Biogas Basics
What is biogas?
Biogas originates from bacteria in the process of bio-degradation of organic material under
anaerobic (without air) conditions. The natural generation of biogas is an important part of
the biogeochemical carbon cycle. Methanogens (methane producing bacteria) are the last
link in a chain of micro-organisms which degrade organic material and return the
decomposition products to the environment. In this process biogas is generated, a source of
renewable energy.
Biogas and the global carbon cycle
Each year some 590-880 million tons of methane are released worldwide into the
atmosphere through microbial activity. About 90% of the emitted methane derives from
biogenic sources, i.e. from the decomposition of biomass. The remainder is of fossil origin
(e.g. petrochemical processes). In the northern hemisphere, the present tropospheric
methane concentration amounts to about 1.65 ppm.
Biology of methanogenesis
Knowledge of the fundamental processes involved in methane fermentation is necessary for
planning, building and operating biogas plants. Anaerobic fermentation involves the activities
of three different bacterial communities. The process of biogas-production depends on
various parameters. For example, changes in ambient temperature can have a negative
effect on bacterial activity.
Substrate and material balance of biogas production
In principle, all organic materials can ferment or be digested. However, only homogenous
and liquid substrates can be considered for simple biogas plants: faeces and urine from
cattle, pigs and possibly from poultry and the wastewater from toilets. When the plant is filled,
the excrement has to be diluted with about the same quantity of liquid, if possible, the urine
should be used. Waste and wastewater from food-processing industries are only suitable for
simple plants if they are homogenous and in liquid form. The maximum of gas-production
from a given amount of raw material depends on the type of substrate.
Composition and properties of biogas
Biogas is a mixture of gases that is composed
chiefly of:
• methane (CH4): 40-70 vol.%
• carbon dioxide (CO2): 30-60 vol.%
• other gases: 1-5 vol.%
including
• hydrogen (H2): 0-1 vol.%
• hydrogen sulfide (H2S): 0-3 vol.%
Like those of any pure gas, the characteristic
properties of biogas are pressure and
temperature-dependent. They are also affected
by the moisture content. The factors of main
interest are:
• change in volume as a function of
temperature and pressure,
• change in calorific value as a function of
temperature, pressure and water-vapor
content, and
• change in water-vapor content as a
function of temperature and pressure.
Biogas is used as an ecologically friendly
and future oriented technology in many
countries Photo: Krieg
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