What Is Bio-Oil?
Biomass, when subjected to high temperature in the absence of oxygen (i.e., pyrolysis), converts into gas, solid char, and liquid products. The liquid product, called bio-oil or pyrolysis oil, is typically brown, dark red, or black in color with a density of about 1.2 kg/liter. Bio-oil has a water content of typically 14–33 wt%, which cannot be easily removed by conventional methods (e.g., distillation); in fact, bio-oil may phase separate above certain water content. The higher heating value of bio-oil is typically 15–22 MJ/kg, which is lower than that for conventional fuel oil (43–46 MJ/kg), mainly due to the presence of oxygenated compounds in bio-oil (Mohan, Pittman, and Steele, 2006).
Pyrolysis is the thermal decomposition of biomass, which occurs in the absence of oxygen or when significantly less oxygen is supplied than needed for complete combustion. Pyrolysis can convert biomass into more useful fuels: a hydrocarbon-rich gas mixture, an oil-like liquid, and a carbon-rich solid residue.
Pyrolysis, in the form of wood distillation, has been used since historical times. For example, ancient Egyptians practiced wood distillation by collecting tars and pyroligneous acid for use in their embalming industry and for caulking boats. In the 1800s, wood pyrolysis to produce charcoal was a major industry for supplying fuel for the industrial revolution, until it was replaced by coal.