Chem395:Bioethanol

Jesse Federman CHE 395 Dr. Walker Term Paper

Bioethanol:The Good, the Bad, the Debate

There are many benefits to using bioethanol as a fuel source over conventional fossil fuels. As the world moves towards renewable energy to meet energy demands, new and innovative ways are being sought to meet this demand. There are technologies being developed that will allow for biomass (waste) to be converted into fuels and other usable chemicals1 ¬, instead of distilling the ethanol produced by plants. These technologies include thermochemical treatments, enzymes and fermentative microorganisms. The purpose of this paper is to show the how ethanol is produced, as well as how it can be used as a sustainable resource globally and nationally through its many industrial applications.

Conventional Fossil Fuels

Fossil fuels are hydrocarbons that are used to produce energy. These hydrocarbons are oxidized to form carbon dioxide and water and are widespread throughout the world. They are used in powering vehicles, aircrafts, and trains for transportation and are also used to generate electricity. There is much controversy concerning Fossil Fuels for several reasons. They are deemed non-renewable resources, meaning that they are finite in their amounts. They also take thousands of years to form, and with the current usage, they are not sufficient in abundance to meet current energy needs. There is concern over global warming and other environmental impacts that Fossil Fuels bring as well.

There are five types of fossil fuels: coal, natural gas, oil shales, petroleum, tar sands. Coal is a sedimentary rock and is essentially the remains of plants and animals that decayed over time. Natural gas is methane gas mixed with several other gasses (ethane, carbon dioxide, nitrogen, propane, butane etc) and is found in oil fields, coal beds and natural gas fields. Petroleum is a liquid found within the earth mixed with hydrocarbons and other compounds. Oil shales are sedimentary rocks with a variety of organic compounds called kerogen. When kerogen is heated, crude oil is produced. Tar sands are sands and clay mixed together with bitumen (polycyclic aromatic hydrocarbons) that yields heavy crude oil.

Ethanol as an Alternative Fuel

The effort to develop bioethanol technology gained significant momentum in the late 1970s as a result of the energy crises that occurred in that decade3. Ethanol has the formula C2H5OH, and it is widely used as an alternative fuel or fuel additive 2 and it is most common renewable fuel today 1. Ethanol is renewable and is not limited in supply and there are two ways in which it’s made. The first is the starch and sugars produced by plants in traditional agricultural crops, and the second is the ethanol made from waste products termed “bioethanol.”3 It is important to note that the term “Biomass” can be used to describe both the starch and sugar products in plants as well as waste products (such as leaves, stalks and husks). The first type of ethanol described is “bad” ethanol because it wastes biomass and the second type of ethanol is “good” because it utilizes the entire biomass.

The first type of ethanol is produced through hydrolysis and fermentation reactions in plants that produce sugars. Some typical sources of these sugars are: corn, wheat, sorghum, sugarcane and willow trees. Wastes from plant cell walls contain carbohydrate polymers (cellulose, hemi-cellulose, lignin etc). For plants to produce sugars from these wastes, they react with acids and enzymes which reduce the carbohydrate polymers into sugar monomers. For example, the cellulose and hemi-cellulose carbohydrates are hydrolysed by enzymes and dilute acids to form sucrose. Sucrose can then be fermented into ethanol. The lignin present in plants can also be used for ethanol production as well using the same acids and enzymes.

The second type of ethanol that is made from biomass wastes requires a more sophisticated knowledge of biomass conversions and many technological advances to make the conversions possible. Instead of only using the sugars from the plant to extract ethanol, the science behind this second type of ethanol seeks to utilize the entire plant - leaves, stalks, husks, peels and other plant parts to produce usable ethanol.

When ethanol is added to gasoline, the fuel becomes more oxygenated. This allows the fuel to fully combust so that air pollution is minimized. Ethanol burns to produce water and carbon dioxide. It is a high octane fuel and has been used to replace lead as an octane enhancer in gasoline. Such ethanol-gasoline blends are available in the United States. The most common of these blends is E10 (10% ethanol and 90% gasoline). Another blend, known as E85 (85% ethanol and 15% gasoline) requires special vehicles to run on it2.

Ethanol is also used for aviation purposes because it costs less than gasoline. The ethanol used in the aviation industry is called Aviation Grade Ethanol (AGE-85). This blend of ethanol is E85 and is used in any type of reciprocating engine aircraft. This new fuel for planes has begun to replace the conventional low lead gasoline used by the aviation industry since WWII4. The advantages that AGE-85 has over conventional fuel is that more torque, and horsepower (about 12%) are produced4, there are lower temperatures for engine operation, and lower maintenance costs because of the increased engine efficiency.

