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Thermal depolymerization of biogas digestate 295

10.3.3 Uncertainty assessment of the costing estimation approach

Reviewing the cost estimation methods utilized in this chapter, it is clear that the esti-
mation methods utilized in this chapter incorporate the scale-of-magnitude estimate
approach (estimations based on capacity of the plant) [60, 71, 72] and the factored
estimated approach (estimations based on Lang factors) [58]. We must accept also that
difficulties in estimating the total cost of each alternative plant containing equipment
required for HTL digestate processing, nutrient recovery, and water treatment consti-
tute an important source of uncertainty. This uncertainty may lead to an over-
estimation or underestimation of the unit digestate processing costs of the
alternative pathways [75].
It is therefore necessary to assess the uncertainties in the unit digestate processing
costs in the alternative pathways due to variations in the unit cost determinants,
namely the annualized capital cost, the annual operating cost, and the digestate feed-
stock rate. Uncertainties in the unit cost estimates for the alternative pathways have
been investigated for 50% variation in the unit cost determinants. To determine the
uncertainty in the unit processing cost, the Monte Carlo simulation method has been
utilized in predicting the probability density of the unit digestate cost, according to the
probability density function defined as follows [76],

1 1 x μ 2

fxðÞ ¼ ffiffiffiffiffip e 2 σ (10.28)
2π σ
where x represents the value of the unit digestate processing cost for specific values of
the annualized capital cost, the annual operating cost, and the digestate feedstock rate;
μ represents the mean value of the unit digestate processing cost; and σ represents the
standard deviation of the unit digestate processing cost.
The likelihood (P) of the unit digestate processing cost to exist within a particular
unit cost interval (a, b) is determined by the area under the curve generated by the
probability density function as follows [76, 77],

ð b
ð
Pa < x < bÞ ¼ fxðÞ dx (10.29)
a
The detailed expressions of the probability density function can be found in Ref. [77].
A thousand different combinations of the annualized capital cost, the annual oper-
ating cost, and the digestate feedstock rate were initially generated using the Minitab
17.1.0 to reduce computational time and the associated unit digestate processing costs
calculated and analyzed.

10.4 Results and discussions

The results of the experimental biocrude yields from the HTL of high moisture
digestate are presented in Table 10.4. Table 10.4 shows that poor values of the yields
of biocrude ranging from 3.7 to 6.8wt% were obtained in the experiments. These poor