2. Carbon Dioxide Emissions
2. Carbon Dioxide Emissions 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.
In 1993, emissions rose by 2 percent, to 1,409.2 million metric tons, as accelerating economic growth and an unusually hot summer raised electric utility coal consumption. (3) Carbon dioxide emissions associated with residential and commercial consumption accounted for 21 million tons of a 28-million- metric-ton increase in energy-related carbon emissions, while the industrial and transportation sectors accounted for the balance.
Table 4 is divided into three sections. The first section, "Energy Consumption," covers carbon emissions from the combustion of fossil fuel. The second section, "Adjustments to U.S. Energy," covers energy-related carbon emissions that are not counted in official energy statistics for the United States as reported by the EIA. These emissions may be included in energy statistics maintained by other organizations, particularly the International Energy Agency. The third section, "Other Sources," covers industrial and process emissions, largely caused by calcining of carbonate rock. Each of these sources is discussed in the following sections.
Over the long term, the level of U.S. energy-related carbon dioxide emissions can be viewed as the outcome of the interaction between three interrelated sets of factors:
Since 1985, energy consumption in the United States has lagged behind economic growth (Figure 4). The U.S. economy grew at an annual rate of 2.3 percent over the period 1985-1993, while energy consumption increased at a lower rate of 1.6 percent per year. The shifting composition of energy production has permitted carbon emissions to lag behind energy consumption since the late 1980s (Figure 5).
Figure 4. Indices of U.S. Gross Domestic Product, Population, Energy Consumption, and Carbon Dioxide
Emissions, 1980-1993
Figure 5. U.S. Energy-Related Carbon Dioxide Emissions by Fuel, 1980-1993
Table 5 illustrates the sectoral composition of carbon dioxide emissions. The transportation and industrial sectors each account for about one-third. 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 this growth, energy consumption for the transportation sector in 1993 was 16 percent greater than in 1980 (Figure 6).
Figure 6. U.S. Energy-Related Carbon Emissions by Sector, 1980-1993
By comparison, industrial sector energy consumption declined during the 1980s and was only slightly higher in 1993 than in 1980. The residential and commercial sectors are smaller sources of emissions. The residential sector accounts for about 20 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 25 percent higher in 1993 than in 1980.
For analytical purposes, carbon dioxide emissions from electric utilities were distributed over the end-use sectors proportionately according to the amount of electricity consumed in each sector. However, trends in emissions from electric utilities can be considered independently. 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 took a leading role, especially in the growing nonutility market. Nuclear power also increased as new capacity came on line and capacity factors increased from 65 to 70 percent. In addition, efficiency improvements (both on the supply side and on the demand side) reduced emissions below the levels that would otherwise have been reached. For example, reported energy savings from demand-side management programs in 1992 were about 32 trillion kilowatthours, an amount equivalent to a little more than 1 percent of electric utility generation in the same year. (4)
Renewable fuels currently account for about 8 percent of total U.S. energy consumption. (5) 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 increases 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.
Carbon emissions in this report were calculated by multiplying energy consumption for each fuel type by an associated carbon coefficient. The result was then modified by subtracting carbon sequestered by nonfuel use. This section describes the derivation of information on energy consumption, carbon coefficients, and carbon sequestered by nonfuel use.
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 1992: 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 1993 were derived from data in the Monthly Energy Review and Petroleum Supply Monthly. (6) Industrial coal consumption, disaggregated by type, was derived from receipts by subsector published in EIA's Quarterly Coal Report. (7)
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. (8) This year, the EIA conducted a major review of its assumptions about carbon coefficients for estimating U.S. emissions. Most of the coefficients for major fuels remained largely unchanged. The most significant changes were for liquefied petroleum gas (LPG), still gas , and jet fuel :
Table 6 lists the factors used in this year's report (EIA94) and compares them with the factors used in last year's report (EIA93). A detailed description of the basis for estimating the new emissions coefficients is included in Appendix A .
Carbon Sequestration. After energy consumption was multiplied by the emissions coefficients shown in Table 6, carbon sequestered through nonfuel use was then deducted from gross energy consumption. Estimates of nonfuel use of fossil fuels were based on data provided in EIA's Annual Energy Review 1993, Table 1.15, "Fossil Fuel Consumption for Nonfuel Use, 1980-1993." (10) 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 4.98 quadrillion Btu in 1993 (Table 7).
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. (11) 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 6. The rationale for the assumptions made for some of the larger products is as follows:
The nonfuel use of energy shown in Table 7 was multiplied by the emissions coefficients and the proportion sequestered shown in Table 6 to determine the amount of carbon sequestered by nonfuel use. The results are shown in Table 8.
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. (12) 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 emission 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.
