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< prev - next > Energy Biogas biogas (Printable PDF)
Biogas is a well-established fuel for cooking and lighting in a number of countries. It is a gas
mixture comprising around 60% methane and 40% carbon dioxide that is formed when organic
materials, such as dung or vegetable matter are broken down by microbiological activity in the
absence of air, at slightly elevated temperatures (most effective between 30 - 40°C or 50 - 60°C).
This is the same process as that which occurs naturally at the bottom of ponds and marshes and
gives rise to marsh gas or methane.
China has over 7.5 million household biogas digesters, 750 large- and medium-scale industrial
biogas plants, and a network of rural 'biogas service centres' to provide the infrastructure
necessary to support dissemination, financing and maintenance. India has also had a large
programme, with about three million household-scale systems installed (Martinot 2003). Other
countries in the South with active programmes include Nepal, Sri Lanka, Kenya, and several
countries in Latin America. As carbon emission levels are becoming of greater concern and as
people realise the benefits of developing integrated energy supply options, then biogas becomes
an increasingly attractive option.
The biogas process is known as anaerobic (without air) digestion, and provides a clean cooking
and lighting fuel that can be produced on a scale varying from a small household system to a
large commercial plant of several thousand cubic metres. Biogas can be used for electricity
generation and powering farm equipment. There are two main types of electricity generation
Microturbines are small gas turbines that burn methane, mixed with compressed air. As
they burn, the hot pressurized gases are forced out of the combustion chamber and
through a turbine wheel, causing it to spin and turn the generator, thus making the
Reciprocating gas engines that have been modified from natural gas engines but which
can handle the larger quantities of carbon dioxide and contaminants that are found in
biogas. They work on a much larger scale, burn efficiently, and deliver between 1MW and
2 MW of electrical power.
The digestion of animal and human waste yields several benefits:
The production of methane for use as a fuel, which reduces the amount of woodfuel
required and thus reduces desertification.
The waste is reduced to slurry that has a high nutrient content, making an ideal fertiliser.
During the digestion process, dangerous bacteria in the dung and other organic matter
are killed, which reduces the pathogens dangerous to human health.
Carbon emissions
In some cases, anaerobic digestion is used to produce fertiliser as the main product, and the
biogas is merely a by-product which is vented from the digester. This has serious negative
environmental impacts as methane is a damaging greenhouse gas. Conversely, when the gas is
burnt, it is one of the few energy processes that is ‘carbon negative’ in that it reduces the amount
of greenhouse gases emitted by the raw material (dung emits methane), making it an attractive
option for those seeking carbon funding for wide-scale dissemination.
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