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In a dry, windy canyon not far from San Francisco, landfill operator Waste Management and German gas refiner Linde are using technology typically found at massive natural gas fields to turn trash into liquid natural gas.

The natural gas is then trucked to dedicated filling stations where it fuels specially converted garbage trucks, eliminating the need for diesel fuel and keeping tens of thousands of tons of greenhouse gases out of the atmosphere each year.

Driving a truck across the top of Altamont Landfill and Resource Recovery Facility's 237 acres of trash just outside Livermore, California, Ken Lewis, director of landfill operations, explains that there are 300 feet (91 meters) of trash—45 million tons of waste—buried underneath. “We own this big pile of trash, and we need to look at it and think, ‘what can we do to get something out of it?’ ” Lewis says.

One solution lies in the decomposition process itself. Over time, the organic waste in the landfill—anything from wood to old food—is consumed by naturally occurring bacteria, producing methane. There’s enough garbage at Altamont to keep the bacteria going for more than 30 years.

A Garbage Mine

All that methane is usually burned as waste gas, to prevent explosions or air pollution. But as demand for alternative fuels has increased, “biogas” generated from human trash and sewage is increasingly viewed as an important resource.

By drilling holes in the mountain of trash, landfill operators can literally mine garbage for the flammable gas. Beginning in the 1980s, American landfills installed small electrical plants that used the methane from garbage to generate power. At Altamont, a 23-year-old facility already generates 6.5 megawatts of electricity, enough to power 8,000 homes.

Still, much of the landfill’s methane goes unused. Waste Management began working with German gas refiner Linde in 2003 on an idea to miniaturize the type of plant usually found at natural gas fields in places such as Alaska and Siberia, scaling it down to handle a tenth of the typical natural gas well’s output without losing too much efficiency.

“Liquefaction of methane has been around for decades,” says Steve Eckhardt, Linde North America’s head of business development for alternative energy. “The challenge here is doing it at a small scale.”

The result is a sun-baked set of compressors, pipes, filters and condensers surrounded by a high chain-link fence. A black pipe two feet (two-thirds of a meter) wide pumps in raw landfill gas, which is about half methane. The other half is a mixture of carbon dioxide, nitrogen, sulfur, and the stinky alcohols and other compounds that make up what Lewis gently calls “landfill perfume.” A set of scrubbers separates out the methane; the rest is burned in a giant smokestack.

The methane is then super-cooled to -260º F (-162ºC) and stored in vacuum-insulated tanks—like giant Thermos bottles—until trucks arrive to move it to local fueling stations. There, it’s pumped into garbage trucks that have been converted to run on liquid natural gas, a clean-burning alternative to diesel.

Overall, Linde estimates the $15.5 million Altamont LNG plant, now approaching its first anniversary, will be keeping 30,000 tons of carbon dioxide out of the atmosphere each year. Even the energy burned to chill the methane, often cited as a major drawback of LNG as a green fuel, comes from the landfill’s methane-fired power plant. The carbon footprint of a garbage truck running on liquid natural gas from Altamont, project officials say, could easily be smaller than that of a plug-in electric car.

Closing the Loop

Because it’s so complicated to transport and store the super-cooled methane, liquid natural gas has limited uses. “It’s not something that’s going to let everyone fill up their car,” says University of Oklahoma chemical engineering professor Richard Mallinson. “But for trucking, where you have a centralized refueling station and you know the routes, natural gas makes sense.” Gas from Altamont’s trash is trucked to cities in the Bay Area and southern California, fueling 350 garbage trucks in cities like Oakland, Long Beach and El Cajon.

Linde and Waste Management hope the work they’ve done to create a small-scale liquid natural gas plant can be used in other places across the country. But not every trash pile is ripe for an LNG plant. The landfill has to be large, to produce enough gas to make the cost of the facility worthwhile, and have the right temperatures, moisture, and type of garbage to support methane-emitting bacteria. “You need a lot of waste, and the right kind of waste,” Lewis says. “There are hundreds of opportunities out of the thousands of landfills in the United States.”