Systematic investigation of oxymethylene ether combustion using electron and synchrotron radiation ionization mass spectrometry

Abstract

Oxymethylene ethers (OMEs) are considered as sustainable alternatives or additives to conventional diesel fuels. They are characterized by their molecular structure CH3O-[CH2O]n-CH3 and can be produced through CO2-neutral processes. Having no direct C-C bonds, OME combustion shows a decrease in particulate matter emissions, such as soot, when compared to standard diesel blends, but they also show an increase in unregulated pollutants, such as aldehydes. The presented Ph.D. thesis systematically studies the oxidation behavior of OMEs with different chain lengths (OME0−5). Speciation data is measured using electron ionization molecular-beam mass spectrometry (EI-MBMS) with high mass resolution in an atmospheric flow reactor and low-pressure laminar flames. To obtain additional isomer-selective speciation analysis, double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy using vacuum ultraviolet (VUV) radiation at the Swiss Light Source is obtained. The combination of both spectroscopy methods provides a comprehensive understanding of OME oxidation processes and a more detailed overview of the species involved. Three consecutive experimental studies are presented in this Ph.D. thesis: Focusing on the impact of the OME chain length on the oxidation behavior, OME0−5 are investigated in atmospheric laminar flow reactors. Subsequently, the oxidation of OME2 and its branched isomer, trimethoxymethane, are compared to comprehend the structural influences on their oxidation behavior. Finally, high-temperature conditions are examined for OME0−4 using low-pressure laminar flames. Overall, oxygenated species are the dominant intermediates for all OMEs. The observed species pool is nearly independent of the OME’s chain length. Identification of significant isomers and detailed mole fraction profiles are provided for several oxygenated species such as methanol, formaldehyde, dimethyl ether, formic acid, methyl formate as well as for several hydrocarbons. Notably, the absence of typical soot precursors underscores OMEs’ potential for clean combustion. The reactivity of OMEs increases with longer chain lengths or branched structures, evidenced by changing peak temperatures of intermediates. The presence of ethanol as a key intermediate is remarkable and indicates unknown or underestimated reaction pathways. Comparison between the linear OME2 and its branched isomer, trimethoxymethane (TMM), reveals differences in the species pools illustrating the structural impacts on combustion and providing direct insights into the decomposition of the examined fuels. The experimental data on OMEn oxidation are compared with the DLR model Concise, revealing its accuracy while also suggesting areas for improvement. Based on this experimental study, enhancements are already implemented in DLR Concise and are highlighted in this Ph.D. thesis. Overall, the study highlights the benefits of combining two complementary diagnostic techniques to obtain a more comprehensive and detailed analysis. The experimental results of this systematic oxidation analysis of OMEs can be used to develop, validate, and optimize reaction models. This research supports the advancement of OMEs as replacements for crude-oil based diesel fuels, contributing to a more sustainable transportation sector.