Synthetic fuels are usually thought of as liquid fuel substitutes for gasoline and diesel fuel made from petroleum sources. In broad context, the source of these synthetics can be any feedstock containing the combustible elements carbon or hydrogen. These include coal, oil shale, peat, biomass, tar sands, and natural gas. Water could be included here because it can be decomposed to produce hydrogen gas, which has the highest heating value per unit weight of any material (excluding nuclear reactions). The conversion of natural gas is treated in a separate article, so this article will emphasize solid feedstocks.
From a practical standpoint, coal, because of its abundance, has received the most attention as a source for synthetic fuels. As early as 1807, a coal-gas system was used to light the streets of London, and until the 1930s, when less expensive and safer natural gas started to flow through newly constructed pipelines, gas piped to homes in the Eastern United States was derived from coal. Kerosene, originally a byproduct from the coking of coal for metallurgical applications, can be considered the first synthetic liquid fuel made in quantity. But once crude oil became cheap and abundant, there was little serious research on synthetic liquid fuels in the industrial world until the Energy Crisis of 1973. The main exceptions to this generalization are the important work on coal conversion in Germany, cut off from oil imports during the two World Wars, and the Sasol Process in South Africa, which produces a synthetic, waxy "crude oil" from indigenous coal deposits.
After 1973 the United States invested heavily in synthetic fuel research and development, hoping synthetics could serve as economical substitutes for crude oil. However, coal conversion is not profitable unless the price of crude oil is over $50 per barrel, which is why the processes developed were mothballed when world crude oil prices fell in the 1980s.
The major chemical difference between natural gas, crude oil, and coal is their hydrogen-to-carbon ratios. Coal is carbon-rich and hydrogen-poor, so to produce a synthetic liquid or gas from coal requires an increase in the hydrogen-to-carbon ratio. Coal's ratio of about 0.8 has to be raised to 1.4 to 1.8 for a liquid, and to over 3 to produce a synthetic gaseous fuel. Natural gas (chiefly methane) has a ratio of 4. This can be done by either adding hydrogen or rejecting carbon.
Addition of hydrogen can involve reacting pulverized coal with hydrogen-rich liquids. More commonly, pressurized hydrogen gas is reacted with coal (hydrogenation) in the presence of a catalyst. The latter scheme also removes many of the noxious sulfurous and nitrogenous impurities in coal by converting them to gaseous hydrogen sulfide and ammonia. Carbon removal entails pyrolysis in the absence of air (coking) to produce varying amounts of gases, liquids, and char, depending on the reaction time-temperature-pressure conditions employed.
Synthetic Fuel Liquids via Gas Intermediates
Liquids can also be synthesized via an indirect scheme where the coal is first gasified in an intermediate step. The coal is pulverized and reacted with steam to produce "water gas," an equimolar mixture of carbon monoxide and hydrogen: The carbon monoxide can then be further reacted with steam and/or hydrogen in the water gas shift reaction: Combining Equation 1 and Equation 2, one can generate hydrogen and carbon dioxide: And adding reactions from Equation 1 and Equation 3 yields just methane:
These operations carry energy penalties, and the heat of combustion released when burning the methane, hydrogen, or carbon monoxide produced is less than the energy that would have been released had the coal been burned directly. To produce heavier liquids, the equimolar mixture of hydrogen and carbon monoxide (water gas, also known as synthesis gas) is the preferred feedstock. Badische Anilin pioneered the synthesis of methanol at high temperature and pressure after World War I:
Shortly thereafter, Fischer and Tropsch discovered an iron catalyst that would convert synthesis gas to a mixture of oxygenated hydrocarbons (alcohols, acids, aldehydes, and ketones) at atmospheric pressure. In the next decade Ruhrchemie developed new cobalt catalysts that could produce a mixture of hydrocarbon liquids and paraffin wax from Fischer Tropsch liquids at moderate pressure. This, plus direct hydrogenation of coal, was the basis of German synfuel capacity during World War II. Finally, in 1955, the South African Coal, Oil and Gas Co. further improved the technology and commercialized the Sasol Process as the basis for South Africa's fuel and chemicals industry. Favorable economics were possible because government mandates and subsidies dictated the use of local coal resources rather than spending scarce hard currency to import foreign crude oil.
