Mathematical Modeling of Fast Biomass Pyrolysis and Bio-Oil Formation. Note II: Secondary Gas-Phase Reactions and Bio-Oil Formation

This paper summarizes
the research activities done at Politecnico
di Milano in the field of the detailed kinetic modeling of fast pyrolysis
of biomass to produce bio-oil. Note I of this work already discussed
biomass characterization and the multistep pyrolysis mechanisms of
reference species. The model is able to provide a detailed composition
of pyrolysis products and char residue. Different critical steps are
involved in this multicomponent, multiphase and multiscale problem.
The first complexity relies in biomass characterization. Then, fast
pyrolysis process involves detailed kinetic mechanisms, first in the
solid phase for the biomass pyrolysis, then in the gas-phase for the
secondary reactions of released products. The complexity of these
kinetic mechanisms requires strong simplifications, thus chemical
lumping procedures are extensively applied. Successive or secondary
gas phase reactions of gas and tar components released during the
pyrolysis process complement the kinetic model, together with the
heterogeneous reactions of residual char. The modeling of fast pyrolysis
process requires a comprehensive description of the coupled transport
and kinetic processes, both at the particle and the reactor scale.
A few examples and comparisons with experimental data validate the
reliability of the overall model. Finally, the composition and physical
properties of the pyrolysis bio-oil are also discussed, with emphasis
on combustion and pollutant emissions.