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Making the best of garbage gas

Methane generated by rotting rubbish in landfill dumps could make a far greater contribution to the world's energy supply. A new way of harvesting the gas should mean that many landfill dumps that till now were thought to be too small to produce usable amounts of the gas will be able to provide a viable supply.

In Europe alone, landfill has the potential to generate as much as 94 billion cubic metres of methane each year. Yet according to the European Commission's energy directorate only about 1% of this is being tapped. The rest of it goes to waste, as landfill operators burn it off to prevent a build-up of dangerous quantities of the flammable gas.

As well as wasting energy, flaring off methane pumps pollutants into the environment. These are caused by impurities in the methane and the low temperature of the flare, says Greg Miller, who set up a firm called SusBus in 1998 in Durham, UK, to look for ways to use methane from landfill gas as fuel for buses.

Methane forms in landfill when organic waste decomposes in the absence of oxygen. At some landfills the gas is collected and used to power vehicles or to heat nearby buildings. Till now this has been practical only for landfills that produce large volumes of methane. But Viktor Popov at the Wessex Institute of Technology, in Southampton, UK, says simple modifications to existing landfills will make it possible to extract methane from any site.
No oxygen required

Methane is usually extracted by sinking pipes or wells into the landfill and sucking the gas out. But if the surroundings are not airtight, sucking out methane also sucks in air. This is not only difficult to separate from the methane; it also means methane production slows down. "You don't want oxygen in there because that would prevent anaerobic digestion," says Popov. As a result the only landfill sites suitable for methane extraction are those that are large and deep enough to restrict the entry of air.

Popov's solution is to cover landfills with a membrane that prevents air contaminating the methane. The membrane consists of three layers: a middle permeable layer sandwiched between two relatively impermeable layers, which would probably be made of clay.

The plan is to pump carbon dioxide, which can itself be extracted from the landfill gas, into the permeable layer so the CO2 is slightly above atmospheric pressure. This creates a barrier that prevents air being drawn into the landfill. As the methane is sucked out of the ground CO2 is in turn sucked into the landfill from the membrane.

"It's an ingenious solution," says Miller. "The biggest stumbling block in getting this methane is to remove the nitrogen," he says. The greatest potential for this new system is in North America, where landfills are still a common way of disposing of rubbish. In Europe the long-term plan is for landfills to be phased out, though this is not likely to happen for some time.

But even when rubbish is no longer being buried in landfills, old dumps will continue to be sources of energy. "After closure a landfill will continue to produce methane for 15 to 20 years," says Popov.

Journal reference: Renewable Energy (vol 30, p 1021)

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