Low temperature hydrothermal pretreatment of algae to reduce nitrogen heteroatoms and generate nutrient recycle streams

A two-stage hydrothermal liquefaction process was used to reduce nitrogen heteroatoms in algae biocrude and generate an aqueous co-product stream for algae cultivation. A low temperature liquefaction pretreatment (PT: 125-225°C, 0.5-30 min) hydrolyzed proteins and partitioned nitrogen to the aqueous phase, which was separated and analyzed for nutrient and toxin concentrations. The retained solids (20%) were subjected to hydrothermal liquefaction (HTL) in the second step at 350°C for 60 min. The low temperature pretreatment was most effective at higher temperatures and holding times (225°C, 15 min), removing 45% of the nitrogen present in the algae solids, while retaining 59% of the solid mass, yet reduced overall bio-oil yields by 35 to 71% relative to single stage HTL. Experiments conducted in two-chamber reactors with increased heating rates, resulted in similar nitrogen removal levels at shorter residence times (250°C, 5 min). When coupled with high-temperature HTL, pretreatment improved the elemental content of the biocrude by increasing the total nitrogen removed and reducing nitrogen heteroatom content by 26-28%, compared to HTL alone. The aqueous co-product stream was analyzed for nutrient availability and indicated potential for recycle and algae cultivation due to its high nitrogen and phosphorus levels, moderate pH, and low inhibitory compound concentrations.