The International Information Center for Geotechnical Engineers

Landfill Gas Monitoring Systems - Landfill Gas

 

 2. Landfill Gas

 The following sections provides background information about landfill gas: what it is composed of, how it is produced, why landfill gas monitoring is important, and the conditions that affect its production. It also provides information about how landfill gas moves and travels away from the landfill site. Finally, the sections present an overview of the landfill gas modeling that might be applied to estimate landfill gas composition.

 

  2.1 Landfill Gas Overview

 LFG (landfill gas) is a natural byproduct of the decomposition of organic material in anaerobic (without oxygen) conditions. Landfills are the second largest human-caused source of methane in the United States, accounting for approximately 18.2 percent of U.S. methane emissions in 2013 (EPA gas portion). Methane is a potent greenhouse (heat trapping) gas with a global warming potential that is 25 times greater than carbon dioxide (EPA, 2015a). One million tons of MSW produces roughly 432,000 cubic feet per day (cfd) of LFG and continues to produce LFG for as many as 20 to 30 years after it has been landfilled. With a heating value of about 500 British thermal units (Btu) per standard cubic foot, LFG is a good source of useful energy, normally through the operation of engines or turbines. Some landfills collect and use LFG voluntarily to take advantage of this renewable energy resource while also reducing GHG emissions. However, its adverse effects to humans as well as environments are also significant today.

 

 Danger of landfill sites

Figure 1. Landfill gas (Diagram from Gazasia)

 

2.2 Types and Amounts of Compounds of Landfill Gas

 Landfill gas is composed of a mixture of hundreds of different gases. By volume, landfill gas typically contains: 45% to 60% methane (CH4) and 40% to 55% carbon dioxide (CO2). Landfill gas also includes small amounts of nitrogen, oxygen, ammonia, sulfides, hydrogen, carbon monoxide, and non-methane organic compounds (NMOCs) such as trichloroethylene, benzene, and vinyl chloride. Specifically, Table 1 represents the gas composition of the Fresh Kills municipal solid waste landfill in USA (Bart et al., 1998). The example shows typical characterization of the composition of the landfill gas and the variability in the gas composition from 250 separate landfill gas samples.

 

 Figure 2

Figure 2. Typical constituents found in municipal solid waste landfill gas; Trace constituents are benzene, toluene, vinyl chloride, trichloroethylene, dichloromethane etc. (G. Tchobanoglous et al, 1993)

  

Figure 3

Figure 3. Typical concentrations of some trace compounds found in landfill gas (G. Tchobanoglous et al, 1993)

 

Table 1. Average landfill gas composition, unit: parts per million by volume (Bart et al., 1998)

 Table 1

 

2.3 Concerns Caused by Landfill Gas

There are several concerns with landfill gas (USACE, 1995):

  • Methane gas is highly combustible, making it potential hazard in the landfill environment or in structures on adjacent properties.
  • Landfill gas is capable of migrating significant distances through soil, thereby increasing the risk of explosion and exposure. Serious accidents resulting in injury, loss of life, and extensive property damage may occur where landfill conditions favor gas migration.
  • As landfill gas is produced, the pressure gradient upward may create cracks and disrupt the geomembrane in the landfill cover.
  • Methane gas is an asphyxiant to humans and animals in high concentrations.
  • Migrating gas may cause adverse effects such as stress to vegetation, by lowering the oxygen content of soil gas available in the root zone.
  • Gas generated at landfills and vented to the atmosphere frequently emits bad odors, causing annoyance to people residing nearby.
  • Emissions of NMOCs in landfill gas may be contributing to the degradation of local air quality. Vinyl chloride from landfills has been found to be present in substantial concentrations in landfill gases, presenting health and safety concerns.
  • Methane gas, a “greenhouse gas,” contributes to the possibility of global warming of Earth’s climate.
  • Uncontrolled landfill gas is a loss of potential resources; instead, it can be a satisfactory fuel for a wide variety of applications.

