FOSSIL FUEL

any of a class of materials of biological origin occurring within the Earth's crust that can be used as a source of energy. Fossil fuels include coal, natural gas, petroleum, shale oil, and bitumen. They all contain carbon and were formed as a result of geologic processes from the remains of organic matter produced by photosynthesis hundreds of millions of years ago. All fossil fuels can be burned with air or with oxygen derived from the air to provide heat. This heat may be employed directly, as in the case of a home furnace, or utilized to produce steam with which to drive a turbogenerator so that it can supply electricity. In still other cases, as, for example, gas turbines used in jet aircraft, the heat yielded by burning a fossil fuel can serve to increase both the pressure and the temperature of the combustion products to furnish motive power. Since the late 18th century, fossil fuels have been consumed at an ever-increasing rate. Today, they supply nearly 90 percent of all the energy consumed by the industrially developed nations of the world. New deposits continue to be discovered, but the reserves of these principal fossil fuels remaining in the Earth are limited. The amounts that can be recovered economically are difficult to estimate and are in large part contingent on their assumed rate of consumption and future value, as well as on technological developments. For example, a coal bed must be no less than 60 cm (24 inches) thick and be buried no more than about 2,000 m (6,560 feet) to be mined economically with available equipment and techniques. Advances in technology, however, may make it possible to mine thinner beds at greater depths at reasonable cost, thereby increasing the amount of recoverable coal. Estimating remaining oil resources is equally difficult. However, as recoverable deposits of conventional (light-to-medium) oil become depleted, it is expected that heavy oil and syncrudes from tar sands and oil shales will be exploited as sources of liquid petroleum on a wide scale.