Text Box: Ongoing Research
Estimated U.S. anthropogenic methane emissions totaled approximately 26.6 million metric tons in 1993, a decrease of more than 1 million metric tons from the levels for each of the previous 3 years (Table 15). Nearly all of this drop can be traced to reduced emissions from energy production and distribution. Emissions from coal mines declined substantially in 1993, as a strike by the United Mine Workers of America (UMWA) reduced underground coal production by more than 50 million short tons. Overall coal production rebounded with the end of the UMWA strike, but production in some of the "gassiest" coal mining regions continued to decline, likely restraining future growth in emissions from this source. Emissions from the oil and gas system also diminished in 1993 as a result of large decreases in the estimated volume of gas vented.
Table 15. U.S. Methane Emissions from Anthropogenic Sources, 1987-1994
(Million Metric Tons of Methane)
NA = not available.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: EIA estimates presented in this chapter. Crop residue burning - U.S. Department of Agriculture, National Agricultural Statistics Service, Crop Production annual reports. Emissions calculations based on Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), pp. 4.50-4.62.
Estimated emissions from landfills declined as well, due to decreases in the volume of waste being landfilled and installation of additional methane recovery systems. However, landfills continued to be the largest source of anthropogenic methane emissions in the United States during 1993, accounting for almost 40 percent of the total. The management of domesticated livestock accounts for a large portion of the remaining U.S. methane emissions, totaling nearly 8.2 million metric tons in 1993, or 31 percent of all methane emissions.
Table 16. U.S. Methane Emissions from Coal Mining and Post-Mining Activities,
1987-1994
(Million Metric Tons of Methane)
NA = not available.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Coal production numbers from Energy Information Administration, Coal Production, DOE/EIA-0118 (Washington, DC, various years), and Coal Industry Annual 1993, DOE/EIA-0584(93) (Washington, DC, December 1994). Methane recovery rates from U.S Environmental Protection Agency, Office of Air and Radiation, Anthropogenic Methane Emissions in the United States: Estimates for 1990, Report to Congress (Washington, DC, April 1993), pp. 3-19 - 3-24; and Identifying Opportunities for Methane Recovery at U.S. Coal Mines: Draft Profiles of Selected Gassy Underground Coal Mines (Washington, DC, September 1994), pp. 6-6 - 6-8. Ventilation data for 1985, 1988, 1990, and 1993 provided by G. Finfinger, U.S. Department of the Interior, Bureau of Mines, Pittsburgh Research Center.
Emissions from coal mines grew steadily between 1983 and 1990, when underground coal production reached a high of 424 million short tons. At that time, emissions from coal mines represented 17 percent of all anthropogenic methane emissions in the United States and 58 percent of emissions from energy production and distribution. By 1993, these shares had fallen to 13 percent and 52 percent, respectively, largely as the result of a 17-percent decrease in underground coal output. Estimates of emissions from coal mines have been revised for the period 1987-1992, compared with estimates presented in last year's report.(Note 2) This is due to a change in the method used for calculating emissions from the ventilation systems of the Nation's gassiest underground mines (see discussion below), the inclusion of measured ventilation data for 1993, and the addition of degasification emissions from four mines that had previously been overlooked. The revised method results in emissions estimates for 1988 through 1992 that are between 100,000 and 200,000 metric tons higher than previously reported.
Methane Formation and Release. The natural processes that create coal also create methane, which is stored in the pore space of solid coalbeds and fills the cracks and fissures within the coal. The volume of methane produced increases with temperature. Because temperature tends to increase with depth, methane content, coal rank, and coalbed depth are correlated.
Other important variables determining methane content in a coalbed include moisture content and porosity. Low porosity coal contains less methane. Methane produced that is not held in the coal pores either migrates to the atmosphere through cracks in the coal seam or is stored in the surrounding strata of the coal seam. Methane stored in coal pores or in fissures within the coal seam migrates to areas of lower pressure. Thus, methane is released into the open spaces created by underground coal mines, or into the atmosphere as overlying strata are removed during surface mining.
The volume of methane released during surface mining is small compared with that emitted from underground coal mining. Surface mines are comparatively shallow, and the organic matter converted to coal close to the surface has been subjected to much lower temperatures than organic matter converted to coal deep underground. Further, due to its thin overburden, the methane in coal seams mined from the surface has had a greater opportunity to migrate to the atmosphere prior to mining. All methane remaining in the coal pores after mining is released when the coal is transported and pulverized for combustion.
Estimation Method. There are four major sources of methane emissions from coal mines: ventilation systems in underground coal mines, degasification systems in underground coal mines, emissions from surface mines, and post-mining emissions during transport and pulverization of coal. In addition, closed and abandoned underground coal mines may have measurable emissions in some cases.
Because methane in atmospheric concentrations above 5 percent is explosive and represents a potential fatal hazard to miners, the Mine Safety and Health Administration (MSHA) requires underground mines to be ventilated and sets standards for methane concentrations in the mines. The MSHA conducts quarterly inspections of underground mines, measuring methane concentrations and airflow both at the mine face and at the fan exhausts. Thus, a fairly reliable set of data exist for estimating emissions from the ventilation systems of underground mines. Estimates of emissions from degasification are more uncertain. Degasification emissions are not monitored by any regulatory agency. Where degasification does occur, the method of disposition (e.g., venting, flaring, sale for energy) may not be tabulated. Also, because coalbed methane has recently been recognized as a valuable energy resource in its own right, coalbed methane may be extracted from a coal seam that will not be mined for several years or may never be mined.
Estimates of emissions from surface mines and post-mining activity are uncertain. Methane from surface mines does not represent a significant health hazard to miners, cannot be practically recovered for energy, and thus is not directly measured. Similarly, post-mining emissions are not measured. However, because coal is pulverized before combustion, all methane contained in the coal pores that has not desorbed during mining or transport will be released at the combustion site.
Ventilation Systems in Gassy Mines. Emissions from ventilation systems in "gassy" mines accounted for 1.8 million metric tons of emissions, or 52 percent of all methane emitted from coal mining in 1993 (Table 16).(Note 3) The Department of Interior's Bureau of Mines has collected data from MSHA quarterly mine inspection reports and developed a database for selected years (most recently 1980, 1985, 1988, 1990, and 1993) containing data for the approximately 200 gassiest mines each year.