EIA's energy data for the United States cover only the 50 States and the District of Columbia. In contrast, energy data produced by the International Energy Agency for the United States cover the 50 States plus U.S. territories, including Puerto Rico, the U.S. Virgin Islands, and Guam. The energy consumption of the U.S. territories is only about 0.5 quadrillion Btu. Because U.S. territories are all islands, their consumption consists primarily of petroleum products. For the territories as a group, oil consumption ranges between 200,000 and 250,000 barrels per day, and coal consumption averages about 300,000 short tons per year.
Table 9 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.
Energy consumption for U.S. territories was converted to carbon emissions using the same emission coefficients applied to U.S. energy data. Carbon emissions for U.S. territories ranged from 9 to 11 million metric tons per year (Table 10). 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.
The term "international bunker fuels " refers to fuel purchased by merchant ships in U.S. ports and by international air carriers. By convention, trade statistics treat sales of bunker fuels as exports by the selling country, because the purchaser promptly hauls the fuel outside national boundaries. This convention is followed by those who prepare international energy statistics, such as the United Nations and the International Energy Agency. (13)
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 9 therefore shows U.S. international bunker fuel usage, based on data published in EIA's International Energy Annual for 1987-1990 and EIA's Fuel Oil and Kerosene Salesfor 1991 and 1992. The amount is about 1.1 quadrillion Btu (or 500,000 barrels per day), largely of residual oil; it accounts for emissions of about 22 or 23 million metric tons of carbon annually (Table 10).
Because EIA data on natural gas "consumption" stem from readings of customer natural gas meters by local gas distribution companies and other natural gas sellers, reported consumption in the United States is consistently 1 to 4 percent less than the amount of natural gas that reportedly enters pipeline service. This difference is called the " balancing item " in EIA's natural gas statistics. Because this gas is, by definition, "unaccounted for," there is no way of knowing its ultimate disposition for certain. Early studies of this issue suggested that much of the discrepancy may have resulted from pipeline leaks, and that the missing gas entered the atmosphere in the form of methane. For safety, liability, and commercial reasons, however, gas transmission and distribution companies have aggressive programs to find and fix leaks, and the handful of careful studies of existing pipeline systems tend to show that pipeline leaks are unlikely to account for as much as 0.5 percent of total natural gas consumption.
If leaks account for only a small portion of the missing gas, the balance must be accounted for by various accounting and tabulation errors and unreported consumption. Accounting and tabulation errors should, in principle, randomly fluctuate between positive and negative values. Because unaccounted for gas always represents more than 1 percent of total consumption, the implication is that there is some unreported consumption, possibly due to systematic metering errors or to underreporting of gas transported by one company for the account of another. (14)
The EIA has assumed that the portion of the "unaccounted for" natural gas that cannot be attributed to pipeline leakage is actually consumption that has the same carbon characteristics as other gas consumption. Pipeline leakage was estimated using emissions coefficients based on studies of leakage from a sample of transmission and distribution pipelines. (15) These assumptions imply the combustion of approximately 0.4 quadrillion Btu per year, accounting for carbon emissions of between 4.0 and 6.0 million metric tons annually (Table 11). The exceptionally low carbon emissions from this source in 1989 and 1990 are due to low "unaccounted for" levels for those years.
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.
Natural gas venting and flaring is a temporary event that occurs early in the field development process-while gas handling facilities are under construction, when the volume of gas produced is very low, or when oil is being produced far from natural gas markets or pipelines. In the United States the strict regulations that control gas flaring , together with the economics of a well-developed gas market, ensure that only about 0.5 percent of gross production is vented or flared. (16)
Natural gas venting and flaring statistics are collected and reported to the EIA by State energy offices. The States use varying methods to compute venting and flaring, ranging from pro rata estimates based on State oil production to requiring operators to fill out detailed reports. Operators rarely meter gas venting or flaring directly; consequently, the volumes disposed of are, at best, estimates. In 1990, the Department of Energy conducted a comprehensive examination of gas venting and flaring. For that study, every U.S. State energy office was asked to determine the proportion of venting and flaring in its State. (17) The results of the study suggested that 87 percent of the natural gas in question was flared, while 13 percent was vented. Adjusting for incomplete combustion, these data suggest an 80 percent/20 percent split between flaring and venting. Thus, it is clear that most reported natural gas is flared.
Table 12 provides EIA estimates of carbon emissions from natural gas flaring. The composition of "wet" associated gas differs materially from the composition of "dry" pipeline-quality natural gas, in that it contains less methane and more natural gas liquids and inert gases. The estimates presented here use Btu conversion factors for "wet" natural gas.
Carbon emissions result not only from combustion of fossil fuels but also from chemical and industrial processes-mostly from the calcination of limestone (calcium carbonate, CaCO3) to form lime (CaO). While emissions from calcination are attributed primarily to the cement industry, limestone and lime are also used in steelmaking, agriculture, water and sewage treatment, glass manufacture, flue gas desulfurization , and many other chemical and industrial processes. Additional carbon is contributed by the production of soda ash (Na2CO3), as well as its use in the manufacture of certain products. Carbon emissions from industrial processes rose in 1993, totaling 17.3 million metric tons (Table 13), because of expanded construction and higher levels of industrial activity in the U.S. economy.
