Alcohol based fuels are of interest for several reasons, but primarily because they are easily produced and because they can be delivered and utilized much like conventional fossil fuels. The major drawback to their implementation is that they still produce greenhouse gases when burned, thought not at the same levels as traditional fuels.
There are four alcohols primarily considered for fuel: methanol, ethanol, propanol, and butanol. Methanol and ethanol are both relatively simple to produce and can both be created from carbon dioxide, which could reduce their net contribution to greenhouse emissions.
Any of these fuels are listed as biofuels if they are produced from feedstocks. Ethanol is commonly produced from sugar cane, potato, and corn. There are some concerns that relying on biofuels will lead to increased food prices.
Ethanol is currently mixed with gasoline in many cases because of its superior octane. 100% ethanol has an octane rating of roughly 99. In testing, it has been shown to reduce emissions of carbon monoxide, particular, and nitrous oxide over conventional fuels. E85 shows reductions in emission of all three contaminants from 12% to 25%.
In 2007, the world used roughly 52 billion liters of ethanol as fuel, primarily in the United States and Brazil. Together, these two countries account for 88% of world ethanol use for transportation. Brazil is the only country to support 100% ethanol fuel vehicles. Brazil is able to use 100% ethanol because it is a warmer climate. In colder locations, it can be difficult to achieve high enough vapor pressures to spark ignition. This is why Europe and the U.S. have instituted the maximum for ethanol fuels at E85 and some places will see seasonal reduction to E70.
There are several drawbacks to using ethanol. First, it is corrosive to plastics and rubbers used in most cars. In order to use gasoline that contains more than 10% ethanol, the rubber and plastic components of most vehicles must be upgraded to resistant forms.
Another drawback of ethanol is its energy density. At 21.1 megajoules per liter, is would require approximately 1.5 liters of ethanol to create the same energy as in a liter of gasoline. It is often pointed out that because alcohol-fueled engines can be more energy efficient, the cars can actually travel a greater distance on the same volume of alcohol as they can gasoline. The reason for this is the high octane of ethanol, which allows for engines with higher compression.
It is important to remember that the carbon dioxide emissions of the ethanol may be lower when burned, but that on an energy per liter basis, ethanol produces 19% more carbon dioxide for energy created than gasoline. In addition, total carbon dioxide emissions starting with the production of food stocks results in much higher greenhouse gas emissions than conventional fossil fuels.
It is estimated that the health costs to the citizens of the United States for burning one billion gallons of gasoline amount to $469 million. For the same quantity of ethanol, costs are estimated at somewhere between $472 million and $952 million when produced from food stock (corn mostly) but at only $123 million to $208 million for cellulosic production (also depending on the type of cellulose used).
Butanol and Propanol
These two fuels are less toxic and less combustible than methanol. However, producing them is more difficult. These alcohols are more energy dense than ethanol and methanol and can be produced from cellulose and waste products of agriculture. In fact, these alcohols could be produced from left over fiber after sugar crops are harvested. In most cases, these alcohols are created by fermentation with either yeast or bacteria.
The bacterium that is most often used is called Clostridium acetobutylicum. It turns cellulose into alcohol, but smells awful. In addition, because some ethanol is produced in the process, and these bacteria begin to die when ethanol reaches 7%, their efficiency is rather poor. Yeast can survive up to 14% ethanol, but are more difficult to grow in general.