To estimate emissions from the ventilation systems of underground coal mines, each mine appearing in any year of the Bureau of Mines data was assigned to one of five coal basins: Northern Appalachia, Central Appalachia, Warrior, Illinois, and Western. Using coal production data reported to the Energy Information Administration (EIA) on Form EIA-7A, "Coal Production Report," total annual production from gassy mines in each basin was calculated. An emissions coefficient per ton of coal mined was derived for each basin in the years 1985, 1988, 1990, and 1993 by dividing reported emissions from gassy mines in each basin by total production from those mines. Emissions factors for intervening years were estimated by interpolating between the 1985, 1988, 1990, and 1993 factors.
This method includes two important revisions from that used to develop estimates for last year's emissions report. Measured ventilation data from the Nation's gassiest mines in 1993 were used to develop factors. Also, in developing the emissions factors, the inclusion of coal production data for all mines that have appeared in any year of Bureau of Mines data, rather than only those which appeared in the 1990 compilation, produced a more accurate reflection of total production within gassy mines.
Ventilation Systems in Nongassy Mines. According to the Bureau of Mines, total emissions from nongassy mines (those mines with emissions below 100,000 cubic feet per day) equal less than 2 percent of all emissions from underground mines.(Note 4) Thus, emissions from "nongassy" mines are of limited consequence, estimated at 30,000 metric tons in 1993 (Table 16).
Using the 1988 Bureau of Mines database, basin level emissions for nongassy mines were estimated by the U.S. Environmental Protection Agency (EPA) at 2 percent of total ventilation system emissions.(Note 5) Dividing this figure by total production for nongassy mines in 1988 provided a set of basin-level emissions factors for all years. These factors were multiplied by annual production to arrive at the emissions total. The use of a single set of emissions factors for all years may bias the annual estimates, but the minuscule quantity of emissions from nongassy mines makes the bias small.
Mine Degasification. Emissions from degasification systems in underground mines are estimated at just under 1.2 million metric tons in 1993. This represents a decline of 29 percent since 1990 and 22 percent from 1992 (Table 16). Degasification systems are employed in the Nation's gassiest mines, mostly located in Alabama, Virginia, and West Virginia. Degasification systems are used when gas volumes are too high to be practically reduced to safe levels by standard ventilation techniques. When a coalbed contains methane in sufficient quantities and purity for commercial sale, a degasification system may be used to recover gas for sale to pipeline companies.
There are approximately 30 degasification systems currently operating in the United States, with 10 mines recovering gas for pipeline sales.(Note 6) Estimates of degasification system emissions are based on estimates developed by the EPA for mines believed to have had such systems in place in 1988.(Note 7) Emissions factors were derived for each coal basin by scaling emissions to production for the mines identified in that survey. The emissions factors were then multiplied by annual production from those mines that are now believed to be operating degasification systems.
Surface Mines. Global average emissions factors recommended by the Intergovernmental Panel on Climate Change (IPCC) were used to estimate emissions from this source. The IPCC provides a range of emissions from 0.3 to 2.0 cubic meters of methane per metric ton of coal mined, developed from studies conducted in the United States, England, France, and Canada.(Note 8) This report adopts the central estimate produced using these factors. For 1993, emissions from surface mines are estimated at 420,000 metric tons. Emissions from this source have remained nearly stable over the past decade (Table 16).
Emissions from U.S. surface mines have not been systematically measured and are believed to be highly heterogeneous. However, emissions from five surface mines have been evaluated by Piccot et al.,(Note 9) using Fourier Transform Infrared (FTIR) spectroscopy.(Note 10) Although the sample remains too small to validate or generalize the results, the mines examined revealed emissions rates rivaling some of the gassiest underground mines in the United States. Extrapolating from these mines, Piccot et al. estimated emissions from surface mines at 388,000 metric tons during 1991, a difference of less than 10 percent from the estimate developed for this report.
Post-Mining Emissions. Post-mining emissions, like those from surface mining, are not measured systematically. Once again, this report relies on global average factors recommended by the IPCC.(Note 11) Coal mined from the surface has a very small in-situ methane content due to the low temperatures under which it was formed and the low pressures at limited depth. The resulting emissions range--0.0 to 0.2 cubic meters of methane per metric ton of coal mined--produces a nearly insignificant midpoint estimate of 40,000 metric tons emitted in 1993 (Table 16).
Emissions from transport and pulverization of underground coal are more substantial. Using the IPCC emissions range of 0.9 to 4.0 cubic meters per metric ton of coal mined, emissions from this source are estimated at just over 500,000 metric tons for 1993 (Table 16).
Inactive and Abandoned Mines. The treatment of closed mines varies, depending on local regulations and whether the mine is expected to be temporarily or permanently inactive. Mines may be completely sealed by backfilling or left open to the atmosphere with cement caps and vent pipes. According to the MSHA, some 6,000 underground coal mines in the United States have been abandoned since the early 1970s. Only a fraction of those are vented, and those that lie below the water table are likely to flood and have limited emissions. However, those that are vented and do not flood may have substantial emissions, and measurements at several abandoned mines have shown emissions rates comparable to those for some of the gassiest active mines in the United States.(Note 12)
Extrapolating from measurements at 20 abandoned underground coal mines, Piccot et al. estimated emissions from this source at approximately 280,000 metric tons. Because the limited data make extrapolation highly uncertain, these estimates are not included in this report. Emissions from those abandoned mines measured in 1993 totaled 25,000 metric tons. Some estimate of emissions from this source probably will be included in future years' reports.
Methane Recovery for Energy. From 1987 to 1991, about 250,000 metric tons of methane were recovered and sold to pipeline companies annually from mines in Alabama and Utah. In May 1992, four mines in western Virginia began recovering and selling gas. By the end of 1993, methane recovered from all mines totaled nearly 500,000 metric tons annually.(Note 13)
Table 17. U.S. Methane Emissions from Oil and Gas Operations, 1987-1994
(Million Metric Tons of Methane)
P = preliminary data.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: U.S. Environmental Protection Agency, Anthropogenic Methane Emissions in the United States: Estimates for 1990 (April 1993); World Oil (February issue, various years); American Gas Association, Gas Facts (various years); Energy Information Administration, Natural Gas Annual, DOE/EIA-0131 (various years); Radian Corporation, Global Emissions of Methane from Petroleum Sources (February 1992); Energy Information Administration, Annual Energy Review 1994, DOE/EIA-0384(94) (Washington, DC, July 1995); Energy Information Administration, Petroleum Supply Annual, DOE/EIA-0340 (Washington, DC, various years).
