| Overview | Motor Gasoline | Jet Fuel | Crude Oil | Liquefied Petroleum Gases | Appendix B Data Tables |
In the October 1994 edition of Emissions of Greenhouse Gases in the United States, the Energy Information Administration developed new emissions coefficients for estimating carbon released from the combustion of fossil fuels in the United States. Emissions coefficients for more than 20 petroleum products were derived, based on their density, heat content, and carbon share. These variables were estimated from the underlying chemical composition of the fuels and, where available, ultimate analyses of product samples [171]. (For a more detailed discussion of how emissions coefficients for fossil fuels were derived, see Appendix A in the 1994 report [172].)
The composition of marketed petroleum products varies over time due to changes in exploration, recovery, and refining technology, economic changes (e.g., changes in the price of oil), or regulatory changes (e.g., requirements for reformulated gasoline in the Clean Air Act Amendments of 1990). To capture the effects of these changes, the emissions coefficients for several important petroleum products have been annualized. The emissions coefficients for motor gasoline, jet fuel, crude oil, and liquefied petroleum gas now vary over time, reflecting changes in their composition and density.
Appendix A of the October 1995 edition of Emissions of Greenhouse Gases in the United States discusses the development of these annualized coefficients in more detail [173]. That appendix also describes revisions in the emissions coefficients for jet fuel and liquefied petroleum gases from those found in the 1994 report. The jet fuel coefficient was revised based on 39 jet fuel samples analyzed by Boeing that were more representative of the fuel currently in use than those samples previously used [174]. The emissions coefficient for liquefied petroleum gas (LPG) was revised to reflect the four paraffinic hydrocarbons included in the EIA taxonomy of LPG: ethane, propane, isobutane, and n-butane. The coefficient developed for the 1994 report had erroneously included pentanes-plus in the LPG category along with the four compounds listed above.
The most important development addressed in this appendix is the calculation of an emissions coefficient for motor gasoline consumed in 1995. For the first time, the effect on carbon emissions of the requirement for reformulated gasoline is quantified. The Clean Air Act Amendments mandated the use of reformulated gasoline in heavily polluted areas beginning in 1995. The impact of this legislation is substantial, with reformulated gasoline representing approximately 25 percent of motor gasoline production in 1995. Some studies predict that the share of reformulated gasoline may grow substantially by 2000 to as much as 100 percent of the gasoline market [175]. Reformulated gasoline has an emissions coefficient about 1 percent lower than typical gasoline. Thus, if the entire gasoline market consisted of reformulated gasoline, U.S. carbon emissions could be reduced by nearly 3 million metric tons. However, because reformulated gasoline contains less energy per gallon than typical gasoline, more gallons must be consumed to travel the same distance. While this will not affect estimates of carbon emissions, because they are calculated on an energy consumed basis, fuel consumption (measured in gallons or barrels) may increase [176].
In addition to updating the emissions coefficient for motor gasoline to reflect the introduction of reformulated fuels, this appendix includes updated emissions coefficients for jet fuel, crude oil, and LPG. Emissions coefficients for all fossil fuels are shown in Table B1.
Almost 20 percent of all U.S. greenhouse gas emissions are attributable to motor gasoline consumption. 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 very 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. This variation takes into account the differing performance requirements of gasoline associated with temperature changes. The density of gasoline has increased slowly over the past decade, across all octane grades and in all seasons [177]. This trend is partially the result of the phase-out of leaded gasoline. 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 must be increased. Aromatic hydrocarbons take the form of CnH2n-2, a lower ratio of hydrogen to carbon than other hydrocarbons typically found in gasoline. Because carbon is much heavier than hydrogen, this lower ratio results in increased fuel density and higher shares of carbon. The result has been an emissions coefficient that 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 emission coefficients for that time period.
The trend toward higher emissions coefficients ended in 1995. That year, reformulated gasoline was consumed in large volumes (about 25 percent of overall gasoline consumption) for the first time. 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, after accounting for 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 19.39 million metric tons per quadrillion Btu in 1995.
From 1995 on, the use of reformulated gasoline in areas with serious ambient air pollution is mandated by the Clean Air Act Amendments of 1990. The three principal additives to reformulated gasoline are MTBE, ETBE, and TAME. These additives differ from the hydrocarbons typically found in motor gasoline due to the presence of an oxygen atom in their molecules. The oxygen atoms reduce the emissions of carbon monoxide and unburned hydrocarbons. In contrast to standard motor gasoline, which has an estimated carbon share of 86.6 percent, these additives have a carbon share of 68.2 to 70.5 percent [178]. MTBE is the most important additive, representing about 10 percent of reformulated gasoline in 1995. It has a density of 59.1 degrees API and a carbon content of 68.2 percent.
To derive an overall emissions coefficient for gasoline consumed during 1995, individual coefficients for motor gasoline consumed in the winter and summer months were developed. These 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 [179].
