Chapter 10
‘SYNTHETIC’ FUELS, OIL SHALE
AND TAR SANDS
New clean coal technologies can substantially improve efficiency and reduce emissions from powerplants. Until they are proven at commercial scale, however, their u entails more risk for utilities than conventional technologies. This additional risk could make it difficult for the new technologies to enter the marketplace quickly, especially given the tight deadlines of the Clean Air Act Amendments of 1990. The Clean Coal Technology Program, the single largest technology development program in the Department of Energy, is designed to help overcome this risk by offering the Federal Government as a financial partner in demonstrating worthy projects.
(National Energy Strategy, Executive Summary, 1991/1992)
The long-term risk of investing in new technologies that have not been demonstrated in multiple commercial applications inhibits the new technology market to such an extent that Federal resources have been needed to help fund commercial demonstration efforts. The Department of Energy's cost-sh
ared Clean Coal Technology Demonstration Program, to which industry has contributed $2 for every $1 of Federal money invested, has evolved from an early focus on emission control systems to an emphasis in its later rounds on highly efficient, environmentally superior advanced power systems.
(Sustainable Energy Strategy, 1995)
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英语外教一对一Coal, petroleum and natural gas are the traditional fossil fuels who direct u today accounts for most of the world's energy consumption. The fuels are rich in carbon and hydrogen. A relatively large amount of energy is stored in them and they have a high calorific value. However, the are not the only fossil fuels found on our planet. As they are depleted, or as their price increas, other fossil fuels can become more attractive for commercial exploitation. Furthermore, the abundant coal can be converted into cleaner and more convenient gaous and liquid fuels, similar to petroleum and natural gas. The non-traditional, or unconventional, fossil fuels are discusd in the prent chapter.
Oil Shale
Oil shale is an inorganic, non-porous rock that contains some organic material in the form of kerogen. In some respects, oil shale is similar to the source rock that produced petroleum. One important difference between oil shale and oil source rock is that the former contains greater amounts of kerogen (as much as 40%) than the latter, which usually contains about 1%. A cond major difference is that oil shale has never been expod to sufficiently high temperatures to convert the kerogen to oil. In a n, we can think of oil shale as being a ‘hybrid’ of oil and coal. Oil shale contains more kerogen than oil source rock, but less than coal. The composition of the oil derived from oil shale is much more similar to the composition of petroleum than to coal. Some oil shales can be ignited, like coal, and they burn with a very sooty, smoky flame much like a coal of very high volatile matter content. However, oil shales are of no interest as solid fuels. Their principal interest is in the possibility of conversion to liquid fuels.
Lean shale contains about 4% kerogen. When heated to 350-400 °C, it yields about 6 gallons of oil per ton of shale. Rich shale may contain up to 40% kerogen and typically yields about 50 gallons of oil per ton.
Two thirds of the world's oil shale rerves are located in the United States. The largest known rerves of hydrocarbons of any kind are the Green River shale deposits in Wyoming, Colorado and
Utah. The rerves are estimated to be 270 billion tons. At 20 gallons per ton of shale, this translates into 130 billion barrels of oil. This is five times as much as the proven rerves of petroleum in the U.S. (e Figure 9-6). However, no commercial production of fuels from oil shale exists today, so their economic recoverability is not well known. It is probably safe to say, however, that oil from shale is not economically competitive with petroleum at current world petroleum prices.
The recovery of oil shale us mining techniques similar to the methods ud in coal mining. The room-and-pillar method is one such approach. Oil is then recovered from the shale by retorting the shale. Retorting involves heating the shale in the abnce of air to temperatures of 500 °C or more. Typically, 75-80% of the kerogen is converted to oil. An alternative to mining is in situ retorting. In this process, holes are bored into the shale deposit underground. By injecting hot gas and air into the shale, the shale can become
‘SYNTHETIC’ FUELS, OIL SHALE AND TAR SANDS183 hot enough for the kerogen to turn into oil underground. In situ retorting eliminates the mining cost, many of the costs for above-ground retorts and liquid handling equipment, and address the problem of disposing of the shale after the oil has been ‘cooked’ out of it. In this ca, since the shale has never been removed from the ground, there is, in a n, no disposal problem at all.
The industrial processing of oil shale began a century ago. During the 1920s, oil shale was an economically important energy source. With the greater availability of low-cost petroleum, the commercial exploitation of oil shale cead. During the energy crisis of the 1970s veral major oil companies made massive investments both in rearch and development and in possible commercial u of oil shale. The Exxon Corporation even began construction of entire new towns in the Green River region. When petroleum prices began falling in the early 1980s, the efforts were abandoned.