Estimates of emissions from the oil and gas system have been revised upward for the period 1987-1992, compared with estimates presented in last year's report.(Note 14) This revision is attributed to a change in the method used for calculating emissions from associated gas vented at oil wells (see discussion below). The revised method results in emissions estimates that are, on average, 6.7 percent higher annually for the period.
Methane Formation and Release. Oil and natural gas are found stored in porous and highly permeable reservoir rock, trapped under low permeability cap rock. Within the reservoir, oil and gas are arrayed vertically according to their densities, oil at the greatest depths and natural gas above. Natural gas is also usually found dissolved in the oil.(Note 15)
The natural gas held in rock reservoirs typically contains between 70 and 100 percent methane. Methane in both oil and gas is produced during the thermal degradation of organic matter in sedimentary rocks.(Note 16) Natural gas may escape from the oil and gas system at several points, including oil wells, oil refineries, natural gas wellheads, gas processing plants, and gas transmission and distribution pipelines. The release of natural gas leads to substantial methane emissions.
Estimation Methods. The U.S. oil and gas system is large (23 trillion cubic feet of withdrawals and 1.7 million miles of pipeline) and structurally diverse. It would be impossible to measure all intentional and fugitive emissions of methane. Instead, a series of emissions factors for system components must be used and scaled to readily available data, such as pipeline mileage and throughput. With the exception of gas vented, this report relies on emissions factors derived from a small sample of system components.
Oil and Gas Production and Processing. In 1993, a highest ever 283,812 natural gas wells produced 16.93 trillion cubic feet of gas. This was approximately 73 percent of total gas production, with the remaining 6 trillion cubic feet captured as associated gas withdrawn from oil wells.
Natural gas extracted at the wellhead is transferred to processing plants through gathering pipelines. As the gas is transferred, leakage from valves, meters, and flanges occurs. Some pneumatic valves use pressurized natural gas as motive power and release gas when reset. Additional emissions occur when pipelines are emptied during maintenance operations. At the gas processing plant, heavy hydrocarbons that are particularly valuable are removed, as are contaminants such as hydrogen sulfide that may damage pipelines and other equipment. At the processing plant, leakage, maintenance operations, and system upsets also result in emissions. System upsets result from sudden increases in pressure that require the release of gas as a safety measure or, failing that, result in a system rupture. Such events are uncommon in the U.S. oil and gas system and contribute only slightly to overall emissions.
The EPA has published emissions factors for the oil and gas system developed from model oil and gas production facilities, transmission lines, and processing plants.(Note 17) Emissions factors for oil and gas wells are based on studies of four model facilities. Factors for gathering pipelines are based on two model transmission line systems (discussed below), and factors for gas processing facilities are based on analyses of three model plants. These factors were modified slightly for scaling purposes. Estimates of emissions from oil and gas wells are based on the number of wells in operation, emissions from gathering pipelines are based on pipeline miles, and emissions from gas processing plants are scaled to gas throughput. For 1993, total methane emissions from oil and gas wells, gathering pipelines, and gas processing were estimated at 1.2 million metric tons, down slightly from 1992 emissions and virtually identical to 1990 emissions (Table 17).
Gas Venting. Often, when a reservoir is developed for oil extraction, associated natural gas is produced at the wellhead. In the United States, because of its well-developed natural gas market and infrastructure, associated gas is usually captured and sold commercially. In 1993, 6 trillion cubic feet of associated natural gas were withdrawn from oil wells.(Note 18) On occasion, however, the flow of associated gas may be too small or intermittent to be of value or have an insufficient heating value to be marketed, or the site may lack the necessary gas gathering and processing facilities. In such cases the gas is vented or flared.
When associated gas is flared, its methane content is converted to carbon dioxide (see emissions estimates in Chapter 2). When vented, methane is released directly into the atmosphere. Each State reports an annual volume of vented and flared gas to the EIA. However, no distribution between venting and flaring is provided.
To estimate emissions from venting, the volume of gas reported as vented and flared by each State is allocated according to a venting share estimate developed for each State in a 1990 Department of Energy study.(Note 19) A national quantity for vented gas is calculated by aggregating the individual State figures. This method improves on that used for previous EIA emissions reports, which applied a national average venting share to all State data. As a consequence, the level of emissions estimates from this source are, on average, 37 percent higher for 1987-1992 using the new method. Some of the largest State sources were underrepresented when a simple national average was used.
On an annual basis, emissions reductions from venting were achieved in 1993. The volume of gas vented was estimated as 30 billion cubic feet, a 30-percent reduction from 1992. The reduction reflects commissioning of gas gathering facilities in areas where gas formerly was lost. As a result, estimated methane emissions from this source declined from 830,000 to 580,000 metric tons (Table 17).
Gas Transmission and Distribution. High-pressure transmission pipelines are used to transport natural gas from production fields and gas processing facilities to distribution pipelines. The gas is sent through gate stations where its pressure is lowered for distribution to end users. Again, leakage from the pipelines, valves, and other equipment is the primary source of emissions. Compressor exhaust, pneumatic devices, and routine maintenance are also sources of methane emissions.
The emissions factors used to calculate emissions from transmission pipelines, gate stations, and distribution pipelines were developed by the EPA. They are the result of studies of 4 transmission systems, 28 gate stations, and 2 companies' distribution networks.(Note 20)
The emissions factors for pipelines have been scaled to mileage. However, with low-emission plastic pipeline being the preferred material for replacement and expansion of distribution systems, emissions may actually increase at a lesser rate than mileage, as older leaky pipe is eliminated. The transmission and distribution of natural gas contributed an estimated 1.4 million metric tons of methane emissions in 1993 (Table 18). This represents a 2-percent decline from 1992 levels, largely due to a reduction in miles of transmission pipeline.
