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Thermal depolymerization of biogas digestate 293

The approach will facilitate an early stage estimation of the ISBL cost (E ISBL ) for
digestate processing for combined nutrient recovery and additional water treatment
operations.
The E ISBL cost in 2018 in US$ was therefore estimated as follows [60, 71, 72],

380,000 U N Q 0:3 CEPCI 2018
E ISBL ¼ (10.20)
CEPCI 2010
where the constant number of 380,000 is a parameter for production processes of
which the capacity is <60,000tons/year, Q is the capacity (specified as the tons of
major product per year), and U N represents the number of “functional” units or
processing steps specified to be two major steps, namely the nutrient recovery oper-
ation and the water treatment operation. The major product has been specified as the
nutrient-concentrated fertilizer. The values for CEPCI 2018 and CEPCI 2010 were
reported to be 589.65 and 532.9, respectively [61]. For this analysis, it has been
assumed that all solids present in the digestate are capable of being recovered as useful
nutrient-concentrated fertilizer.
The capital investment cost (I) of the plant used in processing digestate in existing
systems was subsequently estimated using Eq. (10.9).

10.3.2.2 Annual operating cost estimate for the digestate
processing for both digestate processing and water
treatment operations
l Labor cost and chemical cost
Due to the increased processing complexity, additional personnel were assumed to be
required in conventional digestate processing plants for digestate nutrient recovery and
water treatment, compared to the HTL-digestate processing system. This implies that the
total labor cost for four workers is US$146,688 per year. Because during conventional
digestate handling processing, 0.01kg of H 2 SO 4 will be required for the acid treatment of
1kg of digestate [7], the chemical cost associated with the cost of acid input per year is deter-
mined as follows,

C acid ¼ 72U c,a _ m dig: (10.21)
where C acid is the cost of acid per year and U c,a is the unit acid cost in US$/kg, specified as
US$0.37 per kg in a commercial website (alibaba.com).
Energy cost estimate
l
To estimate the energy cost (E c ), it is necessary to determine the net energy input duty, as
discussed previously. From Fig. 10.1, it can be observed that two major aggregated opera-
tional stages are responsible for the energy demand during digestate processing. These oper-
ational stages are nutrient recovery and tertiary treatment operations. During the nutrient
recovery operation, energy is expected to be consumed during the solid-liquid phase sepa-
ration. The energy consumed during solid phase separation for nutrient recovery is represen-
_
ted as E is in kJ/h. Assuming tertiary water treatment is required, the additional energy duty
required for tertiary treatment operation is represented as _ E it in kJ/h. The total energy duty
_
input into the process, E input , in kJ/h is therefore estimated as follows,