Greenhouse Gases 1987-1994 Chapter 2

U.S. carbon dioxide emissions are mostly (98.5 percent) accounted for by the combustion of fossil fuels, such as coal, natural gas, and petroleum. Because fossil fuels are of considerable economic value and their consumption is carefully monitored, energy-related carbon dioxide emissions can be estimated more reliably than any other emissions source. Table 4 shows trends in U.S. carbon dioxide emissions estimated in million metric tons of carbon. Carbon units can be converted to carbon dioxide (at full molecular weight) by multiplying by 3.667. Between 1993 and 1994, emissions rose by 1.7 percent, to 1,430 million metric tons. Carbon dioxide emissions associated with the transportation sector's consumption accounted for nearly one-half of the 20-million-ton increase in energy-related carbon emissions, and the industrial and commercial sectors accounted for most of the balance (Table 5).

Table 4. U.S. Carbon Dioxide Emissions from Energy and Industry, 1987-1994
(Million Metric Tons of Carbon)

Petroleum                                 576.1 601.6 601.2 589.4 569.9 581.3 582.9 596.8

Coal                                         454.0 479.2 479.8 481.5 475.7 478.5 494.6 496.8

Natural Gas                              251.9 263.1 274.8 273.4 279.1 286.5 296.9 302.6

Total Energy Consumption 1,282.0 1,344.0 1,355.8 1,344.2 1,324.6 1,346.3 1,372.5 1,396.2

Adjustments to U.S. Energy

U.S. Territories 10.0 8.5 9.4 9.3 10.9 9.6 10.0 10.0

Unmetered Gas 5.1 5.2 1.8 1.0 6.0 5.9 4.9 4.6

Total Adjustments 15.2 13.7 11.2 10.2 16.9 15.5 14.9 14.6

Other Sources Cement Production 8.6 8.7 8.7 8.8 8.5 8.6 9.1 9.5

Other Industrial 7.5 8.1 8.3 8.3 8.3 8.3 8.2 8.2

Gas Flaring 1.4 1.7 1.7 1.8 2.1 2.1 1.4 1.5

Total Other Sources 17.6 18.5 18.7 18.9 18.9 18.9 18.7 19.2

Total 1,314.7 1,376.2 1,385.6 1,373.3 1,360.4 1,380.8 1,406.2 1,430.0

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). Emissions coefficients are annualized for coal, motor gasoline, liquefied petroleum gases, jet fuel, and crude oil. Includes emissions from bunker fuels. Totals may not equal sum of components due to independent rounding.

Source: EIA estimates documented in this chapter.

Table 5. U.S. Carbon Dioxide Emissions from Fossil Energy Consumption by End-Use Sector, 1987-1994
(Million Metric Tons of Carbon)

Residential                             251.0 264.8 267.5 253.0 257.1 255.9 271.6 271.6

Commercial                         197.2 207.6 210.0 206.7 206.4 205.5 212.1 216.9

Industrial                             422.7 444.1 445.6 452.4 436.6 453.6 454.0 461.4

Transportation                      411.1 427.5 432.7 432.1 424.5 431.4 436.7 446.3

Total Energy                     1,282.0 1,344.0 1,355.8 1,344.2 1,324.6 1,346.3 1,372.5 1,396.2

Electric Utility*                     452.6 475.9 483.5 476.9 473.5 472.9 490.6 494.9

P = preliminary data.

Notes: Includes energy from petroleum, coal, and natural gas. Electric utility emissions are distributed across consumption sectors. 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.

Source: EIA estimates documented in this chapter.

Emissions Trends

• The growth of U.S. energy consumption, which is determined by, among other things, the rate of economic growth, changes in energy prices, weather fluctuations, and the rate of increase in the efficiency with which the economy uses energy.
• Within energy consumption, the division between the use of fossil fuels and nonfossil fuels (since hydroelectric and nuclear power do not directly produce carbon emissions). For example, during the 1980s, long-delayed nuclear capacity came on line and contributed to an increased share of energy provided by nonfossil fuel sources.
• Within fossil energy consumption, the division between types of fossil fuels, since coal produces almost twice the carbon emissions per unit of energy that natural gas does, while petroleum products occupy an intermediate position. Growth in energy provided by coal consumption tapered off in the late 1980s, while natural gas consumption expanded to reach levels not attained since the 1970s.

During the 1990-1994 period, energy consumption in the United States has lagged behind economic growth (Figure 3). The U.S. economy grew at an average annual rate of 2.0 percent over the period 1990-1994, while energy consumption increased at a lower rate of 1.3 percent. The shifting composition of energy consumption has caused U.S. energy-related carbon emissions to lag behind energy consumption, growing at an average annual rate of 1 percent.