Table 18. U.S. Methane Emissions from Natural Gas Transmission and
Distribution, 1987-1994
(Million Metric Tons of Methane)
P = preliminary data.
Note: Totals may not equal sum of components due to independent rounding.
Sources: Emissions factors derived from U.S. Environmental Protection Agency, Anthropogenic Methane Emissions in the United States: Estimates for 1990 (Washington, DC, April 1993). Pipeline mileage from American Gas Association, Gas Facts (various years).
Oil Refining and Transportation. Methane emissions from refining operations can be attributed to three sources: fugitive emissions, tank farms, and flaring. Fugitive emissions are the result of equipment leakage during the portion of the refining process where methane is separated from the oil. Emissions at tank farms are produced by vapor displacement when oil is transferred to storage tanks upon arrival at the refinery. Additionally, any methane not destroyed by flaring operations is emitted to the atmosphere.
Transportation-related emissions also result from vapor displacement. In this case, however, the emissions occur during loading and unloading of oil to barges and ships. Emissions from pipelines, trucks, and rail cars are exceedingly minor and are not included in this report.
Emissions from oil refining and transportation were estimated using factors provided in a 1992 Radian Corporation report.(Note 21) The emissions factor used in the calculations assumes a methane content of 15 percent for the volatile organic compound vapor associated with oil.(Note 22) Estimated emissions from oil refining and transportation have remained relatively stable for a decade, adding 86,000 metric tons of methane to the atmosphere in 1993 (Table 19).
Table 19. U.S. Methane Emissions from Oil Refining and Transportation,
1987-1994
(Thousand Metric Tons of Methane)
Sources: Radian Corporation, Global Emissions of Methane from Petroleum Sources (Research Triangle Park, NC, February 1992); Energy Information Administration, Annual Energy Review 1994, DOE/EIA-0384(94) (Washington, DC, July 1995); Energy Information Administration, Petroleum Supply Annual, DOE/EIA-0340 (Washington, DC, various years).
If fuel combustion were complete, the only products emitted would be carbon dioxide and water. In fact, combustion is rarely complete, and the burning of fossil or renewable fuels emits other radiatively important gases, such as methane, nitrous oxide, carbon monoxide, nitrogen oxides, and nonmethane volatile organic compounds. Methane emissions from stationary and mobile combustion together account for less than 3 percent of all U.S. methane emissions, with wood burning in residential wood stoves representing 55 percent of that total (Table 20).
Methane Formation and Release. When carbon-based fuels are combusted, much of the carbon is released as carbon dioxide. However, when combustion is incomplete, methane may also be released. The volume of methane released is a function of the efficiency and temperature of the combustion process. Because most stationary sources are large, highly efficient boilers, methane emissions from these sources are limited.
Estimation Methods. The fuel consumed and the method of combustion for stationary sources of methane vary by economic sector. For example, wood consumed in the industrial sector is combusted in boilers with much higher temperatures and efficiency than the wood combusted in stoves in the residential sector. Thus, specific emissions coefficients are identified by fuel and combustion method. These coefficients were obtained from the EPA's Compilation of Air Pollutant Emission Factors and from the IPCC.(Note 23) Emissions coefficients were then combined with consumption data in EIA's State Energy Data Report 1993 and Annual Energy Review 1994(Note 24) to yield overall emissions estimates.
Table 20. U.S. Methane Emissions from Stationary Combustion Sources,
1987-1994
(Thousand Metric Tons of Methane)
*Less than 500 metric tons of methane.
aEstimate. Underlying energy data not available.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: U.S. Environmental Protection Agency, Office of Air and Radiation, Office of Air Quality Planning and Standards, Compilation of Air Pollutant Emission Factors, AP-42 Supplement D (Research Triangle Park, NC, September 1991). Energy Information Administration, State Energy Data Report 1993, DOE/EIA-0214(93) (Washington, DC, July 1995); Monthly Energy Review, DOE/EIA-0035(95/07) (Washington, DC, July 1995); Annual Energy Review 1994, DOE/EIA-0384(94) (Washington, DC, July 1995), p. 267; and Estimates of U.S. Biomass Energy Consumption 1992, DOE/EIA-0548(92) (Washington, DC, May 1994).
Table 21. U.S. Methane Emissions from Mobile Sources, 1987-1994
(Thousand Metric Tons of Methane)
NA = not available.
Sources: Calculations based on vehicle miles traveled from U.S. Department of Transportation, Federal Highway Statistics, various years, Table VM-1. Vehicle emissions coefficients from Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), pp. 1.72-1.82. Distribution of passenger car and light duty truck fleet model years for 1983, 1985, 1988, and 1991 according to data in the Energy Information Administration's þResidential Transportation Energy Consumption Surveysþ for those years. Distribution for passenger cars and light duty trucks in other years computed by interpolation. Distribution of bus and other truck fleet according to model year computed assuming 10-percent attrition per annum of pre-1983 fleet for each year after 1984.
Methane Formation and Release. In automobiles, when the amount of oxygen available is insufficient for complete combustion, methane emissions result. This condition occurs especially in low speed and engine idle situations. Other factors influencing the level of methane emissions include the level of unburnt hydrocarbons passing through the engine, engine maintenance, and post-combustion controls of hydrocarbon emissions, such as catalytic converters.
An increasing share of emissions is generated by light-duty trucks, as consumer tastes shift toward those vehicles. Methane emissions from nonhighway mobile sources account for less than 10 percent of all emissions from transportation. There is some indication that jet planes may draw ambient methane into their engines, combusting it along with fuel and hence reducing net emissions.(Note 25)
Estimation Methods. Mobile sources can be divided into two broad categories: highway sources and nonhighway sources.
Highway Sources. In the United States, highway sources include automobiles, light-duty trucks, motorcycles, buses, and heavy-duty trucks. Emissions factors for these vehicles developed by the IPCC were adopted for this report.(Note 26) Expressed in terms of grams of methane per kilometer traveled, they vary by vehicle type, fuel used, and emissions control technology employed. Due to the effects of evolving environmental regulations and technological development, the model year of U.S. vehicles is an accurate indicator of emissions control technology employed. Thus, emissions estimates require data on miles traveled by vehicle type and model year.
