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Waste to Energy Technologies
By John Rubino
 
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Waste to Energy

Burning trash to generate electricity-or just to get rid of it-is already a common practice. Incinerators are everywhere, and according to the Solid Waste Association of North America, there are now 89 waste-to-energy facilities in United States that burn landfill trash to produce power. In total, they generate 2.7 gigawatts of electricity, enough to power about 700,000 homes. This is renewable energy, but by and large, it's not green. Despite tightening emissions rules, the typical incinerator still puts coal plant levels of CO2 into the air, along with unacceptable amounts of industrial chemicals from the materials being burned. Some greener variations are more interesting:

Reverse Polymerization

Put paper, old tires, plastic, or pretty much any other kind of common trash into a chamber filled with nitrogen rather than oxygen, then zap it with microwaves, and something interesting happens. The material heats up, but without oxygen it can't burn. Instead, molecular bonds break, converting the materials to simpler compounds of lower molecular weight-without releasing dioxins or other toxins. The process exceeds the tightest European emissions standards and produces oils and carbon residue that can be used as fuel. Canadian firm Environmental Waste International is marketing a reverse polymerization process for several different waste streams, including medical waste (it recently sold one machine to the pharmaceutical firm Abbot Labs) and tires.

Gasification

Forest waste can be turned into syngas, which is either processed into other fuels or burned directly to run a turbine. Massachusetts start-up Ze-gen is trying something similar with debris from construction and demolition sites, passing the material through molten metal, which dissociates it into hydrogen and carbon monoxide. The syngas is then burned to power a steam turbine, which produces more than enough power to run the plant. In theory, the process produces far less greenhouse gas than either incinerating the trash or allowing it to become methane in a landfill. If Ze-gen's plant works as promised, it (and about a thousand other entrepreneurs with similar ambitions) will extend the process to other waste streams, turning landfills and transfer stations into power plants and garbage into cheap energy.

Biogas Harvesting

When you bury organic material-or dump it in ponds, as livestock farms tend to do-it doesn't just sit there. Bacteria feast on it, multiply, and excrete a combination of methane and carbon dioxide. Landfills, waste treatment plants, and pig farms all produce gas in this way. Capture it and use it to run a turbine, and you've got yet another source of carbon-neutral power. Already, the United States has over 400 landfills with gas-to-energy plants, with 50 more under construction as of early 2008. And many more are coming. North Carolina has over 1,100 farms with enough pig manure to justify a power plant, while California has over 900 sufficiently large dairy farms.

Plasma

Run an electrical current through a container of ionized gas and you get 30,000-degree lightning in a bottle. When tires, old chemical weapons, scrap metal, or pretty much anything else is exposed to this kind of heat, molecules break apart into constituent atoms, producing a few pieces of glassy stone and syngas that can be used to power the machine. The theoretical value of such a process for the waste disposal business is obvious: Just create a big plasma-torch chamber, feed in the trash, and poof-that's the end of the garbage issue. Obviously it's not that easy, or there would be no need for this article. But plasma's fans claim that it does indeed have this kind of potential-if the cost can be lowered a bit. Plasma-torch disposal is currently twice as expensive as incineration or landfill disposal in most places. So it's only economical where landfill space is extraordinarily tight and air quality standards too stringent for incineration. Parts of Europe and Japan qualify on both counts, and plants are operating there and doing what they promise. Meanwhile, labs around the world are researching ways to lower plasma's cost.

 
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