Ethanol fuel

This entry was compiled, edited and written by: Cutler Cleveland

Ethanol, also called ethyl alcohol, is a volatile, flammable, colorless liquid best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol fuel, or bioethanol, refers to ethyl alcohol used as a transport fuel, mainly as a biofuel additive for motor gasoline. Ethanol production has increased sharply and now accounts for about 5% of world motor gasoline use.

Brazil and United States accounted for 89% of global ethanol use in 2009. Most cars on the road today in the U.S. can use  blends of up to 10% ethanol, and the use of 10% ethanol gasoline is mandated in some U.S. states and cities. Since 1976 the Brazilian government has made it mandatory to blend ethanol with gasoline, and since 2007 the mandatory blend is 25% ethanol and 75% gasoline, or E25 blend.

Most of the ethanol produced today is derived from agricultural crops such as sugar cane, potato, manioc and maize. There is considerable debate about the energy, economic, social and environmental impacts of a large scale effort to replace conventional motor gasoline with ethanol derived from food crops. These concerns center on the amount of arable land required for crops, the energy and carbon balance of the ethanol fuel, and the possible tradeoffs between the use of land to grow food versus fuel,

Developments with cellulosic ethanol production and commercialization may address some of these concerns. Cellulosic ethanol is a biofuel produced from lignocellulose, a structural material that comprises much of the mass of all plants. Cellulosic ethanol thus is produced from the non-edible portion of plants, as opposed to the current method of using the edible starch portion of plants.

Ethanol is a controversial fuel. Proponents argue that ethanol is a renewable fuel that could reduce oil imports, provide additional markets for agricultural products, improve air quality, and slow the emissions of greenhouse gases (GHGs). Critics argue that ethanol from corn requires a large subsidy from fossil fuels that negates much, and possibly all, of its energy and environmental benefits, and that it diverts agricultural land away from the principal task of feeding people.

Chemistry

 Glucose (a simple sugar) is created in the plant by photosynthesis:

6CO2 + 6H2O + light → C6H12O6 + 6O2

During ethanol fermentation, glucose is decomposed into ethanol and carbon dioxide: 

C6H12O6 → 2C2H5OH+ 2CO2 + heat

During combustion ethanol reacts with oxygen to produce carbon dioxide, water, and heat:

C2H5OH + 3O2 → 2CO2 + 3H2O + heat

Ethanol feedstocks

In principle, almost any plant-based material can be an ethanol feedstock.  Theser materials fall into two general categories: starch- and sugar-based ethanol feedstocks and cellulosic ethanol feedstocks.

Starch- and sugar-based feedstocks

The vast majority of today's ethanol is derived from starch- and sugar-based feedstocks. The sugars in these feedstocks are relatively easy to extract and ferment using widely available biochemical conversion technologies, making large-scale ethanol production affordable. Starch-based feedstocks include plants such as corn, wheat, and milo. The starches in these plants are chains of sugars that can be broken down into simple sugars before fermentation. Sugar-based feedstocks, such as sugar cane and sugar beets, contain simple sugars that can be extracted and fermented readily. Corn is the feedstock for more than 90% of current U.S. ethanol production. Brazil, the world's second-largest ethanol producer behind the United States, uses sugar cane as a feedstock.

Cellulosic Ethanol Feedstocks

Plants contain the cellulosic materials cellulose and hemicellulose. These complex polymers form the structure of plant stalks, leaves, trunks, branches, and husks. They are also in products made from plants, such as paper. Cellulosic feedstocks contain sugars within their cellulose and hemicellulose, but they are more difficult to biochemically convert into ethanol than starch- and sugar-based feedstocks. Cellulose resists being broken down into its component sugars. Hemicellulose is easier to break down, but the resulting sugars are difficult to ferment. The plant compound lignin also resists biochemical conversion. Producing cellulosic ethanol is far more challenging then normal ethanol, requring the use of special enzymes or the use of gasification and biomass-to-liquids technologies. The high cost of these enzymes is one of the main limitations on the commercialization of cellulosic ethanol.

