AVAILABLE AND EMERGING TECHNOLOGIES

Office of Air and Radiation June 2011


AVAILABLE AND EMERGING TECHNOLOGIES
FOR REDUCING GREENHOUSE GAS EMISSIONS
FROM MUNICIPAL SOLID WASTE LANDFILLS
2
Available and Emerging Technologies for Reducing
Greenhouse Gas Emissions from Municipal Solid Waste
Landfills
Prepared by the
Sector Policies and Programs Division
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
June 2011
3
Table of Contents
Abbreviations and Acronyms ……………………………………………………………………………………………………….. 4
I. Introduction ……………………………………………………………………………………………………………………………. 6
II. Purpose of this Document ………………………………………………………………………………………………………… 6
III. Description of Municipal Solid Waste Landfills ………………………………………………………………………… 6
IV. Summary of Control Measures ………………………………………………………………………………………………… 8
V. Available Control Technologies for GHG Emissions from MSW Landfills ………………………………….. 10
A. LFG Collection Efficiency Improvement ……………………………………………………………………………… 10
B. LFG Control Devices …………………………………………………………………………………………………………. 12
C. Increase of CH4 Oxidation ………………………………………………………………………………………………….. 17
D. Economic Analysis ……………………………………………………………………………………………………………. 18
VI. Bioreactor Landfill Systems ………………………………………………………………………………………………….. 20
VII. Management Practices …………………………………………………………………………………………………………. 21
EPA Contact…………………………………………………………………………………………………………………………….. 22
References ……………………………………………………………………………………………………………………………….. 23
Appendix A ……………………………………………………………………………………………………………………………… 26
Calculations to Estimate Cost Effectiveness for CO2e Reduced ……………………………………………………….. 26
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Abbreviations and Acronyms
ADEME French Agency for Environmental and Energy Management
ATSDR Agency for Toxic Substances and Disease Registry
BAAQMD Bay Area Air Quality Management District
BACT Best available control technology
Btu British thermal units
CCAR California Climate Action Registry
CCTP Climate Change Technology Program
CEC California Energy Commission
CH4 Methane
CHP Combined heat and power
CNG Compressed natural gas
CO Carbon monoxide
CO2 Carbon dioxide
CO2e Carbon dioxide equivalents
CPTR Cost Incurred Per Metric Ton of Reduced CO2e
DER Distributed Energy Resource
GHG Greenhouse gas
HAP Hazard air pollutants
H2 Hydrogen
H2S Hydrogen sulfide
kW Kilowatts
lb Pound
LFG Landfill gas
LFGcost Landfill Gas Energy Cost Model
LFGE Landfill gas energy
LMOP Landfill Methane Outreach Program
LNG Liquefied natural gas
Mg Megagrams
MSW Municipal solid waste
MT Metric ton
MW Megawatts
MWh Megawatt-hour
N2 Nitrogen
N2O Nitrous oxide
NESHAP National Emission Standards for Hazardous Air Pollutants
NMOC Nonmethane organic compounds
NOx Nitrogen oxides
NREL National Renewable Energy Laboratory
NSPS New Source Performance Standard
O2 Oxygen
ppmv Parts per million by volume
PSD Prevention of significant deterioration
psi Pounds per square inch
RCRA Resource Conservation and Recovery Act
scfm Standard cubic feet per minute
SOx Sulfur oxides
5
SWICS Solid Waste Industry for Climate Solutions
WARM Waste Reduction Model
6
I. Introduction
This document is one of several white papers that summarize readily available information on
control techniques and measures to mitigate greenhouse gas (GHG) emissions from specific
industrial sectors. These white papers are solely intended to provide basic information on GHG
control technologies and reduction measures in order to assist States and local air pollution
control agencies, tribal authorities, and regulated entities in implementing technologies or
measures to reduce GHG under the Clean Air Act, including, where applicable, in permitting
under the prevention of significant deterioration (PSD) program and the assessment of best
available control technology (BACT). These white papers do not set policy, standards or
otherwise establish any binding requirements; such requirements are contained in the applicable
EPA regulations and approved state implementation plans.
II. Purpose of this Document
This document provides information on control techniques and measures that are
available to mitigate GHG emissions from the municipal solid waste landfill sector at this time.
