Fuels are substances that are burned to produce energy. In many practical situations, it can be advantageous first to carry out one or more processing steps on a fuel before it is burned. This might be done to improve the yield of the fuel from its source, to improve the performance of the fuel during combustion, or to mitigate potential environmental problems resulting from using the fuel. Examples include processes to enhance the yield of gasoline from petroleum, to improve gasoline performance in engines, and to convert solid coal into cleaner gaseous or liquid fuels. Some fuels, particularly natural gas and petroleum, also serve as important feedstocks for the organic chemical industry, for producing a host of useful materials. So, fuels can be used in at least three different ways: burned directly to release thermal energy; chemically transformed to cleaner or more convenient fuel forms; or converted to non-fuel chemicals or materials. These uses might appear quite different at first sight, but all have in common the making and breaking of chemical bonds and transformation of molecular structures. The ways in which we use fuels, and their behavior during conversion or utilization processes, necessarily depend on their chemical composition and molecular structure.

The world is now in a transition state between an energy economy that, in most nations, has an overwhelming dependence on petroleum, natural gas, and coal, to a new energy economy that will be based heavily on alternative, renewable sources of energy, including fuels derived from plants. This book covers both. The dominant focus is on wood, ethanol, and biodiesel among the plant-derived fuels, and on coal, petroleum, and natural gas as traditional fuels. If we were to assemble a collection of examples of each, at first sight they would appear to be wildly different. Natural gas, a transparent, colorless gas, commonly contains more than ninety percent of a single compound, methane, at least as delivered to the user. Ethanol, a transparent, volatile, low-viscosity liquid, is a single compound. Petroleum is a solution of several thousand individual compounds. Depending on its source, the color, viscosity, and odor can be very variable. Biodiesel, a lightly colored, moderate viscosity liquid, contains only perhaps a half-dozen individual compounds. Wood, a heterogeneous solid, is usually of light color, but varies in density, hardness, and color, depending on its source. Coals usually are black or brown heterogeneous solids of ill-defined and variable macromolecular structure.