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Report Contents Report#:EIA/DOE-0573(98)
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Biofuel Combustion The carbon found in biofuels is the result of atmospheric uptake. During the combustion of biofuels, there is an immediate release of the carbon in the form of carbon dioxide. Thus, as part of the natural carbon cycle, carbon is reabsorbed over time. Because they produce no net change in the overall carbon budget, such emissions are not included in this report. If the initial flux had been counted, 1998 carbon dioxide emissions estimates would have been approximately 72 million metric tons of carbon higher than reported in Chapter 2 (Table D1). Table D1. Estimated U.S. Carbon Dioxide Emissions from Biofuels, 1990-1998 Emissions are estimated by multiplying Energy Information Administration (EIA) energy consumption data for biofuels by the applicable emissions factors. Carbon dioxide emissions factors for combustion of wood fuels and municipal solid waste are taken from the EIA report, Electric Power Annual 1997.(193) The emissions coefficient for alcohol fuels, 17.99 million metric tons of carbon per quadrillion Btu, was derived specifically for use in this report. Enhanced Oil Recovery Carbon dioxide is injected into petroleum reservoirs for the purpose of retrieving additional oil. Over time, the carbon dioxide seeps into the producing well, creating a mixture of oil, natural gas, and carbon dioxide. If the energy content is sufficiently high, the gaseous portion of this mix will probably be sent to a gas plant. If the energy content is low, the gas is likely to be vented or flared. At this time, there is no basis for EIA to estimate the quantity of added carbon dioxide that is vented or flared. EIA believes that most of the carbon dioxide recovered with the oil is recycled, so that annual emissions are a fraction of the carbon dioxide recovered, which in turn is probably less than the volume of carbon dioxide injected. The annual amount of carbon dioxide used for enhanced oil recovery is probably on the order of 12 million metric tons,(194) and emissions would be some fraction of that figure. Emissions from this source may be included in future reports. "Off Spec" Gases Combustion of "off spec" gases and fuels is not covered as a separate line item in this report, but much of the emissions from this source may be included in the "flaring" category, which is covered in this report, or as industrial consumption of "still gas" by refineries. Overseas U.S. Military Oil Consumption Domestic military energy consumption is incorporated into U.S. energy statistics; however, energy consumption for overseas operations is a more complex issue. The data can either be reported in the national energy statistics of the host country or included in U.S. export statistics if domestic oil is transported to ships and other facilities. In some circumstances, the oil consumption may go unreported. Estimating, even roughly, the quantity of oil consumed for overseas military operations is an uncertain procedure. The Defense Energy Support Center reports that petroleum "sales" (i.e., transfers to military sources and operating units) for fiscal year 1998 totaled approximately 102 million barrels.(195) Of that, approximately 79 percent was acquired domestically and is assumed to be included in U.S. statistics. A reasonable estimate of military oil consumption not reported elsewhere would, therefore, be 21 percent of total military consumption of jet fuel, middle distillates, and residual oil.(196) By this method, excluded emissions for 1997 were estimated at 2.3 million metric tons of carbon (Table D2). Table D2. Estimated Carbon Emissions from U.S. Military Operations Abroad, 1990-1998Forest Fires Forest fires are known to create greenhouse gas fluxes within the atmosphere over extensive time periods. Specifically, forest fires produce carbon dioxide, methane, and nitrous oxide. Considering that carbon uptake occurs with subsequent regrowth (assumed to balance out the initial carbon flux), and because emissions from natural forest fires cannot be distinguished from those from human-induced fires, estimates from this source are not included in this report. Unaccounted for Gas The editions of Emissions of Greenhouse Gases in the United States published by EIA before 1997 included an emissions a category called "unmetered natural gas." In those years, U.S. natural gas producers consistently reported selling about 3 percent more natural gas than U.S. consumers reported buying. In EIA natural gas statistics, this "missing" gas is described as "the balancing item" or "unaccounted for gas." The balancing item can be viewed as the sum of leakage, measurement errors, data collection problems, and undetected over- and underreporting, as well as undetected nonreporting. Only a fraction of this amount can credibly be attributed to leakage from transmission systems. Evidence from the electric utility sector-- where transmission companies report gas sales and electric utilities report gas purchases--suggests that there may be undercounting of natural gas consumption. Estimates of carbon dioxide emissions from this source were