Between 1990 and 1994, U.S. total carbon dioxide emissions increased by a total of 57 million metric tons of carbon, or 4.1 percent. Emissions from natural gas consumption (Figure 4) accounted for a major portion of this increase (29.2 million metric tons); however, this is less than if the additional energy had been provided by coal, which contains more carbon per unit of energy supplied. To demonstrate, in 1994, if the additional energy provided by natural gas had been provided by coal, emissions of carbon dioxide probably would have been 3.5 to 4 million tons higher than they were.

Table 5 illustrates the sectoral composition of carbon dioxide emissions. The transportation and industrial sectors each account for about one-third of total emissions. Emissions from the transportation sector are growing more rapidly, because demand for motor gasoline, jet fuel, and diesel fuel is expanding. Although efficiency improvements have curtailed some growth, the growing popularity of luxury cars, trucks, and recreational vehicles effectively stabilized the U.S. average fuel economy at nearly 17 miles per gallon for all motor vehicles; in 1993 this measure actually worsened slightly.(Note 1) Energy consumption for the transportation sector in 1994 was 3 percent greater than in 1990.

The largest energy-consuming sector in the economy involves industrial activity. Overall energy consumption grew slightly between 1993 and 1994, by about 0.5 percent. Industrial sector energy consumption was only slightly higher in 1994 than in 1980. The residential and commercial sectors are smaller sources of emissions (Figure 5). The residential sector accounts for about 19 percent of U.S. carbon emissions and the commercial sector for about 15 percent. Over time, commercial sector energy consumption and emissions have grown with the expansion of the service sector in the U.S. economy. Commercial energy consumption was 28 percent higher in 1994 than in 1980.

For analytical purposes, carbon dioxide emissions from electric utilities were distributed over the end-use sectors in proportion to the amount of electricity consumed in each sector. However, trends in emissions from electric utilities can be considered independently.

Figure 3. Indices of U.S. Gross Domestic Product, Population, Energy Consumption, and Carbon Dioxide Emissions, 1980-1994

Sources: Carbon dioxide emissions are EIA estimates documented in this report. Energy consumption, gross national product (constant dollars), and population are reported in Energy Information Administration, Annual Energy Review 1994, DOE/EIA-0384(94) (Washington, DC, July 1995), pp. 5, 13, and 367.

Following the world oil crises of 1974 and 1978, electric utilities in the United States switched from oil to other fuels, such as coal and nuclear power. By the early 1990s, however, trends had changed. Coal's share of electric utility generation stabilized at about 55 percent, while natural gas use grew, especially among independent power producers. Nuclear power also increased as new capacity came on line and capacity factors increased to 74 percent. In recent years, efficiency improvements (both on the supply side and on the demand side) have reduced emissions below levels that would otherwise have been reached. Reported energy savings from demand-side management programs in 1993 were about 45 trillion kilowatthours, equivalent to a little more than 1.5 percent of electric utility generation in the same year.(Note 2) This savings is equivalent to about 7.4 million metric tons of carbon per year.

Renewable fuels currently account for about 7 percent of total U.S. energy consumption.(Note 3) Conventional hydroelectric power (which emits no carbon) is the largest single source of renewable energy--supplying nearly half the energy provided by renewables in the early 1990s. Biofuels, which are dominated by wood but also include municipal solid waste and alcohol fuels, collectively account for another major portion of renewable energy. Biofuels emit carbon when burned, but, by convention, emissions from biofuels are presumed to substitute for natural decomposition, and consequently to result in no net increase in emissions. Reported consumption of the remaining renewables (geothermal, solar, and wind) altogether amounts to less than 5 percent of renewable energy consumption and 0.3 percent of total U.S. energy.

Figure 4. U.S. Energy-Related Carbon Dioxide Emissions by Fuel, 1980-1994

Source: EIA estimates presented in this chapter.

Estimating Emissions

Sources of Energy Consumption. The energy consumption data used to make the estimates provided in this report were taken from EIA's State Energy Data Report 1993: Consumption Estimates, where they are detailed by end-use sector (residential, commercial, industrial, and transportation), by fuel type (petroleum [distinguishing 11 products], coal, natural gas, and electricity), and by year. Estimates for 1994 were derived from data in the Monthly Energy Review and Petroleum Supply Monthly.(Note 4) Industrial coal consumption, disaggregated by type, was derived from receipts by subsector published in EIA's Quarterly Coal Report.(Note 5)

Figure 5. U.S. Energy-Related Carbon Emissions by Sector, 1980-1994

Note: Electric utility emissions are distributed across end-use consumption sectors. They are shown separately here only for independent analysis. See Appendix C, Tables C1-C5, for a detailed accounting of the distribution.

