Page 333 - Advances in Eco-Fuels for a Sustainable Environment

288 Advances in Eco-Fuels for a Sustainable Environment

To determine the mass of soluble solids in the post-HTL water, the post-HTL water
was dried at 60°C [56] to constant mass and the mass of soluble solids measured using
a weighing balance (Mettler Toledo, New Zealand) equipped with an electronic scale
with a resolution of 10 3 g. The yield of the soluble solids, in wt% on a dry basis (Y ss ),
was then determined as follows,

m ss
Y ss ¼ 100 (10.4)
m dig: dbð Þ

where m ss represents the mass of the soluble solids present in the post-HTL water in g
and m dig.(db) represents the mass of the digestate on a dry basis in g.
The overall yield of the gaseous products (Y g in wt% db) was determined via mass
balance as follows,

(10.5)
Y g ¼ 100 Y biocrude Y biochar Y ss

10.3 Comparative assessment of the economic
performances of the HTL based one-step processing
of digestate and existing digestate processing
technologies

This chapter has so far initially described the existing digestate processing technolo-
gies, with preliminary discussions emphasizing the complexity of these existing sys-
tems with an alternative HTL based one-step digestate processing technology
proposed. The experimental methods employed in the recovery of useful products
from the HTL processing of high moisture digestate feedstock with special emphasis
on the optimized recovery of biocrude as a viable liquid fuel have also been presented.
However, this chapter is yet to present numerical estimates that will form the basis of
testing the hypothesis that the HTL-based one-step processing pathway will present an
improved economic performance compared to the economic performance of existing
digestate handling systems. It is therefore crucial to test this hypothesis. This is
because a favorable economic performance of the HTL digestate processing pathway
will be instrumental in justifying the proposed large-scale commercial exploration of
the HTL-based one-step digestate processing pathway as a viable intensification
technology.
To therefore compare the economic performances of the alternative digestate han-
dling pathways highlighted in this chapter, the unit cost of processing the high mois-
ture digestate (U d )inUS$/ton, albeit largely approximate, will be estimated. The
alternative digestate processing plants have been assumed to operate for 7200h per
year. To comparatively assess the economic performance of the process involving
the HTL conversion of digestate and the existing digestate processing technology,
the total cost incurred per unit mass of digestate was determined for both processes
as follows,