| Overview | Motor Gasoline | Jet Fuel | Crude Oil | Liquefied Petroleum Gases |
In the October 1994 edition of Emissions of Greenhouse Gases in the United States, the Energy Information Administration
(EIA) developed new emissions coefficients for the estimation of carbon released from the combustion of fossil fuels
in the United States. Emissions coefficients for more than 20 petroleum products were derived from 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.(192) For a more detailed discussion of how emissions coefficients
for fossil fuels were derived please refer to Appendix A of the 1994 report.(193)
The composition of marketed petroleum products varies over time with 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). Thus, a time series of emissions coefficients for fossil fuels has been developed, and the coefficients for motor gasoline, jet fuel, crude oil, and liquified petroleum gases are updated annually to reflect changes in their composition and density (Table B1).
A review of the annual coefficients reveals several trends. First, the density of crude oil entering U.S. refineries has
generally increased over the past decade, so that each barrel of oil now contains a greater mass of oil and, consequently,
a greater weight of carbon. Because the presumed heat content of a barrel of crude oil has not changed, the emissions
coefficient continues to climb slowly.
A second, more dramatic change has been the near elimination of naphtha-based jet fuel consumption. Between 1989
and 1996, the share of naphtha-based jet fuel consumed declined from 13 percent to 0.3 percent.Because kerosene-based
jet fuel has a significantly lower emissions coefficient than naphtha-based fuel, the weighted average emissions
coefficient for jet fuel has now diminished to 19.33 million metric tons per quadrillion British thermal units (Btu), equal
to the coefficient for kerosene-based jet fuel.
Also, in 1995, the emissions coefficient for motor gasoline reversed a decade-long trend of small increases due to
increasing density. This reversal is attributable to the widespread introduction of reformulated fuel into the U.S. motor
gasoline market. The Clean Air Act Amendments mandated the use of reformulated gasoline in heavily polluted areas
beginning in 1995. The impact of the legislation has been substantial, with reformulated gasoline representing
approximately 25 percent of motor gasoline production in 1995 and as much as 30 percent by the summer of 1996.
Some studies predict that the share of reformulated gasoline may grow to as much as 100 percent of the gasoline
market by 2000.(194)
Almost 20 percent of 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 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 different performance requirements of gasoline at different temperatures. The density of gasoline increased slowly and steadily for the decade prior to 1995, across all octane grades and in all seasons,(195) a trend that resulted in part from the phaseout 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 was increased. Aromatic hydrocarbons take the form CnH2n-6, with a lower ratio of hydrogen to carbon than other hydrocarbons typically found in gasoline. Because carbon is much heavier than hydrogen, the lower ratio results in increased 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).
The trend toward higher emissions coefficients ended in 1995, when 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 the density of standard gasoline, and the much lower carbon contents of the principal additives to reformulatedgasoline (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.
The 1996 emissions coefficient for motor gasoline remained stable at 19.38 million metric tons per quadrillion Btu. This
apparently static condition masked some underlying changes in the composition of reformulated fuel and thus the
calculation of a general motor gasoline emissions factor. The methyl tertiary butyl ether (MTBE) content of winter
reformulated gasoline grew from 4.3 percent to 7.3 percent between the winter of 1994-1995 and the winter of 1995-1996. As a result, the emissions coefficient for winter reformulated gasoline declined between 1995 and 1996. That
decline was offset by an increase in the emissions coefficient for summer gasoline, which can be traced to the exclusion
of tertiary amyl methyl ether (TAME) and
ethyl tertiary butyl ether (ETBE) from reformulated gasoline and the resultant increase in the portion of standard
gasoline.
In 1995, the three principal additives to reformulated gasoline were MTBE, ETBE, and TAME. These additives differ
from the hydrocarbons typically found in motor gasoline by the presence of oxygen atoms in their molecules. The
oxygen atoms reduce the emissions of carbon monoxide and unburnt hydrocarbons. In contrast to standard motor
gasoline, which has an estimated carbon share of 86.6 percent, these additives have carbon shares between 68.2 and
70.5 percent.(196) 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 contentof 68.2 percent. By 1996, the average gallon of reformulated
gasoline contained no TAME and only very small amounts of ETBE.
