Experimental study of the influence of hydrogen as a fuel additive on the formation of soot precursors and particles in atmospheric laminar premixed flames of methane

Abstract

We report here the experimental investigation of lightly sooting methane premixed flames with and without hydrogen. Two different approaches were considered to introduce hydrogen in the methane flame, either by keeping the total gas flow rate constant or not. Speciation data were obtained using a set of analytical tools including Gas Chromatography, Fourier-Transform Infrared Spectroscopy, Jet-Cooled Laser-Induced Fluorescence, Laser-induced Incandescence coupled with Cavity Ring-Down Spectroscopy.The results include mole fraction profiles of gaseous species (C 0 -C 16 ) and soot volume fraction (f v ) measured in all studied flames. These results demonstrate that the introduction of hydrogen to the flame insignificantly impacts the maximum mole fractions of small species (<C 6 ) while it strongly influences the concentration of aromatics and f v . Interestingly, this influence is different, depending on how hydrogen was added to the reference methane flame.An excellent linear relationship between pyrene squared concentrations [C 16 H 10 ] 2 and f v is demonstrated in all investigated flames. Moreover, hydrogen addition changes the slope of these data indicating a non-negligible impact of hydrogen on the rate of the soot nucleation process. These data and the observed linearity between f v and [C 16 H 10 ] 2 supports therefore the idea of a soot nucleation step initiated by a dimerization process implicating moderate-size polyaromatic hydrocarbons (PAHs) as pyrene. Finally, we note that PAH mole fractions at the beginning of soot formation highlight similar values regardless the flame conditions, which suggests that the inception of the soot formation might be dependent of a minimum concentration threshold of aromatic precursors required to initiate the soot nucleation process.

Experimental and numerical investigation of the transition from non sooting to sooting premixed n-butane flames, encompassing the nucleation flame conditions

“…Moreover, not all of the considered mechanisms include PAH larger than pyrene. Pyrene has been shown in numerous previous studies that it was a species close enough to actual precursors to give good tendencies [1,26,28,29,75,[94][95][96][97][98]. Thanks to the experimental data provided in the previous sections, the influence of this simplifying assumption is evaluated in Section 3.3 with the introduction of cases using all PAH larger than pyrene for particle inception and PAH adsorption/desorption in the cases using the KAUST mechanism.…”

“…Recently the existence of nucleation flames, burning different fuels and at different pressures, in which soot particles undergo negligible growth with residence time in the flame, has been demonstrated [24][25][26][27][28]. In these flames, the soot particles remain at constant diameter around 2-4 nm along the height above the burner (HAB) [27] and the soot volume fraction increases mainly by its number of smallest measurable soot particles, which are issued from the overall nucleation process [26].…”

Acceleration of a Chemical Reaction due to Nonequilibrium Collisional Dynamics: Dimerization of Polyaromatics

“…A purely chemical growth of aromatics, i.e., the extension of the PAH size by addition of gaseous species, such as acetylene, is insufficient to explain the size of emerging soot particles; the particle size is determined by particle coagulation . The latter assertion was suggested by early transmission electron microscopy of soot particles and phenomenological analysis of their time evolution, which was later corroborated by detailed modeling. , A more recent affirmation comes from high-quality experimental observations of particle nanostructure, showing that particle constituents are largely middle-range (four-to-seven-ring) PAHs. − …”

Hydrogen as a fuel additive in laminar premixed methane flames: Impact on the nucleation and growth of soot particles

“…We recently also showed that the introduction of H2 in a nucleation methane flame (C=1.82), i.e. a flame generating only nascent soot particles [32], drastically reduced or enhanced the formation of aromatic species and the soot volume fraction, depending on the way of introducing H2 (either by addition or substitution to the diluent). However, these flames did not allow studying the soot growth process and the impact of H2 on the soot particles distribution and soot number density.…”