Because the primary GHG emitted by the municipal solid waste landfill industry are methane
(CH4) and carbon dioxide (CO2), the control technologies and measures presented in this
document focus on these pollutants. While a large number of available technologies are
discussed here, this paper does not necessarily represent all potentially available technologies or
measures that that may be considered for any given source for the purposes of reducing its GHG
emissions. For example, controls that are applied to other industrial source categories with
exhaust streams similar to the municipal solid waste sector may be available through
“technology transfer” or new technologies may be developed for use in this sector.
The information presented in this document does not represent U.S. EPA endorsement of
any particular control strategy. As such, it should not be construed as EPA approval of a
particular control technology or measure, or of the emissions reductions that could be achieved
by a particular unit or source under review.
III. Description of Municipal Solid Waste Landfills
The term municipal solid waste (MSW) landfill refers to an entire disposal facility in a
contiguous geographic space where municipal waste is placed in or on land. The term does not
cover land application units, surface impoundments, injection wells, or waste piles. Many MSW
landfills receive other types of waste, such as construction and demolition debris, industrial
wastes, and sludge. The information presented in this paper refers to landfills that primarily
receive MSW, as defined in the criteria for MSW landfills under the Resource Conservation and
Recovery Act (RCRA) regulations (40 CFR Part 258).
According to 2009 data, 54% of MSW in the United States was landfilled, 12% was
incinerated, and 34% was recycled or composted (EPA, 2010a). There were approximately
1,800 op
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8
CH4 and 1,000 metric tons of CO2 would have CO2e emissions of 22,000 metric tons [= (1,000 x
21) + 1,000].
Landfills primarily use the “area fill” method which consists of waste placement on a
liner, spreading the waste mass in layers, and compaction with heavy equipment. Daily cover is
then applied to the waste mass to prevent odors, blowing litter, scavenging, and vectors (carriers
capable of transmitting pathogens from one organism to another). Landfill liners may be
comprised of compacted clay or synthetic materials to prevent off-site gas migration and to
create an impermeable barrier for leachate. A final cover or cap is placed on top of the landfill,
after an area or cell is completed, to prevent erosion, infiltration of precipitation, and for odor
and gas control.
Methane generation in landfills is a function of several factors, including: (1) the total
amount of waste; (2) the age of the waste, which is related to the amount of waste landfilled
annually; (3) the characteristics of the MSW, including the biodegradability of the waste; and (4)
the climate where the landfill is located, especially the amount of rainfall. Methane emissions
from landfills are a function of methane generation, as discussed above, and (1) the amount of
CH4 that is recovered and either flared or used for energy purposes, and (2) the amount of CH4
that leaks out of the landfill cover, some of which is oxidized.
IV. Summary of Control Measures
Table 1 summarizes the GHG control measures presented in this document. Where
available, the table includes emission reduction potential, capital costs, operating and
maintenance costs, and any important details on the applicability of the control.
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Table 1. Summary of GHG Control Measures for MSW Landfills
Measure Applicability CH4
Reductiona
Typical
Capital
Costsb
Typical
Annual
O&M
Costsb
Cost
Effectiveness
($/metric ton of
CO2e reduced)e
Notes/Issues
LFG
Collection
Efficiency
Improvement
All landfills with
gas collection
systems
Varies $24,000/acre $4,100/acre NA Cost and performance
varies depending on the
type of cover material.
Flare All landfills with
gas collection
systems
99% $6 – $25 Emits secondary criteria
pollutant emissions (e.g.
NOx and CO.
No revenue.
Turbine For larger
landfills with
gas collection
systems
99% $1,400/kW
(≥3 MW)
$130/kW $12 – $18 Emits secondary criteria
pollutant emissions (e.g.
NOx and CO).
Generates revenue for
landfills.
Engine 96-98% $1,700/kW
(≥800 kW)
$180/kW $12 – $16
Microturbine 99% $5,500/kW
(≤1 MW)
$380/kW $2 – $13
Small Engine 96-98% $2,300/kW
(≤1 MW)
$210/kW $11
CHP Engine 96-98% $2,300/kW
(≥800 kW)
$180/kW $7 – $57
CHP Turbine 99% NA NA $4 – $51
CHP
Microturbine
99% NA NA $9 – $64
Direct Use
(boilers,
heaters, etc.)
Varies by
technology
$960/scfmc +
$330,000/miled
$90/scfmc,d NA
Biocover All landfills Up to 32% $48,000/acre NA $745 No extensive retrofit.
