METHANE PRODUCTION, RECOVERY, AND
UTILIZATION FROM LANDFILLS.
by
Stephen C. James
and
Chris W. Rhyne
ABSTRACT
Municipal solid waste disposal sites are untapped sources
of methane gas. Landfill gas, which is mainly produced during
anaerobic decomposition, has a volume composition of 40 to 60
percent methane. Other gases produced are carbon dioxide,
nitrogen, and hydrogen sulfide.
The amount of recoverable gas will depend upon two factors:
the ultimate gas production (scf/pound of solid waste) and the
area-depth relationship. Ultimate gas production values have
ranged from .05 to 7.0 scf/pound of solid waste. These estimates
include lysimeter studies conducted at optimal conditions and
include the addition of sewage sludge to municipal solid
waste. It is estimated that 2.53 scf of raw landfill gas
per pound of solid waste will beNobtained at the Environmental
Protection Agency (EPA) gas recovery project at Mountain
View, California. Since this raw landfill gas is 441 percent
methane, 1.1 scf of methane per pound of solid waste will be
obtained.
The other factor concerns the area-depth relationship.
Because of the possibility of air infiltration, a deeper land-
fill is preferred to a shallow landfill, even though volume
is constant. If air infiltrates into the landfill, methane
production will cease or decline to a level where the methane
content of the raw gas is too low for economic recovery.
The heating value of raw landfill gas is approximately 450
BTU/scf. Depending on final use, landfill gas can be compressed,
dehydrated, and stripped of carbon dioxide and nitrogen to pro-
duce a gas with a heating value range from 650-1,000 BTU/scf.
* Mr. Stephen C. James is a sanitary engineer fo'r the Municipal
Environmental Research Laboratory, U.S. EPA in Cincinnati, OH
45268. Mr. Chris W. Rhyne is a sanitary engineer for the
Office of Solid Waste, U.S. EPA, Washington, D.C. 20460.
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Cost estimates for collection, treatment, and yearly
expenses indicate that landfill gas, while not presently com-
petitive with natural gas or oil, is competitive with LNG and
SNG. Thus landfill gas can be an important supplemental or
alternative fuel source.
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METHANE PRODUCTION, RECOVERY, AND
UTILIZATION FROM LANDFILLS
by
Stephen C. James
and
Chris W. Rhyne
INTRODUCTION
The microbiological decomposition process is constantly
occurring as leaves, dead animals, sewage sludge, garbage,
and other matter decay. This process occurs at solid waste
disposal sites, where tons of decomposable organics are placed
daily. The decay of these organic materials results in the
formation of landfill gas.
LANDFILL GAS PRODUCTION
Decomposition—The Basic Process
The process of landfill gas formation is mainly an anaerobic
process, not unlike that of a sewage sludge digester. Aerobic
digestion takes place initially because large quantities of air
are entrained in the waste during placement. The oxygen is
quickly consumed and the process becomes anaerobic shortly after
refuse placement.
Anaerobic refuse decomposition is a continuous process that
stabilizes the organic wastes and results in the production of
methane. The organic material, such as leaves, paper, and food
waste, is used as food for the acid-forming bacteria. This
organic material is then changed by the bacteria to simple
organic material, mainly organic acids. Methane-forming bacteria
then use these organic acids as food and produce carbon dioxide
and methane gas. This process is shown in figure 1.
When the waste stabilization process proceeds normally,
approximately 50 to 60 percent of the gas produced will be
methane. The remainder will primarily be carbon dioxide.
Methane formers grow quite slowly compared to the acid
formers since they obtain very little energy from their food.
This results in the methane formers being very sensitive to
slight changes in environmental factors. The acid formers are
rapid growers and are not so sensitive to environmental condi-
tions. Thus the production of landfill gas is largely depen-
dent upon maintaining optimum conditions for the methane-forming
bacteria.
