Many researchers have been focusing on the bio-oil production
via the HTL process from a wide variety of biomass, including
woody biomass, mainly forest residues [27e30], agricultural waste
such as barley straw, corn stalk, corncob and rice husk [31e34],
animal manure [35e37], municipal solid waste [38e40] and most
recently microalgae [41e48]. Through these studies, it was found
that the yield and quality of the bio-oil resulting from the HTL
process were highly associated with the operating parameters such
as the reaction temperature, retention time, ratio of feedstock and
solvent, etc., as reviewed by Akhtar and Amin [49] as well as the
experimental results reported in the literature [27e48]. The
commonly used temperature range is from 250 C to 400 C; and
usually around 300 C, the maximum bio-oil yield was obtained. At
a low temperature, an incomplete decomposition of biomass gives
a relatively high solid residues yield and a low bio-oil yield. With an
increase in the liquefaction temperature, the yield of bio-oil increases
while the solid residue decreases. However, at the temperature
higher than 300 C, for most feedstocks, a decreased biooil
yield and increased gas fraction yield are observed due to a
secondary decomposition of the bio-oil and Bourdard gas reactions.
However, the solid residue yield may also increase, being caused by
the re-polymerization of the bio-oil components and/or recombination
of high-concentration free radicals to form bio-char.
Retention time is another key factor influencing the bio-oil production
and the overall biomass conversion. Generally, a short
retention time (10e30 min) is preferred, giving a relatively high
bio-oil yield. The ratio of feedstock and solvent also plays an
important role in the biomass liquefaction process. It was observed
that a low ratio of feedstock and solvent led to a high bio-oil yield,
however it was not economically favorable. In contrast, some researchers
also found that a higher biomass concentration might
promote the dehydration/polymerisation of the intermediates
products, which resulted in an increase of the bio-oil yield. Despite
enormous studies conducted and significant progress made, the
underlying mechanism of the effects of operating parameters on
HTL process was so far not fully understood. The optimization of
reaction conditions and knowledge of the interaction of operating
parameters have to be obtained by experiments.