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Page 379 12 Alternative Fuels Introduction Current estimates show that automobiles and trucks account for about 45% of the anthropogenic VOCs (volatile organic compounds), 50% of the NOx (oxides of nitrogen) (Figures 12-1, 12-2, 12-3, 12-4), and 90% of the CO (carbon monoxide) in dries where the ozone NAAQS is not met (OTA, 1989). This is by far the largest category of emissions, and recent studies indicate that these estimates could be low for VOCs and CO (Pierson et al., 1990; Lawson et al., 1990b). So, although strategies to improve air quality should consider all sources, it is dear that reductions in automotive emissions are necessary. Some reductions will occur as older, dirtier vehicles are taken off the road and replaced with new, well-controlled vehicles. However, these reductions will be small, and by the year 2004, the increase in vehicle use is expected to lead to an increase in VOC emissions (OTA, 1989). A longer term solution is necessary. One possibility is using alternative fuels. However, the use of alternative fuels alone will not solve the ozone problems of the most severely affected areas. Moreover, it will not necessarily alleviate the most critical problem associated with motor vehicle emissions, which is the increase in emissions that occurs over time with in-use vehicles (see Chapter 9). The central role of VOCs in the formation of tropospheric ozone suggests that changing fuels could be effective by reducing the reactivity of the VOC emissions, the total mass emitted, or both. Gasoline and the exhaust from conventionally fueled vehicles are highly reactive in the atmosphere because
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Page 380 Figure 12-1 Estimated nationwide VOC emissions by source category, by year. Assumes no regulations other than those in place in 1987. The estimates are representative of the emissions on a typical nonattainment day, multiplied by 365 days per year, rather than estimates of true annual emissions. The baseline does not include reductions due to the limit on gasoline volatility of 10.5 psi Reid vapor pressure (RVP). Stationary sources that emit more than 50 tons per year of VOC are included in the "Large" category. Source: OTA, 1989.
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Page 381 they are rich in aromatics and alkenes. Alternatively fueled vehicles, such as those that run on natural gas or methanol, would have less reactive emissions and hence reduce ozone. There is also the possibility of reducing the mass emission rates of VOCs, CO, and NOx when these fuels are used (Austin et al., 1989; Williams et al., 1989). Electric vehicles would run virtually without emissions. As discussed elsewhere in this report, the relative benefits of VOC and NOx controls vary by location, and this phenomenon is critical to determining the benefits (or lack thereof) of alternative fuel use. For example, reducing the reactivity of VOCs can reduce ozone in NOx-rich urban centers, such as downtown Los Angeles and New York, but would likely provide little help in regions with high VOC-to-NOx ratios, such as Houston and Atlanta. The degree to which the different alternative fuels could improve air quality is discussed below. It should be noted that the choice of alternative fuel cannot be made on environmental considerations alone, nor are different locations going to be affected similarly. Economics, politics, energy security and diversity issues, consumer acceptance, technological advances, and the relative prices of fuels will enter into the choice, but such considerations are beyond the scope of this report. Concern over global warming and the need to improve air quality suggest that the use of alternative fuels, and the choice of those fuels, will need to be considered thoroughly and carefully. Fuel Choices A variety of alternative fuels and technologies are or could become available for automotive use, including natural gas; methanol (and methanol blends); ethanol (and ethanol blends), liquid petroleum gas (LPG), including propane; hydrogen; electricity; and reformulated gasoline. Although much interest has focused on use in light-duty vehicles (cars and fight trucks), most of the fuels also can be used in heavy-duty vehicles. Different technologies also exist to use these fuels: the traditional internal combustion engine (using organic fuels and hydrogen) and electric motors coupled with battery storage or fuel cells (running on hydrogen or methanol). Each of the fuels and the associated technologies will have various environmental effects and could be viable at different times in the future. In the near term (0-5 years) the only alternative fuel that can effect ozone concentrations is reformulated gasoline, whose potential is uncertain. No new technology or distribution system is required, although refining capacity for these fuels is limited. Most major oil companies have environmentally improved, reformulated gasolines for sale in limited markets, targeting those urban areas with the most severe air-quality problems. These fuels are formu-
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Page 382 Figure 12-2 VOC emissions in nonattainment cities, by source category, 1985. Stationary sources that emit more than 50 tons per year of VOCs are included in the ''Large'' category. Total emissions, 11 million tons/year. Source: OTA, 1989.
