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Other Gases: Hydrofluorcarbons, Perfluorocarbons, and Sulfur Hexafluoride
In addition to the three principal gases (carbon dioxide, methane, and nitrous oxide), there are other gases that account for 2 percent of U.S. greenhouse gas emissions when weighted by global warming potential (GWP). These gases are engineered chemicals that do not occur in nature. Although they tend to have very high GWPs, they are emitted in such small quantities that their overall impact is currently small. The Kyoto Protocol has defined three classes of these gases that “count” for emissions estimation: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). This chapter describes emissions sources and gives emissions estimates for HFCs, PFCs, and SF6. As a group, emissions of HFCs, PFCs, and SF6 are rising. In the case of HFCs, the rise in emissions reflects the introduction of HFCs specifically as replacements for CFCs, whose use is being phased out under the Montreal Protocol because they damage the Earth’s ozone layer. CFCs have been widely used as refrigerants, aerosol propellants, and foam blowing agents for many years, but with CFC production virtually ceasing by 1996, HFCs have been introduced into the market to fill the void in many key applications. Emissions of PFCs and perfluoropolyethers (PFPEs) have also been rising in the 1990s (although not as rapidly as HFC emissions), mainly because of the recent commercial introduction of new PFCs and PFPEs both as CFC substitutes and for use in various applications in the semiconductor manufacturing industry. HFCs, PFCs, and SF6 are emitted in small quantities, but they have disproportionate effects because their long atmospheric lifetimes and extreme scarcity in the atmosphere give them extremely large GWPs. SF6 is the most potent of the greenhouse gases, with a GWP of 23,900. PFCs also tend to have particularly high GWPs, falling in the range of 7,000 to 9,000. Among HFCs, HFC-23 is the most potent greenhouse gas, with a GWP of 11,700. Table 30 summarizes U.S. emissions of HFCs, PFCs, and SF6 from 1990 to 1999, and Table 31 shows the corresponding emissions in million metric tons carbon equivalent. Throughout the 1990s, HFC emissions have accounted for roughly one-half of the total carbon-equivalent emissions of HFCs, PFCs, and SF6 combined. The emissions estimates presented in Tables 30 and 31 are taken primarily from the EPA report, Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-1998.56 The 1999 preliminary estimates represent advance estimates developed by EPA and provided to EIA.57 The advance EPA estimates also included some relatively minor revisions to the historical emissions for HFCs, based on more recent runs of EPA’s Vintaging Model. (See discussion on "What's New in This Chapter"). The revisions to the historical estimates are reflected in the emissions estimates presented in this chapter.58
HFCs are compounds containing carbon, hydrogen, and fluorine. They do not destroy ozone. The market for HFCs is expanding as CFCs are being phased out. It is difficult to keep pace with the variety of HFCs that are being developed and the quantities being produced. Consequently, accurate data are difficult to obtain. HFC-23 Although emissions of HFC-23 are relatively small, its high GWP gives it a substantial direct effect. HFC-23 is created as a byproduct in the production of HCFC-22. Small amounts are also used in semiconductor manufacture and as a fire-extinguishing agent. EPA estimates 1998 HFC-23 emissions at 3,420 metric tons.59 Annual emissions dropped by 23 percent between 1992 and 1995 but increased by 49 percent between 1995 and 1998. Consumption of HCFC-22 continues to grow, although at a slower rate than in past years. It continues to dominate the refrigerant market for stationary refrigeration and air conditioning (including chillers, room air conditioners, and dehumidifiers).60 Because of the strong demand, capacity utilization of HCFC-22 production facilities worldwide is estimated at 94 percent.61 The Clinton Administration’s Climate Change Action Plan (CCAP) includes a voluntary program with HCFC-22 producers to reduce HFC-23 emissions, which may help to offset the rising demand for HCFC-22 in the short term. In the longer term, EPA intends to restrict domestic HCFC-22 production by 2010 and to phase out U.S. production entirely, pursuant to U.S. agreements under the Copenhagen Amendments to the Montreal Protocol.62 This will automatically reduce and eventually eliminate HFC-23 emissions from this source. 