Improving Characterization of Anthropogenic Methane Emissions in the United States (2018) / Chapter Skim
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2 Current Inventories of Methane Emissions
2 Current Inventories of Methane Emissions
Pages 37-76
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INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS The United States, like all developed countries, submits a national, annual inventory of anthropogenic GHG emissions and sinks to the United Nations (UN)
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These recalculations may be related to the use of a new emission calculation methodology, updated activity data, emission factors, or other parameters used for one or more years of the time series.
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Although coverage under the GHGRP is not complete, the methane data reported under the program can provide additional insight beyond what can be obtained from the national GHGI, because emissions are estimated using a higher-tiered method and provide specific plant- and component-level emission data for sources across the United States. The EPA uses the GHGRP data to update activity data and emission factors in the GHGI and refine national estimates and trends.
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The SIT provides a common framework developed in a manner consistent with the GHGI methodologies, but allowng for the use of state-specific activity data and emission factors where available. i However, only a few states report methane emissions either through their state regulatory reporting program or employing the SIT to estimate their methane emissions.
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For instance, field studies that quantify local- to r egional-scale emissions may report daily or hourly averages. Discrepancies between these subnational values and those reported by the GHGI may be difficult to explain given the differences in estimation and measurement approaches, as well as the complex spatial and temporal dynamics of methane emissions from various source categories.
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Large spatial differences in emissions from petroleum and natural gas systems and manure management were also observed between the two inventories, suggesting that evaluating emissions using different spatial and temporal scales can influence estimates. Hristov et al.
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(2016) studies are mainly due to different emission factors for the various subcategories of cattle (Figure 2.2)
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rely on EPA data, including the SIT tool.17 However, it does not include methane emissions from key petroleum and natural gas segments and from industrial wastewater. This tool helps users understand which states have the highest methane emissions and identify potentially relevant policies for the methane sources within these states (Figure 2.3)
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Finer-scale, gridded inventories of national methane emissions provide significant value to the scientific community to better characterize and compare inventories and test against top-down emission estimates. Further, gridded inventories have the potential to inform mitigation action at spatial scales relevant to policymakers and industry.
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• Manure management methane emissions have increased by 78 percent since 1990, resulting partially from a shift from smaller dairies and swine production facilities to larger facilities and increasing use of liquid manure systems for both swine and dairy cows, which have higher methane emissions than solid storage or application to pasture, range, and paddock. • Natural gas system emissions reflect the vented, fugitive, and flared emis sions from hundreds of thousands of individual sources.
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Rice cultivation is discussed briefly in Appendix C Enteric Fermentation Beef cattle are the largest contributor of enteric methane emissions (they accounted for 71 percent of 2015 emissions)
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The default emission factors (Ym) for cattle in the United States are 3 ± 1 percent for feedlot cattle (fed high-grain diets)
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. In addition to updating the methodologies, research is needed to define the probabilities of updated emission factors, given that, from a statistical perspective, emissions factors should follow certain probability distributions.
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methodologies with some modifications to account for variable methane conversion factors (MCFs) on a FIGURE 2.4 Percentage and amount of manure management methane emissions from each major livestock category in 2015 as reported by the GHGI (EPA, 2017b)
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More accurate methane emission estimates from manure management could be achieved by collecting more accurate activity data related to the distribution of manure in different manure management systems, updating the maximum methane potential for the various manure groups and methane conversion factors (via taking into account the effect of temperature and storage time on emissions) , as well as evaluating the accuracy of assigning manure distribution based on farm size.
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. Petroleum and natural gas emission estimates are generally based on multiplying estimated or actual counts of specific devices (e.g., wells, compressors, and pneumatic controllers)
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Flares are used to combust gases that might otherwise be vented, such as gases generated during well completions or FIGURE 2.6 Segments of the natural gas industry, including production, gathering and processing, transmission, and distribution, and top methane emission sources for each category. SOURCE: American Gas Association.
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Offshore platforms are inventoried separately from onshore well sites. Studies have demonstrated that a significant fraction of the emissions from the petroleum and natural gas industry originate from a relatively small fraction of outlying sources with higher-than-expected emission factors, that some specific sources responsible for the emissions may change over time (including daily)
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. Pneumatic Production 1.02 Pneumatic controller emission factors are based on controllersa Gas Research Institute (GRI)
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Normal Production 0.34 Normal fugitives consist of fugitive emissions from fugitives gas wells and well-pad equipment. Emission factors are based on the EPA/GRI, methane emissions from the natural gas industry (EPA, 1996a)
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Similarly, the EPA identifies 668 natural gas processing plants in the 2014 GHGI, while the EIA identifies 551 such plants.21 Hence, a publicly available, consistent, and centralized repository, similar to the National Oil and Gas Gateway,22 of key facility-level and component-level activity data at an appropriate spatial scale could improve national methane emission estimates while supporting improved gridded inventories. The EPA has in recent years employed activity data reported through the GHGRP to improve the emission estimates for some of the emission sources in the GHGI.
