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3. Methane Emissions Overview
U.S. anthropogenic methane emissions totaled 26.6 million metric tons of methane (612.8 million metric tons carbon dioxide equivalent) in 2002, a decrease of 0.8 million metric tons of methane from 2001 levels (Table 13). The decline in total methane emissions was primarily the result of a 0.6 million metric ton decrease in methane emissions from landfills, as well as smaller decreases in emissions from coal mining and stationary combustion. The amount of methane generated from the decomposition of waste in landfills was nearly unchanged between 2001 and 2002, but a 0.6 million metric ton increase in methane recovered for energy reduced net emissions from this source substantially. In addition, U.S. coal production dropped by 3.0 percent. Estimated U.S. emissions of methane in 2002 were 4.6 million metric tons below the 1990 level, a decrease equivalent to 106.3 million metric tons of carbon dioxide, or 1.5 percent of total U.S. anthropogenic greenhouse gas emissions. In addition to a 4.1 million metric ton decrease in methane emissions from landfills since 1990, there was also a 1.4 million metric ton decrease in methane emissions from coal mines during the same period (Table 14). The 32.8-percent decline in emissions from coal mining was the result of a 207.3 percent increase in methane recovery from coal mines and a shift in production away from gassy mines. Overall, methane emissions account for about 8.9 percent of total U.S. greenhouse gas emissions when weighted by methane’s global warming potential factor. As a result of revisions in estimation methods for emissions from enteric fermentation and the solid waste of domesticated animals, overall estimated levels of methane emissions in this report are lower than the estimates previously published by the Energy Information Administration (EIA). The downward revision amounts to approximately 0.7 million metric tons in 1990 and 1.2 million metric tons in 2001. Methane emission estimates are much more uncertain than carbon dioxide emission estimates. Methane emissions usually are accidental or incidental to biological processes and may not be metered in any systematic way. Thus, methane emission estimates must often rely on proxy measurements. Estimated U.S. anthropogenic methane emissions for 2002 are based on incomplete data for several key sources; thus, the overall estimate is likely to be revised. Emissions from three of these sources—coal mining, natural gas systems, and landfills—represented three-fifths of all U.S. methane emissions. Thus, comparisons between 2001 and 2002 numbers are more likely to be valid in the context of directional change rather than magnitude of change. For example, because 2002 data on waste generation are not yet available, waste generation has been scaled to economic output as a proxy. Less critical but still important data are also unavailable for natural gas systems, such as miles of gas transmission and distribution pipeline.
Energy Sources U.S. methane emissions from energy sources were estimated at 10.9 million metric tons in 2002 (251.8 million metric tons carbon dioxide equivalent), 0.1 million metric tons lower than 2001 levels and 1.0 million metric tons below 1990 levels (Figure 3). The drop in methane emissions from energy sources since 1990 can be traced primarily to decreased emissions from coal mines and, to a lesser extent, to lower emissions from petroleum systems and stationary combustion. Methane emissions from coal mines dropped by 32.8 percent (1.4 million metric tons) between 1990 and 2002. This decline resulted from the increased capture and use of methane from coal mine degasification systems and a shift in production away from some of the Nation’s gassiest underground mines in Central Appalachia. Between 1990 and 2002, the share of coal production represented by underground mines declined from 41.2 percent to 31.9 percent. Methane emissions from petroleum systems dropped from 1.3 million metric tons in 1990 to 1.0 million metric tons in 2002. A decrease of 0.2 million metric tons in estimated methane emissions from stationary combustion made a smaller contribution to the overall drop in emissions from energy sources between 1990 and 2002. Together, the declines in emissions from coal mining and stationary combustion more than compensated for the increase of 0.9 million metric tons in emissions from the natural gas system, attributed to increasing U.S. consumption of natural gas between 1990 and 2002. Coal Mining The preliminary estimate of methane emissions from coal mines for 2002 is 2.9 million metric tons (Table 14), a decrease of 3.6 percent from the 2001 level. This decrease can be traced to declines in coal production levels, which dropped by 3.0 percent in 2002 after rising by 5.0 percent in 2001. U.S. coal production declined to 1.09 billion short tons in 2002, down from the record 2001 level of 1.13 billion short tons. The decline was primarily the result of a decrease in coal exports and