Source: EIA estimates presented in this chapter.

Table 6. Comparison of EIA Estimates of Carbon Emissions Coefficients at Full Combustion, 1987-1994
(Million Metric Tons per Quadrillion Btu)

1995 Factors                 1994 1987 1988 1989 1990 1991 1992 1993 1994

LPG                             17.16 17.05 17.04 17.07 17.00 16.99 17.00 16.98 17.02

Jet Fuel                         19.74 19.42 19.42 19.41 19.40 19.40 19.39 19.37 19.34

Crude Oil                     20.29 20.13 20.16 20.13 20.16 20.18 20.22 20.23 20.21

Emissions Coefficients. The amount of carbon released when a fossil fuel is burned depends on the density, carbon content, and gross heat of combustion of the fuel.(Note 6) This year, the EIA conducted a review of its assumptions about coefficients for estimating U.S. carbon emissions. Most of the coefficients for major fuels remained largely unchanged. The most significant changes were for motor gasoline, liquefied petroleum gases (LPG), jet fuel, and crude oil (Table 6). For each of these fuels EIA developed an annualized carbon emissions coefficient to reflect changes in chemical composition or product mix over the years. Below is a short description of what is included in the new coefficients. Appendix A contains a more detailed discussion of the methodology for developing the coefficients. Table A1 presents a full listing of all factors for crude oil, natural gas, and the complete slate of petroleum products.

• Motor Gasoline. Motor gasoline consumption is the single most significant source of anthropogenic carbon emissions in the United States. For this report, EIA expanded upon earlier analyses to annualize the coefficient for motor gasoline to capture the long-term change in density of this fuel. Although the difference between using a general coefficient and a set of annualized coefficients makes a minimal difference in current estimates, it is expected to be useful in capturing the consequences of the ongoing shift to reformulated gasoline.
• Liquefied Petroleum Gas. LPG covers a mix of products (principally, ethane, propane, butane, and isobutane). Annualizing the coefficient more accurately reflects the year-to-year shifts in product mix, although the net effect on emissions calculations is slight.
• Jet Fuel. Most of the jet fuel consumed in the United States is the kerosene type; the naphtha type (mostly consumed by the military) makes up the remainder. The annualized coefficient is necessary to reflect the military conversion from naphtha-based JP-4 to kerosene-based JP-8 jet fuel. In addition, since the last issue of this report, the EIA has obtained better data on fuel composition, so that the coefficient is now given as between 19.34 and 19.45 million metric tons per quadrillion Btu, a level somewhat lower than the 19.74 used previously.
• Crude Oil. The annualized coefficient, presented here, reflects long-term shifts in the quality of crude oil input to U.S. refineries. The average density of crude oil refined in the United States declined from 33.74 degrees to 31.39, or by about 7 percent, between 1981 and 1994. The average sulfur content increased from 0.87 to 1.14 percent over the same period. When these and other factors are considered, the annual emissions coefficient for crude oil moves from a low of around 19.94 million metric tons per quadrillion Btu in 1981 to 20.23 million metric tons per quadrillion Btu in the mid-1990s. Previously, EIA's estimates were based on a crude oil coefficient of 20.29 million metric tons per quadrillion Btu. Since the EIA estimates emissions on the basis of end-use consumption of refined products, and only tiny amounts of crude oil are consumed by end users, use of these annualized factors results in minimal changes to EIA's estimates.

Carbon Sequestration. After energy consumption was multiplied by the emissions coefficients shown in Appendix A, Table A1, carbon sequestered through nonfuel use was then deducted from gross carbon emissions. Estimates of nonfuel use of fossil fuels were based on data provided in EIA's Annual Energy Review 1994, Table 1.15, "Fossil Fuel Consumption for Nonfuel Use, 1980-1994."(Note 7) Table 7 lists nonfuel use of fossil fuels by product type. Most nonfuel use of energy occurs in the industrial sector. Nonfuel use of energy was about 5.24 quadrillion Btu in 1994 (Table 7).

