Biogas Plants (GTZ, 1988, 85 p.) 4. Scaling of biogas plants 4.1 Definitions 4.2 Scaling of the digester 4.3 Scaling of gasholder 4.4 Digester/gasholder ratio 4.5 Measuring and test programmes

Biogas Plants (GTZ, 1988, 85 p.)

4. Scaling of biogas plants

4.1 Definitions

To calculate the scale of a biogas plant, certain characteristic parameters are used. These are as follows for simple biogas plants:

- Daily fermentation slurry arisings (Sd),
- Retention time (RT),
- Specific gas production per day (Gd), which depends on the retention time and the feed material.

The following additional concepts and parameters are also used in the theoretical literature:

- Dry matter (DM). The water content of natural feed materials varies. For this reason the solids or dry matter content of the feed material is used for exact scientific work (see table in Fig. 2).

- Organic dry matter (ODM or VS). Only the organic or volatile constituents of the feed material are important for the digestion process. For this reason, only the organic part of the dry matter content is considered.

- Digester loading (R). The digester loading indicates how much organic material per day has to be supplied to the digester or has to be digested. The digester loading is calculated in kilograms of organic dry matter per cubic metre of digester volume per day (kg ODM/m³/day). Long retention times result in low digester loadings. In a simple biogas plant, 1.5 kg/m3/day is already quite a high loading. Temperature-controlled and mechanically stirred large-scale plants can be loaded at about 5 kg/m3/day. If the digester loading is too high, the pH falls. The plant then remains in the acid phase because there is more feed material than methane bacteria.

Example:

Calculation of digester loading
Digester volume (VD): 48001 (4.8 m³) Retention time (RT): 80 days
Daily amount of fermentation slurry (Sd): 60 kg
Proportion of organic matter: 5 %

R = 5x60/100 x 4.8 = 0.625 kg/m3/day

Retention time (RT or t) indicates the period spent by the feed material in the digester. It is chosen by economic criteria. The retention time is appreciably shorter than the total time required for complete digestion of the feed material.

Specific gas production may be quoted for the amount of fermentation slurry, the dry matter, content or only the organic dry matter. In practice, it represents the gas production of a specific feed material in a specific retention time at specific digester temperatures.

Degree of digestion is measured as a percentage. It indicates the amount of gas obtained as a proportion of total specific gas production. The difference from 100% indicates the proportion of feed material which is not yet fully digested. In simple biogas plants, the degree of digestion is about 50 %. This means that half the feed material is not used.

Biochemical oxygen demand (BOD) is an important parameter in effluent treatment. It indicates the degree of pollution of effluents or sewage. The BOD is a measure of the amount of oxygen consumed by bacteria in biological purification.

4.2 Scaling of the digester

The size of the digester - the digester volume (VD) - is determined by the length of the retention time (RT) and by the amount of fermentation slurry supplied daily (Sd). The amount of fermentation slurry consists of the feed material (e.g., cattle dung) and the mixing water.

Example:

30 l dung + 30 l water = 60 l fermentation slurry

The digester volume is calculated by the formula

VD(l) = S_d(l/day) x RT (days)

Example:

Daily supply (S_d): 60 l
Retention time (RT): 80 days
Digester volume (VD):
60 l/day x 80 days = 4800 1 (4.8 m³)

For a specific digester volume and a known amount of fermentation slurry, the actual retention time is given by the formula

RT(days) = V_D (l) -:-S_d (l/day)

Example:

Digester volume (VD): 4800 l
Daily supply (Sd): 60 l/day
Retention time (RT):
4800 l -:- 60 l/day = 80 days

If the digester size is given and a specific retention time is required, the daily amount of feed is calculated by the formula

Sd (l/day) = VD (l) . RT(days)

Example:

Digester volume (VD): 4800 l
Retention time (RT): 80 days
Daily fermentation slurry requirement (Sd):
4800 l -:- 80 days = 60 l/day

If a biogas plant is loaded not daily but at relatively long intervals, the daily supply (Sd) decreases although the fermentation slurry proportion (S) remains the same. The retention time is correspondingly prolonged.

Example:

Digester volume (VD): 4800 l
Fermentation slurry proportion (S): 60 l

1. Daily loading, i.e. S_d = S = 60 l/day:
Retention time (RT):
4800l -:- 60 l/day = 80 days
2. Loading every other day, i.e.
S_d=S 2=30Q/day:
Retention time (RT):
4800 l -:- 30 £/day = 160 days
3. Loading twice a week, i.e.
S_d = S x 2/7 = 17.2 l/day:
Retention time (RT):
4800 l -:- 17.2 l/day = 279 days

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.