Emissions from most industrial processes are estimated by using basic stoichiometric calculations-the mass of the substance that is produced or consumed each year is multiplied by the ratio of the molecular weight of carbon to the molecular weight of the substance, i.e., the fraction of carbon in the substance. This method should, in theory, yield accurate emissions numbers, since it is based on material balances. However, impurities in the raw materials used and uncertainty in production and consumption data may affect the accuracy of the calculations.
Cement manufacturing is the largest nonenergy industrial source of carbon dioxide emissions. The emissions result from the heating of limestone, which constitutes approximately 80 percent of the feed to cement kilns. During cement production, high temperatures are used to transform the limestone into lime, releasing carbon dioxide to the atmosphere.
In the United States, the most commonly used hydraulic cement is Portland cement. To produce Portland cement, at least four chemical elements are needed: calcium, silicon, aluminum, and iron. They are derived from naturally occurring rocks and minerals, such as limestone, clay, shale, and iron ore. The materials are ground to a very fine powder and fed into a rotary kiln, where they are heated at temperatures up to 1,500°C. The product of the kiln is an intermediate material called "Portland cement clinker ." The clinker produced by this process is mixed with gypsum and other chemical additions and ground to a powder to form Portland cement.
It is during the heating in the cement kiln that calcination and the resulting carbon dioxide emissions occur. In this process, one molecule of calcium carbonate is decomposed into one molecule of carbon dioxide gas and one molecule of calcium oxide. Cement manufacture utilizes nearly 100 percent of the calcium oxide obtained from roasting the calcium carbonate. Thus, the measurement of the calcium oxide content in the cement clinker is a good measurement of the amount of carbon dioxide released during production.
To estimate carbon dioxide emissions from cement production, an emissions factor is derived by multiplying the fraction of lime in the cement clinker by a constant that reflects the mass of carbon released per unit of lime. EIA has assumed an average lime content of 64.6 percent, based on the recommendations of the IPCC, (18) yielding an emissions factor of 0.138 tons of carbon per ton of clinker produced. The United States produced 65 million metric tons of cement clinker in 1993, which implies carbon emissions of nearly 9 million metric tons-a 5-percent increase over 1992. (19)
Additional carbon dioxide is released as a result of adding extra lime to Portland cement clinker to make masonry cement, a more plastic cement that is typically used in mortar. To estimate these emissions, a coefficient was derived by a method similar to that used for cement clinker. The fraction of lime in masonry cement not attributable to clinker, assumed to be 3 percent, was multiplied by the ratio of the mass of carbon to the mass of lime, yielding an emissions coefficient of 0.0064 metric tons of carbon per metric ton of masonry cement produced. Emissions from this source were estimated at almost 19,000 metric tons of carbon for 1993-a small portion of the total for both types of cement.
Limestone is also calcined to produce lime for use in agriculture, construction, steelmaking, pulp and paper manufacturing, water purification, and a wide variety of other chemical and industrial processes. Lime is manufactured by heating either limestone or dolomite (CaMg(CO2)3) in a rotary or vertical kiln, which subsequently releases carbon dioxide to the atmosphere. According to the U.S. Bureau of Mines, (20) 17 million metric tons of lime were produced in the United States in 1993, implying carbon emissions of 3.5 million metric tons.
While the production of lime accounts for the majority of carbon emissions associated with the consumption of limestone and dolomite, carbon emissions also result from the direct consumption of these two substances in certain industrial processes. Emissions from those processes totaled 1.2 million metric tons in 1993. Flue gas desulfurization (FGD) is the largest of the processes and the only one experiencing much growth in the 1990s. The FGD process is used primarily in coal-fired electric generating plants to remove sulfur oxides from stack gases either during or after the combustion of fossil fuels.
The wet lime/limestone scrubber is the most widely used FGD system, comprising about 70 percent of all installed FGD capacity. (21) In these systems, flue gas passes through the FGD absorber, where sulfur dioxide is removed by direct contact with an aqueous suspension of finely ground limestone, before it is released to the atmosphere from a stack or a cooling tower. The byproducts of this reaction are either a mixture of calcium sulfate /sulfite or gypsum , which can be sold for use in plaster, cement, and wallboard.