Table 7. U.S. Fossil Fuel Consumption for Nonfuel Use, 1987-1994
(Quadrillion Btu)

Asphalt and Road Oil                                         1.13 1.14 1.10 1.09 1.08 1.10 1.15 1.17

Liquefied Petroleum Gases                                 1.12 1.21 1.26 1.28 1.42 1.45 1.60 1.73

Lubricants                                                         0.35 0.35 0.35 0.37 0.33 0.33 0.34 0.35

Industrial                                                         0.18 0.18 0.18 0.19 0.17 0.17 0.17 0.18

Transportation                                                 0.17 0.17 0.17 0.18 0.16 0.16 0.16 0.17

Petrochemical Feed                                         1.00 1.00 1.00 0.82 1.15 1.20 1.21 1.24

Petroleum Coke                                             0.14 0.15 0.14 0.19 0.16 0.26 0.18 0.18

Special Naphtha                                             0.14 0.11 0.11 0.11 0.09 0.10 0.10 0.08

Other

(Waxes and Misc.)                                         0.23 0.24 0.25 0.23 0.26 0.21 0.20 0.22

Coal                                                             0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02

Natural Gas to Chemical Plants                 0.26 0.29 0.30 0.29 0.22 0.24 0.25 0.27

Total                                                             4.40 4.51 4.53 4.41 4.74 4.91 5.05 5.24

Notes: Asphalt and lubricants are as reported in State Energy Data Report 1993, DOE/EIA-0214(93) (Washington, DC, July 1995). Some slight differences exist between this table and the Annual Energy Review. 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).

Source: Energy Information Administration, Annual Energy Review 1994, DOE/EIA-0384(94) (Washington, DC, July 1995), Table 1.15, p. 33, and underlying estimates.

Not all nonfuel use of fossil fuels results in carbon sequestration. For example, natural gas (predominantly methane, or CH4) is used as a feedstock to make ammonia (NH4). The carbon in the methane is reformed into carbon dioxide and emitted into the atmosphere. On the other hand, petrochemical feedstocks, such as ethane, are made into ethylene and ultimately into polyethylene plastics and numerous other products. The carbon in these products ultimately is sequestered in landfills. Ideally, nonfuel use of fossil fuels would be divided into its constituent applications, and each application would be studied to determine the ultimate fate of the carbon in the fossil fuel. It has not been possible to collect sufficient information this year to adopt this approach in other than a single case.

Instead, as was done last year, the EIA used the Intergovernmental Panel on Climate Change (IPCC) methods and information specific to U.S. industry to determine how much carbon was sequestered by each product shown in Table 7.(Note 8) A "proportion of nonfuel use sequestered" was assumed for each product, usually based on IPCC recommendations but with EIA assumptions for those products for which no IPCC recommendation was available or for which more precise information could be obtained. These assumptions are shown in Table 8. The rationale for the assumptions made for some of the larger products is as follows:

• Asphalt and Road Oil. All carbon in the 1.17 quadrillion Btu of asphalt and road oil was presumed to be sequestered.
• Liquefied Petroleum Gas. This use is derived from sales of LPG (i.e., ethane, propane, butane, isobutane, and pentanes plus) to the chemical industry. For this amount (approximately 1.73 quadrillion Btu), the EIA assumed that 80 percent of the carbon was sequestered.
• Petrochemical Feedstocks. Petrochemical feedstocks include naphtha and other oils used as petro chemical feedstock, still gas not used as refinery fuel, and synthetic gas. This was about 1.2 quadrillion Btu of industrial energy consumption in 1994. Since some of these fuels are used to make products like plastic and rubber, in which carbon will be sequestered for long periods of time, the EIA assumed that 80 percent of the carbon in the feedstock is sequestered.
• Natural Gas. Nonfuel use of natural gas is based on estimates of natural gas feedstocks supplied to the chemical industry to manufacture ammonia (for fertilizer) and other chemicals. This use was about 0.56 quadrillion Btu in 1994. All the natural gas used to manufacture ammonia was assumed to be oxidized during the production process, while the carbon in other chemicals was assumed to be sequestered. Since the mix of these products can change from year to year, the effective sequestration rate also varies. Previously, this had been assumed to be a constant 33 percent, but actually it ranges from 41 to 55 percent in the United States.

Table 8. Rates of Sequestration for U.S. Fossil Fuel Consumption

Motor Gasoline 0.99 --

LPG 0.995 0.8

Jet Fuel 0.99 --

Distillate Fuel 0.99 --

Residual Fuel 0.99 --

Asphalt and Road Oil 0.99 1

Lubricants 0.99 0.5

Petrochemical Feed 0.99 0.8

Aviation Gas 0.99 --

Kerosene 0.99 --

Petroleum Coke 0.99 0.5

Special Naphtha 0.99 0

Other

Aviation Gas Blending Components 0.99 --

Crude Oil 0.99 --

Naphtha <401oF 0.99 0

Other Oil ò401oF 0.99 0

Petrochemical Feed Still Gas 0.99 0

Motor Gasoline Blending Components 0.99 --

Miscellaneous 0.99 --

Natural Gasoline 0.99 --

lant Condensate 0.99 --

Pentanes Plus 0.99 --

Still Gas 0.99 --

Special Naphthas 0.99 0

Unfinished Oils 0.99 --

Unfractionated Stream 0.99 --

Waxes 0.99 0

Coal

Residential and Commercial 0.99 --

Industrial Coking 0.99 0.75

Industrial Other 0.99 --

Electric Utility 0.99 --

Natural Gas

Flare Gas 1 --

Natural Gas 0.995 0.41-0.55

To recapitulate, the nonfuel use of energy shown in Table 7 was multiplied by the emissions coefficients in Table A1 and the proportion sequestered shown in Table 8 to determine the amount of carbon sequestered by nonfuel use. The results are shown in Table 9.