Cellulosic feedstocks suited to ethanol production include the following:

  • Agricultural residue—crop residues such as wheat straw and corn stalks, leaves, and husks
  • Forestry residue—logging and mill residues such as wood chips, sawdust, and pulping liquor
  • Grasses—hardy, fast-growing grasses such as switchgrass grown specifically for ethanol production
  • Municipal and other wastes—plant-derived wastes such as household garbage, paper products, paper pulp, and food-processing waste
  • Trees—fast-growing trees such as poplar and willow grown specifically for ethanol production

The main components of these types of biomass are:

  • Cellulose (C6H10O5)n is the most common form of carbon in biomass, accounting for 40%-60% by weight of the biomass, depending on the biomass source. It forms the main constituent of the cell wall in most plants. It is a complex sugar polymer, or polysaccharide, made from the six-carbon sugar, glucose. Its crystalline structure makes it resistant to hydrolysis, the chemical reaction that releases simple, fermentable sugars from a polysaccharide.
  • Hemicellulose is also a major source of carbon in biomass, at levels of between 20% and 40% by weight. It is a complex polysaccharide made from a variety of five- and six-carbon sugars. It is relatively easy to hydrolyze into simple sugars but the sugars are difficult to ferment to ethanol.
  • Lignin is a complex polymer, which provides structural integrity in plants. It makes up 10% to 24% by weight of biomass. It remains as residual material after the sugars in the biomass have been converted to ethanol. It contains a lot of energy and can be burned to produce steam and electricity for the biomass-to-ethanol process.

Despite the economic and technical challenges facing cellulosic ethanol, the technology has a number of potential advantages compared tostarch- and sugar-based feedstocks.  These include:

  • The use of waste biomass, such as forestry wastes, or by-products of the paper industry, allows for ethanol production without affecting agricultural production, this avoiding the food-versus-fuel problem.
  • Cellulosic biomass from fast-growing perennial energy crops, such as short rotation woody crops and tall grass crops, can be grown on a much wider range of soil types, where the extensive root systems that remain in place with these crops help prevent erosion, and increase carbon storage in soil.
  • Energy crops can often be grown on poorer soils.
  • Cellulosic biomass can be easier to store for long periods of time.
  • Compared to conventional crops, where only small portion of the plant can be used for biofuel production, perennial energy crops can supply much more biomass per hectare of land, since nearly the entire plant can be used as feedstock.

Ethanol production

Two reactions are key to the conversion of biomass to bioethanol. Hydrolysis is the chemical reaction that converts the complex polysaccharides in the raw feedstock to simple sugars. In the biomass-to-bioethanol process, acids and enzymes are used to catalyze this reaction.

Fermentation is a series of chemical reactions that convert sugars to ethanol. The fermentation reaction is caused by yeast or bacteria, which feed on the sugars. Ethanol and carbon dioxide are produced as the sugar is consumed. 

Production from starch and sugar

The basic processes for converting sugar and starch crops are well-known and used commercially today. The steps are:

  1. Biomass Handling. Biomass goes through a size-reduction step to make it easier to handle and to make the ethanol production process more efficient. For example, agricultural residues go through a grinding process and wood goes through a chipping process to achieve a uniform particle size.
  2. Biomass Pretreatment. In this step, the hemicellulose fraction of the biomass is broken down into simple sugars. A chemical reaction called hydrolysis occurs when dilute sulfuric acid is mixed with the biomass feedstock. In this hydrolysis reaction, the complex chains of sugars that make up the hemicellulose are broken, releasing simple sugars. The complex hemicellulose sugars are converted to a mix of soluble five-carbon sugars, xylose and arabinose, and soluble six-carbon sugars, mannose and galactose. A small portion of the cellulose is also converted to glucose in this step.
  3. Enzyme Production. The cellulase enzymes that are used to hydrolyze the cellulose fraction of the biomass are grown in this step. Alternatively the enzymes might be purchased from commercial enzyme companies.
  4. Cellulose Hydrolysis. In this step, the remaining cellulose is hydrolyzed to glucose. In this enzymatic hydrolysis reaction, cellulase enzymes are used to break the chains of sugars that make up the cellulose, releasing glucose. Cellulose hydrolysis is also called cellulose saccharification because it produces sugars.
  5. Glucose Fermentation. The glucose is converted to ethanol, through a process called fermentation. Fermentation is a series of chemical reactions that convert sugars to ethanol. The fermentation reaction is caused by yeast or bacteria, which feed on the sugars. As the sugars are consumed, ethanol and carbon dioxide are produced.
  6. Pentose Fermentation. The hemicellulose fraction of biomass is rich in five-carbon sugars, which are also called pentoses. Xylose is the most prevalent pentose released by the hemicellulose hydrolysis reaction. In this step, xylose is fermented using Zymomonas mobilis or other genetically engineered bacteria.
  7. Ethanol Recovery. The fermentation product from the glucose and pentose fermentation is called ethanol broth. In this step the ethanol is separated from the other components in the broth. A final dehydration step removes any remaining water from the ethanol.
  8. Lignin Utilization. Lignin and other byproducts of the biomass-to-ethanol process can be used to produce the electricity required for the ethanol production process. Burning lignin actually creates more energy than needed and selling electricity may help the process economics.
Production from cellulosic biomass 

There are two ways of producing ethanol from cellulose.  Cellulolysis processes consist of hydrolysis on pretreated lignocellulosic materials, and use enzymes to break complex cellulose into simple sugars such as glucose followed by fermentation and distillation. Gasification transforms the lignocellulosic raw material into gaseous carbon monoxide and hydrogen. These gases can be converted to ethanol by fermentation or chemical catalysis. Both methods include distillation as the final step to isolate the pure ethanol.

