Landfill GHG Emissions vs. Measured Biogas

Some of the regulatory frameworks for landfills are inefficient and outdated with respect to GHG emissions.
By Toraj Ghofrani | April 24, 2021

More than 1,100 municipal landfills in the U.S. account for the third largest anthropogenic source of GHG emissions, according to the U.S. EPA, primarily due to methane emission from biogas. Methane is the main constituent of landfill biogas (approximately 50%), 25 times more potent than carbon dioxide (CO2). The majority of landfill GHG emissions are due to fugitive area source emissions that are difficult to control.Therefore, it is imperative for the methane to be fully captured and converted into renewable energy, rather than contributing to the global warming effect through flaring or fugitive emissions. 
The EPA prescribes the use of the following equations to estimate annual methane emissions from landfill biogas:
• Equation HH1 models how many metric tons of methane a landfill should be generating each year based on the annual tonnage of depositing refuse.
• Equation HH4 measures how many metric tons of methane is recovered at a landfill based on landfill biogas volume and methane concentration.
• Equation HH6 estimates methane emission from a landfill when modeled methane (HH1) is greater than measured methane (HH4).  
• Equation HH8 estimates methane emission from a landfill when modeled methane (HH1) is less than measured methane (HH4).

We create predictive mathematical models in attempt to come up with answers when we do not understand the laws of the intricate mother nature. Such is the case for equation HH1 that predicts how many methanogenic microbes generate methane under landfill’s anaerobic conditions. Considering that there are three times more microbes in our intestine compared to the 30 trillion cells in our bodies, one can only imagine how many microbes are residing in a landfill and why a direct measurement of methane generation by microbes is impractical. 

In the GHG emission estimate, the result of equation HH1 is used as a yardstick to compare with the result of equation HH4. Whether HH4 is greater or smaller than HH1, landfills are assigned fugitive emissions by the EPA using equations HH6 and HH8. The larger the difference between HH4 and HH1, the larger the assigned fugitive emission. To demonstrate this, two landfills (landfill A and landfill B) are compared to one another.

Modeled Methane Generation (HH1) vs. Measured Recovered Methane (HH4)
Both landfills A and B are assumed to be identical, each serving a city of 300,000 population since 1965, growing at a rate identical to that of the national average and producing an average of 2 kilograms of refuse per day per capita. Assuming all other parameters to be identical for the equation HH1, the modeled methane generation for both landfills A and B were estimated to be equal to 22,865 metric tons (MT) for the year 2019 (Figure 1).

To demonstrate the impact of HH4 deviation from HH1 on methane emission estimates, the measured methane (HH4) for landfill A is assumed to be 10%, 20%, 30%, 40% and 50% greater than the modeled methane (HH1), while the HH4 for landfill B is assumed to be 10%, 20%, 30%, 40%, and 50% less than the HH1. HH4 equaling HH1 (0% change) is also considered for both landfills A and B.

Methane Emissions Using Equations HH6 and HH8
Additional assumptions were made to set landfills A and B in equal settings. Both are assumed to flare 20% of the recovered methane with flare destruction efficiency of 99%, while the remaining 80% of the landfill biogas was assumed to be converted to renewable energy. Both landfills A and B were assumed to have identical surface cover systems, including 0.8 hectares (approx. 2 acres) of daily soil cover, 16 hectares of intermediate soil cover, and 65 hectares of final cover system.

The methane emission results using equations HH6 and HH8 are presented in Figure 2, and are as follows.
• When HH4 is equal to HH1 for both landfills A and B, equation HH6 estimates approximately 47 MT of methane emissions to represent the flare emissions only; the fugitive emission is rendered to be zero. Equation HH8, in contrast, estimates approximately 1,571 MT of emission, accounting for both fugitive emission and for flare emission.  
• When HH4 is greater than HH1 for landfill A, equation HH6 estimates the same constant emission of approximately 47 MT to represent the flare emissions, regardless of landfill A’s 10%, 20%, 30%, 40%, and 50% increase in HH4. It is likely for this reason that EPA would not allow the use of equation HH6 when HH4 is greater than HH1. However, equation HH8 estimates methane emission in proportion to landfill A’s 10%, 20%, 30%, 40%, and 50% increase in HH4, that is 1,723 MT, 1,876 MT, 2,028 MT, 2,181 MT, and 2,333 MT, respectively.
• When HH4 is less than HH1 for landfill B, equation HH8 estimates methane emission in proportion to landfill B’s 10%, 20%, 30%, 40% and 50% decrease in HH4 as compared with the HH1—that is 1,418 MT, 1,266 MT, 1,113 MT, 961 MT, and 808 MT, respectively. This seems contrary to common sense, as the less methane is recovered, the more methane is expected to be lost to fugitive emission. It is perhaps for this reason that the EPA would not allow the use of equation HH8 when HH4 is less than HH1. However, when HH4 is less than HH1 for landfill B, the more HH4 decreases as compared to HH1, with the disproportionally larger methane emission estimated by equation HH6. As landfill B’s HH4 decreases to 10%, 20%, 30%, 40%, and 50% of HH1, the HH8 methane emission increases from 1,532 MT to 3,018 MT, 4,504 MT, 5,991 MT and 7,477 MT, respectively.

The EPA’s reliance on HH1 as an absolute yardstick is particularly punishing for progressive landfills that proactively install additional landfill biogas collection wells in a tighter space, or those that may improve landfill cover systems to more effectively recover methane than the model can predict. 

The agency has always been the harbinger of technology that can better protect public safety and the environment.  However, some of the regulatory frameworks that hover over landfills are inefficient with respect to GHG emissions. One example is the requirement for tedious surface emission monitoring for methane leaks. A combination of drone and laser technology is already in use for methane leak detection in the oil and gas industries. These flyover technologies are much easier and faster in identifying real-time methane leaks from the surface of a landfill. 

With these technologies already within reach, the EPA equations HH6 and HH8 can be utilized more realistically to estimate the actual measured emissions, rather than relying solely on modeled emissions.

Contact: Toraj Ghofrani
Civil Engineer, King County Solid Waste Division
[email protected]

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