Two-stage upgrading of hydrothermal algae biocrude to kerosene-range biofuel†
A new pathway to upgrade directly the biocrude derived from Nannochloropsis oceanica hydrothermal liquefaction (HTL) to kerosene-range biofuel was conducted without using a solvent. The influential factors including the catalyst/biocrude ratio (wt/wt%) and temperature were investigated to improve the biofuel's yield, physical properties and component characteristics. The biofuel was characterized at 44.5 MJ kg−1 higher heating value (HHV) with 1.95% N and 0.72% O by two-stage hydrotreated upgrading. The carbon distribution was analyzed and the transfer of components and elements were discussed for a better understanding of the mechanism of the upgrading process. The results of component analyses suggested that the heteroatoms in phenols, pyrroles and indoles were difficult to remove. In order to obtain a further reduction of oxygen and nitrogen contents, they need a specific upgrading. Higher reaction temperature had a facilitative effect on the generation of aromatic hydrocarbons. The two-stage upgrading also improved the aromatic hydrocarbon contents from 9.64% (stage-1) to 17.38% (stage-2). In addition, the content of kerosene-range biofuel with carbon numbers from 8 to 16 was 65.53% in stage-1 upgrading biofuel and 79.02% in stage-2 upgrading biofuel.