Table 9. U.S. Carbon Sequestered by Nonfuel Use of Energy, 1987-1994
(Million Metric Tons of Carbon)

Petroleum Liquefied Petroleum Gases                     15.3 16.5 17.2 17.5 19.3 19.8 21.8 23.5

Asphalt and Road Oil                                             23.3 23.5 22.7 22.5 22.3 22.7 23.7 24.1

Lubricants                                                             1.9 1.8 1.8 1.9 1.7 1.7 1.8 1.8

Other                                                                     22.0 22.3 22.4 19.9 25.2 26.4 25.3 26.0

Petrochemical Feed                                                 15.5 15.5 15.5 12.7 17.8 18.6 18.7 19.2

Petroleum Coke                                                     1.9 2.1 1.9 2.6 2.2 3.6 2.6 2.5

Waxes and Misc.                                                     4.6 4.8 4.9 4.6 5.2 4.2 4.1 4.3

Coal                                                                        0.6 0.4 0.4 0.4 0.4 0.4 0.4 0.4

Natural Gas                                                             3.7 4.2 4.3 4.2 3.2 3.5 3.6 3.9

Transportation Lubricants                                        1.7 1.7 1.7 1.8 1.6 1.6 1.7 1.7

Total                                                                     68.5 70.4 70.5 68.1 73.7 76.0 78.2 81.4

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).

Source: EIA estimates documented in this chapter.

There is also a very small amount of carbon sequestration associated with the combustion of fossil fuels. Using IPCC assumptions, this report assumes that oxidation of liquid and solid fuels during combustion is 99 percent complete, and that 1 percent of the carbon remains sequestered. Oxidation of gaseous fuels (LPG and natural gas) is assumed to be 99.5 percent complete.(Note 9) Conceptually, fuel may be "lost" before combustion due to evaporation, leaks, or spills; it may be subject to incomplete combustion and vented to the atmosphere in the form of volatile organic compounds or particulates; or it may remain at the site of combustion in the form of carbon-containing ash or soot.

In recent years, there have been several estimates of U.S. carbon emissions, some of which differ by as much as 5 percent. Two significant reasons for the differences in emissions estimates (beyond those associated with differences in coefficients) are the definitions of "energy consumption" and "the United States" employed by researchers. Subtle differences in definition can produce variations of several percent in reported energy consumption, and hence in carbon emissions. Some agencies include U.S. territories, such as Puerto Rico, while others exclude U.S. territories. If consumption is estimated as "apparent consumption" based on production plus imports minus exports plus stock change, then statistical discrepancies will be included in consumption. International bunkers are sometimes counted as domestic consumption, and sometimes as exports. This section describes how each of these items is accommodated in this report.

Energy Consumption in U.S. Territories

Table 10 illustrates reported energy consumption in U.S. territories. These data have been published in EIA's International Energy Annual. This table also uses unpublished estimates of oil consumption for Wake Island, American Samoa, and the Pacific Trust Territories, which are included as "Other" in the Asia/ Pacific region in the International Energy Annual.

Table 10. Energy Consumption in U.S. Territories and International Bunkers, 1987-1994
(Quadrillion Btu)

Puerto Rico 0.34 0.30 0.30 0.29 0.37 0.31 NA NA

Virgin Islands 0.12 0.08 0.12 0.11 0.12 0.11 NA NA

American Samoa 0.01 0.01 0.01 0.01 0.01 0.01 NA NA

Guam 0.01 0.01 0.01 0.03 0.03 0.02 NA NA

Micronesia * * * * * * NA NA

Wake Island 0.02 0.02 0.02 0.02 0.02 0.02 NA NA

Total 0.49 0.42 0.46 0.46 0.54 0.47 0.50 0.50

U.S. Bunker Fuels 0.91 1.00 1.06 1.05 1.11 1.17 1.06 NA

P = preliminary data. NA = not available.

