What is Landfill Gas?
Landfill gas (LFG) is created from the decomposition of organic matter buried in the landfill. Typically LFG has a BTU value ranging from 500 – 600 BTU SCF. It primarily consists of:
- Methane (CH 4 ) 40 – 60%
- Carbon dioxide (CO 2 ) 40 – 60%
- Nitrogen (N 2 ) 2 – 5%
LFG also contains a number of trace gases which are responsible for the odors associated with LFG since the primary gases are odorless. Some of the trace gases most frequently detected are:
- Hydrogen sulfide(H 2 S)
- Non-methanogenic Organic Compounds (NMOCs) such as: Vinyl Chloride, Hexane,oluene, Benzene, and Xylenes)
These trace gases are typically associated with other chemical reactions that are concurrent with the decomposition process. It is the H2S and the mercaptans that are usually responsible for the odor associated with LFG.
Although landfill gas is primarily associated with municipal waste landfills, other types of landfills also may release gases to the environment. Most notable are Construction / Demolition Debris (CDD) landfills in which large quantities of gypsum wallboard have been deposited. The introduction of moisture and the reaction with the gypsum wallboard has resulted in the release of H 2 S to the environment.
Landfill Gas Generation Rates
There are several phases of decomposition of organic wastes in the landfill. They all may be occurring at the same time in different parts of the landfill such as completed closed areas vs. active areas:
- Phase I – Aerobic; lasts several weeks to a month until O 2 is depleted; dependent uponmicrobes that thrive only in the presence of oxygen.
- Phase II – Anoxic; lasts several months after O 2 is depleted; converts compounds fromaerobic phase into acids and alcohols creating an acidic environment in the landfill
- Phase III – Unsteady methanogenic; lasts several months to several years; this anaerobic phase features the symbiotic existence of both methane and acid producing microbes andnresults in a more neutral environment in the landfill.
- Phase IV – Steady methanogenic: lasts several years to several decades; this anaerobic phase features a relatively stable production of methane as produced by the microbes that predominate this phase. After a period of time the production of methane begins to decline as the organics (food) supply dwindles. Some refer to this as Phase V, Declining methanogenic.
What Conditions Affect Generation?
There are many conditions that effect the quantity and quality of LFG generation. They include:
- Composition of the waste (relative amounts of food wastes, paper, plastics, yard wastes, glass, metals, leather, ceramics, etc.)
- Compaction of the waste (effectiveness of placement and mechanical compaction; baling)
- Covering of the waste (daily cover; intermediate cover; final cover and closure cap; type of cover: soil, tarps, or spray on stucco-like product)
- Moisture content (natural moisture content of waste as its delivered; rainfall / snowfall; run-on / run-off of stormwater; deliberate introduction of liquid such as recirculation of leachate)
- Particle size (shredding of waste prior to landfilling)
- Age of waste (closed areas vs. active areas)
In general, the more compact the waste, the drier the waste, and the older the waste the lower the rate of gas production. Eventually, the same amount of gas will be produced. This is why, to increase the rate of LFG generation, and reduce the time for the waste mass to stabilize, liquid such as leachate often is reintroduced into the landfill. Computer models have been developed to estimate LFG production rates over time for any given landfill. One of the most common is the LandGEM developed by the USEPA. The model will estimate the generation of CH 4 , CO 2 and NMOCs.
Why is Landfill Gas a Concern?
There are many concerns as it relates to the unabated emission of LFG. Some of these are “acute” concerns, others are “chronic” in nature. The acute concerns are:
- Poisonous atmosphere
Methane is explosive when concentrations range from 5% (lower explosive limit – LEL) to 15% (upper explosive limit – UEL) by volume in air. Above 15% it is flammable. Thus methane presents both an explosion and fire hazard to landfill personnel and nearby structures if it migrates beyond the waste boundaries. In confined spaces, along with CO2, methane is an asphyxiant. Hydrogen sulfide, H2S, is an explosive gas and is toxic at low concentrations.
