Transportation Fuels
From Thermal-FluidsPedia
The most common fuels used for automobiles are still gasoline and diesel oil. They are liquid, are relatively inexpensive, and have good combustion characteristics. Furthermore, the necessary infrastructure for production, storage, and distribution of these fuels is fully in place. These fuels, however, are becoming increasingly more scarce and have been associated with most of our air pollution problems.
In the search for better fuel economy and reduced emissions, much research has been done on the use of alternative fuels. However, these fuels have not found wide acceptance and their use has been limited to fleet and government vehicles. Suitable alternative fuels can only find wide-scale acceptance if they meet many of the same requirements as gasoline and diesel fuels. To get a better understanding of the issues in using alternative fuels, various transportation fuels as well as their advantages and drawbacks are summarized below.
Gasoline (Petrol) is the most common fuel used in vehicles. It is a colorless and volatile liquid made up of many hydrocarbons, but is conveniently represented as a single compound with a molecular structure approximated as C8H17. Like most other liquid fuels, gasoline is derived primarily from crude oil. To aid combustion and reduce hydrocarbon emissions, it is advantageous to add some oxygen to gasoline. The product, called reformulated gasoline, has been found to reduce smog in cities with a large concentration of cars. Additionally, adding some detergents to gasoline will help to prevent the build-up of engine deposits, keeping engines working smoothly and burning fuel cleanly.
Diesel oil, like gasoline, is a mixture of light distillate hydrocarbons. Its boiling point is somewhat higher than that of gasoline, allowing for engine operation at an increased compression ratio with less concern over the engine knock, common in gasoline engines.
Kerosene is a mixture of heavier hydrocarbons; it is mostly used as heating oil. Kerosene is lighter than gasoline and has a greater heating value. It is used as jet fuel in pure distillate (JP5) or is blended with gasoline (JP4). Kerosene is often represented as a single hydrocarbon with the formula C12H26.
Biodiesel is derived from vegetable oils, seeds, animal fats, and algae. Biodiesels are, however, more viscous than fossil fuels; this makes cold-temperature starts more difficult. Because biodiesel is a biomass, it is biodegradable, less toxic, has fewer emissions, and does not contribute to global warming. Biodiesel is marketed as pure biodiesel (B-100), or as a blend of 20% biodiesel and 80% petroleum diesel (B-20).
Natural gas is about 90% methane and 10% heavier hydrocarbons. Its high octane number (ON = 130) allows engines to operate at compression ratios as high as 12:1, compared to 8:1 for gasoline engines. The main problems with methane are its toxicity and lack of odor, which makes leaks undetectable. Furthermore, methane-powered cars have inferior power and energy densities, resulting in roughly 20% less fuel efficiency than gasoline and diesel engines and a relatively low cruising range.
Because of its low energy density, natural gas must be compressed or liquefied before it can be used for vehicles. To produce compressed natural gas (CNG), methane must be compressed to 15-25 MPa (2,400-3,600 psi) and stored in special containers. At ordinary room temperatures, it is impossible to liquefy methane by compression alone and must be cooled to cryogenic temperatures. CNG is generally considered a relatively clean fuel, as the emission of carbon monoxide, hydrocarbons, and particulate matter is substantially less than that of gasoline or diesel fuel. Reduced emissions are offset, however, by an increase in the emission of unburned methane, a potent greenhouse gas which traps heat about twenty times more effectively than carbon dioxide. At this time, methane emission accounts for 13% of all greenhouse gases, but it would become a major concern if natural gas were to become a dominant fuel for transportation.
Liquefied Petroleum Gas (LPG) is natural gas leftover after methane is removed. It is mostly propane, but some butane and higher hydrocarbons are also present. Unlike methane, propane can be liquefied at room temperature and relatively low pressure; this makes it suitable for storage in light fuel tanks and gives cars a driving range comparable to that of gasoline.