To derive an overall emissions coefficient for gasoline consumed during 1996, individual coefficients for motor gasoline
consumed in the winter and summer months, respectively, were developed. The coefficients were based on the
densities of product samples collected by the National Institute on Petroleum and Energy Research, in conjunction with
a carbon share of 86.6 percent as estimated by Mark DeLuchi.(197)
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.(198) The additive portions were defined as the net increase in MTBE,
ETBE, or TAME, as compared with the additives in standard fuel (small amounts of these compounds are also 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 vs. reformulated consumption (approximately 3 to 1). 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. The overall emissions
coefficient may be revised in the future, because data on the average composition of fuel used during October,
November, and December 1996 are not yet available. The winter coefficient for 1996 is based on data for January
through March 1996.
The EIA classifies jet fuel into two categories: "naphtha-based" and "kerosene-based." Kerosene-based jet fuel includes civil-grade Jet A, consumed in commercial jet airliners, which is an important contributor to U.S. greenhouse gas emissions and accounts for more than 4 percent of U.S. carbon dioxide emissions. Naphtha-based jet fuels were used primarily by the military until 1993, when the U.S. Department of Defense began a conversion from naphtha-based JP-4 jet fuel to kerosene-based JP-8 jet fuel. As a result, the share of naphtha-based jet fuel has declined from 12 percent of U.S. jet fuel consumption in 1991, to 7.2 percent in 1993,to 0.3 percent in 1996.(199) The annual emissions coefficients for jet fuel are consumption-weighted averages of the coefficients for kerosene-based and naphtha-based jet fuels (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 has led to a steadily declining emissions coefficient. In 1996, naphtha-based jet fuel consumption dropped to the point at which it no longer has a significant impact on the weighted average computation. Hence, the 1996 weighted-average emissions coefficient is equal to the coefficient for kerosene-based jet fuel (to the second decimal place).
The emissions coefficient for kerosene-based jet fuel was developed from data on 39 samples of Jet A collected and
analyzed by Boeing.(200) 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 19.95 million metric tons carbon per quadrillion Btu. It is based on fewer data
and has a larger margin of error than that for kerosene-based fuel.
Although crude oil 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 its 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.(201) 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.(202)
Data on sulfur content and density obtained from ultimate analyses of 182 crude oil samples were regressed against their carbon content
to produce 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 to calculate the share of carbon in an average barrel of oil during the 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 from EIA's standard energy content for crude
oil (5.8 million Btu per barrel).
The 1996 emissions coefficient for crude oil is 20.25 million metric tons carbon per quadrillion Btu. This is a slight increase over the 1995 value (Table B5), despitethe fact that the carbon content of the crude oil remained constant, because the heat content per barrel was assumed to be constant, and the higher density of crude oil in 1996 indicates more carbon per barrel. In all likelihood, however, the actual heat content per barrel increased sufficiently to offset the increase in density.
The EIA identifies four categories of paraffinic hydrocarbons(203) 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 of these
compounds are also well known, and their emissions coefficients can be calculated easily.
The EIA collects data on consumption of each of the four compounds in LPG and reports them collectively as LPG in the Petroleum Supply Annual.(204) By weighting each compound's individual emissions coefficient by its share of energy consumed, an overall emissions coefficient for LPG can be derived. The different LPG products are used for different purposes: propane is extensively used in the residential and commercial sectors as a heating fuel, whereas ethane and butane are used primarily in the industrial sector as petrochemical feedstocks.Upon investigation, it became apparent that almost all ethane (the lightest, lowest carbon component of LPG) is used as a petrochemical feedstock (a nonfuel use), and that fuel uses predominate for propane. Therefore, this year's report contains new, separate emissions coefficients for fuel use and nonfuel use of LPG. Each coefficient is calculated by summing the total carbon and total energy content of each category of LPG, then dividing total carbon by total energy content (in effect, calculating a Btu-weighted average of carbon emissions). Both the fuel use and nonfuel use emissions coefficients vary slightly over time, but the fuel use coefficient is consistently about 2 percent higher than the nonfuel use coefficient (Table B6).
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