1,2,2,2-Tetrafluoroethane (HFC-134a) HFC-134a, with a GWP of 1,300, is gaining importance as a replacement for CFCs, especially in automotive air conditioners. Emissions in 1990 were estimated at 560 metric tons, but since then they have grown rapidly to 26,850 metric tons in 1998 (Table 30). In 1993, Ford Motor Company sold nearly 40,000 vehicles that used HFC-134a as the air conditioner coolant. Each vehicle used about 2 pounds of coolant.63 Previous models used about 2.5 pounds of CFC-12. Nearly all 1994 and subsequent model year automobiles use HFC-134a as their air conditioner refrigerant. In addition, HFC-134a conversion packages are now available for older cars. Automobile air conditioners are subject to leakage, with sufficient refrigerant leaking (15 to 30 percent of the charge) over a 5-year period to require servicing. On its Form EIA-1605, General Motors (GM) reported total HFC-134a emissions of about 2,150 metric tons from GM-made vehicles on the road in 1998.64 GM based its estimate on an assumed annual leakage rate of 10 percent per year. With GM vehicles accounting for about one-third of the U.S. light-duty fleet,65 the GM emissions estimate implies that total U.S. HFC-134a emissions from mobile air conditioners were equal to about 6,500 metric tons in 1998. Emissions from this source are expected to continue to increase in the near future, as the replacement of vehicles using CFCs proceeds at a rapid pace. In addition to its use in all new automobiles, an automotive aftermarket for HFC-134a has been developing. Spurred by rising prices for CFC-12, 5 million cars were retrofitted for HFC-134a use in 1997.66 This trend toward retrofitting is expected to continue, given that CFC-12 is no longer produced, remaining inventories are being depleted, and CFC-12 prices are rising.67 Furthermore, many of the air conditioners in mid-1990s models (which were among the first automobiles to use HFC-134a) are now due to be serviced. A spokesperson for Elf Atochem North America estimates the U.S. aftermarket for HFC-134a at 45 to 50 million pounds, or roughly 35 percent of total annual demand. He believes that, as the market for HFC-134a matures, the aftermarket will eventually be about twice the size of the original equipment market.68 The automotive aftermarket is already responsible for much of the growth in current HFC-134a demand.69 HFC-134a is also used as a refrigerant in most new refrigerators built in the United States and in commercial chillers, but leakage from these sources is much less than from automotive air conditioners. Leakage occurs primarily during servicing of the units rather than during normal operation. Short-term uses of HFC-134a, on the other hand, are becoming an important source of emissions. Such uses include aerosols and open-cell foam blowing, which are denoted as short term because most of the HFC-134a used will be emitted to the atmosphere within a short period of time. According to the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS), worldwide sales of HFC-134a for short-term applications jumped almost fourfold between 1994 and 1995. However, sales for short-term uses leveled off at 10,000 metric tons in 1996 and then dropped to 7,400 metric tons (or 7 percent of sales for all uses) in 1997. A further drop, to 6,500 metric tons, occurred in 1998.70 New developments in the U.S. market may, however, reverse this recent downward trend. Beginning in September 1998, U.S. regulations require aerosol manufacturers to use propellants with a maximum volatile organic compound (VOC) level of 45 to 55 percent. HFC-134a is one of only two chemicals that meet this requirement (the other being HFC-152a). Also, in January 1999, the major marketers of tire inflaters began requiring the use of nonflammable material, creating additional demand for HFC-134a. Pennzoil was the first company to enter this new market, after removing its hydrocarbon-based canisters and reconfiguring them to use HFC-134a.71 For many years, the HFC-134a market was characterized by excess capacity and low prices, because the transition away from CFC-12 occurred more slowly than producers had expected.72 In 1998 and 1999, however, the market tightened considerably, as evidenced by a series of