Alternative Fuels Alcohols Ethers and Esters
For environmental and supply reasons, attention is turning to the development of cleaner-burning alternatives to hydrocarbon fuels, especially to power vehicles. Some of these alternatives are, at least in principle, renewable in the sense that their production could be sustained indefinitely into the future without resulting in the accumulation of additional carbon dioxide. In the material that follows, we discuss the nature and properties of the major contenders for alternative fuels. In a later section, we take a longer-range viewpoint and consider hydrogen, the ultimate "fuel of the future."
The organic fuels considered here have the inherent advantage over hydrogen—and even over natural gas—that they are liquids under normal temperatures and pressures that bum easily in air to produce considerable heat; like the gasoline and diesel fuels with which they can be blended or which they can replace, they are energy-dense fuels. Since they all contain carbon, however, their combustion releases carbon dioxide.
The alternative fuels for vehicles fall into three classes: alcohols, ethers, and esters. Because they all contain some oxygen, they generally produce a little less energy per liter than do gasoline and diesel fuel. However, their oxygen content results in low emissions of many air pollutants. NOx emissions from these organic liquids are also lower than from pure gasoline because the flame temperature is lower and thus less thermal NO (Chapter 3) is formed.
Ethanol as a Fuel
Ethanol, C2H5OH, also called ethyl alcohol or grain alcohol, is a colorless liquid that has been used as an automobile fuel as far back as the late 1800s; indeed, Henry Ford designed his original cars to run on ethanol.
As a fuel for vehicles, ethanol can be used "neat," i.e., in pure form, or as a component in a solution that includes gasoline. Often these fuels are referred to by the letter E (for ethanol) followed by a subscript that indicates the percentage of alcohol in the gasoline-ethanol mix. In North America, the "gasohol" currently sold is about 10% ethanol and 90% gasoline, i.e., El0, Ethanol and gasoline are freely soluble in each other, so all possible combinations can be produced. Currently in Brazil, E23 is used by all gasoline-powered vehicles. Pure ethanol, E100, is used mainly in Brazil, where about one-eighth of car engines have been designed to use it.
One attractive feature of "oxygenated" transportation fuels such as ethanol is that they result in lower emissions of many pollutants—specifically carbon monoxide, alkenes, aromatics, and particulates—-compared to emissions from combustion of pure gasoline or diesel fuel, particularly from older vehicles that do not have catalytic converters. In North America, however, the turnover of vehicle fleets means that very few cars still on the road emit much CO. The reduction in urban ozone that would result from the lowered emissions of carbon monoxide and reactive hydrocarbons would be countered by increases due to the higher amounts of acetaldehyde (Chapter 5) and vaporized ethanol that would be emitted. This is particularly true for urban areas in which ozone formation is NOx-limited rather than hydrocarbon-limited. However, NOx emissions from engines burning ethanol are lower than from those burning gasoline. Studies in Rio de Janeiro indicate that the concentration in air of the important pollutant peroxyacetylnitrate, PAN (see Chapter 5), which is readily formed from the acetaldehyde emissions, has increased due to the use of ethanol fuel; since Brazilian cars are not equipped with catalytic converters, this finding is not directly relevant to the North American situation. However, measurements in Albuquerque, New Mexico, have established that the use of ethanol as a gasoline additive increased the concentrations of pollutants such as PAN in the air of that city. It is curious that some proponents of ethanol as a fuel point to its ability to reduce CO emissions, which in fact is important only for cars without catalytic converters, but downplay the effects of acetaldehyde emissions by stating that they can always be minimized by use of catalytic converters!
One of the difficulties in using pure ethanol (or pure methanol) as a vehicular fuel is its low vapor pressure: See Figure 8-8, in which the vapor pressure of gasoline-ethanol mixtures is plotted against its composition, with pure gasoline at the left side of the horizontal axis and pure ethanol at the right. Thus, in cold climates, there is very little vaporized fuel available with which to start a cold automobile engine. However, a blend of
100% gasoline 0% alcohoJ
Composition of liquid
85% ethanol and 1 5% gasoline has a high enough vapor pressure (Figure 8-8) to overcome the cold start problem; in North America and Europe, E85-fueled vehicles are now available.
