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.
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:
- quick and easy to procure
- procurement difficult, involving a substantial amount of extra work
Measuring the biomass availability (quantities of excrement and green substrate)
This is a time-consuming, cumbersome 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 only provide indication of seasonal or fodder-related variance if sufficiently long series of measurements are conducted.
Determining the biomass supply via literature data
According to this method, the biomass supply 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 preferable.
Dung yield = liveweight × number of animals × specific quantity of excrements [ kg/d ]
Often, specific quantities of excrement are given in % of liveweight per day, in the form of moist mass, total solids content or volatile solids content
Determining the biomass supply 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 availability via user survey
This approach is necessary if green matter is to be included as substrate.
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. The following table helps to collect all data concerning the energy demand. To get more information about the specific energy demand, for example, see "Biogas appliances".
Table: Outline for determining biogas demand
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| Energy consumers
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Data
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Biogas demand [l/d]
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| 1. Gas for cooking
Number of persons
Number of meals
Present energy consumption
Present source of energy
Gas demand per person and meal
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
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| 2. Lighting
Specific gas consumption per lamp
Number of lamps
Duration of lamp operation
Gas demand
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| 3. Cooling
Specific gas consumption * 24 hours
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| 4. Engines
Specific gas consumption per kWh
Engine output
Operating time
Gas demand
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| 5. Miscellaneous consumers
Gas demand
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| Anticipated increase in consumption (%)
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| Total biogas demand
1st-priority consumers
2nd-priority consumers
3rd-priority consumers
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The following alternative modes of calculation are useful:
Determining biogas demand on the basis of present energy consumption, e.g. for ascertaining the cooking-energy demand. This involves either measuring or inquiring the present rate of energy consumption in the form of wood, charcoal, kerosene and 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 day,
- reference data taken from literature, although this approach involves considerable uncertainty, since cooking-energy consumption depends on local 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 120 l/h and a planned operating period of 3 h/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. In that connection, it is important to attach relative priority values to the various consumers, e.g.:
- priority: applies only when the biogas plant will cover the demand.
- priority: coverage is desirable, since it would promote plant usage.
- priority: excess biogas can be put to these uses.
