Investigation of PAH and soot formation in a dimethyl ether (DME) laminar coflow diffusion flame

Abstract Dimethyl ether (DME) is an alternative fuel with significantly reduced particulate (i.e., soot) emissions. The polycyclic aromatic hydrocarbons (PAHs) and soot formation in pure DME flames have not been fully investigated and there is a need to develop a detailed understanding of soot formation from pure DME without the confounding effect of other more sooting hydrocarbons. Therefore, PAH and soot formation in a pure DME /air laminar coflow diffusion flame were explored in this study for two different initial temperatures of the fuel. Gas chromatography /mass spectroscopy (GC/MS), and time-resolved laser-induced incandescence (TiRe-LII) were used to quantitatively measure PAH concentrations, soot volume fractions and primary particle diameters. The experimental results show that, before the appearance of the soot particles, the gas phase reactions are dominated by PAHs smaller than three rings (A3: phenanthrene and anthracene), which decrease along the centerline as the soot volume fractions increase. The primary particle diameters remain relatively constant along the radial profiles at different flame heights, which suggests a moderate surface growth. The numerical predictions of soot and PAHs (smaller than A3) formation are consistent with the measurements (within a factor of 2.4), while the overprediction of the mole fraction of A3 is near an order of magnitude. The model can also capture the effect of increasing the initial fuel temperatures with reasonable accuracy. The experimental results suggest that PAHs are mainly formed by Hydrogen Abstraction Carbon (Acetylene) Addition (HACA), C1 addition is efficient for monoaromatic while the addition of C3 and species containing cyclopentadienyl moiety are not dominant for PAH growth. A database of the PAHs and soot formation has been created for further investigation of this DME flame.