&EPA
United States Environmental
Protection Agency
Office
Office of Air and Radiation
(6202J)
430-F-01-006
January 2001
LANDFILL METHANE
OUTREACH PROGRAM
ADAPTING BOILERS TO UTILIZE LANDFILL GAS:
FEASIBLE AND COST-EFFECTIVE
The use of landfill gas (LFG) in place
of natural gas in boilers is an estab-
lished and well-tested technology
with a track record of over 20 years of suc-
cess. This fact sheet summarizes the basic
technology issues that a facility manager
may encounter when retrofitting a natural
gas boiler to use LFG.
Over 70 companies have switched to the
use of LFG in their commercial and indus-
trial boilers. These companies recognize
LFG as an attractive renewable fuel that
offers significant cost savings - typically 10
to 40 percent net of conversion costs - in
addition to environmental benefits. A facil-
ity manager that switches to LFG will also
reap the benefits of a secure fuel supply at
a constant and known price.
Facilities that use LFG in their boilers -
"direct" end-users of LFG - can accommo-
date this new fuel through cost-effective
retrofits to existing natural gas and oil-fired
Boilers That Can Be
Successfully Retrofitted
for Landfill Gas
The most typical boiler technology
suitable for retrofitting is the package
boiler used in a variety of commercial
and industrial applications. The two
most common types of package boil-
ers are water wall boilers and fire tube
boilers. These boilers have been
demonstrated to operate successfully
on LFG.
OWater wall boilers use walls consist-
ing of tangential tubes that sur-
round the flame, and are typically
used in larger capacity, high-
pressure applications.
OFire tube boilers pass hot flue gases
through fire tubes immersed in
water, and are typically employed in
smaller capacity, low-pressure
applications.
boilers, while maintaining their units' effi-
ciency. Boilers successfully retrofitted for
LFG range in size from 2 million British
Thermal Units per hour (MMBtu/hour) to
150 MMBtu/hour. The average boiler con-
version can cost as little as several thou-
sand dollars for minor adjustments on
small boilers to tens of thousands for more
elaborate retrofits on larger units.
Retrofit Challenges Are Easily Managed
To successfully retrofit a boiler for LFG
use, certain characteristics of LFG must be
taken into account LFG has about half the
heat content of natural gas (approximately
500 Btu) and burns at a lower temperature
than natural gas due to the greater volume
of nitrogen, carbon dioxide, and moisture
contained in LFG. Minor modifications are
needed to adapt a boiler to the greater gas
flow, higher corrosivity, and lower flame
temperature associated with LFG. All of
these issues, which are described in the fol-
lowing paragraphs, are easily resolved
through cost-effective boiler retrofits.
Greater Volume of Gas Flow
Since the methane content of LFG is half
that contained in natural gas, the gas flow
required to supply the same energy content
with LFG is twice as great To accommo-
date this difference in flow, the valve ori-
fices for fuel control need to be enlarged.
Using a larger fuel valve orifice can mean
additional cost savings since the larger ori-
fice reduces the amount of compression
required to attain the boiler's pressure speci-
fications.
The average lower heating value of LFG
due to its lower concentration of methane,
together with fluctuations in its heating value,
can affect a boiler's flame stability after
switching to LFG. This issue can be addressed
by the application of redundant ultraviolet
(UV) sensors and dual fuel capability.
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UV sensors are standard safety features
that monitor the boiler flame and verify mat
the flame has not been extinguished. Since
the flame from LFG is more difficult to
detect due to its lower temperature, redun-
dant UV sensors should be employed and
equipped with voltage indicators.
Retrofitting the boiler with dual fuel
burners that can accommodate natural gas
as a back-up fuel is another method of
ensuring fuel constancy and flame stability.
Dual fuel burners are fed by separate gas
lines that connect at an intake flow regula-
tor valve, equipped with a Btu content
sensor. The valve regulates the proportion-
ate flow of the two fuels to maintain a con-
stant Btu value of the gas entering the
burner. Although LFG is most often used
together with natural gas, it can also be used
with other fuels like propane or coal.
Lower Flame Temperature
A direct effect of LFG's lower flame tem-
perature is the need to increase the super-
heater size by 20 percent If a flue gas recir-
culation (FGR) feature is used as a control
measure for nitrogen oxides (NOx), flame
temperatures may be even lower, and the
size of the superheater will need to be
increased proportionately.
rode even stainless steel components.