Notes: Energy consumption in Micronesia ranged from 0.002 to 0.004 quadrillion Btu. 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: U.S. Territories: Energy Information Administration, International Energy Annual, DOE/EIA-0219 (various years), and unpublished data included in "Other" countries in the Asia/Pacific region. Data are shown in tables of "Apparent Consumption of Petroleum Products." Data for 1992 and 1993 based on unpublished preliminary information. Bunker Fuels: Jet FuelþOak Ridge National Laboratory, Transportation Energy Data Book (Oak Ridge, TN, various years); Distillate and Residual Fuel Oils, 1987-1990"Energy Information Administration, International Energy Annual, DOE/EIA-0219 (Washington, DC, 1987-1990); Distillate and Residual Fuel Oils, 1991-1993"Energy Information Administration, Fuel Oil and Kerosene Sales, DOE/EIA-0535 (Washington, DC, 1991-1993).

Energy consumption for U.S. territories was converted to carbon emissions using the same emissions coefficients applied to U.S. energy data. Carbon emissions for U.S. territories ranged from 9 to 11 million metric tons per year (Table 11). Because a large portion of reported energy consumption in U.S. territories was from "other petroleum," there is a degree of uncertainty about the correct emissions factor to be used in this area, as well as the reliability of underlying data.

Table 11. Carbon Emissions from U.S. Territories, International Bunkers, and Unmetered Gas Consumption, 1987-1994
(Million Metric Tons of Carbon)

Bunker Fuels                                                 18.9 20.7 21.9 21.7 22.9 24.2 21.8 NA

Unmetered Natural Gas Consumption             5.1 5.2 1.8 1.0 6.0 5.9 4.9 4.6

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).

Source: Estimates documented in this chapter.

International Bunker Fuels

Bunkers, however, are an export without a corresponding import, because the purchasing ship generally burns the fuel on the high seas. EIA energy statistics, which are based on domestic sales of products, treat bunker fuels sales in the same way as sales of other fuels, i.e., as domestic energy consumption. Carbon emissions from bunker fuels are, therefore, already counted in the domestic energy consumption of the United States--primarily as transportation-related consumption of residual oil.

Those who wish to understand the differences between emissions inventories based on international energy statistics and EIA data will, however, need to know the amount of energy consumption and the amount of carbon emissions associated with international bunkers. Table 10 therefore shows U.S. international bunker fuel usage.(Note 10) The amount is about 1.1 quadrillion Btu (or 500,000 barrels per day), largely of residual oil; it accounts for emissions of about 19 to 24 million metric tons of carbon annually (Table 11).

Unreported Natural Gas Consumption

Repairing leaks has become a priority in pipeline operations, due to safety and liability concerns. For this reason, only 0.5 percent of natural gas consumption can be attributed to pipeline leakage. Leaked gas enters the atmosphere in the form of methane. (Estimates of methane emissions from natural gas leakage can be found in Chapter 3 of this report.) While measurement errors and data reporting problems certainly exist in the natural gas industry, these errors ought not to be "tilted" in the direction of gas supply unless there is unreported consumption. The EIA believes that the amount of gas in the "balancing item" less the amount lost to leakage is more likely than not to reflect unreported consumption.

Emissions from this source were estimated by first converting the volume of unmetered consumption into Btu, then multiplying by a carbon emissions coefficient. In 1994, unmetered consumption of 0.32 trillion cubic feet resulted in emissions of approximately 5 million metric tons of carbon (Table 12). Annually, emissions from unreported natural gas consumption tend to fall in the range of 4 to 6 million metric tons, with the exceptions of 1989 and 1990, when the "balancing item" for those years was significantly low.

Table 12. U.S. Natural Gas Consumption and Balancing Item, 1987-1994

(Trillion Cubic Feet)                                                         17.21 18.03 18.80 18.72 19.04 19.54 20.30 20.60

"Balancing Item" (Trillion Cubic Feet)                             -0.44 -0.45 -0.22 -0.15 -0.50 -0.51 -0.41 -0.39

Estimated Gas Leakage (Trillion Cubic Feet)                 -0.07 -0.07 -0.07 -0.07 -0.07 -0.07 -0.07 -0.07

Unmetered Consumption (Trillion Cubic Feet)                 -0.37 -0.38 -0.15 -0.08 -0.43 -0.44 -0.34 -0.32

Estimated Unmetered

Carbon Emissions (Million Metric Tons)                 5.13 5.19 1.78 0.97 5.98 5.94 4.92 4.59

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: Energy Information Administration, Natural Gas Annual, DOE/EIA-0131 (Washington, DC, 1987-1993), and Natural Gas Monthly, DOE/EIA-0130(95/05) (Washington, DC, May 1995), p. 3. Leakage estimates from this report (see Chapter 3). All gas in the balancing item not attributed to leakage is assumed as unreported consumption.

