Plastics
Plastics have been in widespread use in the field of biogas engineering for a long time. Basic
differentiation is made between flexible materials (sheeting) and rigid materials (PE, GRP,
etc.). Diverse types of plastic sheeting can be used for constructing the entire digesting
chamber (balloon gas holders) or as a vessel cover in the form of a gas-tight "bonnet".
Sheeting made of caoutchouc (india rubber), PVC, and PE of various thickness and
description have been tried out in numerous systems. The durability of plastic materials
exposed to aggressive slurry, mechanical stress and UV radiation, as well as their gas
permeability, vary from material to material and on the production processes employed in
their manufacture. Glass-fibre reinforced plastic (GRP) digesters have proven quite suitable,
as long as the in-service static stresses are accounted for in the manufacturing process.
GRP vessels display good gas-tightness and corrosion resistance. They are easy to repair
and have a long useful life span. The use of sandwich material (GRP - foam insulation -
GRP) minimizes the on-site insulating work and reduces the cost of transportation and
erection.
Wood
A further suitable material for use in the construction of biogas systems is wood. It is often
used for building liquid-manure hoppers and spreaders. Wooden digesters require a vapor-
proof membrane to protect the insulation. Closed vessels of any appreciable size are very
hard to render gas-tight without the aid of plastic sheeting. Consequently, such digesters are
very rare.
Gasholders
Basically, there are different designs of construction for gasholders used in simple biogas
plants:
• floating-drum gasholders
• fixe-domes gasholders
• plastic gasholders
• separate gasholders
Floating-drum gasholders
Most floating-drum gas-holders are made of 2-4 mm thick sheet steel, with the sides made of
thicker material than the top in order to compensate for the higher degree of corrosive attack.
Structural stability is provided by L-bar bracing that also serves to break up surface scum
when the drum is rotated. A guide frame stabilizes the gas drum and prevents it from tilting
and rubbing against the masonry. The two equally suitable and most frequently used types
are:
• an internal rod & pipe guide with a fixed (concrete-embedded) cross pole (an
advantageous configuration in connection with an internal gas outlet);
• external guide frame supported on three wooden or steel legs.
For either design, substantial force can be necessary to rotate the drum, especially if it is
stuck in a heavy layer of floating scum. Any gas-holder with a volume exceeding 5 m3 should
be equipped with a double guide (internal and external).
All grades of steel normally used for gas-holders are susceptible to moisture-induced rusting
both in- and outside. Consequently, a long service life requires proper surface protection,
including:
• thorough de-rusting and de-soiling
• primer coat of minimum 2 layers
• 2 or 3 cover coats of plastic or bituminous paint.
The cover coats should be reapplied annually. A well-kept metal gas-holder can be expected
to last between 3 and 5 years in humid, salty air or 8-12 years in a dry climate.
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