Optimization of microwave-assisted hydrothermal pretreatment and its effect on pyrolytic oil quality obtained by an auger reactor

Microwave-assisted hydrothermal (MHT) treatment of biomass has received significant attention owing to energy efficiency during internal energy transfer and the additional benefits to the hydrochar produced in terms of physicochemical composition. Therefore, this study proposes the combination of MHT pretreatment with the fast pyrolysis process, to evaluate and optimize the effect of this treatment on the quality of the hydrochar and, consequently, on the quality of the bio-oil. The optimization of MHT treatment using black spruce was carried out, followed by fast pyrolysis of the hydrochar produced under optimal conditions in an auger reactor at 550 °C to obtain a high-quality bio-oil. As a result, the pretreated biomass showed on the one hand a significant decrease in the ash content by 58% and 43% in the content of the extractives. While on the other hand, the obtained hydrochar showed an increase in the availability of cellulose by 18.5% as a consequence of the reduction in the content of hemicellulose. Accordingly, hydrochar showed an increase in thermal stability during pyrolysis and it produced a higher total bio-oil yield, increasing by 24%. Most importantly, the oil obtained showed a 35% reduction in moisture content. Chemical composition of the oil was qualitatively examined through GC-MS analysis. It was observed that the bio-oil showed a dramatic increase in the relative content of levoglucosan, by 127%. A bio-oil with the characteristics obtained would be a suitable candidate for use in boilers for heating purposes or chemical extraction.