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Simultaneous solid and biocrude product transformations from the hydrothermal treatment of high pH-induced flocculated algae at varying Ca concentrations

Hable, Robert D., Alimoradi, Sirwan, Sturm, Belinda S.M., Stagg-Williams, Susan M.
Algal research 2019 v.40 pp. 101501
algae, biofuels, biomass, calcium, calcium carbonate, carboxylic acids, crystal structure, feedstocks, harvesting, hot water treatment, hydrothermal liquefaction, lipids, pH, phosphorus, tanks, tricalcium phosphate, wastewater
Because of the substantially high growth rate, algae have become a prominent biomass feedstock to produce biofuels and renewable chemicals leading to research on cost-competitive means to cultivate and convert algal biomass. Wastewater effluent streams provide the necessary water and nutrient inputs for algal growth, however, the low lipid, high ash composition of algal solids produced are not ideal for conventional conversion techniques. Hydrothermal liquefaction (HTL) is a holistic algal conversion method that utilizes subcritical water to produce biocrude and high-valued solid product from wastewater-cultivated algal solids. The objective of this study was to understand how calcium, the primary inorganic element of wastewater-cultivated algal solids, affects the final HTL products. The calcium concentration was varied in six algal growth tanks and the algae was harvested via high pH induced-flocculation. The algal biomass was then processed using HTL at 350 °C for 1 h. Calcium carbonate was the primary crystalline structure observed in the algal and HTL solids. One HTL solid sample showed a significant production of tricalcium phosphate. The higher calcium concentrations resulted in near 100% capture of phosphorus in the HTL solids. Increasing the calcium content also had a positive impact on the biocrude including the H/C ratio, amide, alkene, and carboxylic acid content. The results of this study suggest that the inorganic solids collected from high pH, flocculated harvesting may have an in-situ catalytic impact on the biocrude.