U.S. energy production processes also generate small volumes of carbon dioxide emissions. The two principal sources of these emissions are flaring of natural gas and venting of the carbon dioxide that is produced in conjunction with natural gas. When a field is developed for petroleum extraction, any natural gas associated with that field may be flared if its use is not economically justifiable. This is typically the case with a remote site or when the gas is of poor quality or minimal volume. During natural gas production, flaring may be used for disposal of waste products (e.g., hydrogen sulfide), capacity testing, or as a result of process upsets.

This year, the method for estimating emissions from natural gas flaring has been modified and is now based on the volume of vented and flared gas reported to EIA by each State. This composite volume is scaled by a State-specific flaring percentage to ascertain the amount of natural gas flared in that State. The percent flared value is taken from a 1990 Department of Energy study that determined the relative split between venting and flaring for each State.(Note 11) To calculate carbon emissions, the State figures are aggregated, converted into Btu, and then multiplied by the emissions coefficient applicable to natural gas.

As Tables 4 and 13 indicate, natural gas flaring is a minor source of emissions, accounting for only 1.4 million metric tons of carbon in 1993. There is some uncertainty to these estimates given that operators in the field are not required to meter the amount of gas that is vented or flared. Further, methods used by States to determine their vented and flared statistics are not uniform.

Table 13. U.S. Carbon Dioxide Emissions from Gas Flaring, 1987-1994

Total Natural Gas Vented and Flared

(Billion Cubic Feet)                                     85.80 102.18 101.56 111.08 127.64 124.23 86.94 89.80

Btu Content of Flare Gas

(Btu per Cubic Foot)                                 1,112 1,109 1,107 1,105 1,108 1,110 1,106 1,106

Carbon Emissions from Flaring

(Million Metric Tons)                             1.42 1.69 1.68 1.83 2.11 2.06 1.43 1.48

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: Energy Information Administration, Natural Gas Annual, DOE/EIA-0131 (Washington, DC, 1987-1993), and Natural Gas Monthly, DOE/EIA-0130(95/05) (Washington, DC, May 1995); and U.S. Department of Energy, An Evaluation of the Relationship Between the Production and Use of Energy and Atmospheric Methane Emissions, DOE/NBB-0088P (Washington, DC, April 1990).

In addition to energy-related emissions, carbon dioxide is also produced during industrial processes. The primary source of industrial emissions is limestone (CaCO3) calcination to create lime (CaO). These two compounds are basic materials in a variety of manufacturing processes, particularly, cement, iron and steel, and glass. Other sources of industrial emissions include the production and use of soda ash (Na2CO3) and the manufacture of carbon dioxide and aluminum.

For this source category, emissions estimates are based on the compound used in the industrial process. By multiplying the amount of production or consumption of the compound by a carbon coefficient (the relative amount of carbon in that compound), a process-specific estimate is derived. In 1993, industrial processes accounted for 17.3 million metric tons of carbon emissions (Table 14). Preliminary figures for 1994 indicate an increase to 17.7 million metric tons of carbon.

Table 14. U.S. Carbon Dioxide Emissions from Industrial Sources, 1987-1994
(Million Metric Tons of Carbon)

