Bio-oil, commonly referred to as pyrolysis oil, is the primary liquid product generated from the high-temperature thermal decomposition (pyrolysis) of biomass under oxygen-free or oxygen-limited conditions. It is not a single compound, but rather an extremely complex mixture containing hundreds to even thousands of different organic compounds.

These compounds are predominantly oxygenated substances, including molecules with functional groups such as acids, ketones, aldehydes, alcohols, phenols, saccharide, esters, ethers, and furans. In addition to these complex organic components, bio-oil also contains a significant proportion of water.

In terms of physical appearance, bio-oil typically presents as a dark brown, viscous liquid with a distinctive smoky odor. Chemically, it is characterized by high oxygen content, high viscosity, strong acidity, a lower heating value compared to conventional fossil fuels, chemical instability, and corrosiveness toward common metal materials.

Pyrolysis is defined as the process of heating organic materials (in this case, biomass) under completely oxygen-free or strictly oxygen-limited conditions, causing thermal decomposition and chemical recombination.

Unlike combustion (which occurs in the presence of abundant oxygen) and gasification (which takes place with limited oxygen), the objective of pyrolysis is to maximize the breakdown of large biomass molecules into smaller molecular products in the form of gases, liquids, and solids.