Landfill gas is primarily composed of methane and carbon dioxide and contains many other minor components. When uncontrolled, risks posed by landfill gas include: flammability and explosion risks, asphyxiation risks, potential health impacts, odour impacts and climate change potential. To control these risks, the Environmental Protection Agency (EPA) requires that landfill gas is contained and collected to minimise migration off-site and that the collected landfill gas is utilised or, where that is not possible, flared in accordance with best practice.
Landfill gas is generally required to be managed by utilisation or flaring. However, where landfill gas levels are low, it may not be possible to sustain continuous flaring to 1,000șC using the standard type of flare employed at landfills. Standard flares typically cannot meet these flaring criteria where the level of methane in landfill gas falls below about 30% by volume. This is primarily an issue following closure of landfills as methane levels fall over time. The requirement to divert biodegradable municipal waste from landfills will make it an increasing issue during the operational phase of landfills in the future.
Landfill Gas Generation and Composition Over Time
Landfill sites accepting biodegradable waste generate landfill gas during waste decomposition. Generation rates as well as the composition of landfill gas vary throughout the landfill’s life. The waste decomposition process involves several stages during which different groups of bacteria break down complex organic substances such as carbohydrates, proteins and lipids into successively simpler compounds.
At the beginning of the degradation process, bacteria consume any oxygen within the waste and release mostly carbon dioxide, water and heat. Methane production only begins after anaerobic conditions have started in the waste, typically approximately 3 to 6 months after waste placement (EPA, 2010). During peak landfill gas production the bulk gas consists typically of 50 to 60% by volume methane and 40 to 50% by volume carbon dioxide. After all biodegradable substrate has been consumed, landfill gas production slows and the gas composition in the waste returns to normal atmospheric conditions.
Benefits of Managing Low Levels of Landfill Gases
- From a climate change perspective, methane is estimated to be 25 times more potent as a greenhouse gas compared with CO2 i.e. 1 tonne of methane is equivalent to 25 tonnes of carbon dioxide when considered over a 100-year time horizon. Therefore combustion of methane (to form carbon dioxide) can yield significant benefits in terms of minimising the causes of climate change.
- The primary components of landfill gas (methane, carbon dioxide, oxygen, nitrogen and water vapour) are odourless. However, landfill gas also contains a large number of trace components and a number of these are odorous (for example hydrogen sulphide, thiols, aldehydes etc.). Combustion of landfill gas significantly reduces its odour potential.
- Raw landfill gas contains methane and carbon dioxide in concentrations of approximately 60% by volume and 40% by volume respectively. The lateral migration of landfill gas and its subsequent accumulation is a potential acute hazard to receptors located in proximity to landfill sites. Experience has shown that lateral migration of landfill gas in concentrations likely to pose a hazard may occur at distances of up to 100 m in unconstrained geology.
One of the main (and most effective) techniques for the management of lateral migration of landfill gas therefore is effective gas extraction (EPA, 2011).
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