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Page 383 Figure 12-3 NOx emissions an peak concentrations of ozone in non-attainment cities, 1985. Source: Adapted from OTA, 1989. lated to have lower emissions of highly reactive or toxic organic compounds. These fuels are new, and their ability to improve air quality is not dear. Reformulated gasoline is obtained from refined petroleum. As such, it is not considered a true alternative, but it is discussed here because it is viewed as a candidate means for improving air quality. In the middle term (5-20 years), likely candidates to replace gasoline- and diesel-powered vehicles are similar vehicles powered by internal combustion engines that run on methanol, natural gas, or reformulated gasoline. Electric vehicles are another middle-term possibility. It is unclear which fuels and technologies have the greatest potential or will
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Page 384 Figure 12-4 NOx emissions from mobile sources in 1985 as a percentage of total (mobile plus stationary) emissions. LD refers to light duty; HD refuers to heavy duty. Source: OTA, 1989. even be viable for the long term. Reformulated gasoline, methanol, and natural gas would still be available. It is dear that there are ample supplies of both petroleum and natural gas worldwide (Sperling, 1991). Other fuels, in particular hydrogen and electricity, are more attractive from an environmental standpoint. Advances in battery technology could make electric vehicles an attractive alternative to vehicles with internal combustion engines. Bemuse many variables affect which fuel is best, it is important to consider
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Page 385 the attributes of each. Below, the environmental attributes of each fuel are discussed, followed by a summary of the studies that have looked at their effects on air quality. Results of the air-quality studies are then used to estimate the relative effectiveness of each fuel in lowering ozone. For each of the fuels, there are significant uncertainties in the likely environmental benefits. Still, some general conclusions can be developed, and these are discussed at the end of the chapter. Attributes of Alternative Fuels Reformulated Gasoline As the name implies, reformulated gasoline is a refined petroleum product whose composition is similar to gasoline. It can be used in conventional engines with no modification, although achieving the greatest air-quality benefits could require modification of the automotive control system. The composition of the gasoline is altered to make exhaust products less photochemically reactive and toxic and to lower total emissions. This is accomplished by modifying the refining process and by adding oxygenates. The resulting fuel is lower in olefins and aromatics, and has a lower Reid vapor pressure (RVP). (RVP is the constrained vapor pressure of the fuel at 100ºF.) Oxygenates serve two purposes: First, they enhance the fuel's octane rating, which is lowered when the aromatic and olefin content is reduced. Second, they can improve the efficiency of combustion, and the presence of fuel oxygen tends to decrease CO emissions. Commonly used oxygenates include ethers, particularly methyl t-butyl ether (MTBE) and ethyl t-butyl ether (ETBE), and alcohols (methanol and ethanol). The trend is to use the ethers, although ethanol is used extensively in the Midwest, especially to reduce CO emissions in the winter. The addition of alcohols can raise the vapor pressure of the gasoline, increasing evaporative emissions and decreasing the fuels' positive benefits during the summer. Oxygenates also can increase the rate of formaldehyde (HCHO) emissions (Anderson et al., 1989). Oxygenates may comprise a few to more than 15% of the reformulated fuel. Reformulated gasolines have been introduced in limited marketsthose that have severe photochemical smog problems. The potential for using reformulated gasoline to improve air quality is uncertain. First, reformulated gasolines are new, and their compositions could evolve further. Second, it needs to be determined to what degree automobile control systems and fuels can be matched to lower emissions; this would likely require standardization of fuel
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Page 386 composition. Still to be fully explored is the possibility of reducing total mass emissions of VOCs and NOx by using different blends of reformulated gasolines. A major effort, the Auto/Oil Air Quality Improvement Research Program, is under way to help accomplish these goals (Burns et al., 1991). Natural Gas Natural gas is primarily methane (> 90%), with other light hydrocarbons, including ethane, ethene, propane, propene, and butane, as impurities. For use as an automotive fuel, it is compressed and stored at pressures up to 30 megapascals (4500 psi), or liquified. Because liquified natural gas (LNG) requires cryogenic cooling and storage, it has not been used as much as compressed natural gas (CNG). (In this section, reference is made to natural gas vehicles [NGVs.]