Reactivity of solid residue from hydrothermal liquefaction of diatom in oxidizing atmosphere

Abstract

The reactivity of solid residue (char) derived from diatom (Fistulifera sp.) after hydrothermal liquefaction (HTL) reaction at 673 K, 10 MPa, 30 min was investigated during oxidation in air at 553 − 1073 K. Results indicate the derivative thermo-gravimetric (DTG) curve of HTL char showed very small peak below 600 K, indicating HTL reaction effectively decomposed lipid, carbohydrates and protein. The activation energies of the HTL char derived from diatom during combustion up to 1073 K by FWO and KAS methods was 120.0 and 115.3 kJ/mol, respectively, when the char conversion α is 0.2 − 0.5 (i.e. pyrolysis). At higher char conversion (α ≥ 0.5), the activation energy monotonically increased as combustion reaction proceeds. However, this increment of activation energy of HTL char derived from diatom during oxidation stage was much smaller than those of de-oiled microalgae char reported in the literature. The HTL char derived from diatom has porous structure (specific surface area: 293.2 m2/g and total pore volume: 0.672 cm3/g). It was found this porosity is derived both the silicon structure of diatom and remaining carbon after HTL reaction. The total amount of oxygen functional groups (OFG) substantially increased by oxidation at 553 and 573 K. By holding the reaction temperature at 553 K up to 60 min, the total OFG monotonically increased to 1.04 mmol/g. At 573 K, the amount of total OFG peaked 1.08 mmol/g at 30 min and decreased. The decomposition of carboxyl groups is dominant reaction for reduction in total OFG after 30 min at 573 K during oxidation. The tendencies of BET surface area and total pore volume of char agreed with these results of char during oxidation.