Landfill Gas Generation and Emission Models

Transcription

1 Landfill Gas Generation and Emission Models Model Options for Recovery System Design and Greenhouse Gas Inventories This article provides an overview of various landfill gas (LFG) generation and recovery models, vital for the design of LFG control/recovery systems including the U.S. Environmental Protection Agency s (EPA) Landfill Gas Emissions Model (LandGEM), the Intergovernmental Panel on Climate Change (IPCC) Waste Model, and alternatives, like the Capturing Landfill Emissions for Energy Needs (CLEEN) Model as well as the recently-validated California Methane Inventory Model (CALMIM), currently the only field-validated landfill cover emission model that can be used to assess landfill contributions to climate change.

2 Microorganisms generate landfill gas by degrading organic waste in the absence of oxygen. Due to its methane content (approximately percent), LFG, like natural gas, can be burned to generate heat or electricity, and thus serve as a renewable energy resource. 1 However, methane is also a potent greenhouse gas, with a global warming potential 28 times that of carbon dioxide over 100 years. 2 Landfills are the third largest source of U.S. human methane emissions, accounting for 18 percent of the U.S. total. 3 Recently-issued EPA regulations require LFG controls for landfills with a design capacity 2.5 million metric tons of waste. EPA estimates that 115 new or modified landfills, and 731 existing landfills, will need to install LFG controls by When designing these control/recovery systems over can range from percent 7 ), and resulting emissions to the atmosphere, is thus important in assessing the overall contribution of landfills to climate change. Generation/Recovery Models for LFG Control/Recovery System Design The three generation/recovery models to be discussed in detail here (LandGEM, IPCC, and CLEEN 8-10 ) are downloadable, free, and have convenient, user-friendly Excel interfaces. All are based on first-order kinetics, which have been found to reasonably approximate waste degradation and associated methane generation. 11 According to first-order kinetics, methane generation depends on the methane generation potential and methane generation Roughly one quarter of methane generated is not collected and can be emitted to the environment. the next decade, modeling of LFG generation and recovery potential will be crucial. LFG control/recovery systems, although critical in reducing methane emissions, are unfortunately not 100-percent efficient. Reported efficiencies range from percent; EPA recommends a 75-percent value if no site-specific data are available. 5 This means that roughly one quarter of methane generated is not collected and can be emitted to the environment. 6 The amount of methane ultimately emitted to the atmosphere depends on the fraction of methane oxidized by microorganisms to carbon dioxide as LFG passes through the landfill cover. Modeling methane oxidation in landfill covers (which rate. The methane generation potential, or maximum amount of methane that can be produced per mass of waste, depends on the waste s composition and degradable organic content. The methane generation rate encompasses how quickly the waste is degraded and the methane is generated; it depends on the waste s composition, moisture (a function of waste composition, rainfall, and leachate recirculation), ambient temperature, waste particle size, and ph. 12 LandGEM, IPCC, and CLEEN differ in the specificity of their values for methane generation potential and rate. All three models assume a default LFG composition of 50-percent methane/50-percent carbon dioxide. Table 1. LandGEM default values for methane generation potential and rate. Default Type Landfill Type Methane Generation Methane Generation Potential (m 3 /Mg) Rate (yr -1 ) CAA Conventional (Rainfall > 25 in./yr) CAA Arid area (Rainfall < 25 in./yr) Inventory Conventional (Rainfall > 25 in./yr) Inventory Arid area (Rainfall < 25 in./yr) Inventory Wet (bioreactor)

