A vast number of cellulose-, starch-, and sugar-containing plants can be converted to ethanol. For all of these potential raw materials, the production of alcohol by fermentation involves the following steps:
1. Growth, harvest, and delivery of the raw material to an alcohol plant.
2. Pretreatment or conversion of the raw material to a substrate suitable for fermentation to ethanol. This requires little effort when sugar-containing materials such as sugarcane are used; starchy materials need to be cooked to gelatinize the starch, which then must be hydrolyzed to sugars by enzymes. For cellulose-containing raw materials, a more difficult chemical or enzymatic hydrolysis is required.
3. Fermentation of the substrate to alcohol and purification by distillation.
4. Treatment of the fermentation residue to reduce pollution and to recover by-products for use as feed, fertilizer, or energy.
Raw material cost is the major factor in production economics, constituting 60-70 percent of ethanol-manufacturing costs. Year-round availability of raw materials is also highly desirable for alcohol production, whether from crops with a long harvest season, multiple crops with overlapping harvest seasons, or storable crops. There is little value in building an alcohol plant unless it can be operated at full capacity for most of the year. Molasses, most cellulose-based raw materials, and- various grains can be stored without spontaneous degradation, but many agricultural products or wastes cannot. For example, a vast quantity of coconut water, which contains about 2 percent fermentable sugar, is discarded in coconut processing because it is difficult to collect and store this biologically labile waste. Coconut water can be concentrated to a kind of molasses, but the heat required exceeds the caloric value of the alcohol that could be produced from the molasses. Waste fruit and other spoiled or surplus produce are often excellent substrates for alcohol production, but their availability is usually seasonal. In many banana-producing countries, however, significant quantities of culled bananas are available year-round at central locations. In Ecuador, for example, it has been estimated that the yearly volume of waste bananas equals 68,000 tons of fermentable carbohydrate. These bananas, currently dumped, burned, or used as pig feed, could be used for conversion to ethanol.
Raw materials from sugar-, starch-, and cellulose-containing plants will be discussed in turn.
SUGAR CONTAINING RAW MATERIALS
Sugarcane is one of the best candidates for the production of alcohol fuel. It has the considerable advantage of a long history of cultivation and processing, and bagasse from the cane provides more than ertough boiler fuel for a distillery (Figure 6). Moreover, high cane yields- more than 200 tons per ha annually under experimental growing conditions-augur well for improvement of current average yields (50 tons per ha annually) in many parts of the world. For the production of ethanol, either the juice directly extracted from the cane or the residual molasses from sugar refining may be used. When all of the juice expressed from the cane is converted to ethanol, yields of about 70-90 liters per ton of cane are obtained.
When molasses is used to produce ethanol, the yield is about 245 liters of alcohol per ton of molasses. (For each ton of refined sugar manufactured, about 300 kg of molasses are obtained.)
Sugar beet (Beta vulgaris), a temperate crop of little interest to tropical developing countries, is included here for comparison. Beet yields in good growing areas of northern Europe and the United States average 45-50 tons per ha, and the average sucrose content is about 14 percent. The crop requires good, well-drained soil and 500-700 mm of well-distributed rainfall. The harvested plants are processed by cutting off the stem, leaves, and top and bottom parts of the fleshy root, usually mechanically with rotating discs. The middle, sugar containing part of the root is then pulped mechanically and the sucrose extracted. The tops and extracted pulp are valuable sources of animal feed. Fodder beet is the name given to many varieties of beet with lower sugar content than sugar beets that have traditionally been grown for animal feed because of their high yields. They are now receiving attention for their potential as a combined energy and animal feed crop.
Sweet sorghum (Sorghum bicolor) has been grown mainly as a forage crop, but the relatively high amounts of syrup contained in the stalk have encouraged its investigation as a source of fermentable sugar and starch. Because the syrup contains both sucrose and starch, processing for sugar is more complicated and less economic than with sugarcane or beet. However, this limitation need not apply to alcohol fermentation. The plant's potentially high yields (45-80 tons per ha) and adaptability to a variety of soil and climate conditions have made it the subject of recent research attention.
Nipa palm (Nipa fruticans) is a tree 3-4 m tall that grows abundantly in estuarine swamps in Sumatra, Malaya, Sabah, Papua New Guinea, and elsewhere in Southeast Asia and the Pacific (Figure 7). Its sap contains about 15 percent sugar, which can be collected from the mature fruit stalk after cutting off the fruit head. Carefully done, this can be repeated over an extended period, and considerable quantities of sap can be obtained. Pratt et al. report yields of 401 liters per tree per season, which they project as 30,000 liters of juice per hectar. each year. Cultivated palms may produce as much as 0.46 liters per tree per day, equivalent to nearly 8,000 liters of alcohol per acetic each year.
