Ultrasonic pretreatment effects on the bio-oil yield of a slow wood pyrolysis

Research on biomass conversion into energy through pyrolysis has emerged in the past several years. Pyrolysis is a technique for breaking down molecular chains (polymers of cellulose, hemicellulose and lignin in wood) by heat, thus producing small molecules gases and biochar. It is believed to have a real future for green-fuel production and as the means of the revitalization of the Canadian pulp and paper industry. As pyrolysis has recently been successfully optimized through the use of heterogeneous catalysts, attention has been drawn to ultrasound for its ability to cleave lignocellulosic bonds.

In this study, we investigated the use of ultrasound techniques as a pretreatment for biomass before wood pyrolysis with respect to bio-oil yields. Different conditions in terms of frequency (40, 68 and 170 kHz), time (0.5, 1 and 1.5 h) and power (125, 250, 500 and 1000 W) were explored to include the primary action mechanisms of ultrasound: mechanical and sonochemical effects. The combination of using 40 kHz and 170 kHz frequencies in a sequence of 0.5 h at 170 kHz and 1.5 h at 40 kHz and a power of 1000 W has been demonstrated to be the best, achieving a 12% increase in bio-oil yield compared to untreated wood. However, the best energy efficiency was obtained at low power (125 W). Therefore, depending on the final production goal, the ultrasonic conditions could be adjusted accordingly. Finally, the chemical analysis of the oils by gas chromatography indicated no influence of the pretreatment on the final composition of the recovered oils.

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