Miles traveled in personal transportation vehicles (cars and light-duty trucks) are obtained as part of EIA's Residential Transportation Energy Consumption Survey (RTECS).(Note 27) This survey was conducted in 1983, 1985, 1988, and 1991. Emissions for nonsurvey years were estimated by interpolating between the weighted average estimates for survey years. Emissions estimates for 1992 and 1993 rely on fleet age data reported by the American Automobile Manufacturers Association.(Note 28) VMT data for nonhousehold vehicles (fleets, rental cars, etc.), motorcycles, buses, and heavy-duty trucks were obtained from the U.S. Department of Transportation, Federal Highway Administration(Note 29) and reconciled with RTECS data.
Nonhighway Sources. Nonhighway sources include farm equipment, construction equipment, air transport vehicles, ships, and locomotives. The IPCC provides emissions coefficients for these sources in terms of grams of methane per kilogram of fuel consumed.(Note 30) The factors for ships, farm equipment, locomotives, and construction equipment were multiplied by data con tained in EIA's Fuel Oil and Kerosene Sales report.(Note 31)
Factors for jet aircraft and piston powered aircraft were multiplied by data in EIA's Petroleum Supply Annual,(Note 32) and factors for recreational boats were used in conjunction with fuel consumption estimates in the Transportation Energy Data Book.(Note 33)
Table 22. U.S. Methane Emissions from Anaerobic Decomposition in Landfills,
1987-1994
(Million Metric Tons of Methane)
NA = not available.
Note: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994).
Sources: Municipal solid waste landfilled from Franklin Associates, Ltd., Characterization of Municipal Solid Waste in the United States, Worksheets, 1992 Update (prepared for the U.S. Environmental Protection Agency, Municipal Solid and Industrial Solid Waste Division, July 1992), Personal communication with Marjorie Franklin, Franklin Associates, Ltd., May 1994, and Biocycle, Nationwide Survey: The State of Garbage in America 1988- 1994. Emissions calculations based on S.A. Thorneloe et al., "Estimate of Methane Emissions from U.S. Landfills," Prepared for the U.S. Environmental Protection Agency, Office of Research and Development, in departmental review (April 1994), and D. Augenstein, "The Greenhouse Effect and U.S. Landfill Methane," Global Environmental Change (December 1992), pp. 311-328. Methane recovered and flared from S.A. Thorneloe, þLandfill Gas Recovery UtilizationþOptions and Economics,þ presented at the Sixteenth Annual Conference by the Institute of Gas Technology on Energy from Biomass and Wastes, Orlando, Florida (March 5, 1992), and J. Pacey, "Methane Recovery from Landfills," presented at the 1995 Greenhouse Gas Emissions and Mitigation Research Symposium, Washington, DC (June 27-29, 1995).
Methane Formation and Release. The decomposition of organic wastes begins with aerobic bacteria consuming oxygen while converting organic substances to carbon dioxide, heat, and water. This process continues until all available oxygen is depleted. After oxygen depletion, decomposition of organic material continues under anaerobic conditions (the absence of oxygen). In the absence of oxygen, anaerobic bacteria, including methanogens, begin digesting the waste and producing methane.
Methanogenic anaerobes require a fairly narrow range of temperature, pH, and moisture content to maintain significant rates of activity. Because sanitary landfills in the United States are essentially closed systems designed to minimize the entry and exit of moisture, conditions within a landfill are largely a product of the composition of the waste itself. Thus, conditions are likely to vary both across different landfills and within a single landfill, producing methane at different rates and volumes.
Estimation Methods. Because it is nearly impossible to determine the exact composition of waste in every landfill in the United States, developing a national level estimate of methane emissions from landfills requires several assumptions about the average composition of landfilled waste.(Note 35) For the purpose of this report, the assumptions contained in the EMCON Methane Gen eration Model(Note 36) were adopted with minor modifications. This model divides the waste in landfills into three categories: readily decomposable waste, moderately decomposable waste, and slowly decomposable waste, each with its own set of emissions characteristics. For any given year, the share of all waste landfilled falling into each category can be estimated, and its emissions consequences can be calculated.
Because of uncertainty in the estimates of the methane yields for each type of waste, the EMCON model provides both a high methane yield scenario and a low methane yield scenario. For each category of decomposable waste, a time lag until methane generation begins is estimated, as well as a time constant during which the methane yield of the waste is realized. The methane yield represents the total amount of methane that a given amount of waste will produce over its lifetime. Under the low methane yield scenario, slowly decomposing waste will begin producing methane after a 5-year lag and will continue emitting over a 40-year period. Thus, to estimate methane emissions from landfills for 1985, the amount of waste going to landfills in 1940 must be known.
Estimates of Waste Landfilled. Estimates of waste landfilled annually were derived using data from two sources: Franklin Associates and Biocycle magazine.(Note 37) Franklin Associates provide annual estimates of municipal solid waste landfilled beginning in 1960, and Biocycle provides annual refuse estimates beginning in 1988. The Franklin Associates data are developed using a model that estimates trash outputs based on production inputs. The Biocycle data are based on a survey of State agencies, and the State data often include industrial waste, construction and demolition waste, or sewage sludge. The EIA believes that the Biocycle data are more likely to be representative of the waste stream.(Note 38) For the period 1988-1993, Biocycle estimates of waste landfilled are, on average, 1.43 times greater than the Franklin Associates estimates. Thus, the Franklin Associates estimates for the years 1960-1993 have been adjusted upward by that ratio.
No reliable data have been compiled for waste landfilled prior to 1960. Thus, estimates of waste landfilled between 1940 and 1959 were "backcast" using a regression equation developed with gross domestic product (GDP) and total population as independent variables and waste generated as the dependent variable. This backcast is somewhat uncertain, particularly for the years 1941-1945, when the United States was participating in World War II. Increased government spending inflated GDP during those years, and recycling campaigns in support of the U.S. war effort diverted waste away from landfills. However, the waste landfilled in those years has only a small effect on overall emissions estimates.
These estimation methods yield a solid waste landfilled figure of 167.5 million metric tons for 1993, a decrease of 2.5 million metric tons from 1992 and more than 43 million metric tons lower than the 1990 high of 210.6 million metric tons (see Table C10 in Appendix C). The steady decline in waste landfilled can be credited to increased refuse recycling, combustion, and, to a lesser extent, source reduction. According to Biocycle, some 19 percent of all waste generated in 1993 was recycled.
