Effect of process parameters on solvolysis liquefaction of Chlorella pyrenoidosa in ethanol-water system and energy evaluation

In this work, Chlorella pyrenoidosa was converted into bio-oil via solvolysis liquefaction in sub/supercritical ethanol-water system. The influence of reaction temperature (220-300 °C), retention time (0-120 min), solid/liquid ratio (6.3/75-50.0/75 g/mL) and ethanol content (0-100%) on bio-oil yield and property was investigated. The increase of reaction temperature and retention time both improved the bio-oil yield. The bio-oil yield increased firstly and then decreased when the solid/liquid ratio and ethanol content exceeded 18.8/75 g/mL and 80%, respectively. As the reaction temperature <260 °C and retention time <30 min, a soft and unsticky product was insoluble in dichloromethane (DCM) during the extraction process. The chemical composition of the DCM-insoluble product was analyzed by FTIR (Fourier Transform Infrared Spectrometry). The change tendency of O/C and H/C atomic ratio of bio-oil indicated that the addition of ethanol contributed to deoxygenation and hydrogen-donating for bio-oil, due to the dehydration and decarboxylation reaction. 1H NMR (hydrogen-1 nuclear magnetic resonance) analysis indicated that the main chemical compositions of bio-oil were aliphatic functional groups and heteroatomic functionalities (80.00-83.58%). The addition of ethanol enhanced the transesterification to form more ester. The NER (net energy ratio, the ratio of energy output to energy consumption) of solvolysis liquefaction in ethanol-water system (NER < 1) was less than that of hydrothermal liquefaction in sole water system (NER = 1.29), but the NERs of 20% and 40% ethanol content (NER = 0.91, 0.70 for 20% and 40% ethanol content) were larger than pyrolysis technology (NER = 0.66). The high bio-oil yield, better bio-oil property and high NER were achieved at reaction temperature of 300 °C, with retention time of 60 min, solid/liquid ratio of 18.8/75 g/mL and ethanol content of 40% (bio-oil yield = 39.75%, HHV (higher heating value) = 39.31 MJ/kg and NER = 0.70).