Clinker Production 8.62 8.67 8.69 8.75 8.51 8.59 9.09 9.47

Masonry Cement 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

Total 8.64 8.69 8.71 8.77 8.53 8.61 9.11 9.49

Other Industrial

Limestone Consumption Lime Manufacture 3.06 3.31 3.33 3.39 3.36 3.47 3.63 3.70

Iron Smelting 0.49 0.50 0.51 0.47 0.44 0.37 0.31 0.30

Steelmaking 0.10 0.10 0.13 0.08 0.09 0.07 0.13 0.13

Glass Manufacture 0.08 0.06 0.03 0.03 0.03 0.04 0.05 0.06

Flue Gas Desulfurization 0.47 0.45 0.53 0.52 0.55 0.54 0.51 0.56

Total 4.19 4.43 4.53 4.50 4.46 4.49 4.63 4.76

Dolomite Consumption 0.15 0.12 0.08 0.09 0.10 0.08 0.07 0.08

Soda Ash Manufacture 0.79 0.86 0.93 0.92 0.92 0.94 0.91 0.92

Soda Ash Consumption Glass Manufacture 0.38 0.38 0.37 0.36 0.34 0.35 0.35 0.36

Flue Gas Desulfurization 0.02 0.02 0.03 0.02 0.02 0.02 0.02 0.02

Sodium Silicate 0.06 0.05 0.05 0.05 0.05 0.05 0.06 0.06

Sodium Tripolyphosphate 0.04 0.04 0.04 0.04 0.03 0.03 0.03 0.03

Total 0.50 0.50 0.49 0.46 0.44 0.45 0.46 0.47

Carbon Dioxide Manufacture 0.21 0.22 0.23 0.24 0.25 0.26 0.26 0.27

Aluminum Production 1.69 1.99 2.03 2.04 2.08 2.04 1.86 1.67

Total Other Industrial 7.53 8.11 8.30 8.26 8.26 8.27 8.20 8.17

Total 16.17 16.80 17.00 17.03 16.79 16.88 17.31 17.66

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: Methodologies documented in this chapter and numerous sources of trend data. U.S. Department of the Interior, Bureau of Mines, Soda Ash Annual Report (Washington, DC, various years). American Iron and Steel Institute Annual Statistical Report (Washington, DC, various years). U.S. Department of the Interior, Bureau of Mines, Mineral Commodity Summaries (Washington, DC, various years). U.S. Department of the Interior, Bureau of Mines, Cement Annual Report (Washington, DC, various years). U.S. Department of the Interior, Bureau of Mines, Crushed Stone Annual Report (Washington, DC, various years). Chemical Manufacturers Association, U.S. Chemical Industry Statistical Handbook 1993 (Washington, DC, September 1993). Energy Information Administration unpublished survey data, Steam Electric Plant Operation and Design Report, Form EIA-767 (Washington, DC, various years). Freedonia Group, Inc., Carbon Dioxide, Business Research Report B286 (Cleveland, OH, November 1991), and Carbon Dioxide, Industry Study 564 (Cleveland, OH, February 1994).

Cement Manufacture

One mole of calcined limestone produces one mole of carbon dioxide and one mole of lime. Since virtually all of the lime produced is absorbed into the clinker, the lime content of clinker is assumed to be representative of the amount of carbon dioxide that is emitted.

In order to estimate emissions from cement manufacture, a carbon coefficient must be calculated. The EIA has adopted the IPCC recommendation that 64.6 percent of cement clinker is lime.(Note 12) Multiplying this lime content factor by the ratio of carbon produced to lime produced yields the coefficient for cement clinker. A separate coefficient is necessary for estimating emissions from the additional lime used to produce masonry cement. In this case, the amount of lime not accounted for as clinker is assumed to be 3 percent.(Note 13) This factor is then multiplied by the same production ratio of carbon to lime, generating the carbon coefficient for masonry cement.

Production of cement clinker rose to 69 million metric tons in 1994, resulting in carbon emissions of 9.5 million metric tons. Emissions from masonry cement production have stabilized at 0.02 million metric tons of carbon annually over recent years.

Limestone and Dolomite Consumption

As noted above, lime is produced by heating a calcium-rich substance such as limestone or dolomite. This process emits carbon dioxide. As shown in Table 14, lime manufacturing is the predominant source of emissions from limestone and dolomite consumption, contributing over 3.6 million metric tons of carbon in 1993. Preliminary figures for 1994 indicate a slight increase to 3.7 million metric tons of carbon. A rising trend has been developing since 1991. Carbon dioxide emissions from the other consumptive uses totaled approximately 1 million metric tons of carbon in 1993 and have remained relatively stable in recent years.

Soda Ash Manufacture and Consumption

Once manufactured, most soda ash is consumed in glass and chemical production. Other uses include water treatment, flue gas desulfurization, soap and detergent production, and pulp and paper production. As soda ash is processed for these purposes, additional carbon dioxide may be emitted if the carbon is oxidized. Because of the limited availability of specific information about such emissions, only certain uses of soda ash are incorporated into this report. Sodium silicate and sodium tripolyphosphate are included in this category as chemicals manufactured from soda ash and components of detergents. Emissions from soda ash consumption are relatively small and stable, contributing a flux of only 0.46 million metric tons of carbon to the atmosphere in 1993.

Carbon Dioxide Manufacture

Most carbon dioxide produced from wells is injected back into the ground for enhanced oil recovery. This process sequesters the carbon dioxide, at least in the short run. Conceptually, only carbon dioxide produced from wells and diverted to industrial use is emitted to the atmosphere. The Freedonia Group estimates nonsequestering industrial use at 1.3 million metric tons of carbon in 1993.(Note 15) If 20 percent of this industrial use is supplied by wells, emissions can be estimated at 0.26 million metric tons of carbon. Based on the Freedonia report, the 1994 estimate is calculated assuming an annual 4.2-percent increase, implying emissions of 0.29 million metric tons of carbon.

Aluminum Manufacture