Wood was the primary energy source throughout most of human history and remained so up until the industrial revolution. Population increases in Europe reduced the forest cover from 95 percent during the Roman Empire to 20 percent at the start of the industrial revolution (Figure 31.1). The shortage of wood and the energy density of coal combined with the steam engine to usher in the age of fossil fuels (Gore 2009, 52).
Conventional fuels can be classified as solid fuel, coal, liquid fuel, oil, and oil byproducts, such as gasoline, and gaseous fuels, such as methane. Liquid fuels have very high energy content per volume and are easily moved through pipelines and tanker trucks to distribution points where vehicles can have their gas or oil tanks filled up. These characteristics make liquid fuels very useful for transportation. Oil is currently the fuel society uses the most (Gore 2009, 54). Natural gas is the next primary energy source after oil. Natural gas is used primarily in industry but also to heat homes. Natural gas is the cleanest of the conventional fuels because it has fewer impurities and it has a higher ratio of hydrogen to carbon (Gore 2009, 55). When a fuel is burned the fuel is oxidized. The atoms in the fuel are rearranged to a lower energy state with the accompanying release of heat energy. The energy comes from the rearrangement of the hydrogen (H) and the oxygen (O) atoms. Methane is CH4, and oxygen is O2.
Sometimes there are other atoms in a fuel, such as sulfur and or nitrogen. Then the combustion process produces sulfur oxides and nitrous oxides, which create smog. These impurities can be significantly cleaned from the exhaust gases, but usually not completely so. An important concept to understand is that the complete combustion of a fuel produces carbon dioxide (CO2) and water (H2O). There is no way to combust a fuel without producing carbon dioxide.
