Synthetic fuels

DEFINITION: Solid, liquid, or gaseous sources of energy that do not occur naturally

Given the finite nature of the world’s stores of natural petroleum, the development of economically viable, environmentally safe, and renewable synthetic fuels is important for human survival.

Synthetic fuels are normally produced from abundantly occurring natural resources such as coal, tar sands, oil shale, and biomass. One of the main objectives in the production of a synthetic fuel is to eliminate sulfur and nitrogen from the fuel compound, thereby creating an environmentally clean energy source. Oxides of nitrogen and sulfur dioxide are among the most undesirable of common air pollutants. Sulfur dioxide is one of the major causes of acid rain, which is created when sulfur dioxide combines with water vapor in the atmosphere to form sulfuric acid. Similarly, oxides of nitrogen produce nitric acid. These acids fall back to earth in rain and are detrimental to aquatic life as well as botanical life. Synthetic fuel manufacturers thus strive to eliminate these pollutants, as well as others such as carbon monoxide, hydrocarbons, particulates, and photochemical oxidants, from the fuel supply.

Principles of Synthetic Fuel Manufacture

The manufacture of liquid and gaseous synthetic fuels normally involves transforming naturally occurring carbonaceous raw material through a suitable conversion process. The techniques employed include hydrogenation, devolatilization, decomposition, and fermentation. The principal aim in the manufacture of synthetic fuel is to achieve a low carbon-to-hydrogen atomic mass ratio, or a high hydrogen-to-carbon atomic ratio, whenever possible. This results in a clean-burning fuel that releases by-products that are harmless to the environment. For example, pure methane (CH₄), with a molecular weight of 16, has a high hydrogen-to-carbon ratio of 4:1. Methane gas is a common component that is absorbed into coal. The method used to the gas involves fracturing the and exposing it to low pressures. Coal-bed methane is one of the cleanest-burning fossil fuels; the by-products of burning it are simply carbon dioxide and water. Synthetically generated substitute natural gas is more than 90 percent methane. Natural gas (of which methane is the chief constituent) has a hydrogen-to-carbon ratio of approximately 3.4:1, which is also quite high. The ratios for liquefied gas and for naphtha lie between 2:1 and 3:1. (In comparison, the ratios for gasoline and fuel oil are less than 2:1. Bituminous coal has one of the lowest values, with ratio of much less than 1:1.)

Although coal is among the most abundant natural energy sources, it is also among the dirtiest. The composition of this solid is a major disadvantage; it consists of about 70 percent carbon and about 5 percent hydrogen, translating to a highly undesirable carbon-to-hydrogen mass ratio of 14:1. Coal-burning power-generating stations thus spew out large quantities of gases that are harmful to the environment. Despite the use of such emission-reducing devices as electrostatic precipitators, the levels of pollutants emitted by coal-burning plants remain high. Techniques such as and coal yield synthetic fuels that are safer for the environment.

The process of coal involves making coal react with steam at very high temperatures (in the range of 1,000 degrees Celsius, or 1,832 degrees Fahrenheit). This process produces synthetic gas. Three types of synthetic gas are in common use. Low-calorific-value gas (also called producer gas) is used in turbines. Medium-calorific-value gas (also called power gas) is used as a fuel gas by various industries. High-calorific-value gas (also called pipeline gas) is a very good substitute for natural gas and is well suited to economical pipeline transportation. Pipeline gas contains more than 90 percent methane; as a result, it has a high hydrogen-to-carbon ratio.

The process of coal liquefaction is employed to generate a liquid fuel with a high hydrogen-to-carbon ratio; it is also used to obtain low-sulfur fuel oil. Several methods are employed to accomplish coal liquefaction, including direct catalytic hydrogenation, indirect catalytic hydrogenation, pyrolysis, and solvent extraction. While these fuels may be much safer for the environment than the original coal, the process of manufacturing them produces high amounts of carbon dioxide (CO₂) and other greenhouse gasses.

Tar Sands and Oil Shale

Naturally occurring tar sands contain grains of sand, water, and bitumen. Bitumen, a member of the petroleum family, is a high-viscosity crude hydrocarbon. A method known as hot water extraction is used to procure bitumen from tar sands. The bitumen is subsequently upgraded to synthetic in refineries. Synthetic crude oil (also called syncrude) is similar to petroleum and can be obtained through coal liquefaction as well as from tar sands and oil shale.

Large deposits of tar sands are found in Alberta, Canada; the United States has huge reserves of oil shale in Utah, Wyoming, and Colorado. Oil shale is probably the most abundant form of hydrocarbon on earth. Oil shale is a sedimentary rock that contains kerogen, which is not a member of the petroleum family. A popular method known as retorting is used to produce oil from shale. The process involves the method of pyrolysis, which reduces the carbon content in the raw hydrocarbon through distillation. In the twenty-first century, technological advancements have allowed shale oil to become an economically feasible alternative to petroleum. In a process known as hydraulic fracturing, or fracking, water, sand, or chemical are injected into the rock formation under high pressure, forcing out the trapped shale oil. Extracting oil from shale and other rock formations became the most significant oil-producing method in the United Sates in the 2010s. By 2021, 65 percent of all oil produced in the United States came from such methods.

Biomass Fuels and

Like oil and coal, biomass is derived from plant life. Oil and coal, however, are considered nonrenewable resources, as it takes vast periods of time for geologic processes to produce these materials naturally. Because consists of any material that is derived from plant life, it is produced in far shorter spans—one hundred years or less—and is thus considered renewable. Wood is the most versatile biomass resource; farm and agricultural wastes, municipal wastes, and animal wastes are also considered to be biomass. Biomass can be processed into fuel using a variety of methods. Fermentation, for example, yields ethanol, or ethyl alcohol (sometimes called grain alcohol). Other methods include combustion, gasification, and pyrolysis. According to the US Energy Information Administration, 5 percent of the total energy output in the United States in 2022 came from biomass fuels.

Gasohol is a mixture of gasoline and small quantities of ethanol. The mixture burns cleaner than conventional gasoline; however, it can cause damage to plastic and rubber materials used in automobile engines. In the United States, therefore, the Environmental Protection Agency (EPA) permits the addition of only 10 percent by volume to gasoline to create gasohol. Methanol, or methyl alcohol (also called wood alcohol), can also be combined with conventional gasoline to produce cleaner fuel; however, the EPA limits the amount of methane in such mixtures to 3 percent.

Other Fuels

A nonpolluting rocket fuel based on alcohol and hydrogen peroxide has been developed by U.S. Navy research engineers at China Lake, California. This nontoxic homogeneous miscible fuel (NHMF) can be modified and used to drive turbines, which in turn drive alternators that produce electricity. Further developments based on what has been learned about this fuel may permit its use in automobiles. During World War II, moreover, Germany produced synthetic fuels in large quantities to meet its energy demands, employing coal gasification and also creating diesel oil and aviation kerosene using a reconstitution process; this process is still in use in many places.

Although the present abundance of natural petroleum limits the economic competitiveness of most synthetic fuels, the finite nature of the world’s oil supply virtually ensures that synthetic fuels will become increasingly important energy sources. The U.S. Department of Energy and governmental agencies in many other countries thus provide funding to encourage research into the creation of economically viable, environmentally safe, and renewable synthetic fuels.

Bibliography

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