The E10 blend of ethanol in gasoline is now widely available in North America. In order to reduce the evaporation of volatile organic compounds (VOCs) from gasoline—since they contribute to the ozone problem—the United States regulates the maximum vapor pressure of gasoline sold during the summer months. To achieve the lower overall volatility, the amount of (highly volatile) butane in gasoline is being reduced and replaced by substances that have low volatility. Unfortunately, as a minor additive, ethanol is quite volatile and actually increases the vapor pressure of gasoline. (The same phenomenon occurs for methanol-gasoline blends.) This behavior can be understood by reference to Figure 8-8. As ethanol is added to gasoline, the vapor pressure of the mixture rises sharply since the C2 compound behaves in this hydrocarbon environment much like a low-molecular-weight—and therefore volatile—hydrocarbon. In contrast, as a pure liquid, ethanol has a lower vapor pressure than gasoline (see Figure 8-8) since the extensive hydrogen bonding between ethanol molecules in this situation provides a 'glue" that makes it difficult to break the molecules apart and vaporize them.
Another disadvantage of ethanol (which is even more valid for methanol) as a fuel is that the energy it produces per liter combusted is somewhat less than is generated by an equal quantity of gasoline; to travel the same distance, fuel tanks for alcohols would have to be larger. In principle, about 1.25 gal of ethanol are needed to generate the same amount of energy as is obtained with 1 gal of gasoline. In practice, however, the efficiency of the combustion is greater with the alcohols, so the volume penalty is not this large.
0% gasoline 100% alcohol
FIGURE 8-8 Variation in vapor pressure with composition for typical alcohol-gasoline mixtures.
Industrially, ethanol is made by catalytically adding water to petroleum-based ethene, CH2 — CH2, to produce CH3CH2OH. In contrast, ethanol for fuel is produced on a massive scale by the fermentation of carbohydrates in plants. Such bioethanol is produced by the yeast-driven fermentation principally of glucose, C6H12Q6. In North America, most carbohydrate used for ethanol production is derived from the starch in kernels of corn, although some wheat and other grains are also used. In Brazil and some other semitropical countries, sucrose from sugarcane is used. A number of developing countries, including Thailand and China, are producing ethanol from cassava, a woody shrub that produces a tuberous root high in starch content. In the fermentation process, the sunlight-derived energy of the glucose becomes more concentrated in the ethanol product, since some of the carbon is released as carbon dioxide gas:
By comparing oxidation numbers, show that the carbon atoms in ethanol are more reduced—and therefore serve as better fuels when oxidized—than would the carbon atoms in the glucose molecules from which they originated before fermentation. Show also that there is no net change in oxidation number of carbon when going from reactants to products in the fermentation reaction.
Using the enthalpies of formation listed below, calculate the enthalpy change for the fermentation reaction of glucose into ethanol and carbon dioxide. Is the process exothermic or endothermic? From your answers, decide whether the fuel value of the ethanol product is slightly greater or slightly less than that of the glucose from which it is created,
AHf values in kj mol-1: C6Hi2Q6(s) —1273.2
As with all biofuels, the attractions of using ethanol to replace some of the hydrocarbon component in gasoline are that
• the producing country becomes less reliant on imported petroleum;
• air pollution is reduced; and
• the net amount of C02 emitted into the air is reduced.
Using biofuels such as ethanol is thought to counterbalance much of the resulting greenhouse gas emissions because the plants absorb the carbon, used in photosynthesis, that accounts for much of their mass; the atmosphere is thereby depleted of some of its C02. The harvested plant biomass is converted by fermentation into a fuel, which is combusted, and the carbon is released back into the air as carbon dioxide in the same amount that the plant had absorbed to grow. The resulting net change to atmospheric C02 from growing and then burning the fuel would be zero. Since the process can be repeated the next season by growing more biomass in the same field, the fuel would be renewable. In the case of ethanol, the photosynthesis reaction is
6 C02(g) + 6 H2Q(g)-»C6H12Q6 + 6 Oz
The reverse of this reaction is the combustion process for glucose.
