Landfills Case Study
Space Heating with LFG: Small Project-Big Savings
The key to generating LFG for reuse at the landfill was the implementation of the recirculation-to-energy concept.
Michael Cook
On a typical January morning in Minnesota, a landfill operator is working on the compactor’s transmission. Despite the four inches of snow on the ground, he opens the garage doors in the maintenance building to let out some of the excess heat. Unlike previous winters, this year the operator can afford to crank up the heat in the building while working on equipment because the heat is being fueled by a renewable energy source created right on site.
The Crow Wing County Landfill located in northern Minnesota recently installed a boiler fueled by landfill gas (LFG) to provide heat to its two maintenance buildings. With this investment, the landfill has been able to significantly reduce its heating costs and its carbon emissions.
A Small Site with Big Hopes
The landfill serves the county’s nearly 60,000 year-round residents and, during the spring and summer, it serves twice that population as vacationers visit the many resorts dotting the lakes of the county. The landfill provides disposal options for nearly all kinds of waste. The landfill’s current disposal area is a four cell, 22.5 acre area. It began operation in 1991 under the U.S. Resource Conservation and Recovery Act, Subtitle D rules, and was the first in the state of Minnesota to do so. In addition to the active areas of the landfill, there is also an unlined 28-acre landfill that was closed in 1992.
The county constructed Cell 1 along with two leachate treatment ponds (700,000 gallons each) in 1991. In 1995, it added Cell II and a third treatment pond with capacity of 680,000 gallons. Disposal occurred in Cells I and II for 10 years before the landfill built Cell III in 2001. Then in the summer of 2007, it constructed Cell IV and a 2.5 million gallon leachate pond with the eight-acre Cell IV beginning waste filling in 2008.
Crow Wing County has worked hard to construct and operate a facility that is able to manage and maintain environmental liabilities on site. In 1994, it installed a land application system on the closed landfill. Using the treatment capabilities of the leachate ponds, the landfill reduced offsite hauling of leachate by spraying treated leachate over the 11-acre spray field. However, as the facility grew, the need for more leachate storage, treatment and disposal increased.
Recirculation to Energy
As early as 1997, the County started looking at other options for leachate management. That year the County submitted an application to the Minnesota Pollution Control Agency (MPCA) for a leachate recirculation pilot project. At that time, leachate recirculation was not allowed by the MPCA as part of normal landfill operations and could only be performed under the pilot project status. During the first phase of the pilot project, the landfill outlined six goals, including accelerated settlement (maximizing airspace) and accelerated waste stabilization, and identified several potential issues, including odor control and increased LFG generation.
In 1997 the MPCA approved the application, and by April 1998 the county began recirculating leachate. Almost immediately, leachate hauling for offsite disposal ceased, and the county had the ability and capacity to manage all of its leachate onsite.
Accelerated LFG generation became evident over the next few years, so in 2001 the county’s engineering consultant, R.W. Beck, an SAIC company, performed a simple test to measure flow coming from the landfills leachate collection system cleanout risers and other venting risers. A basic anemometer and the landfill’s four-gas handheld meter were used to measure gas flow and LFG composition coming from several of the vents. This test showed an increased pressure build up in the landfill forcing gas out of the cleanout risers and passive gas vents. It also revealed the LFG was approximately 55 percent methane. This simple test sparked the idea for the second phase—Recirculation-to-Energy (RTE).
RTE would allow the county to recirculate leachate in all cells of the landfill without the use of a control cell and was approved by the MPCA as a pilot project in 2002. The county initiated the RTE demonstration to show how recirculation at a small landfill exempt from New Source Performance Standards (NSPS) would lead to an increase in LFG generation and make energy recovery more feasible. Plus, the county added two additional goals to the original six:
Voluntary LFG reuse for energy recovery at a small landfill that otherwise would not require an active system
Voluntary reduction in landfill greenhouse gas emissions through energy recovery or flaring.
As part of the RTE project, the county and R.W. Beck conducted verification field tests on several potential LFG collection locations. Each test yielded positive results and indicated that the landfill was highly active and producing a usable volume of quality LFG. As the RTE concept was developing, the county designed a new 6,400 square foot maintenance building. The promise of significant LFG generation allowed the design of the maintenance building to include installation of tubing for an in-floor heat system. The county planned to install a heat source (boiler) that would be fueled by LFG at some point in the future.
The testing results combined with LFG generation modeling verified that installation of an active gas collection and control system (GCCS) was feasible. The county used the results of the testing to present data to potential industries for LFG reuse projects. Any installation of a GCCS would be designed to fit the reuse project. The end user’s systems would need to be evaluated to determine what the facility’s LFG fuel requirements were and whether upgrades were needed to its equipment. A GCCS would need to be flexible and provide LFG based on the facility’s needs.
Reuse, a Reality
From 2002 to 2007, the county explored several reuse options for direct LFG use and electric generation. Although these options stalled due to the economy, the county realized the potential a GCCS could provide and began the design with flexibility for future reuse in late 2007. Gas collection would be maximized using dual-phase gas wells (including electric pumps), connections to all the leachate collection cleanout risers, and connections to the recirculation laterals. The dual-phase wells included pumps to remove accumulated liquids from the wells due to recirculation, improving LFG collection. The connection to the leachate cleanout risers would allow for gas collection from the bottom of the landfill. This also allowed the county to begin collecting LFG from the newly opened Cell IV soon after waste placement. The recirculation laterals would provide limited LFG collection during the year.
