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U.S. Emissions of Greenhouse Gases in Perspective Appendix A: Estimation Methods Appendix B: Carbon Coefficients Used in this Report Appendix C: Uncertainty in Emissions Estimates Appendix D: Emissions Sources Excluded Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997 Appendix F: Common Conversion Factors
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Overview The first edition of Emissions of Greenhouse Gases in the United States, published in September 1993, applied emissions coefficients developed by Marland and Pippin(151) that had been adopted by the Intergovernmental Panel on Climate Change.(152) Those coefficients, developed for broad international use, covered only the six petroleum product categories in the International Energy Agency's taxonomy. The Energy Information Administration (EIA) collects data on more than 20 petroleum products, and U.S. petroleum products often differ in composition from those consumed abroad. Thus, EIA needed coefficients for the specific products reported in U.S. energy statistics. Further, the emissions coefficients developed by Marland and Pippin were based on a very limited set of fuel samples. In 1994, the EIA developed more specific and accurate emissions coefficients for estimating carbon released from the combustion of fossil fuels in the United States. The EIA developed emissions coefficients for coal by rank (anthracite, bituminous, subbituminous, and lignite) and State of production, using 5,426 coal samples from the EIA coal analysis file. An emissions coefficient for natural gas was generated, based on 6,743 gas samples in a Gas Research Institute database. Emissions coefficients for the petroleum products captured in EIA energy statistics were derived, based on their density, heat content, and carbon share. These variables were estimated on the basis of the underlying chemical composition of the fuels and, where available, ultimate analyses of product samples.(153) In all but a few cases, the revised emissions coefficients differed from those developed by Marland and Pippin by less than 5 percent. The magnitude of potential variation in emissions coefficients for fossil fuels is constrained by the limits imposed by the chemical properties of the hydrocarbon compounds that define the fuels.(154) Although the marginal improvements in accuracy for the emissions coefficients were not large, a number of important insights were gained with regard to fuel composition trends and their effects on emissions coefficients. The composition of marketed petroleum products varies over time because of changes in exploration, recovery, and refining technology, economic changes (e.g., changes in the price of oil), and regulatory changes (e.g., requirements for reformulated gasoline in the Clean Air Act Amendments of 1990). Four petroleum products prone to changes in their composition that have important effects on carbon emissions--either because emissions per unit may change substantially or because great volumes are consumed in the United States--are motor gasoline, jet fuel, crude oil, and liquefied petroleum gases (LPG). Thus, EIA has developed annualized emissions coefficients for these fuels in an effort to capture changes in composition (Table B1). The most important changes have occurred in the emissions coefficient for motor gasoline. Motor gasoline consumption accounts for about 20 percent of all U.S. greenhouse gas emissions. Between 1986 and 1995, small increases in the density of gasoline led to a slowly escalating emissions coefficient. In 1995, a requirement for reformulated gasoline in nonattainment areas implemented under the Clean Air Act Amendments changed the composition of gasoline consumed in the United States. During 1995, 25 percent of all gasoline consumed was reformulated, rising to 32 percent in 1997. Because the additives contained in reformulated gasoline have much lower carbon shares than typical gasoline, the national average emissions coefficient for motor gasoline has reversed the previous decade's trend and declined by about 0.5 percent over the past 3 years. Jet fuel consumption is responsible for more than 4 percent of U.S. carbon dioxide emissions. Like motor gasoline, jet fuel consumed in the United States has undergone a dramatic change in composition over the past decade. Until 1993, two types of jet fuel were widely used in the United States. Kerosene-based jet fuel was generally used in the commercial airline industry and naphtha-based jet fuels were used primarily by the U.S. Department of Defense. The emissions coefficient for naphtha-based jet fuels was about 3 percent higher than that for kerosene-based jet fuel. In 1989, 13 percent of all jet fuel consumed was naphtha-based. By 1996, that figure had fallen to 0.3 percent, and in 1997 total naphtha-based jet fuel consumption was negligible. Thus, the emissions coefficient for jet fuel, weighted by consumption of each fuel type, fell steadily between 1988 and 1996 and has now stabilized at the level of kerosene-based jet fuel. The emissions coefficient for LPG is a weighted average of the emissions coefficient for four paraffinic hydrocarbons: ethane, propane, isobutane, and n-butane. The emissions coefficient for this source varies according to the proportion of each compound consumed and whether the compounds are used as petrochemical feedstock or for fuel. Last year, for the first time, EIA began publishing separate emissions coefficients for LPG fuel use and LPG nonfuel use. LPG emissions coefficients for 1997 are unchanged from 1996. Crude oil consumption in the United States is a very small portion of carbon emissions, because nearly all crude is refined into finished petroleum products. However, crude oil does represent a much larger portion of consumption in the energy statistics of other nations and can offer a "quick and dirty" mass balance approach to estimating trends in national emissions. The EIA has developed a regression equation reflecting the relationship between the density, sulfur content, and carbon content of crude oil. From these data a crude oil emissions coefficient can be calculated. The density of crude oil entering U.S. refineries has risen gradually over the past decade. Thus, each barrel of oil now contains a greater mass of oil and a greater weight of carbon. Because the presumed heat content of a barrel of crude oil has not changed, the emissions coefficient continues to rise slowly. Motor Gasoline As with all petroleum products, the emissions coefficient for motor gasoline is a function of its density and carbon content. This relationship is particularly clear in the case of motor gasoline, because the share of impurities found in the fuel must be kept low to maintain the operating condition of modern automobile engines and limit the environmental impact of vehicle use. Motor gasoline density varies between summer and winter grades and from low octane to high octane. The variation takes into account the differing performance requirements of gasoline associated with temperature changes. Partly as a result of the phaseout of leaded gasoline, the density of gasoline increased slowly and steadily across all octane grades and in all seasons from 1987 through 1994.(155) In order to maintain the "anti-knock" quality and octane ratings of motor gasoline in the absence of lead, the portion of aromatic hydrocarbons used in gasoline was increased. Aromatic hydrocarbons take the form of CnH2n-6, a lower ratio of hydrogen to carbon than in the other hydrocarbons typically found in gasoline. Because carbon is much heavier than hydrogen, the lower ratio results in greater fuel density and higher shares of carbon. As a result, the emissions coefficient for motor gasoline rose slowly from 19.39 million metric tons carbon per quadrillion Btu in 1988 to 19.45 million metric tons carbon per quadrillion Btu in 1994. Table B2 shows the increasing densities and emissions coefficients for that time period. Reformulated gasoline was consumed in large volumes (about 25 percent of overall gasoline consumption) for the first time during 1995. The density of reformulated gasoline is about 1 percent less than that of standard gasoline, and the much lower carbon contents of the principal additives to reformulated gasoline (Table B3) reduce the overall share of carbon in reformulated fuel. Thus, taking into account the 25 percent of fuel consumed with a lower emissions profile, the emissions coefficient for motor gasoline dropped from 19.45 million metric tons per quadrillion Btu in 1994 to 19.38 million metric tons per quadrillion Btu in 1995. By 1997, reformulated gasoline accounted for 32 percent of total gasoline consumption, and the overall emissions coefficient declined to 19.35 million metric tons carbon per quadrillion Btu. To derive an overall emissions coefficient for gasoline, individual coefficients for standard motor gasoline consumed in the winter and summer months were developed. The coefficients were based on the densities of product samples collected by the National Institute on Petroleum and Energy Research (NIPER), used in conjunction with a carbon share of 86.6 percent as estimated by Mark DeLuchi.(156) Emissions coefficients for reformulated fuels consumed during the summer and winter were calculated by the following procedure. First, the carbon share of each additive used in reformulated gasoline was calculated from its chemical formula and combined with the additive's density and energy content, as provided by the California Air Resources Board, to produce individual coefficients for each fuel additive. Next, the reformulated fuel was separated into its standard fuel components and its additive portions, based on fuel samples examined by NIPER.