 

 Table 2. Examples of landfill gas hazards (Gino Yekta, 2014)

 Table 2

 

2.4 Landfill Gas Generation

 Approximately 251 million tons of MSW were generated in the United States in 2012, with about 54 percent of that deposited in landfills (EPA, 2015b). Landfill gas is generated by the natural process of bacterial decomposition of organic material contained in landfills. A number of factors influence the quantity of gas that a MSW landfill generates and the components of that gas. Among them, there are certain processes that form landfill gas including bacterial decomposition, chemical reactions, and volatilization (Gino Yekta, 2014).

  • During bacterial decomposition, organic waste (which includes food waste, green waste, wood and other paper products) is broken down by bacteria naturally present in the waste and in the soil that is used to cover the landfill. Bacteria decompose organic waste in five distinct phases and changing gas composition during each phase.
  • During chemical reactions, Non-Methane Organic Compounds (NMOCs) are created,
  • During volatilization, landfill gases can be created when certain wastes, particularly organic compounds, change from a liquid or a solid into a vapor.
  • Bacterial decomposition has four phases as shown in Figure 4. The composition of the gas produced changes with each of the four phases of decomposition. Landfills often accept waste over a 20 to 30 year period, so waste in a landfill may be undergoing several phases of decomposition at once. This means that older waste in one area might be in a different phase of decomposition than more recently buried waste in another area.

  

Figure 4

Figure 4. Production phase of typical landfill gas (ATSDR, 2001)

 

 Phase I: Aerobic bacteria which live only in the presence of oxygen consume oxygen while breaking down the long molecular chains of complex carbohydrates, proteins, and lipids that comprise organic waste. The primary byproduct of this process is carbon dioxide. This phase could last for days or months, depending on how much oxygen is present. Phase I continues until available oxygen is depleted.

Phase II: Phase II is an anaerobic process which does not require oxygen. In this phase, bacteria convert compounds created by aerobic bacteria into acetic, lactic and formic acids and alcohols such as methanol and ethanol. The landfill becomes highly acidic. As the acids mix with the moisture present in the landfill and nitrogen is consumed, carbon dioxide and hydrogen are produced.

Phase III: Anaerobic bacteria consume the organic acids produced in Phase II and form acetate, an organic acid. This process causes the landfill to become a more neutral environment in which methane-producing bacteria are established by consuming the carbon dioxide and acetate.

Phase IV: The composition and production rates of LFG remain relatively constant. LFG usually contains approximately 50-55% methane by volume, 45-50% carbon dioxide, and 2-5% other gases, such as sulfides. LFG is produced at a stable rate in Phase IV, typically for about 20 years; however, gas will continue to be emitted for 50 or more years after the waste is placed in the landfill.

Gas generation in landfill is affect by the following factors (USACE, 1995):

  • Quantity and composition,
  • Compaction,
  • Age,
  • Presence of oxygen in the landfill,
  • Moisture content (very important), and
  • Temperature

 

 2.5 Migration Mechanisms

 Once gases are produced under the landfill surface, they generally move away from the landfill. Main factors influencing the migration of landfill gas are (Sharma and Reddy, 2004):

  • Molecular effusion
  • Diffusion (response to concentration gradient). Moving from high concentration to low concentration in order to be in equilibrium.
  • Convection (response to pressure gradient).

Molecular effusion occurs at the surface boundary of the landfill with the atmosphere. When the material has been compacted and has not been covered effusion is the process by which diffused gas releases from the top of the landfill. For dry solids, the principal release mechanism is direct exposure of the waste vapor phase to the ambient atmosphere.

Molecular diffusion occurs in gas systems when a concentration difference exists between two different locations within the gas. The diffusive flow of gas is in the direction in which its concentration decreases. The concentration of a volatile constituent in the landfill gas will almost always be higher than that in the surrounding atmosphere, so the constituent will tend to migrate to a lower concentration area.

