|
|
 |
|
|
|
|
Methane Emissions
U.S. anthropogenic methane emissions totaled 28.8 million metric tons in 1999, a decline of about 0.5 million metric tons from 1998 levels (Table 14). The decline is primarily the result of increased methane recovery for energy use at U.S. landfills, and to a lesser extent reductions in emissions from animal waste, coal mining, and petroleum systems. To be eligible for the tax credit included in Section 29 of the Windfall Profits Tax Act of 1980, methane recovery systems at landfills must have been operational by June 30, 1998. As the last recovery projects installed by the tax credit deadline came on line full-time, methane recovery for energy at U.S. landfills rose from 1.7 million metric tons to 2.1 million metric tons. Meanwhile, total U.S. coal production in 1999 fell for the first time in 5 years due to a drop in coal exports and nearly flat demand from electric utilities, lowering methane emissions by about 0.1 million metric tons.30 At the same time, domestic oil production fell, decreasing methane emissions from petroleum systems, and swine populations declined, lowering emissions from the solid waste of animals. Total 1999 methane emissions were 3.0 million metric tons below the 1990 level, a decrease equivalent to 17.1 million metric tons of carbon, or roughly 0.9 percent of total U.S. anthropogenic greenhouse gas emissions. In addition to a 2.3 million metric ton decrease in methane emissions from landfills since 1990, there has also been a 1.4 million metric ton decrease in methane emissions from coal mines since 1990 (Table 14). The 32-percent decline in emissions from coal mining is the result of a fourfold increase in methane recovery from coal mines and a shift in production away from some of the gassiest mines in Appalachia. Overall, methane emissions account for about 9 percent of total U.S. greenhouse gas emissions weighted by global warming potential.31 Table 14. U.S. Methane Emissions from Anthropogenic Sources, 1990-1999 Methane emissions estimates are much more uncertain than carbon dioxide emissions estimates. Methane emissions usually are accidental or incidental to biological processes and may not be metered in any systematic way.32 Thus, methane emission estimates must often rely on proxy measurements. Considerable effort has been devoted to improving estimation methods. For example, the IPCC has convened expert workshops on best practices for emissions estimation. However, with very little additional sample or activity data being gathered, the marginal improvements associated with revised methods are limited. U.S. anthropogenic methane emissions for 1999 also include preliminary data for several key sources; thus, the overall estimate is preliminary.
Methane emissions from two key sources—coal mining and natural gas systems—are preliminary. These sources represent almost one-third of all U.S. methane emissions. Coal production data on a mine-by-mine basis are collected on Form EIA-7A and typically are not finalized until December following the data year. Estimates of methane emissions from ventilation and degasification systems in coal mines in 1999 have been scaled to reflect a decrease in national production centered in Appalachia and the Illinois basins— areas with predominantly underground operations and the preponderance of ventilation and degasification emissions. Estimated emissions from natural gas systems are preliminary because 1999 data for transmission and distribution pipelines are not yet available. U.S. methane emissions from energy sources were estimated at 10.6 million metric tons in 1999, down by 0.13 million metric tons from 1998 and by 1.4 million metric tons from 1990 (Figure 4). Both the decline in 1999 and the drop 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. Emissions from coal mines dropped by 32 percent (1.4 million metric tons) between 1990 and 1999. The decline resulted from 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 Appalachia. Between 1990 and 1999, the share of coal production represented by underground mines declined from 41 percent to 36 percent. Methane emissions from petroleum systems dropped from 1.29 million metric tons in 1990 to 1.04 million metric tons in 1999. A decrease of 0.15 million metric tons in estimated emissions from stationary combustion made a smaller contribution to the overall drop in emissions from energy sources between 1990 and 1999. Figure 4. U.S. Emissions of Methane by Source, 1990-1999 (Million Metric Tons Methane) Together, the declines in emissions from coal mining, petroleum systems, and stationary combustion more than compensated for an increase of 0.41 million metric tons in emissions from the natural gas system, attributed to increasing U.S. consumption of natural gas between 1990 and 1999. Overall, annual estimates of methane emissions from energy sources in this report are about 1 million metric tons higher than those that appeared in previous editions of Emissions of Greenhouse Gases in the United States due to the first-time addition of much more comprehensive estimates of emissions from petroleum systems (see discussion on "Comprehensive Estimates of Methane Emissions from Petroleum Systems Added This Year"). The