Air preheater corrosion can be pre-
vented by coating the preheater with
porcelain and maintaining the tempera-
ture of the exhaust gas at 300 degrees
Fahrenheit or higher. Using steam coils to
pre-heat the combustion air helps keep
the temperature high enough.
Stack corrosion can be prevented by insu-
lating the stack to prevent the exhaust gas
temperature from lowering to the dew point
In addition, the stack should be made of
carbon steel coated with corrosion-resistant
materials such as inorganic zinc. Fuel con-
trol valves and associated piping should use
stainless steel to protect against corrosion.
Proper water circulation needs to be
ensured after the conversion to LFG. The
lower flame temperature of LFG can affect
the circulation in water wall boilers and
cause steam blanketing against the walls of
the steam tubes. On some boilers with low
circulation velocity, the lower energy of the
steam can result in water condensing out of
the steam onto the water wall tube risers
where corrosive impurities may be
deposited. The circulation pattern of the
boiler should be checked independently to
alleviate these concerns.
Corrosion
Corrosion potentially resulting from LFG
use can be circumvented with technically
simple solutions. Air preheaters and stacks
are susceptible to corrosion from chlorine
compounds in the exhaust gas of boilers
that use LFG. Sulfur trioxide (SO,) formed
from the sulfur content in LFG raises the
dew point in boiler exhaust gas to approxi-
mately 280 degrees Fahrenheit If the tem-
perature of the exhaust gas falls below the
dew point, the chlorine in the gas will cor-
Deposits
Deposits of silica, iron, sulfur, and chlorine
are known to accumulate on air preheaters
and flue gas ductwork. The deposits are
easily removed by soot blowing and manual
cleaning during routine maintenance,
Successful Boiler Conversions to LFG
Ajinomoto Pharmaceutical Company
hi 1989, pharmaceutical firm Ajinomoto
partnered with Natural Power and the City
of Raleigh, North Carolina, to develop a
direct-use project to produce steam from
Challenges in LFG Conversions Solutions
Greater volume of gas flow
Use larger orifices on fuel control valves.
Flame stability
Equip ultraviolet sensors with redundant scanners.
Employ dual fuel burneYs.
Lower flame temperature
Increase superheater size.
Corrosion
Insulate preheater and flue stack.
Preheat combustion air with steam coils.
Ensure that water circulation meets
manufacturer's specifications.
LFG. Natural Power pipes LFG recovered
from the city's Wilder's Grove landfill to a
nearby Ajinomoto plant, where the LFG
fuels a gas-fired boiler owned by Natural
Power. Ajinomoto uses the steam produced
by the boiler to heat and supply power for
manufacturing processes. Combustion of
LFG now supplies more than 95 percent of
Ajinomoto's steam needs. The project is
expected to continue until 2020 when the
supply of LFG diminishes below economi-
cally recoverable levels.
The original 800-horsepower boiler was
fired by natural gas prior to the retrofit for
LFG. The boiler now has dual fuel capabil-
ity with separate fuel lines supplying LFG
and natural gas. This boiler has been run-
ning efficiently on LFG for more than 10
years. In 1997, Natural Power added
another LFG-fired boiler, an 800-horse-
power Cleaver Brooks fire tube boiler, also
equipped to operate on both LFG and nat-
ural gas. Both boilers operate on a full-time
basis and require only routine inspection
and maintenance.
Sanitation Districts of
Los Angeles County
The Sanitation Districts of Los Angeles
County, California have used landfill gas
projects since 1984, using the collected gas
to generate electricity. The district's projects
are at four landfills that use nine boilers-
five Zurn water walls and four Kewanee fire
tube boilers. The boilers range in size from
6 MMBtu/hr to more than 300 MMBtu/hr.
Minor retrofits were required to adapt the
natural gas boilers to LFG specifications,
and the boilers have all operated efficiently
and free of corrosion since the conversion.
The revenues derived from generating elec-
tricity and selling it to the grid help defray
the cost of operating the landfill.
For More Information
The LMOP is a voluntary program that
helps landfill owners, project developers,
and communities develop landfill gas use
projects. The LMOP can offer technical
assistance, resource documents, and
other tools to help landfill owners and
operators realize their facility's LFG use
potential. For more information, call 888-
782-7937 or visit the LMOP website at
www.epa.gov/lmop.
Deposits
Remove deposits during routine maintenance.
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