Bioethanol is the most common renewable fuel today, and the "Biofuels Initiative" in the U.S. (by the US Department of Energy), has tried to make cellulosic ethanol “cost- competitive” by 2012 and it is theoretically to be a third of the U.S. fuel consumption by 20305. However, the starch biomass material (corn and sugar cane) is limited. For ethanol to be a successful renewable biofuel and compete with fossil fuels a low –cost, abundant renewable resource is needed5.

Cellulolytic Ethanol Extraction Processes

There are three methods of extracting the sugars from biomass to obtain ethanol2. They are Chemical Hydrolysis (Concentrated Acid and Dilute Acid processes), Enzymatic Hydrolysis and Microbial Fermentation and are explained below.

Chemical Hydrolysis

In the Concentrated Acid Hydrolysis Process a concentrated sufuric acid (70-77%) solution is added to dried biomass. For this process, the temperature is kept constant (50ºC). Water is then added to the acid and this mixture is heated at 100ºC. The product from this reaction goes through a distillation column to separate the mixture.

The dilute acid hydrolysis process is the most efficient of these methods for ethanol production. Here the biomass is hydrolyzed into sucrose molecules from the hemi-cellulose. The processes begins with sulfuric acid (0.7%) added to the biomass at 190˚C. The next stage is supposed to have a higher yield. Using 0.4% sulfuric acid at 215˚C, the liquid undergoes hydrolysis and neutralization so that the product can be recovered.

Another way to perform hydrolysis is to use yeast to heat the mixture and form glucose and fructose (in the presence of an enzyme called invertase). The sugars formed in this process react with another enzyme (zymase) to form ethanol and carbon dioxide. This is a fermentation process and it take about three days at a temperature around 250-300˚C for the batch to ferment. After the fermentation, there is a large amount of water in the ethanol mixture. In order to extract the water, fractional distillation is used. Here, the mixture is boiled and the ethanol vaporizes (it has a lower boiling point of 78.4˚C) before the water (Boiling Point of 100˚C) which allows it to be separated.

Enzymatic Hydrolysis

The cellulose polymers (chains) in the biomass can be broken down into glucose monomers by cellulase enzymes. For example, for cattle and sheep the reaction occurs in their stomachs where enzymes are produced by bacteria (at several different steps). A similar system utilizing lignocellulosic materials can be done to enzymatically hydrolyze the biomass without inhibiting enzymatic activity. This is achieved at a relatively mild condition (~50oC and pH5). All of these methods of treatment require the enzymatic hydrolysis step to achieve high sugar yield for ethanol fermentation6.

Thermophiles and especially thermophilic enzymes have to date gained a great deal of interest both as analytical tools, and as biocatalysts for application in large scale5. Using these enzymes however is costly and as such can limit their use on a large industrial scale5. Despite this, the surge in demand for such enzymes has caused increased production in their availability in such a way that such costs for enzymes are expected to decrease. Since many industries have begun to experience revolutionary changes between fossil fuel availability and that of alternative fuels, the use, and need of microbes and enzymes will increase. There will certainly be a continued and increased need of thermostable selective biocatalysts in the future5.

Microbial Fermentation

Another alternative process is using thermostable enzymes and thermophiles in isolating ethanol. Thermophiles have not yet played any major role in metabolic engineering, due to the limited amount of vectors and tools available for their modification. Instead, well-known mesophiles like S. cerevisiae are used, and has recently been modified with genes from a fungal xylose pathway and from a bacterial arabinose pathway, which resulted in a strain able to grow on both pentose and hexose sugars with improved ethanol yields7.

The microbial process is expensive and alternative methods are under development to make it more efficient. Predominantly, enzymatic cellulose hydrolysis on glucose is carried out by fungi, notably Trichoderma, Penicillium and Aspergillus8. In order to compete with yields from the acid hydrolysis process, more efficient degradation of the biomass (for instance higher temperatures) is necessary. However, many enzymes have been used ranging from thermophiles and hyperthermophiles8.

Global Outlook on Ethanol

Brazil is the largest producer of bioethanol because of its abundance of sugarcane. The sugar-ethanol market in Brazil reaches nearly 7.5 billion US dollars per year9. In 1996–1997, Brazil produced 14.16 billion L (liters) of ethanol from sugar9. There are several problems however with Brazil’s ethanol production and that of similar nations. Once the sugar from the sugarcane is harvested, the rest of the sugarcane plant is no longer used (biomass). The excess sugarcane becomes a nuisance and is disposed of. Instead of utilizing the entire plant and maximizing the land used to grow the crop, only a portion of the true ethanol is obtained.

