Fuels and fuel chemistry

A fuel is any compound that has stored energy. This energy is captured in chemical bonds through processes such as photosynthesis and respiration. Energy is released during oxidation. The most common form of oxidation is the direct reaction of a fuel with oxygen through combustion. Wood, gasoline, coal , and any number of other fuels have energy-rich chemical bonds created using the energy from the Sun , which is released when the fuel is burned (i.e., the release of chemical energy). Chemical fuels or the fossil fuels are useful reserve of fuels and are therefore used extensively to satisfy the demands of an energy-dependent civilization.

Fossil fuels are principally hydrocarbons with minor impurities. They are so named because they originate from the decayed and fossilized remains of plants and animals that lived millions of years ago.

Fossil fuels can be separated into three categories. The first is petroleum or oil. This is a mixture of light, simple hydrocarbons dominated by the fractions with 6 to 12 carbons but also containing some light hydrocarbons (e.g., methane and ethane). Fully half of the energy consumed in the United States is from petroleum used to produce fuels for automobiles, recreational vehicles, home heating, or industrial production.

The principal use of petroleum is the production of gasoline. Over 40% all of all production ends up consumed in automobiles and such. Smaller fractions are turned into fuel oil (27%), jet fuel (7.4%), and other miscellaneous fuels, while the small fraction (about 10%) is used for the synthesis of the thousands of petrochemicals used in our daily lives. Indeed, many food compounds and pharmaceuticals owe their synthesis to a petrochemical precursor.

The second most prominent and naturally most abundant fossil fuel is coal. Coal also originates from decayed vegetative material buried eons ago, but the process is slightly different, being less oxidizing. The resulting material still has some of the original lignin-like structure exhibiting many fused rings and a large fraction of aromatic compounds. Consequently, coal is more of a polymeric substance than petroleum and is found as a solid not a liquid. The carbon to hydrogen ratio in coal is close to 1:1 (depending upon the type of coal), whereas the carbon to hydrogen ration in petroleum is closer to the 1:2 value expected for a hydrocarbon chain.

Minable coal is defined as 50% of the coal in a seam of at least 12 in thickness. The proven reserves of minable coal are sufficient to supply the industrial needs of modern society for the next four to five hundred years. Unfortunately, as a fuel source, coal has many disadvantages. It is a very dirty fuel that produces a large amount of unburned hydrocarbon, particulate, and—most damaging of all—significant quantities of sulfur dioxide byproducts. Indeed, it is the coal burning power plants of the eastern United States that are responsible for much of the acid rain and environmental damage observed in upstate New York and eastern Canada. The other significant disadvantage of coal is that it is not liquid, making it awkward to transport and store and limiting its use in applications like automobiles. A great deal of research has been done on the liquefaction of coal but with little economically viable success.

The third major fossil fuel is natural gas . This is a generic term for the light hydrocarbon fractions found associated with most oil deposits. Natural gas is mostly methane with small quantities of ethane and other gases mixed in. It is hydrogenrich, since methane has a carbon to hydrogen ratio of 1:4. It is also an excellent fuel, burning with a high heat output and little in the way of unwanted pollution. It does produce carbon dioxide , which is a greenhouse gas, but all organic compounds also generate carbon dioxide on combustion. Natural gas is also easy to transport through pressurized pipelines.

All of this would appear to make natural gas the perfect fuel. However, it is not without its drawbacks. This includes the presence of hydrogen sulfide in some gas fields, leading to the term "sour gas." Hydrogen sulfide is the smell of rotten eggs, but if smell were the only problem, this would be of little concern. However, hydrogen sulfide is extremely corrosive to the pipes used to transport natural gas and is a very toxic compound being lethal at levels around 1,500 ppm.

In addition, natural gas is potentially explosive as the gas must be maintained under pressure, and any hydrocarbon, in a gaseous state, can explode. This is in contrast to the use of gasoline, which is a much safer fuel. Nevertheless, both the gaseous and liquid forms of hydrocarbons are much more volatile and represent a hazard compared to coal.

It has been suggested that because all of the oxygen in the atmosphere came from the splitting of carbon dioxide via photosynthesis, the total oxygen content may lead to an estimate of the total carbon reserves. However, it is not the total abundance of fuels that is critical—it is the accessibility from both an engineering and economics standpoint that makes near-term global fuel shortages appear probable.

In addition there are serious flaws in the gross estimate of fuels as a significant amount of the world's carbon reserves are tied up in calcium carbonate rock formations. Loosely speaking, calcium carbonate is limestone and there is abundance of limestone present throughout the world. Accordingly, most industry analysts place available fossil fuel reserves at much lower levels. It is estimated by a wide variety of sources that we will reach maximum oil production in the next twenty years. After that, production will decline worldwide and we will be forced to wean ourselves from an oil-based society. The estimates for coal provide a slightly better prognosis giving a window of about 500 years for consumption of all known reserves.

Natural gas also has one significant advantage over the other fossil fuels: it is "renewable." Natural gas is found as a side product of any decaying material. Methanogenic bacteria—literally, methane-making bacteria—exist in the garbage dumps and waste disposal sites of the industrial world, busily producing methane from garbage. Enough that the Fresh Kills garbage dump on Staten Island, New York is capable of heating 16,000 homes.

Various companies have been exploring the use of hydrogen as a fuel. When used in simple combustion, hydrogen has some of the problems associated with natural gas. It must be stored under pressure and is extremely explosive upon ignition in air (e.g. the Hindenburg disaster, in which a hydrogen-filled airship explosively burned). The type of explosion—the shape of the detonation—also makes hydrogen unsuitable as an alternative fuel for the conventional automobile. The sharpness of the explosion would quickly rattle pistons to pieces.

However, hydrogen does not need to be burned directly with oxygen to provide energy. Fuel cells combine hydrogen and oxygen at electrodes to produce electricity , which can then be used to run an electric motor or a spacecraft. NASA has been employing hydrogen/oxygen fuel cells for years to provide the electricity for both manned and unmanned spacecraft. In addition, fuel cells have the added bonus of providing crew members with fresh drinking water , as the only product of the reaction is pure water. Using fuel cells, hydrogen could potentially be used for conventional automobiles. Questions about storing hydrogen and long-term viability of the fuel cells need to be addressed, but the future of this technology looks promising. At present, however, our economically viable supply of hydrogen is obtained from fossil fuels because hydrogen is released from hydrocarbons during the refining process.

Both the use of sunlight and solar panels to create sufficient electricity to electrolyze water and bacteria capable of splitting water to generate hydrogen and oxygen may make hydrogen the chemical fuel of the future. In addition, research is underway into the use of methanol as a potential partner for a fuel cell, eliminating the need for hydrogen and greatly reducing the difficulties with storage and filling the tank.

The United States leads the world in energy consumption—over 90 quadrillion Btu are used annually in the United States alone (year 2001 estimates).

See also Chemical bonds and physical properties; Energy transformations; Environmental pollution; Petroleum detection; Petroleum, economic uses of; Petroleum extraction; Petroleum, history of exploration