James G. Speight - Encyclopedia of Renewable Energy

See also: Alcohols, Biodiesel, Butanol, Diesel Fuel, Ethanol, Hydroshear Emulsification, Methanol, Propanol.

Various alcohols (C nH 2n+1OH) are used as fuel for internal combustion engines. The first four aliphatic alcohols (methanol, ethanol, propanol, and butanol) are of interest as fuels because they can be synthesized chemically or biologically, and they have characteristics which allow them to be used in internal combustion engines. When obtained from biological materials and/or biological processes, the alcohols are often referred to as bioalcohols (for example, bioethanol). However, there is no chemical difference between biologically produced and chemically produced alcohols.

Most methanol is produced from natural gas, although it can be produced from biomass using similar chemical processes. Ethanol is commonly produced from biological material through fermentation processes. Butanol has the advantage in combustion engines in that its energy density is closer to gasoline than the simpler alcohols (while still retaining over 25% higher octane rating). However, biobutanol is currently more difficult to produce than ethanol or methanol.

One advantage shared by the four major alcohol fuels is the high octane rating which tends to increase the fuel efficiency and largely offsets the lower energy density of vehicular alcohol fuels (as compared to gasoline and diesel fuels), thus resulting in comparable fuel economy in terms of distance per volume metrics, such as kilometers per liter, or miles per gallon.

See also: Alcohol-Blended Fuel.

Ethanol is the predominant fuel produced form crops that, because of favorable properties, has been used as fuel in the United States since at least 1908, along with methanol and gasoline ( Table A-10).

Table A-10Selected physical and chemical properties of methanol, ethanol, and gasoline.

Although early efforts to sustain an ethanol program failed, oil supply disruptions in the Middle East and environmental concerns over the use of lead as a gasoline octane booster renewed interest in ethanol in the late 1970s. At present, extending the volume of conventional gasoline is a significant end use for ethanol, as is its use as an oxygenate. To succeed in these markets, the cost of ethanol must be close to the wholesale price of gasoline, currently made possible by the federal ethanol subsidy. However, in order for ethanol to compete on its own merit,s the cost of producing it must be reduced substantially.

The production of ethanol from corn is a mature technology that holds much potential. Substantial cost reductions may be possible, however, if cellulose-based feedstocks are used instead of corn. Producers are experimenting with units equipped to convert cellulose-based feedstocks, using sulfuric acid to break down cellulose and hemicellulose into fermentable sugar. Although the process is expensive at present, advances in biotechnology could decrease conversion costs substantially. The feed for all ethanol fermentations is sugar - traditionally a hexose (a six-carbon or “C6” sugar) such as those present naturally in sugar cane, sugar beet, and molasses. Sugar for fermentation can also be recovered from starch, which is actually a polymer of hexose sugars ( polysaccharide ).

Biomass, in the form of wood and agricultural residues such as wheat straw, is viewed as a low cost alternative feed to sugar and starch. It is also potentially available in far greater quantities than sugar and starch feeds. As such, it receives significant attention as a feed material for ethanol production. Like starch, wood and agricultural residues contain polysaccharides. However, unlike starch, while the cellulose fraction of biomass is principally a polymer of easily fermented hexose (C6) sugars, the hemicellulose fraction is principally a polymer of pentose (C5) sugars; with quite different characteristics for recovery and fermentation, the cellulose and hemicellulose in biomass are bound together in a complex framework of crystalline organic material known as lignin.

These differences mean that recovery of these biomass sugars is more complex than recovery of sugars from a starch feedstock. Once recovered, fermentation is also more complex than a simple fermentation of hexose sugars. The current focus focuses on the issues of releasing the sugars (hydrolysis) and then fermenting as much of the C6 and C5 sugars as possible to produce ethanol.

There are several different methods of hydrolysis: (i) concentrated sulfuric acid, (ii) dilute sulfuric acid, (iii) nitric acid, and (iv) acid pretreatment followed by enzymatic hydrolysis.

Ethanol is produced from the fermentation of sugar by enzymes produced from specific varieties of yeast. The five major sugars are the five-carbon xylose and arabinose and the six-carbon glucose, galactose, and mannose. Traditional fermentation processes rely on yeasts that convert six-carbon sugars to ethanol. Glucose, the preferred form of sugar for fermentation, is contained in both carbohydrates and cellulose. Because carbohydrates are easier than cellulose to convert to glucose, the majority of ethanol currently produced in the United States is made from corn, which produces large quantities of carbohydrates. Also, the organisms and enzymes for carbohydrate conversion and glucose fermentation on a commercial scale are readily available.

The conversion of cellulosic biomass to ethanol parallels the corn conversion process. The cellulose must first be converted to sugars by hydrolysis and then fermented to produce ethanol. Cellulosic feedstocks (composed of cellulose and hemicellulose) are more difficult to convert to sugar than are carbohydrates. Two common methods for converting cellulose to sugar are dilute acid hydrolysis and concentrated acid hydrolysis, both of which use sulfuric acid.

Dilute acid hydrolysis occurs in two stages to take advantage of the differences between hemicellulose and cellulose. The first stage is performed at low temperature to maximize the yield from the hemicellulose, and the second, higher temperature stage is optimized for hydrolysis of the cellulose portion of the feedstock. Concentrated acid hydrolysis uses a dilute acid pretreatment to separate the hemicellulose and cellulose. The biomass is then dried before the addition of the concentrated sulfuric acid. Water is added to dilute the acid and then heated to release the sugars, producing a gel that can be separated from residual solids. Column chromatographic is used to separate the acid from the sugars.