Methanol is an excellent, high-octane motor fuel, but it was not cost-competitive with gasoline made from cheap, abundant, crude oil. It also has a lower specific energy (miles per gallon) because it is already partially oxidized. In the late 1970s, Mobil Oil announced its "Methanol to Gasoline" Process, which efficiently converts methanol to C2 to C10 hydrocarbons via a synthetic zsm-5 zeolite catalyst. This coal-methanol-gasoline technology will probably enjoy use as crude oil prices rise in the future. Since natural gas (largely methane or CH4 is readily converted into methanol via partial oxidation with pure oxygen, this also offers an alternate to low-temperature liquefaction or pipelining as a way to utilize natural gas deposits that are in remote locations.
There is a parallel technology for the partial oxidation of methane to make ethylene and other olefins. These can then be polymerized, alkylated, and hydrotreated as need be to make hydrocarbon fuels in the gasoline and diesel fuel boiling range. Methane can also be partially oxidized to produce oxygenates, such as methyl tert-butyl ether (MBTE), that are used in reformulated gasoline as blending agents.
Direct Liquefaction of Coal
In comparison to the capital-intensive, multistep gaseous route from coal to gasoline, the direct liquefaction of coal in a single processing step is attractive. While the removal of carbon from coal to generate liquids richer in hydrogen sounds simple, there is not enough hydrogen in coal to yield much useable liquid fuel. Further, the process must have a ready market, such as a nearby smelter, to utilize the major coke fraction. Addition of hydrogen during pyrolysis (hydropyrolysis) increases liquid yields somewhat, but the direct hydrogenation of coal has always seemed a more attractive route. Nevertheless, the Office of Coal Research of the U.S. Deptartment of the Interior sponsored research on this scheme for many years. As a result, the Char-Oil-Energy Development (COED) process was developed by the FMC Corporation in the period 1965–1975. In the COED process, 50 to 60 percent of the coal feed is rejected as char, and finding a market for char is problematic. Working on subbituminous western coals, the Oil Shale Corporation developed a similar scheme (Toscoal Process), which also produced 50 percent solid char.
High-pressure hydrogenation of coal was patented by Bergius in 1918, but the liquids were of low quality. However, by the mid 1930s, plants using on the order of 500 tons per day of low-rank coals and coal pitch had been built in England and Germany. Toward the end of World War II, Germany had 12 large "coal refineries" producing over 100,000 barrels a day of motor and aviation fuels for the war effort. After the war, in spite of the abundance of crude oil in the United States, pilot plant work on coal hydrogenation was continued in by the U.S. Bureau of Mines. This work was further developed by Ashland Oil into the "H-coal" process, and a pilot plant was built in 1980. The hydrocarbon liquids produced were rather high in nitrogen and oxygen, but all heavy materials were recycled to extinction so that no pitch or char was produced.
In the 1920s, I. G. Farben discovered that certain hydrogen-rich solvents, such as tetralin, could dissolve heavy coal components. The heavy extracted liquids could more conveniently be upgraded by hydrogenation, and the extracted residue was converted to metallurgical coke and carbon electrodes. After the war a low-level research effort was undertaken in the United States on solvent-refined coal processes, with primary emphasis on producing boiler fuel. Yields of useful liquids were gradually improved by inserting additional processing steps such as hydrocracking some of the heavier fractions.
Spurred by the OPEC oil crisis, the U.S. Department of Energy encouraged major oil companies to increase efforts to produce liquid fuels from coal, tar sands and shale oil deposits. A key development was the Exxon Donor Solvent Coal Liquefaction Process (EDS). It involves reacting a coal slurry with hydrogenated recycle solvent and hydrogen. The donor solvent transfers some of its hydrogen to the coal, is distilled from the reaction products, hydrogenated, and then recycled to the process. The light ends from distillation are steam-reformed to make process hydrogen. The heavy vacuum bottoms are Flexicoked, and the coke is gasified to provide process fuel gas. Except for some residual carbon in the gasifier ash, very little of value in the coal is wasted.
