Environmental Engineering Reference
In-Depth Information
8.5
Internal gas storage.
the digesting material, a structure is installed horizontally at the top of the
digester, supported by a central pillar. In the case of simple membranes,
EPDM (ethylene propylene dienemonomer) rubber is usually used due to its
good UV resistance and elasticity. For double membranes, soft PVC is used.
An air blower between the two layers constantly inflates the outer
membrane, protecting the inner membrane, which inflates and deflates
with gas volume.
Another possibility is external gas storage, meaning that the storage place
is separate from the digester. Formerly, floating roof gas holders were used.
Gas vessels were placed on tanks filled with water, which provided an elastic
gas-tight seal and the vessel could rise or fall depending on the gas volume.
Nowadays, flexible structures are preferred for external gas storage. For
example, membrane cushions are a convenient and cost-effective solution,
existing in many different configurations (Fig. 8.6 and Fig. 8.7). They can be
placed on the digester roof or elsewhere, but they should be protected by a
double membrane or by a shelter.
The over-pressure in such gas holders is around 0.05 bar. In consequence,
the space requirement for the storage is important: 1Nm 3 of biogas,
containing approximately 6 kWh, occupies about 0.95m 3 . In rare cases,
medium (5-20 bar) or high-pressure (200-300 bar) storages are used, which
allow significant reductions in volume (1Nm 3 may be reduced to less than 4 l
in a high-pressure storage). However, these types of storage require
expensive steel constructions and energy-consuming gas compression and
decompression.
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