. Unless stated otherwise, these are vehicles that use either CNG or LNG. For the most part, the advantages and disadvantages of the two fuels are similar.) VOC emissions from NGVs mimic the fuel, and are largely methane (Table 12-1). Given its very low atmospheric reactivity, methane has great potential for reducing ozone formation. However, impurities in natural gas can greatly reduce its benefit. Ethane and propane contribute up to 25 times as much ozone on a mass basis (grams ozone/grams VOC), as does the less reactive methane, and the presence of alkenes would lead to even less benefit (Carter, 1990b). Assuring a low alkene content of the natural gas is crucial to achieving the maximum benefit of NGVs. Also, the products of incomplete combustion (aldehydes) are highly reactive, although the mass emission rates of these species are likely to be small (Alson, 1988; Austin et al., 1989; CARB, 1989a,b). Emissions of CO from NGVs, which are usually operated under lean-bum conditions (i.e., more air is used than is required for complete combustion of the fuel), are generally much less than from gasoline-powered vehicles (Alson, 1988; Austin et al., 1989). Tests consistently show reductions on the order of 90%. In addition to lowering ambient CO, the decrease in CO emissions lowers, slightly, the ozone formation resulting from the vehicles. This is because CO oxidation produces hydroperoxyl (HO2), resulting in increased nitric oxide (NO) oxidation. It is difficult to state how NOx emissions will compare in an optimized NGV engine, and tests show results in both directions (Alson, 1988; Austin et al., 1989). Meeting the NOx emission standard limits the ability to use lean-burn engines in NGVs or methanol-fueled vehicles. Both fuels can be burned very lean, leading to increased fuel economy and lower engine-out VOC, CO, and NOx emissions. (Engine-out emissions are those
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Page 387 TABLE 12-1 VOC Composition of Exhaust and Evaporative Emissions from Gasoline (Indolene) and Alternative Fuelsa,b VOC Indoline Methanolc Ethanol Liquid petroleum gas Compressed natural gasd M85 M100 Alkanes 0.632 0.224 0.023 0.077 0.797 0.170 Alkenes 0.040 0.007 0.001 0.002 0.062 0.031 Formaldehyde 0.021 0.067 0.050 0.010 0.041 0.023 Aldehydes 0.004 0.004 0.001 0.050 0.005 0.005 Ethene 0.031 0.005 0.001 0.034 0.082 0.017 Toluene 0.199 0.032 0.009 0.023 0.007 0.007 Aromatics 0.059 0.023 0.005 0.010 0.003 0.014 Methyl ethyl ketone 0.015 0.005 0.001 0.002 0.003 0.009 Methanol 0 0.633 0.911 0 0 0 Ethanol 0 0 0 0. 791 0 0 aCompositions given in decimal form. bThe tests relied on very few vehicles, and the individual tests are not likely to be representative of the actual fleet of vehicles operating on each fuel. cM85, 85% methanol by volume; M100, 100% methanol. dThe composition of exhaust is primarily methane, and the composition shown reflects that about 70% of the VOC is nonreactive. Methane comprises a much smaller fraction of the exhaust when other fuels are used. Source: CARB, 1989a.
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Page 388 released prior to catalyst reduction by a catalytic converter.) However, the exhaust is so oxygen-rich and CO-lean that the performance of the NOx reduction catalyst is very poor, leading to higher exhaust NOx emissions. Although it could be possible to reduce the rate of emissions to 0.4 g NOx/mile, estimates indicate that California's standard of 0.2g NOx/mile would not be met using a lean-burn engine (Austin et al., 1989). Given the recent findings that further NOx reductions are effective for reducing ozone, using a lean-burn engine fueled by natural gas (or methanol) could be counterproductive in many environments. One advantage of NGVs, from an air-quality perspective, is that the emissions from a "super-emitting" NGV are not as likely to increase to the same levels as those from a super-emitting conventional vehicle. Engine-out emissions of VOCs, CO, and NOx from NGVs are substantially lower than from conventional vehicles; studies indicate that malfunctioning vehicles account for 10% of the conventional fleet and are responsible for 60% of fleet CO emissions (Lawson et al., 1990a). A second inherent advantage is that evaporative emissions from NGVs would be very small (if any) and only slightly reactive. This could be a substantial advantage if it turns out that evaporative emissions are the reason for the discrepancy between the inventories and ambient measurements discussed in Chapter 8. Refueling emissions also would be small. Methanol Methanol and methanol blends have recently been the subject of great interest. Methanol fuel is primarily methanol, although significant amounts (up to 15%) of other compounds are added for safety and performance reasons. Pure methanol is a colorless, toxic liquid with low vapor pressure and high heat of vaporization. Although the latter two attributes can lower emissions when an engine is warm, they also make it difficult to start a vehicle running on pure methanol when cold, and high cold start emissions ensue. Also, the flame from a pure-methanol-based fire is nearly invisible in daylight, which may be a safety problem. Additives raise the vapor pressure, help cold start, and add color to the flame. The most common additive is unleaded gasoline, from 5 to 15% by volume. The resulting blends are designated by an "M" followed by the volume fraction of the methanol. Thus, M85, the most common blend, is 85% methanol by volume, but there is no standard blend. Dedicated methanol-fueled vehicles (MFVs) and flexibly fueled vehicles have been developed. The flexibly fueled vehicles are designed to operate on any combination of gasoline and M85 fuel, and in this way they ease the transition during periods when methanol is not widely available.
