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.