Environmental Engineering Reference
In-Depth Information
system, energy cannot be lost, only transferred - must be taken into
account when defining a calculation model.
.
To keep track of the energy flows it can be helpful to separate the model
for the whole biogas plant into sub-models of single units of the plant.
This can be carried out on the basis of the process steps defined in
Section 9.2, but different segmentation can also be used. Most
important is application of the rules for the whole balance to each
sub-model to guarantee a consistent methodology.
.
Last but not least, the method for addressing energy 'losses' at the
different stages in the process must be specified. Firstly, 'losses' in this
context must be defined. One suggestion in this regard is that all energy
flows over the system boundaries that cannot be used for practical
applications can be classified as losses, including conversion losses that
occur as frictional heat. Moreover, each biogas loss (e.g. via security
valves, un-combusted methane in the exhaust of a CHP unit or a flare)
can also be defined as an energy loss from the system. However, it
should not be automatically assumed that all differences in the
calculated energy balance that are unaccounted for actually equate to
true losses.
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Data collection will be one of the challenges for successful balancing.
Most data are not readily available and require careful measurement. It
must be remembered that the energy uptake of technical units and
process steps is dependent on the load and in most cases does not
correlate with the data provided by the equipment supplier. For
example, motor-driven equipment is very rarely operated at full load
and thus the energy demand is typically 50-70% of the energy demand
given in the technical specification. Additionally, most devices are only
operated for an average of a few minutes in every hour.
9.4.2 Practical balancing
Once the system and boundaries have been clearly defined and the necessary
data collected, an energy balance evaluation can be carried out. An example
of the energy balance of an agricultural biogas plant connected with a CHP
unit for electricity and heat production is given in Fig. 9.3.
In Fig. 9.3 it is clear that a very large proportion of the energy chemically
bound in the substrates will not be available for external use. In this case,
about 40% is available as electricity and heat. The chemically bound energy
that is not converted into biogas is retained in the solid and liquid residues,
and offers little potential for methane production. The energy flow diagram
clearly indicates the different pathways of 'losses' of energy from the system.
These energy flows must be the starting points for process optimisation, with
measures undertaken to
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