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Thermal depolymerization of biogas digestate 283

Table 10.2 Characteristics of biogas digestate used in the present study [35]

Biogas digestate Measured mean value
Lipid content (wt%, dry digestate basis) 1.60
Protein content (wt%, dry digestate basis) 17.50
Ash content (wt%, dry digestate basis) 39.53
Carbohydrate content (wt%, dry digestate basis) 41.40
Carbon content (wt%, dry digestate basis) 28.64
Nitrogen content (wt%, dry digestate basis) 2.80
Sulfur content (wt%, dry digestate basis) Trace
Hydrogen content (wt%, dry digestate basis) 4.45
Oxygen content (wt%, dry digestate basis) 24.60
Moisture content (wt%, wet digestate basis) 96.98
pH of wet digestate 8.23
Bulk specific gravity at 20°C 1.02
Higher heating value (MJ/kg) 11.82

10.2.2 Design of experiment, experimental procedure,
and statistical analysis
In this study, the effects of the selected operating variables on the biocrude yield and
the determination of the ranges of the selected operating variables that enable the opti-
mization of biocrude yields have been determined using the Box-Behnken experimen-
tal design method. The Box-Behnken experimental design method, which constitutes
a subset of the classic response surface methodologies (RSMs) [40], was selected
based on its efficiency in investigating the individual and combined effects of system
variables [41]. RSMs constitute a group of statistical techniques that enables exper-
imental design for statistical model development, operating variable assessment,
and the estimation of the values of operating variables that will maximize the desirable
responses [42, 43]. RSMs enable a reduction in the number of required experimental
runs while simultaneously ensuring a systematic and efficient variation of selected
levels of the operating variables under investigation [43]. The analysis of the variation
(ANOVA) of the experimental date will be undertaken to provide a statistical basis for
assessing the significance of the operation variables investigated [44]. Utilizing the
Box-Behnken experimental design method, the present study will investigate the
effects of the initial pressure p, the reaction temperature T, and the reaction time t
on the biocrude yield in a hydrothermal liquefaction process of a high moisture
digestate sample. The Box-Behnken design method also ensures that a thorough
understanding of the dependence of the response parameter (biocrude yield) on the
system variables is achieved and thus also helps to determine the values of the oper-
ating variables that will ensure optimized generation of the required response param-
eters by undertaking fewer experimental runs.
To determine the appropriate levels of the conditions of the process variables,
levels presented in previous studies were utilized as sufficient guides. In assessing
the effect of temperature on biocrude yield, a target temperature range of