Several plants have been proposed to be good sources of energy. These include woody crops and grasses/herbaceous plants, starch and sugar crops and oilseeds, fast growing trees such as hybrid poplars, shrubs such as willows, and so forth. Energy crops can be grown on agricultural lands not utilized for food, feed, and fiber. Farmers could plant these crops along the riverbanks, along lakeshores, between farms and nat­ural forests, or on wetlands. These crops could be a good source of alter­nate income, reducing the risk of fluctuating markets and stabilizing farm income. Woody plants, herbaceous plants/grasses, and aquatic plants are different sources for biomass production. The type of biomass selected determines the form of energy conversion process. For instance, sugarcane has high moisture content, and therefore, a “wet/aqueous” bioconversion process, such as fermentation, is the predominant method of use. For a low-moisture content type such as wood, gasification, pyrol­ysis, or combustion are the more cost-effective ways of conversion. Characteristics of an ideal energy crop are mentioned below:

■ Low energy input to produce

■ Low nutrient requirements

■ Tolerance to abiotic and biotic stresses

■ High yield/high conversion efficiency

■ Low level of contaminants

Energy plantations and cropping are means of growing selected species of trees or crops that can be harvested in a shorter time for fuel, energy, and other resources. Each type of popular plant species is dis­cussed in brief, with respect to renewable resources.

Euclayptus. It is a fast growing plant for firewood (see Fig. 2.5). Different species such as Eucalyptus nitens, E. fastigata, and E. globu­lus are used in many countries such as Australia and Brazil. Eucalyptus, an exotic species from Australia, is a versatile tree which adopts itself to a variety of edaphic and climatic conditions. It comes up in different types of soils and climates varying from tropical to warm temperatures and with annual rainfall ranging from 400 to 4000 mm. It grows well in deep, fertile, and well-drained loamy soils with adequate moisture. A large eucalyptus plantation program has been successfully launched in Brazil to serve as the feedstock for its methanol plant. Amatayakul et al. suggest that if eucalyptus wood is used for electricity generation, the cost of electricity generation would be 6.2 US cents/kWh, and con­sequently, the cost of substituting a wood-fired plant for a coal-fired plant and a gas-fired plant would be US $107 and $196 per ton of C, respectively [6]. Eucalyptus plantations could offer economically attrac­tive options for electricity generation and CO2 abatement.

Casuarina. Casuarina is a genus of shrubs and trees of the Casurinacea family, native to Australia and islands of the Pacific. The species involve Casuarina equisetifolia Linn. It is a big evergreen tree with a trunk diameter of 30 cm and height 15 m, and is harvested after 5-7 years (see Fig. 2.6). The plant fixes nitrogen through symbiotic bacteria and thus adds fertility to the soil. It is very useful for afforesting sandy beaches and sand dunes. The wood is used for fuel purposes.

Mimosa. Mimosa leucocephala or kubabul is a fast-growing species known for energy plantation (see Fig. 2.7). It has a very high potential for nitrogen fixation and can be well adapted to poor soils, drought, and windstorms. It can fix up to 500 kg of nitrogen per hectare per annum. It coppices readily, and the sprouts, after harvesting, can grow up to 18 ft in just 1 year. It is also called the wonder tree. Under irrigated condi­tions, it can give fodder yields up to 80-100 ton/(ha • yr). Three different

varieties of this species (Hawaiian, Cunningham, and Brazilian) are commonly used for plantations in Hawaiian, Salvador, and Peru. The Hawaiian and the Cunningham varieties are used for energy plantation in India and Australia, respectively. A Hawaiian plantation of 1.27 hectares can support a 1-MW power plant. In Brazil and the Philippines, it is converted into charcoal that has 70% of the heating value of oil. Charcoal can be used to produce calcium carbide, acetylene, vinyl plas­tics, pig iron, and ferroalloys. The low silica, ash, and lignin contents and high cellulose content make this plant good for paper and pulp materi­als, and also for rayons and cellophanes. It not only gives a prolific fuel — wood yield but is also a nutrient-rich fodder for livestock.

Sugarcane. Sugarcane (Saccharum officinale) is a hardy plant that can tolerate poor drainage, can be cultivated as a rotation crop, and can be maintained for years. It is grown in fertile areas with more than 1000 mm of rain and an abundant supply of water. The ethanol yields from this are in the range of 3.8-12 kL/(ha • yr) [7].

Cassava. Cassava (Manihot esculenta), like sugarcane, is grown in trop­ical climates with an average rainfall of 1000 mm. As it is relatively drought resistant, it can withstand lower annual rainfall. It needs to be grown annually and is difficult to mechanize, and compared to sugar­cane, it is less energy efficient. Ethanol yields are estimated in the range of 0.5-4.0 kL/(ha • yr).

Sorghum. Sorghum embraces a wide variety of plant types and, unlike sugarcane and cassava, is found in the tropical summer rainfall zones. While it can grow in as little as 200-250 mm annual rainfall, maximum yields are obtained in a minimum of 500-600 mm rainfall. Compared to other cereals, it can tolerate high temperatures. Due to its deep root system and low rate of transpiration, it is exceptionally resistant to drought. Ethanol yields of stems and grains of sorghum are in the range of 1.0-5.0 and 2.0-5.0 kL/(ha • yr), respectively.

