What is a synfuel?

To understand how various feedstocks can be converted into liquid synthetic fuels, you first need to understand how fuel works. Oil, and products like gasoline made from oil, are made up of long chains of organic molecules called hydrocarbons (because they contain hydrogen and carbon). When those hydrocarbons are burned, they break down and release energy, which is used to fuel the engines in cars, trucks, planes, etc. Most organic material, including oil, coal, natural gas, plant waste and sewage, contains hydrocarbons. Today's engines were designed to work with oil-derived fuels like gasoline. In order for synthetic fuels to work in those engines, their hydrocarbons have to be restructured so they resemble the hydrocarbons found in petroleum and petroleum products.

There are basically two categories of synthetic fuels, synthetic crude oils (syncrude), and Fischer-Tropsch liquids.The first category includes feedstocks and processes that are used to produce syncrude, or synthetic crude oil. Synthetic crude oil can be used for the same purposes as conventional crude oil. It is used as a raw material. Like conventional crude oil, syncrude must be refined and processed to make the various forms of petroleum-based commercial fuels like diesel, gasoline and kerosene.

The three most popular sources of syncrude are extra-heavy oil, oil shale and oil sands. Each of those materials occurs naturally just like conventional oil, but they have different physical properties and amounts of impurities. For example, oil shale is a rock, and oil sands are a tarry mixture of sand and the oil-containing substance bitumen. These syncrude feedstocks are exposed to various levels of heat, pressure, and physical manipulation to produce a substance with the same arrangement of hydrocarbons as naturally occurring crude oil.

Processing syncrude feedstocks tends to harm the environment. Since they require more processing than crude oil, they create more CO₂ emissions and other pollutants [source: U.S. Department of the Interior]. Also, collecting the feedstock often involves harmful environmental practices like strip mining. One advantage to syncrude fuels as an alternative to oil is that the world contains substantial untapped reserves of extra-heavy oil, oil shale and oil sands. Of course, like oil, those resources are not sustainable. They, too, will run out eventually.

Read on to learn about how Fischer-Tropsch synfuels are produced.

The second type of synthetic fuels, commonly referred to as Fischer-Tropsch liquids, use feedstock that can be converted directly into commercially viable liquid fuels, essentially skipping the syncrude step. The most common feedstocks used to produce Fischer-Tropsch synfuels include natural gas, coal and biomass (plants and plant waste). In F-T synthesis, the feedstock is subjected to very high heat -- 1,900 degrees Fahrenheit (1,037.7 degrees Celsius) or higher -- and pressure to produce a mix of carbon monoxide and hydrogen called synthesis gas (or syngas) [source: Van Bibber]. This step of the process makes Fischer-Tropsch liquid fuels much cleaner than fuels produced from crude or syncrude. Impurities like heavy metals can be easily removed from the gasifier after the syngas is filtered out. Gases like carbon and sulfur can be filtered out so they don't become pollutants when the fuel burns.

Next, the syngas is condensed into liquid form. Again put under high heat and pressure, a catalyst is introduced into the process, usually a compound containing either iron or cobalt. The catalyst triggers a chemical reaction between the hydrogen and carbon monoxide, creating long chains of hydrocarbons. Different catalysts can produce different hydrocarbon structures. These hydrocarbons are then cooled and condensed into liquid form, and filtered. Along with synthetic forms of diesel fuel or gasoline, Fischer-Tropsch synthesis can produce industrial lubricants, kerosene and other products.

Compared to syncrude products, Fischer-Tropsch liquids burn much cleaner. These synfuels have almost no particulate emissions, contain less nitrogen oxide than traditional fuels and fewer carbon monoxide emissions [source: U.S. Environmental Protection Agency]. Of course, those synfuels that use renewable resources as feedstock (like biomass) are better for the environment in the long run than those that use fossil fuels as feedstock.

The U.S. government has a vested interest in synfuels, since it has abundant access to feedstock like coal, natural gas and plant waste. Taking only one variety of synfuel into account, there is an estimated 1.3 gigatons of unused biomass inside the United States that could be used to produce synfuels [source: Coal-to-Liquids Coalition]. So, the U.S. military and other government agencies have been pushing to research synfuels in recent years. Other countries like China and Germany have also made recent investments in exploring synfuel technology [Source: U.S. Energy Information Administration]. While synthetic fuel technology is promising, it costs much more to produce than gasoline from oil. So, it most likely will not replace oil unless oil prices increase dramatically.