United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-91/055 Dec. 1991
&EPA Project Summary
Analysis of Factors Affecting
Methane Gas Recovery from Six
Landfills
Darcy Campbell, David Epperson, Lee Davis,
Rebecca Peer, and Walter Gray
In 1990, EPA's Air and Energy Engi-
neering Research Laboratory (AEERL)
began a research program with the goal
of improving global landfill methane
(CH4) emissions estimates. Part of
AEERL's program Includes developing
a field testing program to gather data to
identify key variables that affect CH4
generation and to develop an empirical
model of CH4 generation based on those
variables. The first step in developing
the field testing program was a pilot
study of six U.S. landfills that have CH,
gas recovery systems. In order to evalu-
ate the effects of climate on CH4 pro-
duction and recovery, the sites were
chosen to represent a variety of mois-
ture and temperature patterns (I.e., hot
and wet, cool and wet, hot and dry).
Landfill gas was tested at each of the
six landfills In order to evaluate the
quality of the gas recovery data avail-
able at each site. The testing program
included assessing the adequacy of on-
site instrumentation and scanning the
landfill surfaces for organic vapors that
would Indicate emissions of CH4. In ad-
dition, information on waste composi-
tion and landfill characteristics was
sought for each landfill. Except for flow
measurements, the test procedures se-
lected for this project were well suited
to the types of gas recovery installa-
tions at the landfills visited. Based on
comparisons between EPA Reference
Method 3C and instrument analyses of
the landfill gas composition, all on-site
analysis instruments appeared to be
operating with reasonable accuracy.
Reviews of calibration procedures and
records indicate that long-term instru-
ment accuracy should be comparable
to the accuracies noted during on-site
testing. A negative correlation between
refuse age and CH4 recovery per ton
was found; a weak positive correlation
was found for normal annual precipita-
tion and CH4 recovery per ton. The re-
sults of this pilot study are sufficiently
encouraging to warrant further data
gathering and analyses.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
Ings of the research project that Is fully
documented In a separate report of the
same title (see Project Report ordering
Information at back).
Introduction
In response to concerns about global
warming, the U.S. Environmental Protec-
tion Agency's (EPA's) Office of Research
and Development (ORD) has initiated a
program to characterize the causes and
effects of global climate change, and to
identify and quantify emission sources of
greenhouse gases. To assist in this under-
taking, EPA's Air and Energy Engineering
Research Laboratory (AEERL) has begun
research to improve emissions inventories
of greenhouse gases in the U.S. and
throughout the world.
One greenhouse gas of particular con-
cern is methane (CH4). Methane's radia-
tive-forcing potential is thought to be much
greater than that of carbon dioxide (C02).
In 1990, AEERL began a research pro-
gram with the goal of improving global
landfill CH4 emissions estimates.
To determine the factors that affect
CH4 generation in landfills on a global ba-
sis, a model is needed that is responsive
to a wide range of climates and types of
waste. Part of AEERL's program to create
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a CH4 landfill emissions database, there-
fore, includes developing a field testing
program to gather data to: (I) identify key
variables that affect CH4 generation; and
(2) develop an empirical model of CH4
generation based on those variables.
Landfills with gas recovery systems
were chosen for this study. The landfill gas
is being collected and measured by the
gas recovery operators; if those data can
be verified to be reasonably accurate, and
if sufficient data are available on the land-
fill itself, the landfill gas measurements
collected over several years may be used
to estimate total CH4 generation.
The first step in developing the field
testing program was a pilot study of six
U.S. landfills that have landfill gas recov-
ery systems. The objectives of the pilot
study were to:
(1) Determine the type and quality of
landfill data on landfill gas recovery
rates, gas composition, and refuse
characteristics available at landfills
with gas recovery systems;
(2) Use these data to determine trends
in the effects of climate, refuse age,
and landfill characteristics on landfill
gas recovery; and
(3) Use the results of the emissions test-
ing and data analysis to assess the
relationship between gas recovery
and gas generation, and the feasibil-
ity of expanding the study to include
other sites.
To meet these objectives, a pilot study
of six sites, chosen to represent a range of
climates, was undertaken. The general pro-
cedures and methodologies planned were:
(1) Identify potential sites;
(2) Visit the landfills to collect data
records from the facility;
(3) Independently measure landfill gas
flow;
(4) Assess accuracy and adequacy of
the data; and
(5) Develop statistical methods for analy-
sis of the data.
Although the CH4 content of the landfill
gas is of most importance at this time,
other constituents were also measured (car-
bon dioxide, oxygen, nitrogen, and
nonmethane organic compounds).
Site Selection and Description
The pilot study included visits to six
landfills in the U.S. to gather data on CH4
recovery rates and factors thought to influ-
ence these rates. The primary criterion in
selecting a landfill for study was that it
have a gas recovery system in place. The
recovery system needed to be well-con-
trolled (i.e., operating under good engi-
neering practices to minimize leaks and
maximize CH4 recovery) so that the CH4
recovery data would be useful in estimat-
ing total CH4 production at the site. In
addition, well-maintained records on rou-
tine monitoring were needed for possible
gas migration at the perimeter and surface
of the landfill.
