Effects of temperature on the formation of lignin-derived oligomers during the fast pyrolysis of Mallee woody biomass

This paper reports the evolution of the composition of bio-oil obtained from the fast pyrolysis of Mallee woody biomass as a function of temperature between 350 and 580 degrees C. Several analytical techniques were used to quantify bio-oil composition. For the volatile components, the results obtained by gas chromatography-mass spectrometry and Karl Fischer titration agree very well with those obtained by thermogravimetric analyses. However, for the heavy components, especially lignin-derived oligomers, synchronous UV-fluorescence spectroscopy and thermogravimetric analysis give more reliable results than the precipitation in cold water. Our results indicate that the accuracy of the precipitation methods to quantify lignin-derived oligomer in bio-oil is limited by the relatively large amounts of small oligomers remaining soluble in a metastable form in cold water. A maximum in the yield of lignin-derived oligomers was observed between 450 and 500 degrees C. In fact, most of the increases in the yield of bio-oil with increasing temperature above 350 degrees C can be explained by the formation of this type of oligomers. Increases in the rates at which the oligomers are formed and increases in their volatility at elevated temperature are the main reasons for the increased presence of oligomers in bio-oil produced at temperatures higher than 350 degrees C.