Another concern is that the waste gas from gasification may contain dioxins, which form when organic material is heated to high temperatures in the presence of chlorine-containing compounds, which are ubiquitous in municipal waste.
"Any attempt to turn garbage into energy will most likely cause the production of significant amounts of dioxin, which many consider the most significant carcinogen known to science," says Ron Saff, a physician in Tallahassee, Florida, and a member of Physicians for Social Responsibility.
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.