Biogas Plants in Animal Husbandry (GTZ, 1989)
 4. Balancing the energy demand with the biogas production
 (introduction...) 4.1 Determining the Energy Demand 4.2 Determining the biogas production 4.3 Sizing the plant 4.4 Balancing the gas production and gas demand by iteration 4.5 Sample calculations

### 4.3 Sizing the plant

The size of the biogas plant depends on the quantity; quality and kind of available biomass and on the digesting temperature.

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 x RT (m³ = m³/day x number of days)

The retention time, in turn, is determined by the chosen/given digesting temperature (cf. fig 5.2).

For an unheated biogas plant, the temperature prevailing in the digester can be assumed as 1-2 K 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 (cf. table 5.7).

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:

a) The volatile-solids content

G = kg VS-input x spec. Gy (solids)

b) the weight of the moist mass

G = kg biomass x spec. Gy (moist mass)

c) standard gas-yield values per livestock unit (LSU)

G = no. of LSU x spec. Gy (species)

Table 4.3 lists simplified gas-yield values for cattle and pigs. A more accurate estimate can be arrived at by combining the gas-yield values from, say, table 3.5 with the correction factors for digester temperature and retention time shown in figure 5.2.

GYT,RT = mGy x fT,RT

GYT,RT = gas yield as a function of digester temperature and retention time
mGy = average specific gas yield, e.g. 1/kg VS (table 3.5)
fT,RT = multiplier for the gas yield as a function of digester temperature and retention time (cf. fig. 5.2)

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:

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 gas/m³ Vd x d)

Ld - TS (VS) input/m³ digester volume (kg TS (VS)/m³ Vd x d)

Then, a calculated parameter should be checked against data from comparable plants in the region or from pertinent literature.

Table 4.3: Simplified gas-yield values for substrate from cattle and pigs (digesting temperature: 22-27 °C) (Source: OEKOTOP)

 Type of housing/ manure Cattle, live wt. 200 - 300 kg Buffalo, live wt. 300 - 450 kg Pigs, live wt 50 - 60 kg manure yield Gas yield (I/d) manure yield Gas yield (I/d) manure yield Gas yield (l/d) (kg/d) RT=60 RT=80 (kg/d) RT=60 RRT=80 (kg/d) RT=40 RT=60 24-h stabling - dung only (moist),unpaved floor (10% losses) 9-13 300-450 350-500 14-18 450-540 300-620 - - - - dung and urine,concrete floor 20-30 350-510 450-610 30-40 450-600 5440-710 2.5-3.0 120-140 150-180 - stable manure (dung + 2 kg litter), concrete floor 22-32 450-630 530-730 32-42 550-740 630-890 - - - Overnight stabling - dung only (10% losses) 5-8 180-270 220-310 8-10 240-300 2290-360 - - - - dung and urine,concrete floor 11-16 220-320 260-380 16-20 260-330 330-410 - - - 1 kg/d moist dung ~35 ~40 ~34 ~40 - - 1 l/d manure ~20 ~25 ~20 ~24 ~50 ~60 1 kg/d manure ~22 ~27 ~22 ~26 - - 1 kg TS/d ~200 ~240 ~200 ~240 ~2270 ~340 1 kg VS/d ~250 ~300 ~250 ~300 ~3350 ~430

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 (Vg 1) and
- hold the gas produced during the longest zero-consumption period (Vg 2).

Vg1 = gc, max x tc, max = vc, max
Vg2 = G x tz, max

gc, max = maximum hourly gas consumption (m³/h)
tc, max = time of maximum consumption (h)
vc, max = maximum gas consumption (m³)
G = gas production (m³/h)
tz, max = maximum zero-consumption time (h)

The larger Vg-value (Vgl or Vg2) determines the size of the gasholder. A safety margin of 10-20% should be added. Practical experience shows that 40-60% of the daily gas production normally has to be stored. Digester volume vs. gasholder volume. (Vd: Vg) The ratio

Vd : Vg

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 occurring most frequently.