the generators burning methane to produce electricity at the Prince William County landfill are housed in a simple industrial building
Landfills are rich in organic material, thanks to the garbage discarded from households and the yard waste tossed into the garbage. Grass clippings, residue in pizza boxes, and the contributions of cats to litter boxes all end up being buried in solid waste landfills.
Today, landfills are carefully designed in cells, with geotextile fabrics and impervious liners to contain the material deposited by trash trucks. A solid clay cap on top of a closed landfill cell will keep rainwater out, and liners underneath will keep groundwater out, but decay of the waste will generate garbage juices ("leachate) within the cell. Modern cells can contain such liquids indefinitely.
Bacteria inside the cell will also generate gases through decomposition of organic material. In addition, discarded materials may vaporize and chemical reactions within a landfill may generate gases. While landfill gases are roughly 50% methane and 50% carbon dioxide, other minor components can have significant impacts. Old landfills, without impervious caps, may literally stink up a neighborhood with sulfides, ammonia, and volatile organic compounds. Landfills that accept construction and demolition debris will have gypsum wallboard, and the sulfur in the calcium sulphate (CaSO4) can result in a "rotten egg" smell.1
The pressure must be released before the clay cap ultimately fractures and "pops." As solid waste is placed within a landfill, a framework of porous plastic pipes is laid down at different levels. When the clay cap seals a cell, the pipes are connected to a vent through the cap. Gas builds up within the cell, migrates to the pipes, and is vented to the outside where it is "flared" (burned).
methane extracted from decomposing organic material in a landfill is burned to reduce explosion risks and to minimize the impact on global warming
Burning the methane extracted from landfills ensures it does not accumulate and form an explosion risk. In addition, methane is a greenhouse gas with greater impact on global warming that carbon dioxide. Burning methane converts the CH4 to CO2.
Methane is also a potential energy source, though the gas generated at landfills has roughly half the energy value of natural gas extracted from underground wells. Large landfills can install small microturbines or fuel cells to burn the methane and generate electricity, in a process that can also minimize odors and reduce global warming. Revenue from sale of the electricity can be used to offset a portion of landfill operating costs. 2
The regional landfill in Suffolk, operated by the Southeastern Public Service Authority, developed a system to utilize landfill gas in the 1990's. Methane was collected at the landfill, then piped 2.3 miles to the Ciba Specialty Chemicals plant (later purchased by BASF) to be burned in the facility's boilers. Starting in 2011, landfill gas was also used to generate electricity sold to the local utility, further reducing pressure at the clay cap and ensuring compliance with Clean Air Act regulations.3
the South Hampton Roads regional landfill (green circle) pipes methane to the BASF (formerly Ciba) chemical plant on the Nansemond River (red circle)
Source: ESRI, ArcGIS Online
Municipal solid waste can also be compressed into Refuse Derived Fuel (RDF) pellets and burned to heat water into steam, which spins turbines to generate electricity. Only large landfills generating sufficient methane can justify the investment required to generate electricity. As of 2013, the Virginia Department of Environmental Quality (DEQ) identified 14 electric generating plants using municipal solid waste and landfill gas as a fuel, all located near the urban areas of Northern Virginia, Richmond, or Hampton Roads.4
Fauquier County has been the most ambitious of the small projects, creating a $3 million, 2-megawatt (MW) merchant power plant in 2004 without grants, subsidies, or tax credits as a precursor to a planned biorefinery, intended to help the community achieve energy independence. The American Recovery and Reinvestment Act (ARRA) then provided over $825,000 to expand the Corral Farm Landfill project.5
electric generating plants using municipal solid waste (MSW) or landfill gas for fuel
Source: Virginia Department of Environmental Quality (DEQ), Electric Generating Plants Currently Operating in Virginia (as of March 25, 2013)
Smaller projects can utilize landfill gas for a variety of purposes, including evaporation of leachate to reduce its volume. The Environmental Protection Agency (EPA) list of electricity-generating sites is broader than the Virginia Department of Environmental Quality list. EPA has identified 30 projects in Virginia using landfill gas at one time or another, including the projects generating electricity from landfill methane.
In Hanover County, a closed landfill generated only a low flow of gas, but it was sufficient for a burner system water-based paint into a dry powder for deposit in the new landfill. EPA also noted that the Frederick County/City of Winchester project to generate electricity at the landfill, triggered in part by community concerns about the development of new power transmission lines bring electricity from distant plants, would recover the public investment in 5-9 years.6
Landfill Gas Energy Projects identified by EPA, including direct utilization of gas as well as generatiion of electricity
Source: Environmental Protection Agency (EPA), Energy Projects and Candidate Landfills
In Prince William County, methane is used to generate electricity, to heat the Fleet Maintenance Building, and also as fuel for the nearby animal shelter. Animals not adopted within the shelter's timeframe are euthanized, and methane from the landfill offsets propane used in the incinerator to dispose of carcasses.7
no money was wasted on esthetics when constructing the original metal structure housing the electrical generator and the flare gas stack at the Prince William County landfill
Source: Prince William County, Landfills as Renewable Energy Sources
terraces and drainage ditches minimize erosion on the top of clay caps, while gas collection systems prevent pressure generated within the cell from creating cracks in the clay caps