Overall Emissions Model Calibration. To apply the EMCON Methane Generation Model, waste landfilled was divided into two categories: waste buried in 105 landfills with gas recovery or disposal systems, and waste buried in all other landfills. Waste in place and methane emissions from landfills with gas recovery systems have been measured or estimated by Thorneloe et al.(Note 39) Those landfills contained 9.4 percent of all waste in place in 1992 and had total methane emissions of 2.1 million metric tons during that year. In the absence of any empirical data, the share of waste landfilled in those 105 landfills was assumed to remain constant, although it is likely that their share has increased over time.
To derive an estimate of emissions from the 9.4 percent of waste assumed to be buried in the 105 landfills with methane recovery systems for years other than 1992, the EMCON model was calibrated to produce the 2.1 million metric tons of measured emissions for 1992. This calibration increased the default methane yields contained in the EMCON model by a factor of 1.96. The revised yields were used in conjunction with the model's original time lags and time constants. The increased yields are plausible, as it probably is most economical to install gas recovery systems at the landfills producing the greatest volumes of methane. Emissions from the remaining 90.6 percent of waste landfilled annually were calculated using the default methodology for the EMCON model (Table 23).
Table 23. EMCON Methane Generation Model Parameters
Source: D. Augenstein, "The Greenhouse Effect and U.S. Landfill Methane," Global Environmental Change (December 1992), pp. 311-328.
Methane Recovery. In 1992, there were 105 U.S. landfills with methane recovery systems in place. Approximately 1.7 million metric tons of methane were recovered at these landfills, with 1.2 million metric tons being combusted for energy use providing some 300 megawatts of electrical generating capacity. The remaining 500,000 metric tons of methane recovered were flared. In 1994, there were 130 known methane recovery systems in operation at landfills in the United States, with 360 megawatts of electrical generating capacity.(Note 40) Assuming a constant ratio of capacity to methane recovery for energy, 1.44 million metric tons of methane were recovered for energy in 1994. The 1993 estimate was interpolated from the 1992 and 1994 figures. Some 550,000 metric tons of methane is estimated to have been flared from recovery systems in 1993.
Methane emissions from agricultural sources totaled 8.7 million metric tons in 1993, almost one-third of all anthropogenic methane emissions. This represents an increase of 410,000 metric tons or 5 percent since 1990 (Table 15). The growth can be traced to increases in emissions from livestock management, which account for 94 percent of all emissions from agricultural sources. Emissions from livestock management are the result of two processes: the breakdown of carbohydrates in the digestive tract of herbivores (enteric fermentation), and the decomposition of animal waste matter. The remainder of methane emissions from agricultural sources can be traced to rice cultivation and the burning of crop residues.
Table 24. U.S. Methane Emissions from Enteric Fermentation in Domesticated
Animals, 1987-1994
(Million Metric Tons of Methane)
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Cattle, sheep, and pig population data provided by the U.S. Department of Agriculture, National Agricultural Statistics Service, Livestock, Dairy and Poultry Service. Goat and horse population figures extrapolated from U.S. Department of Commerce, Bureau of the Census, Census of Agriculture, 1982, 1987, and 1992. Emissions calculations based on U.S. Environmental Protection Agency, Office of Air and Radiation, Anthropogenic Methane Emissions in the United States: Estimates for 1990 (Washington, DC, April 1993), and P.J. Crutzen, I. Aselmann, and W.S. Seiler, "Methane Production by Domestic Animals, Wild Ruminants, Other Herbivorous Fauna, and Humans," Tellus, Vol. 38B (1986), pp. 271-284.
Emissions from enteric fermentation have been increasing steadily since 1990, as the cyclical pattern in animal populations has turned upward. The effect of the increases in animal populations has been magnified by simultaneous increases in animal size and productivity for the largest emitting group, cattle. In 1993, while populations continued to grow, animal sizes have, at least temporarily, stabilized.
Estimates of emissions from enteric fermentation have been revised for the period 1987-1992, compared with the estimates presented in last year's report.(Note 41) This change is the result of adjustments to population data provided by the U.S. Department of Agriculture. These revisions have mixed impacts on the overall emissions estimates, varying from year to year.
Methane Formation and Release. Methane production from enteric fermentation is greatest in ruminant animals, including cattle, sheep, and goats. These animals possess a rumen, or forestomach, that allows them to digest the large quantities of cellulose found in plant material. Symbiotic microorganisms present in the rumen are responsible for this process. A small fraction (5 to 10 percent)(Note 42) of these microorganisms are methanogenic bacteria, which produce methane while removing hydrogen from the rumen. The majority (some 90 percent) of the methane produced by ruminant animals is released through eructation and normal respiration. The remainder is released as flatus.
Although nonruminant animals do produce methane from enteric fermentation in their large intestines, the production is small compared with that which occurs in ruminant animals. However, because of their size and relatively large populations, methane emissions from horses and swine are included in the emissions estimates for this report.
Estimation Methods. Methane production from the digestive processes of domesticated animals is a function of several variables, including quantity and quality of feed intake, the growth rate of the animal, its productivity (reproduction and/or lactation), and its mobility. In order to calculate methane emissions from enteric fermentation in domesticated animals, the animals are divided into distinct, relatively homogenous groups. For a representative animal in each group, feed quality and quantity, as well as growth rate, productivity, and activity levels, are estimated. These variables are combined to derive a methane emissions factor for that animal. The factor is then applied to the total population of the animal group to calculate an overall emissions estimate.
Population data for cattle, sheep, and swine are available from the U.S. Department of Agriculture (USDA), National Agricultural Statistics Service, Livestock, Dairy, and Poultry Branch. The USDA has recently revised the population estimates downward for the years 1980-1986 and 1989-1992.(Note 43) Population estimates for 1987-1988 have been revised upward. These revisions are reflected in overall emissions estimates that vary from previous EIA estimates.(Note 44) Population data for goats and horses were extrapolated from the U.S. Department of Commerce, Census of Agriculture for 1982, 1987, and 1992.(Note 45)
Cattle. The EPA developed emissions factors for several classes and subclasses within the U.S. cattle population as it was composed in 1990.(Note 46) The general taxonomy that EPA used is adopted here. First, cattle are separated into dairy and beef classes. Dairy cattle are further divided into three subclasses: replacement heifers 0-12 months old, replacement heifers 12-24 months old, and mature cows. Beef cattle are divided into six groups: replacements 0-12 months old, replacements 12-24 months old, mature cows, bulls, steers and heifers raised for slaughter under the weanling system, and steers and heifers raised for slaughter under the yearling system.