high levels of coal stocks built up during 2001. Annual U.S. coal consumption was nearly constant between 2001 and 2002. Between 1990 and 2002, methane emissions from coal mines dropped by 32.8 percent from the 1990 level of 4.2 million metric tons. The decline is attributed to three important trends: (1) methane recovery from active coal mines for use as an energy resource increased from 0.3 million metric tons in 1990 to about 0.8 million metric tons in 2002; (2) methane drainage from degasification in active mines decreased by more than 0.2 million metric tons between 1990 and 2002; and (3) methane emissions from ventilation systems at gassy mines dropped by about 0.6 million metric tons between 1990 and 2002 (Table 14). Natural Gas Systems At 6.5 million metric tons, 2002 estimated methane emissions from natural gas production, processing, and distribution were up from the revised estimate of 6.4 million metric tons for 2001 (Table 15). The 1.4-percent increase in emissions can be traced to an increase in gas withdrawals from storage in 2002; however, the 2002 estimate is preliminary, because pipeline data for 2002 had not been finalized as of the publication of this report. The estimated 2002 emissions level is 15.5 percent above the 1990 level, with about two-thirds of the increase attributable to increased mileage of distribution pipelines and one-third attributable to increases in gas withdrawals. Petroleum Systems Methane emissions from petroleum systems are estimated at 1.0 million metric tons in 2002, nearly unchanged from 2001 levels and down by 21.5 percent from 1.3 million metric tons in 1990. Domestic oil production in 2002 was 78.1 percent of the 1990 level, accounting for the decline in methane emissions from this source. Approximately 96.4 percent of all emissions from petroleum systems occur during exploration and production. Of the roughly 1.0 million metric tons of methane emissions annually from this source, 90.3 percent was traced to venting, of which nearly half is attributable to venting from oil tanks (Table 16). A much smaller portion of methane emissions from petroleum systems can be traced to refineries and transportation of crude oil. Stationary Combustion U.S. methane emissions from stationary combustion in 2002 were 0.4 million metric tons, down by 12.3 percent from the 2001 level and 36.0 percent below 1990 levels (Table 17). Residential wood consumption typically accounts for about 85 percent of methane emissions from stationary combustion. Methane emissions are the result of incomplete combustion, and residential woodstoves and fireplaces provide much less efficient combustion than industrial or utility boilers. Estimates of residential wood combustion are, however, very uncertain (for further details, see Energy Information Administration, Documentation: Emissions of Greenhouse Gases in the United States 2002 (to be published). The universe of wood consumers is large and heterogeneous, and EIA collects data on residential wood consumption only at 4-year intervals in its Residential Energy Consumption Survey (RECS). The most recently published EIA data on residential wood consumption are from the 1997 RECS. Updated data on residential wood consumption for calendar year 2002 will be available from the 2003 RECS. Mobile Combustion Estimated U.S. methane emissions from mobile combustion in 2002 were 0.2 million metric tons, down by 1.0 percent from 2001 levels and 2.3 percent lower than the 1990 level (Table 18). Methane emissions from passenger cars have declined since 1990 as older cars with catalytic converters that are less efficient at destroying methane have been taken off the road. However, from 1993 to 1999, rapid growth in the fleet of light-duty trucks and the related increase in methane emissions offset the declines from passenger cars. Since 1999, as growth in the fleet of light-duty trucks has moderated and the penetration of advanced catalytic converters has grown, emissions from both passenger cars and light-duty trucks have declined. Waste Management Methane emissions from waste management accounted for 28.6 percent of U.S. anthropogenic methane emissions in 2002 (Figure 3), down from 37.1 percent in 1990. Landfills represent 91.2 percent of the 7.6 million metric tons of methane emissions from waste management and remain the single largest source of U.S. anthropogenic methane emissions (Table 13). The remainder of emissions from waste management is associated with domestic wastewater treatment. Estimated emissions from waste management would increase if sufficient information were available to develop a reliable estimate of emissions from industrial landfills and industrial wastewater treatment. The U.S. Environmental Protection Agency (EPA) estimates some 1.7 million metric tons of methane emissions from these two sources during 2001 (1.0 million metric tons from industrial landfills and 0.7 million metric tons from wastewater treatment), or about 6.3 percent of total estimated U.S. methane emissions. Landfills Due to record levels of methane