Polycyclic aromatic hydrocarbons (PAH) and soot formation in premixed, laminar n-heptane flames: Effects of oxygenate additives

Polycyclic aromatic hydrocarbons (PAH) and soot formation in atmospheric pressure, premixed, laminar, fuel rich flames of n-Heptane/Oxygen/Argon and n-Heptane/Oxygenate/Oxygen/Argon have been studied as a function of equivalence ratio and oxygenate type. Methanol, ethanol, and Methyl t-butyl ether (MTBE) were used as oxygenate additives. The oxygen weight percent in fuel-oxygenate mixtures was 2.7 percent for each oxygenate studied. Effects of oxygenate concentration have also been investigated. A heated quartz microprobe coupled to an on-line gas chromatography/mass spectrometry (HP 5890 Series II/HP 5972) has been used to establish the identities and absolute concentrations of over 40 major, minor, and trace race species by the direct analysis of samples withdrawn from the flames. Benzene was the most abundant aromatic compound identified. The largest PAH detected were the family of C18 H10's (molecular weight of 226) that include cyclopenta[cd]pyrene and benzo[ghi]fluoranthene, with peak concentrations reaching 8 ppmv and 6 ppmv, respectively. All the additives studied reduced the concentrations of aromatic and PAH species. Soot particle diameters, number densities, and volume fractions were determined using classical light scattering and extinction measurements. A coherent and collimated 514.5 nm laser beam (Spectra Physics 2017) that was mechanically modulated at 1000 Hz and focused was passed through the center of the flame. The scattered light was collected at 90° from the incident beam with a lens, and directed through an iris onto the cathode of a photomultiplier tube. The transmitted and reference lights were measured with photodiodes and recorded with an oscilloscope. Soot particle diameters were under 20 run for all the flames studied. The addition of oxygenates into the reactant mixture also reduced the size and amount of soot formed (soot volume fraction) under the experimental conditions studied. However, the levels of reduction in soot particle diameters and volume fractions were about the same for the different oxygenate additives used.