Study of bio-oil and bio-char production from thermal and catalytic hydrotreatment of simulated pyrolysis oil under mild conditions

Licentiate thesis, 2023

Pyrolysis oil derived from fast pyrolysis of lignocellulosic biomass is a promising alternative energy
source used to produce renewable biofuels. However, the high reactivity of unsaturated oxygenated
compounds in pyrolysis oil results in charring and deactivation of the catalyst under severe
hydrotreatment conditions. This thesis focuses on reactions during stabilization of such oils, specifically,
the undesirable carbon loss to solid char as the main side reaction.
In the first study, mild thermal and catalytic hydrotreatment of a simulated pyrolysis oil, containing a
comprehensive mixture of various oxygenated groups, was performed using NiMo/Al2O3 under mild
conditions of 180-300°C and 60 bar hydrogen in a batch reactor. The solid products were extracted into
soluble and insoluble solid fractions to determine the degree of polymerization. It was found that the
soluble solids transformed into the bio-liquid product and solid insoluble yield was suppressed at
elevated temperatures. It was also accompanied by a higher degree of hydrodeoxygenation (HDO)
causing the stabilization of light oxygenates, and a significant decrease in the formation of heavy
oligomers in the liquid phase. In catalyst-free experiments, the formation of solids was higher and
showed a decreasing trend when increasing the temperature, except during heating where no solids were
observed for the non-catalytic experiment. In the presence of the catalyst, the soluble solids at lower
temperatures (180°C) consisted of macromolecule structures that were rich in sugar derivatives, while
the corresponding insoluble solids were not fully developed into char. Their composition changed to
aliphatic compounds and fully developed char respectively at higher temperatures. Moreover, the
removal of furan and sugar compounds was found to be crucial to reduce the solid char formation.
In the second study, a set of catalysts; Pt/Al2O3, Rh/Al2O3, Pd/Al2O3, Re/Al2O3, and NiMo-S/Al2O3 was
used to stabilize the same simulated pyrolysis oil at identical reaction conditions (180°C) as applied in
the first study. The catalyst screening results showed that Pd/Al2O3 was significantly better in terms of
achieving the highest conversion of pyrolysis oil and producing a high yield (66 wt%) of liquid oil
product. The bio-liquid was mostly composed of low molecular weight compounds such as stabilized
oxygenates and hydrocarbons. It also featured a minimum solid formation (3.3 wt%) in which soluble
polymers were more pronounced. The results also indicated that NiMo-S/Al2O3 was fairly good in
catalyzing reactive compounds, except furans, into stable light oxygenates with the formed solids rich
in heavy insoluble polymers (13.6 wt%). The Rh/Al2O3 was comparable to NiMo-S/Al2O3, however,
Pt/Al2O3 and particularly Re/Al2O3 rendered poor performances with the lowest yields and qualities of
the liquid products, consisting mainly of heavy soluble oligomers, which resulted in a high degree of
polymerization.