Could we vaporise all this trash and get clean energy? (Image: Steve Wilkes / Getty)

From trash to gas

To burn or to zap?

     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 1.600°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.
     Keeping the system small and avoiding the expense of creating plasma makes it affordable for businesses to deploy: excluding the gas burner, the system costs $850,000 and, according to Haber, will pay for itself in four years through savings on electricity, heating and waste disposal charges.

Clean and green?
     Others say that chlorine can cause an additional problem in the extremely hot, oxygen-starved environment of a plasma gasification chamber. "If you pass mixed waste with chlorine in it through a plasma arc, you get metal in the [syn]gas that otherwise shouldn't be there," says Thomas Cahill, an emeritus professor of physics and atmospheric science at the University of California, Davis. These metal pollutants could escape into the environment when the gas is burned, he argues.
     Companies already running gasification systems point out that the process is as clean as you make it: what matters is how efficiently the syngas is scrubbed and how effectively the ash is disposed of. They also say that they operate to strict national or regional standards governing emissions from waste-to-energy power generation.
     "The regulations that they have to comply with are much more stringent and focus on a wider range of toxins than for a conventional power plant," says Marc Wolman of the Massachusetts Department of Environmental Protection in Boston. "If they don't meet these limits they get shut down, period."
     On the issue of dioxins, at least one waste-to-energy company is making reassuring noises. Andreas Tsangaris of the Plasco Energy Group in Ottawa, Canada, which has been running an 85-tonne-per-day waste-to-energy pilot plant since September 2007, says: "We remove virtually all the chlorine before combustion. There is no chance for dioxins to form." The company's own monitoring shows that its emissions, including those of dioxins and heavy metals, have remained at or below the most stringent regulatory limits in North
America and Europe.
     Nevertheless, a newspaper article by Cahill, based in part on his studies of emissions from the smouldering remains of the World Trade Center in New York - which he says are "eerily similar" to those from gasification plants - plus a strongly worded editorial by Saff, had a direct impact on two proposals for high-profile commercial plasma gasification plants in the US. A plant in St Lucie, Florida, has been scaled back significantly, partly in response to environmental concerns, and plans for a similar plant in Sacramento, California, have been delayed indefinitely.
     Another question mark over the green credentials of waste gasification concerns just how efficient these plants are at producing energy and minimising greenhouse gas emissions compared with other methods of waste disposal. A recent study by the Tellus Institute, an independent think tank based in Boston, compared gasification with landfill sites where methane is captured to be burned for energy. It concluded that while gasification produces six times as much energy per tonne of waste as landfill sites, landfills with methane recapture systems save two-and-a-half times as much CO₂ equivalent as the combination of gasification and syngas burning. The Tellus report also found that the energy saved by recycling a given amount of waste is 3.4 times the energy that can be produced through gasifying it.
     Some are opposed to gasifiers on principle. They say their very existence discourages efforts to tackle the garbage crisis at its source. "Once you build a gasifier, you have to feed it," says Tangri. "It creates a financial disincentive to do waste reduction and recycling."
     Ultimately, it may be some time before we realise the full effects, for good or bad, of zapping our rubbish. Few long-term independent studies have been carried out into emission levels, dioxin contaminants and the potential for toxins to leach out from waste ash. Nor is it clear how much energy can be created by gasifying various types of waste, or how reliable energy generation can be, given variations in the waste stream from day to day and in different parts of the world. So far, though, the indications are that gasification is neither the panacea for our waste and energy woes that some are claiming it to be, nor an environmental catastrophe waiting to happen.
     For Kevin Whiting of Juniper, a British waste-processing consultancy based near Dursley in Gloucestershire, the way forward may be on some kind of middle ground. "If there is a market for recyclables, we should recycle as much as is practicable and not take resources from our great-grandchildren," he says. "But if waste can't be recycled, it has an energy value. And the more energy you can generate [from it], the better."
Editorial: Waste gasification needs more light and less heat