Decemb'
er 2009
Adapting Boilers to Utilize Landfill Gas:
An Environmentally and Economically
Beneficial Opportunity
Utilization of landfill gas (LFG) in place of a
conventional fuel such as natural gas, fuel oil,
or coal in boilers is an established practice
with a track record of more than 25 years of
success. In the United States, more than 60
organizations have switched to the use of LFG
in their industrial, commercial, or institutional
boilers, with more than 70 boilers operating
with LFG, either alone or co-fired with other
fuels. Boilers firing LFG range in size from
2 to more than 150 million British Thermal
Units per hour (MMBtu/hr). Companies using
LFG are saving money while protecting the
environment. General Motors fires LFG in
boilers at four of their manufacturing and
. .	Mallinckrodt, Inc. Raleigh, NC
assembly plants and reports that they have
realized energy cost savings of about $500,000 per year at each of the four plants.
This fact sheet discusses the technical and engineering issues associated with using LFG in boilers
designed to burn other fuels. The equipment and operational changes are relatively simple and use
proven technologies, and dozens of firms can engineer and implement a conversion project.
Comparison of Landfill Gas and Natural Gas
Like natural gas, LFG's heating value is derived largely from methane, but unlike natural gas, LFG is
comprised about 50 percent by volume of non-combustible gas, mostly carbon dioxide (C02). LFG is
classified as a "medium Btu gas" with a heating value of about 500 Btu per cubic foot, about half
that of natural gas. Therefore, the volume of LFG that must be handled by the fuel train and burner
is twice that of natural gas. This means that modifications to the fuel train and burner are usually
required to accommodate the higher overall gas flowrate for an equivalent natural gas heating
value. The increased gas flow, however, does not have an appreciable effect on the design and
operation of boiler components downstream of the burner. The added volume of non-combustible
(inert) gas in LFG is equivalent to the inert gas entering a boiler when about six percent of the
flue gas is recirculated to the boiler. Flue Gas Recirculation (FGR) is a widely applied technique for
reducing nitrogen oxide (NOxj emissions from natural gas-fired industrial and commercial boilers,
and boilers can typically operate at recirculation rates of 20-25 percent without adversely affecting
boiler heat transfer and efficiency. This comparison illustrates that the increased flow of LFG as
compared to natural gas will not adversely affect boiler operation, although the burner, controls,
and fuel train will require some modifications.
Adapting Boilers to Utilize Landfill Gas: An Environmentally and Economically Beneficial Opportunity
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LANDFILL METHANE
OUTREACH PROGRAM
Burner, Control, and Fuel
Train Modifications
The equipment for retrofitting a boiler to
burn LF6 is commercially available, proven,
and not overly complex. The decisions that
must be made during engineering and
design are, however, site-specific and may
be somewhat involved. For example, some
installations have retained the original burner
but modified it for LFG (e.g., by installing
separate LFG fuel train and gas spuds) while
maintaining the existing natural gas fuel
train and qas ring to permit LFG/natural gas
„ , ,a ,7 . .	, J, Mallinckrodt, Inc. Raleigh, NC
co-firing. Other installations have replaced
the entire burner, controls, and fuel train with a dual-fuel burner and dual-fuel trains specifically
designed to handle medium Btu gas. In general, the decision to furnish all new equipment is made
based on the owner's preference or because the existing burner and controls are nearing the end of
their useful lives. Additional analysis may be required to determine the amount of LFG compression
that is provided versus the modifications needed for the burner and gas train.
Because LFG is typically a wet gas often containing trace corrosive compounds, the fuel train and
possibly some burner "internals" should be replaced with corrosion-resistant materials. Stainless
steel has typically been the material selected.
The controls associated with fuel flow and combustion air flow need to be engineered to cope with
the variable heat content of LFG. The complexity of the burner management system will depend
upon whether the boiler is to be co-fired with natural gas or oil and whether the boiler is to be
co-fired at all times or if there will be times when it will be fired with LFG only. Today's modern
controls, fast-responding oxygen analyzers, and responsive flame sensors make it possible to fire LFG
with the same level of safety that is characteristic of current natural gas systems.
Boiler Deposits and Boiler Cleaning
in recent years, a family of organo-silicon compounds, known as siioxanes, commonly found in
detergents, shampoos, deodorants, and cosmetics, have gradually found their way into the solid
waste stream and into LFG. Their quantity in LFG is small and varies with the age of the landfilled
material. When LFG is burned, the siioxanes are oxidized to silicon oxide-the primary chemical
compound in sand. After firing boilers for an extended period with LFG, operators report a thin
coating of white powder, described as similar to talcum powder, on some of the boiler tubes and
substantial accumulations of the white powder on portions of the boiler floor. Where the material
collects and how much of it accumulates is likely to be a function of the velocity patterns in
the boiler and the siloxane concentrations in the LFG. One firetube boiler operator reported no
deposits at all, probably due to the high flue gas velocity that is characteristic of the firetube boiler
configuration.
