Investigation of the combustion chemistry in laminar, low-pressure oxymethylene ether flames (OME0–4)

Abstract

Quantitative speciation data for alternative fuels is highly desired to assess their emission potential and to develop and validate chemical kinetic models. In terms of substitute choices for fossil diesel are oxymethylene ethers (OMEs) strongly discussed. Due to the absence of carbon-carbon bonds, soot emissions from combustion of OMEs are low, but significant emissions of unregulated pollutants such as aldehydes emerge.The combustion behavior of OME fuels with different chain lengths, OME0–4, was investigated in laminar premixed low-pressure flames using complementary molecular-beam mass spectrometry (MBMS) techniques. MBMS sampling provides an in-situ access directly into the reaction zone of the flame. Almost all chemical species involved in the oxidation process can be detected and quantified simultaneously. Neat OME0–3 flames were analyzed by electron ionization (EI) MBMS with high mass resolution (R ≈ 3900) providing exact elementary composition. To obtain isomer-specific information, an OME1-doped hydrogen flame and a stochiometric OME4 flame were studied by double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Both, EI-MBMS detection and i2PEPICO spectroscopy, enables a complete overview of all intermediates.The results show a dominance of oxygenated intermediates for all measured conditions. Mole fraction profiles for the most important species are presented (i.e. formaldehyde, methanol, methyl formate and formic acid) and compared to modeling results. Hydrocarbons with more than four carbon atoms were not detected under the investigated conditions. Isomers such as ethanol/dimethyl ether (m/z = 46) and ethenol/acetaldehyde (m/z = 44) could be separated using threshold photoelectron spectra for clear identification and photoionization efficiency curves for quantification. This investigation permits the discussion and analysis of systematic trends, including intermediate species, for the combustion of the studied series of oxymethylene ether fuels.