The global potential for bioethanol production (from major crops, corn, barley, oat, rice, wheat, sorghum, and sugar cane) is of great importance for ethanol’s future as an alternative fuel. According to one study, there are about 73.9 Tg of dry wasted crops in the world that could potentially produce 49.1 GL year−1 of bioethanol10. About 1.5 Pg year−1 of dry lignocellulosic biomass from these seven crops is also available for conversion to bioethanol10. Lignocellulosic biomass could produce up to 442 GL year−1 of bioethanol10.

Based on this study, the total potential bioethanol production (from crop residues and wasted crops) is 491 GL year−1. This means that the current world ethanol production can be ~16 times higher based on this figure. Thus, the potential bioethanol production from this extra amount could replace nearly 32% of the global gasoline consumption (when bioethanol is used in E85 fuel for a midsize passenger vehicles)10.

Additionally, lignin fermentation residue (coproduct of bioethanol made from crops residues and sugar cane bagasse), can potentially generate both 458 TWh (terawatt: 1012 watts∙ hour) of electricity (~ 3.6% of world electricity production) and 2.6 EJ of steam10. The study goes on to say that a primary producer could be Asia which produces up to 291 GL year−1 of bioethanol10 from biomass (waste). Its favorable bioethanol sources are rice, wheat straw, and corn stover10.

The second highest ethanol producer is Europe which produces 69.2 GL of bioethanol, in which most of the bioethanol is derived from wheat10. In North America, corn is the main source of bioethanol, where ~ 38.4 GL year−1 of bioethanol can potentially be produced3. On a global scale, rice can produce 205 GL of bioethanol, making it the largest amount of bioethanol produced from a single biomass source10. The second highest source is wheat, and it can produce 104 GL of bioethanol10.

Food vs. Fuel Debate

The biofuel industry, particularly ethanol has grown considerably and with this growth comes an increased demand for corn. Proponents believe that ethanol has many advantages, however some critics point out that farmers will be unable to satisfy the demand for alternative fuels and maintain the traditional uses of the crop (food for animals and humans).

Parallel to ethanol demands, the price of food all over the world has increased. It is important to note however that the price of oil impacts the cost of food much more than the price of corn or alternative biofuels because oil is used in every stage of food production, packaging and transport. This is where many people disagree on the effectiveness of biofuels.

A Merill Lynch commodity strategist, Fransico Blanch, stated in the Wall Street Journal just recently on March 24, 2008, that oil and gasoline prices will be 15% higher in the future without the use of biofuels. His figures for oil per barrel in the future are over $115 instead of the current $102 dollars and that U.S. gasoline prices will be over $3.70 a gallon.

In a study performed by Searchinger et al., state that the high process associated with biofuel will increase the use of land for agricultural purposes. They also went on to say that emissions from the time it would take to convert land to agriculturally viable land would take nearly 52 years to pay back and the emissions would increase over a period of 30 years by 50%11. Searchinger et al. criticizes the ability of crops to yield enough biomass for biofuel stocks.

In response to this study, Texas A&M’s Agricultural and Food Policy Center found that the notion that biofuels have a “carbon debt” is wrong. According to this study, Searchinger et al. assumes that producing biofuels increases the cost of crops (corn, wheat etc) and promotes the idea of converting more land (protected regions such as the rainforest, marshes and peat land) for agricultural purposes12. This study however, shows that high agricultural costs for production actually reduce the amount of land that is used agriculturally. As a result of these costs, farmers are looking for ways to increase the yields per acre of land they currently use. The Searchinger paper also calculates the “carbon debt” for biofuels such as ethanol, however, this new study points out that the price of oil was never taken into consideration in the study.

In conclusion, alternatives to petroleum-derived fuels are being sought in order to reduce the world's dependence on non-renewable resources1. Better technologies for treating biomass are needed to make ethanol more readily obtainable. This means that the fiber structure of the biomass must be broken down chemically, mechanically or biologically so that cellulase enzymes can remove the lignin and hemi-celluloses from the entire biomass to produce bioethanol. Ethanol has proved itself to be an excellent alternative fuel additive or blend and commercialization has been promising. In countries such as Brazil, the use of less productive methods of ethanol production is not only damaging to the future of ethanol, but is considerably wasteful. It is expected that there will be limits to the supply of crops in the near future. However, lignocellulosic biomass is often seen as an attractive source for future supplies of ethanol1.