Environmental Engineering Reference
In-Depth Information
requires removal of the agitation system. In the case of paddles, the
digester has to be emptied.
.
Hydraulic agitation works by creating a strong hydraulic current that
mixes up the material. The substrate is withdrawn from the digester and
returned with pressure through a nozzle. A powerful pump is needed,
but the same pump can be used for feeding from the holding tank. One
advantage of this system is that wearing parts are outside the digester
and are therefore accessible. Hydraulic agitation is an efficient solution,
but there is a risk of clogging of the system by too dense or fibrous
substrates.
.
Pneumatic agitation functions by the injection of biogas under pressure
at the bottom of the digester. Rising gas bubbles induce vertical
movement in the tank, which causes substrate mixing. The gas nozzles
have to be distributed evenly to avoid static zones in the reactor. This
technique works well for liquid substrates, but it is also applied in
vertical plug-flow digesters for solid substrates such as source-separated
municipal waste.
8.3
Gas storage
Fluctuations and peaks in biogas production commonly occur. To dampen
the effect of variable gas production and to allow a controlled flux to the
transformation unit, biogas is gathered and temporarily stored. Due to this
buffer volume, irregular consumption (e.g. by CHP units) can also be
counterbalanced. Storage facilities need to be gas tight and resistant to
pressure, UV irradiation, temperature variations and harsh weather
conditions such as hail. The storage system also needs to be equipped
with a sensor to detect over- or under-pressure. Different recommendations
for storage capacities can be found in the literature; for example
.
Eder and Schulz (2006) indicate a storage capacity for 5-12 hours of gas
production in the case of co-generation
.
Deublein and Steinhauser suggest (2011) a storage capacity for 12-
18 hours of gas production for co-generation
.
Go¨ risch and Helm recommend (2006) an average storage capacity for
4 hours of gas production.
Usually, the storage volume is chosen as a function of space availability and
budget. Many biogas plants function with 2-3 hours of storage capacity, or
even less in the case of biogas upgrading.
One possibility is to store the biogas directly inside the digester. In this
case, a flexible membrane is used as both digester roof and storage volume
(Fig. 8.5). When the gas production rate is higher than consumption, the
membrane extends. To avoid contact between the deflated membrane and
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