Biofiltration
Bed
Landfills with
passive or no gas
collection
systems
Up to 19% NA NA NA Low cost.
a References provided in section V of this document for the different control measures.
b Costs for collection system & flare, turbines, engines, microturbines, small engines, and direct use obtained from Chapter 4
(Project Economics and Financing) of LMOP’s Landfill Gas Energy Project Development Handbook (EPA, 2010c), Costs
for CHP engines determined by evaluating the engine case study in the handbook as a CHP engine using LMOP’s LFGcost
model (EPA, 2010d).
c Costs for gas compression and treatment.
d Costs for pipeline and condensate management system (if applicable).
e Cost effectiveness obtained from analysis done by BAAQMD for conventional landfills with a medium compacted waste
density (BAAQMD, 2008), with adjustments made to determine the costs per metric ton of CO2e reduced from the
combustion of CH4, instead of the costs per metric ton of CO2e avoided from displacement of power generation. See section
V.D and Appendix A for additional information.
NA = not available
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V. Available Control Technologies for GHG Emissions from MSW Landfills
This section describes the available technologies for controlling GHG emissions from
MSW landfills. The available control technologies are divided into three categories: LFG
collection efficiency improvement, LFG control devices, and increase of CH4 oxidization. An
economic analysis of the control technologies discussed is also included. It should also be noted
that large landfills with emissions exceeding 50 megagrams (Mg) NMOC or more are required
by New Source Performance Standards (NSPS) to control and/or treat LFG to significantly
reduce the amount of toxic air pollutants released. In essentially all cases, controls required by
the NSPS will co-control the GHG emissions.
A. LFG Collection Efficiency Improvement
Collection efficiency is contingent upon landfill design and the manner in which landfills
are operated and maintained. Gas collection efficiency can be improved by implementing
rigorous gas well and surface monitoring and leak identification and repair. Factors contributing
to variability in collection efficiency are discussed below.
There are two types of LFG collection systems, active and passive. Passive systems rely
on the natural pressure gradient between the waste mass and the atmosphere to move gas to
collection systems. Most passive systems intercept LFG migration and the collected gas is
vented to the atmosphere. Active systems use mechanical blowers or compressors to create a
vacuum that optimizes LFG collection (ATSDR, 2001a).
For active gas collection systems, the collection efficiency depends primarily upon the
design and maintenance of the collection system and the type of materials used to cover the
landfill (BAAQMD, 2008). In the background information document for the draft updated
landfill AP-42 chapter, a typical collection efficiency range of 50% to 95% is given with a
suggested average of 75% (EPA, 2008a).
EPA’s Office of Research and Development has completed a field test program using
optical remote sensing technology (EPA’s OTM-10) to quantify LFG collection efficiency.
Sampling was conducted at three MSW landfills to evaluate CH4 emissions across the landfill
footprint to compare to the quantity of extracted gas (i.e., rate of fugitive CH4 vs. rate of
collected CH4). The preliminary results suggest gas collection efficiencies from 36% to 85%
reflecting a range based on landfill design and operational differences. The report is under
review and is expected to be released in 2011.
Higher collection efficiencies may be achieved at landfills with well maintained and
operated collection systems, a liner under the waste, and a cover consisting of a geomembrane
and a thick layer of clay. Studies conducted by the Solid Waste Industry for Climate Solutions
(SWICS) indicate collection systems meeting the requirements of NSPS, Subpart WWW are
often more capable of achieving higher collection efficiencies than collection systems used
solely for energy recovery because it is difficult to optimize gas quality while trying to attain a
high level of gas collection (SWICS, 2009).
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Results of gas collection efficiency studies for various cover materials using flux box
measurements are documented in Spokas et al. (2005). The data were used to develop default
values of percent recovery for the French environment agency (ADEME). These default
collection efficiencies for active gas collection systems are listed in Table 2.
Table 2. LFG Collection Efficiencies for Various Cover Materials
Type of Landfill Cover
Material Gas Collection Efficiency
Operating cell (no final cover) 35%
Temporary cover 65%
Clay final cover 85%
Geomembrane final cover 90%
Gas collection research studies done by SWICS used flux box data, which may
potentially under estimate gas collection efficiency. The resulting collection efficiencies for
landfills with active gas collection systems are summarized below (SWICS, 2009):
• 50-70% (mid-range default = 60%) for a landfill or portions of a landfill that are
under daily soil cover;
• 54-95% (mid-range default = 75%) for a landfill or portions of a landfill that contain
an intermediate soil cover; and
• 90-99% (mid-range default = 95%) for landfills that contain a final soil and/or
geomembrane cover systems.
