Page 197 - Materials Chemistry, Second Edition
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Sustainability of (H 2 ? CH 4 ) by Anaerobic Digestion 185
An efficiency factor of electrical energy conversion into mixing energy equal to
0.75 was considered. All the above-mentioned Equations could be implemented in
an Excel sheet to perform the energy balance for each situation.
3.3 Indirect Energy in Anaerobic Digestion Process
When performing a sustainability analysis of a technology, great care should be
taken in the evaluation of the energy and materials flows. The net energy and
useful energy differ from each other because of the contribution of the total
indirect energy (refers to Fig. 3). In mathematical terms, the useful energy can be
evaluated from the difference between the net energy and the indirect energy.
Equation (12) expresses, in mathematical terms, each contribution that should be
considered to evaluate the total indirect energy having taken into account the GER,
i.e., the sum of all the contributions of the energy life cycle (direct, indirect,
capital, and feedstock energy):
ð12Þ
E ind ¼ E chem þ E mat þ E diren þ E constr þ E main þ E decomm þ E amort
where
is the GER of chemicals (MJ)
E chem
is the GER of construction materials (MJ)
E mat
is the GER of direct energy (MJ)
E diren
is the energy for plant building (MJ)
E constr
is the energy for maintenance (MJ)
E main
is the energy for labor (MJ)
E lab
is the energy for decommissioning (MJ)
E decomm
is the energy for amortization (MJ)
E amort
As previously stated, both direct and indirect energy need to be measured in a
physical energy unit; hence, it is necessary to convert all the material flows into
energy units. In the process, materials that were produced elsewhere are usually
used. This leads to a higher consumption of energy, but without it, the process
cannot take place. The GER allows one to convert and evaluate the energy content
in each kilogram of material and is evaluated in energy units per unit mass of
material. E chem and E mat were evaluated by utilizing the SimaPro 7.2.4 software
(2010) and the Ecoinvent database (Ecoinvent 2007) (Table 2).
Table 2 GER values of the Steel 29,630 kJ/kg (De Benedetti et al. 2007)
construction materials and Polystyrene 105,800 kJ/kg (Buwal 250 1996)
chemicals
NaOH 6 kJ/kg (Ecoinvent 2007)
Water 2 kJ/kg (Ecoinvent 2007)