Impact of Methanol and Butanol on Soot Formation in Gasoline Surrogate Pyrolysis: A Shock-Tube Study

The influence of methanol and butanol on soot formation
during
the pyrolysis of a toluene primary reference fuel mixture with a research
octane number (RON) of 91 (TPRF91) was investigated by conducting
shock-tube experiments. The TPRF91 mixture contained 17 mol % n-heptane, 29 mol % iso-octane, and 54 mol % toluene. To assess the contribution of individual
fuel compounds on soot formation during TPRF91 pyrolysis, the pyrolysis
of argon diluted (1) toluene, (2) iso-octane, and
(3) n-heptane mixtures were also studied. To enable
the interpretation of the TPRF91 + methanol and TPRF91 + butanol experiments,
the influence of both alcohols on soot formation during the thermal
decomposition of toluene and iso-octane was also
investigated in a separate series of measurements. Pyrolysis was monitored
behind reflected shock waves at pressures between 2.1 and 4.2 bar
and in the temperature range of 2060–2815 K. Laser extinction
at 633 nm was used to determine the soot yield as a function of reaction
time. For selected experiments, the temporal variation in temperature
was also measured via time-resolved two-color CO absorption using
two quantum-cascade lasers at 4.73 and 4.56 μm. It was found
that soot formed during TPRF91 pyrolysis is primarily caused by the
thermal decomposition of toluene. Adding methanol to TPRF91 results
in a slight reduction of soot formation, whereas admixing butanol
results in shifting soot formation to higher temperatures, but in
that case, no overall soot reduction was observed during TPRF91 pyrolysis.
Measured soot yields were compared to simulations based on a previous
and an updated version of a detailed reaction mechanism from the CRECK
modeling group [Nobili, A.; Cuoci, A.; Pejpichestakul, W.; Pelucchi,
M.; Cavallotti, C.; Faravelli, T. Combust. Flame 2022; 10.1016/j.combustflame.2022.112073]. Rate-of-production analyses for reactions involving BINS at different
experimental conditions were carried out. Although in the case of
TPRF91 and toluene pyrolysis, no quantitative agreement was obtained
between the experiment and simulation, the comparison nevertheless
shows that the new version of the CRECK mechanism is a significant
improvement over the previous one. In the case of n-heptane decomposition and iso-octane pyrolysis
with and without alcohols, the updated reaction mechanism shows excellent
agreement between simulation and measured soot yields.