preliminary estimate of methane emissions from coal mines for 1999 is 2.88 million metric tons (Table 15), a decrease of 4.6 percent from the 1998 level.33 The decrease can be traced to coal production levels, which fell for the first time in 5 years. U.S. coal production dropped from 1.12 billion short tons in 1998 to 1.09 billion short tons in 1999. The decline was primarily attributable to a 37 million short ton decrease in production in Appalachia, a region dominated by underground mining. In contrast, production in Wyoming, nearly all from surface mines, grew from 314 million short tons to 335 million short tons. Metallurgical coal exports to Europe—a key market for eastern U.S. coal—dropped by 30 percent, and increased nuclear generation east of the Mississippi River dampened growth in the electric utility market.34
Between 1990 and 1999, methane emissions from coal mines dropped by 32 percent from the 1990 level of 4.26 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 0.25 million metric tons in 1990 to about 1 million metric tons in 1999; (2) methane drainage from degasification in active mines decreased by 0.3 million metric tons between 1990 and 1999; and (3) methane emissions from ventilation systems at gassy mines dropped by about 0.4 million metric tons between 1990 and 1999 (Table 15).35
Table 15. U.S. Methane Emissions from Coal Mining and Post-Mining Activities, 1990-1999 Natural Gas Production, Processing, and Distribution At 5.99 million metric tons, 1999 estimated methane emissions from oil and gas production, processing, and distribution rose slightly from the revised estimate of 5.95 million metric tons for 1998 (Table 16). The 0.7-percent increase can be traced in part to a 17-percent increase in gas withdrawals from storage that offset a 1.4-percent reduction in gross gas withdrawals. The 1999 estimate is preliminary, however, because pipeline data for 1999 have not been finalized. The estimated 1999 emissions level is 7.4 percent above 1990 levels, with about 40 percent of the increase attributable to a 10.4-percent increase in gross gas withdrawals and 55 percent due to a 17-percent increase in distribution main pipeline.36 The estimates of methane emissions from natural gas production, processing, and distribution presented here are about 0.1 million metric tons lower than those that appeared in previous editions of Emissions of Greenhouse Gases in the United States because methane emissions from oil wells and oil refining and transport are now included in estimates of methane emissions from petroleum systems (see discussion on "Comprehensive Estimates of Methane Emissions from Petroleum Systems Added This Year"). Table 16. U.S. Methane Emissions from Natural Gas Systems, 1990-1999
Approximately 97 percent of all emissions from petroleum systems occur during exploration and production. Of the approximately 1 million metric tons of emissions annually from this source, 90 percent can be traced to venting, of which nearly half is attributable to venting from oil tanks (Table 17). A much smaller portion of methane emissions from petroleum systems can be traced to refineries and transportation of crude oil. Overall, methane emissions from petroleum systems are estimated at 1.04 million metric tons in 1999, down from 1.11 million metric tons in 1998 and 1.29 million metric tons in 1990. Domestic oil production in 1999 was approximately 81 percent of the 1990 level, accounting for the decline in methane emissions from this source. Table 17. U.S. Methane Emissions from Petroleum Systems, 1990-1999
U.S. methane emissions from stationary combustion in 1999 were 412 thousand metric tons, up by 6.6 percent from 1998 levels but 27 percent below 1990 levels (Table 18). Residential consumption of firewood 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.37 The universe of wood consumers is large and heterogeneous, and wood for residential consumption is typically obtained from sources outside the documented economy. EIA relies on its Residential Energy Consumption Survey (RECS), which is fielded every 3 to 4 years, to estimate residential wood consumption. Residential wood consumption data are derived from the 1990, 1994 and 1997 RECS. Intervening and subsequent years are scaled to heating degree-days. For the third consecutive year, the U.S. experienced warmer than average winter temperatures, driving down estimates of wood consumption from more typical levels seen between 1990 and 1996. Table 18. U.S. Methane Emissions from Stationary Combustion Sources, 1990-1999 Estimated U.S. methane emissions from mobile combustion in 1999 were 244 thousand metric tons, up by 1.5 percent from 1998 levels but 0.6 percent below 1990 levels (Table 19). 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. From 1993 to 1996, however, rapid growth in the fleet of light-duty trucks and the related increase in methane emissions offset the declines from passenger cars. Since 1997, emissions from passenger cars and light-duty trucks have been flat, and the small growth in emissions over that time is attributable to increased fuel consumption by heavy trucks and other forms of transport such as ships, construction equipment, and small commercial aircraft.
