A Look at Proposed Revisions for Municipal Solid Waste Landfill Emissions
1. Introduction
Municipal solid waste landfills (MSWL) are essential facilities that primarily handle household waste while also accepting other wastes such as hazardous materials, municipal sludge, industrial non-hazardous waste, and construction/demolition debris.
Landfill gas (LFG) is a complex mixture of gases produced through the microbial breakdown of organic materials within landfills. This process, known as anaerobic decomposition, occurs in the absence of oxygen and involves various microorganisms breaking down organic matter. The primary components of LFG biogas are methane (CH4) and carbon dioxide (CO2), which together make up the majority of the gas. Methane is a potent greenhouse gas with a much higher heat-trapping potential than CO2, making its management critical for reducing greenhouse gas emissions. Additionally, LFG biogas contains trace amounts of non-methane organic compounds (NMOCs), including volatile organic compounds (VOCs) like benzene and toluene, as well as other gases such as hydrogen sulfide (H2S) and nitrogen oxides (NOx).
Proper management and utilization of landfill gas as well as the accurate reporting of emissions helps to mitigate environmental impacts and offer opportunities for renewable energy production through technologies like landfill gas-to-energy (LFGTE) systems, which capture methane for electricity generation or heating applications.
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2. Landfills are a major source of methane
Municipal solid waste landfills are the third-largest source of human generated methane emissions in the United States, accounting for approximately 14.3% of methane in 2021. Reducing methane emissions from MWSLs is part of the strategy to reduce greenhouse gas emissions for climate change responses.
3. Landfill gas emissions are regulated through AP-42
The document known as AP-42, Compilation of Air Pollutant Emissions Factors from Stationary Sources (AP-42), is a widely used source of emission factors and methods for estimating air emissions from various industries. An emission factor is a representative value used to estimate the mass amount of an air pollutant released based on an associated activity (e.g., pounds of air pollutant generated per volume of gas combusted). In January 2024, the USEPA posted draft updated emissions factors for AP-42, Chapter 2, Section 4 – Municipal Solid Waste Landfills. See link.
AP-42 Section 2.4 includes the following sections relating to municipal landfills emissions:
a.??? Process description related to emissions formation?
b.??? Control technology methods
c.???? Emission calculation methods (equations, conversion factors, models)
d.??? Default concentrations for LFG constituents (e.g., NMOC, benzene, toluene, H2S, etc.)
e.??? Controlled emissions calculation methods and control efficiencies
f.????? Criteria air pollutant emission factors based on amount of methane combusted
g.??? References
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4. AP-42 Section 2.4 Changes
The January 2024 proposed changes will update the current final Section 2.4 dated November 1998. Draft changes from October 2008 were posted on the EPA website but not finalized. Changes include:
a. Updates references to more recent federal regulations such as NSPS/EG, NESHPAPs.
b. Updates Table 2.4-1. Default Concentrations for LFG Constituents.
c. Adds Section 2.4.5 for Source Classification Codes for municipal landfillss.
d. Updates Table 2.2-2 Default Concentrations of Benzene, NMOC, and Toluene Based on Waste Disposal History.
e. Updates Table 2.4-3. Control Efficiencies for LFG Constituents.
f. Table 2.4-4. (Metric Units) Emission Factors for Secondary Compounds Exiting Control Devices – primarily for criteria air pollutants
g. Table 2.4-5. (English Units) Emission Factors for Secondary Compounds Exiting Control Devices – primarily for criteria air pollutants
h. Updates and adds references for Section 2.4
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5. AP-42 Emission Factor Ratings
All AP-42 sections include “Emission Factor Ratings” for emission factors and the control efficiency of emission control equipment. Emission factor ratings in AP-42 serve as indicator of the variability or appropriateness of emission factors for estimating the average emissions for an activity.
The rating system includes the following:
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6. Emission Calculations in Section 2.4
Use of the calculation methods in Section 2.4 include the following steps:
a. Calculate methane emissions metric tons per year (mt/yr) using Equation HH-1 found in 40 CFR 98 subpart HH – Municipal Solid Waste Landfills. Section 2.4 does not include Equation HH-1.
b. Use the Equation HH-1 calculated methane mt/yr value as input to equations to convert methane volume generated to units of cubic meters methane per year (m3/yr).
c. Determine air pollutants to calculate (e.g., nonmethane organic compounds (NMOC), hazardous air pollutants, etc.).
i. Decide whether to use default compound concentrations (ppmv) supplied in Section 2.4 or use site specific sample data for the compounds to be calculated.
ii. For site specific data, adjust air pollutant concentrations using equations based on whether LFG sample dilution or air intrusion into the landfills is applicable. Default compound concentration data is adjusted for infiltration.
d. Use Section 2.4 calculated methane m3/yr value to calculate MSWL generated nonmethane organic compounds (NMOC), individual hazardous air pollutants, and other air pollutants.
e. Adjust generated emissions for each air pollutant based on control technology used and control efficiency using Section 2.4 supplied data, testing data or control equipment manufacturer data.
