(38f) Bio-Crude Quality and Composition from Algal Solids Cultivated in Varying Media, Growth Stage, and Solids Loading

Hydrothermal liquefaction (HTL) has become a promising technique for converting algal biomass to high-value products because it utilizes algae’s natural, wet environment for the conversion making it both energy and cost-effective. Furthermore, the necessary water and nutrients to cultivate algal biomass can be provided in a sustainable and inexpensive manner with the use of wastewater (WW) effluent. Algal biomass harvested from WW can also contain large amounts of inorganics, primarily calcium, depending on the treatment operation at the plant. During the HTL reaction these inorganics produce an abundant, solid co-product. X-ray Diffraction (XRD) has confirmed these solids to be one of three calcium materials: hydroxyapatite (HAp), Ca₅(PO₄)₃OH, tricalcium phosphate (TCP), Ca3(PO4)₂, or calcium carbonate (CaCO₃). The purpose of this presentation is two-fold. First, to demonstrate the impact media and growth stage have on biocrude composition. Second, to understand the role these calcium solids have on biocrude quality.

Conventional HTL reactions at 350°C for 60 minutes were performed on various WW and raceway-cultivated algal solids. To simulate seasonal and regional differences of WW, algae were grown in two separate raceway ponds: one raceway that had N-limited media and the other P-limited. Algal solids were then harvested at different growth stages thus altering their biomolecular content and ultimately varying the biocrude composition. To regulate inorganic content of the algal solids, the algae were harvested via auto-induced flocculation that occurred from a manual increase in media pH. At each growth stage the total Ca:P molar ratio of the algal solids were manipulated to range between 1 and 3 with the addition of lime (Ca(OH)₂), a common chemical used to balance the alkalinity and pH in WW treatment.

GC-MS results displayed dramatic change in oil composition dependent upon growth media, growth stage, and solids formed. Elemental CHN/O analysis indicated little change in N/C or O/C ratio between growth stages and media. However, the H/C ratio did change with the HTL solids produced from the reaction. Thus, further characterization of HTL solids were measured for their catalytic potential. In addition, fundamental reactions of model inorganic and algal compounds were also performed to better understand the mechanism of solid synthesis and biocrude upgrading.