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Thermal depolymerization of biogas digestate 291

and the Cost i,2006 represents the purchase cost of the ith equipment in the reference
year 2006.
Given that the reference purchase equipment cost for the HTL reactor, pressure
pump and the tank for product separation is estimated for the year 2006, the
CEPCI 2006 for the reference year of 2006 has been specified as 478.6 [60]. The
CEPCI 2018 value of 589.65 has been used in calculating the HTL reactor and tank pur-
chase cost estimate for the present year, 2018, because at the time of work the value
constituted the most recent CEPCI 2018 reported [61].

10.3.1.2 HTL-digestate processing operating cost estimation

The annual operating cost for the HTL processing of the digestate (C AC ) has been esti-
mated by considering the associated labor cost, repair and maintenance costs, energy
cost, depreciation cost, overhead cost, chemical cost, and raw material cost.
Labor cost estimation
l
Due to the simplicity of HTL processing, three workers were assumed to be required,
with the process largely automated. Assuming that a factory worker earns US$15.28/h
[62], and assuming each worker puts in 8h per day, the total labor cost can be estimated
to be US$110,016 per year.
l Estimation of repairs and maintenance costs, depreciation cost, and energy cost
The cost of repairs and maintenance was calculated to be 6% of the total investment cost (I)
[63]. Methods used in estimating the depreciation of equipment utilized in a previous study
have been used. According to Vogel [64], for cost efficiency calculations, it is generally suf-
ficient to assume linear depreciation over a 10-year utilization period. The depreciation cost
(D c )in US$ per year is therefore estimated as the capital investment cost (I) divided by the
10-year plant lifespan period. To estimate the energy cost, the energy demand of the HTL-
reactor has been estimated while assuming that the energy consumed during product recovery
is negligible, as stated in Section 10.1. The energy demand during the HTL processing of the
digestate has been considered as the major energy demand component due to the conditions of
high temperature and high pressure usually imposed. The total energy cost was determined by
estimating the energy cost required to maintain the high HTL temperature condition and the
energy cost required by the pump to pressurize the digestate to a high pressure. The total
energy cost per year (E c ) was therefore estimated follows,
_ _
E c ¼ 7200 u h E h + u p E p (10.16)
_
where E h is the rate of heat energy consumed during the HTL process in kJ/h, to maintain the
reactor temperature at high temperature; _ E p is the rate of electrical energy consumed by the
HTL process in kJ/h, to maintain the reactor at high pressure; 7200 is a constant value to con-
vert energy demand from kJ per h to kJ per year; u h is the unit heat energy cost in US$ per kJ;
and u p is the unit electrical energy cost in US$ per kJ. For simplicity, the digestate mixture has
_
been assumed to be an incompressible fluid to simplify the E p calculations.
_ E h for energy consumed in the HTL process in kJ/h is estimated as follows [65],
_
ð
ð
E h ¼ _ m d wc w T 25ðð½ ÞÞ +1 wÞ c b T 25Þފ (10.17)
ð
while _ E p , is estimated as follows [66],