4.4 Digester/gasholder ratio

The form of a biogas plant is determined by the size ratio between the digester and the gasholder (see Figures 11 - 13).

Fig. 11: Digester/gasholder ratio The ratio of the digester volume (VD) and gasholder volume (VG) substantially determines the shape and design of a biogas plant. These two parameters must be calculated before any project is planned. For a digester/gasholder volume ratio of VD:VG = 6:1, a spherical shell is far more economical than a cylinder even in floating-drum plants.

Fig. 12: Dependence of shape on retention time on a floating-drum plant (cattle dung above; pig manure below) Filling volume and gasholder capacity (C = 55 %) are the same in each case. The differences in digester/ gasholder ratios result solely from the differing retention times (RT).

Fig. 13: Dependence of shape on retention time on a fixed-dome plant (cattle dung above; pig manure below) The filling volume and gasholder capacity (C = 55 %) are the same in each case. The differences in digester/ compensating tank ratios result solely from the different retention time (Gd as a result of RT, Figures 7 and 8).

For floating-drum plants with a low digester/ gasholder ratio (1:1 to 3:1), the best shape for the digester is a cylinder. If the ratio is larger, shell and vault structures are worthwhile.

The digester/gasholder ratio depends primarily on:

- retention time (RT),
- specific gas production (Gd),
- gasholder capacity (C).

The digester/gasholder ratio chosen must be correct regardless of the type of plant, otherwise the biogas plant will not serve its purpose.

In a fixed-dome plant, the digester/gasholder ratio corresponds to the size ratio between the net digestion space and the compensating tank above the zero line (see Figure 6): VD: VG corresponds to VD: VO

The examples given below show the importance of the specific gas production for the scaling of the plant and for the digester/ gasholder ratio.

For extensive biogas plant construction programmes, a knowledge of the specific gas production and the necessary gasholder capacity is particularly important. It is then a good plan to carry out measurements and tests of one's own (see Section 4.5).

Examples for the Calculation:

Feed material: cattle dung, amount (D_d):
30 kg/day
Mixing ratio: dung: water = 1:1
Fermentation slurry amount (S_d):
30 kg/day x 2 = 60 l/day
Retention time (RT): 80 days
Digester volume (V_D):
60 l/day x 80 days = 4800 l
Digester temperature (t): 26 - 28 °C
Specific -gas production (G_d) from Fig. 7:
40 l/kg
Daily gas production (G):
40 l/kg x 30 kg/day = 1200 l/day
Gasholder capacity (C): 60 %
Gasholder volume (V_G):
1200 l x 0.60= 720 l

Digester/gasholder ratio:

V_D:V_G= 4800l: 720 l =6.67: 1
Feed material: pig manure, amount (D_d):
20 kg/day
Mixing ratio: manure: water = 1: 2
Fermentation slurry amount (S_d):
20 kg/day x 3 = 60 l/day
Retention time (RT): 80 days
Digester volume (V_D):
60 l/day x 80 days = 4800 l
Digester temperature (t): 26-28 °C

Specific gas production (G_d) from figure 8:
112 l/day
Daily gas production (G):
122 l/kg x 20 kg/day = 2240 l/day
Gasholder capacity (C): 60
Gasholder volume (V_G):
2240 lx 0.60 = 1344 l
Digester/gasholder ratio:
V_D: V_G = 4800 l 1344 l= 3.6: 1

4.5 Measuring and test programmes

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):

Fig. 14: Biogas plants for a test programme for determinantion of gas production The length of the retention time (RT) has the greatest effect on digester size (VD). Test plants may have any shape. However' they should all be identical and should preferably conform to the type to be used later in a biogas programme. The test plants must be filled regularly for at least three months before gas production is measured. The gasholders must be all the larger, the longer the time between tests. Safe spanning of the after-dark hours must be ensured.

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.

Fig. 15: Measuring gas production on the plant In a floating-drum plant, the height of the gasholder is measured (top left), In a fixed-dome plant, the height of the slurry level is measured (top right). The manure is either weighed or measured in litres before introduction to the plant. Containers whose shape is easy to calculate are more accurate. If the lengths are measured in dm (1 dm = 10 cm), the volume in litres is obtained directly.