LFG Energy Project Development Handbook 2-1 

2. Landfill Gas Modeling

Chapter Overview 

Landfill gas (LFG) modeling is the practice of forecasting gas generation and recovery based on past 

and future waste disposal histories and estimates of collection system efficiency. LFG modeling is an 

important step in the project development process, because it provides an estimate of the amount of 

recoverable methane that will be available over time to fuel an LFG energy project. 

LFG modeling is performed for regulatory and non-regulatory purposes. Regulatory applications of 

LFG models for U.S. landfills are conducted to estimate emissions of LFG and its constituents, 

including non-methane organic compounds (NMOCs). These emissions estimates establish the 

requirements for gas collection and control system installation and operation. The modeler inputs 

landfill-specific waste disposal estimates but uses values for input variables (e.g., the methane 

generation rate) as determined by applicable regulations. 

Non-regulatory applications of LFG models typically include any of the following: 

C Evaluating LFG energy project feasibility 

C Determining gas collection and control system design requirements 

C Performing due diligence evaluations of potential or actual project performance 

This chapter covers non-regulatory LFG modeling applications only. EPA does not intend for the 

material presented in this handbook to supersede or replace required procedures for preparing LFG 

models for regulatory purposes. Federal regulations such as the New Source Performance Standards 

(NSPS) require modeling to determine rule applicability and compliance. For regulatory applications, 

the modeler must use the specific procedures, default values, and test methods prescribed in the 

rule. Refer to the appropriate regulations (e.g., the NSPS [40 CFR part 60 subpart WWW] and related 

documentation) for details. 

2.1 Introduction to LandGEM 

The most widely used LFG model is EPA’s Landfill Gas Emissions Model (LandGEM). LandGEM is the 

industry standard model for regulatory and non-regulatory applications in the United States. The 

latest version of LandGEM (v. 3.02) was released in May 2005. This section provides an introduction 

to LandGEM and the first order decay equation and variables employed by the model. 

The First-Order Decay Equation 

LandGEM is a first order decay model. First order decay models assume that landfill methane 

generation is at its peak shortly after initial waste placement (after a short time lag during which 

anaerobic conditions are established in the landfill). They also assume that landfill methane 
2-2 LFG Energy Project Development Handbook 

generation then decreases exponentially (i.e., first order decay) as the organic material in the waste 

decreases as it is degraded by bacteria in the landfill. 

LandGEM assumes that landfill methane generation can be projected using the following first order 

exponential equation: 

QCH4 =   k L = =

n

i j 1

1

1.0

o (Mi/10) (e-ktij) 

Where: 

QCH4 = estimated methane generation flow rate (in cubic meters [m3] per year or average cubic 

 feet per minute [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 megagram [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) 

LandGEM calculates methane generation using the first order decay equation shown above, and it 

calculates LFG generation by dividing methane generation by the estimated percent methane. The 

default methane content is 50 percent, which is both the industry standard value and LMOP’s 

recommended default value. 

Model Inputs 

Of the several variables in the first order decay equation used by LandGEM, only three (Mi, L0, and k) 

require user inputs. The user assigns these variables in a “USER INPUTS” worksheet in LandGEM. 

The following sections describe the three variables and their effects on estimated LFG generation. 

Annual Waste Disposal Rates (Mi). Estimated waste disposal rates are the primary determinant of 

LFG generation in any first order decay-based model, including LandGEM. LandGEM does not adjust 

annual waste disposal estimates to account for waste composition. Adjustments to account for 

waste composition are typically handled by adjustments to the L0 value. 

Potential Methane Generation Capacity (L0). The potential methane generation capacity, or L0, 

describes the total amount of methane gas potentially produced by a metric ton of waste as it 

decays. The values of the theoretical and obtainable L0 range from 6.2 to 270 cubic meters per 

metric ton or megagram (m3/Mg) of waste.1 Except in dry climates where lack of moisture can limit 

methane generation, the value for the L0 depends almost entirely on the type of waste present in the 

landfill. The higher the organic content of the waste, the higher the value of L0. Note that the dry 

 

1 U.S. EPA. 1991. Air Emissions From Municipal Solid Waste Landfills — Background Information for Proposed 

Standards and Guidelines. EPA-450-3-90-011a. p. 3-22. 
LFG Energy Project Development Handbook 2-3

organic content of the waste determines the L0 value, not the wet weight measured and recorded at 

landfill scalehouses, since water does not generate LFG. 

Methane Generation Rate Constant (k). The methane generation rate constant, k, describes the rate 

at which waste placed in a landfill decays and produces LFG. The k value is expressed in units of 

year-1. At higher values of k, the methane generation at a landfill increases more rapidly (as long as 

the landfill is still receiving waste), and then declines more quickly after the landfill closes. The value 

of k is a function of (1) waste moisture content, (2) availability of nutrients for methane-generating 

bacteria, (3) pH, and (4) temperature. Figure 2-1 shows an example gas curve for a landfill with 

approximately 2 million tons waste-in-place expected at closure. The potential gas generation was 

modeled in two scenarios, using identical landfill parameters except that k was varied between a 

value for arid conditions (0.02 yr-1) and a value for wet conditions (0.065 yr-1). The graph 

demonstrates the significant difference in gas generation that can occur based on moisture 

conditions at the site. 

