Elemental analysis and calorific value were determined for alga residue from biorefinery process and three single processes. As shown in Table 5, carbon content of all alga residue samples increased compared with raw material, especially for algal residue from biorefinery process and hydrolysis only process, significant increase of carbon content were observed. In this way, we may say that hydrothermal carbonization happened. The energy densification (ED) and energy yield (EY) of alga residue were evaluated by the method suggested by Yan et al. (2009) to study the efficiency
of the hydrothermal treatment. It can be seen that residue from fucoidan extraction only and alginate extraction only had
lower ED (1.12 and 1.21), and this might be due to the lower temperature applied for the extraction processes, as hydrothermal processing
temperature strongly effect the carbon content in biochar
(Xu et al., 2013). In comparison, alga residue from biorefinery process
and hydrolysis only process had much higher ED (1.55, 1.67),
which is also higher than the reported value of 1.45 for macroalgae
Sargassum horneri (Xu et al., 2013). As expected, the HHVs of residue
from fucoidan extraction only and alginate extraction only
processes were 15.42 and 16.65 MJ/kg, which was lower than
those from biorefinery process (21.23 MJ/kg) and hydrolysis only
processes (1.67). In terms of the energy yield, as mass yield of alga
residue from biorefinery process was low compared with other
three single processes, the energy yield was also relatively low,
33.23%