As shown in Table 3, the mid-range default values for the three cover types identified
above were adopted as the collection efficiencies listed in the GHG reporting rule for MSW
landfills (40 CFR 98, Subpart HH, Table HH-3). The collection efficiency of a passive gas
collection system is assumed to be zero because the pressure gradient is unknown and would
likely vary in time and space.
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Table 3. LFG Collection Efficiencies in the GHG Reporting Rule
Description Gas Collection
Efficiency
Area without active gas collection,
regardless of cover type 0%
Area with daily soil cover and active gas
collection 60%
Area with an intermediate soil cover, or a
final soil cover not meeting the criteria below
to achieve 95% efficiency, and active gas
collection
75%
Area with a final soil cover of 3 feet or
thicker of clay and/or geomembrane cover
system and active gas collection
95%
As shown is Table 3, landfills with final geomembrane covers have higher collection
efficiencies. Changing the final cover material can improve gas collection efficiency. This
technology is applicable for all landfills. Typically, modern landfills with active gas collection
systems have clay or geomembrane covers in place. An additional geomembrane or clay cover
can be added to older landfills with gas collection systems to reduce LFG emissions (BAAQMD,
2008).
B. LFG Control Devices
After collection, LFG may be controlled and/or treated for subsequent sale or use as an
energy source to create electricity, steam, heat, or alternate fuels such as pipeline quality gas or
vehicle fuel. With approximately half the heating value of natural gas (350 to 600 British
thermal units (Btu) per cubic foot), LFG is considered a medium Btu gas. Combustion of LFG is
the most common method used to reduce the volatility, global warming potential, and hazards
associated with LFG. Combustion methods include destruction devices (e.g., flares), electricity
generation units (e.g., reciprocating engines, gas turbines), and energy recovery technologies
(e.g., boilers). During the combustion process, CH4 in LFG is converted to CO2. Since CH4 has
21 times the global warming potential of CO2, combustion reduces the global warming effect of
LFG significantly. Although CH4 has 21 times the global warming potential of CO2, combusting
CH4 reduces the global warming potential only by a factor of 7.6 because the resulting CO2
weighs more than the CH4 by a factor of 2.75. Combustion of LFG also reduces odors and other
hazards associated with LFG emissions. However, combustion units emit secondary criteria
pollutants, such as carbon monoxide (CO) and nitrogen oxides (NOX), as well as hazardous air
pollutants (HAP). Fuel cells are considered a non-combustion treatment option for LFG that
converts the gas to energy.
The control devices frequently used for LFG and the associated control efficiencies are
described in the following sections. It is important to note that all of the technologies discussed
13
below typically require treatment of the LFG prior to entering the control device to remove
moisture, particulates, and other impurities. The level of treatment depends primarily on the type
of control and the types and amounts of contaminants in the LFG. A list of common LFG
constituents is found in Tables 2.4-1 and 2.4-2 of the landfill AP-42 chapter (EPA, 1998a).
Some of the major trace contaminants in LFG that may need to be treated prior to control include
sulfur compounds, such as hydrogen sulfide (H2S), and siloxanes.
Flares
Of the combustion methods, flaring is the most commonly used. However, unlike other
combustion options, flaring does not recover energy. Controlling LFG emissions by flares is
technically feasible for most landfills and many landfills have flares in place. The capital and
maintenance costs associated with flares are relatively low compared to other combustion
technologies. Flares are often used as backup control devices for landfills that have engines or
turbines to generate electricity to limit emissions while these devices are off-line or to respond to
variations in LFG generation.
Two different types of flares are available, open flares and enclosed flares. Open flares
employ simple technology where the collected gas is combusted in an elevated open burner. A
continuous or intermittent pilot light is generally used to maintain the combustion. While open
flares are thought to have combustion efficiencies similar to those of enclosed flares, data are not
available to confirm this because open-air combustion makes them difficult to test. Under NSPS,
Subpart WWW, open flares must meet a minimum Btu content and have a pilot light. For
landfills generating LFG that is unable to meet the Btu content consistently, it may be necessary
to supplement the collected gas with natural gas or another fuel source, which may create an
additional cost for the landfill.