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Page 389 On a mass basis, unburned methanol is the largest component of MFV exhaust whether M85 or M100 fuel is used (Table 12-1). The second most abundant species of methanol exhaust is formaldehyde (HCHO). Gasoline-like hydrocarbon components comprise 35-50% of the exhaust of M85-fueled vehicles (Horn and Hoekman, 1989; Snow et al., 1989; Gabele, 1990). HCHO is often cited as the most deleterious species emitted by MFVs because of its toxicity and high photochemical reactivity. Engine-out HCHO emissions are substantial, up to 600 mg/mile, and to derive any air-quality benefits the catalyst system must remove most of that. Tests of current vehicles show that after catalytic reduction, M85 vehicles emit 30-40 mg HCHO/milethree to six times as much as is emitted by a conventional vehicle (Horn and Hoekman, 1989; Gabele, 1990), although amounts as high as 180 rag/mile have been reported for older (pre-1989) cars and trucks (Snow et al., 1989). Recent tests show that M100-fueled vehicles have 25% higher HCHO emissions than do vehicles with similar control systems fueled by M85 (Gabele, 1990). This is attributable to the poor cold start when using M100. During cold start, the catalyst takes about 30 seconds to light off (become hot enough to cause VOC oxidation), and engine-out HCHO is readily emitted during the transient warming. The high HCHO emissions, if not properly controlled, could negate any potential advantage of MFVs. Cold start is responsible for a bulk of the emissions from MFVs, and various technologies are being studied to help solve this problem. One promising direction is a more closely coupled catalyst that is electrically heated before or during start-up. Tailpipe emissions of HCHO of as little as 4 mg/mile have been measured from an M100 vehicle with an electrically heated catalyst (Hellman, et al., 1989). This is a 55% decrease compared to emissions using an unheated catalyst, and this amount of HCHO emissions is comparable to that from a gasoline-fueled vehicle, although it has not been established that these results can be sustained in vehicles with higher mileage (more than 20,000 miles, for example). The resistively heated catalyst also decreases methanol, although NOx emissions increase. Tests consistently show that CO emissions from MFVs are lower than from their conventionally fueled counterparts (Williams et al., 1989; Gabele, 1990), because of the higher oxygen content of the fuel. Reductions of 50% could be realized. This is, however, a direct function of the control system used, and might not be realized in practice. This amount of CO reduction would lead to substantial decreases in ambient CO and slight decreases (0-2%) in ozone. It is not dear whether methanol use would lead to lower NOx emissions. Limited tests on flexibly fueled vehicles show some NOx reductions, but a dedicated MFV would use a higher compression ratio, which could lead to higher NOx emissions. It is likely that the NOx emissions would be catalytical-
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Page 402 had greater mass emission rates. (The CARB [1989a, b] vehicle tests relied on very few vehicles, and the individual tests are not likely to be representative of in-use vehicles optimized to run on ethanol.) The mass-weighted emissions are predicted to increase ozone compared with emissions from conventional vehicles. LPG emissions would lead to 47% as much ozone, and CNG about 25% as much, after accounting for reactively differences and reductions in mass emissions. Evaporative emissions from LPG and CNG vehicles are expected to be small and unreactive. When weighted by the mass emission rates measured, LPG vehicles would contribute about half as much ozone as conventional vehicles, and NGVs would contribute about one-fourth as much. It has been proposed that gasoline using ethanol as a blending agent (gasohol) would be allowed to have an increased vapor pressure, and hence increased evaporative emissions. Given the minor reactivity reduction of ethanol compared with gasoline, the increased evaporative emissions would negate much of the ozone reduction, and actually could lead to increased ozone production. This is similar to the effect resulting from use of low methanol blends in flexibly fueled vehicles (Figure 12-3). Although Carter and co-workers have not specifically considered the effects on production of pollutants other than ozone, the atmospheric chemistry of the emitted compounds provides much of the information needed to assess the likely response of other compounds. Acetaldehyde is the atmospheric oxidation product of ethanol, and it is also relatively abundant in ethanol-fueled vehicle emissions. Acetaldehyde is a precursor to peroxyacetyl radical and PAN formation. Increased PAN levels can be expected, as has been noticed in Brazil (Tanner et al., 1988; Grosjean et al., 1990b). Natural Gas And Lpg NGVs and LPG vehicles emit relatively small amounts of formaldehyde. Given the low reactivity of their emissions, it is likely their use would reduce atmospheric HCHO and PAN concentrations. It also is reasonable to expect slower NO oxidation to NO2 and HNO3, lowering those species' concentrations. Emissions of toxics (such as benzene) also would be substantially less from NGVs and LPG vehicles than from conventional vehicles. Electricity Electric vehicles are the cleanest of the alternative vehicles. Their emissions are essentially displaced to centralized electricity-generating stations.