3 LandGEM. According to its user information, LandGEM is considered a screening tool. One reason is the limited accuracy of its default values for methane generation potential and rate. Referring to Table 1, LandGEM contains two sets of defaults: the overly-conservative U.S. Clean Air Act, or CAA, defaults (used only to determine whether a landfill is subject to CAA regulations) and the inventory defaults, which can be used for design of LFG control/recovery systems. LandGEM contains only three inventory default values for the methane generation rate, which account for moisture content variability to a limited extent, but do not account for how the rate varies from landfill-to-landfill based on temperature or waste composition. The single default value for methane generation potential does not account for variability in waste composition, either. LandGEM s default values for methane generation potential and rate were based on data from 40 U.S. landfills in the 1980s/early 1990s. 5 The U.S. waste stream composition today, however, differs from earlier decades; for example, food waste rose from 8 percent in 1985 to 15 percent in Since both methane generation potential and rate depend on waste composition, their values have likely changed over the past years. Although the user can input sitespecific values for methane generation potential and rate, such information is often not available. It should be emphasized that the default values should not be used to estimate LFG generation in other countries; developing countries in particular tend to have higher proportions of food waste. Despite its name, LandGEM models methane recovery, not emissions. LandGEM s default methane potential value was based on LFG recovery data. Estimating landfill emissions depends on methane oxidized as LFG passes through the cover, but LandGEM does not consider cover oxidation, because such data were not available at the time of the last model update (2005). 5 Besides methane and carbon dioxide, LandGEM estimates generation of minor landfill gases, such as hydrogen sulfide, toluene, and xylene. IPCC Waste Model. Compared to LandGEM, the IPCC (2006) Waste Model provides much greater specificity in methane generation rate and potential values. It includes a multiphase first-order option, which uses separate rates for rapidly-, moderately-, and slowly-degrading waste, with slowly-degrading waste further subdivided into paper/textiles and wood/straw, as shown in Table 2. Within the four waste categories, specific rates are provided for two temperature ranges (< 20 o C and > 20 o C) and two moisture contents (dry or wet), for a total of 16 default rate values. Conventional landfill methane recovery, Temale Landfill, Ghana. Source: University of Texas at Arlington.

4 Table 2. IPCC Waste Model default methane generation rates (yr -1 ). Waste Category Climate Zone Boreal and Temperate Tropical (mean annual temp. <20 C) (mean annual temp. >20 C) Dry (MAP/ Wet (MAP/ Dry (MAP/ Wet (MAP/ PET* <1) PET >1) <1,000 mm) <1,000 mm) Rapidly-degrading Food, sewage sludge Moderately-degrading Garden, disposable diapers, industrial waste Slowly-degrading Paper, textiles Slowly-degrading Wood, straw Note: *MAP = mean annual precipitation; PET = potential evapo-transpiration. The IPCC Waste Model calculates a weighted-average methane generation potential based on component-specific values included in the model and user-input percentages for each component. Since the values are component-specific (not country-specific), they can be used to estimate methane generation for a landfill anywhere, as long as percentages of components in the waste stream are known. The IPCC Waste Model is best suited as a LFG generation model. To estimate LFG recovery, the user must input a recovery percent. As mentioned previously, methane emissions depend not only on methane recovery, but also on oxidation through landfill cover soils. The IPCC Model assumes 0-percent oxidation unless the user inputs a different value, and thus provides a very conservative estimate of methane emissions. Alternative Models. The Global Methane Initiative provides seven country-specific LFG generation models, available for free on their website, including ones for China, Mexico, and Thailand. 14 Many consulting firms use their own proprietary models. The University of Texas at Arlington has developed the Capturing Landfill Emissions for Energy Needs (CLEEN) Model to provide greater specificity in methane generation variation with temperature and moisture content. Like LandGEM and the IPCC Waste Model, CLEEN assumes first-order methane generation. However, unlike LandGEM (which includes three default methane generation rates representing different moisture contents) and the IPCC model (which includes four combinations of temperature/ moisture content), CLEEN allows users to input specific values of annual average temperature (20 37 o C) and rainfall (2 12 mm/day) in order to calculate a location-specific methane generation rate. Landfill gas collection system, Steuben County, NY. Source: The CLEEN equation for methane generation rate is based on methane generation data from laboratory-scale landfill reactors operated with varying waste compositions, average rainfall rates, and temperatures. 15 To account for the fact that laboratory methane generation rates exceed field rates, a scale-up factor equation was developed to adjust the lab rates to values representative of field conditions. The current scale-up factor equation was based on methane recovery data from 11 landfills in high-income countries. As data become available, a scale-up factor equation for lower-income countries will be developed. When CLEEN model methane generation rates were compared to LandGEM and IPCC rates for six U.S. landfills not included in scale-up factor development, CLEEN rates were closest to actual rates derived from field data in four of six cases. 16

