Page 185 - Materials Chemistry, Second Edition

Sustainability of (H 2 ? CH 4 ) by Anaerobic Digestion 173

part will be used to feed civilization in an appropriate form. In other words, a
technology is sustainable if produces a surplus energy as useful energy. Figure 3
reports a general picture of the energy terms involved in such technology in order
to extract a useful energy from an energy source.
The evaluation of useful energy offers several advantages over the standard
economic analysis (Röegen 1999): primarily, because it assesses the change in the
physical scarcity of energy resources, then because it is a measure of the potential
of such a technology to work in a sustainable way, and finally, because it is
possible to rank alternative energy supply technologies according to their capacity
to produce useful energy. In order to perform a useful energy analysis of such a
technology, it is necessary to evaluate the direct energy and, moreover, the indirect
energy required to produce it. Direct energy is the fuel or the electricity used
directly to run a plant while indirect energy refers to the energy used to produce
materials, to assemble parts of the plant (such as pumps, pipes, valves, etc.), to
produce chemicals and all the other consumables, plus the energy consumed to
produce fuels and electricity (Hammerschlag 2006). Amortization energy is the
energy necessary to rebuild the plant taking into account the recycling or reuse
options. Finally, it is important to take into account the energy used to sustain the
labor to operate the plant. In this context, it is important to state that the energy
terms need to be measured in a physical energetic unit; in some cases, the sum of
direct and indirect energy is named embodied energy (Cleveland and O’Connor
2011).
The sustainability of H 2 ? CH 4 produced by AD is investigated using a scale-
up procedure (Najafpour 2007) along the diameter. The energy produced as H 2 and
CH 4 is referred to their combustion enthalpies. The sustainability of the AD
technology is estimated by evaluating the EROI and EPT parameters.

2.2 Indices to Evaluate Sustainability of (H 2 1 CH 4 )by AD

The energy assessment of a process by LCA approach involves the entire life cycle
of the process, including raw material extraction and processing, manufacturing of
the plant and its assembly, transportation, energy use for the operation of the plant,
such as electrical energy and heat, multiple use of the plant and recycling and/or
final disposal in the so-called decommissioning phase. Attention is here focused on
the technology term in order to evaluate the most suitable technology to convert a
source into an energy service in the most satisfactory sustainable way. This means
that the same energy service could be furnished by using different sources and/or
different technologies. Referring to Fig. 3, all included terms in the indirect energy
need to be considered. The indirect energy will be considered by using LCA
approach evaluating the global energy requirement (GER). The evaluation of the
maintenance, amortization, and decommission energy terms is specific for each
technology; in particular, the amortization term depends on lifetime of the
technology.