Production of bio-oil by direct liquefaction of Spartina alterniflora in ethanol-water co-solvent

Abstract: Biomass is considered to the most promising feedstock with low carbon intensity for producing renewable and clean alternative fuels due to their abundant reserves, wide distribution and carbon neutral. In this study, direct liquefaction of Spartina alterniflora was performed in a 50 mL bath reactor with ethanol and water as a co-solvent. Experiments for optimal operating conditions such as reaction temperature, ethanol volume fraction in ethanol-water co-solvent and solvent to biomass ratio were conducted to understand its influence on liquefaction product distribution. Raw material and bio-oil were analyzed by thermogravimetry analysis and gas chromatography-mass spectrometry (GC-MS). Results showed that thermogravimetric (TG) and differential thermogravimetric (DTG) curves of Spartina alterniflora were almost the same with the increase of heating rate from 10 to 40 ℃/min. However, the temperature where the maximum rate of weight loss occurred, tended to move to higher value. This suggested that there was a hysteresis phenomenon on thermogravimetric characteristics of Spartina alterniflora with respect to the heating rate, which was caused by the combination of time and temperature. The bio-oil yield tended to increase but the solid residue yield tended to decrease with increasing temperature up to 340 ℃, while further increasing temperature increased the residue yield but decreased the bio-oil yield. The similar tendency for bio-oil yield and solid residue yield was observed when increasing ethanol volume fraction in ethanol-water co-solvent between 0 and 100%. As the solvent to biomass ratio increased from 6 to 10 mL/g, the bio-oil yield increased obviously and after that the trend went opposite. Meanwhile, the solid residue yield reduced gradually with the solvent to biomass ratio in the range of 6-10 mL/g and leveled after higher than about 15 mL/g. Optimal operating conditions were reaction temperature of 340℃, ethanol volume fraction in co-solvent of 50% and solvent to biomass ratio of 10 mL/g, resulting the highest bio-oil yield of 44.2% and minimum solid residue yield of 12%, respectively. Compared with single solvent like pure water or ethanol, ethanol-water co-solvent showed an obvious synergistic effect during direct liquefaction of Spartina alterniflora. Not only the yield of bio-oil was enhanced but also the quality of bio-oil was improved with ethanol-water co-solvent as the reaction medium. The result of GC-MS analysis indicated that the bio-oil obtained from direct liquefaction of Spartina alterniflora at optimal operating conditions was a complex and oxygen-containing organic mixture, mainly including acids, phenols, esters and ketones. The major components of bio-oil were phenols (29.62%) and esters (11.27%). The presence of ethanol could promote the formation of esters by reacting with acids through esterification while the majority of phenols were derived from the decomposition of lignin. Ester compounds were similar to biodiesel in composition. Thus, it was desirable to increase the amount of esters in bio-oil for a better bio-oil quality. The energy recover rate of bio-oil obtained from direct liquefaction in ethanol-water co-solvent with 50% ethanol volume fraction reached up to 76.5%, which was significantly higher than that in single solvent. Therefore, ethanol-water co-solvent was a potential medium with higher bio-oil energy recover during direct liquefaction of biomass.