Page 201 - Materials Chemistry, Second Edition

Sustainability of (H 2 ? CH 4 ) by Anaerobic Digestion 189

4.2 The Evaluation of EROI and EPT

In this section, the sustainability of AD is evaluated by EROI and EPT. In this
manner, it is possible to score the H 2 ? CH 4 produced by the AD against other
energy technologies; the AD is equivalent to ‘‘mine mouth’’ for fossil energy
sources.
EROI and EPT are evaluated using the Eqs. (14) and (15). The results are
shown in Figs. 8 and 9, considering two cases: with or without the contribution of
labor in the Total Indirect Energy. The EROI and EPT values versus the reactor
diameter are reported. The sustainability of the technology increases with the
dimensions: without the labor contribution, EROI is higher than 1 for a diame-
ter [1 m; with the labor contribution, the technology is sustainable for higher
diameters than 2 m.
ð14Þ
EROI ¼ E net = E ind
EPT ¼ t d = EROI ð15Þ

In Table 5, the values of EROI and EPT are reported for the two diameters
considered in previous paragraph for the evaluation of useful energy.
In spite of the fact that the energy labor as only food contribution in this
analysis, a great effect of labor on the EROI for low diameters of the bioreactor
was found.
A comparison of the evaluated EROI and EPT of H 2 ? CH 4 values with other
energy technologies ranks the AD technology in a good position among renewable
and fossil energy sources (see Table 6). The sensitivity of EROI and EPT to the
indirect energy of materials has been investigated: the main case previously
described, with polystyrene foam as the insulator, is compared with other cases, in
order to permit a comparison between different insulating materials. Physical and
thermophysical properties are reported in Table 7. In the sensitivity evaluation, net
energy value has been kept constant, i.e., k/s = cost., which means that the
thickness of the insulating walls varies according to thermal conductivity value.

Fig. 8 EROI of an AD EROI
process with and without the 4
labor contribution
3
[#] 2
1
0
0 2 4 6 8 10
Diameter [m]
with labour without labour