Environmental Engineering Reference
In-Depth Information
7.3.3 Quantifying the energy added value of anaerobic
digestion within a biomass chain
Quantifying benefits of AD plants should include issues such as the
substitution of energy, the avoidance of costs related to fertilizer use,
hygiene, odor reduction and protection of the environment. Energy is an
interesting attribute with which to analyze the role of AD within biomass
chains not only because methane as an energy carrier can be regarded as the
most significant and visible outcome of an AD unit, but also because it
allows the translation of other flows related to the benefits of AD (i.e.
different types of nutrients and water) into equivalent units to produce
single unit outcomes. In addition, and in view of concerns related to the
amount of land being used for bioenergy production and other competing
claims, energy savings can be easily translated to equivalent land units,
allowing one to draw conclusions at a higher level of abstraction. Similarly,
using energy as a parameter allows for comparison with other biofuel
production options. The proposed framework for analyzing the role of AD
within a cascade departs from the definition of a reference system followed
by that of the system with AD embedded. The contributive perspective
introduced previously is hereby used to exemplify the proposed approach,
recognizing that similar approaches are valid for the other two perspectives,
i.e. multifunctional and protagonistic perspectives.
Figure 7.3 shows the situation in a system before AD is introduced. As
can be seen, the major flows are inputs to biomass and industrial processes
and outputs in the form of products and by-products. Equation 7.1
expresses the situation of a system without AD as the difference between
energy outputs, in products and by-products, and energy inputs.
¼
E
bal
GJ
yr
E
agriprod
þ
E
induprod
þ
E
by-prod biomprod
þ
E
by-prod ind prod
E
input biom
þ
E
input ind
E
loss
½
7
1
:
where E
agriprod
refers to the energy content of agricultural products,
E
induprod
refers to industrial products coming from biomass and E
by-prod biom
prod
and E
by-prod ind prod
refer to the energy content of the by-products or
residues coming from the agricultural and the industrial activities,
respectively. Further, E
input biom
and E
input ind
refer to the energy input in
both the agricultural and industrial transformation activities, where both
direct and indirect energy inputs should be considered. Finally, the term
E
loss
accounts for biomass-linked energy losses.
Once AD has been introduced into a system, the flows in the system
change as shown in Fig. 7.4. Products and by-products from the agricultural
and industrial systems can be directed to the AD process, which in turn will
Search WWH ::
Custom Search