Energy & Fuels 1993, 7, 27-32
27
Comparative Analysis of the Environmental Impact of Alternative Transportation Fuels Robert F. Klausmeier' Radian Corporation, P.O.Box 201088, Austin, Texas 78720-1088
Dr. Irwin F. Billick Gas Research Institute, 8600 West Bryn Mawr Ave., Chicago, Illinois 60631 Received July 2, 1992. Revised Manuscript Received October 15, 1992
Substituting alternative transportation fuels for gasoline and diesel fuel has been proposed as a method to hasten attainment of the National Ambient Air Quality Standards for ozone and carbon monoxide $0).Light-duty vehicles emit about half of the ozone precursors and almost all of the CO in nonattainment areas and, consequently, have been targeted for additional controls. This paper examines how methanol, compressed natural gas (CNG), and liquified petroleum gas (LPG) will reduce emissions of ozone precursors and carbon monoxide from light-duty vehicles that are currently powered by gasoline. Ozone precursors include nonmethane organic gases (NMOGs) and nitrogen oxides (NO,). Vehicles powered by CNG, LPG, and 100% methanol are likely to emit fewer NMOGs per mile than gasoline-powered vehicles. Vehicles powered by these fuels are likely to emit NO, a t approximatelythe same rate as gasoline-powered vehicles,assuming they have similar emission control devices. As a result, CNG, LPG, and 100% methanol may assist in attainment of the ozone standard if ozone formation is limited by NMOG concentrations in the atmosphere. CNG- and LPG-powered vehicles are likely to emit lower amounts of CO per mile than gasoline-powered vehicles. Thus, they will assist in attainment of the CO air quality standards.
Introduction Emissions from motor vehicles contribute significantly to air pollution problems. Despite new emission standards and advances in motor vehicle emission control technology, many areas in the country are still projected to have air pollution problems in the year 2000 and beyond. This situation has spurred interest in alternative fuels for transportation. Substituting alternative transportation fuels for gasoline and diesel fuel may improve air quality in the United States. The goal of this paper is to analyze the impact of alternative transportation fuels on attainment of the National Ambient Air quality Standards for ozone and carbon monoxide $0). Global warming impacts also are briefly assessed. Although cost and other consumer acceptance factors are not analyzed, all the alternative fuels studied are considered feasible for use by the general public. The term 'alternative fuel" is used throughout this report to mean any non-gasoline or non-diesel fuel. This report concentrates on light-duty applications of alternative fuels, because light-duty vehicles play a much greater role in ozone and CO nonattainment than heavyduty vehicles. The emphasis in this report is on how methanol, compressed natural gas (CNG), and liquified petroleum gas (LPG) compare with gasoline. Reformulated gasoline was not analyzed. The information presented in this report is based on a study for the Gas Research Institute.' ~
~~
(1)Balar, G.F.; Draves, J. A,; Klausmeier, R. F. Radian Corporation.
'Assessment of Environmental, Health, and Safety Issues Related to the Use of Alternative Fuels", February 10, 1992.
0887-0624/93/2507-0027$04.00/0
Impact of Alternative Fuels on Attainment of the NAAQS for Ozone Ozone is formed in the lower atmosphere (troposphere) by photochemical reactions between non-methane organic gases (NMOG) and nitrogen oxides (NO,) and to a lesser extent between NMOGs and carbon monoxide (CO).2 Mobile sources account for about half of the nationwide NMOG and NO, emi~sion.~ NMOGs include non-methane hydrocarbons (NMHCs) and oxygenated volatile organic compounds such as methanol, acetone, ethanol, and formaldehyde. NMOG emissions from gasoline-powered vehicles are composed (2) Seinfeld, J. H. Atmospheric Chemistry and Physics of Air Pollution: John Wiley & Sons: New York, 1986. Manahan, S. E. Enuironmental Chemistry, 3rd ed.; Willard Grant Press: Boston, MA, 1979. (3) National Air Quality and Emissions Trends Report, 1987. USEPA, February 1991. (4) EPAs MOBILE4.1 Emission Factor Model. ( 5 ) Alson, J. A.; Adler, J. M.; Baines, T. M. "Motor Vehicle Emission Characteristics and Air Quality Impacts of Methanol and Compressed Natural Gas". U S . Environmental Protection Agency, Office of Mobile Sources, 1989. (6) Hellman, K. H.; Piotrowski, G. K. "Recent Results from Prototype Vehicle and Emission Control Technology Evaluations Using Methanol Fuel". US. Environmental Protection Agency, Ann Arbor, MI, 1990. (7) Bechtold, R. L.; Miller, M. T.; Hyde, J. D. Ford Methanol FFV Performance/EmissionsExperience in Methanol Fuel Formulations and IN-Use Experiences; Society of Automotive Engineers Inc., 1990. (8) Potential Emissions and Air Quality Effects of Alternative Fuels. Sierra Research, Inc., Sacramento, CA, 1988. (9) California Air Resources Board. Definition of a Low-Emission Motor Vehicle in Compliance with the Mandates of Health and Safety Code Section 39037.05(Assembly Bill 234, Leonard, 1987). May 19,1989. (10) Data Base on Emissions from Methanol Fueled Vehicles. US. Environmental Protection Agency, Ann Arbor, Mi, 1989. (11)Environmental Protection Agency. Analysis of the Economic and Environmental Effects of Compressed Natural Gas as a Vehicle Fuel. Vol. 1 Passenger Cars and Light Trucks, 1990. (12) Bruetach, R. I. US.Environmental Protection Agency, 1990.