recovery for energy at U.S. landfills, estimated methane emissions from landfills dropped to 6.9 million metric tons (159.7 million metric tons carbon dioxide equivalent) in 2002, 8.5 percent below the 2001 level of 7.6 million metric tons and 4.1 million metric tons (37.0 percent) below 1990 levels (Table 19). The dramatic decrease in methane emissions since 1990 is directly attributable to a 4.7 million metric ton increase in methane captured that otherwise would have been emitted to the atmosphere. Of the 5.9 million metric tons of methane believed to be captured from this source in 2002, 3.0 million metric tons was recovered for energy use, and 2.9 million metric tons was recovered and flared. In 2002, methane recovery for energy increasingly took the form of direct use of medium-Btu gas in industrial boilers. The acceleration of this practice was driven by higher natural gas prices, which made landfill gas more competitive. While estimates of methane recovered and disposed of both by flaring and by recovery for energy are drawn from data collected by the EPA’s Landfill Methane Outreach Program, there is less uncertainty in the estimate of methane recovered and used for energy. It is likely that estimates of methane flared are biased downward due to a lack of comprehensive industry data. The rapid growth in methane recovery has been aided by a combination of regulatory and tax policy. The Federal Section 29 (of the Internal Revenue Code) tax credit for alternative energy sources, added to the tax code as part of the Crude Oil Windfall Profits Act of 1980, provided a subsidy roughly equivalent to 1 cent per kilowatthour for electricity generated from landfill gas. However, this tax credit expired on June 30, 1998, and, absent a similar subsidy, the number of additional landfill gas-to-energy projects that are commercially viable may be limited. H.R. 6, the comprehensive energy bill before the U.S. Senate in September 2003, contains new landfill gas-to-energy incentives under Section 45 of the Internal Revenue Code that would run through 2007. Increases in methane recovery have also resulted from the implementation of the EPA’s New Source Performance Standards and Emission Guidelines. These regulations require all landfills with more than 2.5 million metric tons of waste in place and annual emissions of nonmethane volatile organic compounds (NMVOCs) exceeding 50 metric tons to collect and burn their landfill gas, either by flaring or for use as an energy source. Domestic and Commercial Wastewater Treatment
With the U.S. population growing slowly, methane emissions from domestic and commercial wastewater treatment are estimated to have grown by 1.3 percent between 2001 and 2002 to 0.67 million metric tons. This is about 15.6 percent above the 1990 level of 0.58 million metric tons (Table 13). Methane emissions from industrial wastewater treatment are discussed. EIA has revised the estimates of methane emissions from domestic and commercial wastewater since publication of the previous edition of this inventory, Emissions of Greenhouse Gases in the United States 2001. Estimates of the fraction of organic content in wastewater and the portion of wastewater that decays anaerobically have been increased. The resulting estimates show annual emissions nearly four times as large as those previously published. Methane emissions from domestic and commercial wastewater treatment are a function of the share of organic matter in the wastewater stream and the conditions under which it decomposes. Wastewater may be treated aerobically or anaerobically. Because aerobic decomposition does not yield methane, whereas anaerobic decomposition does, the method of treatment is a critical determinant of emissions; however, there is little information available on wastewater treatment methods. Data on flaring or energy recovery from methane generated by wastewater are also sparse. EIA believes that emissions from this source are relatively small, representing 2.5 percent of all U.S. methane emissions in 2002. Thus, emissions are estimated using a default per-capita emissions factor and U.S. population data. Agricultural Sources Estimated agricultural methane emissions decreased by 0.5 percent between 2001 and 2002 due mainly to small decreases in emissions from animal waste and rice cultivation. At an estimated 8.0 million metric tons of methane (183.4 million metric tons carbon dioxide equivalent), methane emissions from agricultural activities in 2002 represent 29.9 percent of total U.S. anthropogenic methane emissions (Table 13). Ninety-four percent of methane emissions from agricultural activities result from livestock management. Sixty-seven percent of these emissions can be traced to enteric fermentation in ruminant animals, and the remainder is attributable to the anaerobic decomposition of livestock wastes. A small portion of U.S. methane emissions result from crop residue burning and wetland rice cultivation. Revisions to the estimation methods for enteric fermentation and the solid waste of domesticated