Because characteristics critical in determining energy intake and hence methane emissions for cattle, such as growth rates and milk production, have increased over the past decade, an attempt was made to incorporate these changes into emissions factors. Average slaughter weights for the respective classes of cattle as reported by USDA were used as a proxy for feed energy intake, thus scaling emissions factors to changes in slaughter weight. A more detailed discussion of the methods employed was included in last year's edition of this report.(Note 47) (For slaughter weights used, see Appendix C of this year's report, Table C12.) The slaughter weight adjusted emissions factors were applied to recently revised population data obtained from the USDA. For each subclass of cattle in a given year, a population figure was derived by averaging USDA estimates for January 1, July 1, and December 31 of that year.(Note 48) This method yielded an estimate of 5.2 million metric tons of methane emitted from enteric fermentation in cattle during 1993, up some 160,000 metric tons since 1990.
Other Animals. In addition to cattle, estimates of methane emissions from enteric fermentation were derived for sheep, pigs, goats, and horses (Table 24). Like the method for estimating emissions from cattle, emissions from these animals were estimated by multiplying per-animal emissions factors by total animal populations. Emissions factors are a function of gross energy intake and methane yield for each animal class. Gross energy intake and methane yield estimates were drawn from the work of Crutzen et al.(Note 49) The emissions factors adopted were 13 kilograms methane yield per head per year for sheep, 1.5 kilograms for pigs, 8 kilograms for goats, and 18 kilograms for horses. Emissions factors for pigs and sheep were applied to population data taken from USDA estimates, while factors for goats and horses were used in conjunction with population estimates developed from the Census Bureau's Census of Agriculture for 1982, 1987, and 1992. Goat and horse populations in intervening years were estimated using straight-line interpolation. For 1993, population data were derived for goats and horses using straight-line extrapolation. Emissions factors were not adjusted for any changes in size or productivity for these animals. Methane emissions from enteric fermentation in animals other than cattle totaled slightly more than 250,000 metric tons in 1993, virtually unchanged since 1990. Emissions reported for horses have been revised downward from last year's report,(Note 50) as additional data from the 1992 Census of Agriculture suggest a decline in horse populations between 1987 and 1992 rather than the previously estimated growth.
Table 25. U.S. Methane Emissions from the Solid Waste of Domesticated
Animals, 1987-1994
(Thousand Metric Tons of Methane)
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Population data for horses and goats extrapolated from U.S. Department of Commerce, Bureau of the Census, Census of Agriculture, 1982, 1987, and 1992. Population data for all other animals from U.S. Department of Agriculture, National Agricultural Statistics Service, Livestock, Dairy and Poultry Branch. Typical animal sizes from U.S. Environmental Protection Agency, Office of Air and Radiation, Anthropogenic Methane Emissions in the United States: Estimates for 1990, Report to Congress (Washington, DC, April, 1993). Cattle sizes adjusted by annual slaughter weight from U.S. Department of Agriculture, National Agricultural Statistics Service, Livestock, Dairy and Poultry Branch. Maximum methane production, and waste management systems used from L.M. Safley, M.E. Casada, et al., Global Methane Emissions from Livestock and Poultry Manure (Washington, DC: U.S. Environmental Protection Agency, February 1992), pp. 24-27. Methane Conversion Factors from Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), pp. 4.33-4.41.
Estimates of emissions from the solid waste of domesticated animals have been revised from the estimates appearing in last year's report.(Note 51) This revision is the result of updated cattle and horse population data from the USDA and the Department of Commerce, respectively. The population changes create slightly higher emissions estimates for the period 1987-1989 and slightly lower emissions estimates (between 15,000 and 30,000 metric tons annually) for 1990-1992.
Methane Formation and Release. Methane is produced when methanogenic bacteria decompose the organic material in the solid waste of animals under anaerobic conditions. The amount of organic material susceptible to decomposition is described as the "volatile solids content." The volume of methane produced when a given amount of volatile solids decomposes under optimal anaerobic conditions is characterized as the maximum methane-producing capacity of the animal waste. Because conditions are rarely optimal, methane production is usually below the maximum.
In addition to varying as a function of the volatile solids content of the waste, methane emissions are driven by the manner in which the waste is managed. Liquid-based waste management systems provide an anaerobic environment as well as the moisture required for methanogenic bacterial cell production and acidity stabilization.(Note 52) In contrast, animal waste left to dry in the fields will decompose in an aerobic environment, minimizing methane production. The share of the maximum methane produced using a particular waste management system is represented by its methane conversion factor.
Estimation Methods. Estimates of emissions from the solid waste of domesticated animals were calculated by linking emissions to the volume of solid waste produced by a given animal, the volatile solids in that waste, and the system in which the waste is handled. The volume of waste produced is in turn a function of the animal's size, diet, and energy requirements. As a proxy for these variables, the typical animal mass of each class of animal was a principal determinant in developing emissions estimates. Typical animal masses for livestock and poultry inventoried in 1990 have been estimated by the EPA.(Note 53) These animal sizes were adopted directly for all animals except cattle. In an effort to capture changes in the size of cattle over the past decade, typical animal masses for the various classes of cattle were adjusted annually, based on average slaughter weight as reported by the USDA (see earlier section in this chapter on "Enteric Fermentation"). Volatile solids produced per kilogram of animal weight, maximum methane-producing capacity of each animal's waste, and share of waste handled in each management system for each animal type were adopted from the work of Safley et al.(Note 54)
Using these data, separate emissions factors were calculated by waste management system used, for beef cattle, dairy cattle, cattle on feed, breeding swine, market swine, broiler chickens, layer chickens, sheep, goats, and horses. Emissions factors for cattle, poultry, swine, and sheep were multiplied by population data obtained from the USDA. Average broiler chicken populations for each year were estimated by multiplying the estimated number of broiler chickens slaughtered annually by 0.1425, based on the recommendation of the USDA's Economic Research Service.(Note 55) Population data for goats and horses were extrapolated from the Census of Agriculture for 1982, 1987, and 1992.(Note 56)
Table 26. Area of Land Harvested for Rice and Estimated U.S. Methane Emissions from Flooded Rice Fields, 1987-1994
Source: Rice area harvested data from U.S. Department of Agriculture, National Agricultural Statistics Service, Crop Production annual reports. Emissions calculations based on Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), pp. 4.46-4.55.