0 1993 American Chemical Society
28 Energy & Fuels, Vol. 7, No. 1,1993
""/
\
'\
\
HDDV = Heavy Duty Diesel Vehicle HDGV = Heavy Duty Gasoline Vehicle LDDV = Light Duty Diesel Vehicle LDGV = Light Duty Gasoline Vehicle
Figure 1. Mobile source relative hydrocarbonemissions (1990). Source: MOBILE4.1.'
almost entirely of NMHCs; however, a substantial amount of the NMOG emissions from vehicles powered by methanol and ethanol will be oxygenated compounds. Mobile sources emit NMOGs as either tailpipe or evaporativeemissions. Tailpipeemhiions occur as a result of incompletecombustionand/or chemical reactionsduring combustion. NO, is largely produced by high-temperature reactions between nitrogen and oxygen present in air. Three major factors determine the level of ozone formation: (1)the concentrationof NO, in the atmosphere; (2) the concentration and reactivity of NMOGs in the atmosphere and subsequent reaction products; and (3) ambient temperature and amount of solar radiation. Accurate estimates of the ozone produced as a result of vehicle emissions must take these factors into consideration. The following discussion first compares the mass emission rate of NMOGs for vehicles powered by different alternative fuels. Then an assessment is made of the reactivity in forming ozone of the NMOGs associated with the different fuels. The estimates of mass emission rates are then combined with estimates of ozone reactivity to derive comparative estimates of the ozone-producing potential of vehicles powered by different fuels. An assessment of the NO, emission rates associated with the different fuels is then provided. Mass of NMOG Emitted from Current and Future Conventionally-FueledMotor Vehicles. As shown in Figure 1,in the year 2010,9076 of the NMOG emissions from mobile sources are estimated to be from light-duty gasoline-powered vehicles. Therefore, these vehicles are the target for additional NMOG controls. To study the impact of different fuels on mass emissions, it is necessary to look at evaporative and exhaust NMOG emissions separately, before they are summed to generate total NMOG emission estimates. The United States Environmental Protection Agency (EPA) has developed models to estimate mobile source fleet emission factors. A t the time this paper was prepared, MOBILE4.1 was the most current model. MOBILE4.1 estimates emission rates for each pollutant in terms of grams per mile. Le., grams emitted per vehicle mile travelled (VMT). Emission factors are calculated for different vehicle types; they then are multiplied by
Klausmeier and Billick appropriate weighting factors to develop a composite emission factor for each VMT in an area. Corrected for actual ambient temperatures and vehicle driving characteristics (e.g., averagespeed)they take into consideration several factors, including the following: vehicle emission control technology (i.e., the emission standards that the vehicles were designed to comply with); owner maintenance and tampering habits; altitude; fuel oxygenate content; fuel volatility; and distribution of model years operating on the highway. A t this time, emission factor models have not been developed for alternatively-fueledvehicles. In some cases, estimatesof realistic in-use emissionshave been developed, but a model with the flexibility of MOBILE4.1 is not available for different fuels. As a result, the expected inuse emission levels of alternatively-fueledvehicles is based upon an analysis of existing data on vehicle emission tests for different fuels under FTP conditions. To perform the analysis, a database was developed containing the results of vehicle emission tests with different fuels. The minimum and maximum exhaust emission rates for each fuel in the database are used to determine likely in-use rates. The minimum value was weighted twice as much as the maximum value and a weighted average was determined. This weighting scheme was used to reflect improvements in vehicle technology. In some cases, a modeled in-use emission factor was available for