Yields and ageing of the liquids obtained by slow pyrolysis of sorghum, switchgrass and corn stalks

• Three types of biomass were pyrolysed at slow heating rates.
• Structural changes of the bio-oils due to ageing were followed by three techniques.
• Rapid structural changes were found to occur within about 48 h of preparation.
• Positive effects on oil stabilisation were found at low storage temperatures (5 °C).
• Addition of solvents decreased changes due to ageing but only at high concentration.

A laboratory scale reactor has been set up to pyrolyse different kinds of biomass under slow heating rates and a range of peak temperatures. Yields of solid and liquid products (i.e. char and oil) were measured allowing the effects of temperature and biomass feedstock to be determined. Small differences in char and oil yields were found between the three biomass feedstocks.Pyrolysis oil samples were collected and characterised with size exclusion chromatography (SEC), ultra violet fluorescence (UV-F) and infra red (IR) spectroscopies to study their structural changes as a function of time after collection, i.e. to assess the ageing of the bio-oils. The effects of several variables on the stability of product liquids were examined: e.g. pyrolysis temperature, storage temperature, solvent addition, type of biomass feedstock.Rapid structural changes in the oil samples were found to occur within about 48 h of preparation. Ageing of oils is thought to be caused by polymerisation reactions taking place in the product liquids. The positive effects on the oil stabilisation due to low storage temperatures (5 °C) and the addition of a solvent (methanol or acetone) were confirmed. However, in order to stop the ageing process completely, the concentration of these solvents in the final pyrolysis oil–solvent mixture needed to be greater than 25% (w/w). At lower solvent concentrations bio-oil ageing was slowed down but could not be suppressed altogether.