To Better Understand the Formation of Short-Chain Acids in Combustion Systems

Abstract

While their presence at the outlet of IC engines has been attested, the formation of short-chain monocarboxylic acids, formic (HCOOH), acetic (CH₃COOH), propionic (C₂H₅COOH) or acrylic (C₂H₃COOH)) acids has very rarely been reported in laboratory combustion systems. In order to better understand their potential formation, detailed kinetic mechanisms have been proposed and tested. A first model has been used to simulate lean (equivalence ratios from 0.9 to 0.48) laminar premixed flames of propane stabilized at atmospheric pressure. It was found that amounts up to 40 ppm of formic acid, 25 ppm of acetic acid and 1 ppm of C₃ acids, mainly acrylic acid, can be formed. A quite acceptable agreement has been obtained with the scarce results from the literature concerning oxygenated compounds, including aldehydes and acids. A reaction pathways analysis demonstrated that each acid is mainly derived from the aldehyde of similar structure, with a dominant role of OH. radicals. Based on this first one, a second model has allowed us to simulate a flame of propane doped by toluene and to show, as it was experimentally observed, an enhancement of the formation of C₃ acid, which could be due to the addition of OH. radicals to cyclopentadienone. A third model has been proposed to qualitatively explain the formation of acids during the pre-ignition phase (temperatures below 1100 K) in an HCCI (Homogeneous Charge Compression Ignition) engine alimented by a n-heptane/iso-octane mixture (equivalence ratio of 0.3). Noticeable amounts of monocarboxylic acids could derive from the secondary reactions of ketones or cyclic ethers, which are important products of the oxidation of alkanes at low temperature. These amounts remain too low compared to what is actually observed at the outlet of engines.

Keywords:

• HCCI engine
• Kinetic modeling
• Laminar flame
• Monocarboxylic acids

Acknowledgments

C. Mounaïm-Rousselle and A. Dubreuil from the LME in Orléans are gratefully thanked for providing data for the simulations in an HCCI engine.

Notes

^a

Thermochemical data are given in cal, mol, K units.

C₂ species

These reactions were mainly proposed by Konnov (2006).

The rate constants are given (k = A Tⁿ exp(− E_a/RT)) in cc, mol, s, cal units.