Methanol (also called methyl or wood alcohol) is the simplest alcohol. Many consider methanol to be the fuel of choice. It is liquid, like gasoline, but has a higher octane rating (ON = 100), and can be produced by utilizing a variety of methods from gasoline, natural gas, and other fossil fuels in large quantities and at a low cost; it may also be produced from the distillation of wood chips, garbage, and animal manure. Since methanol has a high H/C ratio, it is a relatively clean fuel and, except for aldehydes, methanol emissions are considerably lower than those of gasoline.
Unfortunately, methanol has several drawbacks which limit its use as a viable transportation fuel. Methanol has an invisible flame. To enhance flame luminosity, methanol is usually mixed with a small percentage of gasoline and is often used as M85 (85% methanol, 15% gasoline) instead of pure methanol (M100). Methanol is so toxic that even ingesting a small amount can cause blindness or death. Another problem is that methanol is corrosive to aluminum and other materials commonly used in seals and pipe fittings for transporting gasoline. Therefore, methanol cannot be transported through existing pipelines and must be trucked. Methanol has only half the energy density of gasoline, which means for a given driving range about twice the amount of fuel will be needed. Because of these limitations, the number of methanol vehicles has declined steadily in recent years. Because methanol can be converted to hydrogen at relatively low temperatures, it remains an attractive fuel for producing the hydrogen needed to operate fuel cell vehicles.
Ethanol (also called ethyl or grain alcohol) is made through the biochemical conversion (fermentation) of sugar and perennial grasses (such as miscanthus), or by the hydration of ethylene from petroleum. Ethanol can be used directly in fuel cells, used in the pure form or mixed with gasoline to power internal combustion engines.
Since ethanol is a biomass, there is no net carbon dioxide emission and it does not contribute to global warming. Like gasoline, ethanol is a liquid, making it possible to use in existing fuel tanks, fuel pumps, and fueling stations. Engine performance is also comparable to that of engines using gasoline. NOx emissions are lower and formaldehyde higher, but other emissions are about the same as gasoline and slightly greater than methanol.
Ethanol has a lower heating value than gasoline and burns at a lower A/F (9:1 as compared with 15:1 for gasoline). So, for a given engine more fuel can be introduced per cycle, compensating for its lower heating value, and an overall higher power output is possible. Ethanol has a very high octane rating (ON = 113) and is therefore most suitable for use in high-compression engines. Ethanol, like methanol, burns with an invisible flame. To increase visibility, ethanol is often blended with gasoline to produce gasohol (a mixture of 10% ethanol and 90% gasoline), E85 (85% ethanol, 15% gasoline), and E95 (95% ethanol, 5% gasoline). E85 is usually used for light-duty applications, whereas E95 is best for heavy-duty vehicles. Gasohol is not common in the United States.
Hydrogen is considered by some to be the fuel of the future; it is the simplest and most abundant element in the universe. Pound for pound, hydrogen has the highest energy content of all fuels, and when burned in oxygen the only by-product is water. Hydrogen is also an ideal fuel to produce electricity – through fuel cells – directly eliminating carbon dioxide and other pollutants that are associated with burning of fossil fuels.
There are, however, a number of factors that prevent hydrogen from being utilized as the most promising transportation fuel, and some even have dismissed hydrogen as hype. Unlike petroleum and methane, hydrogen cannot be mined but must be produced in a number of ways, ranging from hydrocarbon reformation, to coal gasification and pyrolysis, to the electrolysis of water. Although hydrogen is a clean fuel, converting conventional fuels to hydrogen does not necessarily improve air quality. Cleanliness depends on how the hydrogen is produced. If hydrogen is produced by reforming ethanol, then the large amount of energy required for the distillation of ethanol will take away most of the hydrogen advantage; if hydrogen is produced by reforming fossil fuels, then most of its environmental benefits disappear as well. Renewable sources such as solar, wind, and hydropower are ideal because they can electrolyze water in large quantities without polluting the atmosphere. Nuclear sources are another option.
