Background
The world faces many new energy related challenges as we move further into the 21 st century. Among these challenges are the declines in easily obtained fossil fuel reserves, and more importantly, the carbon emissions that using fossil fuels create. The burning of fossil fuels produces around 21.3 billion tons of carbon dioxide per year. It is estimated that natural processes absorb about half of that amount, so there is a net increase of 10.65 billion tons of atmospheric carbon dioxide per year. 1 Carbon dioxide is one major greenhouse gases that contribute to global warming and climate fluctuation.
One possible solution that would ameliorate some of both of these crucial problems would be to switch at least some of our motor fuels from traditional fossil fuels such as oil and gasoline to so-called biofuels. The ability of plants to capture CO 2 from the atmosphere and convert it into harvestable biomass is the cornerstone of biofuel production. Biofuels are a possible answer to the carbon pollution of traditional fossil fuels because they are considered carbon neutral. Such fuels are potentially because they do not result in a net increase in atmospheric .This means that the carbon that is released by burning the fuel is eventually taken up again and converted into the biomass used to produce new fuel. The carbon does not permanently remain in the atmosphere, and therefore biofuels are considered a better alternative to fossil fuels.
Biofuels consist of a fairly wide variety of fuels that are converted from biomass. Probably the most widespread and highly visible biofuel is ethanol. Ethanol currently comprises at least 10% of what we commonly use as fuel in a gasoline powered vehicle. Most automobiles in the United States run on blends of up to 10% ethanol (E10). Most gas pumps have some visible indication on the pump that the gasoline being dispensed has at least 10% ethanol mixed with the gasoline. Pumps that do not have ethanol mixed with the gasoline also have very visible stickers indicating that they do not dispense any ethanol along with the gasoline. Flexible-fuel automobiles and trucks use gasoline/ethanol blends ranging from pure gasoline up to 85% ethanol (E85). There are already approximately 11 million E85-capable vehicles on U.S roads. 3 The ethanol is used as a fuel extender, and as an additive to increase the octane rating of gasoline fuel. In the United States, most of the ethanol used for fuel is derived from corn which is grown specifically for use in ethanol production. 4
Some nations such as Brazil produce ethanol by the fermentation of sugars from sugar cane. Research in Brazil has centered on the introduction of better producing varieties of sugar cane, and creation of policies that support sustainable land management. 5 In the United States, most ethanol production has involved the use of corn as a feedstock. Ethanol production in the U.S. topped 4 billion gallons in 2005 and consumed 1.4 billion bushels of corn, valued at $2.9 billion. This represents the third largest demand for U.S. corn after animal feed and export markets. With additional construction of ethanol plants and increasing ethanol demand, ethanol fuel production exceeded 7.5 billion gallons before the year 2012 target set forth in the Energy Policy Act of 2005. 6
Corn and sugar based ethanols are considered "first generation" biofuels. Due to increasing global demand, biofuel production is expected to increase significantly, mainly due to the expansion of first-generation biofuels in developing nations such as Brazil, China, and the United States. 7
One response of corn growers to the need for more and more corn for ethanol has been to intensify their farming practices. One of the largest practices is to adopt a "high yield" cropping system. High-yield cropping systems require fossil-fuel inputs to substitute human and animal labor and to maximize capture and conversion of solar radiation into crop biomass. Inputs to agricultural systems that require fossil fuel in their manufacturing process include fertilizer, seed, pesticides, and machinery. Fossil fuel also is required for application of fertilizers and pesticides, and for irrigation. Because fossil fuel combustion results in carbon dioxide emissions, any energy inputs raise the level of carbon dioxide pollution, and decrease the net benefits of corn ethanol even further. Another potential negative aspect of corn ethanol production is the potential for indirect land use change and associated carbon dioxide production from clearing of carbon-rich natural ecosystems for crop production. There has been a marked increase of land that is marginal being cultivated to ethanol corn production in the United States. 8
Generally speaking, first-generation biofuels consume much more water throughout their life cycle, when compared with other energy carriers. In fact, corn based biofuels exhibit much greater water footprints, sometimes by two or three orders of magnitude, than other energy carriers This is mainly a consequence of the high direct water use during feedstock production. Nevertheless, different biofuels have radically different water requirements, depending on the feedstock, the region where it is produced, and the production practices adopted. 9
Most guidance for calculation of the carbon footprint of biofuels, and most published carbon footprints, presume that biomass fuels are carbon neutral. However, it is recognized increasingly that this is incorrect: biomass fuels are not always carbon neutral. Indeed, they can in some cases be far more carbon positive than fossil fuels. The nature and the magnitude of overall biofuel carbon contributions depend on the biofuel feedstock, the mode of feedstock production, the agricultural practices adopted during feedstock production, the environmental and socioeconomic context of biofuel production, the stage of the biofuel's life-cycle, and the policies in place during biofuel production, use, and trade. 7
There are seven stages to a biofuels "life cycle". They are: 1) Site selection and preparation, 2) Feedstock growth and preparation, 3) Feedstock transportation, 4) Feedstock treatment and biofuel production, 5) Biofuel transportation, 6) Biofuel storage and dispensing, and 7) Biofuel combustion. 7 There are efficiencies to be gained in any of these steps, but my classes will focus on the feedstocks and the production of the different biofuels. We will also discuss the first and second stages as part of our Agriculture and Farming units, and thereby better tie the agricultural portion of the production of ethanol to the uses and actual production o the fuels themselves.
