Fossil fuel consumption has long been the most practical means of energy use in areas of transportation and electricity production, especially among developed nations. From the historical exploitation of coal since the Industrial Era to todays uses of natural gas and petroleum products, the United States and many European countries rely on a high level of fuel use. Although the energy sources used to produce these fuels are almost entirely non-renewable, research is being conducted in search of processes through which fossil fuels can be more efficiently utilized. Alternate, renewable fuel resource prospects are also being evaluated.
Fears of a devastating energy crunch are being expressed as the antiquation of numerous coal processing plants continues. Research is being conducted to find methods in which replacement plants can more efficiently process coal, all the while reducing pollutant emissions (Sloan, McMullan, & Williams, 1997). Scientists have experienced preliminary success in a number of new processing techniques. Two techniques involving the use of a fluidized bed of inert materials such as liquefied sand have proven to be more environmentally friendly than current practices, although each process suffers from limiting factors including economic drawbacks and physical restrictions of factory machinery (Sloan et. al., 1997). Scientists are conducting exergy analyses in which they can better optimize the function and design of steam turbines commonly used in these plants (Rosen & Dincer, 2003).
A third technique seems to be a more practical means of coal combustion, as efficiency is significantly increased while sulfur emissions are nearly eliminated completely. The technique, termed integrated gasification combined cycle (IGCC), employs both thermal and chemical reactions to produce a relatively efficient gas combustible in a conventional gas turbine. Sulfur is sequestered during the IGCC process, so pollutants are far diminished (Sloan et. al., 1997). A downside of IGCC processing is that it is currently uneconomical to employ the process on a widespread basis. Furthermore, fossil fuels must still be used to feed the processes of clean coal production. Because fossil fuels are non-renewable resources, sustainable fuel sources must be researched to solve future energy concerns.
Some countries already enjoy successes in their research for such fuels. The United States and Brazil have both achieved scientific breakthroughs in converting vegetative feedstock into ethanol, a highly combustible alcohol suitable for use in a converted automobile engine (Myers, 1992). Starch from United States fisld corn can be fermented to form ethanol and carbon dioxide gas. Proteins from the corn can be incorporated into animal feed so that reduced amounts of biomass go unused. Although ethanol burns cleanly and efficiently, large-scale processing of corn into ethanol is currently unlikely, as the infrastructure demanded for ethanol production is not well-developed and collection systems of suitable corn material are inefficient. Brazil, on the other hand, has had a long-standing ethanol production program. With the establishment of the countrys National Alcohol Plan in 1975, Brazil has been a world leader in ethanol production, although the program has lost funding in recent years (Kheshgi, Prince, & Marland, 2000). By the late 1980s, around nine million vehicles were powered fully or partially by ethanol products. Brazils processing program involves sugarcane as an appropriate source of fermentable sugar; sugarcane has a sugar content of up to 47% dry weight (Kheshgi et. al., 2000). Ethanol burns at an efficiency of nearly 87%, more than double that of conventional coal (Sloan et. al., 1997). This efficiency makes future research in ethanol processing an intriguing research prospect, with yields expected to increase threefold using a sustainable approach to agricultural cropping (Kheshgi et. al., 2000).
The amount of sugar in sugarcane is astonishing, hence its function as a source for ethanol production. At the same time, however, more than half of the plants biomass is inaccessible for immediate fermentation. Scientists are performing research in the enzymatic processing of cellulose into glucose, which can then be fermented to produce ethanol. Preliminary findings show that enzymatic activity may successfully lyse cellulose molecules into glucose, thus breaking the chemical bond that typically renders cellulose useless to the fermentation process (Robinson, 1980). True cellulose is simply a polymer composed of many glucose units and can be broken up relatively easily with known enzymes. However, cellulose is found in conjunction with hemicelluloses and lignin in nature. Glucose sugars are therefore joined with pentose sugars sugars with a five-carbon skeleton and the enzymatic lysing process is hampered. Cellulose must be given a treatment before enzymatic activity begins.
Many methods of acid hydrolysis for processing of vegetative residues into suitable animal fodder have been pursued since the 1940s (Robinson, 1980). Processes can be used to separate pentose sugars, glucose, and lignin from wood and other plant material. In 1978, Dr. G. Tsao of Purdue University formulated an efficient process to separate the various structural components of cellulose (Robinson, 1980). The source plant is milled and immersed into a mixture containing strong sulfuric acid. This separates the pentose sugars from the other elements; these sugars can then be fermented to produce liquid alcohol. A solvent is applied to the remaining cellulose structure to dissolve the lignin. The residual cellulose material is again bathed in sulfuric acid. As with the pentose sugar, the acid separates the glucose, which is also fermented to produce alcohol (Robinson, 1980). Although the processes exist, the products of hydrolysis of cellulose pale in yield to those of cornstarch.
Methanol, another alcohol-based fuel, is currently being produced from natural gas, with potential in a conversion process from vegetative sources. In order to produce methanol, materials must be subjected to pyrolytic processes under anaerobic conditions and be further upgraded into fuels before being burned within a fuel cell. Methanol appears to be a less expensive solution than ethanol, as it is believed that biomass-produced methanol could cost under a dollar per gallon (Kheshgi et. al., 2000).