As part of the flexibility for future reuse, a condensate knockout pot (KOP) was designed with an additional future use connection. This structure is used to further reduce condensate from the LFG stream prior to its destruction location. The KOP would feed a flare skid, and the diversion pipe could feed a compressor system or electric generator. This system or generator could be installed in a new storage building.
The flare skid required two additions from typical flare skids—a diversion pipe on the main piping of the flare skid downstream of the blower and an additional flow control valve. The flow control valve would provide the ability to create a back-pressure on the LFG flow and force LFG through the diversion pipe to a LFG fueled boiler.
Crow Wing County planned to install a LFG fueled boiler in the new storage building to provide the heat source to the in-floor heating system and heat for the new storage building. Essentially, the boiler required needed to be a typical commercial-sized boiler, so based on these requirements, finding the appropriate boiler was difficult. Manufacturers do not typically provide boilers of this size to run on LFG because typical direct use LFG projects use large industrial size boilers that have been built to handle LFG from larger landfills. In addition, commercial boilers run on natural gas and are typically too expensive to convert the unit to handle LFG.
The requirements for the in-floor heat system were less than one million British thermal units (MMBTU), less than five inches of water column (in-w.c.) gas pressure, and a flow of less than 30 standard cubic feet per minute (SCFM). A small commercial boiler would easily meet these requirements. However, the heat output would be less than the design capacity of the boiler because the methane content of LFG was approximately 50 percent of natural gas.
The design moved forward using a typical indoor atmospheric commercial boiler that was able to meet the requirements. The landfill operator was prepared for additional maintenance necessary to ensure the boiler would work continuously. The maintenance essentially consisted of cleaning the burner deck and draining the piping as condensate accumulated.
During 2008, the county installed the GCCS, including the boiler and flare skid, and commissioned the flare skid in December. Gas flow at start up was approximately 350 SCFM and has since stabilized around 250 SCFM, and methane content has remained about 50 percent. The boiler did not run during the first winter because the winter start up made it difficult to work out the system “bugs” and balance the collection field to stabilize flow, which was necessary to ensure optimum operation of the boiler. This delay allowed additional work to be completed on the boiler prior to use. The pilot system used LFG but required a modification to use natural gas. Safety pressure switches were installed to shut the boiler down should the incoming fuel pressure fall out of the specified ranges. If the gas pressure rises above the set value of 7 in-w.c. or below 3 in-w.c., the boiler automatically shuts down. Generally, this only occurs when the flare skid shuts down. The pressure in the fuel line drops to zero shutting down the boiler and stopping flow.
The boiler first fired in October 2009 and continued in operation with minimal down time until February 2010. A particulate build-up on the burner deck reduced efficiency. At this time, the county added a few additional modifications. It installed a continuous methane analyzer to measure methane destroyed at the flare and allow the operator the ability to automatically shut down the boiler should methane content drop below 45 percent at which point the boiler loses the ability to maintain a steady flame. Due to the particulate build-up, a particulate scrubber was installed to reduce the potential for recurrence. The county also registered the GCCS on the Climate Action Reserve (CAR), which requires each destruction device to have a flow meter, so a flow meter was installed for the boiler.
GCCS by the Numbers
The county installed the GCCS at a cost of approximately $1.2 million. The boiler and mechanical work portions were about 10 percent of the total. The estimated annual operational cost for the entire GCCS is approximately $60,000. The annual operating cost for the boiler portion of the GCCS is estimated at $1,500.
The county does not currently have a revenue stream from the sale of their LFG and is still actively seeking potential reuse projects. However, the use of the boiler has allowed the county to save money by offsetting the natural gas they would normally use for heat. During peak months of operation (January and February 2009), the county saved $1,000 or more compared to previous years by heating the two buildings with the LFG boiler. The ambient temperatures for the winter of 2009/2010 were comparable to previous years. Natural gas is still required as both a backup and as a pilot gas with minimal used during the winter months.
Although the county does not receive revenue from LFG sales, they are registered with CAR. In 2009, the county entered an agreement with TerraPass through a request for proposal process for the purchase of its verified carbon offset credits. That year the county verified 17,249 metric tons of emissions reduction in conjunction with TerraPass and First Environment (an emissions reduction verifier) and earned nearly $100,000 in the sale.
Smaller landfills, like Crow Wing County, typically do not produce the LFG capacity to install large industrial boilers or electric generators that could be used in combination with heat recovery systems. However, an installation of an active GCCS could provide an opportunity for the installation of a commercial boiler system, which has the potential to be used in several ways, including an in-floor heat system, forced air heat system and leachate heating system during winter providing additional treatment. The Crow Wing County Landfill has been considering the use of boiler heat for further treatment in their leachate ponds.
The key to generating LFG for reuse at the landfill was the implementation of the RTE concept. This allowed the County to recirculate leachate knowing that the accelerated LFG generation would be reused. The recirculation resulted in the production of higher quantity and quality of LFG than would have otherwise been produced without it.
Michael Cook is a civil engineer in the solid waste practice of R. W. Beck (Seattle, WA), a wholly owned subsidiary of Science Applications International Corporation (SAIC). Based in the Minnesota office, his work is mainly focused on providing facility siting and design, permitting, environmental reporting and construction management services for municipal solid waste landfills in the Midwest. He has also performed due diligence services related to a multi-state energy production operation that generates power from multiple LFG sites. He can be reached at (651) 289-2519 or [email protected].