Optional Parts of Biogas Plants
Heating systems
Normally, because of the rather high involved costs, small-scale biogas plants are built
without heating systems. But even for small scale plants, it is of advantage for the bio-
methanation process to warm up the influent substrate to its proper process temperature
before it is fed into the digester. If possible, cold zones in the digester should be avoided. In
the following, a number of different ways to get the required amount of thermal energy into
the substrate are described. In principle, one can differentiate between:
• direct heating in the form of steam or hot water, and
• indirect heating via heat exchanger, whereby the heating medium, usually hot water,
imparts heat while not mixing with the substrate.
Direct heating
Direct heating with steam has the serious disadvantage of requiring an elaborate steam-
generating system (including desalination and ion exchange as water pretreatment) and can
also cause local overheating. The high cost is only justifiable for large-scale sewage
treatment facilities.
The injection of hot water raises the water content of the slurry and should only be practiced
if such dilution is necessary.
Indirect heating
Indirect heating is accomplished with heat exchangers located either inside or outside of the
digester, depending on the shape of the vessel, the type of substrate used, and the nature of
the operating mode.
4. Floor heating systems have not served well in the past, because the accumulation of
sediment gradually hampers the transfer of heat.
5. In-vessel heat exchangers are a good solution from the standpoint of heat transfer as
long as they are able to withstand the mechanical stress caused by the mixer,
circulating pump, etc. The larger the heat-exchange surface, the more uniformly heat
distribution can be effected which is better for the biological process.
6. On-vessel heat exchangers with the heat conductors located in or on the vessel walls
are inferior to in-vessel-exchangers as far as heat-transfer efficiency is concerned,
since too much heat is lost to the surroundings. On the other hand, practically the
entire wall area of the vessel can be used as a heat-transfer surface, and there are
no obstructions in the vessel to impede the flow of slurry.
7. Ex-vessel heat exchangers offer the advantage of easy access for cleaning and
maintenance.
While in Northern countries, often a substantial amount of the produced biogas is consumed
to provide process energy, in countries with higher temperatures and longer sunshine hours,
solar-heated water can be a cost-effective solution for heating. Exposing the site of the
biogas plant to sunshine, e.g. by avoiding tree shade, is the simplest method of heating.
Pumps
Pumps become necessary parts of a biogas unit, when the amounts of substrate require fast
movement and when gravity cannot be used for reasons of topography or substrate
characteristics. Pumps transport the substrate from the point of delivery through all the
stages of fermentation. Therefore, several pumps and types of pumps may be needed.
Pumps are usually found in large scale biogas units.
Types of pump
There are two predominant types of pump for fresh substrate: centrifugal pumps and
positive-displacement pumps (reciprocating pumps). Centrifugal pumps operate on the
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