Chemical and physical characterisation of biomass-based pyrolysis oils: Literature review

Biomass-based pyrolysis oils are complex mixtures of mainly organic compounds and water. The determination of their physical and chemical properties and chemical composition is a challenge for researchers. Characterisation of biomass pyrolysis oils has been studied at many universities in North America and Europe in the 1980s and 1990s. The existing literature on the analytical methods used for these oils is reviewed in this report. The physico-chemical properties, such as water content, acidity, density, viscosity, heating value and stability, are important in terms of utilisation, storage and handling of oils. In the analyses, standard methods as such or as modified and, in addition, self-developed methods have been used. Standard fuel oil analyses are not often suitable as such for biomass-based pyrolysis oils. For characterising the chemical composition, the bio-oils have first been mainly fractionated into different classes. Solvent extraction and adsorption chromatography are the most general methods used. In solvent extraction, the oils have often been divided into acidic, phenolic, basic, hydrocarbon and aqueous fractions or water-soluble and -insoluble fractions. In adsorption chromatography, the oils have been fractionated into different hydrocarbon and polar fractions. The fractions obtained have been analysed with various chromatographic and spectroscopic methods. Gas chromatography/mass spectrometry (GC/MS) technique is the analytical method most widely used and well adaptable for the fractions. For high-molecular-mass and highly polar compounds liquid chromatographic (LC) techniques as well as infrared (FT-IR) and nuclear magnetic resonance (1H NMR and 13C NMR) spectroscopies are more suitable due to the low volatility of pyrolysis oils. For whole pyrolysis oils, LC techniques, primarily size exclusion chromatography and FT-IR and FT-NMR spectroscopies have proved to be useful methods, giving information on molecular weight, functional groups and aliphatic and aromatic structures and ratios. Direct mass spectrometric techniques (MS), such as molecular-beam MS and MS/MS, are rapid and interesting tools for the characterisation of the oils and for the investigation of the pyrolysis process. In-depth characterisation of the complicated organic composition of pyrolysis oils requires the use of various techniques. The oils contain organic compounds such as acids, aldehydes, anhydrosugars, alcohols, phenolic compounds, esters and hydrocarbons, and, in addition, high-molecular, apparently lignin-derived substances, depending on feedstock, process conditions and recovery techniques.