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Emissions of methane from biogas less hazardous than previously believed

17.03.2006


There is a great interest in biogas as an environmentally friendly alternative to oil and other fossil fuels. However, biogas has been criticized for releasing methane. As a greenhouse gas, methane is 20-25 times worse than carbon dioxide. But new calculations now show that biogas is nevertheless better than fossil fuels as long as the methane emissions are lower than 10-20 percent. It was previously believed that the cut-off point was about 5 percent.

“The margins are thus considerably greater than was previously assumed. Normally methane emissions from biogas are about 2-3 percent,” says Maria Berglund, researcher on Environmental and Energy Systems at Lund University in Sweden.

The point of departure for her dissertation, Biogas production from a systems analytical perspective, has been to study how biogas actually behaves from the point of view of energy and the environment.



“If you digest manure, which is what about half of all biogas is produced from, the margins are as great as 30 percent. This means you avoid the methane emissions that otherwise are connected with traditional handling of manure,” says Maria Berglund.

According to Maria Berglund, other calculations that compare the environmental impact of biogas with different fuels have had a narrower perspective. For instance, they have been based on a specific biogas facility or a single raw material, like manure, energy crops, or household waste.

Another important conclusion is that there is in fact very considerable variation within the group termed biogas as regards environmental impact. One should therefore take care not to draw general conclusions about biogas, since the environmental impact depends a great deal on what raw material is used and what alternative uses are available for that source material.

“For example, household waste always has to be treated in some way. Treatment can involve anaerobic digestion, but composting and incineration are also possible. All forms of waste treatment have environmental impacts, though in varying degree, but this is seldom mentioned in the public debate,” Maria Berglund points out.

Nor do environmental analyses usually factor in what happens to the residual products after the anaerobic digestion is completed. Often analysts only look at how much gas a certain raw material generates and the environmental impact of biogas production as such.

“If we look at manure once again, it generates relatively little biogas in relation to other raw materials. On the other hand, digested manure has great potential as fertilizer. Digested manure works even better than undigested manure,” she says.

Kristina Lindgärde | alfa
Further information:
http://www.lth.se

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