5 Like the IPCC model, CLEEN is basically a methane generation model. Recovery and oxidation are simply calculated as percents of methane generation, based on user inputs. CALMIM Emission Model for Assessing Landfill Contributions to Greenhouse Gases CALMIM focuses on emissions and does not model LFG generation. Default methane concentrations at the base of the cover are provided for daily, intermediate, and final covers; the user can input site-specific values if known. CALMIM then models microbial methane oxidation through the cover, accounting for local changes in cover soil temperature and moisture, which impact oxidation rates. Based on the latitude/ longitude of the landfill site, global simulation models are used to estimate location-specific daily values of temperature and rainfall. CALMIM considers the impacts on emissions of cover composition, cover thickness, and LFG recovery systems. Field validation was completed in 2014 using data from 40 landfills at 29 international sites (North and South America, Europe, Asia, Africa, and Australia). Pearson s r values were > 0.8 for 25 of 29 sites, showing strong correlation between measured and modeled values. Conclusion For design of LFG control/recovery systems, the IPCC Waste Model is recommended, because it applies to landfills in any country, and comprehensively considers impacts of waste composition on methane generation, although its consideration of climate impacts is limited. For assessing landfill contributions to climate change, CALMIM currently represents state-of-theart emissions modeling. em Melanie L. Sattler is an associate professor and Arpita Bhatt is a post-doctoral fellow, both with the University of Texas at Arlington. sattler@uta.edu and arpita.bhatt@uta.edu. References 1. Spokas, K.; Bogner, J.; Chanton, J. A process-based inventory model for landfill CH 4 emissions inclusive of seasonal soil microclimate and CH 4 oxidation; J. Geophys. Res. 2011, 116, G Chapter 8: The Physical Science Basis. In Anthropogenic and Natural Radiative Forcing; Intergovernmental Panel on Climate Change, See Chapter08_FINAL.pdf. 3. U.S. Greenhouse Gas Inventory Report: ; U.S. Environmental Protection Agency, See usinventoryreport.html#_ga= Factsheet: EPA s Air Rules for Municipal Solid Waste Landfills; U.S. Environmental Protection Agency, See 5. Background Information Document for Updating AP42 Section 2.4 for Estimating Emissions from Municipal Solid Waste Landfills; EPA/600/R ; U.S. Environmental Protection Agency, See 6. Chapter 3: Solid Waste Disposal. In IPCC Guidelines for National Greenhouse Gas Inventories; Intergovernmental Panel on Climate Change (IPCC), See gl/pdf/5_Volume5/V5_3_Ch3_SWDS.pdf. 7. Chanton, J.P.; Powelson, D.K.; Green, R.B. Methane Oxidation in Landfill Cover Soils, is a 10% Default Reasonable?; J. Environ. Qual. 2009, 38, Landfill Gas Emissions Model (LandGEM), Version 3.02; U.S. Environmental Protection Agency, See enter-products#software. 9. IPCC Waste Model; Intergovernmental Panel on Climate Change, See iges.or.jp/public/2006gl/vol5.html. 10. Capturing Landfill Emissions for Energy Needs (CLEEN) Model; University of Texas at Arlington, See 3A &c=eugzstcatdllvimen8b7jxrwqof-v5a_cdpgnvfiimm&r=xzypiwgi_skp_je8koknvq&m= vstqbef4nzwxz0sohzzec6p3d5m7uwpviw92h818yoo&s=bhgerszbwr3witp7uqyehsyopbn BkgCJ6vI511SOhqs&e=. 11. Landfill Gas Emissions Model (LandGEM) Version 3.02 User s Guide; EPA-600/R-05/047; U.S. Environmental Protection Agency, See Barlaz, M.A.; Ham, R.K.; Schaefer, D.M.; Isaacson, R. Methane production from municipal refuse: A review of enhancement techniques and microbial dynamics; Crit. Rev. Environ. Sci. Technol. 1990, 19 (6), Studies, Summary Tables, and Data Related to the Advancing Sustainable Materials Management Report; U.S. Environmental Protection Agency, See Landfill Gas Modeling Tools; Global Methane Initiative, See index.aspx?sector=msw. 15. Karanjekar, R.V.; Bhatt, A.; Altouqui, S.; Jangikhatoonabad, N.; Durai, V.; Sattler, M.L.; Hossain, M.D.S.; Chen, V. Estimating methane emissions from landfills based on rainfall, ambient temperature, and waste composition: The CLEEN Model; Waste Manage. 2015, 46, ; doi: /j.wasman ; available online at Bogner, J.; Spokas, K. Improving Landfill Methane Emissions Estimates with a Field-Validated Model: CALMIM. Presented during the Environmental Research & Education Foundation Webinar, See