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9
Other Alternative Fuel Options
This report has focused so far on the major part of the panel’s statement of task,
namely, liquid fuels for transportation that use biomass and coal as feedstocks. To
address the requirement of the statement of task regarding competitive fuels, the panel
reviewed other potential fuels that could be available over the next 25 years. This final
chapter briefly discusses other fuel technologies and their advantages and disadvantages.
Compressed natural gas (CNG) is reviewed first, and then liquid fuels that can be
produced from syngas, including gas-to-liquid (GTL) diesel, dimethyl ether, and
methanol. The chapter discusses technology implications of using hydrogen in fuel-cell-
powered vehicles.
Chapter 4 discussed how coal or coal and biomass gasification produces syngas,
which can be converted to diesel and gasoline or to methanol, which can be converted to
gasoline. Syngas can also be produced by reforming natural gas. Only if large supplies of
inexpensive domestic natural gas were available—for example, from natural-gas
hydrates—would the United States be likely to use natural gas as feedstock for
transportation-fuel production. Chapter 4 discussed how methanol can be produced from
coal synthesis gas, but the panel believes that the best approach is to convert synthesis
gas to methanol and use methanol-to-gasoline technology to produce gasoline, which fits
directly into the existing U.S. fuel-delivery infrastructure. Hydrogen has the potential to
reduce U.S. greenhouse-gas emissions and oil use, as discussed in two recent National
Research Council reports, Transitions to Alternative Transportation Technologies--A
Focus on Hydrogen (NRC, 2008) and The Hydrogen Economy: Opportunities, Costs,
Barriers, and R&D Needs (NRC, 2004); but it is a long-term option.
COMPRESSED NATURAL GAS
In 2007, the main U.S. uses for natural gas were apportioned as follows: electric-
power generation, 30 percent; industrial use, 29 percent; residential use, 20 percent; and
commercial use, 13 percent (EIA, 2008). Only 0.11 percent was used as fuel in
transportation vehicles. It is the primary feedstock for fertilizers and petrochemicals. The
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cleanest and most efficient hydrocarbon fuel, natural gas is environmentally superior to
coal for electric-power generation, and for similar reasons it could be a sound choice for
transportation fuels.
The chapter on fossil fuel of the National Research Council report America’s
Energy Future: Opportunities, Risks, and Tradeoffs (NRC, forthcoming) indicates
estimates U.S. natural gas resources. Current natural-gas consumption needs are met
mainly by domestic production. A switch to natural gas for a large segment of U.S.
transportation use would probably trigger increased importation of natural gas or fuels
produced from natural gas.
Technologies for producing transportation fuels from natural gas are ready for
deployment by 2020. If natural gas were used for transportation instead of for electricity,
there would be a potential to supply roughly one-fifth to one-fourth of transportation
needs from North American natural-gas reserves, but only with investment in distribution
infrastructure. Supplying more would require importing natural gas.
Compressed natural gas (CNG) fuels natural-gas vehicles (NGVs). Natural gas is
not a liquid fuel and it must be compressed to supply sufficient fuel for a vehicle. In
2008, there were over 150,000 NGVs and 1,500 NGV fueling stations in the United
States. Natural gas is sold in gallons of gasoline equivalent (gge); a gallon of gasoline
equivalent has the same energy content (124,800 BTU) as a gallon of gasoline. NGVs are
more expensive than hybrid or gasoline vehicles. The Civic GX NGV has a
manufacturer’s suggested retail price of $24,590 compared with $22,600 for the hybrid
sedan and $15,010 for the regular sedan (Rock, 2008).
Of all the fossil fuels, natural gas produces the least carbon dioxide (CO2) when
burned because it contains the lowest carbon:hydrogen ratio. It also releases smaller
amounts of criteria air pollutants. NGVs emit unburned methane (which has a higher
greenhouse forcing potential than CO2), but this may be compensated for by the
substantial reduction in CO2 emission. Dedicated NGVs emit less carbon monoxide (CO),
nonmethane organic gas, nitrogen oxides (NOx), and CO2 than gasoline vehicles.