Table 19. U.S. Methane Emissions from Mobile Sources, 1990-1999 Methane emissions from waste management account for 32 percent of U.S. anthropogenic methane emissions (Figure 4). This portion has been declining from its 1990 level of 36 percent due to a 2.3 million metric ton drop in emissions from landfills. Landfills represent 98 percent of the 9.1 million metric tons of methane emissions from waste management and remain the single largest source of U.S. anthropogenic methane emissions (Table 14). The remainder of emissions from waste management are associated with domestic wastewater treatment. Estimated emissions from waste management would increase if sufficient information were available to estimate emissions from industrial wastewater treatment.38
Despite a record level of municipal solid waste reaching U.S. landfills in 1999 (Figure 5),39 estimated methane emissions from landfills dropped to 8.94 million metric tons, 3.7 percent below the 1998 level of 9.29 million metric tons and 2.3 million metric tons or 21 percent below 1990 levels (Table 20). This dramatic decrease is directly attributable to a 2.7 million metric ton increase in methane captured that otherwise would have been emitted to the atmosphere. Of the 3.6 million metric tons of methane believed to be captured from this source, 2.07 million metric tons were recovered for energy use. The remainder was recovered and flared. While estimates of methane recovered and disposed of in both manners are drawn from data collected by EPA’s Landfill Methane Outreach Program,40 there is less uncertainty in the estimate of methane recovered and used for energy because these projects are reported directly by their principals. In contrast, estimates of methane recovered and flared are derived from sales data reported to EPA by flare manufacturers. Figure 5. U.S. Solid Waste Generated and Landfilled, 1990-1999 (Million Short Tons) Table 20. U.S. Methane Emissions from Landfills, 1990-1999 The rapid growth in methane recovery has resulted from a combination of regulatory and tax policy. The Federal Section 29 (of the Internal Revenue Code) tax credit for alternative energy sources, including landfill gas, added to the tax code as part of the Crude Oil Windfall Profits Act of 1980, provides an inflation-adjusted credit that currently is equivalent to $6.00 per barrel of oil equivalent of qualified fuels. However, the tax credit for new facilities expired on June 30, 1998, and, absent a similar subsidy, the number of additional landfill gas-to-energy projects that are commercially viable is limited. More recently, increases in methane recovery have resulted from the implementation of EPA’s New Source Performance Standards and Emission Guidelines, which 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 as an energy resource.41 Currently, Section 45 of the Internal Revenue Code contains a provision that grants a tax credit—valued at approximately 1.7 cents per kilowatthour—for electricity generated from wind, closed-loop biomass, or poultry waste.42 Representative Camp has introduced a bill, H.R. 3466, to the House Committee on Ways and Means that would qualify landfill gas as an alternative energy source under Section 45. The bill would also provide an equivalent subsidy to other energy uses of landfill gas. Passage of this bill during the current session of Congress is uncertain. Absent this or an alternative source of subsidy, it is likely that the preponderance of future methane recovery at landfills may result in flaring. Domestic and Commercial Wastewater Treatment
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. If it is treated aerobically, methane emissions will be low. Under anaerobic conditions, methane emissions will be high. There is little data available on wastewater treatment methods. Data on flaring or energy recovery from methane generated by wastewater is also sparse. EIA believes that emissions from this source are relatively small, representing on the order of 0.6 percent of all U.S. methane emissions. Thus, emissions are estimated using a default per-capita emissions factor and U.S. population data. With the U.S. population growing slowly, methane emissions from domestic and commercial wastewater treatment are estimated to have grown by 0.9 percent between 1998 and 1999 to 0.16 million metric tons. This is about 9.3 percent above the 1990 level of 0.15 million metric tons (Table 14). The EPA is conducting research in this area. If additional information becomes available, EIA will review it and revise the estimation method accordingly. At an estimated 9.0 million metric tons, methane emissions from agricultural activities represent 31 percent of total U.S. anthropogenic methane emissions. (Table 14). Ninety-four percent of methane emissions from agricultural activities result from livestock management. About 64 percent of these emissions can be traced to enteric fermentation in ruminant animals and the remainder to anaerobic decomposition of livestock wastes. A small portion of U.S. methane emissions result from crop residue burning and wetland rice cultivation. Estimated agricultural methane emissions dropped slightly between 1998 and 1999 due mainly to a decrease in swine populations that lowered emissions from the solid waste of domesticated livestock. Enteric Fermentation in Domesticated Animals