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Page 403 Utility boilers emit, in comparison to internal combustion engines, very little VOCs and CO. NOx emissions depend on the type of fuel and control technology used. Krupnick et al. (1990) used an airshed model to find the benefits of using 500,000 to 1.5 million electric vehicles in Los Angeles (1.5 million vehicles would be about 17% of the light-duty fleet). Peak ozone reductions of as much as 4.1% were found. This study also showed that using electric vehicles would lead to almost three times the reduction as that from using M85 vehicles. A study by Hempel et al. (1989) found similar results, assuming a much larger portion of electric vehicles in fleet. Of the alternatives, electric vehicles are the only vehicles that, unequivocally, will lead to large NOx reductions in urban areas. Regional effects are less certain and will depend on how electricity is produced. Increased NOx emissions from electricity-generating stations could lead to increased regional ozone that can be transported into cities. The expected change in NOx emissions is an important consideration, given that mobile sources are the dominant source of NOx, and recent studies show that ozone formation in some areas is NOx-limited (Chameides et al., 1988; Milford et al., 1989). Also, concentrations of particulate matter and organic nitrates can be effectively reduced by lowering NOx emissions (Russell et al., 1988b; Milford et al., 1989). Incremental Voc Reactivity A recognized limitation of the studies conducted so far is the uncertainty about the emission composition of exhaust from conventional and methanol-fueled vehicles in the future. The studies discussed above use forecast emissions data or data from limited measurements of prototype methanol-fueled and flexibly fueled vehicles. The limited availability of emissions data has led to an alternative approach to estimating air quality that is applicable not only to methanol but to the other fuels as well. Instead of using a simulated exhaust mixture, the sensitivity of the ozone-forming potential of increased emissions of individual compounds is predicted (Carter and Atkinson, 1989b; Carter, 1991; Russell, 1990). Then the contributions of each emission species, multiplied by the fraction of that species in the exhaust, can be added to find the net emissions reactivity. This allows the flexibility of rapidly estimating impacts of future vehicles as the data become available. This also can guide manufacturers and regulators toward what constitutes a cleaner fuel and how fuels compare. A shortcoming of this approach is that it does not fully account for the nonlinearities in the chemistry of ozone formation. Carter (1991) and Carter and Atkinson (1989b) used an EKMA-type model
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Page 404 with a chemically explicit mechanism to find the incremental reactivity (IR) of individual organic compounds. They define IR as R is a measure of the ozone formation and DORG is the change in the organic gas input. Carter and Atkinson (1989b) defined R as the maximum of the difference between the ozone concentration and NO concentration during the simulation (see also Chapter 5). In essence, this is the local sensitivity of ozone formation to emissions of specific organic gases, and it is given as the maximum number of moles of ozone formed by a one-mole increase of carbon in VOCs. A summary of their calculation results is given in Table 12-3 for some species typically emitted from alternate and conventional vehicles. Note that the incremental reactivities of many compounds are sensitive to VOC/NOx. Some, such as toluene, have a negative sensitivity at high VOC/ NOx because of NOx and radical scavenging by oxidation products. Chang and Rudy (1990) found similar results from a similar study with a less explicit mechanism. An important finding of both studies was that the maximum incremental reactivity of the organics was found at a VOC-to-NOx ratio of about 6. Two aspects of the above studies must be noted. First, they used predominantly single-day simulations, whose results depend on how initial conditions and emissions scheduling are treated. Second, the emissions and other inputs do not correspond to a particular air basin, and ozone responses to changes in VOCs are location