Figure 2-1. LFG Generation Variance by k Value 

0

500

1,000

1,500

2,000

200220072012201720222027203220372042

Year

LFG Generation (scfm)

k=0.065

k=0.02

Moisture conditions within a landfill strongly influence k values and reflect the climate at the site as 

well as the contents of disposed waste and landfill design and operating practices. Waste decay 

rates and k values are very low at desert sites, tend to be higher at sites in wetter climates, and 

reach maximum levels under moisture enhanced “bioreactor” conditions. Annual precipitation is 

often used as a surrogate for waste moisture due to the lack of information on moisture conditions 

within a landfill. Air temperature can also affect k values, but to a lesser extent. Internal landfill 

temperatures are relatively independent of outside temperatures and typically remain in the range of 

approximately 30 to 60°C (85 to 140°F) except at shallow, unmanaged landfills in very cold 
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climates (e.g., landfills located in areas above 50 degrees latitude). For such landfills, waste decay 

rates and k values tend to be lower. 

The k value can also be expressed as a half-life, t1/2. The half-life is the time required for half of the 

remaining methane generation potential to be produced (i.e., half of the deposited waste to decay 

and produce LFG). 

Refer to the LandGEM User’s Manual for additional details on model use. 

Model Outputs 

LFG Generation. After the model inputs are selected, the user can turn to the “RESULTS” worksheet 

in LandGEM to find model outputs. The outputs include annual waste inputs, waste-in-place, and 

generation of total LFG, methane, carbon dioxide, and NMOCs. LFG and methane generation 

estimates are the output parameters that are used for non-regulatory LFG predictions. Once the LFG 

and methane generation is estimated, the collection efficiency must be estimated to determine the 

expected amount of LFG available for an LFG energy project. 

2.2 Estimating LFG Gas Recovery 

Estimating Collection Efficiency 

Collection efficiency is a measure of the gas collection system’s ability to capture generated LFG. The 

LFG generation predicted by the model can be multiplied by the percent collection efficiency to 

estimate the volume of LFG that can be recovered for flaring or use in an LFG energy project. 

Although rates of LFG capture can be measured, rates of generation in a landfill cannot be 

measured; therefore, considerable uncertainty exists regarding actual collection efficiencies 

achieved at landfills. 

To help address the uncertainty surrounding collection efficiencies, EPA has published estimates of 

reasonable collection efficiencies for U.S. landfills that meet U.S. design standards2 and that have 

“comprehensive” gas collection systems. EPA defines a “comprehensive” LFG collection system as a 

system of vertical wells and/or horizontal collectors providing 100 percent collection system 

coverage of all areas with waste within one year after the waste is deposited. According to EPA, 

collection efficiencies at such landfills typically range from 60 to 85 percent, with an average of 75 

percent most commonly assumed.3 Most landfills, particularly those that are still receiving wastes, 

will have less than 100 percent collection system coverage. In such cases, LFG modelers commonly 

use a “coverage factor” to adjust the estimated collection efficiency. The coverage factor adjustment 

is applied by multiplying the collection efficiency by the estimated percentage of the fill areas with 

 

2 Landfills that meet or exceed the requirements in the 40 CFR Parts 257 and 258 RCRA Subtitle D Criteria. 

3 U.S. EPA. 1998. Volume 1, Chapter 2, Section 2.4: Municipal Solid Waste Landfills. In AP42 Compilation of Air 

Pollutant Emission Factors. Fifth Edition. p. 2.4-6. 

http://www.epa.gov/ttn/chief/ap42/ch02/final/c02s04.pdf. 
LFG Energy Project Development Handbook 2-5

wells. This adjustment also should be applied to areas where wells are not fully functioning or are 

watered in. 

Collection efficiency estimates for sites with an operating gas collection and control system are 

typically based on information regarding current or recent conditions. Historical and future collection 

efficiency estimates can be made based on current conditions or other information regarding 

historical and planned collection system installations/expansions. Collection efficiency usually 

increases after site closure when disposal operations no longer interfere with LFG system operations 

and a final cover is installed. Sites without collection systems installed are typically assumed to be 

planning to install a comprehensive system, unless there is site-specific information to suggest a 

different value. 

Estimating LFG Recovery 

The final step in the modeling process is to calculate LFG recovery based on LandGEM projections of 

LFG generation and the modeler’s estimated collection efficiency. This step is done outside 

LandGEM and can be accomplished by setting up a spreadsheet that provides the LFG generation 

estimates, the collection efficiency estimates, and LFG recovery estimates (product of generation 

and percent efficiency). Table 2-1 shows a recommended format for displaying results and the waste 

disposal estimates on which they are based. 