The sap begins fermenting (because of wild yeasts) as soon as it tapped, and, if is not used quickly, fermentation will proceed to acetic acid. This autofermentation could be an advantage in small-scale pro" ceasing, since no additional yeast is needed. The main disadvantages of nipa are the inaccessibility of its wild stands and the difficulty of working large areas of the swampy terrain the plant prefers. Cultivated stands may require heavy clay soils in land that would otherwise be suitable for rice.
The starch-containing plants from which alcohol has been made are cassava, sweet potatoes, Irish potatoes, yams and edible aroids, and the cereal grains.
Cassava (Manihot esculenta), a New World plant that has been introduced into many tropical countries, is a staple food for several hundreds of millions of people. Cassava roots contain a high proportion of starch, and it is possible to obtain relatively high yields of 40-50 tons per ha without expensive fertilizer or supplementary irrigation on soils unsuitable for growing more demanding crops, such as maize. Although production levels under traditional farming systems are around 10 tons per ha, research into improved varieties and cultural practices has shown that annual yields of 80-100 tons are theoretically possible (Figure 8).
The plant is resistant to pests and can be stored in the ground for several months after maturity until it is needed. Once harvested, the storage life of the root - which is not a storage organ capable of dormancy - is limited, and in many countries it is processed for storage by chipping or flaking and drying. Large amounts of dried cassava are exported to Europe from Thailand and other countries as a source of carbohydrate for animal feed. These characteristics make it an excellent smallholder crop and one of the most promising sources of starch for alcohol fuel. Also, the foliage can be used for animal feed (and human food in some cultures). The main advantage of cassava and other starchy root crops over sugarcane is the high content of fermentable material, which can yield higher amounts of alcohol per hectare. The disadvantages are that an additional step is required to convert the starch to sugar, and the plants yield little combustible residue so that fuel must be provided to heat the still or produce the steam for recovering the ethanol from the fermentation mash.
Sweet potatoes (Ipomoea batatas) are similar to cassava in their high yields of fermentable starch per hectare. They require better soil and water supply than cassava, but have a wider range of latitude and altitude and being a prostrate plant can withstand windy conditions. Their productivity is lower than cassava, but they have a much shorter growing season, and two or three crops can be produced per year. Large-scale production of sweet potatoes for alcohol production is being planned in Indonesia.
Irish potatoes (Solanum tuberosum) are a temperate or highland
tropical crop requiring low temperatures for effective deposition of starch.
Yields are lower than those of other tropical root crops, but the high food
value of potatoes makes them important staples.
Yams (Dioscorea spp.) are grown in limited areas of the tropics, particularly in the humid parts of West Africa and the Caribbean. The plants require good, well-drained soil and ample and well-distributed rainfall. They are labor intensive, requiring not only tilling and weeding but also staking to support their vines, all of which increases the costs of production. They can produce as much as 90 tons per ha freshweight. But yams are highly prized food, and current production falls short of demand. It is unlikely that they will ever compete with cassava or sweet potatoes for fuel alcohol purposes.
Taro (Colocasia esculenta) and tannia (Xanthosoma sagzttifolium), two edible aroids, are widely grown as starchy food staples in sites such as drainage ditches, where the root development of other crops is impaired by the lack of oxygen caused by waterlogging. These conditions, and their demand for food use, are likely to limit the potential of these plants as fuel alcohol substrates.
OTHER STARCH-CONTAINING TROPICAL PLANTS
Sago (Metroxylon sagu and Metroxylon rumphii) is the common name of two species of palm trees found widely in Southeast Asia that deposit about 20 percent by weight of starch in their trunks as food for the reproductive phase of their lives. McCann estimates that about 15 boles could be harvested per year per hectare, yielding about 150 kg of starch each, with a potential yield of around 1,350 liters of ethanol per hectare per year.
Sago palms reproduce vegetatively by suckers, which can easily be used in cultivation of the plants. Research in Indonesia and elsewhere is being undertaken to investigate the full potential of these plants.
The false banana, ensete (Musa ensete), is a food staple of southwestern Ethiopia and northern Uganda. Like the sago palm, it accumulates large quantities of starch in the bole, which the inhabitants of these areas ferment for a number of days in pits before consuming it
Although the potential of sago and ensete for large-scale fuel production is not clear, they may have important local applications,
Wheat, maize, rice, rye, and barley are widely used to produce potable alcoholic beers, wines, and spirits, and indeed the cereal grains contribute most of the nongrape alcoholic beverages around the world. The starch in the grain is typically broken down to component sugars, first by a process of malting in which the grain is soaked in water to germinate so that the enzyme maltase is produced and the starch broken down to the disaccharide maltose. The grain "malt" is then dried and can be stored before grinding for fermentation.
The per-hectare productivity of these grains for fuel alcohol is quite low compared with other possible substrates; however, if present restrictions imposed by the requirements of commercial alcohol beverage producers are removed, high-starch-yielding varieties could be produced. The economic viability of grain fuel alcohol production is likely to depend on the income from by-product use, which is likely to be much less in developing countries than in industrialized countries with large modern animal feeding industries.
