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Thermal depolymerization of biogas digestate 301

Table 10.6 Predicted and experimental results for enhanced biocrude yield

Y biocrude , wt% db Y biochar , wt% Y ss , wt%
(predicted. Y biocrude , wt% db (exp. db (exp. Y g , wt% db
yield) db (exp. yield) yield) yield) (exp. yield)
6.69 7.00 44.6 10.3 38.1

of only 4.7%. From Table 10.6, it is clear that the proposed HTL-based one-step
digestate processing approach will facilitate not only the comprehensive treatment
of the digestate, but the proposed pathway will also enable the generation of useful
products of not only biocrude but biochar as well. It is clear that up to 670 and
110tons/year of biochar and biocrude can be generated if 5000tons of high moisture
digestate (96.98% wt water) are made available per year. Also, this implies that
assuming biocrude has a higher heating value of 34MJ/kg [35], as much as 360GJ
of energy can be made available onsite annually via the direct combustion of the
biocrude product with the insoluble solid (biochar) suitable for use as cheap adsor-
bents and soil additives, as discussed briefly in Section 10.1.

10.4.3 Comparative economic assessments of the HTL digestate
processing pathway and the (existing) conventional
digestate processing pathway

The summarized costs for each digestate processing pathway, for a base-case digestate
feed rate of 5000tons per year, are presented in Table 10.7. It is noted that the overall
results presented in Table 10.7 are subject to the base scenario conditions specified in
Section 10.3.
Table 10.7 shows that the annualized investment cost accounts for the bulk of the
total costing requirements in the existing digestate handling process and the proposed
one-step HTL digestate processing technology. Table 10.7 shows that the annualized
investment cost accounts for 46% and 33% of the total costing requirement in the

Table 10.7 Estimated costing components of the proposed one-step digestate processing
pathway and the existing digestate handling process for nutrient recovery and water treatment

One-step Existing digestate processing
Cost components pathway pathway
Maintenance cost (kUS$) 46.39 409.15
Chemical cost (kUS$) 0 1.85
Labor cost (kUS$) 110.0 146.69
Energy (utility) cost (kUS$) 18.67 7.67
Annualized investment 125.82 1109.78
(kUS$)
Depreciation cost (kUS$) 77.31 681.91
Overhead (kUS$) 10.30 41.81
Total cost (kUS$) 388.49 2415.51