Convert Active System to Passive System with Vent Flares
Many landfills operate a gas collection system to comply with state and federal solid waste requirements to prevent or mitigate landfill gas (LFG) migration beyond the facility property boundary or to reduce odors but are not required to combust the LFG in a flare under air quality regulations. Such a landfill may be able to convert its active system to a passive system with vent flares. Vent flares can achieve periodic methane destruction when there is sufficient LFG flow and methane quality to initiate the flares, thereby mitigating some greenhouse gas (GHG) emissions and odor issues. Vent flares are also commonly used to mitigate odor from leachate cleanout and passive vents, as well as trenches installed outside of the edge of waste at landfills to intercept LFG migration—these vent flares can supplement an active system, instead of replacing it.
GHG Mitigation Potential:
- Methane destruction is greater than 99 percent when operating.1 A passive vent flare operates periodically based on gas flow and quality.
- Methane collection efficiency depends on the radius of influence of each vent, how many vent flares are installed and the capacity of each vent flare.
- In addition to collected methane, some methane may be oxidized depending on cover type. For landfills without an active gas collection system, methane oxidation rates range from 10 percent in areas without final cover to 35 percent in areas with final cover.2
Example
Bartholomew County, Indiana 6, 7, 8
Bartholomew County manages both a closed site and an active site near Columbus, Indiana. The closed site stopped accepting waste in 1999 with about 1.5 million tons of waste in place. A passive flaring system was installed over time from 1993 through 2003 as portions of the site were closed. The county decided on passive flaring to help promote vegetation growth and to control gas migration. Only small-scale LFG usage would have been possible with such low LFG generation.
The site has 25 passive wells with three cleanouts and 12 flares. The flares are connected to vertical wells through plastic manifolds that link the wells to above-ground vents. The flares are spark-ignited using solar panels, while a filter and flame arrestor are used for safety. The low-cost system mitigates LFG odors with annual maintenance costs of about $100.
Criteria:
- Landfill has gas methane content above 30 percent for efficient flare combustion, adequate gas pressure to move gas to the flare and gas flow between approximately 2 and 140 cubic feet per minute, depending on the flare model.
- Odor is a concern or landfill wants to minimize hazards related to flammable or toxic gas migration (e.g., nearby residences, structures, brownfield redevelopments).
- LFG is not prone to escaping laterally through surrounding soil.
Pros:
- Methane destruction is still achieved, at least in the areas of the landfill where this equipment is installed.
- If LFG from individual wells does not have sufficient quantity or quality to support combustion or justify cost of a solar flare installation, gas from multiple vents can be combined and routed to a remote solar flare.
Cons:
- Since LFG is not under active vacuum, it may migrate to onsite structures or residential areas nearby and create hazardous or explosive conditions.
- Monitoring and recordkeeping requirements for flare temperature and LFG flow rate may be burdensome depending on state and federal air regulations.
Economic Considerations:
- Vent flare costs – for flares installed on vertical wells, assume density of one well per acre. Costs are less than active gas collection systems but more than free venting.
- Vent flare costs range between $2,840 and $5,100 (for LFG flow rates ranging from 5 to 140 cubic feet per minute).3, 4
- If needed, the cost for a data logger/thermocouple kit and associated software to track the flame pilot can be $1,500.5
- Blower and skid equipment may be able to be sold for scrap.
References
1 LSC Environmental Products, LLC. Combustion Efficiency and Regulatory Compliance of CF-5 Landfill Gas Flares. March 2011. http://solar-spark.com/assets/PDFs/CF-5-CombustionRegulatory-B-3-11-.pdf.
2 U.S. EPA. Documentation For Greenhouse Gas Emission and Energy Factors Used in the Waste Reduction Model (WARM). Management Practices Chapters. November 2020. https://www.epa.gov/sites/default/files/2020-12/documents/warm_management_practices_v15_10-29-2020.pdf.
3 Smith Gardner, Inc. Quotes obtained from vendor LFG Specialties, LLC (an APTIM company), January 12, 2021, and vendor EPG Companies Inc., November 4, 2019.
4 Diamond Scientific. Landfill & Bio Digester Equipment webpage. https://diamondsci.com/collections/landfill-bio-digester-equipment. Accessed July 20, 2018.
5 Smith Gardner, Inc. Quote obtained from EPG Companies, Inc., November 4, 2019.
6 U.S. EPA. LMOP Landfill and Landfill Gas Energy Project Database. July 2021. https://www.epa.gov/lmop/lmop-landfill-and-project-database.
7 Cummins Inc. Landfill Gas Utilization and Feasibility Study. June 28, 2013. http://www.bcswmd.com/pdf/FinalLandfillGasReport.pdf.
8 Siesel, Heather. Bartholomew County Solid Waste Management District. Email correspondence. October 25, 2021.