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296 Advances in Eco-Fuels for a Sustainable Environment

biocrude yields are not unexpected because biocrude yields are largely dependent on
the presence of lipid molecules in the feedstock. The low lipid content of the digestate
feedstock (1.6% wt dry basis) implies that low biocrude product yields will be
expected. Based on the experimental results presented in Table 10.4, an empirical rela-
tion highlighting the relationship between the biocrude yield (Y biocrude ) and the pro-
cess variables, namely target temperature T, holding time t, and initial pressure p, was
fitted as follows,

ð
Y biocrude ¼ 19:79 + 0:0002t +0:1538T +1:618p 0:00158 p tð Þ 0:003545 p TÞ
2
2
0:000052 T tÞ +0:000098t 0:000235 T 0:0542 p 2
ð
(10.30)
2
The coefficient of determination (R ) for the fitted relation in Eq. (10.30) was deter-
mined to be 0.9693, indicating that only 3.07% of the experimental trend is not well
described by the fitted relation.

10.4.1 Effects and statistical significance of operating conditions
of the HTL process investigated

Using Eq. (10.30), three-dimensional surface plots representing the combined effects
of the system variables on the biocrude yield and two dimensional planar plots rep-
resenting the individual effects of each operating variable on the biocrude yield for
clarity, are shown in Figs. 10.3A–C and 10.4A–C, respectively.
Fig. 10.3A shows the combined effect of the holding time and initial pressure on the
biocrude yield, at a target temperature of 300°C. It is observed for a given initial pres-
sure condition that increasing holding times results in reduced biocrude yields. How-
ever, for a given holding time, the biocrude yield increases with increasing pressure.
This observation agrees with the study of the work of Qu et al. [78], where it was dem-
onstrated that prolonged holding times result in decreasing biocrude yields. This is due
to the decomposition of the liquid biocrude to produce gases. The gasification of the
biocrude oil is due to the volatilization lower molecular weight compounds present in
the biocrude oil mixture [79]. The favorable effect of increasing pressures on the
biocrude yield agrees with the results reported in the work of Isa et al. [80], in which
an increasing pressure led to increasing biocrude yields. This is because of the reduced
gasification of the biocrude product [81] and the decreased solvent polarity as the
reactor pressure is increased [82]. The decrease in the solvent polarity enables the sol-
vent to penetrate efficiently into the biomass components, thus enhancing the much
needed biomass solvation and extraction processes [82, 83].
Fig. 10.3B shows the combined effects of the target temperature and initial reac-
tor pressure on biocrude yield at a constant holding time of 30min. It is observed
that at a given target temperature, the biocrude yield increases with the initial reac-
tor pressure to a maximum value at the highest pressure investigated. However, at a
given initial reactor pressure, the biocrude yield increases with the target temper-
ature to a maximum value and then decreases. This feature of the effect of the