Ethanol is the name for a chemical compound which resembles ethane (C2H6), but has one hydrogen atom replaced by a hydroxide ion, thus giving C2H5OH. Most ethanol is produced at present by fermenting crop sugars (such as sugarcane in Brazil, or corn in the United States) in a highly aqueous solution, since, if the solution is insufficiently diluted, the microorganisms that carry out the fermentation cannot survive. In order to make a transportation fuel that is 99% or more pure ethanol, the producer must then distill the solution to remove the water, which is a highly energy-intensive process; estimates vary, but the total energy consumption in an ethanol plant may represent on the order of 50 to 70% of the energy content per unit of ethanol, and much of this energy is devoted to distillation. The energy content per unit volume for ethanol is lower than that of gasoline (21.3 MJ/L or 75,700 Btu/gal net energy content, compared to 32.5 MJ/L or 115,400 Btu/gal), so any comparisons of ethanol and gasoline must be conducted on an equivalent energy content basis.

It is possible to combust pure ethanol in a spark-ignition (SI) engine with appropriate modifications, or to blend it with gasoline and sell to consumers under the label "EXX," where XX stands for the percentage ethanol in the blend. SI engines can combust up to 10% ethanol without modification. Above this ratio, the engine must be modified to function well with the higher proportion of ethanol. In flex-fuel ethanol vehicles, the engine detects the proportion of ethanol and adjusts combustion accordingly. Different countries have taken different approaches. In Brazil, the passenger car fleet includes a mixture of vehicles that run on 100% ethanol, vehicles that use a fuel made up of a fixed ratio of gasoline to ethanol, and flex-fuel vehicles that can adapt to changing proportions. In the United States, vehicles that can run on over E10 ethanol are flex-fuel vehicles, and can combust ethanol mixtures up to E85.

While world ethanol production is currently only a small fraction of the total output of gasoline, there has been robust growth in recent years. U. S. production of ethanol grew from 7.6 to 18.5 billion liters from 2001 to 2006 (2.0 to 4.9 billion gallons). Brazil produced 17.0 billion liters (4.5 billion gallons) in 2006, making it the second largest producer in the world after the United States. Note that the energy content of the total world ethanol production of 51.1 billion liters (13.5 billion gallons) of ethanol in 2006 is

1.1 EJ, compared to 179 EJ content in all crude oil production in 2005. There is room for further growth in output before limitations on the total size of corn or sugar cane crops would curtail production. As an indicator of the potential upper bound on output in the United States, dedicating the entire 2005 corn crop to ethanol production would have displaced 12% of the gasoline demand in that year, compared to 1.7% that was actually displaced. Dedicating all corn to ethanol would of course be impossible, but by expanding corn production, reducing exports, or finding substitutes for corn in other applications, substantial growth could be achieved.

One attraction of ethanol production in the United States is that it serves as a petroleum multiplier, in that it takes a relatively small petroleum input to the life cycle (e. g., for truck or rail transportation to move the corn crop or ethanol product), and converts this input into a much larger quantity of ethanol, in terms of energy equivalent. This process requires large inputs of other fossil energy sources, but since gas and coal are used for these inputs and the United States primarily relies on domestic reserves for these fuels, ethanol helps the U. S. balance of trade.

As corn-based ethanol production has grown in the United States, so has the debate about whether or not it is environmentally beneficial, based on ethanol’s NEB: both positive and negative values have been reported. This debate is, unfortunately, misguided in a number of ways:

• The outcome of whether or not the NEB is positive or negative depends largely on the specific process studied. There is a large amount of variability in ethanol production processes, and it would be difficult to create a study that truly represented the “average” process. Rather than attempting to generalize whether the NEB is positive or not, the research community should attempt to identify best practices and apply them not only to ethanol production across the board but to other emerging biofuels.

• Even those studies that see a positive NEB for ethanol find it to be modest (e. g., 8 or 9 units of energy in for 10 units out), confirming that ethanol is at best a stepping stone toward more effective biofuels in the future, rather than an end in itself. Again, research effort should be applied elsewhere. Note that this limit on NEB does not extend to other ethanol feedstocks such as the use of sugarcane for ethanol production in Brazil. The Brazilian process has a much better NEB: for every unit of energy expended to grow and process the sugarcane into ethanol, approximately three units of energy content are delivered in the fuel. However, sugarcane is a tropical crop and is not available to temperate regions such as the United States.

• The debate over energy inputs and outputs is a distraction from a much more important concern, namely, whether diverting too much corn to ethanol, and achieving modest environmental gains at best, will eventual have repercussions on basic food supplies that the United States as a country comes to regret. Already, U. S. corn prices have increased markedly in recent years, leading to a knock-on effect on many other food prices.

Overall, it is clear that, for ethanol production in temperature regions to have a real positive effect on the environmental bottom line, the ethanol production process will need to be changed to reduce energy input required, use nonfossil resources, use non­crop raw materials, or some mixture of the three. These points are revisited at the end of this section.

Updated: October 27, 2015 — 12:09 pm