Capital requirements and fuel-cycle energy and emissions impacts of potential PNGV fuels. Page: 1 of 6
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smaller than 10 pm (PM10), sulfur oxides (SOX), methane (CH4), nitrous oxide (N20), and carbon
dioxide (CO2). The three greenhouse gases (GHGs) (CH4, N20, and CO2) were weighted by their
global warming potentials to estimate C02-equivalent GHG emissions. Emissions of the five urban
air pollutants were further separated into total emissions and urban emissions to provide a better
indication of human exposure to air pollution that would result from a given combination of 3X fuel
and propulsion system technologies.
The IMPACTT model was used to estimate annual energy consumption and emissions
production by conventional and 3X vehicles by considering vehicle stock and usage and emission
rates from GREET runs. In IMPACTT, age-based tailpipe emissions rates were obtained from
EPA's MOBILE5b and PARTS models for conventional SI and CI engines operating on gasoline
and diesel fuel, respectively. Average operational emissions rates for nonconventional engines and
fuels were estimated on the basis of the assumptions presented below. Although MOBILE5a and
PART5 are under criticism for not predicting emissions accurately, they are still the most widely
used models in the United States.
Emissions standards are an important reason for considering alternative propulsion systems
in the PNGV program. In the United States, new vehicles must meet the federal Tier 1 emissions
standards. Tier 2 standards, requiring a further 50% reduction (beyond Tier 1 standards) in vehicle
emissions for model-year 2004 and beyond, may be adopted in 1999 (vehicles in California must
now meet stricter low-emission vehicle standards). It is generally believed that 3X vehicles will be
subject to Tier 2 standards for VOC, CO, and NOx and the ultra-low-emission vehicle standard for
PM. For this study, we assumed that RFG-fueled SIDI engines would meet Tier 2 standards. All
other SIDI engines (fueled with methanol, ethanol, CNG, LNG, and LPG) were assumed to meet
Tier 2 standards. If an alternative fuel offers inherently lower emissions than RFG, emissions
reductions were assumed for that fuel. We further assumed that 3X CIDI engines fueled with RFD
would at least meet Tier 2 standards for NOX, CO, and VOC and the ultra-low-emission vehicle
standard (i.e., 0.04 g/mi) for PM. As with SIDI alternative fuels, CIDI fuels that offer inherently
lower emissions were assumed to achieve greater reductions than RFD.
Fuel-Cycle Energy and Emissions Estimates
Figures 2-4 display percent changes in urban emissions of VOC, SO,, and PM0 for each of
the propulsion system/fuel combinations examined. Each figure depicts results for a single pollutant
as a series of curves showing annual percentage increases or decreases from the reference scenario
forecast. Curves that are all but indistinguishable are combined to aid interpretation. Note that each
propulsion system/fuel alternative was examined in the context of a scenario that contained a
significant portion of conventional vehicles. Thus, emissions were computed for a combination of
conventional and 3X technologies, so results are less striking than would be the case for 3X
Nitrogen Oxides (NOX). Urban NOX emissions are not shown because of relatively small
differences among the alternatives. The four CIDI fuels (RFD, DME, FT50, and B20) were
assumed to meet equivalent Tier 2 emission standards and thus were essentially equivalent to results
for RFG, methanol, ethanol, LPG, and the gaseous fuel alternatives. Methanol and gasoline fuel
cells offer the largest reduction in urban NOX emissions - 35% in 2030. H2 fuel cells achieve a
somewhat lower NO,, reduction because of their relatively higher upstream emissions.
CO. Reductions in urban CO emissions range from essentially zero to about 35% in 2030;
fuel cells achieve the greatest reductions, and SIDI engines on any of six fuels achieve the lowest.
Given the CI engine's proven record of relatively low CO emissions, it is not surprising that diesel-
like fuels (RFD, DME, FT50, and B20) achieve the second-best CO reduction, approximately 28%
VOCs. Urban VOC emissions reductions range up to approximately 37% (Figure 2). H2 fuel
cells are the clear leader in reducing VOC emissions, methanol fuel cells are a close second and
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Johnson, L.; Mintz, M.; Singh, M.; Stork, K.; Vyas, A. & Wang, M. Capital requirements and fuel-cycle energy and emissions impacts of potential PNGV fuels., article, March 11, 1999; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc623953/m1/1/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.