Conversion of high-ash microalgae through hydrothermal liquefaction
Natural microalgae (NM, Scenedesmus) cultivated by utilization of exhaust gas from municipal solid waste combustion power plant were used for the biofuel production thought hydrothermal liquefaction (HTL). The high-ash NM underwent acid-washing to obtain the deashing microalgae (DA). The HTL experiments were carried out at different temperature from 260℃ to 340℃ with NM and DA. Products derived from NM and DA were examined by various techniques in order to identify the influence of the ash on the hydrothermal decomposition behavior. The results show that the bio-oil yield including heavy oil and light oil is in the range of 17.59 - 22.09% for NM and 24.30 - 31.14% for DA, respectively. The relative content of N-heterocyclic ring components in DA-derived light oil is much higher than that of NM-derived light oil, conversely, the relative content of ketones is lower in DA-derived light oil. Large quantities of long chain fatty acids were detected in DA-derived heavy oil, while no component was detected in the NM-derived heavy oil in the experiment. The concentration of NH4+ in the aqueous phase derived from DA was in the range of 2724 mg/L to 3680 mg/L, which should be returned to microalgae cultivation phase as nutrient. The DA-derived hydrochars are in the region of sub-bituminous with the high heating value of 17.66 - 20.18MJ/kg. For natural microalgae, the deashing pretreatment before HTL is of great significance for the improvement in the biocrude yield and quality, as well as the biomass conversion efficiency, nitrogen utilization and the hydrochar quality.