INFORM November/December 2025

inform November/December 2025, Vol. 36 (10) • 17

Summary of Bio-oil Properties

Property

Description

Oxygen Content

35–50%

Acidity (pH)

2–4

Functional Groups

Alcohols, acids, ketones, aldehydes, phenols

Stability

Poor; reactive, polymerizes on aging Partially water-soluble, polar Low (many heavy compounds)

Solubility Volatility

Sulfur/Nitrogen Content

Low (feedstock-dependent)

the bio-oil. Finally, whole fast pyrolysis bio-oil is effective as a fuel oil substitute without any upgrading. PHYSICAL PROPERTIES Bio-oils are dark, multicomponent mixtures with a distinctive smoky odor that flow freely. As the pyrolysis process condi tions and the feedstock composition affect the properties of bio-oils. But high water (15-30 weight percent) and oxygen (35-50 weight percent) content determine the physical prop erties of bio-oils. Consequently, the lower heating value (LHV) is much lower than other hydrocarbon liquids (in the range of 13-18 MJ/kg compared to 40-44 MJ/kg for gasoline and diesel) and the density of bio-oils is typically in the range of 1.1-1.3 g/ ml at ambient conditions. A fraction of the oxygenated species are organic acids, so bio-oils tend to be very acidic with a pH in the range of 2-4. Finally, the high oxygen content of bio-oils makes them immiscible with hydrocarbons like gasoline and diesel but miscible with oxygenates like alcohols and ketones. Bio-oil viscosity varies greatly (10–1000 centipoise at 40 °C) depending on the solids content, water content, and molecular weight distribution (specifically the oligomer con tent). But bio-oils are thermally unstable and continue to undergo chemical reactions even at mild storage and pro cessing conditions. Reactive components like aldehydes and ketones are prone to polymerization and increase viscosity. Condensation reactions combine two reactive components, like phenolics, to produce larger molecules that increase viscosity and more water that causes the bio-oil to phase separate. Initially, bio oil viscosity can be at the low end of the range, but the vis cosity increases slowly over time during storage (aging) or rapidly upon re-heating as the reactive components polym erize and potentially carbonize. Increasing temperature to reduce viscosity and improve the flow through an upgrading process may work for hydrocarbons but not for bio-oils. CHEMICAL PROPERTIES Bio-oils are also chemically complex mixtures with a broad molecular weight distribution and a wide boiling range. Researchers used gel permeation chromatography to mea sure the molecular weight distribution of bio-oils (https://doi.

org/10.1016/j.jaap.2024.106354). Roughly 50 percent of the bio-oil components are between 0-200 g/mole. Distillation studies showed that 25-50 volume percent of bio-oils boil above 300 °C (https://doi.org/10.1039/c6ra21134h) but the poor thermal stability of bio-oils can yield up to 20 vol percent of the starting material to form non-volatile residuals. Basic bio-oils contain light oxygenates, lignin-derived phe nolics, and carbohydrate-derived furans and sugars. Specific chemical components include alcohols, aldehydes, ketones, car boxylic acids, esters, phenolics, and anhydrosugars. Gas chro matography with mass spectrometric detection (GC/MS) is a common method for determining bio-oil chemical composition. The figure below shows the bio-oil chemical compositions that result from various feedstock chemical compositions pro duced from catalytic fast pyrolysis. Chemical composition mea sured by GC-MS identifies hundreds of unique compounds. Researchers determined the relative amounts of each com Fast Pyrolysis Fast Pyrolysis Vapor Phase Upgrading Catalytic Fast Pyrolysis Reactive Catalytic Fast Pyrolysis Hydropyrolysis Bio-oil Biomass + Heat Catalyst Added H ₂ Added High Pressure

Other

Nitrogen Compounds

Phenols

PAH

Monoaromatic

Furan

Carbonyl

Aliphatic

Alcohol

Acid

100 90 80 70 60 50 40 30 GC/MS Analysis (Area%)

20 10 0

Pine

MSW

Plastic

Biosolids

Corn stover

Miscanthus

Hybrid poplar

Chemical composition of bio-oils produced from various feedstocks determined by GS/MS analysis. Source: Wang, K., et al. , E Tech , 5(1), 183–188, 2017.