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4.3 Sizing the plant
The size of the biogas plant depends on the quantity; quality and kind of available biomass and on
the digesting temperature.
Sizing the digester
The size of the digester, i.e. the digester volume (Vd), is determined on the basis of the chosen
retention time (RT) and the daily substrate input quantity (Sd).
Vd = Sd x RT (m³ = m³/day x number of days)
The retention time, in turn, is determined by the chosen/given digesting temperature (cf. fig 5.2).
For an unheated biogas plant, the temperature prevailing in the digester can be assumed as 1-2 K
above the soil temperature. Seasonal variation must be given due consideration, however, i.e. the
digester must be sized for the least favorable season of the year. For a plant of simple design, the
retention time should amount to at least 40 days. Practical experience shows that retention times of
60-80 days, or even 100 days or more, are no rarity when there is a shortage of substrate. On the
other hand, extra-long retention times can increase the gas yield by as much as 40%.
The substrate input depends on how much water has to be added to the substrate in order to arrive
at a solids content of 4-8%.
Substrate input (Sd) = biomass (B) + water (W) (m³/d)
In most agricultural biogas plants, the mixing ratio for dung (cattle and/or pigs) and water (B: W)
amounts to between 1: 3 and 2: 1 (cf. table 5.7).
Calculating the daily gas production (G)
The amount of biogas generated each day (G, m³ gas/d), is calculated on the basis of the specific
gas yield (Gy) of the substrate and the daily substrate input (Sd).
The calculation can be based on:
a) The volatile-solids content
G = kg VS-input x spec. Gy (solids)
b) the weight of the moist mass
G = kg biomass x spec. Gy (moist mass)
c) standard gas-yield values per livestock unit (LSU)
G = no. of LSU x spec. Gy (species)
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