Biogas Utilization
Gas production
If the daily amount of available dung (fresh weight) is known, gas production per day in warm
tropical countries will approximately correspond to the following values:
• 1 kg cattle dung 40 liters biogas
• 1 kg buffalo dung 30 liter biogas
• 1 kg pig dung 60 liter biogas
• 1 kg chicken droppings 70 liter biogas
If the live weight of all animals whose dung is put into the biogas plant is known, the daily gas
production will correspond approximately to the following values:
• cattle, buffalo and chicken: 1,5 liters biogas per day per 1 kg live weight
• pigs, humans: 30 liters biogas per day per 1 kg weight
Conditioning of biogas
Sometimes the biogas must be treated/conditioned before utilization. The predominant forms
of treatment aim at removing either water, hydrogen sulfide or carbon dioxide from the
raw gas:
Reduction of the moisture content
The biogas is usually fully saturated with water vapor. This involves cooling the gas, e.g. by
routing it through an underground pipe, so that the excess water vapor condenses at the
lower temperature. When the gas warms up again, its relative vapor content decreases. The
"drying" of biogas is especially useful in connection with the use of dry gas meters, which
otherwise would eventually fill up with condensed water.
Reduction of the hydrogen-sulfide content
The hydrogen sulfide in the biogas combines with condensing water and forms corrosive
acids. Water-heating appliances, engines and refrigerators are particularly at risk. The
reduction of the hydrogen sulfide content may be necessary if the biogas contains an
excessive amount, i.e. more than 2% H2S. Since most biogas contains less than 1% H2S, de-
sulfurization is normally not necessary.
For small- to mid-size systems, de-sulfurization can be effected by absorption onto ferric
hydrate (Fe(OH)3), also referred to as bog iron, a porous form of limonite. The porous,
granular purifying mass can be regenerated by exposure to air.
The absorptive capacity of the purifying mass depends on its iron-hydrate content: bog iron,
containing 5-10% Fe(OH)3, can absorb about 15 g sulfur per kg without being regenerated
and approximately 150 g/kg through repetitive regeneration. It is noteworthy that many types
of tropical soils (laterite) are naturally ferriferous and suitable for use as purifying mass.
Another de-sulfurization process showing good results has been developed in Ivory Coast
and is applied successfully since 1987. Air is pumped into the gas store at a ratio of 2% to 5
% of the biogas production. The minimum air intake for complete de-sulfurization has to be
established by trials. Aquarium pumps are cheap and reliable implements for pumping air
against the gas pressure into the gas holder. The oxygen of the air leads to a bio-catalytic,
stabilized separation of the sulfur on the surface of the sludge. This simple method works
best, where the gas holder is above the slurry, as the necessary bacteria require moisture,
warmth (opt. 37°C) and nutrients.
In industrialized countries and for large plants, this process has meanwhile reached
satisfactory standard. For small scale plants in developing countries, however, using an
electric pump becomes problematic due to missing or unreliable electricity supply. Pumping
in air with a bicycle pump works in principle, but is a cumbersome method that will be
abandoned sooner or later.
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