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300 Advances in Eco-Fuels for a Sustainable Environment

to be the least statistically significant process variable for temperature variations rang-
ing from 250°C to 350°C. The high calculated F-value of the initial pressure variable
may be explained by recognizing the crucial role of pressure in facilitating the much
needed maintenance of water present in its subcritical state. The high statistical sig-
nificance of holding time reinforces the magnitude of the unfavorable effect of exces-
sively long holding times on biocrude yields. While the importance of temperature to
the HTL processes cannot be disputed, the recognition of the sufficiency of target tem-
peratures as low as 250°C for biocrude production suggests that reaction temperatures
constitute the least significant operating condition in the tested experimental temper-
ature range of 250–350°C. The P-value is a less commonly utilized indicator of sta-
tistical significance, with a P-value <.05 indicative of high statistical significance. It
is clear that the conclusions deduced from P-value, with respect to the statistical sig-
nificance of the operating variables, also agree with conclusions deduced from the
F-value, as discussed earlier.

10.4.2 The operating conditions for a maximum biocrude yield

Using the fitted relations presented in Eq. (10.30) and applying the optimization mod-
ule of the Minitab software, the target temperature, reaction time (inclusive of holding
time), and initial pressure for the maximum biocrude yield have been determined to be
290°C, 83min (inclusive of heating time), and 5MPa, respectively. A review of the
literature suggests that the experimentally determined optimal target temperature
for the maximum biocrude yield from the high moisture digestate in this chapter is
similar to the optimal target temperature conditions reported in most previous studies
in the literature. Previous hydrothermal liquefaction studies demonstrated maximum
biocrude production at temperatures close to 290°C. Goudriaan and Peferoen [87],He
et al. [88], and Jindal and Jha [89] reported optimal target temperatures of 300°C,
295°C, and 280°C respectively. However, variations in the optimum initial reactor
pressure and reaction time were observed in the literature. The variability of the opti-
mum initial pressure and reaction time reported in the literature is clearly a reflection
of the dependence of the pressure and reaction time on the peculiar reactor design and
performance. Reactor design specifications such as the reactor geometry and heating
rate vary significantly in the literature.
Table 10.6 shows the predicted yield and the experimental yield of biocrude at the
estimated optimum conditions. The yields of the insoluble solid biochar, soluble solids
in the post-HTL water, and gaseous product streams generated under the specified
processing conditions have also been experimentally determined.
As shown in Table 10.6, the experimentally determined results for the optimum
biocrude yield are 7.0wt% on a dry basis. Under the specified conditions of the tem-
perature, initial pressure, and holding time necessary for maximum biocrude produc-
tion, the yields of the insoluble solid (biochar), soluble solids present in the post-HTL
water, and the gas phase products were determined to be 44.6wt%, 10.3wt%, and
38.1wt%, respectively. At the conclusion of the experiment, the final reactor pressure
recorded was measured as 17MPa.
Table 10.6 also shows that experimentally determined maximum biocrude yield
was close to the predicted optimum biocrude yield with a relative absolute deviation