the particular combination of fuel and vehicle technology. If so, this value was used if it exceeded the weighted average. All the light-duty vehicles in the database used similar emission control technologies. All vehicles had three-way catalysts (TWC). All the liquid fueled vehicles used closedloop fuel metering systems that maintain stoichiometric combustion conditions. Most of the gaseousfueled vehicles used mechanical systems that were adjusted to maintain stoichiometricconditions. A few gaseous fueled vehicles were closed-loop controlled. The emission control technologies used on vehicles in the database were appropriate for current Federal and California vehicles. Future Federal and California requirements, such as the Low Emission Vehicle (LEV) program, will force manufacturers to use advanced technologies, such as electricallyheated catalysts. As a result, the emission levels for vehicles fueled by all the fuels will drop in the future. The values presented in this paper for different fuels should be considered on a relative basis; the absolute values may change, but the rankings should remain about the same. Figure 2 summarizes information on NMOG exhaust emissions from light-duty vehiclesburning differentfuels. The figure shows a breakdown of the compounds that are part of the total NMOG emission: NMHC, formaldehyde, and methanol. For reference purposes, two estimates are presented for NMOG exhaust emissionsfrom gasoline-poweredvehicles. The high estimate was generated by MOBILE41 for a fleet composed almost entirely of 1994 and newer automobiles. The low estimate for gasoline-powered vehicles equals the exhaust emission standard for 1994 and newer vehicles. Few data are available on NMOG emissions due to evaporative losses for the different alternative fuels. Consequently, estimates were made based on available
Alternative Transportation Fuels
Energy & Fuels, Vol. 7, No. 1,1993 29
0.8
0.6
t
Figure 2. N M H C exhaust emissions from light-duty vehicles (refs 5-12). Table I. Estimated NMHC Evaporative Emissions 1981 + Light-Duty Vehicles (Summer) emission factor (almi) hot soak running refueling anddiurnal l& loss total fleet LDGV (94+) 0.18 0.16 0.07 0.41 ~~
(MOBILE4) M85” MlOO CNG, Bifuelc CNG, dedicatedd LPG, dedicatedf
~
0.06 0 0.14b 0 0
0.05 0 0.146 0 od
0.05 0 0.02b 0 e
0.16 0 0.30 0 0
0 Source: EPA.13 b Asaumed to be equal t o MOBILE4 emissions with RVP = 8.0 psi. Assumes 80% natural gas, 20% gasoline operation. d Assumes no fugitive emissions. e Small amount of refueling losses. Data not available. f LPG bifuel will be similar to CNG bifuel. LDGV = light-duty gasoline vehicles. 8 Note: No emissionstandardhas been established for running losses, originally assumed to be zero.
Table 11. Evaporative Methanol Emissions. from 1981 and Newer Methanol-Fueled Vehicles fuel M85 MlOO
methanol (almi)
a Hot soak, diurnal, running, and refueling losses. FFV 19941999technology projection (13-EPA 89). Optimized MlOO projection (13-EPA 89).
data and engineeringjudgment. Separate estimates were made for the two main constituents of NMOG NMHC and alcohol. Table I shows estimated evaporative NMHC losses based upon available data and engineeringjudgment. MlOO and dedicated CNG-powered vehicles should have no NMHC evaporative losses. Dedicated LPG vehicles will not have hot soak, diurnal, or running losses, but they will have small but measurable refueling losses. As shown, substantial NMHC evaporativeemissionsare expected from vehicles fueled with M85. Apparently the small portion of gasoline that is mixed with the fuel dominates the vapor space, sometimes resulting in evaporative losses similar to those of vehicles burning 100% gasoline. Table I1 presents estimated methanol evaporative emissions (hot soak, and diurnal losses, running and refueling losses). The value for M85 (0.25 g/mi) is the in-use estimate by EPA.13 The low value for MlOO (0.072 (13)Environmental ProtectionAgency. Analysis of the Economic and Environmental Effects of Methanol as an Automobile Fuel, 1989.