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Environmental Factors Affecting the Decomposition Process
Table 1 summarizes the optimal conditions for anaerobic
digestion. Unfortunately, the landfill decomposition process
is different from a sewage sludge digestion process because
critical environmental factors (temperature, pH, and moisture)
cannot be economically controlled.
Temperature control is a key factor for successful anaerobic
stabilization of organic matter because sudden temperature changes
greater than 2 degrees centigrade will result in losing the
buffering capacity and possibly incapacitating the digester.
The temperature should also be controlled in the range of 29 to
37 degrees centigrade so that optimum gas production may be
achieved. Although the temperature in the landfill cannot be
controlled, it has been determined that the internal temperature
of many landfills falls within the optimum temperature range for
gas production. It has also been observed that the core tempera-
ture of deeper landfills is not affected by diurnal temperature
fluctuations.
The moisture content required for optimum anaerobic decom-
position has been reported to be greater than 60 percent. This,
again, often occurs in the landfill situation, although many
landfills with far lower percentages of moisture have been found
to produce large quantities of gas. Moisture addition at land-
fills has been proposed to enhance gas production; however,
potential leachate problems (ground and surface-water contamina-
tion) have precluded this approach on a large scale.
The optimal operating range for pH is from 6.8 to 7.2.
Many landfills report lower pH levels but still produce signi-
ficant quantities of gas. It is believed that the pH within
a landfill does not fall below 6.2 when methane is produced.
The factor which is probably most critical to the landfill
stabilization process, particularly when methane gas recovery
is anticipated, is air infiltration. Whenever methane gas is
removed from a landfill, there is a tendency for air infiltration
due to leakage through the recovery wells and landfill surface.
Air is toxic to the methane-forming bacteria and thus will
stop the production of methane gas. Here the physical configura-
tion (depth) of the landfill becomes an important factor because
the oxygen in the infiltrating air is consumed in the upper
portion of the landfill and does not hinder the anaerobic pro-
cess at the bottom of the landfill. Depths greater than 100
feet are ideal for landfill gas recovery. Depths as low as
30 to 40 feet are suitable for gas recovery, but more control
over minimizing air infiltration is needed.
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LANDFILL GAS RECOVERY
In the last 10 years, landfill operators, owners, and
engineers have become increasingly aware of the potential
hazard caused by the methane component of landfill gas. Methane
migrating through soils adjacent to the landfill has on occasion
collected in nearby structures and ignited, resulting in struc-
tural damage, injuries, and even deaths.
Control and Recovery Methods
Actual recovery of landfill gas for methane resulted from
efforts to stop the migration of gas to adjacent properties.
The first control methods used to prevent landfill gas migra-
tion were peripheral trenches filled with porous media or
peripheral vent pipes which allowed gas to vent to the atmos-
phere. These control methods were found to be generally
ineffective.
Recently, the technology has advanced to the point that
most new control systems are power exhaust vent systems com-
posed of wells and a header connected to an exhaust blower.
This advance in the technology coupled with the impending
natural gas shortage was the catalyst necessary to launch
the Los Angeles Sanitation District's Palos Verdes Gas Recovery
Project. The project is cosponsored by Reserve Synthetic
Fuels (formerly NRG Nu Fuel). Reserve Synthetic Fuels has
constructed a molecular sieve treatment facility which takes
raw, saturated landfill gas with a heating value of approxi-
mately 500 BTU/scf and sweetens it to 1,000 BTU/scf. "The
upgraded gas is then injected into a nearby Southern California
Gas Company main.
At the Palos Verdes landfill, the gas is recovered from
6 to 8 wells to meet a specific demand. The withdrawal rate
is up to 300 scfm per well, and each well is over 100 feet
deep. Very little trouble with air infiltration occurred due
to the depth of the recovery well. However, severe corrosion
in the regeneration heat exchanger system occurred because of
the presence of chlorinated hydrocarbons in the raw landfill
gas. The use of a corrosion-resistant nickel alloy in the
heat exchanger coupled with changes in the absorbent material
in the pretreating towers has eliminated the corrosion problem
Factors Affecting Recovery
The amount of gas recoverable at a site is dependent upon
the specifics of site construction and the operational aspects
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of a gas recovery system. Site geology is important because
it can permit containment of the landfill gas. A landfill
located in a clay formation or lined with a clay or synthetic
material will inhibit the movement of landfill gas from the
site and thus increase the potential for gas recovery.