No alcohol is commercially produced by fermenting cellulose, although this is the subject of intensive research efforts. A limited amount of methanol is produced from cellulose by destructive distillation in some developing countries.
Two types of cellulose materials are potentially available in large quantities for alcohol production, should this technology prove practical on a large scale: cellulose-containing agricultural wastes (such as straws, cotton stalks, and other crop wastes) and sawmill wastes and wood grown either specifically for fuel purposes or for multipurpose use. These two types of fuel feedstock have been described in recent NRC publications: Food, Fuel, and Fertilizer from Organic Wastes and Firewood Crops: Shrub and Tree Species for Energy Production. Recent interest in fast-growing trees for fuel has provided an incentive for research on growth characteristics and yields on various soils at different levels of rainfall. Some species show spectacular yields under optimal conditions, Eucalyptus grand is in Sao Paulo State averages nearly 50 tons per ha of air-dry wood annually, and this level is also achieved with Gmelina arborea (though it is used mainly for pulpwood) in Amazonia and with Leucaena leucocephala in the Philippines. With active selection programs and better understanding of the role of soil microorganisms and nutrients, as well as the moisture regime, yields of 100 tons per ha of air-dry wood are being confidently predicted. This technology for growing fuelwood in the tropics (combined with practical methods of fermenting the cellulose to ethanol) could revolutionize the liquid fuel supply potential of tropical countries, whose high sunlight intensity gives them a comparative advantage over temperate regions. Eucalyptus can be grown in 4-5 year rotations, compared with 15-20 year rotations typical of temperate deciduous and evergreen forests.
Cellulose-containing raw materials should contain 50-60 percent hydrolyzable polysaccharides, preferably high in hexoses so that a 4050 percent yield of fermentable sugars can be attained.
Ethanol production from sugar-based raw materials is well known and widely used. With regard to starch-based raw materials, although the conversion of most cereal starches to ethanol is common practice, many tropical starches have not yet been thoroughly evaluated. Commercially practical methods for converting cellulose to ethanol are not yet available.
Methanol from biomass requires the refinement of existing techniques for converting coal or natural gas. The use of biomass in this application will demand the establishment of extensive, well-organized plantations.
Lignocellulose feedstocks such as wood, grasses, and crop residues contain cellulose, hemicellulose, and lignin. The proportions of these components vary among lignocellulose materials. Typical ranges are:
The removal of hemicellulose and its hydrolysis to 5-carbon sugars is relatively easy. Hemicellulose from biomass is currently the principal source of the chemical furfural, a feedstock for the preparation of dyes, lacquers, and synthetic resins. Although hemicellulose is not now used as a source of ethanol, this conversion can probably be achieved without great difficulty.
Cellulose, however, is embedded with lignin, which inhibits biological attack. Thus, chemical or mechanical pretreatment is used to separate the lignin from the cellulose to facilitate cellulose hydrolysis.
Mechanical pretreatments include ball-milling, two-roll milling, and steam explosion. Chemical pretreatments include acid, alkali, and solvent disruption of the lignin-cellulose complex. Biological treatment has also been tested.
REFERENCES AND SELECTED READINGS
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Anonymous. 1981. Ethanol from sago palms. Pacific Islands Monthly 52(2):62.
Anonymous. 1981. Mechanised system lifts constraints on cassava production. International Agricultural Development June:20-21.
Anonymous. 1981. New genes promise a sweet future for cassava. New Scientist September 3:599.
Anonymous. 1982. Plant will convert whey to fuel-grade ethanol. Chemical and Engineering News September 13:7.
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Gabrielson, E. 1982. Cattail Rhizome Derived Alcohol. Final report. Contract FG0281AF92016. Available from NTIS, Springfield, Virginia, USA.
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Maldonado, O., Rolz, C., and de Cabrera, S. S. 1975. Wine and vinegar production from tropical fruits. Journal of Food Science 40:262-267.
Marzola, D.L., and Bartholemew, D.P. 1979. Photosynthetic pathway and biomass energy production. Science 205:555-559.
McCann, DJ. 1980. Village-Based Fuel Ethanol Plants. APACE Research Centre, Rotherwood, Australia.
McClure, T.A., and Scantland, D.A. 1979. Can sweet sorghum supply food, fiber, feed, fiber and fuel? Sugar Journal 41(10):37-41.
Meissner, J. 1982. Sugar crop: the Jerusalem artichoke. Renewable Energy News 4(11) :1.
Pratt, D.S., Thurlow, L.W., Williams, R.R., and Gibbs, H.D. 1913. The nipa palm as a commercial source of sugar. The Philippine Journal of Science 8(6):377-398.
Saxon, E.C. 1981. Tuberous legumes: preliminary evaluation of tropical Australian and introduced species as fuel crops. Economic Botany 35(2):163-173.
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