included in early reports, on the grounds that there was an element of systematic underreporting of consumption in the balancing item. Since 1995, however, the sign of the balancing item has changed, and reported consumption now usually exceeds reported production. This development reduces the credibility of the undercounting theory, and consequently this report no longer carries "unmetered natural gas" consumption as a source of emissions. It is possible, however, that there had been an element of undercounting of natural gas consumption before 1996, which may be on the order of 1 percent (3 million metric tons) of natural gas emissions. Fermentation During the fermentation process, complex organic compounds are decomposed through a variety of chemical reactions. The most common is the anaerobic conversion of sugar into carbon dioxide and alcohol. Fermentation does not create a net flux of emissions, however, because the carbon dioxide produced is of biological origin. Lead Smelting Smelting of lead includes a stage in which limestone undergoes calcination. As described in Chapter 2, carbon dioxide is released as a byproduct of the calcination reaction. Emissions estimates cannot be calculated for this report because there are no known statistics regarding the amount of limestone used in lead smelting. EIA is currently researching alternative data sources in an effort to include estimates of these emissions in future reports. Industrial Wastewater Treatment Methane emissions from industrial wastewater treatment are believed to be a function of the volume of wastewater generated, the organic content of the wastewater, and the method used to treat the wastewater. Methane emissions will be much more greater if the wastewater is treated anaerobically (in the absence of oxygen) than if it is treated aerobically. Because data on volumes of wastewater generated by industry and the methods for treating that wastewater are limited, EIA does not present estimates of methane emissions from industrial wastewater. There is anecdotal evidence that very little industrial wastewater is treated anaerobically. Further, when industrial wastewater is treated anaerobically, the methane generated may be flared or recovered for energy use. Thus, 500,000 metric tons is likely to be at the high end of the emissions estimate range. Abandoned Coal Mines The Mine Safety and Health Administration estimate that some 7,500 underground coal mines have been abandoned in the United States since 1970.(197) Measurements taken from 20 abandoned mines showed a total of 25,000 metric tons of emissions.(198) Data gathered from these mines suggest a range in emissions from abandoned mines of 25,000 to 700,000 metric tons.(199) U.S. EPA is currently developing a comprehensive database of abandoned mines in the U.S. This database will include date of abandonment, specific emissions, seam thickness, mine depth, mining method, and ventilation emissions. Upon completion, this data should provide the ability to develop improved estimation methods. Until then, existing estimates are too uncertain to appear in this report. Methane Emissions from Industrial Wastewater Industrial operations in the United States use immense quantities of fresh water. Many industries, particularly food processing industries, discharge wastewater containing large volumes of organic material. Some portion of the organic material may decompose anaerobically, particularly if the industrial facility deliberately uses anaerobic decomposition as a treatment strategy. However, estimating emissions from this source presents certain significant challenges, because there is no clear mechanism for determining accurately the following:
Anecdotal evidence suggests that beer manufacturers produce large volumes of organic matter and cause it to decompose anaerobically so that they can recover methane for energy purposes. It is possible that methane emissions from industrial wastewater are, in fact, a large emissions source, but there is insufficient evidence currently available to make a plausible estimate. Nitrous Oxide Emissions from Industrial Wastewater Just as industrial wastewater may contain large volumes of organic matter, so, under certain circumstances, industrial wastewater may be a source of nitrogen, leading ultimately to nitrous oxide emissions. However, the problems associated with estimating methane emissions from industrial wastewater are even more difficult with respect to nitrous oxide emissions from industrial wastewater. The nitrogen content of industrial wastewater is more problematic, and the extent to which bacterial action converts the nitrogen into nitrous oxide (as opposed to molecular nitrogen or nitrogen oxides) is more uncertain. Methane Emissions from Wetlands Wetlands are a known source of methane. Environments low in oxygen, combined with abundant organic matter, are conducive to the creation of methane, and wetlands meet both criteria. Wetlands cover approximately 274 million acres of land in the United States and are a potentially important source of atmospheric methane. The stock of natural wetlands in the United States has diminished considerably over the past two centuries, which should, in principle, have reduced methane emissions from wetlands (EIA is not aware of research proving or disproving this principle). A recent study of wetland losses concluded that the United States had lost approximately 30 percent of its wetlands between colonial times and the mid-1980s. Almost all of the loss has occurred in the lower 48 States, which have lost 53 percent of their original wetlands.