Optimization of ultrasound-assisted liquefaction of solid digestate to produce bio-oil: Energy study and characterization

Abstract

Among the possibilities for industrial waste valorization, liquefaction is gaining interest as it may provide alternative energy and high value-added products. In this context, this work focuses on the production of bio-oil through ultrasound (US)-assisted direct liquefaction. For the reaction, polyethylene glycol (PEG) and crude glycerol were selected. As this process is conducted by raising reaction temperature, US provided it, while shortening reaction time, through the cavitation phenomenon. For liquefaction reaction optimization, a response surface methodology (Box-Behnken design) was performed. As independent variables, US-amplitude, reaction time and solvent-to-biomass ratio were selected. On the other side, bio-oil yield, high calorific value (HCV) and energy consumption were chosen as dependent responses. Optimal results showed a bio-oil yield of 34.17% (reached in<20 min), HCV of 28.44 MJ/kg and energy consumption (US) of 11.477 kJ. Moreover, differences between predicted and experimental values were found to be negligible. Bio-oil was also characterized using Fourier transform infrared (FT-IR) and chromatography-mass spectrometry gas (GC–MS). Both techniques showed a profile rich in phenols and poly-oils, which can be used as precursors for industrial products, i.e. polymers. Finally, to check the impact of liquefaction on solid digestate, scanning electron microscopy (SEM) analysis was carried out. Results showed an increase in porosity, fragment and conglomerate. It may be concluded that the use of US as auxiliary energy in solid digestate liquefaction, to produce bio-oil, provides energy saving. Thus, the proposed valorization path aids consolidating the concept of circular economy through an efficient biorefinery model.