Natural-gas engines are more fuel-efficient than gasoline engines. The main
benefit of CNG in the past was its low price (about 80 percent that of gasoline on the gge
basis). Transport and distribution of natural gas are relatively inexpensive because the
infrastructure for industrial and household use already exists (Yborra, 2006).
Despite a possibly advantageous fuel-supply situation, NGVs still have a lot of
hurdles to overcome. The two main challenges faced by NGVs are insufficient refueling
stations and inconvenient on-board CNG tanks that take up most of the trunk space. An
NGV market can be analyzed by using the vehicle-to-refueling-station index, defined as
the ratio of the number of NGVs (in thousands) to the number of natural-gas refueling
stations. According to Yeh (Yeh, 2007) “using techniques including consumer preference
surveys and travel time/distance simulations, it has been found out that the sustainable
growth of alternative fuel vehicles . . . during the transition from initial market
development to a mature market requires the number of alternative-fuel refueling stations
be a minimum of 10 to 20 percent of the number available for conventional gasoline
stations.” A thriving NGV market tends to have an index of 1; this gives rise to a
problem: new stations are not being opened, because of the lack of users, but few people
use NGVs, because of the lack of refueling stations.
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A key disadvantage of NGVs is their low range. The average range of a gasoline
or diesel vehicle is 400 miles, and the range of an NGV is only 100-150 miles, depending
on the natural-gas compression. Because of the dearth of natural-gas refueling stations,
the current prevalent choice is to use a bifuel NGV that can run on both natural gas and
gasoline. The problems associated with bifuel engines include slightly less acceleration
and about 10 percent less power than a dedicated NGV because bifuel engines are not
optimized to work on natural gas. Furthermore, warranties on new gasoline vehicles are
severely reduced if they are converted to bifuel NGVs. The most important barriers for
NGVs might be a public perception that CNG is a dangerous explosive to have on one’s
vehicle and a perception that self-service refueling with a high-pressure gas is too risky to
offer to the general public.
About 22 percent of all new public-transit bus orders are for NGVs. Buses and
corporate-fleet cars that stay in town have been the main market for NGVs, and both uses
are mainly in response to the Clean-Fuel Fleet Program set up by the Environmental
Protection Agency to reduce air pollution.
ALTERNATIVE DIESEL
Syngas-production technology has been discussed in the context of coal-to-liquid
(CTL) fuels. The GTL process for producing diesel is similar to the indirect liquefaction
of coal. Instead of producing syngas via gasification of coal or coal and biomass, the
syngas is produced by steam reforming of natural gas. As with CTL, synthesis gas can be
converted to a distillate and wax with a catalytic modification of the Fischer-Tropsch
process1 discussed in Chapter 4. The distillate and wax are hydrocracked to produce high-
quality diesel and naphtha, and other streams that form the basis of such specialty
products as synthetic lubricants. Although it is technically difficult, the naphtha can also
be upgraded to gasoline.
Naphtha is an ideal feedstock for the manufacture of chemical building blocks
(for example, ethylene), and GTL diesel is a high-quality automotive fuel or blending
stock (Johnson-Matthey, 2006). GTL is an option for producing diesel from “stranded”
natural gas like that which exits in the Middle East and Russia. However, a couple of
those plants would begin to swamp the chemical naphtha market with material.
Hypothetically, converting natural gas to GTL diesel has several advantages over
the use of CNG. All diesel vehicles can run on GTL diesel, and this gives gas producers
access to new market opportunities. The range of diesel vehicles is much higher than that
of NGVs because of diesel’s higher fuel density. Engine efficiency and performance are
not compromised by the adjustment for GTL diesel. GTL diesel can be shipped in normal
tankers and unloaded at ordinary ports (The Economist, 2006).
There are several commercial GTL plants, including those of Sasol in Nigeria and
Qatar and Shell in Malaysia and Qatar that produce GTL diesel; and a number of
companies, including World GTL and ConocoPhillips, have plans to build GTL plants in
the next several years. The economics of GTL plants are closely tied to the price of
1
Most evaluations of CTL assume the use of iron-based Fischer-Tropsch catalysts largely because of
impurities. GTL typically uses rhodium-based catalysts that do not have the poor selectivity of the iron-
based catalysts and do not produce olefinic stocks.
