Taming Bio-Oil: Selective Pretreatment for Inorganic Control and Stabilization

Doktorsavhandling, 2026

Fast pyrolysis bio-oil (FPBO) is a complex, oxygen-rich liquid whose inorganic impurities and instability present major challenges for storage, handling, and catalytic upgrading. Addressing these limitations is critical for improving the viability of FPBO as a renewable feedstock capable of displacing fossil-derived fuels. This thesis investigates the chemical form, phase association, and accessibility of inorganic species in FPBO, alongside controlled dewatering as a strategy for stabilization.
Comprehensive characterization demonstrates that inorganic elements in FPBO do not constitute a homogeneous dissolved population. Instead, K and Mg are predominantly water-associated and finely dispersed, whereas Ca is largely retained in the organic phase and strongly associated with coarse particulate matter. Fe is also largely retained in the organic phase, but displays the most complex behavior, with mixed phase partitioning and particle-size distributions indicative of multiple coexisting chemical forms. P shows a distribution pattern that more resembles that of Fe rather than K, Mg, or Ca. These differences fundamentally govern the accessibility of inorganic species under mild pretreatment conditions.
The heterogeneous nature of inorganic speciation translates directly into selective interactions with solid sorbents. Zeolites and acidic ion-exchange resins remove 77-91% of Fe as well as alkali and alkaline earth metals, while γ-alumina exhibits pronounced selectivity toward P, reducing it by >90%. No single sorbent enables comprehensive removal of all inorganic species; however, combining materials with complementary surface acidity and interaction mechanisms substantially broadens overall removal across element groups.
In parallel, azeotropic distillation using mesityl oxide is demonstrated as an effective method for deep dewatering of FPBO. Dewatering reduces water content to ~1 wt.%, with significant drop of acidity. This suppressed reactivity as demonstrated during accelerated aging - without altering bulk molecular structure.
By combining targeted inorganic removal with controlled dewatering, FPBO properties can be tailored to improve stability and compatibility with downstream upgrading, thereby supporting reliable co-processing with fossil-derived refinery streams.