CEPIS/OPS/OMS

Landfill Gas Movement, Control, and Uses

How does a landfill produce gas? Why is control
Of this substance necessary, and how can it be accomplished?
And how is energy recovered from landfill gas?

Lesson Two provided an overview of sanitary landfilling. The decomposition processes that occur in landfills were described. Decomposition produces methane and carbon dioxide, which are the principal components of landfill gas, This gas, if allowed to migrate uncontrolled from a landfill, may result in dangerous conditions within buildings the gas may enter. Uncontrolled release of the gas to the atmosphere may cause air pollution
This lesson focuses on landfill gas management, control procedures, and monitoring techniques.
Municipal landfill gas composition is controlled primerily by products of microbial reactions in the landfill. In general, a ladfill will go throung three different stages with diferent bacterial types predominating in each stage.
Solid waste initially decomposes aerobically; the primary gas product is carbon dioxide. As the oxygen is used up, facultative and anaerobic microorganisms predominate.
These bacteria continue to produce carbon dioxide, but the process proceeds into second-stage anaerobic decomposition, where both methane and carbon dioxide are produced at approximated a 50-50 ratio. In addition, other compounds are produced and additional chemicals are released into the landfill atmosphere by volatilization.

Typical landfill gas composition is shown in Table 1.

By Philip O'Leary and Patrick Walsh

O`Leary and Walsh are solid waste specialists with the University of Wisconsin-Madison.

Trace Constituents of Gas

Over the last several years, concern has grown regarding the release of potential air pollutants from landfills. Monitoring of landfill gas for trace constituents has found varying concentration of higher molecular weight hydrocarbons. The results of a study by Rettenberger (1987) are summarized in Table 2. Note that wide ranges in concentrations were found. The measurements were made on landfill gas that had not mixed with the atmosphere.
In the late 1980s, the state of California and is air management districts required an assessment of the chemical characteristic of gaseous emissions from landfills throughout the state. Special "Sniffer" equipment measured the characteristics of landfill gas escaping though landfill covers. Air pollution monitoring equipment was employed to test the quality of emissions from vents, flares, and energy recovery facilities. Results of these assessments are available from the California Air Resources Board. Monitoring is ongoing.
In the South Coast Air Quality Management District (the Los Angeles area), operators of existing sites must comply with a total organic emission standard on the surface of the landfill. If the standard is exceeded, gas control and handling equipment must be installed. Operators of closed landfills are not allowed to do any excavation at the site until an emission control plan is approved.

Table 1

Typical Landfill Gas Composition
Component %
Methane 47.4
Carbon Dioxide 47.0
Nitrogen 3.7
Oxygen 0.8
Paraffin hydrocarbons 0.1
Aromatic-cyclic hydrocarbons 0.2
Hydrogen 0.1
Hydrogen Sulfite 0.01
Carbon monoxide 0.1
Trace compounds 0.5
Sources: Ham, 1979  

Safety practices

Methane gas concentrations in excess of 5% are explosive. Landfill gas can asphyxiate. A person who enters an enclosure containing it.
The following safety precautions are recommended for landfill operators.

It is imperative that operators receive safety training and that gas monitoring equipment and other safety devices be properly calibrated and maintained.


Why gas control is needed

The need for gas movement control is primarily to prevent the gas from damaging plants and property or causing injury to people. Methane generated in landfills has killed vegetation; it displaces oxygen from the root zone.
More importantly, gas has accumulated in buildings and, if methane concentrations exceed the lower explosive limit of 5%, there is danger of a methane gas explosion.
If one suspects methane gas has accumulated in a building, the fire department should be alerted immediately. Most fire departments have explosive-gas has accumulated in a building, the fire department should be alerted immediately. Most fire departments have explosive-gas detection equipment. Methane gas entry points into a building may be through cracks, construction joints, sub – surface utility service openings, and almost any other weak spot in the basement wall or building floor.
Methane, being lighter than air, will tend to accumulate near the ceiling. If the source of the methane cannot be controlled immediately, venting of the building should continue, an alarm system should be sounded, and the building evacuated.

Mechanics of gas movement

The mechanics of gas movement through refuse and soil are extremely complicated. The gas will lend to migrate from the landfill on a path through the refuse and surrounding soils that offers the least resistance.

