Alternatively, other processing methods such as lipid extraction
and fast pyrolysis have also been exploited to utilize SCG for the
liquid fuel production. Thus, it is worthwhile to compare the HTL
with other pathways. Table 4 presents the properties of the oil
derived from SCG by using HTL, fast pyrolysis and solvent extraction,
respectively. Compared to the fast pyrolysis oil, the crude biooil
obtained from the HTL process has a higher content of carbon
and hydrogen while a lower content of water and oxygen, consequently
an oil product with a higher HHV. It is concluded that the
HTL can provide a larger capability of water and oxygen removal
and a more hydrophobic bio-oil product with a lower acidity than
fast pyrolysis, while the fast pyrolysis is able to obtain a higher yield
of the crude bio-oil (54.9%) than HTL (47.3%). However, achieving
such a high oil yield requires a high temperature (>500 C). More
importantly, HTL is advantageous over the fast pyrolysis in that it
can utilize feedstock with any moisture level without any predrying
processes. Compared to the coffee oil extraction the for
biodiesel production with a yield of 15.0%, HTL produced a higher
yield of oil product. It not only can “extract” lipid content but also
generate oil from other components such as carbohydrate and
protein in SCG. The comparison indicates HTL to be a more suitable
technology to produce liquid fuels from SCG.