POWER July 2015 - 69

FUELS
How Trash Becomes Cash
A 2013-released survey on America's garbage
conducted by Columbia University
estimates the U.S. generated nearly 400
million tons of municipal solid waste (MSW)
in 2011, about 63.5% of which was landfilled,
22.6% recycled, 6.3% composted,
and 7.6% combusted in waste-to-energy
facilities.
When MSW is buried in a landfill, a complex
series of reactions occur in which
anaerobic microorganisms decompose a
portion of the organic fraction (by some
estimates, 150 kg to 250 kg of organic
carbon per metric ton) of the waste, emitting
landfill gas (LFG) as a byproduct. As
gas formation is influenced by a number
of factors-such as waste composition,
landfill storage height and density, air temperature,
atmospheric pressure, and precipitation
levels-landfill gas composition
varies. Typically, it is composed of methane
(35% to 55% by volume), carbon dioxide
(30% to 44%), nitrogen (5% to 25%), oxygen
(0% to 6%), and water vapor.
An LFG collection system basically entails
a series of perforated tubes (or wells)
drilled into a landfill that are interconnected
by a plastic piping system. Gas sucked
into the collection system using a blower
or vacuum is saturated with water, which
must be removed before further processing
(Figure 5). The typical dry composition of
low-Btu gas is 57% methane, 42% carbon
dioxide, 0.5% nitrogen, 0.2% hydrogen,
and 0.2% oxygen, along with a significant
number of other compounds in trace quantities.
Collection
efficiencies from capped landfills
provided with adequate LFG collection
systems in the U.S. can be more than 75%,
says environmental consulting firm SCS Engineers.
Data
volunteered by the nation's landfills
that is curated by the Environmental Protection
Agency's Landfill Methane Outreach
Program (LMOP) suggests that reciprocat5.
The landfill gas-to-energy process. After dewatering, LFG can be used directly
or further processed into higher-Btu gas. Courtesy: WT Energy
ing engines are most widely used to convert
gas to power (Figure 6). This is likely
because reciprocating engines offer electrical
efficiencies of between 36% and 42%;
are tolerant of low-Btu gases, gas changes,
and contaminants; and require relatively
low investment costs.
Leading vendors among the reciprocating
engine set include Caterpillar and GE
Jenbacher. Combined cycle and gas turbine
users mostly pick Caterpillar subsidiary Solar
Turbine's Mars, Centaur, Mercury, Taurus,
or Saturn generators, though larger projects
like the 23.5-MW Arbor Hills Landfill
in Michigan uses a Siemens gas turbine.
Choices among microturbine users vary
between Ingersoll-Rand and Capstone, the
data suggests.
vehicle fleet with compressed natural gas. It
is also looking at converting LFG into synthetic
diesel.
A Risky Business
Like most power projects, LFGTE projects
do sometimes fail, said industry expert Jeffrey
Pierce, project director at environmental
engineering firm SCS Engineers.
One problem involves the over-projection
of recoverable LFG. " When landfill gas recovery
is over-projected, the LFGTE facilJuly
2015 | POWER
ity does not operate at its full capacity, and
project revenues are not adequate to fully
cover debt service and operating costs, "
he said. While advances in LFG recovery
modeling have been made, only a few consultants
have the depth and breadth of experience
required to select site-specific LFG
modeling coefficients.
Waste Management's Kelly also underscored
the importance of good siting. An energy-producing
landfill project that is too far
away from a grid connection or an end user
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tends to be uneconomical, she said.
Then there are challenges that involve interconnecting
with the local electric power
company. " While some measure of standardization
of interconnection requirements has
been accomplished, interconnections are still
approved on a case-by-case basis, " Pierce explained.
" It is generally not difficult, nor extremely
costly, to satisfy a utility's technical
requirements on the power plant side of the
utility meter. It is, however, sometimes difficult
to determine exactly what will satisfy the
69
6. Driving engines. EPA data (in MW)
from March 2015 show which technologies
the nation's 2.28 GW of landfill gas to energy
projects (operational, planned, or shut
down) have picked to convert landfill gas to
power. Projects under construction and that
are planned to date only plan to use reciprocating
engines. Source: EPA LMOP
■ Reciprocating engine ■ Gas turbine ■ Combined
cycle ■ Steam turbine ■ Microturbine
■ Stirling cycle engine
4.96
0.07 (Stirling)
142.53
206.10
252.72
1,398.05
http://www.powermag.com
POWER July 2015

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