Waste Sector Emissions: Landfill Gas Recovery Continues to Stabilize U.S Landfill GHG Emissions, but Is the Data Incomplete?

 

     As more U.S. landfills install gas recovery systems the total greenhouse gas emissions from landfills continues to decrease. Further gas recovery should lead to more declines in the future. The U.S. is well into post-peak landfill emissions but there are questions about emissions accounting in landfills and they are more likely to be underreported.

     The article in Utility Dive (Waste Dive) summarizes waste sector emissions reductions after release of the Waste Chapter of EPA’s 2021 Emissions Report:

 

“Overall emissions from waste declined by about 2.3 million metric tons of CO2e, mostly due to increasing recovery in landfills,..”

 

Source: U.S. GHG Inventory. U.S. EPA. April 2023. Chapter 7, Waste. Main Text_vFR (epa.gov)

Emissions from waste incineration are counted as energy emissions rather than waste emissions but if added to the total waste emissions would make up about 7% of that total. Anaerobic digestion from biogas facilities was added this year to the emissions totals but is a small addition, about 0.12% of total waste sector emissions. Waste Dive notes that the waste sector represents a little more than 2.7% of U.S. GHG emissions (closer to 3% if incinerator emissions are added) but landfills are the country’s third largest source of methane emissions, estimated at 15% of total methane emissions. Ray Huff of SCS Engineers cited ESG-related pressure from investors and shareholders and improving monitoring technology as influencers for the emissions reductions. Indeed, it should be noted that the revolution in methane detection equipment and mitigation strategies for oil and gas facilities should also help improve landfill gas emissions characterization and improve recovery rates and efficiency. Waste Dive cites “new fuel incentive programs and greater scrutiny on methane emissions,” as spurring renewed efforts in landfill emissions reduction.  

Source: U.S. GHG Inventory. U.S. EPA. April 2023. Chapter 7, Waste. Main Text_vFR (epa.gov)

 

Landfill Gas Components and Landfill Emissions Uncertainty

 

     Landfill gas is a type of biogas. The composition of different biogases is variable. The composition of landfill gas is also quite variable, but methane generally makes up about half of it and CO2 is the other half, with smaller amounts of nitrogen, oxygen, ammonia, sulfides, hydrogen, carbon monoxide, and volatile organic compounds (VOCs) such as trichloroethylene, benzene, and vinyl chloride, some of which are known toxins or carcinogens. Early regulatory and gas capture efforts focused on reducing VOCs but have concurrently addressed methane and other components. The amount of organic content in the waste is the biggest determinator of methane emissions. Methane content of manure ponds and anaerobic digestors are higher per unit of waste than from landfills due to the high organic content. Waste composition homogeneity also makes the gas more homogenous and closer to pure methane. Manure is sometimes added to digestors and landfills to accelerate decomposition. Sections of a landfill undergoing anaerobic decomposition will produce more methane. The phase of peak anaerobic decomposition is a function of age since time of burial.

     Since landfill emissions are not point-sourced but come from a large section of land, EPA began addressing landfill gas emissions with modeling. I think that with better integrated methane monitoring utilizing satellites, LiDAR, more continuous monitoring, and ground-sourced monitoring, landfill emissions could be better quantified which could significantly improve the EPA models. It has been argued that due to the limitations of such modeling the methane and CO2 emissions at landfills could be much higher than predicted by the models. Better data should resolve these issues in time.

     In the Waste Chapter EPA explains variations in emissions by landfill as follows:

 

“Methane generation and emissions from landfills are a function of several factors, including: (1) the total amount and composition of waste-in-place, which is the total waste landfilled annually over the operational lifetime of a landfill; (2) the characteristics of the landfill receiving waste (e.g., size, climate, cover material); (3) the amount of CH4 that is recovered and either flared or used for energy purposes; and (4) the amount of CH4 oxidized as the landfill gas – that is not collected by a gas collection system – passes through the cover material into the atmosphere. Each landfill has unique characteristics, but all managed landfills employ similar operating practices, including the application of a daily and intermediate cover material over the waste being disposed of in the landfill to prevent odor and reduce risks to public health. Based on recent literature, the specific type of cover material used can affect the rate of oxidation of landfill gas (RTI 2011). The most used cover materials are soil, clay, and

sand. Some states also permit the use of green waste, tarps, waste derived materials, sewage sludge or biosolids, and contaminated soil as a daily cover. Methane production typically begins within the first year after the waste is disposed of in a landfill and will continue for 10 to 50 or more years as the degradable waste decomposes over time.”

