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Home arrow Resources arrow Renewable Solutions arrow Biomass and Biogas
Biomass and Biogas E-mail
Tuesday, 13 March 2007
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Animal manure can be processed in an anaerobic digester to create natural gas as well as solid and liquid agricultural fertilizer. (Photo credit: Mike Locke)

People have been burning garbage for a long time, and until recent technology and new systems became available, it was generally a bad idea. Open-pit trash burning gave way to municipal solid waste incinerators and so-called “waste-to-energy” facilities, but all of them involved the burning of general household, industrial, and even medical garbage – spewing toxic chemicals like dioxins and PCBs into the air.

Biomass gasification and anaerobic digesters are a totally different way of turning waste materials into fuels for energy – and with state-of-the-art pollution control devices, they are almost completely clean and renewable sources of electricity.


Biomass Gasification

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All biomass (paper, actricultural waste, wood scraps, etc.) can be combusted to generate electricity. (Photo credit: Leopoldo, Flickr.com)

A biomass gasification facility turns plant biomass, including yard and garden waste, vegetable scraps, recycled paper, agricultural waste, and wood refuse from construction, into a pressurized gas mixture. This pressurized gas is then passed through a cleanup system to remove particulate matter and tar vapor, and finally used to turn turbines in a power plant. The remaining gas is a mixture of carbon monoxide, hydrogen, water vapor, and carbon dioxide. Finally, the particulates and tar are put through a process called pyrolysis, which converts them to carbon monoxide.

There is still research being conducted to improve biomass gasification, as there are still some drawbacks with the process. Hydrogen can be removed and stored for use in hydrogen fuel cells. There is also the problem of carbon monoxide emissions. And as mentioned above, biomass gasification does produce some carbon dioxide, although because it is from a renewable organic source it has a short atmospheric residence time and therefore does not contribute to global warming.

Overall, biomass combustion provides a renewable, steady, gap-free flow of electricity to the power grid.

Anaerobic Digestion

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Biogas, a renewable form of natural gas, can be burned for heat or electricity. (Photo credit: Ian Anthony)

Anaerobic digestion is a process that turns plant biomass, including yard and garden waste, vegetable scraps, recycled paper, agricultural waste, and wood refuse from construction, into methane (natural gas). This renewable gas fuel is called “biogas.” The gas is then burned for electrical generation or captured for household use in stoves, gas fireplaces, etc. Production of renewable methane can allow old coal plants to be converted to natural gas. An anaerobic digester is also an excellent component of a cogeneration facility that provides both heat and electricity.

Byproducts of anaerobic digesters include hydrogen, hydrogen sulfide, carbon dioxide, and acidogenic and methanogenic digestate. Acidogenic digestate is a solid waste composed of chitin and lignin (the “woody” parts of plants) and makes an excellent compost. Methanogenic digestate is a nutrient-rich liquid that can also be used as an agricultural fertilizer, depending on the quality of the materials put into the digester.

There are some drawbacks to anaerobic digesters, mainly relating to the type of feedstock used. If the feedstock is full of toxic chemicals (such as pesticide, wood preservatives, etc), then it will be highly concentrated in the digestate. If the digestate cannot be used as a fertilizer, it becomes waste once more.

Because anaerobic digesters are relatively low-tech compared to some other forms of electricity generation, they provide an excellent opportunity for developing countries to sustainably enhance their economies and social infrastructure.


Click here to read about Prairie Biofuels, an organization that is working to raise awareness of how biofuels could simultaneously stop global warming and restore the tallgrass prairie, a threatened ecosystem.

Sources:

Wikipedia, the free encyclopedia (articles on Biomass and Anaerobic digestion)
The State of Oregon - Department of Renewable Resources
US EPA - Biomass Utilization and Atmospheric Protection