Non-renewable energy resources are sources of energy drawn from nature that cannot be replenished. These include wood, coal, petroleum and radioactive material (for nuclear fission). The possibility of exhausting these energy resources is an increasingly serious problem in the modern world, as consumption increases with expanding worldwide industrialization and the growing world population. The extraction and refinement of non-renewable energy resources has often caused pollution and interfered with the aesthetics and utility of the environment. Awareness of the scope of the problem has encouraged conservation efforts and the use of alternative fuel sources.

Wood is one of the oldest sources of energy, and for centuries seemed inexhaustible. With the onset of the Industrial Revolution, however, the amount of available wood was not enough to meet the needs of increasing numbers of factories and mills. In addition, other uses of wood (e.g., in furniture...