Page 198 - Materials Chemistry, Second Edition

186 B. Ruggeri et al.

An analogous discussion should be made about direct energy: the scheme
process should be followed for direct energy. The direct energy such as, e.g.,
electricity could be taken off from the grid; each unity of electricity has determined
an energy expenditure that occurs elsewhere in order to be produced it (power
plant construction, grid maintenance etc.). The indirect contribution of direct
energy is calculated using a GER value expressed in an appropriate unit, e.g., kJ/
kWh in the case of electricity. In the AD process here analyzed, the term E diren is
zero because the energy is produced in loco with a cogeneration plant using a
quote of the biogas produced by the AD process itself. In the calculation of Eq.
(12), E constr is the energy consumed for the plant building, while E decomm is the
energy consumed for decommissioning: in the first case, it was considered that a
workman works for 4 months to assemble the plant and 2 months to disassemble it.
The energy consumed for maintenance operation (E main ) is evaluated as 15 % of
the total energy expenditure for construction materials.
It is intrinsically difficult to evaluate the energy consumed for labor E lab, and
generally, it is often disregarded; in this case, the scoring procedure is only valid
for comparisons of technologies in the same category, i.e., capital-intensive or
labor-intensive. In the present evaluation, it is calculated according to the state-
ment of Sect. 2.6 considering the GER of a typical meal in the industrial world a
worker needs to eat twice a day each workday in order to have the power necessary
to work on the AD process, this was considered for 365 days per year.
Finally, E amort is the energy necessary for amortization, and it is evaluated as
the energy necessary to reconstruct the plant equal to E mat .

4 Sustainability Evaluation

In this section, the result of the evaluation of the net and useful energy of the two-
stage anaerobic fermentation process producing H 2 ? CH 4 is reported by evalu-
ating the above energy terms. All the energy terms are expressed as energy unit per
volume evaluated over the operation time.

4.1 Net Energy and Useful Energy Production

An example of the evaluation of the net energy is reported in Table 3. It shows the
following case study: AD of organic refuse taken from a local market, operated at
35 °C as the working temperature in batch mode producing H 2 and CH 4 in a two-
steps digester. The substrate was pretreated with 2N NaOH at pH = 12 for 24 h.
Regarding the volume of bioreactors, we considered the volume of bioreactor
where hydrogen was produced as 1/20 of the volume of CH 4 for all the situations;
results are reported to the diameter of methane bioreactor. Table 3 shows the net
energy in different situations for the AD system under consideration.