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

### (introduction...)

All extension-service advice concerning agricultural biogas plants must begin with an estimation of the quantitative and qualitative energy requirements of the interested party. Then, the biogas-generating potential must be calculated on the basis of the given biomass incidence and compared to the energy demand. Both the energy demand and the gas-generating potential, however, are variables that cannot be very accurately determined in the planning phase.

In the case of a family-size biogas plant intended primarily as a source of energy, implementation should only be recommended, if the plant can be expected to cover the calculated energy demand.

Since determination of the biogas production volume depends in part on the size of' the biogas plant, that aspect is included in this chapter.

Fig. 4.1: Balancing the energy demand with the biogas production (Source: OEKOTOP)

### 4.1 Determining the Energy Demand

The energy demand of any given farm is equal to the sum of all present and future consumption situations, i.e. cooking, lighting, cooling, power generation, etc. With deference to the general orientation of this manual, emphasis is placed on determining the energy demand of a typical family farm.

Experience shows that parallel calculations according to different methods can be useful in avoiding errors in calculating the gas/ energy demand.

Table 4.1: Outline for determining biogas demand (Source: OEKOTOP)

 Energy consumers data Biogas demand (l/d) 1. Gas for cooking (Chapter 5.5.3) Number of persons ............. Number of meals ............. Present energy consumption ............. Present source of energy ............. Gas demand per person and meal (Table 5.17) ............. Gas demand per meal ............. Anticipated gas demand ............... Specific consumption rate of burner ............. Number of burners - ............. Duration of burner operation ............. Anticipated gas demand ............... Total anticipated cooking-gas demand ............... 2. Lighting (Chapter 5.5.3) Specific gas consumption per lamp (Table 5.20) ............. Number of lamps ............. Duration of lamp operation ............. Gas demand ............... 3. Cooling (Chapter 5.5.3) Specific gas consumption X 24 h (Table 5.22) ............. ............... 4. Engines (Chapter 5.5.4) Specific gas consumption per kWh ............. Engine output ............. Operating time ............. Gas demand ............... 5. Miscellaneous consumers Gas demand ............. ............... Anticipated increase in consumption (%) ............... Total biogas demand ............... 1st-priority consumers ............... 2nd-priority consumers ............... 3rd-priority consumers ...............

The following alternative modes of calculation are useful:

Determining biogas demand on the basis of present consumption

. . ., e.g. for ascertaining the cooking-energy demand. This involves either measuring or inquiring as to the present rate of energy consumption in the form of wood/charcoal, kerosene and/or bottled gas.

Calculating biogas demand via comparable-use data

Such data may consist of:

- empirical values from neighboring systems, e.g. biogas consumption per person and meal,

- reference data taken from pertinent literature (cf. chapter 5.5), although this approach involves considerable uncertainty, since cooking-energy consumption depends on local culture-dependent cooking and eating habits and can therefore differ substantially from case to case.

Estimating biogas demand by way of appliance consumption data and assumed periods of use
This approach can only work to the extent that the appliances to be used are known in advance, e.g. a biogas lamp with a specific gas consumption of 1201/h and a planned operating period of 3 in/d, resulting in a gas demand of 360 l/d.

Then, the interested party's energy demand should be tabulated in the form of a requirements list (cf. table 4.1). In that connection, it is very important to attach relative priority values to the various consumers, e.g.:

1st priority: applies only when the biogas plant will cover the demand.
2nd priority: coverage is desirable, since it would promote plant usage.
3rd priority: excess biogas can be put to these uses.

### 4.2 Determining the biogas production

The quantity, quality and type of biomass available for use in the biogas plant constitutes the basic factor of biogas generation. The biogas incidence can and should also be calculated according to different methods applied in parallel.

Measuring the biomass incidence (quantities of excrement and green substrate)

This is a time-consuming, somewhat tedious approach, but it is also a necessary means of adapting values from pertinent literature to unknown regions. The method is rather inaccurate if no total-solids measuring is included. Direct measurement can, however, provide indication of seasonal or fodder-related variance if sufficiently long series of measurements are conducted.

Determining the biomass supply via pertinent-literature data
(cf. tables 3.2/3.3)

According to this method, the biomass incidence can be determined at once on the basis of the livestock inventory. Data concerning how much manure is produced by different species and per liveweight of the livestock unit are considered preferable.

Dung yield = liveweight (kg) x no. of animals x specific quantity of excrements (in % of liveweight per day, in the form of moist mass, TS or VS).

Determining the biomass incidence via regional reference data

This approach leads to relatively accurate information, as long as other biogas plants are already in operation within the area in question.

Determining biomass incidence via user survey

This approach is necessary if green matter is to be included as substrate.
It should be kept in mind that the various methods of calculation can yield quite disparate results that not only require averaging by the planner, but which are also subject to seasonal variation.

The biomass supply should be divided into two categories:

Category 1: quick and easy to procure,
Category 2: procurement difficult, involving a substantial amount of extra work.

Table 4.2: Outline for determining biomass incidence (Source: OEKOTOP)

 Source of biomass Moist weight TS/VS weight (kg/d) (kg/d) Animal dung Number of cattle: ............ Dung yield per head ....... ....... Amount collected ........... Dung yield from cattle ....... ....... Number of pigs: .............. Dung yield per pig ....... ....... Amount collected: ........... Dung yield from pigs ....... ....... Sheep, camels, horses etc................. ....... ....... Green matter 1. grass, etc. ....... ....... 2.................................... ....... ....... Night soil Number of persons: .................. Dung yield from night soil ....... ....... Total biomass incidence ....... ....... Category 1 ....... ....... Category 2 ....... .......

