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 degrees 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 degrees C for 60 min. The low temperature pretreatment was most effective at higher temperatures and holding times (225 degrees 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 degrees 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. (C) 2015 Elsevier B.V. All rights reserved.