Table 2

Hydrocarbons in Landfill Gas
In mg/m3, based on airless landfill gas
Ethane C2 H6 0.8-48
Ethylene C2H4 0.7-31
Propane C3H8 0.04-10
Butane C4H10 0.-23
Butylene C4H8 1-2
Pentane C5H12 0-1
Hexane C6H14 3-18
Cyclohexane C6H12 2-6
Heptane C7H16 3-8
Octane C8H18 0.05-75
Noname C9H20 0.05-400
Cumene C9H12 0-32
Decane C10H22 0.2-137
Undecane C11H24 7-48
Dodecane C12H26 2-4
Tridecane C13H28 0.2-1
Benzene C6H6 0.03-7
Tolene C7H8 0.2-615
Source: Rettenberger, G, "Trace Composition of Landfill Gas," Proceedings of International Symposium on Process, Technology, and Environmental Impact of Sanitary Landfill, Cagliani, Sardinia, Italy (Oct. 19-23, 1987)

Gas will migrate further through a sand and gravel soil than through a silt or clay soil than through a silt or clay soil. The rate of migration will be strongly influenced by weather conditions: when barometric pressure is falling, gas will tend to be forced out of the landfill into the surrounding soil formations. As pressure rises, gas may be retained within the landfill for a short time period as new pressure balances are established. Figure 1 shows the variations in landfill gas pressure gradients through a landfill cap as barometric pressure changes.
Wet surface soil conditions and frozen ground may prevent the gas from escaping into the atmosphere at the edge of the landfill – possibly causing the gas to migrate event further from the landfill.
Maximum migration distance of methane gas is difficult to predict. Computer models by Farquhar with Melcalfe (1982) can predict the approximate migration patterns from existing landfills. An example is shown in Figure 2.
In order to simulate methane migration, data about the landfill plus soil information must be input to the computer program. Maximum migration distances of greater than 1,500 feet have been observed.


The effect of caps

Controlling gas movement at a landfill begins with a study of the local soils, geology, and nearby area. For example, if the landfill is surrounded by a sand or gravel soil and if buildings are close to the landfill, the movement of gas into this area will need to be controlled by engineering methods.

On the other hand, a landfill surrounded by clay in an isolated location may not need as stringent a control system.
But be careful. The clay cap installed upon completion of a landfill for the purpose of excluding moisture infiltration and restricting leachate generation will, at the same time, tend to contain the landfill gas.
The pressure gradient that results will force the gas to move laterally and into the areas surrounding the landfill. Even a narrow sand seam in a clay formation can transmit a large quantity of gas.
Probes are used to detect the migration of methane gas in the formations around a landfill; a typical probe is shown in Figure 3.
The probe is installed by boring a hole into the ground to at least the same depth as the landfill. A perforated pipe is place into the hole and the space between the original soil and pipe is filed with sand. Clay is packed around the pipe near the ground surface to prevent air leaking into the probe. Two types of measurements are conducted.
Gas pressure is measured with a gauge or manometer. A positive reading indicates that landfill gas is being moved past the probe by pressure built up within the landfill. A negative pressure reading would be expected if a probe was installed near a landfill gas recovery well.
The concentration of methane in the soil atmosphere is also measured by using a calibrated meter. A concentration greater than 5% methane shows areas where migration may have dangerous consequences if the gas enters a building.
Since the migration patterns and the methane concentrations will change rapidly, frequent measurements are required to obtain an accurate picture of the gas migration pattern. At sites where there is a high degree of concern about gas migration endangering residences, daily measurements should be conducted until the crisis period has passed.
At some sites, multi-level probes are installed in order to obtain a more accurate three-dimensional picture of gas movement. Some typical monitoring results are summarized in Table 3. Note high methane concentrations when the barometric pressure is low.


Gas vents and recovery systems

Passive vents and active gas pumping systems are used to control landfill gas migration. Passive system rely on natural pressure and convection mechanisms to vent the landfill gas to the atmosphere.
Shallow gas venting trenches, or gas venting pipes, installed within the landfill and vented to the atmosphere, have been used to allow gas from interior regions of the landfill to escape, these natural vents may be equipped with flares to burn off the gas in order to prevent odor problems.
In a number of instances, passive vents have not effectively removed landfill gas from under the cover. This has resulted in vegetative stress and accompanying erosion problems on the landfill cover. The failure of the passive vents is generally attributed to the fact that there is a significant risk of methane accumulating in buildings, passive systems are not considered reliable enough to be the sole means of protection. Active systems, which are discussed below, should be utilized in moderate or high risk areas.

