Thursday, December 16, 2010

The Many Sources of Methane and Biogas

Methane is a combustible gas, which exists in two ways as part of the natural biological cycle when it is known as biogas, or biomethane, or as a mineral or fossil-fuel gas when it is called "natural gas".


As a mineral fuel it can be extracted from the earth's crust in the form of natural gas, or from waste sludge and biological sources including organic waste as biogas. Methane is widely accepted as the cleanest burning of all fossil fuels, but the extraction process itself requires careful consideration of the natural environment and local community.

Methane has been recently identified as a powerful greenhouse gas (GHG), and it has also been shown that its atmospheric concentration has been steadily increasing over the past 300 years. It is described as sustainable when it is produced by renewable energy (non mineral) sources, when it does not add to the GHG effect. All mineral sources are non-sustainable sources because they contribute to the GHG effect.

Methane is found trapped in air bubbles in ice cores, and measuring the quantity allows us to work out how much was present in the past. Studies of bubbles from the Greenland ice cap show that the concentration of methane in the air remained steady for 10,000 years, up until about 300 years ago.

If really large "burps" were released, quite substantial climatic changes would probably ensue. Methane is currently emitted in enough volume to make any reductions significant in terms of the overall GHG picture. Geochemical evidence for this process has been observed in numerous marine sediments along the continental margins, in methane seeps and vents, around methane hydrate deposits, and in anoxic waters.

There are two major ways in which methane is removed from the environment and these are aerobic oxidation by a specialized group of bacteria, and atmospheric oxidation.

Biogas /biomethane is produced by anaerobic oxidation by a specialized group of archaea, or methanogens.

Methane is emitted during the production and leaks from the distribution of natural gas and petroleum, and is released as a by-product of coal mining and incomplete fossil fuel combustion. Over many centuries as coal deposits are formed, some methane is absorbed by the coal and that is the usual source from which it is found in mines.

Biogas methane is released as vegetation rots under water. This is is happening in millions of square miles of rice fields throughout Asia.

It is combustible, and mixtures of about 5 to 15% in air are explosive. It is the main constituent of natural gas, a fossil fuel . Methane is far more lethal as a greenhouse gas - assuming that one believes in all this - than carbon dioxide. It is between 20 and 23 times more potent than Carbon Dioxide in terms of its GHG climate warming potential, depending upon how you look at its potency, although it does not last forever in the atmosphere.

Cows produce huge volumes of it. Contrary to what might be expected, organic cows produce less milk than conventionally farmed cattle, so their methane output per litre of milk tends to be much higher than the non-organic type.

When we say that methane is a much more potent greenhouse gas than CO2 we refer to the fact that eight for weight, it traps 21 times more heat energy. However, to get this in proportion methane is just one of six gases held responsible for global warming. Carbon dioxide is the second most abundant greenhouse gas behind water vapor.

There are huge quantities of methane in the permafrost. Permafrost in northern Siberia is reported to be 1,600 meters, (5,250 feet) thick and in in northern Alaska it is estimated at 650 meters (2,100 feet) thick Permafrost lies beneath about 80 percent of Alaska, and a higher percentage of Siberia. Permafrost on land, though, is as cold as -12.4 degrees Celsius.

Why do the GHG potencies vary so much between gases? Gases in the atmosphere, such as methane, absorb light energy and different gases absorb different amounts of energy.

Gas pipeline inspection is essential to maintain gas distribution systems, and minimize leakage, but it is a huge task. Just looking at the figures for Germany alone, with a network of more than 40,000 km of long-distance gas transport pipelines. Also, the need for a fast and remote monitoring system is apparent, to identify damage quickly.

Emissions from animal wastes depend on the method of storage or management of those wastes. When manure is stored as slurry, much of the decomposition is anaerobic and methane is produced in significant quantities. These emissions can and should be controlled by specific regulations.

Anaerobic digesters at local organic waste landfill sites or installed as part of large PFI systems capture and destroy the methane or clean it to comply with consumer standards. Large numbers of these plants installed throughout Europe would help to reverse the upward trend in methane discharges to the atmosphere. Nevertheless, even if large numbers of AD Plants were built rapidly, anaerobic methane oxidation will always remain biogeochemically important because methane is such a potent greenhouse gas in the atmosphere.

Steve Evans writes about methane biogas and runs the united kingdom anaerobic digestion web site

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