Enhancement of nutrient recovery from microalgae in hydrothermal liquefaction using activated carbon

For large-scale biofuel production from microalgae, the following outcomes are needed to reduce environmental impact and biofuel production cost 1) extraction of a large amount of lipid (bio-oil), 2) recovery of almost all nutrients and their reuse for cultivation, and 3) effective utilization of solid residue (char). We investigated the possibility of using activated carbon to enhance recovery of bio-oil and nutrients in hydrothermal liquefaction (HTL). Microalga Fistulifera solaris was reacted at 573 K and 10 MPa for 0-60 min with or without commercial activated carbon (0.5-5.0 wt.% of microalgae) in an autoclave reactor. Results show 0.5 wt.% activated carbon can substantially promote total nitrogen recovery in the aqueous phase, probably because of catalytic effect of activated carbon on the decarboxylation reactions to produce amines. Larger amount of activated carbon (> 1.0 wt.%) decreasd bio-oil recovery due to adsorption. A kinetic study of nitrogen recovery in the aqueous phase showed the apparent reaction order was 3 and the reaction rates were 1.1 × 10− 7 s− 1 and 5.6 × 10− 8 s− 1 with and without activated carbon, respectively. 0.5 wt.% of activated carbon has an insignificant influence on bio-oil and phosphorus recovery.