Sizing a biogas plant
The size of the biogas plant depends on the quantity, quality and kind of available biomass
and on the digesting temperature. The following points should be considered
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 × RT [ m3 = m3/day × number of days ]
The retention time, in turn, is determined by the chosen/given digesting temperature. For an
unheated biogas plant, the temperature prevailing in the digester can be assumed as 1-2
Kelvin 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.
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:
10. The volatile solids content VS
G = VS × Gy(solids) [ m3/d = kg × m3/(d×kg) ]
11. the weight of the moist mass B
G = B × Gy(moist mass) [ m3/d = kg × m3/(d×kg) ]
12. standard gas-yield values per livestock unit LSU
G = number of LSU × Gy(species) [ m3/d = number× m3/(d×number) ]
The temperature dependency is given by:
Gy(T,RT) = mGy × f(T,RT)
where
Gy(T,RT) = gas yield as a function of digester temperature and retention time
mGy = average specific gas yield, e.g. l/kg volatile solids content
f(T,RT) = multiplier for the gas yield as a function of digester temperature T and
retention time RT
As a rule, it is advisable to calculate according to several different methods, since the
available basic data are usually very imprecise, so that a higher degree of sizing certainty
can be achieved by comparing and averaging the results.
Establishing the plant parameters
The degree of safe-sizing certainty can be increased by defining a number of plant
parameters:
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