(48e) Simultaneous Solid, Phosphorus Recovery and Biocrude Production through Hydrothermal Liquefaction of Algae

Subcritical water has become a highly regarded method for converting biomass, primarily algae, to an energy dense biocrude product. This conversion technique, more commonly referred to as hydrothermal liquefaction (HTL), still faces challenges of nutrient recovery, producing a practical biocrude substitute, and generating value-added coproducts before becoming a sustainable and commercially viable method. Recovering the phosphorus (P) in the solid biochar could be beneficial for secondary applications.

Previous work has shown that the inorganic content of the growth media, in particular Ca, P, and Si, and pH during harvesting dictate the fate of P in the HTL products. This paper expands on the idea of solid P-recovery via in situ synthesis of calcium phosphates present in the biochar, as seen previously in mixed culture, wastewater-cultivated algae. HTL was performed on diatoms, a unique group of algae that harness Si naturally within their cell walls, and mixed cultures of diatoms and Chlorella kessleri. Powder x-ray diffraction (XRD) characterized both the starting algal solids and resulting HTL biochar while acid digestions and inductively coupled plasma optical emission spectrometry (ICP) closed the P balance of HTL reaction.

In addition, it is hypothesized the algal carbohydrates, proteins, and lipids also play a role in the morphology and structure of the biochar. Model compounds were individually reacted via HTL, in the presence and absence of calcium phosphates, and calcium and phosphate ions. The resulting biochar, aqueous coproduct, and biocrude were characterized. Gas chromatography mass spectrometry, CHN/O elemental analysis, and simulated distillation analysis of the biocrude product were analyzed to identify potential catalytic upgrading properties of the calcium phosphate biochar.