Co-liquefaction of microalgae and synthetic polymer mixture in sub- and supercritical ethanol

Co-liquefaction of microalgae (Spirulina) and synthetic polymer (HDPE, high-density polyethylene) in sub- and supercritical ethanol was investigated in a stainless steel autoclave (1000 mL) at different reaction temperatures (T), Spirulina/HDPE ratio (R1), (Spirulina + HDPE)/ethanol ratio (R2) and solvent filling ratio (R3). Results showed that the addition of Spirulina to HDPE liquefaction could make the conversion conditions of HDPE milder. The yield of bio-oil obtained at 613 K with a 1/10 R2 and a 2/10 R3 was increased by 44.81 wt.% when the R1 was raised from 0/10 to 4/6. Meanwhile, the synergetic effects (SE) between HDPE and Spirulina were increased from 0 to 30.39 wt.%. Further increasing R1 resulted in a decrease in SE. The yields of bio-oil increased with increasing R2 firstly and then declined. An opposite trend was observed for the yield of residue. The effect of R3 to the yields of liquefaction products was similar to that of R2. The content of C and H in bio-oils reduced with increasing R1, while the content of O increased. The bio-oil from pure Spirulina liquefaction runs mainly consisted of oxygen-containing compounds, such as carboxylic acids, esters and ketones. But the major components of bio-oil from co-liquefaction of Spirulina and HDPE mixture were similar to those of pure HDPE-derived bio-oil, in which aliphatic hydrocarbons dominated.

► Co-liquefaction characteristics of Spirulina and HDPE were investigated.
► The addition of Spirulina could make the conversion conditions of HDPE milder.
► Significant synergetic effects between Spirulina and HDPE increased bio-oil yield.
► The presence of HDPE also improved the quality of Spirulina-derived bio-oil.
► Adhesion between Spirulina and HDPE could impede the heat and mass transfer.