Liquid fuels
Liquid fuels are those combustible or energy-generating molecules that can be harnessed to create mechanical energy, usually producing kinetic energy; they also must take the shape of their container. Most liquid fuels, in widespread use, are or derived from fossil fuels; however, there are several types, such as hydrogen fuel (for automotive uses), which are also categorized as a liquid fuel.
This article deals primarily with the concept of liquid fuels in relation to ground transport. However, others such as rocket fuel also play an important role in the economy.
Fossil fuels
Fossil fuels which are also liquid fuels come from dead animals and plants which died many millions of years ago. The most notable of these is gasoline.
Gasoline
Gasoline is the most widely used liquid fuel. Gasoline, as it's known in United States and Canada, (known as petrol in Britain, Australia, New Zealand, and many English-speaking countries) is made of hydrocarbon molecules forming aliphatic compounds, or chains of carbons with hydrogen atoms attached. However, many aromatic compounds (carbon chains forming rings) such as benzene are found naturally in gasoline and cause the health risks associated with prolonged exposure to the fuel.
Production of gasoline is achieved by distillation of crude oil. The desirable liquid is separated from the crude oil in refineries. Crude oil is extracted from the ground in several processes, the most commonly seen may be beam pumps. To create gasoline, petroleum must first be removed from crude oil.
Gasoline itself is actually not burned, but the fumes it creates ignite, causing the remaining liquid to evaporate. Gasoline is extremely volatile and easily combusts, making any leakage extremely dangerous. Gasoline for sale in most countries carries an octane rating. Octane is a measure of the resistance of gasoline to combusting prematurely, known as knocking. The higher the octane rating, the harder it is to burn the fuel, which allows for a higher compression ratio. Engines with a higher compression ratio produce more power (such as in race car engines). However, such engines actually require a higher octane fuel.
Diesel
Conventional diesel is similar to gasoline in that it is a mixture of aliphatic hydrocarbons extracted from petroleum. Diesel may cost more or less than gasoline, but generally costs less to produce because the extraction processes used are simpler. Many countries (particularly in Europe, as well as Canada) also have lower tax rates on diesel fuels.
After distillation, the diesel fraction is normally processed to reduce the amount of sulfur in the fuel. Sulphur causes corrosion in vehicles, acid rain and higher emissions of soot from the tail pipe (exhaust pipe). In Europe, lower sulfur levels than in the United States are legally required. However, recent US legislation will reduce the maximum sulphur content of diesel from 3,000 ppm to 500 ppm by 2007, and 15 ppm by 2010. Similar changes are also underway in Canada, Australia, New Zealand and several Asian countries.
A diesel engine is a type of internal combustion engine which ignites fuel by compressing it (which in turn raises the temperature) as opposed to using an outside source, such as a spark plug.
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 gives about 10% less energy than ordinary diesel. NOTE: As with alcohols and petrol engines, taking advantage of biodiesel's high cetane rating potentially overcomes the energy deficit compared to ordinary number 2 diesel.
Alcohols
Alcohols are a useful type of liquid fuel because they combust rapidly and are often cheap to produce. However, their acceptance is hampered by the fact that their production often requires as much or even more fossil fuel than they replaced.
Methanol
Methanol is the lightest and simplest alcohol, produced from the natural gas component methane. Its application is limited due to its toxicity. Small amounts are used in some gasolines to increase the octane rating. Methanol-based fuels are used in some race cars and model airplanes.
Methanol is also called methyl alcohol or wood alcohol, the latter because it was formerly produced from the distillation of wood.
Ethanol
Ethanol, also known as grain alcohol or ethyl alcohol, is most commonly used in alcoholic beverages. However, it may also be used as a fuel, most often in combination with gasoline. For the most part, it is used in a 9:1 ratio of gasoline to ethanol to reduce the negative environmental effects of gasoline.
There is increasing interest in the use of a blend of 85% fuel ethanol blended with 15% gasoline. This fuel blend called E85, has a higher fuel octane than most premium gasolines. When used in a modern Flexible fuel vehicle, it delivers more performance to the gasoline it replaces. [1]
Ethanol for use in gasoline and industrial purposes may be called a fossil fuel because it is synthesized from the petroleum product ethylene, which is cheaper than production from fermentation of grains or sugarcane.
Butanol
Butanol is an alcohol which may be used as a fuel with the normal combustion engine, typically as a product of the ferment of biomass with the bacterium clostridium acetobutylicum (also known as the Weizmann organism). This process was first delineated by Chaim Weizmann in 1916 for the production of acetone from starch for making TNT.
The advantages of butanol are its high octane rating (over 100) and high energy content, only about 10% lower than gasoline, and subsequently about 50% more energy-dense than ethanol, 100% more so than methanol. Butanol's only major disadvantages are its high flashpoint (95 °F or 35 °C), toxicity (note that toxicity levels exist but are not precisely confirmed), and the fact that the fermentation process for renewable butanol emits a foul odour. It also doesn't help efficiency that the Weizmann organism can only tolerate butanol levels of up to 7% or so, compared to 14% for ethanol and yeast. NOTE: Making butanol from oil produces no such odour, but the limited supply and environmental impact of oil usage defeats the purpose of alternative fuels. The cost of butanol is about $0.57-$0.58 per pound ($1250-$1320 per ton or $8 approx. per gallon) - so another drawback is its high cost in proportion to ethanol (approx. $1.50 per gallon) and methanol.
On June 20 2006, DuPont and BP announced that they were converting an existing ethanol plant to produce 9 million gallons of butanol per year from sugar beets. DuPont stated a goal of being competitive with oil at $30-$40 per barrel without subsidies, so the price gap with ethanol is narrowing.
Hydrogen
Hydrogen as a fuel is a feasible option for future use as a fuel. Liquid hydrogen is an important consideration because it has a higher density than its gaseous counterpart. Liquid hydrogen would be stored in cryogenic tanks. Its application would be most useful in fuel cells where hydrogen would react with oxygen (obviously this is readily available in the air) to create electricity which would power the vehicle.
Unfortunately, widespread use of liquid hydrogen is several decades away. Their application is plagued with several serious problems including production, which may still involve fossil fuels, durability of the fuel cells to common roadway conditions such as bumps, the impracticality of conversion of older cars and difficulties with storage and handling.
Converting energy from another form of fuel to hydrogen would lead to unavoidable losses, so the final user would get, from burning hydrogen, far less energy than he could get directly using the fuel. However, hydrogen can easily be produced through electrolysis of distilled water, by electric current. Providing the electric current from a renewable power source would provide a completely clean hydrogen producing process.