With regard to aesthetics, methane migrating into the root zone of trees or shrubs may result in the death of the vegetation. If the vegetation is part of a landscaping plan or otherwise used as a screening buffer then the vegetation may have to be replaced at a cost to the owner.
The chronic concerns are:
- Potential impact on local groundwater quality
- Climate change
Over time, the trace constituents in LFG may interact with local groundwater regimes such that some of the organics may contaminate the groundwater.
Methane is considered a greenhouse gas that is 20 times more potent than CO2 and thus contributes to concerns regarding global warming.
Because of the concerns discussed above, it is prudent to install a network of LFG monitoring / compliance probes usually at the perimeter of the landfill property. These are monitored on a periodic basis (often quarterly) just as groundwater monitoring wells are monitored periodically. The probes are often designed and installed as the geology dictates, measuring methane concentrations at one or more levels beneath the surface. This will give early warning if there is a LFG migration issue at the facility.
Buildings and structures can be manually monitored coincident with the compliance probe monitoring, or continually monitored with alarm type devices depending upon the use of the structure.
Confined spaces such as manholes, pump stations, meter pits, and tanks are monitored prior to entry under established confined space entry protocols. Problem areas then can be ventilated to create a safe atmosphere for entry workers.
Landfill Gas Venting or Collection
For smaller landfills and/or for older, closed landfills well past their prime methane production years, LFG may be removed with passive landfill gas vents that simply allow for the escape of the LFG through the cover of the landfill.
Typically, though, most modern landfills are permitted and constructed such that LFG is mechanically removed through a system of:
- Vertical extraction wells
- Horizontal trenches
- Valved wellheads
- Above ground or below surface piping
- Blower stations
After the LFG is mechanically removed from the landfill, it is then directed to other equipment such as flares to destroy the LFG or processing equipment to create a product for beneficial use.
(*See image on right for a schematic diagram of a modern engineered landfill)
Destruction of Landfill Gas
In cases where the LFG cannot be economically recovered, LFG is often burned off in a flare to eliminate odors and substantially reduce emissions of CH4 and other organic trace gasses. There are two types of flares in general use:
- Utility flares (often called candlestick flares)
- Enclosed flares (also called ground flares)
Utility flares are open flame devices that cannot be easily monitored regarding combustion efficiency. However, studies have shown that utility flares typically have a 98% efficiency in the combustion of NMOCs and 99% or greater efficiency in the combustion of flammable gases (CH4 and H2S).
Properly operating enclosed flares have the same or better combustion efficiency but can be fitted with monitoring devices to corroborate the efficiency of the device with regard to the destruction of NMOCs and flammable gases.
Beneficial utilization of landfill gas
Where there is a demonstrated economic benefit from the utilization of LFG, several different technologies are available for consideration to determine what is the best with regard to return on investment:
- Direct use
- Electrical generation
- Processing to pipeline quality
- Processing to LNG / CNG
- Production of Ethanol
Direct use of LFG may be applicable where gas flows are low or where the recovered LFG has a low BTU value and little processing of the LFG is necessary. Examples are:
- Heaters for garages or maintenance facilities
- Flower or vegetable greenhouses
- Leachate evaporation
Electrical generation with LFG as the fuel, primarily for sale to the local grid, is a very common and, still growing application. The most prevalent technology employs internal combustion engines to produce the electricity. Gas turbines also have been used, but have had issues with the trace amounts of siloxanes that may be present in the LFG.
Processing the LFG to pipeline quality also is a promising use and has been done successfully. The treated / enhanced LFG can be directly injected into an existing gas company pipeline, or piped several miles away to a customer with a heavy natural gas demand (hospital, factory, chemical plant).
Another use that is gaining in popularity is to process the gas to LNG or CNG for fleet vehicle usage. This is being done by both commercial / private landfills and municipal landfills in the United States.
The production of ethanol from LFG has been successfully accomplished at one landfill in north central United States.
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