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Methods for calculating hydrochloric acid (from chlorinated compounds) from combustion equipment are included in Section 2.4. Site specific analysis of LFG for total chloride is considered the best source of data.
EPA’s e-GGRT system supplies Excel spreadsheets for various GHG reporting subparts including the subpart HH, Equation HH-1 calculations. Equation HH-1 yields metric tons of methane per year using inputs related to operations of the MSWL (e.g., year started operation, quantity and type of wastes disposed) and estimates of degradation factors and used of waste aeration.
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7. Landfill Gas Emissions Model (LandGEM)
Section 2.4 of AP-42 references a model known as Landfill Gas Emissions Model(s) that can be used to estimate emissions from municipal landfills. LandGEM uses a first-order decomposition rate equation to estimate emissions from anaerobic decomposition of waste. LandGEM is a macro-enabled Microsoft Excel file that uses visual basic for applications (VBA). The model can estimate emissions using site-specific data or built-in default parameters that are based on empirical data from U.S. landfills.
LandGEM includes Equation HH-1 from 40 CFR 98 subpart HH.
Emission rates calculated include:
a. Total landfill gas
b. Methane
c. Carbon dioxide
d. Nonmethane organic compounds
e. Criteria air pollutants
f. Various hazardous air pollutants
The model does not calculate emissions based on emission controls such as flares and enclosed combustion devices.
The current version of LandGEM, Version 3.03, was released in 2020. A beta version 3.1 of LandGEM was released in December 2023 for review, testing and comments. The beta version incorporates changes proposed for Section 2.4 of AP-42. The beta version was made available only for testing and should not be used for regulatory purposes.
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8. Criteria Air Pollutants
Section 2.4 includes emission factors for criteria air pollutants (CAP). CAP emission factors are supplied for combustion sources (e.g., enclosed combustion devices) using LFG. Notably, the draft document proposes CAP emission factors for enclosed combustors and flares for nitrogen oxides, NMOC and carbon monoxide that have an emission factor rating listed as “Highly Representative.” We expect this would be like an emission factor rating of “A. Excellent”.
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9. Landfill Gas Control Methods
Emission control methods and associated emission control efficiencies listed in Section 2.4 for landfill gas (LFG) include:
a. Combustion by flare, enclosed combustion units and thermal incinerators.
b. Use purified LFG as fuel for boilers used generate steam and recover heat energy
c. Use purified LFG as fuel gas for turbines and internal combustion engines that generate electricity.
d. Compress purified LFG and send it to a sales pipeline.
The document uses the term flares to mean open-tipped flares and enclosed flares (i.e., enclosed combustion devices). Flares are given a “Typical Control Efficiency” of 99.2% for NMOC emissions. Also, a control efficiency range of 90-99%+ is listed. These values are the same as the current version of Section 2.4 dated October 2008.
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10. EPA Proposed Updates to AP-42 Section 2.4 Emissions Factors
The US Environmental Protection Agency (EPA) recently proposed updates to AP-42, Section 2.4 emission factors for Municipal Solid Waste Landfills (MSWLs).
The changes include:
a. Reference to use 40 CFR 98 subpart HH, Equation HH-1 (not included in AP-42 itself) and other equations to convert calculated MSWL methane emissions into mass amounts of other air pollutants.
b. Default concentrations default values for landfill gas (LFG) constituents like nonmethane organic compounds (NMOC) and hazardous air pollutants and other air pollutants. However, site-specific data is preferred for better accuracy.
c. Updated control efficiencies for technologies used to control LFG emissions, such as flares and enclosed combustors.
d. References to the Landfill Gas Emissions Model (LandGEM) to calculate uncontrolled LFG emissions from MSWLs.
e. Updated references.
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Cimarron Takeaways
The proposed rule changes herein focus mainly on the methodology of calculating emissions. However, the most significant change to the rules above is the potential rule change regarding the new emissions factors for enclosed flares. Having not been included in AP-42 standards in previous years, the industry standard for “Low NOx” enclosed flare systems has been 0.06 lb/MMBtu of NOx. The proposed rule change attempts, for the first time, to include enclosed flare emission guidelines but has set them at .035 lb/MMBtu of NOx, which is drastically lower than current Low NOx systems. If this change does go into effect, Cimarron’s Certified Ultra Low Emission Burner (CEB) is capable of achieving NOx emissions as low as .018 lb/MMBtu. Considered the “Best Available Control Technology” (BACT) in California’s South Coast Air Quality Management District (SCAQMD), the CEB is an excellent choice for landfill gas emission control.
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