Table 2-1. LFG Generation and Recovery Projections

Disposal 

Rate 

Wastein-Place LFG Generation Collection 

Efficiency LFG Recovery 

Year 

(tons/year) (tons) (scfm) (m3/hr) (%) (scfm) (m3/hr) 

Year 1 

Year 2 

Year 3 

Year X 

(final year 

modeled) 

scfm: standard cubic feet per minute 

The LFG recovery projections can also be displayed graphically. Both LFG generation and recovery 

can be displayed as line graphs in an “X-Y scatter” graph, showing LFG flow at 50 percent methane 

(Y-axis) in each year since the landfill opened (X-axis). For sites with operating collection systems and 

recovery data, the graph can be used to display actual recovery as dots. The graph can be used to 

compare projected to actual recovery for model calibration, which involves adjusting model k and L0

values so that the projected LFG recovery rates closely match actual recovery.4 Figure 2-2 shows a 

 

4 This handbook does not cover procedures for model calibration. LMOP recommends seeking the help of an 

experienced professional LFG modeler to perform model calibration. 
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sample model output graph for a landfill that opened in 1980, installed a gas collection system in 

2003,5 and will stop accepting waste at the end of 2011. 

Figure 2-2. LFG Generation and Recovery Rates 

LFG Generation Projected LFG Recovery Actual LFG Recovery

1985 1990 1995 2000 2005 2010 2015 2020 2025 2030

1,200

LFG Flow at 50% Methane (scfm)

1,000

600

400

0

1980

Special Considerations for Bioreactor and Leachate Recirculation Landfills 

In recent years, certain landfills have been designed and managed to deliberately introduce liquids 

into the waste in a controlled manner. This is done in order to speed up the waste decay process and 

shorten the time period of LFG generation. Landfills that achieve 40 percent moisture content in the 

waste through the controlled introduction of liquids (other than leachate and condensate) are 

considered “bioreactor” landfills according to EPA air regulations.6 Landfills that introduce liquids 

(most commonly leachate and condensate) but achieve waste moisture contents less than 40 

percent are considered “leachate recirculation” landfills. Bioreactor studies conducted by EPA’s 

Office of Research and Development provide additional information. 

The introduction of liquids into the landfill causes significant increases in waste decay rates and k 

values. This increase in k will cause gas generation to increase more rapidly while the landfill is 

receiving waste and decrease more rapidly once disposal stops, but will not change total LFG 

generation over the long term. Because only the rate of LFG generation is affected, L0 values should 

theoretically be unaffected by liquids introduction. LandGEM provides a default k value of 0.7 for 

modeling bioreactor landfills (i.e., the “inventory wet” value). LMOP, however, recommends assigning 

 

5 LFG recovery starts at known or projected date of the installation of the gas collection and control system. 

6 “Bioreactor” is defined in the municipal solid waste landfill National Emission Standards for Hazardous Air 

Pollutants, 40 CFR part 63, subpart AAAA. 
LFG Energy Project Development Handbook 2-7

a k value of 0.3 for bioreactors based on a University of Florida study completed shortly after 

LandGEM was released. No single k value is recommended or appropriate for leachate recirculation 

landfills because the impact of leachate recirculation on LFG generation varies depending on the 

amount of liquids added and the moisture content of waste achieved. 

Sometimes, only a portion of a landfill’s total site is designed and operated as a bioreactor or 

leachate recirculation landfill. In such cases, the bioreactor or leachate recirculation portion should 

be modeled separately from the remainder of the site, using waste disposal inputs for these areas 

only. 

2.3 Model Limitations 

Various factors can affect the accuracy of LFG recovery projections: 

C Limited or poor quality disposal data. Significant model error can be introduced if good 

disposal data are not available. 

C Atypical waste composition. Waste composition data are often not available to determine if 

unusual waste composition is a cause of model inaccuracy. 

C Poor quality flow data and/or inaccurate estimates of collection efficiency used for model 

calibration. Model calibration requires both accurate estimates of collection efficiency and 

good quality flow data that are representative of long-term average recovery. 

C Inaccurate assumptions. Inaccurate assumptions about variables such as future disposal 

rates, site closure dates, wellfield buildout, expansion schedules, or collection efficiencies 

can result in large errors in predicting future recovery. 

C Limitations due to the structure of LandGEM. For example, LandGEM cannot accommodate 

changes in k or L0 values in the same model run. Changing landfill conditions that cannot be 

modeled as a result of this limitation include the following: 

< Application of liquids to existing waste 

< Variations in waste composition over time 

< Installation of a geomembrane cover 

The LFG modeler should be aware of the potential for model error due to the above-listed factors and 

use appropriately conservative model inputs to avoid significant overestimation of LFG recovery. 

Accurate estimates that do not overestimate recoverable methane are critical to the proper design 

and financial success of LFG energy projects.