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304 Advances in Eco-Fuels for a Sustainable Environment

technologies were also explored in this chapter. The hydrothermal liquefaction reac-
tion has been achieved using a batch hydrothermal reactor characterized by a heating
rate of 3.5°C/min. Our experimental results demonstrated that it is possible to generate
up to 7wt% (based on dry basis of solids in the wet digestate) of biocrude at a target
temperature, an initial reactor pressure, and a reaction time of 290°C, 5MPa, and
83min, respectively. Yields of other hydrothermal liquefaction product streams under
the aforementioned conditions, namely the insoluble solid biochar, the soluble solid in
the post-HTL water, and the gas phase products, were determined to be 44.6wt%, dry
basis, 10.3wt%, dry basis, and 38.1wt%, dry basis (based on dry basis of solids in the
wet biogas digestate), respectively.
It was also demonstrated that for the base-case scenario of a digestate feedstock rate
of 5000tons per year, the unit digestate processing cost for the proposed HTL-based
one-step processing system and the existing digestate were US$77.7 and US$482.1,
respectively. Statistical investigations incorporating uncertainties further reinforced
the hypothesis that the HTL-based one-step processing pathway may constitute a
cheaper digestate handling pathway. Crucially, the economic favorability of the alter-
native HTL-based one-step digestate processing pathway was established and thus
could provide a compelling basis for the translation of the conceptual ideas presented
in this chapter to practical digestate management in the future.

Acknowledgments

Okoro Oseweuba Valentine gratefully acknowledges the financial support of the University of
Otago via the Otago Doctoral scholarship.

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