Convection is the movement of landfill gas in response to pressure gradients developed within the landfill. Gas will flow from higher to lower pressure regions and from the landfill to the atmosphere. Where it occurs, the convective flow of gas will overwhelm the other two release mechanisms in its ability to release materials into the atmosphere. The rate of gas movement is generally orders of magnitude faster for convection than for diffusion. For most cases of landfill gas, diffusive and convective flows occur in the same direction. The transport mechanisms above are affected by the following factors:

 

  • Permeability,
  • Depth of groundwater,
  • Condition within the waste,
  • Moisture content,
  • Human-made features, and
  • Landfill daily cover and cap systems.

 

  

 2.6 Landfill gas modeling

Engineers generally use one of two methods to quantify gas emissions from landfill: they either estimate the emissions or measure them (ATSDR, 2001). In order to estimate landfill gas emissions, engineers conduct calculations or use models to predict the rate at which sources may release chemicals to the air. LFG modeling is the practice of predicting gas generation and recovery based on past and future waste disposal histories and estimates of collection system efficiency (EPA, 2015b). It is very important in the project because it provides an estimation of the amount of recoverable LFG that will be generated over time. In addition, LFG estimation is needed in performing risk evaluations, demonstrating compliance with regulatory limits, obtaining permits, and designing emission control systems for solid waste landfills. There are several approaches for predicting and analyzing LFG.

The EPA’s LandGEM (EPA, 2005) is a Microsoft Excel-based software application to calculate estimates for methane and LFG generation. The model uses the first-order decay equation and assumes that methane generation is at its peak shortly after initial waste placement (after a short time lag when anaerobic conditions are established in the landfill). The model also assumes that the rate of landfill methane generation then decreases exponentially (first-order decay) as organic material is consumed by bacteria.

 

 Equation 1

 Where:  

          QCH4= estimated methane generation flow rate (in cubic meters per year or average cfm)

i = 1-year time increment

n = (year of the calculation) – (initial year of waste acceptance)

j = 0.1-year time increment

k = methane generation rate (1/year)

Lo= potential methane generation capacity (m3 per Mg or cubic feet per ton)

Mi= mass of solid waste disposed in the ith year (Mg or ton)

Tij= age of the  jth section of waste mass disposed in the ith year (decimal years)

 

Even though LandGEM is the most widely used for LFG modeling and is the industry standard for regulatory as well as non-regulatory applications in the United States, LandGEM may not be appropriate for other countries with significantly different climates or a different landfill waste types. Thus, numerous international LFG models are designed to include adjustments to account for limits to LFG generation and collection caused by conditions at dump sites. Table 3 below represents several models including their assumptions, weak points, and strengths.

 

 Table 3. List of different of LFG prediction model and their specification (revised from Kamalan et al., 2011)

 Table 3

  

 Estimating LFG generation is a critical component of a project assessment and conceptualization because the results are used to estimate the size of the project, project design requirements, expected revenue, and capital and operating costs. However, accurately predicting the total LFG and methane generation can be difficult for many stakeholders because it requires selection and use of an appropriate LFG model among several options, consideration of local conditions that affect LFG generation, and an understanding of the uncertainty inherent with LFG modeling. The value of LFG estimates also heavily depends on the quality of data used in the model; accurate consideration of factors such as annual waste composition, estimated growth rates disposal rates, and the participation of an experienced LFG modeler. The LFG amount also could be estimated by methane generation since methane is big content of Landfill gas. The default methane content of LFG is 50 percent, which is both the industry standard value and LMOP’s recommended default value (EPA, 2015b). Table 4 shows the qualitative comparison of methane generation models by Hans Oonk (2010).

 

 Table 4. Summary of the landfill methane generation models, from ++: very good to --: very poor (Hans Oonk, 2010)

 Table 4

 

 In some cases, engineers will actually measure the gas emissions from landfills. Measuring gas emissions from an entire landfill is a challenging task, primarily because landfill emissions can occur over a surface that spans hundreds, or even thousands, of acres. The landfill gas monitoring is discussed in following sections in detail.

  

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