GWine
Gasoline
Mobile4 %+
HCSTD
MB5
MlOO
94+
Figure 3. Estimated total NMOG from lightduty vehicles (exhaust plus evaporative losses with the ambient temperature between 60 and 84 OF). Values are based on either the weighted average emission levels in the database (2 X low value + high value/3), or modeled in-use value, whichever is higher.
g/mi) was EPA’s estimate for an optimized vehicle. The high value was the same as the value for M85. Figure 3 shows estimates of the total exhaust and evaporative reactive NMOG emissions from lightiduty vehicles during periods when the ambient temperature ranges between 60 and 84 OF. Methanol emissions are indicated by the shaded area. Because they have low NMOG emissionsin the exhaust and negligible evaporative emission losses, dedicated CNG vehicles are estimated to emit the smallest amount of NMOG emissions. The total emission values for M85 and MlOO vehicles include methanol. The totals for M85 are greater than the MOBILEC1estimate for gasoline-powered vehicles. Ml00 vehicles are estimated to emit about the same mass of NMOG as gasoline-powered vehicles. Dedicated LPG vehicles are likely to have lower NMOG emissions than gasoline-poweredvehicles. Bifuel LPG and CNG vehicles will emit much greater amounts of NMOGs than dedicated LPG and CNG vehicles because of evaporative NMHC losses. DeterminingOzone FormationPotentialof NMOG Emissions for Different Fuels. Along with the mass of NMOGs emitted, it is also important to consider the reactivityof these emissions. T w o methods are commonly used to determinethe ozone formation potential of NMOG emissions: (1)the OH radical reactivities and (2)the Carter reactivity factors. The OH radical approach is based on the fact that most hydrocarbons react with the hydroxyl radical.14 The rate constants for these reactions have been measured or can be easily calculated. Thus, this method allows the assembly of a reactivity scale for a given alternative fuel based on the amount and type of emissions. The drawbacks of this method lie first in the fact that other reactive pathways which may be available to the hydrocarbons are not considered, and second, the reactivities themselves are based on reactivities of the OH radicals with a pure hydrocarbon, not the mixture usually encountered in airsheds. This method thus underestimates the production of ozone from species which take long periods of time to react. Counteractingthis, however, is that this method (14) See ref 2.
Klausmeier and Billick
30 Energy & Fuels, Vol. 7, No. 1, 1993 5
Table 111. Relative Amount of Ozone Emitted
fuel gasoline
MOBILE 94+ gasoline 94+ HC STD M85 Ml00 bifuel CNG dedicated CNG bifuel LPG dedicated LPG
reactivityb NMOG (g of 03/g total ozone relative emissionso of NMOG) (a of Os/mi) amount 4.012 3.771 1.00 0.940 0.530
4.012
2.126
0.56
1.330 0.730 0.450 0.150 0.530 0.230
3.517 2.437 composnc 1.504 composnc 2.970
4.680 1.780 1.580 0.230 1.980 0.680
1.24 0.47 0.42 0.06 0.53 0.18
0 NMOG emissions are based on those obtained from the database and shown in Figure 3; they include exhaust and evaporative emissions. b Reactivity valuesare basedon NMHC for gasoline, CNG, and LPG and NMOG for M85 and M100. Composition is used to indicate that the dual fuel vehicle evaporative emissions were assumed to be similar to gasoline and the exhaust emissions were a composite of CNG and gasoline.