Enclosed flares typically employ multiple burners within fire-resistant walls, which allow
them to maintain a relatively constant and limited peak temperature by regulating the supply of
combustion air (ATSDR, 2001b). Enclosed flares can be tested for destruction efficiency of
NMOC and HAP. The background information document for the draft updated landfill AP-42
chapter provides an NMOC control efficiency range of 86% to 100% for flares, with an average
of 97.7% (EPA, 2008a). A report published by California’s Bay Area Air Quality Management
District (BAAQMD) states that flares typically have CH4 destruction efficiencies of greater than
99.5% (BAAQMD, 2008). Under NSPS, Subpart WWW, enclosed flares are considered to be
incinerators and are required to have a minimum NMOC control efficiency of 98% by weight.
In California, flares are required to have minimum CH4 destruction efficiencies of 99% (CCR,
Article 4, Subarticle 6, Section 95464(b)(2)(A)(1)).
Electricity Generation
Internal combustion engines are the most widely used technology for the conversion of
LFG to electricity. Advantages of this technology include: low capital cost, high efficiency, and
adaptability to variations in the gas output of landfills. The operation of reciprocating engines at
low pressure (12-30 pounds per square inch (psi)) also yields less condensate than operation at
14
higher pressure (60-160 psi) (Potas, 1993). Internal combustion engines are primarily used at
sites where gas production can generate 100 kilowatts (kW) to 3 megawatts (MW) of electricity,
or where sustainable LFG flow rates to the engines are approximately 50 to 960 cubic feet per
minute (cfm) at 50% CH4 (EPA, 2010d). For sites able to produce more than 3 MW of
electricity, additional engines may be added.
Turbines are an alternative to internal combustion engines. Turbines using LFG require a
dependable gas supply for effective operation, and are generally suitable for landfills when gas
production can generate at least 3 MW, or where sustainable LFG flow rates to the turbines are
over approximately 1,050 cfm at 50% CH4 (EPA, 2010d). Typically, LFG-fired turbines have
capacities greater than 5 MW. Advantages of this technology when compared to internal
combustion engines include: a greater resistance to corrosion damage, relatively compact size,
and lower operation and maintenance costs. When compared with other generator options,
turbines require additional power to run the plant’s compression system.
Microturbines can be used instead of internal combustion engines for LFG energy
conversion. This technology generally works best for small scale recovery projects that supply
electricity to the landfill or to a site that is in close proximity to the landfill. Single microturbine
units have capacities ranging between 30 and 250 kW, and are most suitable for applications
below 1 MW, or where sustainable LFG flow rates to the microturbines are below approximately
350 cfm at 50% CH4 (EPA, 2010d). Sufficient LFG treatment is generally required for
microturbines and involves the removal of moisture and other contaminants (EPA, 2010c).
In general, turbines have a higher CH4 destruction efficiency (greater than 99.5%) than
internal combustion engines (roughly 96%) (BAAQMD, 2008). For landfills subject to NSPS,
Subpart WWW, control technologies are required to have a minimum control efficiency of 98%
by weight NMOC reduction or an outlet concentration of 20 parts per million by volume (ppmv),
dry basis as hexane at 3% O2, of NMOC. In California, LFG control devices other than flares
must achieve a CH4 destruction efficiency of at least 99% by weight; and lean burn internal
combustion engines must reduce the outlet CH4 concentration to less than 3,000 ppmv, dry basis,
corrected to 15% O2 (CCR, Article 4, Subarticle 6, Section 95464(b)(3)(A)). Lean burn internal
combustion engines are not defined within this California regulation; however, the NSPS for
stationary spark ignition internal combustion engines (40 CFR 60, Subpart JJJJ) defines lean
burn engines as any two-stroke or four-stroke spark ignited engine that does not meet the
definition of a rich burn engine. Rich burn engines are defined as any four-stroke spark ignited
engine where the manufacturer’s recommended operating air/fuel ratio divided by the
stoichiometric air/fuel ratio at full load conditions is less than or equal to 1.1.
Cogeneration
Cogeneration, also known as combined heat and power (CHP), is the use of LFG to
generate electricity while recovering waste heat from the LFG combustion device. The thermal
energy recovered is usually in the form of steam or hot water that can be used for on-site heating,
cooling, or process needs. Cogeneration systems are typically more efficient and often more cost
effective than separate systems for heat and power (EPA, 2008b). Combustion technologies
15
generally suitable for CHP include internal combustion engines, gas turbines, and microturbines.
There are also boiler/steam turbine applications where LFG is combusted in large boilers for
steam generation that is then used by turbines to create electricity (EPA, 2010c).