To evaluate the effects of climate on
CH4 production and recovery, sites were
sought in geographic regions representing
a variety of moisture and temperature pat-
terns (i.e., hot and wet, cool and wet, hot
and dry). Initial recommendations provided
by landfill gas recovery experts in the U.S.
were used to identify potential sites. Final
site selection was influenced largely by the
assurance that long-term gas production
and refuse composition data were avail-
able at the site, the suitability of the site for
sample acquisition, and the landfill
operator's willingness to cooperate in the
study.
A landfill survey form was sent to the
operators of selected sites prior to visiting
them so that they could begin gathering
the records. Sites were visited between
Augusts and 24, 1990.
Landfill Gas Test Procedures
and Results
Landfill gas was tested at each of the
six landfills in the pilot study in order to
evaluate the quality of the gas recovery
data available at each site. The testing
program included assessing the adequacy
of on-site instrumentation and scanning
the landfill surfaces for organic vapors that
would indicate emissions of CH4.
The following test procedures were
used:
(1) EPA Reference Method (RM) 3C was
used to determine CH4, CO2, nitro-
gen (N2), and oxygen (O2) levels;
(2) EPA RM 25C was used to test for
nonmethane organic compounds
(NMOC); and
(3) EPA RM 4 was used to test for the
volume percentage of moisture.
Initially, the volumetric flow rates of landfill
gas were to be tested using EPA RM 2.
However, mechanical difficulties were en-
countered at the landfills, making this test
impossible. In lieu of this test, calibration
records of on-site flow measurement in-
struments were obtained for three of the
six landfills. Tests for the presence of or-
ganic vapors near the landfill surface were
conducted using an organic vapor ana-
lyzer (OVA).
The on-site gas analyzer measurements
were compared to the results of EPA RM
3C testing and found to be reasonably
accurate; three of the CH4 analyzers ex-
hibited relative accuracies better than 10%,
and five were within 12%. In general, all of
the instruments were observed to be oper-
ating in a manner consistent with good
operating practices.
Statistical Methods
Development and Results
The ultimate objective of this research
program is to determine which variables
relating to refuse characteristics, landfill
characteristics, or climate are significant
determinants of gas production. This pilot
study addressed a small number of sites,
and the results were not intended to be
representative of all landfills. Rather, the
study was intended to provide the basis for
development of statistical methods for use
in a larger study, to identify data quality
issues, and to look for trends.
The data obtained from each landfill
consisted of computer printouts or hand-
written data sheets listing total gas flow,
percent CH4 composition of the total gas
flow, and other information applicable to
the individual landfills. The data were usu-
ally in the form of daily averages of hourly
flow rates, and were reported for each on-
line gas recovery unit.
The descriptive data for each landfill
are summarized ir Table 1. The average
CH4 flow in standard cubic feet per minute
(cfm)* was calculated from daily averages
supplied by site operators. Although the
between-landfill variation is large, ranging
from 590 to 3477 cfm (16.71 to 98.47 m3/
min), the day-to-day variability is relatively
small, as shown by the coefficients of varia-
tion, which were generally below 10% ex-
cept for Landfill 6 (12.4%).
One objective of this study was to de-
termine if sufficient data were available for
a time series analysis of CH4 emission
rates from individual landfills. Methane re-
covery is a relatively new process, and
none of these landfills had records for CH4
emissions of sufficient length (several
years) and completeness for time series
analysis. It is highly probable that emis-
sions are autocorrelated so that any at-
tempt to find correlations between CH4
recovery and weather data on a daily or
monthly basis is likely to be confounded by
autocorrelations in the data. Since the
strength of autocorrelation decreases with
averaging period, only annual averages
were used in the statistical analysis of the
relationship between long-term CH4 emis-
sions and weather data between landfills.
The annual CH4 averages were correlated
to annual averages of temperature and
precipitation obtained from 30 years of
* Flow rate at 25°C and 1 atmosphere
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T»bl0 1.
Summary Statistics for Each Landfill Calculated from Daily CH4 and Weather Data
Landfill
Parameter
Analysis period
Number of wells
Average well depth (m)
Number of hectares
Refuse mass (10* Mg)
Average landfill depth (m)
1990 average age (years)
Total Methane Flow
Number of days
Mean (trf/min) (cfm)
Standard deviation
Coefficient of variation (%)
Temperature
Mean (°C) during analysis period
30-year normal
Precipitation
Total (in.) (cm) during
analysis period
30-year normal
1
5/89 to 4/90
45
14
35
6.3
67
8
194
55.36 (1995)
2.12
3.80
7.34
7.51
31.7(80.5)
28.8 (73.2)
2
10/89 to 7/90
65(44 VA)*
14
55
6.1
26
10
302
18.04 (637)
1.19
6.60
7.67
9.28
34.0 (86.4)
35.6(90.4)
3
8/89 to 7/90
31
23
51
7.3
66
10
314
40.07(1415)
2.32
5.80
10.51
12.23
43.9(111.5)
42.4(107.7)
4
7/89 to 6/90
111
21
57
13.8
56
9.50
85
98.47(3477)
1.33
1.40
24.96
23.96
40.0 (101.6)
61.4(156.0)
5
1/90 to 8/90
102
34
32
10.9
46
15
209
24.86 (878)
1.70
6.80
16.12
17.12
9.0 (22.9)
17.0(43.2)
6
5/89 to 4/90
68
10
40
2.6
10
7
37
16.71 (590)
2.07
12.40
16.57
16.18
16.6 (42.2)
17.9(45.5)
*VA = very active; other wells were primarily for odor control.