8
4
t
#byy
M+
CNQ
Table IV. VOC/NO, Ratio Effect on Ozone Formation. ~~~~
overestimates the ozone produced by not compensating for competing reactions. The Carter reactivity factors or maximum incremental reactivities (MIRa) are based on the incremental reactivity of each species emitted.15 The incremental reactivity is an estimate of the amount of ozone formed when a small increment of hydrocarbon is added to a polluted sample. The amount of ozone formed is calculated under various VOC/NO, ratios. The maximum incremental reactivity is then the incremental reactivity where the maximum incremental amount of ozone was produced across the range of VOC/NO, ratios. The method is, however, dependent upon the accurate speciation of the exhaust emissions and knowledge of some of the chemistry involved in ozone formation. The MIRs are currently under consideration for regulatory use for evaluating mobile source emissions. The subsequent analysis in this report is based on the use of MIRs. The estimates of the grams of ozone emitted per gram of NMOG were multiplied by the estimates of the grams NMOG emitted per mile and an overall grams ozone per mile was estimated. Table I11 summarizes the calculation of the grams ozone per mile. The results are shown graphically in Figure 4. As shown, emissions from dedicated CNG vehicles are expected to contribute the least to ozone formation. Dedicated LPG vehicles also appear to have significant advantages over gasoline. Estimated ozone emissions per mile for the other fuels, including bifuel CNG and LPG, appear to be much greater. In fact, M85 appears to have little or no advantage over gasoline. If gasoline is reformulated to obtain a 10-2076 emission reduction, then M85 is likely to be worse than gasoline. NO, Emissions for Different Alternative Fuels. The other precursor component in the atmospheric formation of ozone is oxides of nitrogen (NO,). NO,, specificallyNOS,plays a critical role in the photooxidation of hydrocarbons to form 0z0ne.l~ Current modeling of ambient ozone formation indicates that the amount of ozone formed is dependent upon the NMOG/NO, ratio. Maximum ozone formation is reported (15) Carter, W. P. L.; Lowi, A., Jr. A Method for Evaluation the Atmospheric Ozone Impact of Actual Vehicle Emissions. Presented at the International Congress and Expositions, 1990.
Lw
*m Figure 4. Calculated grams of ozone emitted per mile travelled based on estimated emissions values and MIRs. M+
~
no. of moles of 03 formed plus moles of NO oxidized per carbon atom of comDound low medium high very high 6 10 16 40 0.54 0.031 0.018 0.007 0.22 0.12 0.069 0.019 0.14 0.084 0.027 -0.031 1.65 0.64 0.33 0.14 2.04 0.61 0.39 0.14 2.02 0.62 0.31 0.054 0.082 0.011 4,002 -0.002 0.52 0.04 -0.036 -0.051 1.61 0.32 0.091 -0.025 3.28 0.77 0.32 0.051 1.83 0.55 0.29 0.098 0.27 0.12 0.066 0.029 ~~
compound ethane n-butane n-octane ethylene propylene trans-2-butene benzene toluene m-xylene formaldehyde acetaldehyde methanol OSource: ref 16.
to occur at a VOC/NO, ratio near 8:1.15J6 Table IV presents the amount of ozone produced for different NMOGs at varying NO, ratios as calculated using the carbon 4 chemistry mechanism. These data show that the production of ozone is high for small ratios and is maximum between 6 and 10. At high VOC/NO, ratios the production of ozone diminishes and can become negative. These results indicate that reducing NO, emissions in some airsheds may be detrimental, because it may result in ratios which are favorableto greater production of ozone. Alternatively, in cases with high VOC/NO, ratios, reducing NO, may result in larger oxone reductions than reducing NMOGs. Area-specific studies are generally needed to determine how NO, control would affect the amount of ozone produced. Figure 5 compares estimated emissions from light-duty vehicles burning different alternative fuels with emissions from light-duty gasoline-powered vehicles. Considering that gasoline-powered vehicles can meet much more stringent emission levels than the 0.4 g/mi NO, standard, none of the alternative fuels appears to offer clear advantages in reducing NO, emissions from light-duty vehicles. Emission rates lower than the in-use estimates for gasoline were observed for all the fuels; however, (16)Schleyer,C. H.;Koehl, W. J.ComparisonofGasolineandMethano1 Vehicle Emissions Using VOC Reactivity. Presented at the International Fuels and Lubricants Meeting and Exposition, Tulsa, OK, 1990. (17)DeLuchi, M. A. Greenhouse Gas Emissions from LPG, Gasoline, Diesel Fuel, Methanol, CNG, and Electric Vehicles. Division of Environmental Studies, University of California-Davis, 1990.
Alternative Transportation Fuels
Energy & Fuels, Vol. 7, No. 1, 1993 31
10
RohmncoValuro Rango of Obwwod Vaiwo
-
8
I P S
1,
8 M+
NOxStd
4
Sld 1oQo
Figure 5. Estimates NO, emissions from light-duty vehicles (refs 5, 6, 8-10, 12).
