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There's
untapped renewable energy potential lying within America's
small and medium-sized landfills with a significant
upside for both public and private owners. Until recently,
however, the opportunity was considered too risky for
the investment required.
By
Scott Coon and Fred Doran
Whereas common
knowledge suggests you need 2 or 3 million tons of MSW
in place to justify a landfill gas to energy project,
recent developments might allow green power projects
at hundreds of smaller landfills.
Based on
the accepted solid waste management practices of leachate
recirculation and landfill gas (LFG) control, a new
concept - recirculation to energy (RTE) - relies on the
reintroduction of leachate back into the waste volume
to increase bioactivity and, as a result, to boost gas
generation. Measured LFG generation values are up to
4 to 10 times above those of the dry-tomb landfills,
providing sufficient quantities for energy recovery - and
the higher moisture content is the key.
This practice
has not previously been given credibility at small or
medium-sized landfills because the power produced was
thought to be insufficient to balance the cost investments
of putting the conversion system in place. Capital costs
of well over a million dollars per megawatt for a power
plant fueled by LFG eliminated small producers from
consideration. In addition, many smaller landfills do
not need LFG collection systems for environmental compliance,
thereby further increasing net costs.
New studies
have shown that, if RTE were to be used at smaller sites,
the potential impact could be substantial. In fact,
it is estimated that the annual effect on greenhouse
gas emissions at one landfill could be equal to removing
roughly 10,000 cars from the roads or creating a mature
forest of nearly 13,000 trees. This untapped market
segment consists of nearly 400 landfill facilities throughout
the US that are considered either small or medium in
size. Because of their size, they are considered below
the current Environmental Protection Agency's (EPA)
New Source Performance Standards (NSPS), which stipulate
that landfills with an ultimate capacity of less than
2.5 million megagrams (Mg) are generally not required
to install LFG emissions controls.
So, these
non-NSPS landfills are emitting gases that, instead
of being harnessed for energy, are being wasted and,
worse, could be impacting the atmosphere and global
health. Conversely, if these landfills employed the
RTE concept, they would reduce greenhouse gas emissions;
vented LFG contains the powerful greenhouse gas methane,
which has more than 20 times the damaging effect on
the environment than does carbon dioxide.
Produce
Renewable Energy
With many
states now requiring greater percentages of generated
power to be from renewable sources, this is a win-win
alternative and reduces the rate at which landfills
are expanding by reclaiming landfill airspace. Through
an increasing the rate of biodegradation and settlement,
RTE increases the life cycle for the landfill cells
and delays the need for expansion permitting and construction.
Leading
by Example
Crow Wing
County, located near Brainerd, MN, is among the first
in the US to initiate RTE at a small landfill.
Crow Wing
County Landfill (CWCLF) is a municipally owned, rural
landfill that receives about 45,000 tpy of MSW and has
an ultimate capacity of 1.3 Mg, which is well below
the NSPS threshold. Through a unique bioreactor project
being tested over the past six years, however, this
landfill found it could serve as a model for many of
the non-NSPS landfills in the country. Their goal is
to make it economically viable to capture the landfill
gas and produce electricity. This is being done by accelerating
the landfill gas production through leachate recirculation
under the RTE concept.
Leachate
(see the article "Recirculation:
The Future is Stable," by Fred Doran, in
the May/June 2002 issue of MSW Management) is
the liquid generated in a landfill when precipitation
falls on and percolates through the waste and then is
collected at the base of the landfill. When this liquid
is recirculated or pumped back into the landfill, there
are several proven benefits, including accelerated LFG
production, increased rates of landfill settlement (which
means the settled areas can be refilled, ultimately
extending the life of the landfill cell and slowing
the need for expansion), shortened time necessary for
waste stabilization, leachate treatment, and reduced
leachate management costs.
Although
there are several ways to recirculate leachate, CWCLF
uses horizontal injection trenches and spray applications
that distribute both untreated and treated leachate
throughout the cells.