Emissions coefficients for reformulated fuels consumed during the summer and winter were calculated using 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 [180]. The additive portions were defined as the net increase in MTBE, ETBE, or TAME compared with standard fuel, since 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 developing independent coefficients for both standard and reformulated fuel, each season’s coefficients were combined by weighting according to the ratio of standard versus reformulated consumption (approximately three to one). 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. This overall emissions coefficient may be revised, because data on the average composition of fuel used during October, November, and December 1995 are not yet available, and thus the winter coefficient for 1995 is based on data for January through March only.
The EIA classifies jet fuel into two categories: “naphtha-based” and “kerosene-based” jet fuel. Kerosene-based jet fuel includes civil-grade Jet A, consumed in commercial jet airliners. It is an important contributor to U.S. greenhouse gas emissions, accounting for about 4 percent of U.S. carbon dioxide emissions. Naphtha-based jet fuels were used primarily by the military until 1993, when the Defense Department began a conversion from naphtha-based JP-4 jet fuel to kerosene-based JP-8 jet fuel. This conversion has resulted in the share of naphtha-based jet fuel used in the United States declining from 12 percent of U.S. jet fuel consumption in 1991 to 7.2 percent in 1993 and further to 1 percent in 1995 [181]. The annual emissions coefficients for jet fuel represent a consumption-weighted average of the coefficient for kerosene-based jet fuel and the coefficient for naphtha-based jet fuel (Table B4). Because the emissions coefficient for naphtha-based jet fuel is about 3 percent higher than the coefficient for kerosene-based jet fuel, the near elimination of naphtha-based fuels from the marketplace has led to a steadily declining emissions coefficient for jet fuel.
The emissions coefficient for kerosene-based jet fuel was developed by using data from 39 samples of Jet A collected and analyzed by Boeing [182]. The average density of these samples is 44.5 degrees API gravity, and the average carbon share is 85.8 percent. The resulting emissions coefficient is 19.33 million metric tons per quadrillion Btu. The emissions coefficient for naphtha-based jet fuel is based on less empirical data and has a larger margin of error than that for kerosene-based fuel. However, the small share of jet fuel consumed that is naphtha-based limits the effect of this error on the overall uncertainty of a weighted jet fuel coefficient. According to ASTM standards, naphtha-based jet fuel has a density between 45 and 57 degrees API gravity [183]. To develop an emissions coefficient, a density of 49 degrees API is assumed. A carbon content of 85.8 percent is assumed, based on an estimated sulfur content of 0.1 percent and a hydrogen content of 14.1 percent [184]. These inputs produce an emissions coefficient for naphtha-based jet fuel of 19.95 million metric tons carbon per quadrillion Btu.
Crude oil composition is highly heterogeneous; however, the share of carbon in a fixed amount of crude oil (e.g, a gallon or barrel) varies 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 [185]. Thus, the annual emissions coefficient for crude oil is pegged to these two variables.
In the November 1994 edition of Emissions of Greenhouse Gases in the United States, Appendix A describes the methods used to derive a relationship between crude oil density, sulfur content, and the percentage of carbon in crude oil [186]. Using ultimate analyses of 182 crude oil samples, the sulfur content and density of the samples was related to their carbon content. Regression analysis produced the following equation, which is used to estimate the carbon content of crude oil:
Annualized emissions coefficients are developed by inserting the average density and sulfur content for crude oil entering U.S. refineries for each year from 1985 through 1995 to provide the share of carbon in an average barrel of oil during each year. After the share of carbon is derived, it is used in conjunction with fuel density to estimate the total mass of carbon in a barrel of crude oil. An emissions coefficient per unit of energy is then calculated, using EIA’s standard energy content for crude oil of 5.8 million Btu per barrel.
The 1995 emissions coefficient for crude oil is 20.22 million metric tons carbon per quadrillion Btu, a slight increase over the 1994 value. The increase can be traced to a small rise in the density of crude oil and a small reduction in sulfur content, which increased the share of carbon in crude oil from 84.97 percent to 84.99 percent (Table B5).
The EIA identifies four categories of paraffinic hydrocarbons [187] as liquefied petroleum gases (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 (1). Thus, for example, the carbon share of ethane, C2H6, which has an atomic weight of 30, is 80 percent. The densities for these compounds are also well known, allowing emissions coefficients to be calculated easily.
The EIA collects data on consumption of each of these compounds and then reports them as LPG in the Petroleum Supply Annual [188]. By weighting each compound’s individual emissions coefficient by its share of energy consumed, an overall emissions coefficient for LPG is derived. The results of this procedure are shown in Table B6. As the table illustrates, the emissions coefficient is dominated by propane, which has represented about three-fifths of LPG consumption between 1988 and 1995. The overall emissions coefficient has moved in a range smaller than ±0.2 percent in that time frame, with nearly all variation the result of changes in the ratio between consumption of higher hydrocarbons isobutane and n-butane, and the shorter chain compound ethane.
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