CNET también está disponible en español.

Ir a español

Don't show this again

HolidayBuyer's Guide
Tech Industry

Powering cities on landfill waste

A start-up is opening a plant that will convert municipal waste into electricity through a cleaner process than burning. Photo: Turning waste into energy

Buried in your municipal dump are megawatts of electricity, a resource investors are throwing money and technology at to get.

As interest in unearthing alternative energy sources grows, the technique of converting municipal solid waste into electricity is getting another look and, in some cases, a 21st century makeover.

Start-up Ze-gen is in the process of securing $4.5 million in financing from venture capital firm Flagship Ventures, according to Ze-gen CEO Bill Davis. The money will be used to operate a pilot waste-to-energy facility in New Bedford, Mass., and to fund other projects. The facility is expected to be completed this month.

Using municipal waste to produce energy or reduce pollution is not an unusual practice. Landfill operators siphon off methane gas that is given off by decomposing organic trash. Incinerators burn garbage to produce steam that turns a turbine that makes electricity.

Ze-gen's plant, currently under construction, is an attempt to demonstrate that different technologies--in this case gasification--can be used to convert waste to energy in a cost-effective and less polluting way.

"Even if you don't care about the environmental considerations and just look at it from an economic standpoint, there is a lot of economic potential put into the ground which could be put to use," Davis said.

Davis is one of a wave of entrepreneurs trying to build commercial-scale alternative energy systems. And many entrepreneurs and investors, motivated by concerns about energy security and environmental sustainability, are revisiting old power-generation ideas, including waste-to-energy.

Davis calculates that the 300 million tons of municipal solid waste produced in the U.S. each year could generate tens of thousands of megawatts of power, worth $28 billion.

The process Ze-gen is testing is cleaner than incinerating trash and avoids the production of methane--a potent greenhouse gas--from landfills, said Jim Matheson, a general partner at Flagship Ventures.

"In all the hubbub about energy, clean tech and sustainability more broadly, we think waste has been overlooked both from the dimension of the energy value of the waste and the environmental impact," Matheson said.

Waste-to-energy technologies can be particularly compelling in places like New England and other metropolitan areas that generate a lot of trash, don't have the space for landfills, and have a high energy demand.

Waste not, want not
Ze-gen's New Bedford facility is designed to handle a specific kind of waste stream--the debris from construction and demolition sites, rather than household waste--which will bring in a more uniform input.

Rather than burn the construction debris, which looks like wood mulch once it's processed, it will pass through molten metal at a very high temperature. This gasification process chemically changes the trash to synthetic gas, or syngas, a combination of mostly carbon monoxide and hydrogen.

"We extrude it through a large mechanical feeder with a hydraulic feeder mechanism. Just imagine a tube of toothpaste, pushing a continuous stream below the surface of the (molten metal) bath," Davis explained.

Once captured, the syngas can be burned to power a steam turbine to make electricity. The facility will be able to process 10 to 50 tons of construction debris per day. Initially, the debris will be burned off rather than converted to electricity as the efficiency of the system is tested.

But a facility like this one has the potential to gasify 450 tons of waste and generate 30 megawatts of electricity a day--enough power for roughly 10,000 households--in addition to the 8 megawatts needed to run the plant itself.

There are now 89 waste-to-energy facilities in United States that burn landfill trash to produce power. In total, they create 2,700 megawatts of electricity, enough to power about 2.3 million homes, according to Lori Scozzafava, deputy executive director of the Solid Waste Association of North America. Three more facilities are now being considered, although no gasification plants are operating on a commercial scale, she said.

"There is growing interest in waste-to-energy because of the need for renewable energy, which it is, and because it's sustainable and it's indigenous," Scozzafava said. "It allows waste to be managed closer to where it's produced."

Other processes for converting waste to fuels include ethanol, methane from trash and manure, as well as biodiesel from animal fats. Many of these techniques are being tested to operate at a commercial scale, as part of the clean, or green tech, investment run-up.

Using municipal solid waste as a feedstock hasn't yet taken off because in the past there wasn't much interest in alternative fuels and people underestimated the material handling requirements, Ze-gen's Davis said.

A treatment center in New Bedford will pay Ze-gen a "tipping fee" to take its construction debris, which would otherwise have to be hauled somewhere else. But Davis said the process would be cost-effective even if a waste-to-energy facility operator wasn't paid. It's also conceivable that facilities would have to pay for waste in the future.

The process, if successful, could also qualify for renewable-energy credits or be used to sell carbon offsets, said Flagship Ventures' Matheson.

"Since we're reducing the carbon footprint of incineration and landfill quite dramatically, we think the (facility) should qualify for carbon offsets," he said.

By the end of the year, Ze-gen should have an idea of how its plant is performing, including the quality of the syngas, which varies depending on the process. The modular design of the plant means it can increase production quickly, assuming sufficient capital, and can be used with different types of garbage.

"Given that gasification is proven at a chemical level...it is going to be the next big thing (in waste to energy)," Davis said. "It's just a question of seeing who does it most efficiently."