Specific gas production Gp
i.e. the daily gas generation rate per m3 digester volume Vd, is calculated according to the
following equation
Gp = G ÷ Vd [ (m3/d) / m3 ]
Digester loading Ld
The digester loading Ld is calculated from the daily total solids input TS/d or the daily volatile
solids input VS/d and the digester volume Vd:
LdT = TS/d ÷ Vd [ kg/(m3 d) ]
LdV = VS/d ÷ Vd [ kg/(m3 d) ]
Then, a calculated parameter should be checked against data from comparable plants in the
region or from pertinent literature.
Sizing the gasholder
The size of the gasholder, i.e. the gasholder volume Vg, depends on the relative rates of gas
generation and gas consumption. The gasholder must be designed to:
• cover the peak consumption rate gcmax (->Vg1) and
• hold the gas produced during the longest zero-consumption period tzmax (->Vg2)
Vg1 = gcmax × tcmax = vcmax
Vg2 = Gh × tzmax
with
gcmax = maximum hourly gas consumption [m3/h]
tcmax = time of maximum consumption [h]
vcmax = maximum gas consumption [m3]
Gh = hourly gas production [m3/h] = G ÷ 24 h/d
tzmax = maximum zero-consumption time [h]
The larger Vg-value (Vg1 or Vg2) determines the size of the gasholder. A safety margin of 10-
20% should be added:
Vg = 1.15 (±0.5) × max(Vg1,Vg2)
Practical experience shows that 40-60% of the daily gas production normally has to be
stored.
The ratio Vd ÷ Vg (digester volume ÷ gasholder volume) is a major factor with regard to the
basic design of the biogas plant. For a typical agricultural biogas plant, the Vd/Vg-ratio
amounts to somewhere between 3:1 and 10:1, with 5:1 - 6:1 occuring most frequently.
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