Figure 1
shows how LFG generation at CWCLF has been accelerated
through leachate recirculation and how it significantly
lowers the threshold using industry standard generating
equipment. This figure illustrates the net output power
capacity of a Caterpillar 3516 engine generator set
since this equipment is in wide use. RTE at CWCLF advanced
through comprehensive pump-down testing that measured
the current rate of LFG production and allowed development
of a model for future production at the site. A portable
blower system attached to vertical vent wells, leachate
collection cleanouts, and recirculation laterals was
used. The results show that stable and predictable gas
flows will increase nearly four times the traditional
generation rate seen at traditional landfills, which
minimizes the contact of liquids with the refuse volume.
View
Figure 1
A similar
boost in production - although the rate will vary due
to several factors (e.g., rainfall, tonnage received,
waste composition, and liquid distribution) - can be expected
at all of the non-NSPS sites using leachate recirculation.
Note, however, that the ultimate volume of LFG produced
does not change over the life of the landfill. Instead,
large volumes of LFG are produced in a shorter peak
period, which makes collection and recovery more economically
feasible while shortening the duration of long-term
care.
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| A
portable blower system attached to vertical vent
wells, leachate collection cleanouts, and recirculation
laterals was used. |
It
is important to point out that for a smaller RTE landfill,
economical beneficial use of LFG is contingent upon
continued leachate recirculation and a steady or growing
wastestream. Doug Morris, solid waste coordinator with
Crow Wing County, terms this riding the wave.
A reduction in either factor would dramatically decrease
LFG generation and jeopardize project viability. This
is illustrated by Figure 1, indicating a significant
decline at the projected date of landfill closure. Long-term
regulatory acceptance and permitting of recirculation
and assurance of system waste flow is crucial.
Another critical
challenge landfill managers at smaller RTE landfills
must face is how to effectively capture the LFG being
emitted. Elevated moisture content impacts the flow
of LFG within the waste, sometimes blocking its withdrawal.
In order to recover the maximum quantity of gas, moisture
distribution levels within the landfill must be carefully
controlled to maximize LFG production without flooding
collectors. Methods to extract LFG might need to be
flexible and incorporate innovative options, such as
trench header and drain/cleanout collectors.
More Power
to You
The final
step of tapping into the RTE resource is to determine
how to best use the energy available. Initially CWCLF
had hoped to at least produce enough power to become
self-sufficient. Managers anticipated enough LFG to
heat the maintenance shop and possibly meet the electricity
demand at the site. With the subsequent field test findings,
however, the landfill has enough LFG to supply offsite
sales of gas or green power. Options considered included
these:
- direct
gas or electricity sale to the local plastics industry
- direct
sale of electricity to the local utility, and
- wheeling
electricity to other energy providers.
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| Pump-down
testing measures current LFG production at the landfill. |
The second
option was selected based on the least amount of risk
and lowest implementation cost. A purchase power agreement
is currently being negotiated to provide a baseload
capacity of 800 kW and to benefit the utility from future
renewable power resources.
Although
many states have instituted renewable portfolio standards
for utility generation, the standing of LFG fueled power
in recent Minnesota legislation is not clear.
At press
time, landfill stakeholders are involved in providing
comments to the Public Utility Commission supporting
LFG as a renewable energy source along with biomass,
waste-to-energy, and wind power.
RTE does
have the support of the Minnesota Pollution Control
Agency (MPCA), which has applauded the environmentally
friendly benefits of non-NSPS landfills tapping into
LFG to make power. Methane is a powerful greenhouse
gas and also a good fuel source, so to beneficially
use this gas to produce electricity is environmentally
responsible, says Ron Swenson, supervisor of the Regional
Environmental Management Division of MPCA.
It's a great
idea for landfill managers who want to pursue this approach
to garner such support from area regulatory agencies
and utilities that might take an interest.
Moving
Forward
In summary,
the long-held belief that landfill power production
is only an option for the large landfills has been proven
false. In fact, the RTE process has several proven benefits
for small and midsize landfills. It can lengthen the
life cycle of your existing landfill, benefit the environment
by capturing greenhouse gases being emitted, and turn
the gas into renewable electricity.
Combined,
all of the non-NSPS landfills across the nation could
be taking steps to make a contribution to these goals.
If they analyze their options and look at the big picture,
they might find that they're not so little when it comes
to this new approach to landfill management.
Scott
Coon is a senior mechanical engineer for R. W.
Beck in Madison, WI. Fred Doran is a senior environmental
engineer for R. W. Beck in Minneapolis, MN.
MSW
- May/June 2004
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