The CH4 control efficiency for cogeneration is directly linked to the electricity generation
unit combusting LFG. Landfills subject to NSPS, Subpart WWW, must meet the same
requirements for cogeneration as those listed above for electricity generation.
Direct Use
Landfill gas may be used to offset traditional fuel sources such as natural gas, coal, and
fuel oil used in industrial, commercial, and institutional applications. Direct use of LFG is
primarily limited to facilities within 5 miles of a landfill. There are, however, facilities that have
used LFG as a fuel at distances greater than 10 miles. Direct use applications for landfills
include: boilers (LFG used solely or co-fired with other fuels), direct thermal technologies (e.g.
dryers, heaters, kilns), and leachate evaporation. Innovative uses of LFG include heating
greenhouses, firing pottery, glassblowing, metalworking, and heating water for an aquaculture
(fish farming) operation (EPA, 2010c).
Control efficiencies of CH4 for LFG direct use applications vary depending on the type of
technology employed. For landfills subject to NSPS, Subpart WWW, control technologies are
required to have a minimum control efficiency of 98% by weight NMOC reduction or an outlet
concentration of 20 parts per million by volume (ppmv), dry basis as hexane at 3% O2, of
NMOC. In addition, if a boiler or process heater is used as the control device, the collected LFG
must be routed into the flame zone.
Alternate Fuels
Purification techniques can be used to convert LFG to pipeline-quality natural gas,
compressed natural gas (CNG), or liquefied natural gas (LNG). Purification of LFG for the
production of natural gas typically involves the removal of inert constituents by adsorption
(molecular sieve), absorption with a liquid solvent, and membrane separation. The production of
pipeline-quality gas includes processing LFG to increase its energy content and pressurizing the
pipeline that is connected to the gas production facility (CCTP, 2005).
The conversion of LFG to CNG and LNG require similar processes, and the resulting
products can be used as vehicle fuel. First, the corrosive materials are removed through the use
of phase separators, coalescing filters, and activated carbon adsorbents. Next, water and O2 are
removed. A cryogenic purifier is then used to remove CO2, which yields high quality gas that is
over 90% CH4 (CCTP, 2005).
The type of LFG alternative fuel production and end use will affect the CH4 control
efficiency. For landfills subject to NSPS, Subpart WWW, control technologies are required to
have a minimum control efficiency of 98% by weight NMOC reduction or an outlet
concentration of 20 parts per million by volume (ppmv), dry basis as hexane at 3% O2, of
16
NMOC. If the collected gas is routed to a treatment system, including purification and
conversion devices, then vented gases from the treatment system must meet these requirements.
Fuel Cells
A fuel cell is an electrochemical cell that converts energy from a fuel into electrical
energy. Electricity is generated from the reaction between a fuel supply and an oxidizing agent.
The products of basic fuel cell reactions are CO2, water vapor, heat, and electricity (Vargas,
2008). The difference between a battery and a fuel cell is that in a battery, all reactants are
present within the battery and are slowly being depleted during the use of the battery. In a fuel
cell, reactants (fuel) are continuously supplied to the cell (CEC, 2003). Fuel cells are used in a
variety of applications to generate clean electricity without the use of combustion such as in
generating transportation fuels for car, boats, and buses. Also fuel cells can serve as a power
source in remote locations such as spacecraft, remote weather stations, parks, and in military
applications. Fuel cells running on hydrogen are compact and lightweight and have no major
moving parts.
For LFG applications, fuel cells use hydrogen from CH4 to generate electricity (EPA,
1998b). Fuel cells have an advantage over combustion technologies in that the energy efficiency
is typically higher without generating combustion by-products such as NOX, CO, and sulfur
oxides (SOX) (EPA, 1998c). If fuel cells are used to generate electricity from landfill CH4, then
a gas cleanup system is required to ensure that the catalyst within the fuel cell is not
contaminated by trace constituents that are present in LFG. Trace constituents include sulfur and
chlorine compounds which can inhibit performance and poison the catalyst (NREL, 1998).
EPA’s Office of Research and Development conducted a review of fuel cells for LFG
applications. The phosphoric acid fuel cell was identified as most appropriate because it is
commercially available and has been successfully demonstrated at two landfills. Other types of
fuel cells (molten carbonate, solid oxide, polymer electrolyte membrane) may also be applicable
for LFG applications as further fuel cell development is conducted. The first demonstration of a
fuel cell was at the Penrose Landfill in California. The second was at a Connecticut landfill.