### 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.

### 4.4 Balancing the gas production and gas demand by iteration

As described in subsection 4.1, the biogas/ energy production (P) must be greater than the energy demand (D).

P>D

This central requirement of biogas utilization frequently leads to problems, because small farms with only a few head of livestock usually suffer from a shortage of biomass. In case of a negative balance, the planner must check both sides - production and demand - against the following criteria:

Energy demand (D)

Investigate the following possibilities:
- shorter use of gas-fueled appliances, e.g. burning time of lamps,
- omitting certain appliances, e.g. radiant heater, second lamp,
- reduction to a partial-supply level that would probably make operation of the biogas plant more worthwhile.

The aim of such considerations is to reduce the energy demand, but only to such an extent that it does not diminish the degree of motivation for using biogas technology.

Energy supply - biogas production (P)

Examine/calculate the following options/ factors:

- the extent to which the useful biomass volume can be increased (better collecting methods, use of dung from other livestock inventories, including more agricultural waste, night soil, etc.), though any form of biomass that would unduly increase the necessary labor input should be avoided;

- the extent to which prolonged retention times, i.e. a larger digester volume, would increase the gas yield, e.g. the gas yield from cattle manure can be increased from roughly 200 1/kg VS for an RT of 40 days to as much as 320 1/kg VS for an RT of 80-100 days;

- the extent to which the digesting temperature could be increased by modifying the structure.

The aim of such measures is to determine the maximum biogas-production level that can be achieved for a reasonable amount of work and an acceptable cost of investment.

If the gas production is still smaller than the gas demand (P < D), no biogas plant should be installed.

If, however, the above measures succeed in fairly well matching up the production to the demand, the plant must be resized according to subsection 4.3.

### 4.5 Sample calculations

Energy demand (D)

Basic data
8-person family, 2 meals per day. Present rate of energy consumption: 1.8 1 kerosene per day for cooking and fueling 1 lamp (0.6 1 kerosene = 1 m³ biogas).

Desired degree of coverage with biogas
Cooking: all
Lighting: 2 lamps, 3 hours each
Cooling: 60 I refrigerator

Daily gas demand (D)
Cooking
1. Present fuel demand for cooking:
1.21 kerosene = 2 m³ gas
2. Gas demand per person and meal:
0.15 m³ biogas
Gas demand per meal: 1.2 m³ biogas
Cooking-energy demand: 2.4 m³ biogas
3. Consumption rate of gas burner:
175 l/h per flame (2-flame cooker)
Operating time: 2 x 3 h + 1 h for tea
Biogas demand: 7 h x 3501 = 2.5 m³

Defined cooking-energy demand:
2.5 m³ biogas/d

Lighting
Gas consumption of lamp: 120 1/h
Operating time: 2 x 3 h = 6 h
Biogas demand: 0.7 m³/day

Cooling (60 l refrigerator)
Specific gas demand: 30 1/h
Biogas demand: 0.7 m³/day
Total biogas demand: 3.9 m³/d

 1st priority: cooking 2.5 m³ 2nd priority: 1 lamp 0.35 m³ 3rd priority: 1 lamp/refigerator 1.05 m³

Biomass supply/Biogas production (P)

Basic data
9 head of cattle, 230 kg each, 24-h stabling,
green matter from garden as supplement.

Daily biomass incidence
Animal dung, calculated as % liveweight (as per 1.) or as daily yield per head (as per 2.) as listed in pertinent literature..

1. Dung as % liveweight
Daily yield per head of cattle: 10% of
230 kg = 23 kg/d
Volatile solids/d: 1.8 kg VS per day and animal
Total yield: 207 kg/d (16 kg VS/d)

2. Manure yield on per-head basis
Dung yield per head of cattle: 15 kg/d
Urine: 9 1/d
Volatile solids: 9% = 2.1 kg VS/d
Total yield: 216 kg/d (19 kg VS/d)

Useful percentage: 75%
The lowest values are used as the basis of calculation.

Green matter: 20 kg agricultural waste with 30% VS.

Total biomass incidence 170 kg/d (18 kg VS/d)

Category 1: cattle 150 kg (12 kg)
Category 2: green matter 20 kg ( 6 kg)

Sizing the plant

Basic data (calculation for category 1)
Daily biomass: 150 kg/d
VS: 12kg/d
TS-content: 12%
Soil temperature: max 31 °C, min. 22 °C, average 25 °C

Digester volume (Vd)
Retention time (chosen): RT = 60 d (at
25 °C, i.e. f = 1.0)
Substrate input: Sd = biomass + water
Digester TS-content: = 7% (chosen)
Daily water input: Wd = 100 kg
Sd= 100+ 150=250 l
Digester volume: Vd = 250 1 x 60 d = 15 000 = 15 m³

Daily biogas yield
G = kg/d VS x Gy,vs .
= 12 kg/d x 0.25 = 3.0 m³/d
G = kgid biomass x Gy (moist)
= 150 x 0.02 = 3.0 m³/d
G = number of animals x Gy per animal x d
= 9 x 0.35 = 3.2 m³/d

Anticipated daily biogas yield = 3.0 m³/d
Balancing the biogas production and demand
Demand: 3.9 m³/d
Production: 3.0 m³/d

Changes/accommodations
On the demand side: 1 less lamp, reducing the demand to 3.55 m³
Production side: increasing the digester volume to 18 m³, resulting in a retention time of 75 days (f = 1.2) and a daily gas yield of 3.6 m³

Plant parameters

Digester volume: Vd = 18m³
Daily gas production: G = 3.6 m³
Daily substrate input: Sd = 2501
Specific gas production:
Gp = G : Vd
Gp = 3.6 (m³/d): 18 m³ = 0.2 m³/m³ Vd x d