temperature. The soot-to-total radiation ratio rises from 13.6% at OI = 17% for pure HD5 to 58.3% at OI = 33% for pure e-LG. Maturity parameters converge to beta approximate to 0.6-0.7 and Em approximate to 0.21-0.25 at OI >= 21%, while OI = 17% exhibits anomalous dispersion consistent with a low-temperature, locally oxygen-limited combustion regime. A linear correlation (R-2 = 0.81) between normalized soot loading and normalized mean temperature confirms soot-driven radiative cooling as the governing thermal-radiative mechanism across the full blending range. These results provide actionable experimental guidance for the safe and efficient integration of carbon-neutral PtX-LPG into existing combustion infrastructure, while supplying a validated dataset for soot formation model development in Power-to-X fuel systems. Novelty and significance statement This study provides the first systematic characterization of soot formation, maturity, and radiative coupling across the full HD5/e-LG blending range (0%-100%) under oxy-combustion conditions (OI = 17%-33%), filling a critical gap in the experimental basis for renewable LPG deployment. The novelty lies in demonstrating that, despite the significant chemical disparity between the alkane-dominated HD5 and the isobutane-dominated e-LG, the thermal-radiative response of all intermediate blends collapses onto a single fuel-composition-invariant linear scaling (R-2 = 0.81), extending a known single-fuel T-sootloading correlation to a chemically disparate binary system, and establishing that soot-driven radiative cooling governs the thermal-radiative balance independently of blend ratio and fuel class under sufficient oxygen availability. This result is significant because it supplies a compact, single-parameter predictive framework for soot-radiation coupling in Power-to-X fuel systems and provides quantitative benchmark data for soot formation and radiation model validation across the HD5/e-LG transition regime.