Both demonstrations used a 200 kW phosphoric acid fuel cell manufactured by ONSI
Corporation (EPA, 1998b). The energy efficiency for the demonstration at the Connecticut
landfill was 37% at 120 kW and could have been higher if the waste heat had been utilized. The
trace constituents removed in the gas clean up system were flared. An environmental and
economic evaluation of a commercial fuel cell energy system concluded that there is a large
potential market for fuel cells in this application. The major disadvantage is that the cost is
higher compared to combustion technologies such as internal combustion engines and turbines.
For landfills subject to NSPS, Subpart WWW, control technologies are required to have a
minimum control efficiency of 98% by weight NMOC reduction or an outlet concentration of 20
parts per million by volume (ppmv), dry basis as hexane at 3% O2, of NMOC. If the collected
gas is routed to a treatment system, including conversion devices, then vented gases from the
treatment system must meet these requirements.
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C. Increase of CH4 Oxidation
The technologies to increase the CH4 oxidation rate include biocovers and biofiltration
beds. The principle of these technologies is the use of methanotrophic bacteria, which oxidize
LFG, specifically CH4, to water, CO2, and biomass. Methanotrophic bacteria possess the CH4
mono-oxygenase enzyme that enables them to use CH4 as a source of energy and as a carbon
source. These bacteria are usually found in agricultural soils, forest soils, and compost. These
technologies are primarily in the research and development phase, rather than widespread
application. The details of these two technologies are discussed below.
Biocovers
A biocover is an additional final cover that functions as a CH4 oxidation enhancer to
convert CH4 into CO2 prior to venting to the atmosphere. A biocover is composed of two
substrate layers: a gas dispersion layer and a CH4 oxidation layer. The gas dispersion layer is an
additional permeable layer of gravel, broken glass, or sand beneath the porous media of the CH4
metabolizing layer. This layer is added to evenly distribute the uncaptured LFG to the CH4
oxidation media and to remove excess moisture from the gas. The CH4 oxidation media can be
made of soil, compost, or other porous media. This media is usually seeded with methanotrophic
bacteria from the waste decomposition.
This control technology does not require extensive retrofit and is applicable to all
landfills, including uncontrolled and older landfills with passive or active collection systems.
The biocover itself is not known to affect the functionality of an existing or new gas collection
and control system. In addition, it has low secondary criteria pollutant emissions. Biocovers can
be used as additional final cover to improve the CH4 oxidation rate. According to Abichou et al.
(2006), biocover applications increased the average CH4 oxidation rate up to 32%.
Biofiltration Beds
Similar to biocovers, biofiltration beds aim to further oxidize CH4 from passively
collected LFG. The collected LFG is passed through a vessel containing CH4-oxidizing media
prior to venting to the atmosphere or to a control system. This control technology is only
feasible for small landfills or landfills with passive gas collection systems due to the size of the
biofiltration bed required to treat an air/LFG mixture. In addition, due to the nature of passive
gas collection systems, this technology lacks the ability to control and monitor the LFG
collection. According to Morales (2006), a pilot project shows that the radial biofiltration bed
design has a CH4 oxidation rate of 19%.
A benefit of using a biofiltration bed compared to LFG combustion is that biofiltration
beds produce only CO2 and water vapor. Unlike other combustion-based mitigation measures, a
biofiltration bed does not emit secondary pollutants such as NOX, SOX, and particulate matter.
This technology requires few safety controls for operation, and no start up or shut down
procedures.
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D. Economic Analysis
The economic analysis for GHG control technologies is based on a cost effectiveness
value, which is defined in this paper as the cost to remove one metric ton of CO2e. The cost of
LFG control technologies can be estimated using the Landfill Gas Energy Cost Model
(LFGcost), which was developed by EPA’s Landfill Methane Outreach Program (LMOP) (EPA,
2010d). This model includes direct and indirect costs associated with LFG energy (LFGE)
projects. The direct costs are the costs for equipment, including basic treatment of LFG, and
installation. The indirect costs include costs for engineering, design, and administration; site
surveys and preparation; permits, right-of-ways, and fees; and mobilization/demobilization of
construction equipment. Costs estimated by LFGcost are based on costs for average project
sites. Individual landfills should adjust costs based on site-specific parameters and conditions.
The types of LFG control projects included in LFGcost, Version 2.2 (EPA, 2010d) are as
follows: