Production of Bio-oil from Rice Stalk Supercritical Ethanol Liquefaction Combined with the Torrefaction Process

The rice stalk (RS) was pretreated using torrefaction in a fixed-bed reactor at 200, 240, and 280 °C, respectively. The torrefied rice stalk (TRS) was liquefied in a batch autoclave with supercritical ethanol as the medium at 325 °C and 14–15 MPa for a residence time of 60 min to obtain bio-oil. The TRS was analyzed via Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was observed from SEM results that the compact, uniform, and lamellar structure of RS was broken down, whereas the FTIR and chemical composition analysis results showed that the thermal decomposition of hemicellulose was the main reaction that occurred between 200 and 280 °C. Gas chromatography–mass spectrometry (GC–MS) results showed that the bio-oil obtained from TRS at a temperature of 200 °C had the highest ester content of 30.60% and the lowest acid content of 0.35% and also the alcohols of bio-oil reached the maximum value of 20.56% at a temperature of 240 °C. The water content slightly decreased from 2.23 to 1.31% when the torrefaction temperature was increased from 200 to 280 °C, and the heating value reached a maximum value of 32.53 MJ/kg at a temperature of 200 °C; however, the bio-oil yield gradually decreased from 55.03% of non-torrefied RS to 49.80% of TRS at 200 °C to 38.56% of TRS at 280 °C.