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natural gas, and their viability depends on inexpensive stranded gas. GTL diesel is
viewed mainly as an alternative to liquefied natural gas for monetizing associated natural
gas or large natural-gas accumulations like the one in Qatar. The high cost of producing
GTL makes it unlikely that GTL processes will be developed in the United States unless
an abundant and inexpensive source of natural gas, such as natural-gas hydrates, is found.
METHANOL
Methanol, an alcohol, is a liquid that can be used in internal-combustion engines
to power vehicles. During the late 1980s, it was seen as a route to diversifying the fuels
for the U.S. transportation system by converting natural gas from remote fields around
the world to methanol and transporting the methanol to the United States to be used in the
transportation system. That strategy was seen by energy planners as a way to convert
what was cheap remote natural gas (around $1.00 per thousand cubic feet) to a
marketable product. Today, it is used mainly as a commodity chemical and is produced
primarily from natural gas.
Methanol has a higher octane rating than gasoline and is therefore a suitable neat
fuel (that is, 100 percent methanol) for internal-combustion engines (for example, in
racing cars). In practical terms, the penetration of methanol into a transportation system
for light-duty vehicles that are fueled mainly by gasoline would require the construction
of a distribution system and the use of flexible-fuel vehicles that could run on a mixture
of gasoline and methanol. The use of a mixture of 85 percent methanol and gasoline
(M85) would avoid the cold-start problem caused by methanol’s low volatility, but
methanol has about half the energy density of gasoline, and this affects the range that a
vehicle can achieve on a full tank of fuel. Other drawbacks of methanol include its
corrosivity, hydrophilicity, and toxicity. Methanol can cause various harmful effects to
human health, including blindness and death if ingested, absorbed through the skin, or
inhaled (Fisher Scientific, 2008). It would present substantial environmental, safety,
health, and liability issues for station owners if it were introduced on a wide scale. One
means of avoiding the infrastructure would be to convert the methanol to gasoline.
DIMETHYL ETHER
Dimethyl ether (DME) is a liquid fuel, at low pressure, with properties similar to
those of liquefied petroleum gas. When burned, it produces less CO and CO2 than
gasoline and diesel because of its lower carbon:hydrogen ratio. DME contains oxygen, so
it requires a lower air:fuel ratio than gasoline and diesel. DME has a higher thermal
efficiency than diesel fuel,3 so it could enable higher-efficiency engine design. The
presence of oxygen in the structure of DME minimizes soot formation (Arcoumanis et
al., 2008). Other exhaust emissions—such as unburned hydrocarbons, NOx, and
particulate matter—are also reduced. The California Air Resources Board emission
standards for automotive fuel are surpassed by DME; it is an ultraclean fuel.
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At present, the preferred route and most cost-effective method for producing
DME is through the dehydrogenation of methanol from synthesis gas, which is a mixture
of CO and hydrogen. The basic steps for producing DME are as follows:
1. Syngas production by steam reforming of natural gas or by partial oxidation of
coal, oil residue, biomass, or a combination of those.
2. Methanol synthesis with the use of copper-based or zinc oxide catalysts.
3. Methanol dehydrogenation to DME with the use of a zeolite-based catalyst.
The DME fuel produced is unsuitable for spark-ignition engines because of its
high cetane number; but it can fuel a diesel engine with little modification. DME has
properties similar to those of GTL diesel, including (Yao et al., 2006; Arcoumanis et al.,
2008; Kim et al., 2008) good cold-flow properties, low sulfate content, and low
combustion noise.
The principal advantage of using DME as an automotive fuel is that it is a clean-
burning fuel that is easy to handle and store. It has thermal efficiency and ignitability
similar to those of conventional diesel. As in the case of other potential alternative fuels,
the primary challenge to the use of DME as an automotive fuel is the need for an
infrastructure for its distribution. Disadvantages of using DME include low viscosity,
poor lubricity, a propensity to swell rubber and cause leaks, and a heating value lower
than that of conventional diesel.
