Turning Trash into Electricity

Recently I had the opportunity to tour the Covanta “waste-to-energy” facility in Montgomery County, Md., where I saw first-hand how one county’s trash is converted to electrical power, economically and practically pollution-free.

Montgomery County’s 1,800 tons of garbage per day are collected at a transfer station and shipped to the Resource Recovery Facility (RRF) in huge rectangular steel waste containers loaded onto rail cars. These cars, on a dedicated rail line, pass through radiation detectors to prevent contamination from radioactive sources in discarded medical equipment or industrial gauges.

Once the trash arrives at the plant, trucks transfer it to a holding pit, dumping it into a covered pile that reaches three or four stories high. From steel cross-beams overhead, two 10-foot claws slowly open as they descend on the stinking gray mass, dropping, closing and pulling up giant “handfuls” of ripped plastic bags. Rising to the roof, the claws slide to the edge of a feed chute, open and dump their contents into to a roaring furnace.

Trash into steam; steam into electricity

And that’s when the real fun begins. Using a sophisticated air circulation and moving grate design, the trash moves slowly down the grate and burns, heating water in steel tubes in the boiler. The heated water continues on through a superheater, where it’s converted into steam. The steam pushes the fins of a giant turbine, creating electricity.

Afterwards, the cooled steam flows through a state-of-the-art air pollution control system, where 99 percent of the toxic gases, such as sulfur dioxide and hydrogen chloride, are trapped, and tiny dust particles are filtered out.

Meanwhile, huge chunks of solid scrap metal that didn’t burn get recycled and reclaimed for other uses.

Trash volume reduced 90%

The only thing left is ash, reduced to about 10 percent of the original volume of trash. This gritty gray waste material gets loaded into enclosed containers and hauled by dedicated rail to a lined landfill, where rain can’t leach down through it and contaminate the groundwater.

The Montgomery County RRF generates up to 63 megawatts (MW) of electricity. That’s plenty of power for its own energy needs, with enough left to export to the county’s electrical grid, making a profit for the privately owned facility.

My only question is: Why isn’t every county in every state in every country doing this?

Holly Martin writes about science and technology for business and non-profit clients in advanced materials, biotech, energy, manufacturing, nanotech, and R&D.

Note: This post originally appeared on the Suite101.com Engineering Blog.

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