price increases. Driven in part by a demand surge triggered by an unusually hot summer in 1999, prices nearly doubled, rising from a low of $1.50 per pound to $2.50 per pound by September 1999. For the rest of 1999 and the first half of 2000, the market stabilized, with only one minor price increase in early 2000. A number of HFC-134a producers are undertaking modest capacity expansion projects, including DuPont, ICI Klea, and Honeywell (formerly AlliedSignal). More significant additions of new capacity are likely to be needed, however, given that capacity is increasing by only 2 to 3 percent per year, while global demand is growing by 10 percent. SRI International predicts that global demand will reach 20 million pounds by 2001; and according to a representative of Elf Atochem, the market will face significant supply shortages unless more investment in new capacity is undertaken over the next several years.73 The required capacity will presumably be built, but it is possible that the expansion in supply will lag behind the growth in demand. Anticipating and planning for demand growth has proven to be a difficult challenge for producers, who must manage as best as possible an unprecedented transition from an established product (CFC-12) that is now under a global ban to a new product (HFC-134a). In the long term, consumption and emissions of HFC-134a are expected to continue rising rapidly, although it is possible that capacity constraints may act as a brake on consumption in the near term. 1,1-Difluoroethane (HFC-152a) As a non-ozone-depleting substance with a GWP of 140, HFC-152a is an attractive potential replacement for CFCs. It can be used as a blowing agent, an ingredient in refrigerant blends (e.g., in R-500), and in fluoropolymer manufacturing applications. There are no HFC-152a emissions associated with the latter application, because the HFC-152a is consumed in the manufacturing process. In 1996, 5 million pounds of HFC-152a was consumed in fluoropolymer manufacturing.74 HFC-152a is also compatible with the components used in aerosol products. Unlike CFCs, however, HFC-152a is flammable. Only one U.S. company (DuPont) produces HFC-152a, using the trade name Dymel-152a. DuPont probably was producing HFC-152a at nearly full capacity in 1994, corresponding to production of about 8,000 metric tons. In 1995 the company reported having doubled its production capacity from 1992 levels, to 15,875 metric tons.75 The company reported 1994 HFC-152a emissions of 180 metric tons on its Form EIA-1605 indicating fugitive emissions during production of 2.3 percent. By 1997, however, DuPont’s reported emissions dropped to only 36 metric tons. Other HFCs Other HFCs with considerable radiative forcing potential include HFC-125 (C2HF5), HFC-143a (C2H3F3), HFC-227ea (C3HF7), and HFC-236fa (C3H2F6), with 100-year GWPs of 2,800, 3,800, 2,900, and 6,300, respectively. The EPA estimates 1998 emissions of HFC-125 at 1,120 metric tons and of HFC-143a at 490 metric tons. Emissions of these HFCs are small but growing rapidly, as they continue to find applications as substitutes for CFCs. HFC-125 is used in the blend R-410A, which is designed to replace HCFC-22 as the refrigerant of choice for stationary refrigeration and air conditioning applications. Some manufacturers have already introduced air conditioners that use R-410A, but as yet the product has captured only 2 percent of the market. As the phaseout of HCFC-22 begins to gain momentum, however, Honeywell expects a rapid increase in the demand for R-410A.76 HFC-236fa is also used as a refrigerant, in particular by the U.S. Navy for shipboard applications.77 Other HFCs and HFC blends are also likely to gain market share as a result of the phaseout, because no single product is suited for all applications. For example, each potential replacement product has an optimal operating temperature range; hence, the refrigerant best suited for use in ice cream freezers will differ from the best choice for milk coolers.78 In addition to replacing HCFC-22 in stationary air conditioning and refrigeration applications, HFCs are expected to gain new markets as foam blowing agents. CFCs have already been phased out of this market, having been replaced by HCFCs (primarily HCFC-141b). However, HCFCs are to be banned as foam blowing agents by 2003 under the Montreal Protocol. Among the potential replacements, HFC-245fa appears to be