(200) Ten States-- Arkansas, California, Connecticut, Illinois, Indiana, Iowa, Kentucky, Maryland, Missouri, and Ohio--have lost 70 percent or more of their original wetland acreage. By the mid-1980s, a total of approximately 119 million acres had been lost from the original U.S. total. An update of the wetlands study indicates that 654,000 acres were converted from wetlands to other uses between 1982 and 1987, and that an additional 431,000 acres were converted between 1987 and 1991.(201) Extrapolating from these data, it is estimated that wetlands in the United States are currently destroyed at a rate of approximately 86,000 acres per year. Wetlands, also known as swamps and marshes, have historically been drained or filled in for agriculture, land development, and mosquito control, although it is currently illegal to drain or fill a wetland without a permit from the U.S. Army Corps of Engineers. It is difficult to find information on the conversion of other land categories to wetlands. It is assumed that the number and extent of wetland creations is small enough to leave the above loss estimates essentially unchanged. Estimates of global methane fluxes from wetlands suggest that methane emissions from temperate-zone wetlands are minimal--typically between 5 and 10 million metric tons of methane per year for worldwide temperate-zone wetlands (which include U.S. wetlands)--when compared with estimated global wetlands emissions of 110 million metric tons.(202) The U.S. share of all temperate-zone wetlands is about 57 percent, and temperate-zone wetlands lost during the 1980s accounted for about 0.5 percent of U.S. wetlands at the beginning of the period. Consequently, the reduction in natural methane emissions from wetlands lost might be on the order of 0.57 � 0.005 � 5 to 10 million metric tons of methane, or from 10,000 to 20,000 metric tons of methane annually over the decade. Land Use Changes Affecting Methane and Nitrous Oxide The scientific literature suggests that both grasslands and forest lands are weak natural sinks for methane and weak natural sources for nitrous oxide. Natural soils apparently serve as methane sinks: well-aerated soils contain a class of bacteria called "methanotrophs" that use methane as food and oxidize it into carbon dioxide. Experiments indicate that cultivation reduces methane uptake by soils and increases nitrous oxide emissions. One report indicates that methane uptake in temperate evergreen and deciduous forests in the United States ranges from 0.19 to 3.17 milligrams (measured in carbon units) per square meter per day, equivalent to the uptake of 36.8 to 624.4 metric tons of methane per million acres per year. The range is larger for agricultural lands: 0.2 to 6.3 milligrams per square meter per day. Estimates for methane uptake resulting from the abandonment of farmland range from 0.6 to 6.1 milligrams per square meter per day. While all of these ranges are wide, the total amount of methane in question is less than 1 percent of methane emissions from anthropogenic sources. Of all the greenhouse gases discussed in this report, the least amount of data is available for nitrous oxide. It is known that conversion of forests and grasslands to cropland accelerates nitrogen cycling and increases nitrous oxide emissions from the soil. It is not known with certainty by how much.(203) Some estimates have been made of the difference between fertilized and unfertilized soils. According to one study, unfertilized soils produce emissions of 0.25 to 0.35 milligrams (measured in nitrogen units) per square meter per day, while emissions from fertilized soils range from 0.6 to 1.65 milligrams per square meter per day.(204) Thus, abandoning fertilization should reduce nitrous oxide emissions by 0.35 to 1.3 milligrams per square meter per day--the equivalent of 86 to 321 metric tons of nitrous oxide per million acres per year. Applying this figure to the 35 million acres of cropland idled between 1982 and 1992 implies a reduction in nitrous oxide emissions ranging from 3,010 to 11,235 metric tons annually. In principle, however, about three-quarters of the reduction in emissions from this source should be captured by reduced application of nitrogen fertilizers; thus, reporting emissions reductions using this method would result in significant double counting of units already included in the agriculture statistics in Chapter 4. If such estimates are to be applied to emissions inventories, a problem of crediting the uptakes applies. Removing an acre of farmland from production in a particular year creates a permanent annual methane sink that will absorb small additional amounts of methane each year thereafter, or at least until the use of the land changes. The method that should be used to credit such permanent reductions to a particular year is not obvious. |
