Biogas is the gas generated when bacteria degrade biological material in the absence of oxygen, in a process known as anaerobic digestion. Biogas is a mixture of methane (also known as marsh gas or natural gas, CH4) and carbon dioxide. It is a renewable fuel which has for a long while been produced from organic waste treatment, however, recently to combat high oil prices/meet sustainability targets in some countries, such as Germany, food or selected energy crops have been digested to produce it.

Biogas is growing in popularity and availability in Europe through programs like Biogasmax, which is creating a network of biogas demonstrations starting in Stockholm, Gothenburg, Lille, Rome, and Berne. In the UK the government's Defra Demonstrator Project is funding several fully operational biogas plants.

Biogas is a mixture of about 60-70% methane (natural gas), 30-40% carbon dioxide and other trace gases, such as hydrogen sulfide.

Biogas is also produced in bogs and wetlands where large amounts of rotting vegetation may accumulate. Consisting mostly of methane gas, it is the same as "natural gas", commonly burned in domestic boilers and in commercial use via our town gas mains, and in barbecues.

It is one of the renewable (or “alternative”) energies worldwide which we will need to use once oil production has peaked. Biogas is a very effective fuel once it has been scrubbed of corrosive contaminants and compressed to a high pressure. In this form it is known as CNG (Compressed Natural Gas), and cars and buses can run on this without any major engine modifications. The only disadvantage when compared with diesel or petrol fuel is that compressed natural gas vehicles require a greater amount of space for fuel storage than conventional gasoline power vehicles. (Don't confuse CNG with LPG (Liquified Petroleum Gas) which is the liquid from of methane and has to be kept at low temperatures.)

Biogas is typically used in factory boilers and in engine generator sets to produce electricity and heat. If internal combustion engines are fuelled with this gas to produce electricity, the facility can use the electricity or export it to the power grid.

Another source of biogas is landfills. At the landfill site, large mounds of garbage are buried under the surface.

Biogas can be produced quite simply at a domestic scale and often is in China, India, Nepal and many other developing countries. A typical home might cook for an hour per day on gas from home waste sources.

Cooking using biogas is much cleaner than burning wood, cutting the risk of contracting respiratory and eye diseases caused by the smoke inherent in traditional methods. Women in particular benefit from this cost-effective and clean energy use.

However, efficient use of biogas is more readily accomplished at larger scales in Anaerobic Digestion Plants.

Anaerobic Digestion

Anaerobic digestion is basically a simple process carried out in a number of steps that can use almost any organic material as a substrate - it occurs in animal digestive systems, marshes, rubbish dumps, septic tanks and the Arctic Tundra.

Anaerobic digestion has been traditionally used in the developed nations at sewage treatment plants for sludge stabilization. More recently, applications have been implemented for treating municipal solid wastes, industrial wastes and manures specifically for the renewable energy that can be created.

When anaerobic digestion is carried out as a renewable energy production process it is carried out in large tanks or “reactors”. The feed liquid enters from the bottom of the reactor and biogas is produced while liquid flows up through the sludge blanket. There are a large number of ways in which this basic process can be carried out. One such process is called the UASB process. Many full-scale UASB plants are in operation in Europe using waste water from sugar beet processing and other dilute wastes that contain mainly soluble carbohydrates (Bioenergy Systems Report, 1984).

An anaerobic digester is operated at a controlled temperature, pH and loading rate to encourage the growth of microorganisms that eat the organic matter and produce biogas and a more biologically stable liquid effluent.

Between 25% and 40% of the energy in the biogas is converted to electricity when it is burnt in an engine. The remaining available energy is converted to heat that is used to heat the materials in the anaerobic digester and for other purposes, such being piped as hot water to heat homes, shops or greenhouses. These Anaerobic Digestion Plants and known as Combined Heat and Power plants (CHPs). After being specially treated, biogas can also be fed into the existing natural gas network.

To separate the biogas from the components carbon dioxide and hydrogen sulphide it is compressed to six to eight bar and fed in at the ground level of an absorption column, in a process called scrubbing.

The digested mixture of liquids and solids left after AD are called “bio-slurry” and “bio-sludge” or “liquid” and “solid digestate”. These are mainly used as organic fertiliser for crops. Many new uses for these materials have been proposed and some have met some success.

Energy from biogas is a green or renewable energy, and its production reduces greenhouse gas emissions by reducing reliance on fossil fuels. In addition, biogas systems reduce greenhouse gas emissions by avoiding the methane otherwise produced from stored manure.

Conclusion

Fertilizer and pesticide rely on natural gas and oil based chemicals for production, and farm machinery is run on liquid fossil fuels. This will become increasingly less economically viable.

The simple equation we see in front of us is that unless we start to use this additional and largely untapped source of energy from our wastes, higher crude oil prices will continue to lead to higher food costs.

Join with us at Friends of Biogas to promote the biogas cause!

If you have not seen it already - visit our web site at:
http://www.anaerobic-digestion.com
and out Forum at:
http://www.forum.anaerobic-digestion.com