Currently, most commercial hydrogen is produced by steam reformation of natural gas. Natural gas is a particularly good candidate for producing hydrogen at the wellhead. Instead of piping crude or natural gas, large reformers can be installed to strip off the hydrogen and ship it through the pipelines. The byproduct of the reformation process, mainly carbon dioxide, can then be sequestered by injecting it back into the gas field, raising the pressure and improving extraction efficiency (See Fossil Fuels, Enhanced Recovery).
Hydrogen safety and storage are two other concerns most often raised in regard to hydrogen. Hydrogen is highly explosive and burns easily with an invisible flame. When compared to most other gaseous fuels, hydrogen’s flammability range is unusually broad, from 4 to 75 percent. Natural gas, in contrast, burns between 5 and 15 percent and requires much less energy to ignite. Table 1 compares the ignition characteristics of hydrogen with those of methane and gasoline.
Table 1. Ignition Characteristics of various gaseous and liquid fuels Hydrogen Methane Gasoline Ignition Temperature 585 C 540 C 230-480 C Flammability Limits 4-75% by vol. 5-15% by vol. 1.4-7.6% by vol. Explosion Limits 20-65% by vol. 6-14% by vol. 1-7.6% by vol. Energy Density 120 MJ/kg 50 MJ/kg 44 MJ/kg 8.5 MJ/L 40 MJ/L 21 MJ/L Ignition Energy 0.02 mJ 0.2 mJ 0.2 mJ Source: Flynn, T., Cryogenic Engineering, Second Ed., CRC Press, 2005.
Hydrogen storage is also a problem, as hydrogen must be compressed to very high pressures, cooled to very low temperatures, or stored as chemical or metal hydrides, adding considerable weight to the storage container. Hydrogen has a low energy density; one kilogram of hydrogen contains about the same amount of energy as a gallon of gasoline and about one third that of natural gas. Another concern is hydrogen embrittlement. Due to the small size of its molecules, hydrogen is far more prone to leak out, causing pipes to deteriorate faster. Existing distribution channels used for transport and delivery of gasoline are not suitable for hydrogen and the cost of developing new systems is very high.
Aside from these considerations, some concerns have recently been raised in regards to the effect of potential gas leaks to the environment. Normally, hydrogen is stored at high pressures and about 10% of all hydrogen manufactured is leaked through the atmosphere. Being so light, hydrogen rapidly rises through the atmosphere, eventually reacting with oxygen to form water vapor. The additional water makes the stratosphere wetter, cooling the lower atmosphere, particularly in the polar regions where most hydrogen is converted to water vapor. This would disrupt the ozone layer, causing 7-8% more depletion over the poles (1).
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References
(1) NaTrompme, et al., “Potential Environmental Impact of a Hydrogen Economy on the Stratosphere,” Science, 300, 1740-1742, 2003.
(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) If hydrogen is burned in an internal combustion engine then, besides water vapor, some NOx will also be produced. The high temperatures of hydrogen flames cause air to be ionized into atomic nitrogen and oxygen, which subsequently react to produce nitric monoxide. When hydrogen is used in fuel cells, the reaction is at low temperatures, and the only product is water vapor.
Further Reading
Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004.
Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005.
Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001.
Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003.
Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002.
Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004.
Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science.
Transportation Research Part C: Emerging Technologies, Direct Science Elsevier Publishing Company; this journal focuses on scholarly research on development, application, and implications in the fields of transportation, control systems, and telecommunications, among others.
Fuel Cells Bulletin, Direct Science Elsevier Publishing Company, Fuel Cells Bulletin is the leading source of technical and business news for the fuel cells sector.
International Journal of Hydrogen Energy, Direct Science Elsevier Publishing Company, Quarterly journal covering various aspects of hydrogen energy, including production, storage, transmission, and utilization, as well as economical and environmental aspects.
External Links
US Department of Transportation (http://www.dot.gov).
US Department of Energy (http://www.doe.gov).
US Environmental Protection Agency (http://www.epa.gov).
National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (http://www.nrel.gov/vehiclesandfuels/hev).
FreedomCar (http://www.eere.energy.gov/vehiclesandfuels).