The depth of the landfill is important because of air
infiltration. A minimum depth of 30 to 40 feet is required
for satisfactory recovery. This depth limitation is both an
economic and an engineering consideration. At a depth less
than 30 feet, the withdrawal rate must be kept low so that
air infiltration will not stop methane production. This
hurts the economics of gas recovery since the quantity of
recoverable gas is limited.
Mountain View, California
In June 1975, the U.S. Environmental Protection Agency's
Office of Solid Waste partially funded a project to demonstrate
the feasibility of recovering methane gas from a landfill of
"normal" depth. The landfill at Mountain View, California,
was chosen for its 40-foot depth. Additional participation
came from the Pacific Gas and Electric Company. The primary
objectives of this project were:
1. to determine the optimum withdrawal rate for a
shallow landfill;
2. to determine the gas quality at the chosen rate;
3. to determine the optimum well spacing;
4. to determine the effect of additional moisture
on gas production; and
5. to evaluate the applicability of various modes of
gas utilization.
In order to accomplish the above objectives, the demonstra-
tion phase was conducted. Two test pumping wells were constructed
with perforations at two depths (middle and bottom of the land-
fill) . A monitoring system was also installed to determine the
pressure gradient under both static and pumping situations. The
ensuing testing program revealed a great deal of information,
especially concerning air infiltration. It was found that air
moved freely in and out of the surface of the landfill as
barometric pressure rose and fell. This was unexpected, as it
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was thought that 2 feet of compacted clay cover would severely
reduce air infiltration. Consequently, when the pumping rate
of 200 scfm at these 40-foot wells commenced, the methane con-
tent of the gas went from 52 percent by volume to 32 percent
in 20 days. Nitrogen, during the same period, rose from 12
percent to 39 percent. After several more months of testing
it was found that at a pumping rate of 50 scfm, gas containing
44 percent methane, 34 percent carbon dioxide, 21 percent
nitrogen, and 1 percent oxygen could be consistently withdrawn.
Gas composition for this period is provided in Table 2. This
composition was then used to evaluate the various upgrading and
utilization options available.
LANDFILL GAS UTILIZATION
By far, the most inexpensive utilization option is direct
usage eith onsite or offsite by a nearby industrial user.
Direct utilization at Mountain View was not feasible since
there was no nearby user.
The decision was made by Pacific Gas and Electric Company
to inject the gas into a transmission line that ran across the
landfill site. Since the raw gas was deficient in several
respects (low heating value, saturated, and presence of carbon
dioxide, nitrogen, and oxygen), upgrading by removal of
contaminants was needed. Two conditions were required for
injection into the utility pipeline. First, the value of
the mixed gases had to remain about 975 BTU/scf in the
service area. Second, the average heating value solid to a
customer had to be within + 2 BTU/scf of that shown on the
customer's utility bill.
Gas Upgrading
Various upgrading schemes from simply dehydration to
dehydration, carbon dioxide, and nitrogen removal are available.
The more contaminants removed, the greater the heating value,
but the cost to produce the treated gas would also increase.
The raw gas at Mountain View has a heating value of 450
BTU/scf as compared to 1,000 BTU/sf for natural gas. Because
the volume of natural gas in the pipeline was much greater than
the volume of upgraded landfill gas to be added, it was decided
to upgrade the raw gas to approximately 700 BTU/scf. This
upgrading would be economically feasible and would meet the two
aforementioned criteria. To reach this quality, dehydration
and carbon dioxide removal by the molecular sieve process was
required. Figure 2 shows the basic flow diagram for this pro-
cess.
