4.3 Scaling of gasholder

The size of the gasholder - the gasholder volume (VG, see Figure 6)—depends on gas production and the volume of gas drawn off.

Fig. 6: Digester and gasholder Each biogas plant consists of a digester (VD) and a gasholder (VG). For calculation purposes, only the net digester volume or gas space is relevant. In the fixed-dome plant (C), the net gas space corresponds to the size of the compensating tank (Vo) above the zero line. The zero line is the filling limit.

Gas production depends on the amount and nature of the fermentation slurry, digester, temperature and retention time (Figures 7,8).

Fig. 7: Gas production from fresh cattle manure depending on retention time and digester temperature

The curves represent averages of laboratory and empirical values. The values vary a wide range owing to differences in the solids content of the dung, animal feeds and types of biogas plant. Regular stirring increases gas production. The 26-28 °C line is a secure basis for scaling in the majority of cases.

Fig. 8: Gas production from fresh pig manure depending on retention time and digester temperature

The curves represent averages of laboratory and empirical values. The measured values show an even wider range of variation than in the case of cattle dung. Particularly large variations occur if antibiotics are added to the feed. The 26-28 °C curve is a realistic guide for the planning of a plant.

Gas production is encouraged by high, uniform temperatures (e.g., 33°C), long retention times (e.g., 100 days) and thorough mixing of the slurry.

Gas production is adversely affected by low and fluctuating temperatures (15-25 °C), short retention times (e.g., 30 days) and poor mixing.

Example:

1 kg of cattle dung yields only 15 lof biogas in a retention time of 30 days at a digester temperature of 20 °C. If the retention time is increased to 100 days and the digester temperature to 33 °C, 1 kg of cattle dung gives 54 lof biogas (Figure 7). The size of the gasholder is determined, primarily by the amount of gas drawn off and when it is drawn.

Examples:

A refrigerator operating round the clock consumes all the gas produced on a given day. The gasholder merely has to compensate for fluctuations in the,daily volume of gas produced.
A water pump consumes the entire daily gas production in a few hours. The gasholder must every day collect the entire daytime and night-time production and compensate for daily production fluctuations.

The ratio of gasholder volume (VG) to daily gas production (G) is called the gasholder capacity (C).

Example:

Gasholder volume (VG): 1.5m³ (1500l)
Daily gas production (G): 2.4 m³
Gasholder capacity (C):
1.5 m³ 2.4 m³ = 0.625 = 62.5 %.

The required gasholder capacity and hence the required gasholder size is an important planning parameter. If the gasholder capacity is insufficient' part of the gas produced will be lost. The remaining volume of gas will not be enough. If the gasholder is made too large, construction costs will be unnecessarily high, but plant operation will be more convenient. The gasholder must therefore be made large enough to be able to accept the entire volume of gas consumed at a time. It must also be able to accept all the gas produced between consumption times. Furthermore, the gasholder must be able to compensate for daily fluctuations in gas production. These fluctuations range from 75 % to 125 % of calculated gas production.

Calculation examples for gasholder size:

Daily gas production: 2400 l
Hourly gas production: 2400 -:- 24 = 100 l/h
Gas consumption

from 0600 to 0800 hrs	=2h
from 1200 to 1400 hrs	=2h
from 1900 to 2100 hrs	=2h
Duration of gas consumption:	6 h

To simplify the calculation, uniform gas consumption is assumed. Hourly gas consumption:
2400 l -:- 6 h = 400 l/h

Gas is also produced during consumption. For this reason, only the difference between consumption and production is relevant to the calculation.

D_G = 400 l/h - 100 l/h = 300 l/h

The necessary gasholder size during consumption is therefore:

V_G(1)=300l/h x 2h=600l.

The longest interval between periods of consumption is from 2100 to 0600 hrs (9 hours). The necessary gasholder size is therefore:

V_G(2) = 100 l/h x 9 h = 900 Q.

V_G(2) is the maximum relevant gasholder size. With the safety margin of 25%, this gives a gasholder size of

V_G = 900 l x 1.25 = 1125 £.

The required gasholder capacity is thus:

C = 1 125 l -:- 2400 l= 0.47 = 47 %

Daily gas production: 2400 l
Hourly gas production: 100 l/h
Gas consumption

from 0530 to 0830 hrs	=3h
from 1830 to 2000 hrs	=1.5h
Duration of gas consumption:	4.5 h

Gas consumption per hour:

2400 l -:- 4.5 h = 533 l/h.

Difference between gas production and consumption:

D_G = 533 l/h -100 l/h = 433 l/h.

Hence the necessary gasholder size during consumption is:

V_G(1)= 433 l/h x 3 h = 1299 l.

The necessary gasholder size in the intervals between consumption results from the period from 0830 to 1830 hrs (10 h). The necessary gasholder size is therefore:

V_G(2) = 100 l/h x 10 h = 1000 Q.

V_G(1) is the larger volume and must therefore be used as the basis. Allowing for the safety margin of 25 %, the gasholder size is thus

V_G = 1299 l X 1.25 = 1624 Q.

The required gasholder capacity thus works out as
C = 1624 l -:- 2400 l= 0.68 = 68 %.

Fig 9: Graphic determination of required gasholder volume in accordance with the first example, page 21/22. Working steps: 1. Plotting of gas production curve (a) and gas consumption curve (b). 2. Plotting of gas consumption times. 3. The gasholder curve (thick line) is determined by parallel shifting in accordance with the numbered arrows (1-9). The value VG does not yet include the safety margin of 25 %

Fig. 10: Graphic determination of the required gasholder volume in accordance with the second example on page 23/24. The safety margin of 25 % for fluctuating gas production must be added to the value VG. The distance H can also be regarded as the height of the floating gas drum. Experience shows that about the same volume of gas per hour is produced day and night.

A gasholder capacity of 50-60% is normally correct for peasant households in Third World countries. A capacity of 70 % or even more must be allowed only where not more than one meal a day is cooked regularly or where eating habits are highly irregular.