The use of FGD in power plants has expanded in recent years to help meet stricter air quality requirements established by the 1990 Clean Air Act Amendments. FGD capacity has grown from 60 gigawatts of nameplate capacity in 1985 to 71 gigawatts in 1992. (22) Although highly efficient at sulfur dioxide removal, FGD is associated with increased carbon dioxide emissions for two reasons. First is the use of limestone as a sorbent. The chemical reaction of calcium carbonate (in the limestone) and sulfur dioxide produces calcium sulfate, carbon dioxide, and oxygen. In 1992 about 5 million tons of limestone were used, resulting in carbon emissions of about 533 thousand metric tons. (23) Second is the effective derating of power plant efficiency by 1 or 1.5 percent because of the additional energy required by the FGD process itself. (24) The additional carbon dioxide emissions accrued are implicit in the fuel used, and they are accounted for in electric utility emissions from energy consumption.
In addition to its use in FGD, limestone is also used as a flux in iron smelting and steel making to remove impurities from the raw materials. Small amounts of dolomite are used for this purpose as well. Limestone is also heated in furnaces to make glass.
Soda ash is an alkali used primarily for making glass and manufacturing chemicals, including baking soda (sodium carbonate). Lesser amounts are also used for making soap and detergents, for making pulp and paper, and for water and sewage treatment. Natural soda ash is made by calcining trona ore (sodium sesquicarbonate, Na2CO3·NaHCO3·2H2O) in a rotary kiln, during which one mole of carbon dioxide is released for each mole of soda ash produced. In some areas, soda ash is made by carbonating, and then calcining, sodium carbonate-bearing brines. However, the carbon dioxide generated in this process is recovered for use during the carbonation stage of production and thus is not emitted to the atmosphere. It is also possible to make soda ash artificially from salt and limestone, but this method is more costly and generates more pollution than the natural processes. The United States has produced only natural soda ash since 1986, and in the years prior it is estimated that artificial soda ash accounted for only 10 percent of total U.S. production. (25)
The U.S. Bureau of Mines reports that 9.4 million metric tons of soda ash were produced in the United States in 1992. About 90 percent of this amount was produced from calcining trona ore, implying carbon emissions of approximately 1 million metric tons.
The use of soda ash in certain chemical and industrial processes may contribute additional carbon dioxide to the atmosphere, since soda ash, as its chemical formula indicates, retains a carbon atom after calcination . In 1992, 3.1 million metric tons of soda ash were used to make glass, releasing 350,000 metric tons of carbon. The production of the chemicals sodium silicate and sodium tripolyphosphate from soda ash contributed an additional 80,000 metric tons of carbon. It is possible that other or even all uses of soda ash result in carbon emissions. If this is indeed the case, then the range of possible emissions would be 430,000 metric tons of carbon (from glass, sodium silicate, and sodium tripolyphosphate production) to 711,000 metric tons (assuming that all of the 6.28 million metric tons of soda ash consumed in the United States in 1992 generated carbon emissions). However, emissions estimates in this report reflect only those uses for which detailed information was available.
Industrial carbon dioxide is produced for use in food processing and refrigeration, beverage carbonation, chemical manufacturing, enhanced oil recovery, and various other applications. While most of the carbon dioxide produced is eventually emitted to the atmosphere, carbon dioxide used for enhanced oil recovery is injected back into the ground and can be considered sequestered (although it is still unclear whether it escapes to the atmosphere at some point).
In 1990, consumption of carbon dioxide for purposes other than enhanced oil recovery was 4.4 million short tons, equivalent to emissions of about 1.2 million metric tons of carbon. (26) However, manufacture of carbon dioxide is, with a few exceptions, a secondary operation integrated with the production of other chemicals, particularly, ammonia and hydrogen. Since the principal feedstocks for carbon dioxide manufacture are natural gas and naphtha , the byproduct carbon dioxide produced is accounted for in carbon for nonfuel use that is not sequestered. Carbon dioxide produced from dedicated wells, natural gas wells, and natural gas processing represents only 20 percent of total U.S. production capacity. (27) Assuming that the remaining 80 percent is accounted for in other emissions categories, emissions from this source are estimated at 240,000 metric tons of carbon per year. Demand for industrial carbon dioxide is expected to increase by 5 percent annually through 1995, implying carbon emissions of 280,000 metric tons of carbon for 1993.
Metallic aluminum is produced by the electrolytic reduction of pure alumina (aluminum oxide, Al2O3). During production, alumina is dissolved in a bath of molten cryolite (3NaF·AlF3) and electrolyzed to form metallic aluminum. Carbon contained in the anodes of the reduction cells is oxidized by the liberated oxygen and released to the atmosphere as carbon dioxide. It is estimated that 1.5 to 2.2 metric tons of carbon dioxide are emitted for each metric ton of aluminum produced. (28) Primary production of aluminum in the United States in 1992 was 4.04 million metric tons, which implies carbon emissions of 1.7 to 2.4 million metric tons (Table 14). The primary source of carbon in the anodes is petroleum coke . Anode manufacture should therefore be counted as a nonfuel use of petroleum coke that does not lead to sequestration.
