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Many landfills across the country are installing gas control systems because of state and federal
regulatory requirements. The federal government has developed laws and regulations that govern the
operation and landfill gas emissions through the reduction of ozone precursors (volatile organic
odorous compounds. States may also have specific landfill regulations, which must be as strict as
or stricter than federal regulations.

Odor complaints or potential safety and health concerns may also prompt landfill gas collection. Sulfide
emissions are a common source of landfill odor complaints. At older landfills or at smaller landfills
exempt from federal and state regulations, uncontrolled releases of landfill gases can pose potential
safety and health concerns (e.g, explosion hazards). In such cases, the landfill might implement landfill
gas control measures, even if they are not required by federal or state regulations. Some landfills have
also implemented voluntary gas collection and control or treatment systems to recover landfill gas for
energy production.

What are the important components of a landfill gas control plan?

The goal of a landfill gas control plan is to prevent people from being exposed to landfill gas buildings
and homes in the community. Technologies used to control landfill gas at the landfill or in the
community can be applied separately or in combination. The NSPS rule specifies the type of information
that must be included and the criteria the collection and control systems must meet.

Federal Requirements Under Subtitle D of Resource Conservation and Recovery Act (RCRA)
for Landfill Gas Migration Control

Since October 1979, federal regulations promulgated under Subtitle D of RCRA— which regulates the
design, construction, operation, monitoring, and closure of MSW landfills—have required controls on
migration of methane in landfill gas. These regulations do not address other components of landfill gas.
In 1991, EPA issued standards for landfill design and performance that apply to MSW landfills active on
or after October 9, 1993. The standards require methane monitoring and establish performance
standards for methane migration control. Monitoring requirements must be met at landfills not only
during their operation, but also for a period of 30 years after closure.

Landfills affected by RCRA Subtitle D are required to control gas by establishing a program to
periodically check for methane emissions and prevent off-site migration. Landfill owners and operators
must ensure that the concentration of methane gas does not exceed:

•        25% of the LEL for methane in the facilities' structures (1.25% by volume)
•        The LEL for methane at the facility boundary (5% by volume)

Permitted limits on methane levels reflect the fact that methane is explosive within the range of 5% to
15% by volume of concentration in air. If methane emissions exceed the permitted limits, corrective
action (installation of a landfill gas collection system) must be taken. The Subtitle D RCRA regulations
for MSW landfills can be found in 40 CFR Part 258.

Under NSPS/EG of the CAA, EPA requires affected landfills to collect and control landfill gas. The
NSPS/EG target reductions in the emissions of landfill gas due to odor, possible health effects, and
safety concerns. The rules use NMOCs (which contribute to local smog formation) as a surrogate for
total landfill gas to determine if control is required. Landfills meeting certain design capacity and
gas for energy. Landfills that meet both of the following criteria must collect and control landfill gas
emissions.
Landfill Gas

•        Capacity: design capacity greater than or equal to 2.5 Mg and 2.5 million cubic meters.
•        
Emissions: annual NMOC emission rate greater than or equal to 50 Mg.

The basic requirements are the same for both existing and new landfills. Existing landfills are defined as landfills that received waste after November 8, 1987, and began construction before May 30, 1991.
New landfills are defined as landfills that began construction, reconstruction, or modification on or after May 30, 1991. These are subject to the NSPS. The CAA regulations (NSPS/EG) for MSW landfills
found in 40 CFR Part 60

How do I collect the landfill gas?

Landfill gas can be collected by either a passive or an active collection system. A typical collection system, either passive or active, is composed of a series of gas collection wells placed throughout the
landfill. The number and spacing of the wells depend on landfill-specific characteristics, such as waste volume, density, depth, and area. As gas is generated in the landfill, the collection wells offer
preferred pathways for gas migration. Most collection systems are designed with a degree of redundancy to ensure continued operation and protect against system failure. Redundancy in a system may
include extra gas collection wells in case one well fails. The system-specific components for passive and active gas collection systems are discussed below.

•        
Passive Gas Collection Systems. Passive gas collection systems use existing variations in landfill pressure and gas concentrations to vent landfill gas into the atmosphere or control system.
Passive collection systems can be installed during active operation of a landfill or after closure. Passive systems use collection wells, also referred to as extraction wells, to collect landfill gas.
The collection wells are typically
constructed of perforated or slotted plastic and are installed vertically throughout the landfill to depths ranging from 50% to 90% of the waste thickness.

If groundwater is encountered within the waste, wells end at the groundwater table.
Vertical wells are typically installed after the landfill, or a portion of a landfill, has been closed.
A passive collection system
may also include horizontal wells located below the ground surface to serve as conduits for gas movement within the landfill. Horizontal wells may be appropriate
for landfills that need to recover gas promptly (e.g, landfills with subsurface gas migration problems), for deep landfills, or for active landfills. Sometimes, the collection wells vent directly to the atmosphere.