Converting cellulosic biomass to ethanol is currently too expensive to be used on a commercial scale. Research seeks to improve the efficiency and economics of the ethanol production process by focusing on the two most challenging steps:

Cellulose hydrolysis. The crystalline structure of cellulose makes it difficult to hydrolyze to simple sugars, ready for fermentation. Researchers are developing enzymes that work together to efficiently break down cellulose. 

Pentose fermentation. While there are a variety of yeast and bacteria that will ferment six-carbon sugars, most cannot easily ferment five-carbon sugars, which limits ethanol production from cellulosic biomass. Researchers are using genetic engineering to design microorganisms that can efficiently ferment both five- and six-carbon sugars to ethanol at the same time.

Heating and octane values

One liter of ethanol contains 66% of the energy content as one liter of gasoline, which means that cars that use ethanol require one third more fuel by volume to travel the same distance. Pure ethanol has a high octane value, which improves the performance of gasoline by reducing the likelihood that engine knock problems will occur.

Ethanol fuel mixtures

Ethanol fuel mixtures have "E" numbers that describe the percentage of ethanol in the mixture by volume, for example, E85 is 85% anhydrous ethanol and 15% gasoline. Gasoline is the typical fuel mixed with ethanol but there are other fuel additives that can be mixed, such as an ignition improver used in the E95 Swedish blend. Low ethanol blends, from E5 to E25, are also known as gasohol, though internationally the most common use of the term gasohol refers to the E10 blend.

E10 is a fuel mixture of 10% anhydrous ethanol and 90% gasoline that can be used in the internal combustion engines of most modern automobiles and light-duty vehicles without need for any modification on the engine or fuel system. E10 blends are typically rated as 2 to 3 octane higher than regular gasoline and are approved for use in all new US automobiles, and are mandated in some areas for emissions and other reasons.

E20 contains 20% ethanol and 80% gasoline, while E25 contains 25% ethanol. These blends have been widely used in Brazil since the late seventies.

E85 is a mixture of 85% ethanol and 15% gasoline, and is generally the highest ethanol fuel mixture found in the United States and several European countries, particularly in Sweden, as this blend is the standard fuel for flexible-fuel vehicles. This mixture has an octane rating of about 105, which is significantly lower than pure ethanol but still much higher than normal gasoline 87 octane. There are more than 1,900 public E85 fuel pumps in the U.S. in 2010, mostly concentrated in the Midwest, with the largest single state being Minnesota.

Fuel economy

Ethanol contains about 34% less energy per unit volume than gasoline, therefore in theory, burning pure ethanol in a vehicle will result in a 34% reduction in miles per US gallon, given the same fuel economy, compared to burning pure gasoline.  However, there are several factors that offset the lower energy value of ethanol. Since ethanol has a higher octane rating, the engine can be made more efficient by raising its compression ratio. In fact using a variable turbocharger, the compression ratio can be optimized for the fuel being used, making fuel economy almost constant for any blend. For E10 (10% ethanol and 90% gasoline), the effect is small (~3%) when compared to conventional gasoline, and even smaller when compared to oxygenated and reformulated blends.  However, for E85 (85% ethanol), the effect becomes significant. E85 will produce lower mileage than gasoline, and will require more frequent refueling. Actual performance may vary depending on the vehicle. Based on EPA tests for all 2006 E85 models, the average fuel economy for E85 vehicles resulted 25.56% lower than unleaded gasoline.

Energy balance

One of the most controversial aspects of ethanol is its net energy balance.  This refers to the energy content of a fuel compared to the energy required to produce the fuel.  This comparison is reflected in the metric known as energy return on investment (EROI), a ratio calculated by dividing the amount of useful energy produced by a given process, divided by the energy used in that process. research in the 1970s and 1980s suggested that the EROI for ethanol from corn was less than 1, i.e., that it took more energy to make a gallon of ethanol than was actually in the ethanol itself.  This result was due to the large amounts of energy used to grow the corn--including the energy in the machinery, fertilizers, chemicals, etc--and the energy required to build and operate the biorefinery. An EROI loess than 1 means that ethanol would not be a viable long run, large scale commercial transportation fuel.