Several technical problems must be overcome for successful large-scale u of oil shale in the future. The composition of oil from oil shale is sufficiently different from that of petroleum that liquids derived from oil shale cannot be ud as direct substitutes for petroleum. Oil derived from oil shale has less carbon and hydrogen and more nitrogen, oxygen and (sometimes) sulfur than petroleum. Petroleum refinery operations would have to be modified to accommodate oil shale liquids as a feedstock. Hydrogen must be added to the oil during processing. Furthermore, special care must be taken to remove the nitrogen and sulfur during processing, to avoid the formation of SO x and fuel NO x when the oil is eventually burned. When oil shale is retorted, the inorganic portion of the shale expands considerably. The spent shale remaining after retorting has no commercial value, but it mus
t be dispod of in an environmentally acceptable manner. Ideally, the spent shale is placed back in the mine, refilling the mined-out cavity and helping to prepare the area for land reclamation. Becau of the popcorn effect, the volume of spent shale is greater than the volume of the mine from which it was taken. Thus even if the mine were completely refilled, there would still exist some amount of spent shale for which alternative disposal methods must be sought. In spite of the problems, liquid fuels derived from oil shale can become important alternatives to petroleum, when the world economic situation –particularly the price of crude oil – becomes favorable.
Tar Sands
Tar sands are grains of sand or, in some cas, porous carbonate rocks that are intimately mixed with a very heavy, asphalt-like crude oil called bitumen. The bitumen is much too viscous to be recovered by traditional petroleum recovery techniques. Tar sands contain about 10-15% bitumen, the remainder being sand or other inorganic materials.
The estimated world-wide resources of tar sands are about three times the known petroleum rerves. The world's largest deposit of tar sands is near Athabasca, in Alberta,
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Canada. Other large deposits exist in the former Soviet Union and in Venezuela. In the United States, small deposits of tar sands are found in Utah.
If tar sand is heated to about 80 °C, by injecting steam into the deposit in a manner analogous to that of enhanced oil recovery, the elevated temperature caus a decrea in the viscosity of the bitumen just enough to allow its pumping to the surface. Alternatively, it is sometimes easier to mine the tar sand as a solid material. When the mined tar sand is mixed with steam and hot water, the bitumen will float on the water while the sand sinks to the bottom of the container, allowing for easy paration. Heating the bitumen above 500°C converts about 70% of it to a synthetic crude oil. Distilling this oil gives good yields of kerone and other liquid products in the middle distillate range. The remainder of the bitumen either thermally cracks to form gaous products or reacts to form petroleum coke.
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‘Synthetic’ Fuels (Synfuels)
Current estimates of the lifetimes of domestic oil and gas rerves are about a decade or two, as shown in Figure 9-6. More detailed information, including that for coal, is summarized in Tables 10-1 and 10-2. The numbers shown are larger than tho of proven rerves, as they should be (e Cha
pter 6), but by no more than a factor of two. Despite the subtle differences in the definition of “undiscovered recoverable resources” and “demonstrated rerve ba” – which we do not need to discuss here – it is obvious that coal will be around for veral more centuries. So what is to become of it?
TABLE 10-1
Undiscovered recoverable resources of U.S. crude oil and natural gas
英语介绍自己Region Crude oil
109 barrels
Natural gas 1012 cubic feet
Onshore and State Waters33.3254.0 Alaska13.257.9
Rocky Mountains and Northern Great Plains 4.515.2
Gulf Coast 4.282.5
深圳mba
Pacific Coast 3.511.0 Federal Offshore16.1145.1 Alaska 3.416.8
Gulf of Mexico8.6103.3
Total49.4399.1 [Source: Energy Information Administration.]