the strongest contender at present.79 Honeywell is building a world-scale plant in Louisiana for the production of HFC-245fa, which will become fully operational by 2002. Semi-commercial quantities of the product will be available from the plant in 2000. Honeywell is also considering the possibility of building a second HFC-245fa plant overseas, to serve the European and Asian markets,80 and is developing blends that combine HFC-245fa with other materials to enhance its cost/performance ratio. To date, however, the foam blowing industry has failed to signal a clear preference for HFC-245fa or other alternatives. Instead, it continues to rely primarily on HCFC-141b while waiting to see which of the possible replacement candidates emerges as the preferred alternative.81 For some applications, non-fluorochemical alternatives (e.g., hydrocarbons) have been identified.82
Perfluorocarbons are compounds composed of carbon and fluorine. PFC emissions are not regulated, although their high GWPs (6,500 for perfluoromethane and 9,200 for perfluoroethane) have drawn the attention of the CCAP. PFCs are also characterized by long atmospheric lifetimes (up to 50,000 years); hence, unlike HFCs, they are essentially permanent additions to the atmosphere. As byproducts of aluminum production, they arise during discrete periods of process inefficiency. Emissions can be reduced by improving process efficiency. The Voluntary Aluminum Industrial Partnership, aimed at reducing PFC emissions from the aluminum industry, is a CCAP initiative. The principal quantifiable source of PFCs is as a byproduct of aluminum smelting. The EPA estimates U.S. emissions from aluminum production at 1,420 metric tons of perfluoromethane and 120 metric tons of perfluoroethane in 1998. U.S. primary aluminum production has been increasing since 1994, and the trend is expected to continue as the automobile industry expands its use of aluminum.83 Another source of PFC emissions is semiconductor manufacturing. Perfluoromethane and perfluoroethane are used as etchants and cleaning agents in semiconductor manufacturing. The United States consumed an estimated 800 tons of perfluoroethane and perfluoromethane in 1995.84 For 1998, the EPA estimates total emissions of all greenhouse gases from semiconductor manufacturing at 2.1 million metric tons carbon equivalent.85 It is difficult to assess trends in PFC emissions from the semiconductor industry. On the one hand, the continued rapid expansion of the worldwide semiconductor market may lead to increased PFC use and emissions. On the other hand, industry efforts to curb emissions may help to offset these market forces to some extent. A number of semiconductor manufacturing firms have joined an EPA program to reduce PFC emissions voluntarily.86 In 1999, the World Semiconductor Council, comprising manufacturers from Europe, the United States, Japan, Taiwan, and South Korea, voluntarily committed to reduce emissions of PFCs by 10 percent from 1995 levels. In addition, a number of PFC distributors are developing PFC emissions control equipment.87 Abatement and other control options are commercially available, and substitute chemicals that result in reduced emissions are being adopted.88 A variety of other perfluorinated compounds are beginning to be used in the semiconductor industry, including C3F8 (manufactured by 3M), C4F10 (with a GWP of 7,000), C6F14 (with a GWP of 7,400), NF3 (manufactured by Air Products), and CHF3.
SF6 is used as an insulator for circuit breakers, switch gear, and other electrical equipment. In addition, its extremely low atmospheric concentration makes it a useful atmospheric tracer gas for a variety of experimental purposes. It is also a fugitive emission from certain semiconductor manufacturing processes, and it is used as a cover gas during magnesium production and processing, to prevent the violent oxidation of molten magnesium in the presence of air. SF6 has a high GWP of 23,900, but it is not emitted in large quantities. The EPA’s estimates indicate a gradual increase in U.S. emissions between 1990 and 1995, from 1,160 metric tons to 1,570 metric tons. Between 1995 and 1998 emissions held steady at 1,570 metric tons.89 If you would like to receive any information relating to any of our greenhouse gas reports via e-mail, click here and subscribe by entering your e-mail address.
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