Experimental Study of Subcritical Water Liquefaction of Biomass: Effects of Catalyst and biomass Species

In this work, hydrothermal liquefaction (HTL) of wood industry residues (wood, bark, sawdust) and macroalgae for producing biofuels has been investigated under subcritical water conditions (at temperature of 300 C), with and without the presence of catalyst. The effects of catalyst and biomass type (woody and non-woody) on the biomass conversion, bio-crude yield, and the qualities of products were studied. The results suggested that the addition of potassium carbonate as catalyst showed a positive effect on bio-crude yield, especially for wood, where it was enhanced to 47.48 wt%. Macroalgae showed a higher biomass conversion and a lower bio-crude yield than other woody biomass investigated in the present study, irrespective of whether the catalyst was used. Meanwhile, the effect of catalyst on macroalgae was less significant than that of woody biomass. The heating values and thermal stability of all bio-crudes were analyzed. The results showed that the higher heating values (HHVs) were in the range of 24.15 to 31.79 MJ/kg, and they were enhanced in the presence of catalyst, except for that of the macroalgae. The solid residues were characterized by heating value, SEM and FTIR. It was found that the addition of K2CO3 lowered the solids quality in terms of the heating values, while it did not have apparent effect on the functional groups of solid residues. SEM analysis of the raw biomass and solid residues revealed that the char formation for wood, sawdust and macroalgae had initially finished when they were treated in hot compressed water at 300 C, while conversion of bark had not completed yet.