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Gas Recovery Economics
Table 3 shows the cost estimate for landfill gas recovery
at Mountain View. This estimate is based on a landfill gas
flow of 1 million cubic feet per day. If the pilot-scale
project is successful/ a production rate of 5 million cubic
feet per day is anticipated. This increased gas production
rate should allow for a 20 percent decrease in energy costs.
Thus the cost per million BTU should be reduced to around
$2.00. The data for Table 3 was based on an overall efficiency
of 70 percent, a 12 percent cost of capital, a 10-year life,
and a salvage value of 30 percent.
While landfill gas economics indicate that energy costs
will be more than the current price of natural gas and oil,
this technology holds promise because the economics are com-
petitive with SNG or LNG. Thus, landfill gas can be an
important supplemental or alternative fuel source.
BIBLIOGRAPHY
Blanchet, M.J. Recovery of methane due from north California
landfill. The Oil and Gas Journal, 74(46): 82-85, Nov. 15,
1976.
Blanchet, M.J. [Pacific Gas and Electric Company, San Francisco].
Treatment and utilization of landfill gas; Mountain view
project feasibility study. Environmental Protection Publication
SW-583. [Washington], U.S. Environmental Protection Agency,
1977. 115p.
Bowerman, F.R., N.K. Rohatigi, K.Y. Chen, and R.A. Lockwood.
A case study of the Los Angeles County Palos Verdes landfill
gas development project. Environmental Protection Publication
EPA/600/3-77/047. Cincinnati, U.S. Environmental Protection
Agency, 1977. 114p. (Distributed by National Technical
Information Service, Springfield, Va. as PB-272 241).
Carlson, J.A. [City of Mountain View, Calif.]. Recovery of
landfill gas at Mountain View; engineering site study. Environ-
mental Protection Publication SW-587d. [Washington], U.S.
Environmental Protection Agency, 1977. 63p. (Distributed by
National Technical Information Service, Springfield, Va., as
PB-267.373.)
Dair, F.R. Methane gas generation from landfills. APWA
Reporter, 44(3): 20-23, Mar. 1977.
Eberhart, R.C., ed. Proceedings; a symposium on the utiliza-
tion of methane generated in landfills, Laurel, Md., March 9-10,
1978. Sponsored by U.S. Department of Energy. The Johns Hopkins
University. Applied Physics Laboratory.
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FIGURE 1.
DIAGRAM OF WASTE STABILIZATION
Organic
Matter
Acid-Forming'
Bacteria
Organic
Acids
Methane-Forming
Bacteria
co
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FIGURE 2.
RAW LANDFILL GAS UPGRADING PROCESS AT MOUNTAIN VIEW, CALIFORNIA
Raw
Gas
-*
Compressor
->
Air
Cooler
->
Knock-Out
Drum
-»
Molecular
Sieves
-»
Treated
Gas
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TABLE 1.
OPTIMAL CONDITIONS FOR ANAEROBIC DECOMPOSITION
Anaerobic Coaditions
Temperature
pH
Moisture Content
Toxic Materials
No Oxygen (Air)
85 - 100° F (29 - 37° C)
6.8 - 7.2
Greater Than 40 Percent
None
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TABLE 2.
MEASURED GAS COMPOSITION AT MOUNTAIN VIEW
Average High Low
Methane 44.03 46.49 41.38
Carbon Dioxide 34.20 36.80 30.73
Nitrogen 20.81 23.51 19.98
Oxygen and Argon 0.96 1.69 0.48
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TABLE 3
COST ESTIMATE FOR LANDFILL GAS RECOVERY AT MOUNTAIN VIEW, CALIFORNIA
Equipment Cost Installed Cost
Molecular Sieves $245,000 $368,000
Compression 200,000 350,000
Wells and Gathering System —• — — — 70,000
Total Installed Cost $788,000
Yearly Costs $/Year
Maintenance 25,000
Manpower 30,000
Fixed Charges 195,000
Feedstock Costs 22,320
Total 272,320
Energy Output, MMBTU/yr 97,650
Energy Costs, $/MMBTU $2.79
MC1709
SW-710
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