Published on Nov 15, 2016
The use of ethanol as an alternative motor fuel has been steadily increasing around the world for a number of reasons. Domestic production and use of ethanol for fuel can decrease dependence on foreign oil, reduce trade deficits, create jobs in rural areas, reduce air pollution, and reduce global climate change carbon dioxide buildup. Ethanol, unlike gasoline, is an oxygenated fuel that contains 35% oxygen, which reduces particulate and NOx emissions from combustion. Ethanol can be made synthetically from petroleum or by microbial conversion of biomass materials through fermentation.
In 1995, about 93% of the ethanol in the world was produce by the fermentation method and about 7% by the synthetic method. The fermentation method generally uses three steps: (1) the formation of a solution of fermentable sugars, (2) the fermentation of these sugars to ethanol, and (3) the separation and purification of the ethanol, usually by distillation.
Fermentation involves microorganisms that use the fermentable sugars for food and in the process produces ethanol and other byproducts. These microorganisms can typically use the 6-carbon sugars, one of the most common being glucose. Therefore, biomass materials containing high levels of glucose or precursors to glucose are the easiest to convert to ethanol. However, since sugar materials are in the human food chain, these materials are usually too expensive to use for ethanol production. One example of a sugar feedstock is sugarcane. Brazil developed a successful fuel ethanol program from sugarcane for a number of reasons: (1) Brazil traditionally relied heavily on imported oil for transportation fuels, which caused a severe economic drain on the country; (2) Brazil can attain very high yields of sugarcane; and (3) Brazil has also experienced periods of poor sugar markets.
As a result, the Brazilian government established programs supportive of the industry with the result that Brazil has been able to successfully produce and use sugarcane for fuel ethanol production. Although fungi, bacteria, and yeast microorganisms can be used for fermentation, a specific yeast (Saccharomyces cerevisiae also known as Bakers’ yeast, since it is commonly used in the baking industry) is frequently used to ferment glucose to ethanol. Theoretically, 100 grams of glucose will produce 51.4 g of ethanol and 48.8 g of carbon dioxide. However, in practice, the microorganisms use some of the glucose for growth and the actual yield is less than 100%. Other biomass feedstocks rich in sugars (materials known as saccharides) include sugar beet, sweet sorghum, and various fruits. However, these materials are all in the human food chain and, except for some processing residues are generally too expensive to use for fuel ethanol production.
Another potential ethanol feedstock is starch. Starch molecules are made up of long chains of glucose molecules. Thus, starchy materials can also be fermented after breaking starch molecules into simple glucose molecules. Examples of starchy materials commonly used around the world for ethanol production include cereal grains, potato, sweet potato, and cassava. Cereal grains commonly used in the US for ethanol production include maize and wheat.
Approximately 475 million tonnes of maize were produced in the world in 1990 with about 200 million produced in the US. Approximately 8 to 9 million t, or 4% of US maize grain went into ethanol in 1990. A bushel of maize grain (25.3 kg or 56 lb. at 15% moisture) can produce from 9.4 to 10.9 L (2.5 to 2.9 gallons) of pure ethanol, depending on the technology used. Starchy materials require a reaction of starch with water (hydrolysis) to break down the starch into fermentable sugars (saccharification).
The production of ethanol from cellulosic materials is more complicated than the processes employed for starch- or sugar-based ethanol, because the complex cellulose-hemicellulose-lignin structure in which cellulosic materials are found needs to be broken up before fermentation can begin. The cellulosic ethanol conversion process consists of two basic steps: pretreatment and fermentation. This two-step process increases the complexity of, and processing time required for, converting the cellulosic biomass into ethanol, relative to the processes used to convert corn or sugarcane to ethanol.