Babassu. Babassu (Orbignya sp.) is a palm popular in Brazil for the ethanol derived from it. The mesocarp of coconut is the raw material for ethanol production, with an estimate of 0.24 kL/(ha • yr).

Oil-bearing crops. Vegetable oils are the most promising alternatives to diesel fuel. About 97% of all oil-bearing plants are grown in tropical and subtropical climates. There has been some research into the use of plant oils from sunflower, peanut, rapeseed, soybean, and coconut oils as biofuels in unmodified/slightly modified engines. Seed-based oils are shown to lead to slightly higher fuel consumption, probably due to their calorific value [8]. About 14% of the oil supplied in the world market is palm oil, yielding an average 3.4 ton/(ha • yr) of oil [9]. Individual palm seeds, however, are capable of producing much higher yields. The extrac­tion of palm kernel oil increases fuel oil yields by 10%. Current culti­vation is mostly in lowland humid tropics such as Malaysia, West Africa, and Indonesia. While the conditions to grow coconut palms are similar to oil palms, the yield potential of coconut palms has not yet been devel­oped to that potential. Soybeans and peanuts are annual leguminous crops that are used as sources of both oil and protein. Soybeans thrive best in subtropical climates. The individual varieties differ greatly in terms of their reaction to the length of a day and normally can be grown in a limited geographical area. Peanut cultivation requires an ambient temperature for growth, as less than optimal temperatures are known to result in poor yields. Due to its deep root system, it is relatively resist­ant to drought. It is also a suitable crop for mixed cultivation along with oil palms and corn. In terms of calorific value of seed, oil plants such as Simmondsia chinensis, Pittosporium resinifreum, Ricinus communis, Jatropha curcas, and Cucurbita foetidissima are found to be ideal. Buffalo gourd (Cucurbita foetidissima), a desert-adapted plant, pro­duces high-quality oil and fermented starch. The oil has a high ratio of unsaturated to saturated fatty acids. Crude protein and fat content in the whole seeds is 32.9% and 33%, respectively [8]. With a seed yield of 3000 kg/ha and estimated 16% hydrocarbon, about 35 barrels of crude oil could be produced per hectare, in addition to carbohydrate from roots, forage from vines, and protein-rich oil cakes. Jojoba (Simmondsia chinensis) is a shrub that grows naturally in the United States and Mexico. Its seeds contain about 50% of oil by weight and does not decrease with long-term storage. The oil is remarkably resistant to degradation by bacteria, probably because it cannot cleave and metab­olize the long-chain esters it contains (mostly hydrocarbons containing 38-44 carbon atoms). Jojoba oil has potential uses as a fuel and chem­ical feedstock, and can also be used as a replacement for vegetable oils in foods, hair oils, and cosmetics since it does not become rancid.

Additionally, it can be used as a source of long-chain alcohols for antifoaming agents and lubricants. The hydrogenated oil is a white, hard crystalline wax and has potential uses in preparation of floor and automobile waxes, waxing fruit, impregnating paper containers, and manufacturing of carbon paper and candles. Physic nut (Jatropha curcas), a tropical American species, is a large shrub, or a small tree. The seeds yield 46-58% oil of kernel weight and 30-40% of seed weight. In trade, this oil is called curcas oil. All parts of the plant exude sticky, opalescent, acidic, and astringent latex, containing resinous substances. The bark of this plant is a rich source of tannin (31%) and also yields a dark-blue dye. Now Jatropha oil, a semidrying oil, is in high demand for use as biodiesel in Asian countries. It is employed in preparation of soaps and candles and used as an illuminant and lubricant. In China, a varnish is prepared by boiling the oil with iron oxide, and in England, it is used in wool spinning. The oil is used for medicinal purposes for skin diseases, for rheumatism, as an abortifacient, and it is also effective in dropsy, sciatica, and paralysis.

Miscanthus. Miscanthus, a thin-stemmed grass, has been identified as an ideal fuel crop as it gives a high dry-matter yield (see Fig. 2.8). Under adequate rainfall conditions, light-arable soils give good yield. It has been found that dark-colored soils produce better yield than light-colored soils. It has been evaluated as a bioenergy crop in Europe for over 10 years and is grown in several European countries. Annual harvesting ability, low mineral content, and good energy yield per hectare are desirable characteristics. It is propagated as rhizomes planted in double rows about 75 cm apart, with 175-cm gaps between the rows. While disease control is not a significant issue, weed control measures are important. In Germany and Denmark, yields are 13-30 ton/ha for 3- to 10-year-old plantation [10].

Panicum. Panicum virgatum or switchgrass (see Fig. 2.9) is another thin-stemmed herb that has been used as a model plant [10]. It is a C4 species, and though it has lower moisture content than wood, it has similar calorific value. It has been found suitable for the development of

ethanol for petrol replacement. The low ash and alkali content makes it a suitable fuel for combustion.