specific. Russell et al. (1991; 1992) conducted a similar analysis using multiday airshed calculations for Los Angeles. However, the chemical mechanism employed was more lumped (see Chapter 5) than that used by Carter and Atkinson (1989b). Russell et al. (1991; 1992) calculated local sensitivities to lumped organic classes, methanol, and formaldehyde (Table 12-4), relative to ozone sensitivity to CO emissions. Individual compound sensitivities multiplied by the exhaust compositions given in Table 12-1 yield the relative reactivity of the exhaust. If the exhaust emissions rates also are used, the relative reactivities per mile can be calculated. These measures, for methanol and other fuels, are given in Table 12-5. Comparison of the incremental reactivity method with the model results that look specifically at switching to methanol show general agreement. VOC emissions from M100 would be a little more than half as reactive in terms of ozone formation as would emissions from conventional fuels, and M85 would
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Page 405 TABLE 12-3 Incremental Reactivities of CO and Selected VOCs in Alternative Fuels as a Function of the VOC/NOx Ratio for an Eight-Component VOC Mix and Low-Dilution Conditions, Moles Ozone/Mole Carbon Compound VOC/NOx, ppbC/ppb 4 10 20 Formaldehyde 2.42 0.77 0.24 Ethane 0.024 0.031 0.015 n-Butane 0.10 0.031 0.052 Ethene 0.85 0.64 0.30 Propene 1.28 0.61 0.25 Toluene 0.26 0.04 -0.058 m-Xylene 0.98 0.32 0.012 Methanol 0.12 0.12 0.055 Ethanol 0.18 0.14 0.038 Carbon monoxide 0.011 0.018 0.010 Source: Carter and Atkinson, 1989b be about 70% as reactive. This assumes that the total emissions of VOCs are about the same, that HCHO emissions are not excessive (<30 mg/mile), and that emissions are limited to areas that are not NOx-limited (VOC/NOx < 10). Further air-quality benefits depend on lowering HCHO emissions. Regulatory Implementation of Alternative Fuel Use Because of the considerable contribution of mobile source emissions to
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Page 406 TABLE 12-4 Ozone Peak and Exposure Reactivities of Compounds Relative to Carbon Monoxide. Airshed (Russell, 1990)a Peak Exposure Carter (1989) CO 1 1 1 Aldehydes >C2 64 64 93b Alkanes 16 11 9.5c Alkenes 67 83 51d Aromatics 51 82 53e Ethene 71 67 78 Formaldehyde 119 148 180 Toluene 15 24 25 Methanol 17 14 17 aAirshed model results were combined with the population distribution to find the sensitivities of both the peak ozone and ozone exposure as VOC emission rates are increased. Exposure, in this case, is defined as the ozone concentration multiplied by the population density for ozone concentrations in excess of the NAAQS concentration of 120 ppb. bused incremental reactivity for acetaldehyde. cUsed 50% C4-C5 alkanes, and 50% C6 + alkanes from Carter (1989) dUsed equal portions of C4-C5 and C6 alkenes from Carter (1989) eUsed equal portions of di- and tri-alkylbenzenes from Carter (1989) California's air-quality problems, controlling these emissions has been a key aspect of the overall air pollution strategy of the California Air Resources Board (CARB). In recent years, CARB has adopted various measures to reduce mobile source emissions through better control of in-use emissions and more stringent emission standards. This process was furthered in September, 1990 with CARB's approval of the "low-emission vehicles and dean fuels" regulations (Resolution 90-58), which were approved by the California Office of Administrative Law on August 30, 1991. The low-emission vehicles and clean fuels regulations, an integral part of
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Page 407 TABLE 12-5 Relative Reactivities of Emissions from Gasoline and Alternative Fuels Fuel Cartera (1991) Dunkerc(1990) Russell (1990)/Williams et al. (1989)b Reactivity/gram (exhaust + evap)c Reactivity/gram (exhaust + evap) nonmethane VOCc Reactivity/mile Total emissionsc Gasoline(indolene) 1 1 1d 1d E95f 0.84 2.0 E85g 0.81 2.1 M85h 0.73 0.63 0.93 0.73 (0.55)e Methanol 0.54 0.97 0.64 0.58 (0.48)e Liquid propane gas 0.83 0.47 Compressed natural gas 0.44 0.24 aCarter (1991) used measurements conducted by CARB (1989a, b) to derive fuel-based reactivities;bThe measurements by Williams et al. (1989) are used for a flexibly fueled vehicle fueled on M85 and M100. The relative reactivities of the emissions components are taken from Russell. Dunker (1990) used the same emissions profiles and modeled the exact composition, but did not use relative reactivities.;cReactivities are relative to gasoline (or indolene) being 1. Indolene is commonly used as a test fuel;dCornposition profile developed by Sigsby et al. (1987);eThe first value is for exhaust formaldehyde emissions as measured; the second, in parentheses, is for a low-formaldehyde (5 mg/mile)-emitting vehicle;fE95, 95% ethanol and 5% gasoline;gE85, 85% ethanol and 15% gasoline;hM85, 85% methanol and 15% gasoline.
