Alcoholic Fuels: An Overview

INTRODUCTION
Alcohol-based fuels have been important energy sources since the 1800s. As early as 1894, France and Germany were using ethanol in internal combustion engines. Henry Ford was quoted in 1925 as saying that ethanol was the fuel of the future [1]. He was not the only supporter of ethanol in the early 20th century. Alexander Graham Bell was a promoter of ethanol, because the decreased emission to burning ethanol [2]. Thomas Edison also backed the idea of industrial uses for farm products and supported Henry Ford’s campaign for ethanol [3]. Over the years and across the world, alcohol-based fuels have seen short-term increases in use depending on the current strategic or economic situation at that time in the country of interest. For instance, the United States saw a resurgence in ethanol fuel during the oil crisis of the 1970s [4]. Alcohols have been used as fuels in three main ways: as a fuel for a combustion engine (replacing gasoline), as a fuel additive to achieve octane boosting (or antiknock) effects similar to the petroleum-based additives and metallic additives like tetraethyllead, and as a fuel for direct conversion of chemical energy into electrical energy in a fuel cell.

Alcohols are of the oxygenate family. They are hydrocarbons with hydroxyl functional groups. The oxygen of the hydroxyl group contributes to combustion. The four most simplistic alcoholic fuels are methanol, ethanol, propanol, and butanol. More complex alcohols can be used as fuels; however, they have not shown to be commercially viable. Alcohol fuels are currently used both in combustion engines and fuel cells, but the chemistry occurring in both systems is the same. In theory, alcohol fuels in engines and fuel cells are oxidized to form carbon dioxide and water. In reality, incomplete oxidation is an issue and causes many toxic by-products including carbon monoxide, aldehydes, carboxylates, and even ketones. The generic reaction for complete alcohol oxidation in either a combustion engines or a fuel cell is



It is important to note this reaction occurs in a single chamber in a combustion engine to convert chemical energy to mechanical energy and heat, while in a fuel cell, this reaction occurs in two separate chambers (an anode chamber where the
alcohol is oxidized to carbon dioxide and a cathode chamber where oxygen is
reduced to water.)

ETHANOL
Ethanol (also known as ethyl alcohol) is the most common of alcohols. It is the form of alcohol that is in alcoholic beverages and is easily produced from corn, sugar, or fruits through fermentation of carbohydrates. Its chemical structure is
CH3CH2OH. It is less toxic than methanol. The LD50 for oral consumption by a rat is 7060 mg/kg [5]. The LD50 for inhalation by a rat is 20,000 ppm for 10 hours [6]. The NIOSH recommended exposure limit is 1000 ppm for 10 hours [7]. Ethanol is available in a pure form and a denatured form. Denatured ethanol contains a small concentration of poisonous substance (frequently methanol) to prevent people from drinking it. Ethanol is a colorless liquid with a melting point of –144°C and a boiling point of 78°C. It is less dense than water with a density of 0.789 g/ml and soluble at all concentrations in water. Ethanol is frequently used to form blended gasoline fuels in concentrations between 10–85%. More recently, it has been investigated as a fuel for direct ethanol fuel cells (DEFC)and biofuel cells. Ethanol was deemed the “fuel of the future” by Henry Ford and has continued to be the most popular alcoholic fuel for several reasons: (1)it is produced from renewable agricultural products (corn, sugar, molasses, etc.) rather than nonrenewable petroleum products, (2) it is less toxic than the other alcohol fuels, and (3) the incomplete oxidation by-products of ethanol oxidation (acetic acid (vinegar) and acetaldehyde) are less toxic than the incomplete oxidation by-products of other alcohol oxidation.

BUTANOL
Butanol is the most complex of the alcohol-based fuels. It is a four-carbon alcohol
with a structure of CH3CH2CH2CH2OH. Butanol is more toxic than either methanol or ethanol. The LD50 for oral consumption of butanol by a rat is 790 mg/kg. The LD50
for skin adsorption of butanol by a rabbit is 3400 mg/kg. The boiling point of butanol is 118°C and the melting point is –89°C. The density of butanol is 0.81 g/mL, so it is more dense than the other two alcohols, but less dense than water. Butanol is commonly used as a solvent, but is also a candidate for use as a fuel. Butanol can be made from either petroleum or fermentation of agricultural products. Originally, butanol was manufactured from agricultural products in a fermentation process referred to as ABE, because it produced Acetone-Butanol and Ethanol. Currently, most butanol is produced from petroleum, which causes butanol to cost more than ethanol, even though it has some favorable physical properties compared to ethanol. It has a higher energy content than ethanol. The vapor pressure of butanol is 0.33 psi, which is almost an order of magnitude less than ethanol (2.0 psi) and less than both methanol (4.6 psi) and gasoline (4.5 psi). This decrease in vapor pressure means that there are less problems with evaporation of butanol than the other fuels, which makes it safer and more environmentally friendly than the other fuels. Butanol has been proposed as a replacement for ethanol in blended fuels, but it is currently more costly than ethanol. Butanol has also been proposed for use in a direct butanol fuel cell, but the efficiency of the fuel cell is poor because incomplete oxidation products easily passivate the platinum catalyst in a traditional fuel cell.

PROPANOL
Although propanols are three carbon alcohols with the general formula C3H8O, they are rarely used as fuels. Isopropanol (also called rubbing alcohol) is frequently
used as a disinfectant and considered to be a better disinfectant than ethanol, but it is rarely used as a fuel. It is a colorless liquid like the other alcohols and is flammable. It has a pungent odor that is noticeable at concentrations as low as 3 ppm. Isopropanol is also used as an industrial solvent and as a gasoline additive for dealing with problems of water or ice in fuel lines. It has a freezing point of –89°C and a boiling point of 83°C. Isopropanol is typically produced from propene from decomposed petroleum, but can also be produced from fermentation of sugars. Isopropanol is commonly used for chemical synthesis or as a solvent, so almost 2M tons are produced worldwide.

CONCLUSIONS
In today’s fuel market, methanol and ethanol are the only commercially viable fuels. Both methanol and ethanol have been blended with gasoline, but ethanol is the current choice for gasoline blends. Methanol has found its place in the market as an additive for biodiesel and as a fuel for direct methanol fuel cells, which are being studied as an alternative for rechargeable batteries in small electronic devices. Currently, butanol is too expensive to compete with ethanol in the blended fuel market, but researchers are working on methods to decrease cost and efficiency of production to allow for butanol blends, because the vapor pressure difference has environmental advantages. Governmental initiatives should ensure an increased use of alcohol-based fuels in automobiles and other energy conversion devices.

From:
Alcoholic Fuels
Shelley Minteer
Saint Louis University
Missouri