Pretreatment is necessary to prepare cellulosic materials for a subsequent hydrolysis step which converts the hemicellulose and cellulose into sugars. Typical pretreatment involves a chemical pretreatment step (e.g., acid) and a physical pretreatment step (e.g., grinding). These steps make the cellulose more accessible to enzymes that catalyze its conversion to sugars in a subsequent step and begin the breakdown of hemicellulose into sugar. Following pretreatment, the conversion of cellulose to sugar is completed using a chemical reaction called hydrolysis, normally employing enzymes secreted by certain organisms (typically fungi or bacteria) to catalyze the reaction. The pretreatment and hydrolysis process usually results in one co-product, lignin, which can be burned to generate heat or electricity. Using lignin instead of a fossil-based energy source to power the conversion process reduces cellulosic ethanol’s life-cycle greenhouse gas (GHG) emissions, compared to corn-based ethanol. (This is also an example of biomass substitution for fossil fuels; for more information
Once the sugars have been obtained from the cellulosic materials, they are fermented using yeast or bacteria in processes similar to those used for the corn-based ethanol production. The liquid resulting from the fermentation process contains ethanol and water; the water is removed through distillation, again similar to the corn-based ethanol process. Finding the most effective and low-cost enzymes for the pretreatment process and organisms for the fermentation process has been one of the main areas of research in the development of cellulosic ethanol.2
The type of feedstock and method of pretreatment both influence the amount of ethanol produced. Currently, one dry short ton3 of cellulosic feedstock yields about 60 gallons of ethanol.4 Projected yields with anticipated technological advances are as high as 100 gallons of ethanol per dry short ton of feedstock
The increased complexity and longer processing time associated with producing ethanol from cellulosic materials also makes cellulosic ethanol more expensive to produce than corn- or sugarcane-based ethanol. As of early 2009, no commercial-scale facilities in the United States were producing cellulosic ethanol and costs will remain largely uncertain until the technology is demonstrated at a commercial scale.
In 2006, U.S. Department of Energy (DOE) researchers reported achieving a cellulosic ethanol production cost of $2.25 per gallon. At this cost, cellulosic ethanol is competitive with petroleum-based gasoline when oil prices are near $120 per barrel. Two key factors that shape the cost of producing cellulosic ethanol are the high capital costs and uncertain feedstock costs. • High capital costs A first-of-its-kind cellulosic ethanol plant with a capacity of 50 million gallons per year is estimated to cost $375 million, roughly 6 times the capital cost of a similarly sized corn ethanol plant.
These high initial investment costs can present a considerable hurdle to deployment, especially given the greater risk associated with investments in new technologies.
As the technology matures, future plants are expected to have reduced capital costs.15 • Uncertain feedstock costs Like all biofuels, costs of cellulosic ethanol are highly sensitive to feedstock costs. Therefore, estimating biomass supply costs is critical to estimating future cellulosic ethanol prices. Future feedstock production costs are uncertain and predictions depend on the assumptions made by analysts. Some predict that as the cellulosic ethanol industry matures, establishing a larger market for cellulosic crops and allowing feedstock producers to gain experience, costs could decline. On the other hand, as demand increases for cellulosic materials and the supply of low-cost waste products
Cellulosic ethanol is not yet produced at a commercial scale in the United States. Public and private efforts continue to support research on cellulosic ethanol, and technological advances are expected to reduce costs and improve production methods. As of early 2009, no commercial-size cellulosic ethanol facilities were in operation in the United States. However a number of demonstration plants are in operation and a number of commercial-size facilities are expected to begin production by 2011.16 In 2007, the DOE funded six facilities with annual plant production goals ranging from 11.4 million to 40 million gallons of cellulosic ethanol.17 Although two of the funded companies canceled their plans to move forward due to economic difficulties, the remaining four companies intend to begin production by 2010-2011 and, together, produce a minimum of 70 million gallons of cellulosic ethanol per year.
In 2007, the National Academy of Sciences found that the United States, using currently available crop residues as a feedstock, could produce about 10 billion gallons of cellulosic ethanol per year. This value assumes a production yield of 60 gallons of cellulosic ethanol per dry short ton, requiring the use of 160 million dry short tons of crop residues. If technological improvements increase production yields to 90 gallons per dry short ton, as some studies expect, annual production volumes could be about 14 billion gallons of cellulosic ethanol per year.18 In addition to production of ethanol, cellulosic materials are also being examined as a way to produce other biomass-based substitutes for existing fossil fuels (e.g., gasoline, diesel, and jet fuel) and biobutanol.
Like the cellulosic ethanol production process, the thermochemical process that produces biomass-based replacements for existing fossil fuels is not yet at commercial scale, and research in this area is ongoing with the support of the DOE. Biobutanol, like ethanol, is an alcohol-based fuel that can be produced from biomass feedstocks. Biobutanol can be added to gasoline at higher blending quantities than ethanol.