Unfortunately, a large quantity of water must be used in fermentation in order to solubilize the starch from which the glucose is obtained; otherwise, the yeast dies if it is present in concentrated alcohol. Indeed, the greater the percentage of alcohol in the mixture, the slower the rate of conversion. A total inhibition of fermentation occurs when the alcohol solution reaches about 8-11% ethanol by volume (i.e., about one-tenth of the aqueous solution). For this reason, only dilute solutions of alcohol can be produced by fermentation. However, a dilute solution of ethanol in water (equivalent in alcohol content to that in wine) will not burn.
To be used as a vehicular fuel, almost all the water must be removed from the ethanol solution produced by fermentation. Consequently, the solution is distilled to separate the alcohol from the water. Distillation is a very energy-intensive process, since the watery mixture must be constantly kept at a boil. What ultimately results from repeated distillations is not pure ethanol but a solution of 95.6% ethanol and 4.4% water (by volume). The last vestiges of water cannot be removed by more distillation; however, this removal can be accomplished by a process involving a molecular sieve that also uses heat energy when the sieve is dried so that it can be reused. Many of Brazil's vehicles operate on hydrous ethanol, i.e., 95% C2H5OH.
The crux of the controversy about whether or not bioethanol is a renewable fuel is that heat generated by burning a large amount of fuel is needed to distill the ethanol from the water. In the modem production of ethanol from corn in the United States and Canada, a nonrenewable fuel—either coal or natural gas—is burned to generate the heat required in the distillation process. As a result of this combustion, a large amount of carbon dioxide—a significant fraction of that produced when the alcohol is later burned as a fuel—is released into the atmosphere at this stage. However, if, as is done in Brazil, biomass crop residues (called bagasse in the case of sugarcane) rather than a fossil fuel are used to power the distillation, the carbon dioxide that they release upon combustion is reabsorbed by growth of such material the next season, so there is very little net addition of C02 into the air from this step. However, the particulate air pollution from the smoke that can accompany biomass combustion restricts its use.
Many scientists and policymakers have attempted to add up the pluses and minuses of greenhouse gas release and absorption, as well as of energy production and consumption, in generating ethanol for fuel in North America; they have compared these findings to the corresponding values for gasoline in order to determine whether or not ethanol is truly a renewable fuel. Their conclusions about whether the overall balances for ethanol relative to gasoline are positive or negative depend largely on the assumptions they make, although all agree that the size of the difference is relatively small. The analyses are complicated by the fact that commercial materials such as com gluten feed, com oil, and dried distiller grains, obtained from the nonstarch component of the corn kernels, are co-produced with ethanol in the distillation step of the com mash. Presumably, some of the fossil-energy usage and greenhouse gas emissions in the process ought to be associated with the co-products, rather than assigning it all to the alcohol, since the co-products displace other substances on the market that would require energy to produce. Most analyses conclude that modern North American ethanol production from com requires about two-thirds the amount of fossil fuel that would be required to generate the same amount of energy in the form of gasoline produced from conventional petroleum sources.
We can conclude, then, that the production and use of ethanol derived from carbohydrate biomass in North America reduces by about one-third the amount of fossil fuel per se that is required to produce energy for vehicles. In essence, the energy of ethanol is derived from a combination of two parts fossil liiel and one part captured solar energy; thus the production of ethanol from corn in North America is largely the conversion of natural gas or coal into a convenient vehicular fuel. Ethanol causes about 86% of the greenhouse effect enhancement of the gasoline that it displaces. This greenhouse gas percentage exceeds that for the fossil fuel it consumes in production mainly because it includes the contribution from the nitrous oxide gas that is emitted as a by-product when fertilizers are used to grow the corn; another contributor is the carbon dioxide released when nitrogenous fertilizers are made synthetically.
Although bioethanol produced from sugarcane has a better energy balance than that produced from corn, sugarcane requires considerable water to grow. Although irrigation is not required in the parts of Brazil where it currently is produced, growing sugarcane in some other countries is placing a burden on their water resources.
