The Why Files The Why Files --

Putting decay to work: Making methane from trash

From dump to pump: "Green garbage" center opens in Ohio

When stuff rots but oxygen is absent, bacteria produce methane, which many landfills collect to fuel electric generators. But because landfill gas contains many contaminants, these generators can pollute the air.

If contaminants could cheaply be removed from landfill gas, this waste product could make a clean fuel for cars and trucks.

Large open lot by landfill with two gas pumps
Photo: Courtesy Firmgreen
This big landfill in Columbus, Ohio, is capturing part of the methane it generates to power a few vehicles, but there are big plans to expand the collection.

Having finished our preamble, we turn to a facility at the Columbus, Ohio, landfill that in September started cleaning landfill gas and storing the methane as compressed natural gas (CNG), which makes an excellent fuel for specially-equipped cars and trucks – a gasoline replacement made from pure garbage!

Diagram of refrigerated container that purifies gas
Using a low-temperature “washing” process, a landfill in Ohio has begun cleaning gas generated by rotting garbage. The gas is already being used in vehicles, and eventually for the natural gas pipelines.
Image: Courtesy Firmgreen

Rotten luck!

The cleaned methane is already being used to power a car and a truck owned by Solid Waste Authority of Central Ohio, the landfill’s owner; another eight vehicles are scheduled to be converted to CNG in the next couple of years.

White Honda Civic with powered by gas from trash logo
Photo courtesy of SWACO
This Honda Civic is one of two of SWACO’s compressed natural gas vehicles (CNG). Over the next three years, the company plans to convert a minimum of nine fleet vehicles to CNG, and hopes to interest state and city governments in adding CNG vehicles to their own fleets. According to SWACO, the CNG will be available at a gasoline equivalent rate of less than $3 per gallon, and even less if you’re willing to stop by their landfill to fill ‘er up.

Eventually, the authority plans to expand the process to capture the remaining 92 percent of the landfill gas, with the CNG going to more vehicles, or into a natural-gas pipeline. “We can produce the equivalent of 250,000 gallons of gasoline each year now,” says John Remy, director of communications for the waste authority. “In the second phase, we will have 3 million gallons equivalent.”

Why would a landfill operator be interested in extracting and cleaning methane? Because methane:

is an excellent (and merchantable) source of energy for heat, transport, or electricity;

can explode in the basements of neighboring houses if it’s trapped in the landfill; and

is about 20 times more effective than carbon dioxide in causing global warming.

Three methane wells connect to a buried horizontal pipe
Photo: Courtesy Firmgreen
Unlike carbon dioxide, which is soluble in water and therefore likely to waft up from the moist soil of a landfill, a great deal of less soluble methane remains trapped in the ground. In this cross section of a garbage dump, perforated pipes relieve a bloated trash mound of its excess methane. Here the invisible gas is depicted in a lovely shade of green.

Methane (CH4), the major component of natural gas, is a metabolic product of ancient bacteria called methanogens. Because methanogens thrive only when oxygen is absent, they are called anaerobes, and the decay process is called anaerobic digestion.

Pure brilliance

Although landfill gas has long been collected and burned, FirmGreen, Inc., which designed and installed the separator, claims advantages over competitors. According to Tony Wong, the company director of business development, the problem is purity -- all sorts of gases can emerge from a garbage dump, and the blend can vary from day to day -- yet to power vehicles or pump the gas into the natural-gas supply, the methane must be about 95 percent pure.

The cleaning process relies on cold carbon dioxide, says Wong. "When we go to minus 40 ° F to minus 60 ° F, something really elegant happens. The carbon dioxide that was already present in the gas -- we are not adding any solvent that later has to be removed -- starts to liquefy, forming a mist that magically falls through gravity,” and this mist removes contaminants. (Carbon dioxide is also used as a solvent to remove caffeine from coffee.)

Yellow loader  on mound faces towards green horizon
A front-end loader rolls over a vast mound of unsightly… energy? Though it remains a blight on the verdant landscape of the island of Kauai in Hawaii, landfills like this one could supply energy that alterative fuel proponents hope will help make the world a greener place.

Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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