(157) The additive portions were defined as the net increase in MTBE, ETBE, or TAME as compared with the additives in standard fuel, because small amounts of these compounds are present in standard gasoline. The emissions coefficients for standard gasoline and for each of the additives were then weighted by their proportion in reformulated fuel to arrive at a coefficient for reformulated fuel in each season. After independent coefficients were developed for both standard and reformulated fuel, each season's coefficients were combined by weighting according to the ratio of standard to reformulated consumption. The combined summer and winter coefficients were then weighted on the basis of seasonal consumption, with just over half occurring in summer, to derive an overall emissions coefficient for motor gasoline. Jet Fuel The EIA collects data on two classes of jet fuel: naphtha-based and kerosene-based. In 1989, the share of U.S. jet fuel consumption attributed to naphtha-based fuel was 13 percent, and the remainder was kerosene-based jet fuel. The emissions coefficients for these two fuel classes differed, with an emissions coefficient of 19.95 million metric tons carbon per quadrillion Btu for naphtha-based fuel and 19.33 million metric tons carbon per quadrillion Btu for kerosene-based fuel. A jet fuel coefficient was developed by weighting the emissions coefficient for each class of fuel on the basis of its consumption. By 1993 the share of naphtha-based jet fuel had dropped to 7.2 percent, most of which could be credited to consumption in military aircraft. The U.S. Department of Defense then began a conversion from naphtha-based JP-4 jet fuel to kerosene-based JP-8 jet fuel, fearing that the increased demand for reformulated motor gasoline could inhibit refinery production of naphtha-based jet fuel. By 1995 naphtha-based jet fuel represented less than 1 percent of consumption, and today only negligible amounts are used.(158) Thus, the 1997 emissions coefficient for jet fuel is simply that of kerosene-based jet fuel and can be expected to remain nearly stable for the foreseeable future. The emissions coefficient for kerosene-based jet fuel was developed by using data from 39 samples of Jet A collected and analyzed by Boeing.(159) The average density of the samples was 44.5 degrees API gravity, and the average carbon share was 85.8 percent. The resulting emissions coefficient is 19.33 million metric tons per quadrillion Btu (Table B4). Crude Oil Although crude oil composition is highly heterogeneous, the share of carbon in a fixed amount of crude oil (e.g., a gallon or barrel) varies somewhat systematically with such commonly available identifying characteristics as density and sulfur content. Because the economic value of a barrel of crude oil is largely a product of the oil's density and sulfur content, these values are regularly recorded. Further, the EIA maintains detailed data on the average density and sulfur content of crude oil entering U.S. refineries.(160) Thus, the annual emissions coefficient for crude oil is pegged to these two variables. Ultimate analyses of 182 crude oil samples were used to derive a relationship between crude oil density, sulfur content, and the percentage of carbon in crude oil. The sulfur content and density of the samples was regressed against their carbon content to produce the following equation, which is used to estimate the carbon content of crude oil: Percent Carbon = 76.99 +
(10.19 × Specific Gravity)
The 1997 emissions coefficient for crude oil is 20.24 million metric tons carbon per quadrillion Btu, a slight decrease from the 1996 value. The change can be traced to a relatively large jump in the sulfur content of crude oil, which rose from 1.15 percent to 1.25 percent, its highest level in at least two decades. The increase in sulfur content more than offset a very small increase in the density of crude oil and lowered the share of carbon in crude oil entering U.S. refineries form 84.98 percent to 84.91 percent (Table B5). Liquefied Petroleum Gases The EIA identifies four categories of paraffinic hydrocarbons as LPG: ethane, propane, isobutane, and n-butane. Because each of these hydrocarbons is a pure paraffinic compound, their carbon shares are easily derived by taking into account the atomic weight of carbon (12) and the atomic weight of hydrogen (one). Thus, for example, the carbon share of ethane, C2H6, which has an atomic weight of 30, is 80 percent. The densities of these compounds are also well known, allowing emissions coefficients to be calculated easily. The EIA collects data on consumption of each compound and then reports them as LPG in the Petroleum Supply Annual.(161) By weighting each compound's individual emissions coefficient by its share of energy consumed, an overall emissions coefficient for LPG is derived. LPG may be used as fuel or as a petrochemical feedstock. About three-quarters of the carbon in petrochemical feedstocks will be sequestered. Further, if the mix of paraffinic hydrocarbons used for petrochemical feedstock differs substantially from those used for fuel, using a single emissions coefficient for LPG will bias estimates of emissions. More than 95 percent of all ethane and just under 85 percent of butane consumed goes to nonfuel uses. In contrast, nearly all LPG used as fuel is propane. Thus, the emissions coefficient for LPG used as fuel is 17.20 million metric tons carbon per quadrillion Btu, which is the emissions coefficient for propane (Table B6). On the other hand, the carbon emissions coefficient for LPG for nonfuel use is pulled down to 16.86 million metric tons carbon per quadrillion Btu by the large presence of the lighter ethane and its emissions factor of 16.25 million metric tons per quadrillion Btu.
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