A shock tube study of formic acid ignition delay times and species time histories

This study explores the oxidation chemistry and ignition characteristics of formic acid (FA), highlighting its potential as a hydrogen carrier and e-fuel. Our investigation encompasses experimental measurements of ignition delay times (IDTs) and time-histories of CO and CO₂ during FA oxidation in a shock tube, facilitated with laser diagnostics. Acknowledging the notable uncertainties in prior investigations, particularly related to the low vapor pressure and dimer formation tendency of FA, we employed a diagnostic for the measurement of the initial concentration of FA. Shock tube experiments were carried out at two reflected-shock pressures, around 1.7 and 3.5 bar, two equivalence ratios, 0.72 and 1.47, and temperatures ranging from 1194 to 1658 K. The experimental results demonstrated a strong dependence of IDTs on temperature and pressure, whereas dependence on equivalence ratio was found to be insignificant. Seven literature kinetic models were tested against the experimental data to evaluate their predictive capability. Among these, four models demonstrated good performance in predicting IDTs and species time-histories with some discrepancies at certain conditions. Sensitivity and rate-of-production analyses highlighted the importance of unimolecular decomposition, H-abstraction, and HOCO consumption pathways, with H + O₂ = O + OH as the main promoting reaction while HOCO + O₂ = CO₂ + HO₂ as the main inhibiting reaction. The primary consumption pathway of FA involves OH attacking the acidic hydrogen, yielding HOCO, which then decomposes to generate CO and OH. The rate constants of HOCO decomposition channels significantly contribute to the discrepancies observed in models. These reactions have proven to be crucial in CO and CO₂ time histories and are major components of the primary FA consumption pathway.