Page 199 - Materials Chemistry, Second Edition

Sustainability of (H 2 ? CH 4 ) by Anaerobic Digestion 187

Table 3 Net energy D = 4m D = 10 m
evaluation in the case of 4
and 10 m diameter, E H2+CH4 (kJ/L) 1,123 1,123
respectively E dir (kJ/L) 171 161
E net (kJ/L) 952 962

The useful energy production E u can be calculated as follows:
ð13Þ
E u ¼ E net E ind
where E ind is the total indirect energy consumed.
An example of the evaluation of the useful energy for the same situation is
reported in Table 4.
From Table 4, one can see that the increase in the useful energy E u along the
diameter from D = 4mto D = 10 m is only of 7 %.
In order to show the linkage between all the energy terms as contributions to the
sustainability of AD, an analogical model of the process is presented in Fig. 7.
This figure highlights the linkage between: (1) the energy production due to the
knowledge of the technology; (2) the direct energy consumption necessary to run
the technology; (3) the indirect energy; (4) the useful energy, i.e., the energy that
the technology gives to society in a sustainable way.
It is interesting to conduct a detailed examination of the percent values: the
theoretical available energy evaluated as the LHV of organic waste is 100 %. The
percent value drops to 48 % as produced energy. This depends on the present
knowhow on the fundamentals of AD technology or, in other words, the current
knowledge on biochemistry and microbiology does not permit better results to be
obtained. The percent value further decreases to 41 % as net energy, considering
that the present technology of heat exchanger and electricity production technol-
ogy have lead to an optimization of the system, the 7 % is consumed as direct
energy. In the classical energy analysis approach, 41 % of energy is delivered to
society and no other aspects need to be considered.

Table 4 Useful energy
D = 4m D = 10 m
evaluation in the case of 4
and 10 m diameter, E net (kJ/L) 952 962
respectively E chem (kJ/L) 120.5 120.5
E mat (kJ/L) 90 36
E diren (kJ/L) 0 0
E main (kJ/L) 18 7
E amort (kJ/L) 210.5 156.5
E lab (kJ/L) 29 1.8
E constr ? E decomm (kJ/L) 0.3 0.02
E ind (kJ/L) 468 322
E u (kJ/L) 484 640