Effect of oxymethylene ether-2-3-4 (OME2-4) on soot particle formation and chemical features

The reduction of carbon emissions is leading to alternative and renewable energy sources. Solar or wind sources can only be regulated to a limited extent and fluctuate considerably due to environmental conditions, which could be overcome by chemical energy storage solutions. Synthetic fuels are chemical energy carriers that can be produced using excess renewable electrical energy and can be directly used or easily stored. Different synthetic fuels have been investigated recently as a potential alternative or additive for fossil diesel and gasoline. Of the synthetic fuels, oxymethylene ethers (OMEs) have proven to be suitable candidates for compression-ignition engines while additionally reducing soot emissions when used either as a neat fuel or blended with fossil fuels. However, the effect of varying OME chain length on this soot reduction effect and the structure of the particulate matter has not yet been systematically investigated. This study therefore compares the soot reduction potential and analyzes the physicochemical particle features in laminar premixed eflames fueled with pure ethylene and ethylene/OME2-4 flames at four equivalence ratios, from lightly to heavily sooting conditions. Particle size distribution (PSD) measurements and numerical investigations (Conditional Quadrature Method of Moments—CQMOM) were conducted to study particle formation and growth. The results indicate a reduction in the total number and size of particles at all equivalence ratios, while the number of nanoparticles remains almost unchanged. The CQMOM model predicts the general shape of the experimentally measured PSD for both ethylene and OME-blended flames. Further, carbon particulate matter was thermophoretically sampled and its chemical structure was analyzed. The nanostructure of soot was investigated using ultraviolet–visible and Raman spectroscopy, revealing slightly higher aromaticity for the pure ethylene soot, whereas OME2-4 particles exhibited increased reactivity, as evidenced by thermogravimetric analysis. Fourier-Transform Infrared (FTIR) spectroscopy analysis showed that carbon particulate matter produced from OME2-4-doped flames contained higher amounts of oxygen, mainly as C = O. Mass spectrometry confirmed the presence of oxygen-containing functional groups in OME2-4 particulate only.