The efficiency of a passive collection system partly depends on how well the gas is contained within the landfill. Gas containment can be controlled and altered by the landfill collection system design.
Gas can be contained by using liners on the top, sides, and bottom of the landfill. An impermeable liner (e. g, clay or geosynthetic membranes) will trap landfill gas and can be used to create
preferred gas migration pathways. For example,
installing an impermeable barrier at the top of a landfill will limit uncontrolled venting to the atmosphere by causing the gas to vent through
collection wells rather than the cover
.

The efficiency of a passive collection system also depends on environmental conditions, which may or may not be controlled by the system design.
When the pressure in the landfill is inadequate to
push the gas to the venting device or control device, passive systems fail to remove landfill gas effectively
.

High barometric pressure sometimes results in
outside air entering the landfill through passive vents that are not routing gas to control devices. For these reasons, passive collection systems
are not considered reliable enough for use in
areas with a high risk of gas migration, especially where methane can collect to explosive levels in buildings and confined spaces.

It is fairly common for landfills to combust gas due to odor concerns, for example, even if not the landfill is not subject to regulatory requirements. Passive gas collection systems may be used to comply
with Federal Regulations NSPS/ EG only at landfills where cells are lined in accordance with Subtitle D of RCRA to prevent gas migration.

•        
Active Gas Collection. Well-designed active collection systems are considered the most effective means of landfill gas collection (EPA 1991). Active gas collection systems include vertical
and horizontal gas collection wells
similar to passive collection systems. Unlike the gas collection wells in a passive system, however, wells in the active system should have valves to regulate gas flow
and to serve as a sampling port. Sampling allows the system operator to measure gas generation, composition, and pressure.

Active gas collection systems include
negative static pressure systems, vacuums or pumps that move gas out of the landfill and piping that connects the collection wells to the negative static
pressure system, often vacuum systems are used. Negative static pressure systems pull gas from the landfill by creating low negative pressure within the gas collection wells. The
low pressure in the
wells creates a preferred migration pathway for the landfill gas
. The size, type, and amount of static pressure required in an active system to pull the gas from the landfill depend on the amount of
gas being produced. With information about landfill gas generation, composition, and pressure, a landfill operator can assess gas production and distributions changes and modify the negative static
pressure system and collection well valves to most efficiently run an active gas collection system. The system design should account for future gas management needs, such as those associated with
landfill expansion.

What components are in an Effective Active Gas System Designed?

An effective active gas collection system incorporates the following design elements (EPA 1991):

•        Gas-moving equipment, including negative static blowers, fans, vacuums and piping, capable of handling the maximum landfill gas generation rate.

•        Collection wells placed to capture gas from all areas of the landfill. The number and spacing between each extraction well depends on the waste type, depth, and compaction; the pressure gradients   
created by the negative static pressure fans, vacuums; and the moisture content of the gas.

•        The ability to monitor and adjust flow from individual extraction wells. Inclusion of a valve, pressure gauge, condenser, and sampling port at each collection well allows a landfill operator to monitor
and adjust pressure and to measure gas generation and content.

What methods are available to treat landfill gas after collection?

Some passive gas collection systems simply vent landfill gas to the atmosphere without any treatment before release. This may be appropriate if only a small quantity of gas is produced and no
people live or work nearby. More commonly, however, the collected landfill gas is controlled and treated to reduce potential safety and health hazards. (A landfill may be required to do so by law, such as
the NSPS/ EG and PSD GHG. Common methods to treat landfill gas include combustion and non combustion technologies, as well as odor control technologies.

•        Combustion.
Combustion is the most common technique for controlling and treating landfill gas. Combustion technologies such as direct fired enclosed flares, incinerators, regenerative
thermal oxidizers, boilers, gas turbines, and internal combustion engines thermally destroy the compounds in landfill gas. Over 98% destruction of organic compounds is typically achieved. Methane is
converted to carbon dioxide, resulting in a large reduction in greenhouse gas.
Combustion is most efficient when the landfill gas contains between 1% and 20% methane by volume.
Regenerative thermal oxidizers operate fuel free on 1% while flares are limited to 15-20% by volume. At the higher limits of 20% methane concentration will readily form a combustible mixture with ambient
air, so that only an ignition source is needed for operation. At landfills with less than 20% methane by volume will need to use other technologies i.e, regenerative thermal oxidizers or catalytic thermal
oxidizers otherwise supplemental fuel (e. g, natural gas) is required to operate open flares, greatly increasing operating costs. When combustion is used, several different types of combustion methods
can be chosen: open or enclosed flares and regenerative thermal oxidizers.

  • Open flame flares (e. g, candle or pipe flares), the simplest flaring technology, consist of a pipe through which the gas is pumped, a pilot light to spark the gas, and a means to regulate the gas flow.
    The simplicity of the design and operation of an open flame flare is an advantage of this technology. Disadvantages include inefficient combustion, aesthetic complaints, and monitoring
    difficulties. Sometimes, open flame flares are partially covered to hide the flame from view and improve monitoring accuracy.

  • Enclosed flame flares are more complex than open flame flares. Nevertheless, most flares designed today are enclosed, because this design eliminates some of the disadvantages associated
    with open flame flares. Enclosed flame flares consist of a single or multiple burners enclosed within fire- resistant walls that extend above the flame. Unlike open flame flares, the
    amount of gas and air entering an enclosed flame flare can be controlled, making combustion more reliable and more efficient.