 

     Variations in waste composition and age of cells also complicates the EPA emissions models which depend on many assumptions. Oxidation factors are another source of uncertainty. The oxidation of methane can vary from 0-35%. The EPA models apply an oxidation factor of 10% when there are no reported oxidation factors. Estimated CO2 recovery via flaring and estimated waste-to-energy emissions accounting can lead to more uncertainty. Overall, the EPA gives a 10% uncertainty factor for landfill emissions recovery estimates, but they do plan to improve modeling and refine calculations as some of the bigger uncertainties are better quantified.

     The Environmental Integrity Project, Sierra Club, and Chesapeake Climate Action Network sued the EPA claiming their methodologies were undercounting carbon monoxide, VOCs, and nitrogen oxides by 25% and undercounting methane emissions as well. The EPA did a consent decree agreeing to change their estimation methods for CO, VOCs, and NOx but not for methane. Landfill operators are aiming to improve emissions monitoring at their facilities which should help to better quantify emissions.  

 

New Data, Trends, and Opportunities

 

     The U.S. Energy Information Administration (EIA) estimates that in 2021 about 232 billion cubic feet (Bcf), or about 636 MMCF per day, of landfill gas was collected at 311 U.S. landfills and burned to generate about 9.4 billion kilowatthours (kWh) of electricity. This was equal to about 0.2% of total U.S. utility-scale electricity generation in 2021. Municipal Solid Waste landfills made up 85% of emissions and Industrisl landfills made up the other 15%. The U.S. has about 1500 operational landfills and 3200 closed landfills.

     Early deployment of landfill gas recovery systems led to peak U.S. landfill gas emissions in the early 1990’s. Since then, annual emissions from landfills have been down trending. Since 1994 they have dropped around 35%, more or less steadily but with some fluctuations.

 

     The graph below from the EPS’s Landfill Methane Outreach Program (LMOP) shows annual emissions reductions growth since 2000. Unfortunately, it also shows that growth in landfill emissions reductions has been steadily dropping since 2017 and in 2021 was back at 2010-2011 levels. Emissions reduction grew the most in 2013-2020. The LMOP site suggests that there are still many opportunities to capture landfill gas at several hundred facilities around the country. LMOP reports that there were 546 gas recovery projects operating in U.S landfills.  

 

LMOP Annual Emission Reductions (2000-2021)

Source: Accomplishments of the Landfill Methane Outreach Program. U.S. EPA. April 2023. Accomplishments of the Landfill Methane Outreach Program | US EPA

 

Raw or lightly processed landfill biogas can be burned onsite for some processes but for others like fueling natural gas vehicles it must be significantly processed, which is expensive. Fully processed biogas is known as renewable natural gas (RNG). Though expensive it has become a popular carbon offset investment which generates renewable subsidies in the form of energy credits and also a higher market price for buyers who also buy it as a carbon offset. 

References:

 

Landfill greenhouse gas emissions appeared to decline slightly in 2021, but real-world data remains scarce. Jacob Wallace. Waste Dive. April 14, 2023. Landfill greenhouse gas emissions appeared to decline slightly in 2021, but real-world data remains scarce | Waste Dive

U.S. GHG Inventory. U.S. EPA. April 2023. Chapter 7, Waste. Main Text_vFR (epa.gov)

Biomass explained: Landfill gas and biogas. U.S. Energy Information Administration. Biogas-Renewable natural gas - U.S. Energy Information Administration (EIA)

Accomplishments of the Landfill Methane Outreach Program. U.S. EPA. April 2023. Accomplishments of the Landfill Methane Outreach Program | US EPA

EPA agrees to reevaluate MSW landfill emissions measurements in effort to settle lawsuit. Megan Quinn. Wate Dive. February 13, 2023. EPA agrees to reevaluate MSW landfill emissions measurements in effort to settle lawsuit | Waste Dive

EPA Underestimates Greenhouse Gas Emissions from U.S. Landfills by at Least 25 Percent. Environmental Integrity Project. December 9, 2021. Environmental Integrity EPA Underestimates Greenhouse Gas Emissions from U.S. Landfills by at Least 25 Percent