2
2.w 0
Guollno
G880hO
MobllU 94+
94+ CO Std
ME5
MlOO
CNG
LPG
Figure 7. CO emissions from light-duty vehicles (refs 5,6,8-10, 12).
emission rates equal to or higher than the in-use estimates also were observed for most of the fuels. One can conclude that light-duty vehiclescan be designed to bum alternative fuels such as CNG, LPG, or methanol and meet current and future emission standards; but it appears unlikely that large reductions in NO, emissions from gasoline vehicle levels are possible.
provided as a reference. One is the MOBILE4.1 estimate for 1981 and newer vehicles; the other is the CO emission standard for 1981 and newer vehicles. CNG-powered light-duty vehicles appear to have lower CO emissions than vehicles powered by other fuels. Test data indicate that CO emissions from CNG-powered vehicles are likely to be very low in actual use, because CO emission levels are less sensitive to vehicle technology or tampering. In fact, low CO levels are achievable without emission controls. It is possible to run a CNG engine rich (too much fuel) which greatly increases CO emissions, but these cases should be identified in most inspection1 maintenance programs or preventive maintenance checks. LPG vehicle emissions are higher than CNG vehicle emissions but are lower than the CO standard for 1981 and newer light-duty vehicles. When M85 and MlOO vehicles emissions are compared with the CO emission standard, there appears to be no clear advantage for those fuels. The data are not adequate to project a CO emission value comparable to the MOBILE4 estimate for M85- and M100-powered vehicles. Because most of the emission tests were performed on low-mileage, well-maintained vehicles, it is likely that actual in-use emissions for those fuels would be much higher.
Attainment of the NAAQS for Carbon Monoxide
Greenhouse Gases and Global Warming
Exceedances of the NAAQS for carbon monoxide (CO) are less widespread than exceedances of the ozone standard, and there has been a significant downward trend in ambient CO concentrations. However, several tough CO attainment problems remain. Areas with extreme temperature and altitude conditions, such as Alaska and Colorado, are not projected to attain the CO standard without additional controls. About 70% of the nationwide CO inventory is from mobile sources. And most (917%) of the mobile source CO emissions are from light-duty gasoline-powered vehicles (see Figure 6). Note that CO is emitted directly and is not formed by atmospheric reactions. Figure 7 shows the range of CO emissions that were observed for light-duty vehiclespowered by different fuels. Two estimates of gasoline-powered CO emissions are
The emission of greenhouse gases (GHGs) and their potential impact on global warming continue to be an area of concern. In the US.,mobile sources are estimated to emit about 30% of the GHGs.18 The following are estimates of the amount of GHGs emitted for each of the alternative fuels considered. The values presented include emissions over the entire fuel production, transportation, distribution, and use processes. The Greenhouse Effect. The sun heats the earth by short-wave (high energy) radiation which the earth then reradiates to space as long-wave (low energy) radiation. In other words, the earth degrades the higher energy radiation into lower energy radiation which then escapes to space.
LODV(1K)
0LWV(O1K)
i HDDV = Heavy Duty Diesel Vehicle HDGV = Heavy Duty Gasoline Vehicle LDDV = Light Duty Diesel Vehicle LDGV = Light Duty Gasoline Vehicle
Figure 6. Mobile source CO emissions (1988). Source: MOBILE4.4
(18)Fisher, Diane, C. Reducing Greenhouse Gas Emissions With AlternativeTransportation Fuels. EnvironmentalDefense Fund. April 1991.