As mentioned previously, a time lag of some 50 years between the introduction of a new energy source and its widespread commercial acceptance and u has been typical in recent
‘SYNTHETIC’ FUELS, OIL SHALE AND TAR SANDS185 history. So it may take another 50 years for society to witness the u of renewable energy sources – such as solar and wind energy – on a large, world-wide scale. It ems likely that a gap will develop between the time when natural gas and petroleum become scarce, and thus expensive, and the time when alternative energy sources will provide a major portion of the world's energy needs (e Chapters 16 and 17). We have already en that the renewable energies contribute today much less than 10% of U.S. and world supply (e Figures 5-13 and 5-14) while fossil fuels contribute 90% or so. The day when this situation will be reverd appears to be at least 50 years away.ielts考试费用
TABLE 10-2
Demonstrated rerve ba of coal in the U.S. (in 109 short tons)
Region/State Anthracite Bit/Subbit Lignite% surface Total Appalachian7.4101.2 1.126.6109.7 Pennsylvania7.221.70.0––West Virginia0.036.00.0––Ohio0.023.70.0––Interior0.1131.213.628.3144.8 Illinois0.090.00.0––Western0.0211.429.840.2241.2 Montana0.0104.115.8––Wyoming0.068.50.0––Total7.5443.844.533.7495.7 [Source: Energy Information Administration.]
The energy source that may provide the means for bridging this gap is coal. But it will do so only if it can be ud in an environmentally acceptable way. Technology has been developed to convert coal into ‘synthetic’ gaous or liquid fuels - synfuels. There is really nothing synthetic about them, as there is in synthetic (vs. natural) rubber. But this term has taken root and we'll adopt it here as well.
Coal itlf is of cour well established as a fuel. The Industrial Revolution would not have been possible without it. But it pollutes the environment more than oil or gas. We must go then through the trouble of converting it into a cleaner and more convenient fuel. There are veral ways and reasons to do that. Let's look at the reasons first.
There exists an enormous investment in equipment that burns gaous or liquid fuels; cars and truck
s, locomotives, airplanes, many home furnaces and some power plant boilers are familiar examples. Our society has neither the economic resources nor the manufacturing capacity to replace all of that liquid or gaous fuel equipment in a short time span. Therefore, liquid fuels will continue to be needed until new energy strategies – such as solar homes and vehicles – are developed and become widely accepted.
Also, liquid and gaous fuels are much easier to purify than are solid fuels. For example, the sweetening of a sour gas is a straightforward process that allows the removal
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of esntially all sulfur; but the beneficiation of coal for sulfur removal can be a cumbersome process that never succeeds in completely removing all sulfur from coal. (A conquence is the significant investment needed for flue gas desulfurization systems in coal-fired power plants; e Chapter 11) The conversion of coal to liquid or gaous fuels offers opportunities for removing undesired components at the same time.
Finally, there are many convenience factors associated with the u of fluids rather than solids. Fluids are often easier to transport than solids; they are usually much easier to handle, with pipes, v
alves and meters. They can provide an instant on/off capability in a heating system. Particularly in domestic applications, oil or gas are far more convenient for the average houholder than is coal and, not surprisingly, there is an overwhelming preference for oil or gas, rather than coal, for home heating.
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mbsThere are esntially two approaches to coal conversion. They are known as coal gasification or coal liquefaction, depending on the type of product desired. In either ca, synfuels production involves some strategy for the addition of hydrogen to hydrogen-deficient solid coal to form the relatively hydrogen-rich liquid or gaous fuels (e Illustrations 7-1, 8-1 and 9-1). The relationship between the hydrogen content and the physical state of the fuel is illustrated in Table 10-3.
TABLE 10-3
Relationship between hydrogen content of typical fuels and their physical state
Fuel(Typical)
端午节的来历与习俗C/H mass ratio
kos
(Typical)
C/H molar ratio State
Bituminous coal15 1.25Solid
Crude oil90.77Liquid
Gasoline60.50Liquid
Natural gas30.25Gaous
Coal Gasification. In coal gasification the principal source of hydrogen is high-temperature steam. Steam is easily generated from inexpensive, abundant water. (In coal liquefaction, on the other hand, hydrogen is usually added as molecular hydrogen, H2, which can be expensive to produce on a large scale.)
The objective of coal gasification is to convert coal to a combustible gas, suitable for u as a fuel. The simplest way to obtain a combustible gas from coal is to carbonize the coal, that is, to heat it in the abnce of air. We have already en that heating coal in this way will drive off a variety of volatile substances. After compounds that would be liquids at ordinary temperatures are allowed to conden, the remaining gas is mainly a mixture of methane and hydrogen, with a calorific value in t
形容词顺序he range 550-700 BTU/ft3. (Recall that the calorific value of natural gas is 1000 BTU/ft3.) This gaous product is called coal gas.
Coal gas was first ud in 1798 as a fuel for gas lamps that illuminated a factory owned by James Watt (better known to us as the inventor of the first practical steam engine; e Chapter 4). The first company established in the United States to manufacture and