Pyrolysis, or thermal conversion of biomass into tars, liquids, and chars, offers potentials well beyond solely methanol production (Piskorz, Scott, & Radlein, 1988). Through pyrolysis, most all biomass can be concentrated into corrosive fuels, although the oils processed from biomass cannot be further refined into fuels suitable for transportation purposes. Pyrolytic oils are suited for electricity and heat generation (Robinson, 1980). Herein lays the potential of pyrolysis as an avenue to fossil fuel replacement. Coal need not be burned; as such, pyrolysis may eliminate current high demand for mining activities. Once scientists can overcome the relative inefficiencies of pyrolytic processes compared to fossil fuel refinements, pyrolysis can become a process that relies on sustainable, renewable growth of vegetative life forms. Sources of such life abound in many regions of the earth.
Tropical forests are home to nearly half of the earths biological diversity and enjoy a perennial growing season (Myers, 1992). Likewise, it is in these regions where the potential for taking advantage of a fast-growing source of biomass is most likely to arise. Eucalyptus trees of Australia and Ipilipil trees of the Amazon serve as promising sources of biomass. Eucalyptus is already grown on a large scale in a number of countries for timber and forestry needs (Myers, 1992). Ipilipil (Leucaena leucocephala) is gaining in popularity, as countries like the Philippines are beginning to exploit the products from the plant. L. leucocephala can grow to a height of nearly 30 feet within the first two years of development, while a plantation with an area of 12,000 hectares can supply an estimated energy equivalent of one million barrels of oil a year (Myers, 1992, p. 252). With proper funding, tropical countries would have the opportunity to develop highly efficient pyrolytic processes and could ultimately experience economic benefit, all the while utilizing pre-disturbed land to save old growth tropical forest from further destruction. By virtue of the Ipilipil trees high nutrient content, plantations could also serve as a high-quality pastureland for the burgeoning cattle industry.
Pyrolysis experiments have been conducted in North America on a variety of wood sources, including various poplar and maple species (Piskorz et. al., 1988). Experiments conducted in the 1980s in Ontario, Canadas University of Waterloo have shown pyrolytic potential for four species of temperate trees. Liquid yields of all four species range from 70-80% of the dry weight biomass when subjected to temperatures between 480ø and 550ø C. In all four species of tree, the yield and composition of pyrolytic oils is nearly identical (Piskorz et. al., 1988). Indeed, it seems to be the process not the plant that has the largest impact on oil production.
If the result of pyrolysis is indeed the function of process and not of plant origin, then the process could easily be used to convert agricultural wastes such as cornhusks and leaves into synthetic fuels. As of the 1970s, about three-fourths of crop residues were tilled back into the soil in order to decrease erosion and maintain fertility (Robinson, 1980). However, with a cover crop approach to agriculture in which a nitrogen-fixing legume is planted along with the corn, it would no longer be necessary to incorporate the crop residues back into the soil. The legume could serve the same function as the recycled crop residue. The biomass of corn residue amounts to a range of six to eleven tons of vegetative matter per hectare (Robinson, 1980), thus accounting for a huge potential reservoir of biomass produced each year in the United States. Collection of crop residues could indeed prove valid in the process of pyrolysis.
However promising research for alternate, renewable sources of bioenergy may be, few are close to fruition on a commercial scale. Of all the discussed methods of fuel production, only ethanol has so far been popular, its popularity in Brazil limited by the popularity of cheap, non-ethanol-fueled import cars and uncertain subsidies provided by the government. Research must continue and funding must be dedicated toward the creation of efficient processing of biomass into biofuel. In the meantime, improvements must be made in current coal-powered electricity generation systems, both to prolong the lifespan of our coal reserves and to reduce pollution caused by burning coal. Energy production is essential to modern human society, hence the validity of further research and investment in the field.
Literature Cited Kheshgi, H.S., Prince, R.C., & Marland, G. (2000). The potential of biomass fuels in the context of global climate change: Focus on transportation fuels. Annual Review of Energy & the Environment, 25 (1), 214-230. Myers, N. (1992). The primary source: Tropical forests & our future. New York: W.W. Norton & Company. Piskorz, J., Scott, D.S., & Radlein, D. (1988). Composition of oils obtained by fast pyrolysis of different woods. In Pyrolysis oils from biomass: Producing, analyzing, and upgrading (pp. 167-178). Washington, D.C.: American Chemical Society. Robinson, J.S. (ed.) (1980). Fuels from biomass: Technology and feasibility. Park Ridge, NJ: Noyes Data Corporation. Rosen, M.A. & Dincer, I. (2003). Survey of thermodynamic methods to improve the efficiency of coal- fired electricity generation. Proceedings of the Institution of Mechanical EngineersPart A Power and Energy, 217 (1), 63-73. Sloan, E.P., McMullan, J.T., & Williams, B.C. (1997). Clean coal technologies. Proceedings of the Institution of Mechanical EngineersPart APower and Energy, 211 (1), 95-107.
