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292 Advances in Eco-Fuels for a Sustainable Environment

_ m d ΔP
_ E p ¼ (10.18)
ρ dig: η
In Eqs. (10.17), (10.18), c w and c b represent the specific heat capacities of water and dry
digestate (biomass) specified as 4.18kJ/kgC [65] and 1.25kJ/kgC, respectively [65]; _ m d is
the mass flow rate of the digestate in kg/h; w represents the fractional moisture content of the
digestate feedstock specified to be 0.9698; T represents the optimal HTL reaction temper-
ature for enhanced biocrude yield, in °C; ΔP represents the pressure increase imposed by the
pump in kPa; and η represents the pump efficiency, which is assumed to be 0.3 because stud-
ies show that the average pumping efficiency in large manufacturing plants can be <0.4
[67, 68].
The determined heat energy duty is assumed to be satisfied using natural gas at a cost of
US$2.48 per GJ [69] while the unit cost of the electrical energy is specified to be US$ 0.0681
per kWh [70]. The unit energy costs were utilized to estimate the total cost per year (E c )
arising from energy consumption.
Estimation of chemical cost, raw material cost, and overhead cost
l
Because the digestate is freely available and its transformation to useful products is
achieved in the absence of chemical inputs, the cost associated with chemical input has been
specified to be zero. Additional overhead cost (V c )inUS$ per year for extra cost components
not associated with the repair and maintenance of the chemical plant was estimated. This cost
component includes the cost associated with the purchase of internal staff requirements such
as onsite office stationaries and toiletries and was estimated as follows [64],

V c ¼ 0:05 D c + L c + E c Þ (10.19)
ð
where D c , L c , and E c represent the annual depreciation and the annual labor and energy costs
in US$ per year, respectively.

10.3.2 Cost estimation methods for the existing conventional
digestate processing pathway
10.3.2.1 Total capital investment cost estimate for both digestate
processing and water treatment operations
Studies related to this subject matter are burdened with significant costing uncertainties
and limited accuracies with regard to accurate capital cost and operating cost estimates
attributed to the anaerobic digestate processing systems [6, 9]. Also, some previous
studies have focused largely on the cost associated with nutrient recovery from digestate
[6, 9] for fertilizer production, in the absence of the significant cost consideration that
may be required in tertiary treatments. These tertiary treatments may be undertaken
prior to the utilization of the liquid phase as purified water moreso in countries charac-
terized by extensive environmental protection policies, as stated above.
Due to limited information of costing parameters attributed to the combined nutri-
ent recovery operations and water treatment operation, Bridgewater’s chemical engi-
neering capital cost estimation approach has been utilized in estimating the capital
cost required using existing digestate handling systems [71]. This approach is based
on the number of functional units or major processing stages present in the system.