Biocrude oil preparation by hydrothermal liquefaction of chlorella cultivated in biogas digestate from chicken manure and its characteristic

Abstract: Improper treatment of biogas slurry results in serious environmental pollution. Cultivating algae using the biogas slurry is a promising strategy. By doing this, we can realize the reuse of nutrients, the further treatment of wastewater and the biomass production. In this study, the produced Chlorella cultivated in the biogas slurry of chicken manure was used as feedstock for biocrude oil production through hydrothermal liquefaction (HTL). An orthogonal design was applied to investigate the effects of operational parameters on biocrude oil production, including the holding temperature (250, 290 and 330 ℃), the retention time (30, 60 and 90 min) and the total solid content (15%, 20% and 25%). The characteristics of products and element migration during HTL were analyzed. The highest biocrude oil yield reached up to 23.83% under a temperature of 330 ℃, a retention time of 60 min and a total solid content of 15%. The low yield of biocrude oil in this study may result from the low content of lipid (1.00%) and high content of ash (55.06%). The reaction conditions significantly affected the biocrude oil yields and chemical distribution of HTL products. The carbon recovery, hydrogen recovery and nitrogen recovery of the biocrude oil were 16.13%-31.14%, 19.18%-34.89% and 5.97%-14.32%, respectively. The highest carbon recovery was achieved under the condition of 330 ℃, 60 min and 15%, and the lowest nitrogen recovery was achieved at the condition of 250 ℃, 30 min and 15%. The increased carbon and hydrogen recovery of biocrude oil were mainly due to the increase of the biocrude oil yield. Carbon (48.74%-60.43%), hydrogen (46.81%-62.13%) and nitrogen (74.84%-82.67%) were effectively recovered in the aqueous phase. The high nitrogen recovery in the aqueous phase was mainly due to the promotion of the denitrification during the HTL process. The high nitrogen distribution in the aqueous phase had a harmful effect to biocrude oil, nitrogen content of which needed to be further decreased. Gas chromatograph-mass spectrometer (GC-MS) was chosen to analyze the organic groups in the biocrude oil. The hydrocarbons content in the biocrue oil was 16.14%-24.91%. The highest hydrocarbon content was obtained under the condition of 330 ℃, 30 min and 25%. However, the high content of oxygenates and nitrogen containing compounds in the biocrude oil decreased the quality of biocrude oil. Hence, the further deoxygenation and denitrogenation of the biocrude oil were maybe required before its application to the transport fuel. A thermogravimetric analyzer (TGA) was used to simulate the distribution of boiling points in the biocrude oil. The results indicated that the biocrude oil contained a lot of high molecular weight compounds. Based on the analysis, the biocrude oil seemed suitable for the production of lubricating oil. The concentration of total organic carbon, the total phosphorous and the ammonia nitrogen in the aqueous phase were 24 360-47 760, 107-270 and 1 218-3 629 mg/L, respectively, and the pH value was 8.53-9.15. The aqueous phase rich in nutrients could be recycled for algae cultivation. In addition, the main gas products CO2 (>93%) could be used as carbon asset for algae cultivation. This study provides a potential approach for the biofuel production from Chlorella cultivated in biogas slurry.