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This paper presents an investigation of the production of crude bio-oil, char, and pyrolytic gases from the fast pyrolysis of mallee woody biomass in Australia. The feedstock was ground, sieved to several narrow particle size ranges, and dried prior to pyrolysis in a novel laboratory-scale fluidized-bed reactor. The effects of pyrolysis temperature (350−600 °C), and biomass particle size (100−600 μm), on the yields and composition of bio-oil, gas, and char are reported. In agreement with previous reports, the pyrolysis temperature has an important impact on the yield and composition of bio-oil, char, and gases. Biomass particle size has a significant effect on the water content of bio-oil. It is interesting to note that the temperature for maximum bio-oil yield, between 450 and 475 °C, resulted in an oil with the highest content of oligomers and, consequently, with the highest viscosity. Such observations suggest that the conventional viewpoint of pyrolyzing biomass at temperatures over 400 °C to maximize bio-oil yield needs to be carefully reevaluated, considering the final use of the produced bio-oil. The increases in oil yield with increasing temperature from 350 to 500 °C were mainly due to the increases in the production of lignin-derived oligomers insoluble in water but soluble in CH2Cl2. The yield and some fuel properties of the pyrolysis products were compared with those herein obtained for pine as well as those reported in the literature for other lignocellulosic feedstocks but using similar reactors.

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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 °C. Several analytical techniques were used to quantify bio-oil composition. For the volatile components, the results obtained by gas chromatographymass 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 biooil 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 °C. In fact, most of the increases in the yield of bio-oil with increasing temperature above 350 °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 °C.

Quantification of functional groups (carbonyl, carboxyl, hydroxyl, phenolics) in biomass-derived pyrolysis oils is crucial to advance our understanding of bio-oil compositional changes during production, storage, aging, and upgrading. Traditionally, most of the methods reported in the literature on this subject are based on titration. There are very few studies on the use of spectroscopic techniques for the quantification of functional groups in bio-oils. The distribution of functional groups between the volatile and the heavy fraction is also very poorly understood. The content of functional groups in the volatile fraction estimated by GC/MS was compared with their content in the total oil determined by titration and 31P NMR. The carbonyl groups are almost equally distributed between the volatile and the oligomeric fractions. The content of total phenols varies between 1.6 and 3.1 mmol/g. It is important to note that between 85 and 95% of the phenols in bio-oil are in the form of oligomers. The content of carboxylic acids varies between 1.1 and 2.1 mmol/g. Between 52 and 66% of these acids were detectable by GC/MS, and the rest is in the oligomeric form. These results confirm that the GC/MS-detectable fractionalthough it only represents around 30 wt % of the whole oilcontains more than half of the very reactive carbonyl and carboxyl functional groups of the oil. Our results suggest that as an average 56% of all the oxygen derived from the carbohydrate fraction that is collected in the oil is in the form of water. Around 20% is in the form of carbonyl groups, close to 12% is in the form of carboxylic groups, and only 17% is in the form of OH in aliphatic chains. This result clearly shows the importance of dehydration reactions (close to 70% of the oxygen in the oil is in the form carbonyl or water). The oil was studied by FT-ICR-MS. The heavy fraction is composed of oligomeric materials with up to 29 carbon atoms and 17 oxygen atoms. The Van Krevelen plots of the nonvolatile fraction show for the first time the existence of heavy unknown water-soluble oligomers produced by the gradual dehydration of cellulose primary depolymerization products. This unknown fraction is herein called “pyrolytic humin”. The oils were also analyzed by 1H NMR, FTIR, and UV fluorescence spectroscopies. 1H NMR results confirm that, with appropriate calibrations, this technique could be used to quantify the content of phenols and water. The correlations observed between FTIR spectra and titration results confirm that, with appropriate calibrations, this technique can be used for the quantification of water, carboxylic acids, and phenolics in bio-oils. A good correlation was obtained between the total content of phenols measured by Folin–Ciocalteu and the area of the UV fluorescence peaks.

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