Switchgrass has been identified to be a good model bioenergy species, due to its high yield, high nutrient-use efficiency, and broad geographi­cal distribution. Further, it also has good attributes in terms of soil quality and stability, cover value for wildlife, and low inputs of energy, water, and agrochemicals. Evaluation of the use of switchgrass with coal in existing coal-fired boilers and the handling, operation, combus­tion, and emission characteristics of the co-firing process have been studied. Switchgrass has supplied up to 10% of the fuel energy input. In comparison to the use of corn for the source of bioethanol, switchgrass has been found to generate 15 times more efficiency of energy produc­tion, and it is predicted that switchgrass may entail more profits than conventional crops for a specific area [10].

Hemp. Hemp is a member of the mulberry family that includes mul­berry, paper mulberry, and the hop plant (see Fig. 2.10). It has a cellu­lose content of about 80% and has been grown for the production of medicinal, nutritional, and chemical production. Hemp is the earliest recorded plant cultivated for production of textile fiber. It has a low-moisture content for biomass feedstock [11].

Artocarpus hirsute and Ficus elastica. Stem and leaf samples of A. hirsute and F. elastica have been evaluated for their potential as a renewable energy source. Stem and leaf samples of F. elastica and A. hirsute were evaluated for polyphenol, oil, and hydrocarbon contents. F. elastica

shows the maximum accumulation of protein (24.5%), polyphenol (4.2%), oil (6.1%), and hydrocarbon (2.0%) contents. The leaf of F elastica has been identified to be a good renewable energy source [12].

Calotropis procera. Latex obtained from C. procera could be hydro­cracked to obtain hydrocarbons under severe thermochemical condi­tions. Instead, biodegradation is a less energy-intensive technique for latex degradation. Enhancements in the heptane level have been found in C. procera latex that was subjected to different fungal and bacterial treatments, compared to those of untreated ones. Nuclear magnetic res­onance (NMR) and fourier transform infrared spectroscopy (FTIR) analyses reveals that the latex has undergone demethylation, dehy­drogenation, carboxylation, and aromatization during microbial treat­ment. Petroleum obtained by hydrotreatment of the biotransformed latex is proposed to be used as fuel [13]. Some of the important latex­bearing plants are Hevea brasiliensis, Euphorbia sp., Parthenium agen — tatum, Pedilanthus macrocarpus, F. elastica, and Manihot glaziorii. Several resin-rich plants such as Cappaifera multijuga (diesel tree), Copaifera langsdorffi, Pinus, Dipterocarpus, Shorea sp., and Pithospo — rum resiniferum produce prolific terpene and oleoresins, and are as such very desirable fuel crops. Woody and herbaceous plants have spe­cific growth conditions, depending on the soil type, soil moisture, nutri­ent content, and sunlight. These factors determine their suitability and growth rates for specific geographical locations. Cereals such as wheat and maize, and perennial grasses such as sugarcane have varied yields with respect to the climatic conditions. Depending on the habitat, plants differ in their characteristic makeup. Their cell walls have varying amounts of cellulose, hemicellulose, lignin, and other minor components. The relative proportion of cellulose and lignin is one of the selection cri­teria in identifying the suitability of a given plant species as an energy crop. Herbaceous plants are usually perennial, having a lower proportion of lignin that binds together with cellulose fibers. Woody plants charac­terized by slow growth are composed of tightly bound fibers resulting in their hard external surface. Generally, cellulose is the largest component, representing about 40-50% of the biomass by weight; the hemicellulose portion represents 20-40% of the material by weight. Cellulose is a straight-chain polysaccharide composed of D-glucose units. These units are joined by p-glycosidic linkage between C-1 of one glucose unit and C-4 of the next glucose unit. The number of D-glucose units in cellulose ranges from 300-2500. Hemicellulose is a mixture of polysaccharides, composed almost entirely of sugars—such as glucose, mannose, xylose, and arabi — nose—and methylglucuronic and galacturonic acids, with an average molecular weight of <30,000 g. Cellulose is crystalline, strong, and resist­ant to hydrolysis, whereas hemicellulose has a random, amorphous struc­ture with little strength. It is easily hydrolyzed by dilute acid or base.

A complete structure of lignin is not well defined because the lignin structure itself differs between plant species. Generally, lignin consists of a group of amorphous, high-molecular-weight, chemically related compounds. Phenylpropanes, three carbon chains attached to rings of six carbon atoms, are the building blocks of lignin. These might have one or two methoxyl groups attached to the rings. Sugar/starch feedstocks, such as cereals, have been traditionally used in biochemical conversion of biomass to liquids such as ethanol. High-cellulose content of biomass is generally more efficient and therefore preferred over the lignin-rich biomass for conversion of glucose to ethanol. Depending on the end use and type of bioconversion preferred, the choice of the plant species varies. In northern Europe, the C3 woody species especially grown on short rotation coppice, such as willow and poplar, and forestry residues, are used [14]. In Europe, there is wide interest in the use of oilseed rape for producing biofuel [15]. Brazil was one of the first countries to begin large-scale fuel alcohol production from sugarcane.