Page 188 - Materials Chemistry, Second Edition

176 B. Ruggeri et al.

CH 4 +CO 2
H 2 +CO 2

CH
4
CO
2 nd Gas 2
ORGANIC 1 st reactor up-grade
WASTE Pre-treatments reactor UNIT
UNIT H 2
CO 2

DIGESTION UNIT
Digestate
treatment
Fig. 4 Schematic view of the process involved in AD technology. The AD stages taken into
consideration for the EROI Analysis are shown inside the block: pretreatment unit and digestion
unit (two stages in series for the production of biohydrogen and biomethane, respectively)

the production of the biogas, as reported in the dashed area of Fig. 4. We have
excluded the organic waste stock tank from the analysis, because it is independent
of the technology used to valorize the refuse. The gas up-grading unit has also
been excluded because it depends on the use of the gas and could be different
(Ryckebosch et al. 2011); the biogas could be used to produce heat, electricity or
as fuel for transportation purposes.
Referring to the layout of energy flow reported in Fig. 1, the present analysis
concerns the sustainability evaluation of the AD technology as producer of an
intermediary energy carrier such as (H 2 ? CH 4 ) biogas as one of several candidate
technologies as shown in Fig. 2. In EROI terms, the AD technology is equivalent
to ‘‘mine mouth’’ for fossil sources. Bottom-up approach was used for the energy
estimation of the technology along with a detailed process analysis to evaluate the
input–output quantities. We decided to go ahead in this manner, which is different
from an up-down approach (Carniege-Mellon 2009), because the scope was to
analyze and compare different technologies in order to select the most energy
sustainable one, and these technologies in many cases are still at an ‘‘infancy’’ or
developing stage, and it is therefore difficult to find overall data on the technology.

2.5 Time Boundaries

Regarding the time boundary of the technology, we selected 15 years, with 300
working days per year as operational time of the plant. During this period, the
generated energy as H 2 ? CH 4 and all the spent energy were computed. We
considered that 4 months were necessary for the site preparation and plant