Table 3

Subsurface Methane Gas Concentration
Day Weather

Barometric Condition Presure (Inches of Hig. At Sea levelÇ)

1

20

Probe No.

4

400

2

3

Distance from Landfill (feet)

100 200

1 Clear

30.3

20

16

5

0

2 Clear

30.0

21

15

4

0

3 Cloundy

29.6

29

18

6

1

4 Rain

29.3

48

39

30

21

5 Rain

29.9

50

26

26

18

6 Clear

29.9

30

12

12

6

7 Clear

29.9

21

9

9

3


Active gas recovery systems

Active gas collection systems remove the landfill gas under a vacuum from the landfill or the surrounding soil formation, with the gas being literally pumped out of the ground.
These systems may provide migration control or recover methane for energy recovery may provide migration control or recover methane for energy recovery purposes. Both approaches employ gas recovery wells and vacuum pumps. A pipe network is built to interconnect wells and blower equipment.

When the primary purpose is migration control, recovery wells are constructed near the perimeter of the landfill. Depending on site conditions, the wells may be placed in the waste or in the soil formation immediately adjacent to the landfill. An example of both approaches is shown in Figure 4.
The location will depend on site access, the type of soil formation around the site, and the type of waster in the landfill.
At landfills where the waste has been placed up to the property line, there may not be sufficient space to place the wells and colecction lines outside the waste. The surrounding soil formation must also be evaluated before deciding where to place the wells.
A sandy soil will be more permeable to gas than a clay soil and, therefore, more suitable for well installation. Some wastes may contain materials, for example large pieces of concrete, that prevent the drilling of boreholes and the installation of recovery wells within the landfill. In extremely high risk areas, the wells may be installed in the soil formation before the solid waste is placed in the landfill so that the control system is operational before waste is deposited.


Construction details

Construction details for a gas recovery well are shown in Figure 6. Borehole diameters are generally two to three feet. Larger diameter holes provide more surface area at the refuse-gravel interface and require less suction for gas removal. This configuration is above ground, therefore, providing easy access to piping. An alternative design is for the interconnection between the well and header pipe to be entirely below ground, and is best suited for installation at landfills where all equipment must be out of sight.
Well depths ranging from 50% to 90% of the refuse thickness are common, except where groundwater conditions are encountered, and then the well is terminated at the water table. The well casing is usually some type of plastic pipe that is slotted.
It is important that the wells be individually valved so the vacuum applied to each well can be regulated. Gas probes are employed to monitor the performance of the control wells.
If migration problems continue near a particular well, the control valves are adjusted to pump more gas with that well. Conversely, if methane measurements show low concentrations, the vacuum is reduced so that the well draws in less air.
At some landfills, it is cost-effective to install gas recovery wells throughut the landfill and recover the gas for its energy value. In addition to the wells that may be constructed along the landfill´s perimeter for migration control, wells are placed in a grid pattern to recover gas that might otherwise escape through the landfill cover.
Wells are connected, as shown in Figure 6, to a collection system that carries the gas to energy recovery equipment.
Before constructing an energy recovery system, tests are usually conducted to predict the quantity and quality of gas that may be available. Testing is important because wide variations have been observed in gas generation rates. Some landfills-especially depending on the type of special wastes that may have been received-will have gases with chemical characteristics that require special handling. A pumping observed in gas generation rates. Some landfills-especially depending on the type of special wastes that may have been received-will have gases with chemical characteristics that require special handling. A pumping test is conducted by installing a gas recovery well and a number of monitoring probes in the landfill. The well is pumped until the gas flow stabilizes. Chemical characteristics of the gas are measured to determine methane content and the concentration of other chemicals: concurrently, the probes are monitored for pressure drop and methane content.
Using the results from one or more test wells, a design can be prepared for a full-scale gas recovery system. In addition to pumping test results, other considerations when designing the recovery system are: impermeable layers or walls within the landfill which may retard gas movement, and regions within the landfill where liquids may fill the recovery wells.


