Medieval Baghdad had tar-paved streets and a doctor who distilled kerosene
Long before the oil age, the streets of Baghdad were paved with tar from nearby petroleum fields, and the Persian physician al-Razi wrote down how to distil crude oil into kerosene. He also described a lamp burning mineral oil, which he called the naffatah. Marco Polo later reported wells near Baku yielding hundreds of shiploads.
A fuel is anything that reacts to release energy as heat or work. The term first covered chemical sources like wood and oil, but now also stretches to nuclear fission and fusion. It is distinct from things that merely store energy, such as batteries, springs, flywheels or water held behind a dam. Even our bodies run on fuel, oxidising organic molecules inside cells in the process known as cellular respiration.
Humanity's fuel story starts with fire. Homo erectus was burning firewood nearly two million years ago, and controlled fire at Swartkrans in South Africa dates back as far as 1.5 million years, though nobody knows which hominid lit it. For most of history people burned only plant material and animal fat. Charcoal, used for smelting metal since at least 6,000 BCE, gave way to coke from coal in the 18th century as European forests ran short, and it survives today in barbecue briquettes.
The steam engine of 1769 made coal king, powering ships and locomotives, and by the 19th century gas made from coal lit London's streets. In 2005 coal still generated 40 percent of the world's electricity. Fuels are usually sorted as solid, liquid or gas, and as natural or manufactured. With liquids, curiously, it is the vapour that burns rather than the fluid. Propane ranks as the world's third most used motor fuel, and fuel gases have smelly odorisers added so that a dangerous leak gets noticed.
Energy density helps explain history's shifts: wood holds about 10 to 20 megajoules per kilogram, and more concentrated, flexible fossil fuels overtook water power during the Industrial Revolution. In 2007 petroleum, coal and gas together supplied over 86 percent of world primary energy. Biofuels such as bioethanol and biodiesel have since grown, although how carbon neutral they truly are remains widely debated.
Source: Fuel
