The process takes solid material and heats it to temperatures up to 1,400 Celsius (2,552 F) -- far hotter than an incinerator -- which causes most matter to shift into a gaseous state. That gas is then converted into a synthetic fuel called syngas, largely free of pollutants, that can be burned in an electricity-producing turbine.

The materials that do not convert to gas, including some metals and minerals, shift to a liquid state and when they cool turn into slag, a stable rocklike substance. Slag's stability means its contents do not leach out into their surroundings, so it could be safely used in construction material.

The challenge is how to take a process that works with a uniform input -- coal -- and make it run smoothly with the hodgepodge of materials that make their way into a garbage truck.

"We're really trying to understand the variability that is in municipal solid waste," said Kelly Fletcher, advanced technology leader in sustainable energy at GE's research centre in Niskayuna, New York.

"Not to be cute about it, but garbage in, garbage out," Fletcher said in a phone interview. "We have to really understand what it is that our gasification system is going to get, in terms of the feedstock."

Environmental groups have long opposed incinerating waste -- which releases polluting gases into the atmosphere and creates ash that can be hazardous -- but some are open to the idea of gasifying municipal solid waste.

"We are open to technologies that would deal with MSW in a way that doesn't have the downsides of incineration and created a useful product," said Dave Hamilton, director of global warming and energy at the Sierra Club, in Washington. "We're interested in looking at it."

Many companies around the world, including Waste Management Inc the largest US trash hauler, already produce energy from garbage by capturing the methane gas emitted by decomposing trash in landfills and burning it.

The gasification approach cuts out the landfill -- a key concern in crowded urban areas -- and prevents the trash from decomposing and producing methane, which has more than 20 times the heat-trapping potential of carbon dioxide.

"If you can intercept it from turning into methane, then doing something else with it is probably a better route," said Scott Sklar, president of The Stella Group, a Washington-based green energy consulting and design firm.

Energy experts said there are no garbage-to-energy gasification plans currently operating in the United States, although privately held Plasco Energy Group late last year opened a site in the Canadian capital of Ottawa, which is capable of processing 100 tonnes of municipal trash a day.

Cities in Florida, California, Louisiana and Michigan are contemplating or planning waste gasification facilities.

Fletcher estimated that GE is about five to ten years away from making garbage gasification a paying business.

(For more coverage on the business of waste, click on http://www.reuters.com/news/globalcoverage/recycle)