In 1999, estimated methane emissions from enteric fermentation in domesticated animals remained essentially unchanged at 5.4 million metric tons (Table 21). Because cattle account for about 95 percent of all emissions from enteric fermentation, trends in emissions correlate with trends in cattle populations. The flat trend results from a slowly decreasing cattle population, offset by a slowly increasing average size for cattle. Average cattle size (excluding calves) reached a 20-year high in 1999. Animal size is a principal determinant of energy intake requirements, which relate directly to methane emissions. Emissions remain 4.6 percent above 1990 levels, primarily due to an average 6.4-percent growth in average cattle size between 1990 and 1999.43 Meanwhile, cattle populations have fluctuated in a cyclical pattern, settling in 1999 at levels very similar to those seen in 1990. Table 21. U.S. Methane Emissions from Enteric Fermentation in Domesticated Animals, 1990-1999 Solid Waste of Domesticated Animals
Estimated methane emissions from the solid waste of domesticated animals declined from 3,086 thousand metric tons in 1998 to 3,026 thousand metric tons in 1999 (Table 22). The decrease was the result of a 4-percent drop in populations of market swine and a 7-percent decline in populations of breeding swine, which decreased methane emissions by 75 thousand metric tons. In the absence of the population declines, overall methane emissions from the waste of domesticated animals would have increased slightly. There has also been 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.44 EIA does not have sufficient data to substantiate that belief at this time. If true, however, it would likely change the trend in emissions from this source from slightly negative to slightly positive. Despite the apparent declines, estimated 1999 emission levels were still approximately 337 thousand metric tons above 1990 levels due to a general increase in the size of cattle over the last decade and a 12-percent increase in the population of market swine. Table 22. U.S. Methane Emissions from the Solid Waste of Domesticated Animals, 1990-1999 Estimated methane emissions from U.S. rice cultivation continued to grow in 1999, due to an increase in the acreage harvested. In 1999, estimated emissions reached 0.50 million metric tons, the highest level since the early 1980s and 23 percent above 1990 levels (Table 14). Because data for rice harvested in Florida were unavailable for 1999, EIA assumed that the level of cultivation for Florida remained stable from 1998. This is likely to have a very limited effect on overall emission estimates, because Florida accounts for less than 1 percent of all rice harvested in the United States. Crop residue burning, being the smallest contributor to agricultural greenhouse gas emissions, represents on the order of 0.1 percent of total U.S. methane emissions. Estimated 1999 methane emissions from the burning of crop residues were 0.04 million metric tons, down by 2.8 percent from 1998 levels (Table 14). The small decrease is attributable mainly to reduced corn, soybean, and wheat production. Emissions remain 10 percent (3,000 metric tons) above 1990 levels. The 1999 estimate of methane emissions from crop residue burning reflects a small modification to the estimation method. In last year’s report, the portion of California rice crop residues assumed to be burned was raised from 3 percent used for all other crops to a range that declined from a high of 99 percent in 1990 to 50 percent in 1996 and beyond. The assumption has been revised again to reflect improved data collected by the EPA.45 The most recent EPA estimate of the portion of rice crop residues in California that is burned ranges from 43 percent in 1990 down to 16 percent in 1997 and up to 19 percent in 1998 and 1999. The effect of this change on the overall estimate of methane emissions from crop residue burning is limited, however, because less than 2 percent of emissions from this source are attributable to California rice crop residues. The preliminary estimate of methane emissions from U.S. chemical production in 1999 is 77 thousand metric tons, 21 thousand metric tons higher than in 1990 (Table 23). The 1999 number remains preliminary pending updated production data for five chemicals: methanol, carbon black, ethylene, ethylene dichloride, and styrene. Methane emissions from chemical production were essentially flat in 1997 and 1998. Table 23. U.S. Methane Emissions from Industrial Processes, 1990-1999
With production of pig iron declining by 4 percent, production of sinter dropping by 6 percent, and production of coke falling by 9 percent, estimated methane emissions from iron and steel production were 54 thousand metric tons in 1999, down by 3 thousand metric tons from 1998 levels and 13 percent below the 1990 level of 62 thousand metric tons (Table 23). A general pattern of reduced iron and steel production has resulted in flat or declining methane emissions from this source over the past decade. If you would like to receive any information relating to any of our greenhouse gas reports via e-mail, click here and subscribe by entering your e-mail address.
URL: http://www.eia.doe.gov/oiaf/1605/ggrpt/methane.html Need Help Now? |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