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. These unaccounted-for emissions may be due to inaccurate activity data or high-emitting sources that are not accounted for in current emission factors; they may also be due to missing emission source categories.
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estimated these unassigned methane emissions to be about 19 percent of the entire natural gas supply chain emissions. Similarly, Zavala-Araiza et al.
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Priority emission subcategories for improvement are those whose emissions esti mation primarily relies on activity data and/or emission factors that originated from the 1996 EPA/Gas Research Institute study, including pneumatic controllers, fugitive emissions, and engine exhaust emissions through comprehensive data measurement and analysis. Employing additional research to investigate the benefits and accuracy resulting from the use of facility- versus component-level emission data would further the development of national-scale emission inventories for petroleum and natural gas sources.
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The IPCC (1996, 2006) methodologies have historically been used for both GHGI and GHGRP estimates for landfill methane emissions.
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The 90 percent allows for 10 percent annual oxidation in cover soils and has often been coupled in the United States (e.g., California GHG reporting) with an assumption of 75 percent biogas recovery "efficiency." Concerns with this approach include the reliance on limited data, the 10 percent annual oxidation (discussed below)
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Soil oxidation rates can range from negligible (too hot or cold, too wet or dry) to greater than 100 percent of the methane transport rate from the anaerobic zone (e.g., net uptake of atmospheric methane by cover soils with high oxidation capacities)
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for site-specific methane emission estimates for reporting sites. However, there are four different sets of re 28 "N" is for normal conditions of 0°C temperature and 101 kPa pressure used globally.
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can result in methane emission estimates ranging from several times to as much as 30-fold too high. National harmonization and statistical analysis of waste generation, diversion, 29 See ars.usda.gov.
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Greenhouse Gas Inventory methodology based on IPCC (2006) was never field validated for emissions, relies on the mass of waste in place as the major driver for emissions, assumes that methane generation rates peak in the year following disposal, and does not directly consider m ajor mitigation strategies such as increased density of biogas recovery wells or increased seasonal oxidation in thicker cover materials.
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coal mines (black dots) in the United States.
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Currently, the GHGI estimates from this source are based on GHGRP data obtained from underground mines that report the emissions and quarterly measurement data obtained from MSHA for the remaining mines. For ventilation systems, methane emission calculations rely on the measurements of the flow rate and the methane content in the ventilation 68
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Surface Mining and Post-Mining Activities Methane emissions in the GHGI from surface mining and post-mining activities30 are estimated by multiplying basin-specific surface mine coal production by basin-specific gas content of the surface-mined coal (occurring up to about 76 meters deep) and by one emission factor for the United States.
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The EPA classifies abandoned underground mines into three categories: venting mines, flooding mines, and sealed mines, with the estimation methods modified for each category to reflect different emission levels. Emission estimates from abandoned underground mines take into account the emissions during the active phase of the mine and use emission rate decline curves that have been developed for abandoned coal mines.
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. 33 The 2015 methane emissions from petroleum refining were estimated to be approximately 0.03 Tg in the GHGI or 0.3 percent of the total estimated emissions from petroleum and natural gas systems (EPA, 2017b)
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They found methane leakage at nearly every home, even when the natural gas appliances were not used. Extrapolating these results statewide, the authors estimated unaccounted-for methane emissions from the residential sector to be about 0.2 percent of residential natural gas consumption.
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New research and estimation techniques may provide a more robust estimate of methane emissions. However, application of current emissions and activity data to develop a consistent time series back to 1990 can be difficult if the updated activity data and emission factors are not available for the entire time series or if new emission factors are not applicable to earlier years due to changes in technology and/or practices in the industry.
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FUTURE U.S. METHANE EMISSIONS Projecting future methane emissions is extremely challenging.
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, projections of activity data (e.g., natural gas production from the Energy Information Administration) , and current mitigation policies and programs (e.g., Landfill Methane Outreach Program, Natural Gas STAR, Clean Air Act, and Clean Fuels Programs, as well as state-level programs)
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Future climate could be factored into future emission estimates using a combination of climate projections and field-validated process-based modeling for specific sources. Activity data and emission factors that serve as proxies for projecting future methane emissions, as well as current inventory methodologies, need to be robustly investigated regarding correlations to measured emissions from key categories.
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