Work has also continued on the solvent-refined coal + hydrocracking concept (the NTSL, or non-integrated, two-stage liquefaction process), and a pilot plant was operated by Amoco, DOE and the Electric Power Research Institute (EPRI) from 1974 to 1992.
In addition to coal, there have been extensive post-oil-crisis studies aimed at utilizing the extensive western oil shale deposits. Oil shale utilization has many problems common to coal conversion and, in addition, extensive inorganic residues must be disposed of. The products are also high in nitrogen, and this increases refining costs. Tar sands constitute a final hydrocarbon reserve of interest. Here the problems are more tractable. The sands can be extracted with hot water to produce a material similar to a very viscous crude oil. It is refined as such, and thus Great Canadian Oil Sands, Ltd. has been able to justify operation of several very large tar sands refineries at Cold Lake, Alberta. The major process involves fluid coking of the heavy bottoms from distillation.
ECONOMIC AND ENVIRONMENTAL OUTLOOK
The coal conversion efficiency to synthetic, pipeline-quality natural gas or liquid crude oil is in the 60 to 70 percent range. This means that only 60 to 70 percent of the latent heat energy in the coal can be obtained by burning the product of the conversion. However, for the lower Btu per cubic foot products of water gas and coke oven gas, conversion efficiencies can reach over 95 percent.
The reason for the poor conversion efficiency to synthetic fuels is the high energy cost in liberating hydrogen from water (thermal dissociation, electrolysis) and, when distillation does not involve water, the partial combustion needed to produce the gas (CO). The price of crude oil would have to rise to around $40 to $50 a barrel, or need government subsidies of $20 to $30 a barrel, for liquid synthetic fuel to be competitive. Because the conversion to gaseous fuels is less complex and costly than liquid fuels, the subsidy or rise in natural gas prices would not have to be as dramatic.
Besides the economic feasibility problem, synthetic fuels face significant environmental hurdles. During direct liquefaction, heavy, high-boiling polyaromatics organics are produced. Scientists are trying to eliminate these carcinogenic fractions by recycling them through the liquefaction process. Production by the indirect method is less problematic because it tends to produce fewer toxic chain hydrocarbons. There is also a significant release of solid, liquid, and gaseous residual waste that comes from the boilers, heaters and incinerators, or as part of the processing stream.
The process will adversely affect air quality by releasing nitrogen oxides, sulfur oxides, carbon monoxides and other particulates into the atmosphere. Better control of the conversion conditions and better control of emissions can make the process cleaner, yet technology cannot do anything to curb carbon emissions. Since much of the carbon in coal is converted to carbon dioxide in the synthesis process, and is not part of the synthetic fuel itself, the amount of carbon dioxide that will be released to the environment during combustion is 50 to 100 percent more than coal, and around three times more than natural gas.
Since most systems use tremendous amounts of water, the production of synthetic fuels will have a detrimental effect on water quality as well. It will require major technological advances to more effectively handle waste streams—waste-water treatment systems, sulfur recover systems and cooling towers—to make synthetic fuels an acceptable option from an environmental perspective. This emission control technology will be expensive, only adding to the economic disadvantages the synthetic fuel market already faces.
As crude oil reserves dwindle, the marketplace will either transition to the electrifying of the transportation system (electric and fuel-cell vehicles and electric railways), with the electricity being produced by coal, natural gas, nuclear and renewables, or see the development of an industry to produce liquid fuel substitutes from coal, oil shale, and tar sands. It might also turn out to be a combination of both. The transition will vary by nation and will be dictated strongly by the fuels available, the economic and technological efficiencies of competitive systems, the relative environmental impacts of each technology, and the role government takes in the marketplace.
John Zumerchik Herman Bieber
See also: Hydrogen; Natural Gas, Processing and Conversion of.
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