Landfill gas

Landfill gas is a complex mix of different gases created by the action of microorganisms within a landfill.

Production

Landfill gas production results from chemical reactions and microbes acting upon the waste as the putrescible materials begins to break down in the landfill. The rate of production is affected by waste composition and landfill geometry, which in turn influence the bacterial populations within it, chemical make-up, thermal characteristics, entry of moisture and escape of gas.

The spatially heterogeneous nature of most landfills means that there will be a wide range of physical conditions and biological ecosystems co-existing simultaneously within most sites. This heterogeneity, together with the frequently unclear nature of the contents, makes landfill gas production more difficult to predict and control than standard industrial bioreactors for sewage treatment.

Due to the constant production of landfill gas, the increase in pressure within the landfill (together with differential diffusion) causes the gas's release into the atmosphere. Such emissions lead to important environmental, hygiene and security problems in the landfill. Several accidents have occurred, for example at Loscoe, England in 1986, where migrating landfill gas, which was allowed to build up, partially destroyed the property. An accident causing two deaths occurred from an explosion in a house adjacent to Skellingsted landfill in Denmark in 1991. Due to the risk presented by landfill gas there is a clear need to monitor gas produced by landfills. In addition to the risk of fire and explosion, gas migration in the subsurface can result in contact of landfill gas with groundwater. This, in turn, can result in contamination of groundwater by organic compounds present in nearly all landfill gas.

Landfill gas is approximately forty to sixty percent methane, with the remainder being mostly carbon dioxide. Landfill gas also contains varying amounts of nitrogen, oxygen, water vapour,hydrogen sulphide, and other contaminants. Most of these other contaminants are known as "non-methane organic compounds" or NMOCs. Some inorganic contaminants (for example mercury) are also known to be present in landfill gas. There are sometimes also contaminants (for example tritium) found in landfill gas. The non-methane organic compounds usually make up less than one percent of landfill gas. In 1991, the US EPA identified ninety-four non-methane organic compounds including toxic chemicals like benzene, toluene, chloroform, vinyl chloride, and carbon tetrachloride. At least forty one of the non-methane organic compounds are halogenated compounds (chemicals containing halogens: typically chlorine, fluorine, or bromine). General options for managing landfill gas are: flaring, boiler (makes heat), internal combustion engine (makes electricity), gas turbine (makes electricity), fuel cell (makes electricity), convert the methane to methyl alcohol, clean it enough to pipe it to other industries or into natural gas lines.

The Environmental Protection Agency estimates that there are approximately six thousand landfills in the United States. Most of these landfills are composed of municipal waste, and, therefore, producing methane. These landfills are the largest source of anthropogenic methane emissions in the United States. These landfills will contribute an estimated four hundred and fifty to six hundred and fifty billion cubic feet of methane per year (in 2000).

Monitoring

Main article: Landfill gas monitoring

Some of the gases produced by landfills are hazardous and monitoring techniques have been developed. Flame ionization detectors can be used to measure methane levels as well as total VOC levels. Surface monitoring and sub-surface monitoring as well as monitoring of the ambient air is carried out. Under the Clean Air Act of 1996, it is required that many large landfills install gas collection and control systems, which means that at the very least the facilities must collect and flare the gas. The Environmental Protection Agency estimates that another 600 landfills could support gas to energy projects. The Environmental Protection Agency has also established the Landfill Methane Outreach Program. This program was developed to reduce methane emissions from landfills in a cost-effective manner by encouraging the development of environmentally and economically beneficial landfill gas-to-energy projects.

Federal regulations under Subtitle D of RCRA formed in October 1979 regulate the siting, design, construction, operation, monitoring, and closure of MSW landfills. Subtitle D now requires controls on the migration of methane in landfill gas. Monitoring requirements must be met at landfills during their operation, and for an additional 30 years after. The landfills affected by Subtitle D of RCRA are required to control gas by establishing a way to check for methane emissions periodically and therefore prevent off-site migration. Landfill owners and operators must make sure the concentration of methane gas does not exceed 25% of the EL for methane in the facilities' structures and the LEL for methane at the facility boundary.

Landfill gas migration

Landfill gas migration due to pressure differentials and diffusion can occur. This can create an explosion hazard if the gas reaches sufficiently high concentrations in adjacent buildings.

Landfill gas use

Main article: Landfill gas utilization

The gases produced within the landfill can be collected and flared off or used to produce heat or electricity. The City of Sioux Falls, South Dakota installed a landfill gas collection system which collects, cools, dries, and compresses the gas into an 11-mile pipeline. The gas is then used to power an ethanol plant operated by POET Biorefining.

The number of landfill gas projects, which convert the methane gas that is emitted from decomposing garbage into power, went from three hundred and ninety nine in 2005, to five hundred and nineteen in 2009 according to the Environmental Protection Agency. These projects are popular because they control energy costs and reduce greenhouse gas emissions. These projects collect the methane gas, which is released with twenty times the global warming potential of carbon dioxide, and treat it, so it can be used for electricity or upgraded to pipeline-grade gas. These projects power homes, buildings, and vehicles.

Waste Management uses landfill gas as an energy source. Their landfill gas-to-energy projects create enough energy to power four hundred thousand homes every day. This energy production offsets almost two million tons of coal per year. These projects also reduce greenhouse gas emissions into the atmosphere. Waste Management currently has 110 landfill gas-to-energy facilities.This is a good substitute of natural gas and run the vehicles more efficiently.

Opposition

Large projects often cost millions of dollars. Some environmental groups claim that the projects do not produce renewable power because trash (their source) is not renewable. The Sierra Club opposes any government subsidies for them. The Natural Resources Defense Council (NRDC) believes that government incentives should be directed more towards solar, wind, and energy-efficiency efforts.

Environmental effects

Landfill gases have an influence on climate change. The major components are CO₂ and methane, both of which are greenhouse gas.

Microbial oxidation

When landfill gas permeates through a soil cover, a fraction of the methane in the gas is oxidized microbially to CO₂.

See also

• Anaerobic digestion
• Biogas
• Biodegradability
• Flue gas
• Relative cost of electricity generated by different sources
• Underground coal gasification

External links

• "Primer on Landfill Gas as "Green Energy"". Energy Justice Network. http://www.energyjustice.net/lfg/. Retrieved 2010-04-25. 
• Koch, Wendy (2010-02-25). "Landfill Projects on the rise". USA Today. http://www.usatoday.com/money/industries/energy/2010-02-24-landfill-energy_N.htm. Retrieved 2010-04-25. 
• "Landfill Gas to Energy". Waste Management. http://www.thinkgreen.com/landfill-gas-to-energy. Retrieved 2010-04-26-2010. 
• "Landfill Gas". Gas Separation Technology LLC. http://www.gassep.com/lfg.htm. Retrieved 2010-04-26. 
• "Landfill Gas Control Measures". Agency for Toxic Substances & Disease Registry. http://www.atsdr.cdc.gov/hac/landfill/html/ch5.html. Retrieved 2010-04-26.