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Operators' experiences to date indicate
that annual cleaning is sufficient to avoid
operational problems related to silicon oxide
accumulation. More frequent cleaning may
be necessary as future installations encounter
higher LFG siloxane concentrations or when
low gas velocities exist in the boiler, either
because of boiler design or continuous
operation well below full capacity. In all cases,
the silicon oxide powder is easily removed
from surfaces by brushing or water washing.
Other Considerations
In designing and assessing the economic
feasibility of projects utilizing LFG in boilers,
, r . ¦ i	. .. . c. Mallirickrodt, Inc. Raleiqlh, NC
several factors in addition to the boiler retrofit	3
must be considered. For example, the quantity of LFG available must be considered and compared
to the facility's steam needs and boiler capacities. Factors such as pipeline right-of-way issues and
the distance between the landfill and the boiler will influence costs and the price at which LFG
can be delivered and sold to the boiier owner. Because LFG is generally saturated with moisture,
gas treatment is needed before the LFG is introduced into the pipeline and subsequently the boiler,
to avoid condensation and corrosion. Additionally, condensate knock-outs along the pipeline are
necessary as condensation in the main pipeline can cause blockages. Fortunately, the level of
LFG clean-up required for boiler use is minimal, with only large particle and moisture removal
needed. Other compounds in LFG, such as siloxanes, do not damage boilers or impair their functior.
Generally, LFG clean-up and compression systems are located at the landfill and are often installed
by a developer rather than by the boiler owner. LFG compression provided at the landfill must be
sufficient to compensate for pipeline pressure losses and provide sufficient pressure at the boiler
to permit proper function of the fuel controls and burner. Proper attention to burner selection or
burner modification for low-pressure operation can minimize the LFG compression costs.
Is My Boiler a Candidate for Landfill Gas Retrofit?
Virtually any commercial or industrial boiler can be retrofitted to fire LFG, either alone or co-fired
with natural gas or fuel oil. The firing profile is a primary consideration, regardless of the boiler type,
since the fuel cost savings associated with LFG must offset the costs of the LFG recovery (if a LFG
collection system is not yet in place), the gas clean-up equipment, and the pipeline. Operation at
substantial load on a 24-hour/7 day-per-week basis or something approaching continual operation
is generally important to the economic viability of a potential project.
Both the smaller, lower-pressure firetube package boilers and larger, higher-pressure watertube
package boilers are already in operation with LFG. Older field-erected brick set boilers have also
been retrofitted for LFG fuel. Many major boiler manufacturers, such as Cleaver Brooks, Babcock Et
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Wilcox, Nebraska, and ABCO, are represented
in the population of boilers that have been
converted for LFG service. Similarly, leading
burner manufacturers (e.g., Todd, North
American, and Coen) have provided specially
designed LFG burners or have experience
modifying standard natural gas burners for
LFG service.
Mallinckrodt, Inc. Raleigh, NC
Examples of Successful Boiler
LFG Energy Projects
NASA Goddard Space Flight Center. In early
2003, NASA's Goddard Space Flight Center in
Greenbelt, Maryland, began firing LFG in two
Nebraska watertube boilers, each capable of producing 40,000 pounds per hour of steam. The gas
is piped approximately five miles from the Sandy Hill Landfill to the boiler house at Goddard. NASA
modified the burners and controls to co-fire LFG, natural gas, and oil; however, LFG provides the
total firing requirement for approximately nine months of the year. Later, a third boiler also began
utilizing LFG. NASA estimates an annual savings of more than $350,000. Current NASA plans call
for LFG use to continue for at least 10 years, with a possible extension to 20 years. LMOP Partners
Toro Energy and CPL Systems developed and implemented the project.
Cone Mills White Oak Plant, The LFG retrofit project at textile manufacturer Cone Mills' plant in
Greensboro, North Carolina involved a very old (circa 1927) field-erected brick set boiler. In this
instance, the developers chose to install two new, multi-fuel burners supplied by Coen Company, Inc.
Full operation began in early 1997, with a steaming capacity of 30,000 pounds per hour from the
LFG fuel. Additional steam is provided as needed by co-firing with natural gas or fuel oil. The gas is
supplied to the Cone Mills plant via a three-mile pipeline originating at Greensboro's White Street
Landfill, The project is a partnership between the City of Greensboro, Duke Solutions (now part of
Ameresco, Inc.), and Cone Mills.
Information about additional projects can be found at the project profiles section of the LMOP
website at www.epa.gov/lmop/lmop-landfill-and-project-database. The photographs in this
document depict a boiler retrofitted to burn LFG, courtesy of Mallinckrodt, Inc. in Raleigh, North
Carolina.
Where Can I Obtain Further Information?
LMOP is a voluntary program that helps landfill owners, project developers, and communities
develop LFG energy projects. LMOP offers technical support that includes finding a landfill,
estimating gas generation, analyzing project economics, and providing other tools to help landfill
owners and operators realize their facility's LFG use potential. For more information, visit the LMOP
website at www.epa.gov/lmop.
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