gives: Non-petroleum fossil fuels When petroleum is not easily available, chemical processes such as the Fischer–Tropsch process can be used to produce liquid fuels from coal or natural gas. Synthetic fuels from coal were strategically important during World War II for the German military. Today synthetic fuels produced from natural gas are manufactured, to take advantage of the higher value of liquid fuels in transportation. Natural gas Natural gas, composed chiefly of methane, can be compressed to a liquid and used as a substitute for other traditional liquid fuels. Its combustion is very clean compared to other hydrocarbon fuels, but the fuel's low boiling point requires the fuel to be kept at high pressures to keep it in the liquid state. Though it has a much lower flash point than fuels such as gasoline, it is in many ways safer due to its higher autoignition temperature and its low density, which causes it to dissipate when released in air. Biodiesel Biodiesel is similar to diesel but has differences akin to those between petrol and ethanol. For instance, biodiesel has a higher cetane rating (45-60 compared to 45-50 for crude-oil-derived diesel) and it acts as a cleaning agent to get rid of dirt and deposits. It has been argued that it only becomes economically feasible above oil prices of $80 (£40 or €60 as of late February, 2007) per barrel. This does, however, depend on locality, economic situation, government stance on biodiesel and a host of other factors- and it has been proven to be viable at much lower costs in some countries. Also, it yields about 10% less energy than ordinary diesel. Analogous to the use of higher compression ratios used for engines burning higher octane alcohols and petrol in spark-ignition engines, taking advantage of biodiesel's high cetane rating can potentially overcome the energy deficit compared to ordinary Number 2 diesel. Alcohols Generally, the term alcohol refers to ethanol, the first organic chemical produced by humans, but any alcohol can be burned as a fuel. Ethanol and methanol are the most common, being sufficiently inexpensive to be useful. Methanol Methanol is the lightest and simplest alcohol, produced from the natural gas component methane. Its application is limited primarily due to its toxicity (similar to gasoline), but also due to its high corrosivity and miscibility with water. Small amounts are used in some types of gasoline to increase the octane rating.