|Biogas Plants (GTZ, 1988)|
|4. Scaling of biogas plants|
The aim of a measuring and test programme is to determine the specific gas production obtained at specific retention times.
Since digester temperature affects gas production, the latter should be measured at both the coldest and hottest time of the year.
The programme consists of a set of at least four biogas plants of different sizes. A given filling volume results in different retention times, in turn yielding different amounts of gas production for one and the same filling volume.
Example (Figure 14):
Filling volume: 30 kg manure and 30 lwater; 60 l/day
Retention times (RT) chosen: 30, 45, 60 and 90 days
Required digester volume:
RT(30): VD = 30 x 60 = 1800 l (1.8 m³)
RT(45): VD = 45 x 60 = 2700 l (2.7 m³)
RT(60): VD = 60 x 60 = 3600 l (3.6 m³)
RT(90): VD = 90 X 60 = 5400 l (5.4 m³)
Specific gas production is determined by dividing the daily volume of gas measured by the amount of slurry loaded into the plant (30 kg).
The results are plotted in a curve(like Figure 7 and 8) and are used for the scaling and calculation of the digester and gasholder volumes.
If a test programme is too expensive or complicated, the actual gas production values can also be derived from the results of measurement of a number of existing plants. For this purpose, the volume of gas stored must be measured before and after each consumption (Figure 15). Measurements must be effected for at least three consecutive days and nights.