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

Abstract

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.

Application of Microwave Energy to Biomass: A Comprehensive Review of Microwave-Assisted Technologies, Optimization Parameters, and the Strengths and Weaknesses

“…This is because the heat transfer in microwave-assisted hydrothermal processes occurs from the inside to the outside, via direct coupling of electromagnetic waves with the dipoles present in the reaction mixture. Correspondingly, the core of the sample generally remains at a higher temperature than the surface during the heating process [105]. A smaller particle size can lead to a higher surface area-to-volume ratio, which can improve heat transfer and increase the reaction rate.…”

Hydrothermal Treatment of Biomass Feedstocks for Sustainable Production of Chemicals, Fuels, and Materials: Progress and Perspectives

“…Microwave-assisted hydrothermal treatment is widely used for conversion of biomass into value-added products, namely biochemicals (e.g., furfural, sugars, and carboxylic acids), biofuels, and biomaterials (e.g., hydrochars) − (Table ).…”

Effect of Acid Pretreatment on the Primary Products of Biomass Fast Pyrolysis

“…Most studies on pyrolysis of pretreated biomass showed enhanced production of levoglucosan, while the production of phenolic and furans compounds have been decreased [23,26,29]. However, a few studies have reported otherwise for the phenolic [30,31] or furan (especially furfural) compounds [25,32]. The higher yield of the phenolic compounds from the untreated biomass compared to the pretreated biomass is attributed to the inorganic constituents in the biomass, which may catalyse cracking of lignin and oligomers and thereby enhance the formation of phenolic monomers [1].…”