Because most of the ethanol produced in the United States is made from corn and the energy required by ethanol distilleries comes mostly from fired power plants, there has been considerable debate on the environmental benefits of corn-based biofuels in replacing traditional fossil fuels. Concerns relate to the large amount of land required for crops and its impact on , as well as issues regarding its real and also issues regarding water use and pollution due to the needs of the corn crop itself all the way to the final ethanol production. 10 The use of corn as a fuel source is controversial on several points, and attempts are being made to utilize feedstocks other than the starch in corn kernels for ethanol production. 11
One promising alternative feedstock for ethanol production is cellulose. 12 Cellulose is an with the ( 6 1 0 5) n, a polysaccharide consisting of a linear chains of Β(1→4) linked units. The length of individual cellulose chains varies greatly. Cellulose is the most abundant organic polymer on Earth, and is an important structural component of the primary of .
The linkage pattern of the glucose sub-units in cellulose is different than the glucose-glucose bonds in starches, glycogen and other carbohydrates. Unlike starch, cellulose is a straight chain polymer. The multiple hydroxyl side groups on the glucose from one chain form with oxygen atoms on an adjacent chain, holding the chains firmly together side-by-side and forming fibers with high tensile strength. NNN
Cellulose that comes from plants is usually found in a mixture with , , and other substances. The character and relative ratios of these other substances varies from plant species to plant species. The hemicelluloses and lignin serve as binding agents, holding the cellulose fibers in an even stronger matrix which ultimately gives plant their durability and strength.
Many physical properties of cellulose depend on its chain length and the number of hydrogen bonds between glucose units that make up a cellulose fiber. The degree of lignification of the individual cellulose fibers also determines to a great extent the overall strength of the cellulose structure being constructed. Cellulose from wood has typical chain lengths between 300 and 1700 units and are heavily lignified to neighboring cellulose units, while cotton and other plant fibers have chain lengths ranging from 800 to 10,000 units and have much less lignin within the fibers. 13
Cellulose is available in many forms, including agricultural waste and native grasses. One of the main proposed strategies for cellulosic biofuel production is widespread planting and harvesting of perennial grasses, such as switchgrass (Panicum virgatum L.) or miscanthus (Miscanthus X giganteus). One study suggested that net carbon dioxide savings relative to fossil fuels of greater than 200g of carbon dioxide /m 2/yr may be expected for biomass (switchgrass) conversion to ethanol. 14 Studies have been conducted and shown that the use of some agricultural wastes for ethanol production is also environmentally sustainable. 15
Production quantities of cellulosic ethanol have consistently failed to meet expectations, mostly because those production expectations were set based on amounts of cellulosic material available on fields rather than the ability of technology at the time to turn biomass into fuel. 12
One of the main problems with cellulosic ethanol production is that, unlike the starch sugars in grains, the complex polysaccharides in the cellulose of plant cell walls are locked within the lignin. 16 For advanced biofuels to be economically competitive, scientists must find inexpensive ways to release these polysaccharides from their lignin bindings and reduce them to fermentable sugars that can be synthesized into fuels. Studies are being conducted to determine how to optimize the use of agricultural waste to produce more accessible cellulose for ethanol production. 17
A key consideration when assessing biofuel sustainability is the extent to which a biofuel provides a net-energy gain when compared with the conventional fossil fuels it displaces. Two commonly used indicators are the energy return on investment (EROI) and the percentage fossil energy improvement. The EROI is the ratio of the total energy supplied by biofuel combustion (Eout) to the total energy used during biofuel production (Ein). Values of EROI greater than 1 imply net-energy gains. The percentage fossil energy improvement provides a measure of the amount of nonrenewable energy used during the life cycle of a biofuel. The EROI and percentage fossil energy improvement are usually measured as the carbon output through the complete life-cycle of the biofuel. 7
Biobutanol is another alcohol that can be derived from biomass, and has a greater energy content than ethanol. It is also considered a second generation biofuel. Because of its greater energy content per molecule, butanol been proposed as a better alternative fuel than either corn or cellulose-based ethanol. Butanol has more energy per unit volume than ethanol, and approaches gasoline in its energy content. 18
New discoveries could make the alternative fuel butanol more attractive to the biofuel industry by lowering production costs and eliminating some of the production bottlenecks. 19 Bacterial strains have been identified that can ferment biobutanol directly from cellulosic sources. 20,21 This would allow butanol to be produced in great quantities without some of the limitations of ethanol. Because of its higher energy content, butanol could potentially eclipse ethanol as the biofuel of the future. A switch from corn-based ethanol to either cellulosic ethanol or butanol would lower the environmental impact of biofuel production. 22
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