More recent research indicates that ethanol produced from corn now has a positive net energy balance, i.e., it has an EROI > 1. A number of studies report an EROI of about 1.3.  The improved EROI is due to the fact that the amount of energy needed to produce ethanol from corn has significantly decreased because of improved farming techniques, more efficient use of fertilizers and pesticides, higher-yielding crops, and more energy-efficient conversion technology.

Ethanol supporters argue that the positive net energy balance demonstrates the feasibility and desirability of corn-based ethanol as a replacement for conventional motor gasoline. This claim should be viewed with caution. An EROI of 1.3 is still far lower than the EROI for conventional motor gasoline, so a large shift to corn ethanol would divert large amounts of energy from other productive uses to fuel production. Moreover, it is not even clear that an EROI = 1 is the minimum threshold for the viability of an energy supply system.  Additional energy "downstream" from a feedstock (crude oil, corn) and its processing into fuel (motor gasoline, ethanol) is needed to deliver and energy to the point of end use, and to build and maintain the infrastructure necessary to use the fuel. Research suggests that the minimum EROI for a fuel at the wellhead/farm gate must be at least 3 in order to generate a sufficient surplus of energy to support the downstream infrastructure.  Since the EROI for corn ethanol is less than 3, it must be subsidized by fossil fuels in order to be useful.

The energy balance for ethanol produced from sugar cane in Brazil is more favorable, with an EROI of about 8. Sugar cane is a more energy dense feedstock compared to corn, and the cultivation of sugar cane in Brazil uses less fossil fuel compared to corn cultivation in the U.S. In addition, most biorefineries in Brazil use a cogeneration process that produces steam and electric/mechanical energy from sugarcane bagasse.

Carbon balance

Another point of contention regarding ethanol is its impact on global climate change.  What is the net effect on climate if we substitute a gallon of conventional motor gasoline with a gallon of ethanol?  Conventional "wells to wheels" analysis of corn-based ethanol suggests a 20% reduction in greenhouse gas (GHG) emissions compared to conventional gasoline.

Some scientists question whether corn-based ethanol will produce any reduction in GHG emissions. Their argument looks like this: If increased production of corn-based ethanol in the U.S. raises corn prices and accelerates the conversion of rainforests and conservations lands to farmland worldwide, greenhouse emissions and loss of the carbon sink associated with such deforestation and disruption must be counted towards the biofuel's total emissions. Several analyses possible land use changes suggest that an increase in corn-based ethanol could actually produce an increase in GHG emissions.

Food versus fuel

Yet another issue associated with ethanol production from corn in the U.S. is its potential to exacerbate world hunger. The argument goes like this. An increase in the demand for corn to produce ethanol will prevent U.S farmers from meeting the twin demands of both renewable fuels and traditional uses like livestock and poultry feed, food processing, and exports. In addition, increase demand for ethanol will put pressure on the supply of corn, raise global grain prices and thus hurt the world's poor. Some question the ethics underlying the use of valuable agricultural land to produce fuel for gas guzzling SUVs when people in the world are hungry or malnourished.

The counter argument is that ethanol does not hurt the human food supply or dramatically alter food prices for a number of reasons. First, ethanol production does not reduce the amount of food available for human consumption because it is produced from field corn which is primarily fed to livestock and is undigestible by humans in its raw form. The ethanol production process produces not only fuel but valuable livestock feed products, so-called "distiller's grains." 

Second, a negligible volume of U.S. corn is exported to undernourished populations. The overwhelming majority of U.S. corn, including exported corn, feeds livestock—not humans. Third, while a rise in the price of corn and other agricultural commodities can adversely  impact food prices, it also provides more opportunity for  subsistence farmers around the world that have been devastated by depressed global commodity prices.  Fourth, some of the issues of hunger and poverty that are attributed to biofuels are more appropriately linked to structural problems of corporate concentration and inequalities in agricultural trading systems.

Sources

 

This entry was compiled, edited and written by: 

Terms of Use:

The text of this article is original work done by the author(s) and editor(s) listed on the article.  The text of this article is freely available for non-profit educational purposes.  Complete attribution must accompany any reproduction or derivative use, and such attribution must include a link to the original Energy Library source material.  Commercial and non-educational use of material from The Energy Library is prohibited without prior approval from the owners of The Energy Library.