HYDROGEN
Hydrogen, like electricity, is an energy carrier that can be generated from a wide
variety of sources, including nuclear energy, renewable energy, and fossil fuels.
Hydrogen also can be made from water via the process of electrolysis, although this
appears to be more expensive than reforming natural gas. Used in vehicles, both
hydrogen and electricity make efficient use of energy compared with liquid-fuel options
on a well-to-wheel basis. As generally envisioned, hydrogen would generate electricity in
a fuel cell, and the vehicle would be powered by an electric motor2. Developments in
battery technology that may make plug-in hybrid electric and all-electric vehicles feasible
will be discussed in several forthcoming National Research Council reports.
Hydrogen fuel-cell vehicle (HFCV) technology has progressed rapidly over the
last several years, and large numbers of such vehicles could be introduced by 2015.
Current HFCVs are very expensive because they are largely hand-built. For example, in
2008, Honda released a small number of HFCVs named FCX Clarity which cost several
hundred thousands of dollars to produce (Fackler, 2008). However, technological
improvements and economies of scale brought about by mass production should greatly
reduce costs.
2
Hydrogen also can be burned in an internal combustion engine (ICE), but the overall efficiency is much
lower than a combination of fuel cells and a motor. It would be difficult to store enough hydrogen on-board
to give an all-hydrogen ICE vehicle an acceptable range. The BMW hydrogen ICE also can use gasoline.
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This section provides a synopsis of the National Research Council report
Transitions to Alternative Transportation Technologies—A Focus on Hydrogen (NRC,
2008), by the Committee on Assessment of Resource Needs for Fuel Cell and Hydrogen
Technologies. The committee concluded that the maximum practical number of HFCVs
that could be operating in 2020 would be about 2 million, among 280 million light-duty
vehicles in the United States. By about 2023, as costs of the vehicles and hydrogen drop,
HFCVs could become competitive on a life-cycle basis. Their number could grow rapidly
thereafter to about 25 million by 2030, and by 2050 they could account for more than 80
percent of new vehicles entering the U.S. light-duty vehicle market. Those numbers are
not predictions by that committee but rather a scenario based on an estimate of the
maximum penetration rate if it is assumed that technical goals are met, that consumers
readily accept HFCVs, and that policy instruments are in place to drive the introduction
of hydrogen fuel and HFCVs through the market transition period.
The scenario would require that automobile manufacturers increase production of
HFCVs even while they cost much more than conventional vehicles and that investments
be made to build and operate hydrogen fueling stations even while the market for
hydrogen is very small. Substantial government actions and assistance would be needed
to support such a transition to HFCVs by 2020 even with continued technical progress in
fuel-cell and hydrogen-production technologies.
A large per-vehicle subsidy would be needed in the early years of the transition,
but the number of vehicles per year would be low (Table 9-1) (NRC, 2008). Subsidies per
vehicle would decline with fuel-cell costs, which are expected to drop rapidly with
improved technology and economies of scale. By about 2025, an HFCV would cost only
slightly more than an equivalent gasoline vehicle. Annual expenditures to support the
commercial introduction of HFCVs would increase from about $3 billion in 2015 to $8
billion in 2023, at which point over 1 million HFCVs could be joining the U.S. fleet
annually. The cost of hydrogen also would drop rapidly, and, because the HFCV would
be more efficient, it would cost less per mile to drive it than to drive the gasoline vehicle
in about 2020. Combining vehicle and driving costs suggests that the HFCV would have
lower life-cycle costs starting in about 2023. After that, there would be a net payoff to the
country, which cumulatively would balance the prior subsidies by about 2028.
Substantial and sustained research and development (R&D) programs will be
required to reduce the costs and improve the durability of fuel cells, develop new on-
board hydrogen-storage technologies, and reduce hydrogen production costs. R&D
investments are shown in Table 9-1. These programs should continue after 2023 to
reduce costs and improve performance further, but the committee did not estimate that
funding.