Handling the collected gas

Collected landfill gas can be directly vented to the atmosphere, bummed, or directed to an energy recovery system.
Venting is usually done through a stack to provide atmospheric dispersion and minimize the potential of odor problems. If odor problems or the presence of undesirable air contaminants justify it, the gas may be directed through a burner for combustion.
If the methane concentration exceeds 15% and will support a flame, the gas can be burned directly by a flare. Figure 7 shows a flare used in California that provides a high degree of destruction, consequently limiting any undesirable emissions. Some sites generate gases with low methane concentrations; however, to burn these gases, the aid of a supplemental fuel (such as natural gas) is needed. This can greatly increase the operation cost of the landfill gas control system.
An alternative approach for cleaning up the landfill gas is to pass it through a carbon filter which will capture the trace contaminants.
When the methane gas concentration is greater than approximately 35%, it may be worthwhile to recover the energy from the gas. Landfill gas containing 50% methane has a heating value of 505 Btus/standard cubic foot; this compares to 1,030 Btus for natural gas.


Energy recovery method

The method of energy recovery will primarily depend open the available energy markets.
If a factory or large building is near the landfill, it may be practical to pipe the gas directly into a boiler at the facility. The landfill gas is passed through filters to remove moisture and possible hydrogen sulfide and then injected into the furnace in in combination with the regular boiler fuel which may be coal, oil, or natural gas.
Boiler fuel is possibly the simplest approach for using landfill gas, but availability of a boiler near a landfill is not common. When deciding how far to transport the gas, the cost of a pipeline between the site and the boiler must be compared to the value of the gas.
Natural gas pipelines are located nearby some sites. Several different processes, including liquid solvent extraction and molecular sieves, are being employed to remove the carbon dioxide and other noncombustible constituents in landfill gas. The gas is upgraded to pipeline quality and injected into the natural gas distribution network. The landfill operator is paid by the natural gas utility for the value of the fuel. Since the landfill gas is entering a public utility network, it is imperative that undesirable contaminants be controlled before injection into the pipeline.
Often a boiler or pipeline is simply not available as a market for the methane gas. However, landfill gas can be directed to an engine-generator system for electricity production. Almost all landfills have electrical service and the generated power can be put back into the electric grid.
Figure 8 shows the components of a typical electrical generation system that utilizes landfill gas. The gas is only provided minimal treatment before being used as fuel in the gas turbine. A typical turbine generator system will produce 3.3 megawatts of electricity consuming 1.600 standard cubic feet per minute of 500 Btus per cubic foot of landfill gas, Internal combustion engines are also being utilized to operate the generators.
Since the methane content of the gas will directly affect the performance of the turbine, it is important that the site operator closely regulate the gas collection system. The generation of electricity from landfill gas is expected to greatly increase.
Other options for utilizing landfill gas include vehicle fuel, and experiments are being conducted on fuel cells. As with any resource recovery project, the key to beneficial use of landfill gas is finding a good market for the recovered energy.


Future developments

Many landfill operators can be expected to install some type of landfill gas management system in order to recover energy and to control emissions. Fuel cells, an advanced electrical generation system currently utilized on satellites, are being tested for use at landfills. These systems may prove to be more energy efficient than current recovery systems and require less maintenance.
Landfills in the future may be operated in a manner that enhances decomposition. This would result in higher rates of landfill gas generation over a shorter time period than is currently observed at landfills. These enhancement approaches would result in a greater need to carefully manage landfill gas but would have as a benefit making more and better quality gas available for energy recovery. Consequently a source of revenue would be available to help offset the costs of the more sophisticated facilities.


Lesson assignments

  1. Describe the processes taking place in a landfill that result in landfill gas generation (also see Lesson 2).
  2. What are the environmental hazards with landfill gas and why is it necessary to control gas?
  3. Describe safety procedures to protect employees who may be working in the presence of landfill gas.
  4. Propose at least two ways for controlling gas produced in the landfills.

References

Bogner, Jean E. "Gas Movement Through Fractured Landfill Cover Materials," proceedings of the Ninth Annual Madison Waste Conference Department of Engineering Professional Development, University of Wisconsin – Madison, September 9-10. 1986

Ham, Robert "Recovery, Processing and Utilization of Gas from Sanity Landfills, "EPA" 600/2-79-001,1979

Metcalle, D., "Modelling Gas Transport from Waste Disposal Sites," M.A.Ss Thesis, Department of Civil Engineering. University of waterloo, Waterloo, Ontario, 1982.

Rettenberger, G., Trace Composition of Landfill Gas, "Proceedings of International Symposium on Process. Thechnology, and Environmenttal Impact of Sanitary Landfill, Cagliari, Sardina, Italy (Octuber 19-23, 1987)