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生活ゴミでエネルギ問題を解決する（New Scientists）
生活ゴミをメタンなどの燃えるガスに変換する技術とのこと。
http://www.newscientist.com/article/mg20227051.500-zapping-rubbish-into-ecofuel.html?DCMP=NLC-nletter&nsref=mg20227051.500
Could your trashcan solve the energy crisis?
AT FIRST glance, 303 Bear Hill Road in Waltham, Massachusetts, doesn't look like the scene of an environmental revolution. But packed into a shipping container in the car park of this modest suburban commercial building is a compact piece of technology that its maker IST Energy insists can turn even the filthiest waste into clean, green energy. "Trash will move from being a liability to an asset, providing a clean source of energy that can be used right where it is produced," says Stuart Haber, the company's CEO.
IST is not alone in this revolution. It is one of a growing number of companies and research groups around the world working on gasification - a process that zaps household waste into energy and which, its advocates say, produces few or no harmful emissions. Yet as pilot gasification plants begin to spring up around the world, this apparent environmentalist's dream is not being universally welcomed. Opponents argue that the process is far from clean and that its track record in terms of energy efficiency and emissions can hardly be considered green. Not to mention the fact that it encourages the throwaway society that the environmental movement has been trying so hard to get rid of. So what is the real story? Is vaporising trash the answer to our energy and waste-disposal woes, or an environmental wolf in sheep's clothing?
The idea of converting waste into energy has been around for decades. Heat from garbage-fuelled incinerators can generate steam that drives a turbine that in turn drives an electrical generator. Now fears over energy security and climate change, combined with the rising cost of dealing with the world's waste, are raising the possibility of disposing of household trash using higher-energy methods once reserved for hazardous materials such as medical waste and asbestos.
Gasification, and its cousin plasma gasification, involve heating waste to a high temperature inside a sealed chamber. This is done in the near absence of oxygen, so organic components in the waste do not burn but instead reform into syngas, a mixture of carbon monoxide and hydrogen. This can be filtered and chemically "scrubbed" to remove toxic particles and gases, and then burned to produce energy or converted into other fuels such as methane, ethanol or synthetic diesel. All that's left to dispose of at the end is ash, dirty filters and chemicals from the scrubbing process, which can be treated and sent to landfill or into the sewers.
Gasification yields more energy per volume of trash than incineration, but the possibilities don't end there. Adding an arc of superheated plasma to the mix can increase that yield further. Plasma gasification vaporises waste at much higher temperatures - up to 10,000 °C compared with up to 1600 °C for normal gasification - which ensures that more of the organic waste is gasified.
In this kind of gasification, plasma arcs are created by passing a high-voltage current through a chamber filled with an unreactive gas such as nitrogen (see diagram). As the current flows through the enclosed space, it tears electrons from the gas to form a superheated plasma that rips apart the molecules in whatever is fed into the chamber. "It's like a continuous bolt of lightning that disintegrates almost anything that crosses its path," says Daniel Cohn of the Massachusetts Institute of Technology, who has been working on plasma gasification since the 1980s and now sits on the board of InEnTec, another waste-to-energy company.
Plasma gasification is like a continuous bolt of lightning that disintegrates almost anything in its path
A further advantage of this technique is that the very high temperatures cause the waste to end up not as fine ash but as a glassy solid, which could in principle be used as filler in the construction industry. And while the power required to run InEnTec's pilot plant in Richland, Washington, amounts to one-third to half of the power it produces, Cohn insists that the process is financially viable. He says syngas can be converted to ethanol and synthetic diesel at costs that can compete with petroleum-based equivalents. "We think we can produce fuel at a cost of about $2 a gallon of gas equivalent," he says. If he's right, trash could become the new oil.
Pilot gasification plants are being set up at various sites in the US, Canada, France, the UK and Portugal, most of them using the plasma technique. Japan already has two commercial plasma plants, but these are focused primarily on simply disposing of household waste rather than generating energy from it.
While these new plants will all be large installations, IST Energy believes that small is the way to go. Its container-sized non-plasma GEM system (short for Green Energy Machine) can convert almost 3 tonnes of municipal waste a day into enough syngas to heat and power an office building holding 500 people.
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