The individual and synergistic impacts of feedstock characteristics and reaction conditions on the aqueous co-product from hydrothermal liquefaction

We examined the individual and synergistic impacts of reaction conditions and microalgal feedstock characteristics, including previously unreported combinations of variables, on the yield and properties of aqueous co-product (ACP) from hydrothermal liquefaction. Explicitly, we measured the effects of temperature (150 to 350^∘C), reaction time (1 to 100min), slurry concentration (30 and 120 g L_rxn ⁻¹), biochemical composition (5.2 to 28.5 wt.% lipid, 14.7 to 50.9 wt.% protein), and species identity (Nannochloropsis, Chlorella, and Spirulina) on ACP characteristics. Measured properties included gravimetric yield, elemental (CHNSOP) recoveries, NH₄ ⁺-N and PO₄ ³⁻-P recoveries, and pH. The impacts of slurry concentration and species identity on the properties of ACP produced from microalgae are examined in-depth for novel combinations of reaction variables, with all probed variables affecting ACP yield and composition. Temperature exhibited the most influence, followed by time (at 200^∘C) and biochemical composition (at 300^∘C). Lower slurry concentration led to increased ACP yields and recovery of NH₄ ⁺-N, total nitrogen, PO₄ ³⁻-P, total phosphorus, and sulfur; in fact, the data suggested that increased slurry concentration promotes Maillard reactions that inhibit NH₄ ⁺-N recovery in the ACP and promote N recovery in the biocrude. High-lipid, 30 g L_rxn ⁻¹ slurries reacted at 200^∘C for 31.6 min are a potential “win-win” set of conditions for both maximizing key ACP-recyclability metrics while limiting N and S recovery in the biocrude to <5 and <8%, respectively, and increasing the yield of high-quality biocrude. The results herein further illuminate ways in which reaction conditions and feedstock characteristics for HTL could be manipulated to engineer the ACP for desired nutrient content for direct recycling.