livestock have lowered EIA’s estimates of methane emissions from agriculture for 1990 by approximately 0.7 million metric tons and for 2001 by 1.2 million metric tons. Aside from the magnitude of the change, the most noticeable effect of the revised methods was to alter the trend line in emissions from enteric fermentation. Rather than increasing slowly between 1996 and 2000, emissions from this source are now estimated to have declined slightly during that period. Enteric Fermentation in Domesticated Animals In 2002, estimated methane emissions from enteric fermentation in domesticated animals were unchanged from the 2001 level of 5.0 million metric tons (Table 20). Because cattle account for 95.1 percent of all emissions from enteric fermentation, trends in emissions correlate with trends in cattle populations. Between 2001 and 2002, cattle populations were nearly constant, with small declines in beef cattle populations offset by small increases in dairy cattle populations. Estimated methane emissions from enteric fermentation in 2002 are 4.1 percent below 1990 levels. The animal population data and methodologies used to estimate methane emissions from enteric fermentation have been revised for the 2002 inventory, affecting the estimates for all years. The revised population estimates eliminate previous double counting of beef cattle in feedlots, in turn reducing emissions from enteric fermentation in beef cattle for 1990 through 2002. In addition, the adjusted methane emissions factors reflect greater detail that the EPA has incorporated into the Tier 2 methodology outlined in the “good practice guidance” of the Intergovernmental Panel on Climate Change. Together, these changes alter the estimated trend in emissions from this source. Using both the old and new methods, estimated emissions from enteric fermentation emissions peaked in 1995; however, previously published estimates showed emissions largely flat between 1996 and 2001, whereas the new methods show a decrease in emissions since 1996.
Solid Waste of Domesticated Animals Estimated methane emissions from the solid waste of domesticated animals decreased from 2.49 million metric tons in 2001 to 2.47 million metric tons in 2002 (Table 21). This small drop was the result of decreases in beef cattle and swine populations. The drop offset a larger trend over the past decade: in 2002, emissions from the solid waste of domesticated animals were 0.5 million metric tons above 1990 levels, an increase of 27.6 percent. Between 1990 and 2002 there was a shift of swine populations to larger livestock operations, which are believed to be more likely to manage waste using liquid systems that tend to promote methane generation. To capture this shift and other trends in the management of the solid waste of domesticated animals, EIA revised its estimation methods for this source. The typical animal mass was revised for all animal categories, including revisions to swine sizes to conform more closely with USDA classifications. In addition, EIA has updated volatile solids factors and altered the distribution of waste management systems to reflect the general shift to larger, more managed farms. Together, these changes lowered annual estimates of methane emissions from the solid waste of animals by about 0.6 million metric tons. Rice Cultivation Estimated methane emissions from U.S. rice cultivation declined to 0.46 million metric tons in 2002 from 0.47 million metric tons in 2001. The drop was the result of a 3.3-percent decrease in the number of acres harvested. Arkansas, Missouri, Louisiana, and Texas all saw decreases in acres harvested. Methane emissions from rice cultivation in 2002 were 13.4 percent higher than in 1990 (Table 13). Burning of Crop Residues Crop residue burning, the smallest contributor to agricultural greenhouse gas emissions, represents less than 0.2 percent of total U.S. methane emissions. Estimated 2002 methane emissions from the burning of crop residues were 0.05 million metric tons, down by 4.7 percent from 2001 levels but still 3.0 percent above 1990 levels (Table 13). The small decrease is attributable mainly to declines in corn, sorghum, barley, wheat, and soybean production.
Industrial Sources Chemical Production The preliminary estimate of methane emissions from U.S. chemical production in 2002 is 65.6 thousand metric tons, 2.1 percent more than in 2001. The increase is attributable to increased production of carbon black, ethylene, and styrene, more than offsetting a drop in methanol production. Methane emissions from chemical production in 2002 were 18.1 percent above their level in 1990 (Table 22). Iron and Steel Production With production of pig iron and sinter dropping, methane emissions from iron and steel production fell by 3.6 percent between 2001 and 2002, to the lowest levels in more than 20 years. Emissions in 2002, at 46.5 thousand metric tons, were 24.7 percent below the 1990 level of 61.7 thousand metric tons (Table 22).
Released: October 2003 |
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