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Page 408 CARB's Long-Range Motor Vehicle Plan, established stringent, reactivity-based exhaust emission standards for new passenger cars, light-duty trucks and medium-duty vehicles and required that any dean alternative fuels needed by these vehicles be made available to the public. Under the low-emission vehicle regulations, four categories of low-emission vehicles, each certified to a particular set of exhaust emission standards, will be phased in during the mid-1990s. In order of increasing stringency, the vehicle categories are • Transitional Low-Emission Vehicle ("TLEV") • Low-Emission Vehicle ("LEV") • Ultra-Low-Emission Vehicle ("ULEV") • Zero-Emission Vehicle ("ZEV") The standards for all four categories of low-emission vehicles represent significant reductions in emissions compared to previous standards. Table 12-6 summarizes the 50,000-mile certification standards for passenger cars and small light-duty trucks. The regulations also promulgated emission standards for light-duty trucks above 3750 pounds loaded vehicle weight (LVW) and for medium-duty vehicles and engines. A regulatory problem that has plagued alternative fuels and reformulated gasoline has been how to treat all fuels equally, without apparent bias that is unwarranted on an air-quality basis. One strategy to account for the lower ozone-forming potential of alternative fuels, yet allow for all fuels and technologies to compete on an equal regulatory basis, is to use a "reactivity-adjusted" emission standard. CARB (1990) has adopted regulations that use the incremental reactivity of each compound in automobile exhaust (see Table 5-5) to calculate a Reactivity Adjustment Factor (RAF). The RAF is the reactivity of the alternative fuel exhaust compared to that of the baseline fuel. If the RAF is less than 1, proportionally more mass can be emitted, such that the total ozone-forming potential is equivalent to that from the baseline fuel. For example, if the RAF of the alternative fuel is 0.5, then 2 g of alternative fuel emissions would have the same ozone-forming potential as 1 g of baseline fuel emissions. At present, the method for calculating the reactivities of individual compounds involves using an EKMA-type box model, exercised for about 75 trajectories representing different days in different cities (Carter, 1991). The simulations are, in effect, less than one day long, which may cause the results not to reflect actual airshed behavior. As expressed elsewhere in this report, it is strongly suggested that multiday simulations using grid-based models be used for control strategy analysis. Application of a grid-based model over a three-day period found good agreement with the box model results, but the
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Page 409 TABLE 12-6 California's 50,000 Mile Certification Standards for Passenger Cars and Light-Duty Trucks < 3750 lb.. Loaded Vechicle Weight (g/mi). Category NMOGa COb NOxc HCHOd Conventional 0.25e 3.4 0.4 0.015f Transitional Low Emission Vehicle 0.125 3.4 0.4 0.015 Low- Emission Vehicle 0.075 3.4 0.2 0.015 Ultra-Low-Emission Vehicle 0.040 1.7 0.2 0.008 Zero-Emission Vehicle zero zero zero zero aNMOG: Non-methane organic gases bCO: Carbon monoxide cNOx: Oxides of nitrogen dHCHO: Formaldehyde eStandard is for non-methane hydrocarbons fApplies to methanol vehicles only> less reactive compounds were relatively more reactive over the three days in the grid-based model than in the box model (Russell et al., 1991a, b). The box model's simulations of less than a day do not fully account for the multiday buildup and carryover of the less reactive compounds. While there are differences in the results of the two models, the studies to date support the use of reactivity scaling. However, there are important areas for further investigation, such as the more widespread use of advanced models and mechanisms, studies of the impact of different meteorological conditions and locations, and the development of uncertainty estimates. Summary Alternative fuels have the potential to improve air quality, especially in
