Page 187 - Materials Chemistry, Second Edition

Sustainability of (H 2 ? CH 4 ) by Anaerobic Digestion 175

such process to produce energy (Murphy et al. 2011). It is a ratio between two
energy quantities and is therefore dimensionless. In mathematical terms, EROI is:
EROI ¼ TNEP=TIES ð1Þ

TNEP is an acronym for Total Net Energy Produced: it represents the energy
generated minus the direct energy necessary to run the plant itself. Direct energy,
in general terms, is the electrical energy which should be produced in loco or taken
from the grid and fuel (solid, liquid, or gas) to produce heat. According to Murphy
et al. (2011), TIES is the Total Indirect Energy Spent elsewhere in the economy for
the construction of the plant and for its operation. It includes the following
energies: to produce the plant sections (vessel, pumps, valves, etc.), to produce the
consumables, to prepare the site, to assemble the plant, to replace parts or to
upgrade and, finally, to spend the energy for decommissioning. In addition, as
indirect energy, we have to take into account the energy used to support the labor
force in charge to the plant and the amortization energy as reported in Fig. 3.
It is important to point out that EROI should not to be confused with energy
efficiency conversion, which is well depicted by First and Second Laws of classical
thermodynamics, i.e., going from one form of energy to another one, such as
upgrading oil in a refinery or converting diesel oil to electricity. EROI is only
loosely related, at least in the short term, to the concept of return of monetary
investment, but this aspect has not been considered in the present chapter. In the
present case, the ‘‘energy source’’ refers to the organic refuse produced along the
alimentary chain which is considered renewable and present in the world as long as
the Humanity will exist.
A mathematical formula for EPT is:
ð
EPT ¼ TIES= TNEP=t d Þ ð2Þ
TNEP and TIES have the same meaning as that of EROI; t d is the facility’s
operation time.
Straight lines are usually used in the a priori estimation of EPT. For the
evaluation of EPT, we have considered all the indirect energy including the
amortization term, as spent during the construction time. Different assumptions
can be made depending on the technology under study.

2.4 Spatial Boundaries

The selection of appropriate boundary conditions is a fundamental point in a
sustainability analysis. The use of different boundaries in fact means using dif-
ferent inputs or outputs of the system under study and incomparable results could
be generated (Murphy et al. 2011). In the present analysis, we adopted the LCA
methodology to define the boundaries of the system. This methodology includes
all the operations necessary to run a plant, from the introduction of the refuse till to