32 Energy & Fuekr, Vol. 7,No.1, 1993
However, some atmosphericgasesabsorb some of the lower energy radiation and redirect it back toward the earth. This redirected radiation then warms the earth. The GHGs act as a barrier which impedes radiation escape to the upper atmosphere. The gases which are considered to be principal contributors to the greenhouse effect are carbon dioxide (COP), nitrous oxide (NZO), methane (CH4), and water vapor. Ozone and chlorofluorocarbons (CFCs) also may play a role but to a limited extent. Other gases such as carbon monoxide (CO), nitrogen oxides (NO,), and the nonmethane hydrocarbons play an indirect role by changing the concentrations of the other gases. Calculationof the Global Warming Index. Because vehicle emissionsaccount for a substantial fraction of GHG emissions and because C02 was the first GHG implicated, GHG impact is denominated in terms of C02 emitted per mile. This relation, known as the global warming index (GWI), is expressed in terms of equivalent grams of carbon dioxide emitted per mile traveled. The GWI is calculated by multiplying the emissions of the GHGs by weighting factors. Typically, the following equation is used to calculate the GWI17 GWI = CO,
+ aCH4 + bN,O + cC0
where COZ,CH4, N20, and CO are expressed in grams per mile and the parameters a, b, and c are weighting factors. The weighting factor for C02 is 1,while that for methane can vary from 10 to 80, and for N2O from 200 to 400. (No range has been reported for CO.) Greenhouse gases are emitted during the entire production, distribution, and end use processes which a fuel undergoes. To account for all the emissions which are present, each of these categories must be included in an analysis. They are as follows: end use: emissions from the combustion of the final fuel product; distribution: emissions from the delivery of fuel to local retailers; production: emissions from the actual fuel production process; recovery of fuel feedstock emissions from the acquisition of replacement feedstock; transportation of feedstock: emissions from the delivery of new feedstock to the production plants; and assembly and manufacture of materials used in vehicle production: emissions based on the needs of specific vehicle assembly and manufacturing needs. A review of different studies on estimated GHG emissions found that the primary variable affecting the overall estimates was the methane weighting factor (i.e., a factor that is applied to methane emissions to calculate C02 equivalent emissions). Figure 8 shows overall GHG emissions (a combination of fuel production and vehicle use emissions) as a function of the methane weighting factor.18 Natural gas vehicles appear to have the lowest GHG emissions,even at high weighting factors. Obviously, the margin is greater at low weighting factors. Methanolfueled vehicles have lower GHG emissions than gasoline at high weighting factors; they have about the same emissions as gasoline at low weighting factors. The above results assume methanol is produced from natural gas. Producing methanol from coal results in much greater GHG emissions than the other alternative fuels. Similar concerns apply to substitute natural gas.
Klausmeier and Billick
400
'e . 300
*
t
Naunl OU
m
200
loo
1
OL
0
20 30 40 50 60 CH4 Weighting Factor (vs C02)
10
I
70
Figure 8. Estimated GHG emissions-LDV's COz equivalent for fuel production and vehicle use (ref 18).
Conclusions Impact of Alternative Fuels on Attainment of the NAAQS for Ozone. Efforts to attain the NAAQS for ozone would be enhanced if vehicle fleets in nonattainment areas consumed certain alternative fuels instead of gasoline. Dedicated CNG vehiclesappear to have the greatest ozone benefits. LPG and MlOO vehicles also offer significant ozone benefits. However, dual-fuel CNG or LPG vehicles and M85 vehicles (vehicles designed to burn mixtures of 85 ?& methanol and 1594 gasoline) may not be much better than gasoline vehicles. The primary reason for this is that evaporative non-mthane organic gas (NMOG) emissions from storage of gasoline in the vehicle greatly increase their overall contribution to ozone formation. This conclusion assumes that each fuel will displace a similar amount of gasoline. It does not consider consumer acceptance or infrastructure issues that will impact the market penetration of an alternative fuel. Impact of Alternative Fuels on Attainment of the NAAQS for Carbon Monoxide (CO). Both dedicated and dual-fuel CNG vehicles emit much less CO than gasoline-powered vehicles, so their use will help an area attain the CO NAAQS. LPG vehicles also appear to produce lower amounts of CO than gasoline-powered vehicles, but they emit greater amounts than CNGpowered vehicles. Available data are not adequate to project the impact of methanol-fueled vehicles on CO. Preliminary data show that methanol-powered vehicles (bothM85 and M100) will emit much more CO than CNGpowered vehicles. Impact of Alternative Fuels on Global Warming. Global warming has been attributed to the emissions of greenhouse gases including carbon dioxide (COz),methane (CHd), and nitrous oxide (N20). Overall, CNG-powered vehicles are estimated to produce less greenhouse gases per vehicle mile traveled than similar gasoline-powered vehicles. Although they may produce more methane, CNG vehicles emit significantlyless (30%) C02, thereby lowering their total greenhouse gas emissions compared to gasolinepowered vehicles. Vehicles powered by methanol produced from natural gas are estimated to produce less greenhouse gas emissions than gasoline-powered vehicles, but more than CNGpowered vehicles. However, if methanol is produced from coal, the use of methanol vehicles will increase greenhouse gas emissions relative to gasoline.