data, as well as to other landfill param-
eters.
Table 2 shows the Pearson correlation
coefficients between annual CH4 flow rates
and CH4 flow rates per unit mass with the
annual and long-term (normal) weather data
and other landfill parameters for the six
landfills. Only two correlations with CH4
flow rate were found to be significant at the
95% confidence level, and no correlation
coefficients were significant with the CH4
flow rate per unit mass. The low number of
significant correlations can be attributed,
at least in part, to the low number of obser-
vations. The normal annual precipitation
correlated fairly well with the 1 -year annual
mean CH4 flow rate and, even though it
was not significant for the CH4 flow rate per
unit mass, it had the largest positive corre-
lation coefficient. The correlation coeffi-
cient for refuse mass with CH4 flow rate
was just under the cutoff point for signifi-
cance at the 95% confidence level, but its
value of 0.71 suggests that perhaps with
more data it would be significant.
Conclusions
Except for flow measurements, the test
procedures selected for this project were
well suited to the types of gas recovery
installations encountered at the landfills
visited. Alternative flow measurement meth-
ods that are more appropriate to the site
conditions must be identified if flow mea-
Tabl* 2. Correlation Coefficients of CH4 Recovery Variables with Landfill Parameters and
Summarized Weather Data (n=6)
Dependent Variables
Independent Variables
Annual temperature (1989-1990)
Normal annual temperature
Annual precipitation (1989-1990)
Normal annual precipitation
1990 mean age of landfill
Number of wells
Tons of refuse
Mean depth of landfill
Area of landfill
Volume of landfill
Mean well depth
Annual Methane
Recovery Rate
0.56
0.51
0.55
o.er
-0.15
0.37
0.71
0.62
0.37
0.74'
0.10
Annual Methane
Recovery
Rate per Unit Mass
0.12
0.01
0.33
0.25
-0.80"
-0.15
-0.18
0.26
-0.04
0.24
-0.58
Correlation coefficient significant at 95% confidence level.
Correlation coefficient significant at 90% confidence level.
surements are desired in the future. Since
all sites record flow data, however, a qual-
ity assurance program could be used to
determine the acceptability of the on-site
data.
Based on comparisons between the
RM 3C and instrument analyses of the
landfill gas composition, all on-site analy-
sis instruments appeared to be operating
within reasonable accuracy ranges. Re-
views of calibration procedures and records
indicate that long-term instrument accu-
racy should be comparable to the accura-
cies noted during on-site testing.
•^ U.S. GOVERNMENT PRINTING OFFICE: W»2 - 648-080/401M
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Although the results of this pilot study
are sufficiently encouraging to warrant fur-
ther data gathering and analyses, some
limitations need to be recognized. The main
problem was that the collection efficien-
cies of the CH4 recovery systems were not
known. Where emission control was one
(or the only) reason for the collection
system's existence, efficiency appeared to
be high. However, this is a qualitative as-
sessment based on visual inspection of
the landfills and an assessment of operat-
ing practices at the landfills.
One key piece of information is missing
from most landfills: the average composi-
tion of the refuse. Waste composition un-
doubtedly contributes to data variability but,
unfortunately, it is not possible to get com-
position information for most landfills in the
U.S. Also, it is impossible to fully account
for differences in the structure and operat-
ing characteristics of landfills. All of these
unknowns contribute to the variability of
the CH4 flow rate data. Although it should
be possible to explain some of the variabil-
ity, a certain amount will always remain.
It is likely that the functional relation-
ship between CH4 per ton of refuse and
age and climate are nonlinear, or that in-
teractions between these variables pro-
duce nonlinearities. With a larger sample,
it may be possible to identify these
nonlinearities, and fit the data to the appro-
priate model.
D. Campbell, D. Epperson, L Davis, P. Peer, and W. Gray are with Radian Corp.,
Research Triangle Park, NC 27709.
Susan A. Thorn eloe is the EPA Project Officer, (see below).
The complete report, entitled "Analysis of Factors Affecting Methane Gas Recovery from
Six Landfills," (Order No. PB92-101351/AS; Cost: $35.00, subject to change) will be
available only from:
National Technical Information Seivice
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
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Penalty for Private Use $300
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