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.