Everyday rubbish could provide a valuable source of energy thanks to a clever new invention. Smelly methane（沼气）gas expelled from landfill sites containing tones of rotting household waste could be used to produce a valuable energy resource. In Europe alone land fill tips produce 94 billion cubic meters of methane per year but currently only one percent of what could be a valuable source of energy is used, the rest is burnt off to prevent an explosive build up of gas. Methane gas forms when organic matter breaks down and is drawn off from large rubbish tips by pipes sunk into the middle of the waste. Until now this process wasn’t possible on small sites because the pipe allowed oxygen to seep in and that stop the breakdown process. Viktor Popov of the Wessex Institute of Technology has now developed a system to modify existing landfill sites by making them airtight while the methane is extracted from sunken wells. The site is covered by a special three-layered skin whose two outer layers are made of clay with a pressurized（加压的）layer of carbon dioxide gas in the middle. As the methane is sucked out through a central well it is replaced by carbon dioxide from the middle layer rather than oxygen, allowing the breakdown process to continue. Unfortunately there are no plans to use the power of methane even though Popov claims his invention could provide a valuable source of energy over the next 20 years. 小题1:The passage mainly introduces ______ to us.
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