Kinetic Modeling of the Ignition of Droplets of Fast Pyrolysis Bio-oil: Effect of Initial Diameter and Fuel Composition

Fast biomass pyrolysis
is an effective and promising process for
high bio-oil yields, and represents one of the front-end technologies
to provide alternative, sustainable fuels as a replacement of conventional,
fossil-based ones. In this work, the effect of droplet initial diameter
on the evaporation and ignition of droplets of crude fast pyrolysis
bio-oil (FPBO) and FPBO/ethanol blend (50% vol) at ambient pressure
is discussed. The experimental tests were carried out in a closed
single droplet combustion chamber equipped with optical accesses,
using droplets with a diameter in the range of 0.9–1.4 mm.
The collected experimental data show a significant effect of droplet
diameter and initial fuel composition on the evaporation and combustion
of the droplets. At the same time, 1-dimensional modeling of the evaporation
and ignition of different droplets of crude FPBO and its blend with
ethanol is performed to understand the complex physical and chemical
effects. To this purpose, an 8-component surrogate was adopted, and
a skeletal mechanism (170 species and 2659 reactions) was obtained
through an established methodology. The comparison of numerical and
experimental results shows that the model is able to capture the main
features related to the heating phase of the droplet and the effect
of fuel composition on droplet temperature and evaporation, particularly
the increased reactivity following ethanol addition and the variation
of diameter with time. Also, a sensitivity analysis highlighted the
reactions controlling the autoignition of the droplets in the different
conditions. It was found that the autoignition of pure FPBO droplets
is governed by dimethyl furane (DMF), because of its high volatility
and in spite of not being the most abundant species. On the other
side, ethanol chemistry drives the gas-phase ignition in the case
of the blended (50/50 v/v) mixtures, due to its higher volatility
and reactivity.