Energy flows in biogas plants: analysis and implications for plant design - The Biogas: Science, Production and Applications - page 214

9.2.7 Post-processing, storage and transportation of
residues

Last but by no means least, further processing of liquid and solid residues
can give rise to an energy demand. Liquid residues usually leave the
fermenter by gravity into a storage tank. For the purposes of storage
pumping, transportation (by pipes or by tanker lorry) and mixing
(homogenisation before pumping out of a storage tank) can demand
electricity or transport fuel depending on the local situation. Compared with
the total energy demand of the whole biogas plant, this energy demand is
usually very low. It only becomes significant when further processing of the
liquid or solid residues is employed for solid/liquid separation or for pellet
production and water purification. For solid/liquid separation of residues
from fermentation, Arndt and Wagner (2009) report an electricity demand
of about 1.2 kWh el per kg of wet residue. Further drying before pelletisation
requires about 1.5 kWh th plus 0.05 kWh el per kg of residue, for a residue
with 20-30% dry solids (Arndt and Wagner, 2009). Subsequent pelletisation
can require between 0.25 and 0.35 kWh el per kg of dried residues (Arndt and
Wagner, 2009). There is a further electricity and heat demand for processing
of the liquid phase after separation, but the typical energy demand values
cannot be established due to the diversity of processes that are employed;
these technologies are, in any case, not yet widely used.

9.2.8 Process control equipment and infrastructure

The amount of energy required for the operation of process control
equipment and infrastructure depends on the local situation. Process control
typically has a low electricity demand in most biogas plants. The energy
demand of the infrastructure is also typically low, but can be significant in
terms of heat as well as electricity if offices and visitor facilities are
frequently used, or if 24-hour operation of the plant requires large lit areas.
The local conditions in this respect must therefore be taken into account in
the evaluation of the energy demand involved in process control and
infrastructure.

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9.3 Energy supply for biogas plants

The provision of energy supply for a biogas plant is an important issue that
affects both the economics of the project and the ecological footprint of the
plant. It is therefore advisable to gather as much information as possible on
the energy demands of the plant before planning the energy supply. The
minimum information necessary in this respect is