¦EMISSIONS Figure 1. Methane Generation Modeled Using Equation HH1
the larger the assigned fugitive emission. To demonstrate this, two landfills (landfill A and landfill B) are compared to one another.
1965 Population=300,000 S=1965 T=2019 MCF=1 DOC=0.2 DOCF=0.5 F=0.5 k=0.057 Cumulative Methane Generation — U.S. Population Growth
22,865
% Population Growth
1.2%
25,000
20,000
1.0% 15,000
0.8% 0.6%
10,000
0.4% 5,000
0.2%
0 2019
2017
2015
2013
2011
2009
2007
2005
2003
2001
1999
1997
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
1973
1971
1969
1967
1965
0.0%
Methane in Metric Tons
1.4%
Landfill GHG Emissions vs. Measured Biogas Some of the regulatory frameworks for landfills are inefficient and outdated with respect to GHG emissions.
M
BY TORAJ GHOFRANI
ore 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,
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 approxi-
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26 BIOMASS MAGAZINE | ISSUE 2, 2021