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Page 410 some urban areas, by reducing concentrations of the precursors to ozone and other photochemical oxidants. However, alternative fuels alone will not solve the air quality problems experienced by major cities. They could be an effective addition to ozone abatement strategies, but they must be considered in combination with other possible controls. Also, there are significant uncertainties left to be resolved, many of which stem from the inability to predict what the emissions will be from alternatively fueled vehicles in the future. Also, it must be stressed that the use of alternative fuels other than electricity and natural gas may have little effect on the increase in emissions that occurs over time with in-use vehicles, particularly its "super-emitters." It is this emissions deterioration that is the most central aspect of the motor vehicle emissions problem (Chapter 9). Alternatively fueled vehicles could reduce mass emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) and could decrease the atmospheric reactivity of the emissions (by using methanol or natural gas). There are some regions where alternative fuels could work effectively and others where little benefit would result. Furthermore, the effectiveness of these fuels could vary within a single airshed, depending on the VOC/NOx ratio. Their use must be considered for each location separately. The expected air quality benefits from each of the choices are not equal. Estimates of the potential benefits can be derived from emissions composition and from recent studies on compound reactivities (or ozone-forming potential). The studies to date have been limited to a few cities under a few conditions, so increased study is warranted. Electric vehicles would give the greatest improvement in air quality by virtually eliminating VOCs, CO, and NOx (net NOx emission changes would depend on the source). They would lead to ozone reductions in virtually any region. The fossil fuel alternatives and reformulated gasolines should lead to ozone reductions in areas with low VOC/NOx, such as downtown Los Angeles and New York City. However, their effects in areas with high VOC/NOx (such as Houston, Atlanta, or regions downwind from urban centers) would be minimal, unless NOx emissions are also reduced. The benefits in intermediate regions must be explored further. Natural gas is the cleanest of the fossil fuels, and natural gas vehicles appear to have the lowest reactivity VOC emissions. Vehicles in areas that have carbon monoxide (CO) problems would especially benefit from use of natural gas. However, those vehicles are usually designed to burn fuel-lean, and might not be able to meet proposed NOx standards. If cold-start problems are overcome, burning essentially pure methanol gives substantial reductions in exhaust and evaporative emission reactivity, although the air quality benefits are degraded if M85 (or lower percent methanol) fuel is
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Page 411 used or if formaldehyde emissions are not effectively controlled. Flexibly fueled vehicles could provide an easy transition mechanism to extensive fleets of methanol-fueled vehicles, but they will not offer the air quality benefits of dedicated vehicles. Increased mass emissions from flexibly fueled vehicles using low-methanol-blend fuels could counter the decreased reactivity of those emissions. Also, formaldehyde emissions must be controlled over the life of the vehicle. Reformulated gasolines offer the easiest transition to a cleaner fuel and studies in progress indicate that properly reformulated gasolines can meet or surpass reductions in the emission reactivity of methanol-gasoline blends (e.g. M85). Ethanol would provide less benefit and might even be detrimental (Table 12-5). Recent studies indicate that mobile-source emissions have been seriously underestimated (Chapter 9). Also, if the excess emissions are from ''super-emitters,'' this could affect the choice of alternative fuels used. Not enough is known to assess this issue in detail, although these findings could support the use of naturally cleaner fuels, such as electricity and natural gas. On the other hand, if the inventories have underestimated the VOC emissions from stationary and biogenic sources, the benefits of using alternative fuels would be less than currently predicted. Although the degree to which alternatively fueled, vehicles can improve air quality is not known, the use of alternative fuels can become part of an effective ozone control strategy. Certain applications now exist where these fuels can be used effectively. Heavy-duty-vehicle use of methanol or natural gas appears promising. It is not clear which choice is the best for light-duty vehicles, and fthat decision depends on location and goals.
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Representative terms from entire chapter: