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)
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