Methane Formation and Release. Methane is produced by bacteria during the anaerobic decomposition of organic material in flooded rice fields. However, not all of the methane produced in rice fields is released to the atmosphere. Between 60 and 90 percent of the methane is oxidized by other bacteria in the soil, and additional methane is leached into groundwater. The majority of methane that does reach the atmosphere does so via diffusive transport through the rice plants during the growing season. Bubbling from the soil and diffusion through the water column are alternative, though less common, pathways for emissions.
Estimation Methods. The amount of methane produced in rice fields is controlled by a complex set of variables related to the biological and physical characteristics of the flooded soil, as well as agricultural management techniques. Studies have shown that emissions vary, depending on the temperature and pH of the soil, season of year, irrigation practices, and application of chemicals or organic matter.(Note 57)
For the estimates presented in this report, studies of rice fields in California,(Note 58) Louisiana,(Note 59) and Texas(Note 60) were used to derive a daily emissions rate range of 0.1065 to 0.5639 grams of methane per square meter of land cultivated. This range was chosen to incorporate the emissions variables typical of U.S. rice cultivation. The high and low ends of this range were applied to the growing season length and the harvested area for each State that produces rice. For Louisiana and Texas, where a second or "ratoon" crop of rice is produced after the initial harvest, the area harvested for the ratoon crop was included in the calculation.
Methane Formation and Release. Crop residues are used for such purposes as fodder, land supplementation, and fuel. When residues are not put to such uses, they are sometimes burned. As with all types of biomass, crop residues are rich in carbon. (Actual carbon contents can range from 40 to 50 percent of dry matter.(Note 61)) Consequently, burning these residues releases carbon dioxide and, due to incomplete combustion, small quantities of other gases, including methane.(Note 62)
Estimation Methods. Methane emissions from burning crop residues were estimated by determining the quantity of carbon released, then applying the appropriate emissions factor for methane. The calculation used to derive the amount of carbon released is a function of the amount, carbon content, and combustion efficiency of the burned residue (see Table C11 in Appendix C). In the United States, the practice of burning crop residues has not been inventoried, and some States have enacted legal bans on this type of burning. Due to the lack of data necessary for calculating emissions estimates, this report assumed that 10 percent of crop residues are burned, in keeping with the default methodology recommended for developed countries by the IPCC.(Note 63) This figure is highly uncertain, however, and the actual amount of burning in the United States is likely to be much lower. Because the estimate was created by multiplying crop production by a series of stable factors, all fluctuations in estimated emissions from residue burning can be attributed to annual variations in crop production.
On a global scale, methane emissions from industrial processes are minor when compared with emissions from the combustion of fossil fuels. The IPCC estimates that industrial processes account for 3 percent of total global methane emissions related to fossil fuels.(Note 64) In the United States, however, this figure is more significant, with methane from industrial processes accounting for 15 percent of all fossil-fuel-related methane emissions. All combustion-related methane emissions are detailed in a previous section of this chapter.
This report provides estimates of methane emitted as a byproduct of the production of certain chemicals and of iron and steel. In 1993, these processes produced a total of 123,000 metric tons of methane, divided nearly equally between chemical and iron and steel production (Table 27). Emissions from industrial processes in the United States have remained nearly unchanged for the better part of a decade, as increases in emissions from chemical production have been offset by decreases in emissions from iron and steel production.
Table 27. U.S. Methane Emissions from Industrial Processes, 1987-1994
(Thousand Metric Tons of Methane)
*Based on total U.S. production of metallurgical coke, including non-iron and steel uses.
P = preliminary data. NA = not available.
Note: Totals may not equal sum of components due to independent rounding.
Sources: American Iron and Steel Institute, Annual Statistical Report (Washington, DC, various years); Chemical Manufacturers Association, U.S. Chemical Industry Statistical Handbook (Washington, DC, various years), p. 223; Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), p. 2.6.
Methane Formation and Release. The process of feedstock cracking is used in the production of certain chemicals. The feedstock is an organic compound, and when it is heated, its molecular bonds are "cracked." Depending on the type of feedstock used and the temperature of the reaction, various permutations of fractures may occur, resulting in the production of specific chemicals. Methane is one of the byproducts that may be formed in this process.
Estimation Methods. The IPCC has published emissions factors for methane emitted during the manufacture of ethylene, ethylene dichloride, styrene, methanol, and carbon black.(Note 65) Multiplying the emissions factors by annual production figures provides the emissions estimates for each chemical. There is a degree of uncertainty in the emissions estimates. The emissions factors used in the calculations were developed for global application, but U.S. manufacturers, while complying with air pollution regulations to control emissions of volatile organic compounds, may also be reducing the amount of methane that is released.
Methane Formation and Release. Coke, sinter, and pig iron are the basic materials for production of iron and steel. In order to estimate methane emissions from this source, the individual processes for manufacturing those inputs must be examined. When coal is heated in the absence of oxygen, impurities are driven off and coke is produced. Methane is one of the gaseous byproducts of this process. Pollution controls required on coke ovens to reduce emissions of volatile organic compounds typically eliminate methane as well. However, some leakage may occur. Coke is then combined with iron ore and flux materials to form sinter. In this process, the coke is burned to create heat, causing the sinter to agglomerate. Methane is emitted as a byproduct of the agglomeration. Coke and iron (in the form of ore, pellets, and sinter) are then added to flux materials in a blast furnace and reduced into iron, slag, and exhaust gases (including methane). The exhaust gases are recycled as fuel, but small amounts may be vented.
Estimation Methods. Emissions factors for the production of coke, sinter, and pig iron recommended by the IPCC(Note 66) were multiplied by production data supplied by the American Iron and Steel Institute(Note 67) to produce emissions estimates.