The rapid recent growth in world production of ethanol is illustrated by the green curve in Figure 8-9. Currently, massive amounts of bioethanol— over 16 billion liters annually—for use as vehicular fuel are produced from sugarcane in Brazil; unfortunately, the resulting air and water pollution and soil erosion are massive. Smaller quantities of ethanol are obtained from cane in Zimbabwe and from corn and grain in some midwestern American states and, recently, in Canada. As of 2005, more than 16 billion liters of ethanol fuel were also being produced annually from the starch of corn and grain in the midwestern United States; that amount is expected to more than double by 2009. About 15% of the corn crop in the United States and about half the sugarcane crop in Brazil were used to produce bioethanol in 2005. Many farmers in the United States and Canada have provided major political
FIGURE 8-9 Annual global ethanol (light green curve) and biodiesel (dark green curve) production. [Source: Data from L R. Brown et at., Vital Signs 2006-2007 (New York: W.W. Norton, 2007).]
support for the production and use of ethanol in gasoline, in particular for the government subsidies required to make it economically competitive with petroleum.
Can ethanol produced from corn ever replace petroleum worldwide? The growing area required to do this would be about twice the arable land used for all food crops today, so clearly the answer is "no." Indeed, in developed countries, current energy consumption rates exceed the energy generated in food crops. Thus alcohol from fermentation of corn is unlikely to become a major fuel replacement for gasoline.
An emerging technology for biomass ethanol production uses cellulose and hemicellulose components of plants, rather than starch, as the abundant feedstock from which sugars are produced and fermented. The hope is that such cellulostic ethanol can be produced in the future in larger amounts and at cheaper price in terms of energy and dollars than that obtained from starch sources. The main components in the woody plants being considered for cellulostic ethanol are cellulose (35-50%), hemicellulose (25-30%), and lignin (15-30%). Cellulose is a long polymer of the C6 sugar glucose (see Figure 3-11), approximate formula (CH20)n, whereas hemicellulose consists of shorter polymer chains consisting mainly of sugars such as xylose that contain five, rather than six, carbon atoms. Lignin is an unfermentable component of the biomass.
In order to be converted into alcohol, the woody biomass must first be pretreated in order to break the seal of lignin and to disrupt the crystalline cellulose structure, to enable enzymes to reach and react with the cellulose and hemicellulose within. A number of different pretreatment techniques, some of them physical and some chemical (such as treatment with steam or dilute acid, or ammonia), are available, but all those developed to date are relatively expensive. Once the pretreatment has liberated the cellulose, enzymes are available to depolymerize it via hydrolysis to glucose, which then is fermented to ethanol:
biomass -> (CH20)n -» (CH20)6 -* ethanol pretreatment cellulose enzyme glucose fermentation hydrolysis
Until recently, the enzymes used to hydrolyze cellulose were expensive to produce, but this problem has now been overcome, leaving pretreatment as the most expensive step in the sequence. Unfortunately, the C5 sugars that are the chief product of the depolymerization of hemicellulose are not fermented to ethanol by naturally occurring enzymes, though genetically engineered yeasts have now been produced that ferment both C5 and C6 sugars, albeit producing a very dilute (<5%) alcohol solution.
The biomass for bioethanol production could, for example, be switchgrass—a perennial wild grass that was once widespread in the Great Plains of the United States—grown for this purpose; agricultural residue, such as the stalks and other nonkernel parts of corn plants; or wood, waste paper, or municipal solid waste. However, nitrogen fertilization—with its accompanying nitrous oxide release—would be required to maintain switchgrass production. Short-rotation hardwood crops—grown on land that is currently out of production or on marginal cropland—require substantially fewer fertilizers and pesticides than do corn and switchgrass; together with waste from other forestry and agricultural operations, these crops can produce much more biomass per unit area than corn.
A real advantage to the production of cellulose for bioethanol is the combustion of the mechanically dewatered lignin component of the biomass to fuel the distillation process. This greatly reduces the amount of fossil fuel required to about 8%, and the net greenhouse gas emissions to 12%, of those involved in the energy-equivalent gasoline cycle. Presumably, substantial fossil-fuel reductions would also be achieved in producing ethanol from corn itself if the stalks, cobs, etc., rather than coal and/or natural gas, were used to fuel the distillation.
Continue reading here: Methanol
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