The National Research Council study determined the consequent reductions in
U.S. oil consumption and greenhouse-gas emissions that could be expected in this
scenario. HFCVs can yield large and sustained reductions in U.S. oil consumption and
greenhouse-gas emissions, but several decades will be needed to realize those potential
long-term benefits. Figure 9-1 compares the oil consumption that would be required in
this scenario with a reference case based on Energy Information Administration high oil-
price projections, which include the recent increases in corporate average fuel economy
standards. By 2050, HFCVs could reduce oil consumption by two thirds. Greenhouse-gas
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emissions would follow a similar trajectory if hydrogen produced from coal in large
central stations were accompanied by carbon separation and sequestration.
TABLE 9-1 Costs of Implementing Hydrogen Fuel-Cell Vehicles
By 2023 (breakeven year), the government will have spent about $55 billion:
$40 billion for the incremental cost of HFCVs.
$8 billion for the initial deployment of hydrogen-supply infrastructure.
$5 billion for research and development.
There will be 5.6 million HFCVs operating.
By 2050,
There will be more than 200 million HFCVs operating.
There will be 180,000 hydrogen stations.
There will be 210 central hydrogen-production plants.
There will be 80,000 miles of pipeline.
Industry will have profitably spent about $400 billion on hydrogen infrastructure.
The study then compared those reductions with the potential impact of alternative
vehicle technologies (including conventional hybrid-electric vehicles) and biofuels oil
consumption and greenhouse-gas emissions. Over the next 2 decades, those approaches
could deliver much greater reductions in U.S. oil use and greenhouse-gas emissions than
could HFCVs, but hydrogen offers greater longer-term potential. Thus, the greatest
benefits will come from a portfolio of research and development in technologies that
would allow the United States to nearly eliminate oil use in light-duty vehicles by 2050
(Figure 9-2). Achieving that goal would require substantial new energy-security and
environmental-policy actions in addition to technological developments. Broad policies
aimed at reducing oil use and greenhouse-gas emissions will be useful, but they are
unlikely to be adequate to facilitate the rapid introduction of HFCVs.
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REFERENCES
Arcoumanis, C., C. Bae, R. Crookes, and E. Kinoshita. 2008. The potential of di-methyl
ether (DME) as an alternative fuel for compression-ignition engines: A review.
Fuel 87:1014-1030.
Fackler, M. 2008. Latest Honda Runs on Hydrogen, Not Petroleum. New York Times,
June 17, 2008. Accessed on August 28, 2008 at
http://www.nytimes.com/2008/06/17/business/worldbusiness/17fuelcell.html?_r=
1&oref=slogin.
Fisher Scientific. 2008. Material Safety Data Sheet: Methanol. Accessed on February 3,
2009 at http://fscimage.fishersci.com/msds/14280.htm.
Kim, M.Y., S.H.Yoon, B.W. Ryu, and C.S. Lee. 2008. Combustion and emission
characteristics of DME as an alternative fuel for compression ignition engines
with a high pressure injection system. Fuel 87:2779-2786.
NRC. 2004. The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs.
Washington: The National Academies Press.
———. 2008. Transitions to Alternative Transportation Technologies--A Focus on
Hydrogen. Washington: The National Academies Press.
Rock, B. 2008. An overview of 2007 American 2007 natural gas vehicles Helium, Inc.
Accessed on September 2 at http://www.helium.com/items/451632-an-overview-
of-2007-american-2007-natural-gas-vehicles.
Yao, M., Z. Chen, Z. Zheng, B. Zhang, and Y. Xing. 2006. Study on the controlling
strategies of homogeneous charge compression ignition combustion with fuel of
dimethyl ether and methanol. Fuel 85:2046-2056.
Yborra, S. 2006. Taking a second look at the natural gas vehicle. American Gas Aug/Sep
2006:32-36.
Yeh, S. 2007. An empirical analysis on the adoption of alternative fuel vehicles: The case
of natural gas vehicles. Energy policy 35:5865-5875.
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Appendixes
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