  • Other enclosed combustion technologies such as regenerative thermal oxidizers, boilers, process heaters, turbines, and internal combustion engines can be used not only to
    efficiently destroy organic compounds in landfill gas, but also to generate useful energy or electricity.

Some public concerns have been raised about whether the combustion of landfill gas may create toxic chemicals. Combustion can create acid gases such as SO2 and NOX. The generation of dioxins has
also been questioned. EPA investigated the issue of dioxin formation and concluded that the existing data from several landfills did not provide evidence showing significant dioxin formation during landfill
gas combustion. Because of the potential imminent health threat from other components of landfill gas, landfill gas destruction in a properly designed and operated control device, such as a thermal
oxidizers or energy recovery unit, is preferable to uncontrolled release of landfill gas.

  • Non combustion. Non combustion technologies were developed in the 1990s as an alternative to combustion, which produces compounds that contribute to smog, including nitrogen oxides, sulfur
    oxides, carbon monoxide, and particulate matter. Non combustion technologies fall into two groups: energy recovery technologies and gas-to-product conversion technologies.

Numerous pretreatment methods are available to address the impurities of concern for a specific landfill. After pretreatment, the purified landfill gas is treated by non combustion technology options.

  • Energy Recovery Technologies use landfill gas to produce energy directly. Currently, the phosphoric acid fuel cell (PAFC) is the only commercially available non combustion energy recovery
    technology. The PAFC system consists of landfill gas collection and pretreatment, a fuel cell processing system, fuel cell stacks, and a power conditioning system. Several chemical reactions occur
    within this system to create water, electricity, heat, and waste gases. The waste gases are destroyed in an oxidation system.

  • Gas-To-Product conversion technologies focus on converting landfill gas into commercial products, such as compressed natural gas, methanol, purified carbon dioxide and methane, or liquefied
    natural gas. The processes used to produce each of these products vary, but each includes landfill gas collection, pretreatment, and chemical reactions and/ or purification techniques and requires
    very large gas quantities to become viable. Most must use combustion technology to destroy gaseous wastes.

  • Odor Control Technologies. Odor control technologies prevent odor-causing gases from leaving the landfill. Installing a landfill cover will prevent odors from newly deposited waste or from
    gases produced during bacterial decomposition. Covering a landfill daily with soil can help reduce odors from newly deposited wastes. More extensive covers are installed at landfill closure to
    prevent moisture from infiltrating the refuse and encouraging bacterial growth and decomposition. Vegetative growth on the landfill cover also reduces odors. Combustion is another
    technique that can eliminate landfill gas odors by thermally destroying the odor-causing gases. Venting landfill gas through an active carbon filter is another technology used to reduce odors.

What beneficial uses are there for collected landfill gas?

Landfill gas is the single largest source of man-made methane emissions in the United States, contributing to almost 40% of methane emissions each year (EPA 1996). Consequently, a
growing trend at landfills across the country is to use recovered methane gas from landfills as an energy source. Collecting landfill gas for energy use
greatly reduces the risk of explosions, provides
financial benefits for the community, conserves other energy resources, and potentially reduces the risk of global climate change.

What landfills can be used for gas recovery and how is energy generated from landfill gas?

The feasibility of installing a landfill gas recovery system depends on factors such as landfill gas generation rates, the availability of users, and the potential environmental impacts. Many different
landfill types with
varying gas production rates and composition can support energy recovery projects. There are, however, several guidelines to consider when assessing the feasibility of generating
energy from landfill gas.

If feasible, energy recovery can be implemented by
use of combustion - or non combustion - based technologies. Combustion-based technologies that recover energy include regenerative thermal
oxidizers with low waste heat micro turbines, boilers, process heaters, gas turbines, and internal combustion engines. For example, landfill gas can be piped to a nearby industry, commercial business,
school or government building where it is combusted in a boiler to provide steam for an industrial process or heat for a building. It may be combusted in an industrial process heater to provide heat for a
chemical reaction. The
electricity can be used to meet power needs at the landfill or a nearby facility, or the electricity may be sold to the power grid.

The choice of which type of combustion device to use (e. g, regenerative thermal oxidizer with micro turbine, boiler, gas turbine, internal combustion engine) depends on what users are located near the
landfill, site-specific technical and economic considerations, and sometimes environmental impacts. Other options include
internal combustion engines, however if the landfill is in a non attainment
area for ozone, then NOx emissions may be a barrier to using an internal combustion engine
.

Non combustion energy recovery systems are also available, but are not used as widely used. Fuel cells are a promising new technology for producing energy from landfill gas that does not
involve combustion. This technology has been demonstrated and in the future may become more economically competitive with other options. One option that does not involve combustion of landfill gas at
or near the landfill is purifying the landfill gas to remove constituents other than methane, producing a high British thermal unit (Btu) gas that can be sold as pipeline quality natural gas. While the high Btu
gas is eventually combusted, it would not contribute to any emissions near the landfill. Another option is using compressed landfill gas as a vehicle fuel.
Landfill Gas Compliance and Landfill Gas Usage
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