Ahindra Nag, Ph. D., is a Senior Assistant Professor in the Department of Chemistry at the Indian Institute of Technology, Kharagpur. He has 21 years of teaching experience and has published 60 research papers in major national and international journals. He is the author of three other books: Analytical Techniques in Agriculture, Biotechnology, and Environmental Engineering; Environmental Education and Solid Waste Management; and Foundry Natural Product Materials and Pollution.

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.

The two most promising processes for industrial production of ethanol from cellulosic materials are two-stage dilute-acid hydrolysis (a chemi­cal process) and SSF (an enzymatic process). Advantages and disad­vantages of dilute-acid and enzymatic hydrolyses are summarized in Table 3.3. Enzymatic hydrolysis is carried out under mild conditions, whereas high temperature and low pH result in corrosive conditions for acid hydrolysis. While it is possible to obtain a cellulose hydrolysis of close to 100% by enzymatic hydrolysis after a pretreatment, it is diffi­cult to achieve such a high yield with acid hydrolysis. The yield of con­version of cellulose to sugar with dilute-acid hydrolysis is usually less than 60%. Furthermore, the previously mentioned inhibitory compounds are formed during acid hydrolysis, whereas this problem is not so severe for enzymatic hydrolysis. Acid hydrolysis conditions may destroy nutri­ents sensitive to acid and high temperature such as vitamins, which may introduce the process together with the lignocellulosic materials.

On the other hand, enzymatic hydrolysis has its own problems in comparison to dilute-acid hydrolysis. Hydrolysis for several days is nec­essary for enzymatic hydrolysis, whereas a few minutes are enough for acid hydrolysis. The prices of the enzymes are still very high, although a new development has claimed a 30-fold decrease in the price of cellulase.

Crop description. Carica papaya L. (see Fig. 4.22)—commonly known as papaya, pawpaw, melon tree, papayier, lechosa, or mamon—belongs to the family Caricaceae and grows in tropical to subtropical areas. Native to South America, now the crop is widely distributed through­out the tropics. Papaya is a short-lived rapidly growing plant (not a true tree) having no lignified tissues. The seeds contain 25-29% oil [77, 179]. The oil contains mainly unsaturated fatty acids, around 70.7%, and may contain toxic components that make it unusable in human foods [75]. Fatty acid composition of the oil includes oleic acid (79.1%) and palmitic acid (16.6%) [179].

Main uses. Papaya is mainly used as fresh fruit, and for the production of drinks, jams, and so forth. In some places, the seeds are used for treat­ment against worms [181]. The green fruit is also a commercial source

Figure 4.22 Caricapapaya L. (Photo courtesy of Barbara Simonsohn [www. barbara-simonsohn. de/ papaya. htm].)

of the proteolytic enzymes papain and chymopapain—the former find­ing use in a wide range of industries, particularly brewing for haze removal, and the latter in medicine. Oil extraction from the seeds could improve the viability of the industry in countries where papaya is cul­tivated for papain production and processing. The seeds constitute around 22% of the waste from papaya puree plants [182]. No references about its use as a biodiesel source have been found so far.

Methanol can be produced from resources such as coal, natural gas, oil shell, and farm waste, which are abundant worldwide. But methanol from natural gas is unlikely to provide a large greenhouse benefit, not more than a 10% reduction in emissions with quite optimistic assump­tions. It is not considered as a main raw material to produce methanol. For countries having vast reserves of coal but small oil deposits, methanol from coal can provide an indigenous substitute to oil. But this method has an adverse effect on greenhouse gases and is very expen­sive, requiring capital investments that can increase the price by 50%.

In India, there is an abundant production of sugarcane. The govern­ment can divert this feedstock to produce methanol. The production of methanol by using water and methane is shown in Fig. 7.9, and by using methane and a catalyst in Fig. 7.10.

Producing methanol from methane with the technology available today generally involves a two-step process. Methane is fuel reacted with water and heat to form carbon monoxide and hydrogen—together called synthesis gas. Synthesis gas is then catalytically converted to methanol. The second reaction unleashes a lot of heat, which must be removed from the reactor to preserve the activity of the temperature- sensitive catalyst. Efforts to improve methanol synthesis technology

Figure 7.9 Conversion of methane o ethanol.

focus on sustaining the catalyst life and increasing reactor productiv­ity. As a novel alternative to the two-step method, a chemical catalysis that mimics biological conversion of methane by enzymes is being devel­oped. The iron-based catalyst captures a methane molecule, adds oxygen to it, and ejects it as a molecule of methanol. If this type of conversion could be performed on a commercial scale, it would eliminate the need to first reform methane into a synthesis gas, which is a costly, energy­intensive step. Conversion of coal to methanol is simpler and cheaper as compared to its liquefactions to gasoline.

Advantages of methanol.

1. 1% methanol in petrol is used to prevent freezing of fuel in winter.

2. Tertiary-butyl alcohol is used as an octane improving agent.

3. Because of the excellent antiknock characteristics of the fuel, it is very suitable for SI engines.

4. Isopropyl alcohol is used as an anti-icing agent in carburetos.

5. Addition of methanol causes a methanol-gasoline blend to evaporate at a much faster rate than pure gasoline below its boiling point (bp).

6. Due to an increase in emission levels of conventional fuels, the per­centage of O3 in the atmosphere is increasing. This increase in the O3 in the atmosphere might cause biomedical and structural changes in the lungs which might cause chronic diseases. O3 content of even between 0.14 and 0.16 ppm temporarily affects lung function if the person is exposed to it for 1-2 h. An annual crop yield is also reduced if exposed to O3; some trees suffer injury to needles or leaves,

□ Gasoline Methano

(Indolene) (M85)

and lower productivity or even die. High content of O3 has disturbed the natural ecological balance of species in national forests in California. The effects of methanol on O3 emission as compared with petrol is shown in Fig. 7.11.

The reaction products of the electrochemical reaction in a fuel cell are water, electricity, and heat. The heat energy released in a fuel cell stack is approximately equal to the electrical energy generated and must be managed properly to maintain the fuel cell stack temperature at the optimal level. If this thermal energy (waste heat) is properly utilized, it will considerably increase the efficiency of a fuel cell system. In low- temperature (<200oC) fuel cells (PEMFC, AFC, and PAFC), the stack is cooled by supplying excess air in low power (<200-W) systems, whereas a liquid coolant (deionized water) is used for large-size systems. The waste heat carried out by the coolant is utilized for cogeneration (space heat­ing, water heating, etc.). In high-temperature (<6000C) fuel cell (MCFC and SOFC) systems, all the heat of reaction is transferred to the reac­tants to maintain the stack temperature at the optimal level. The ther­mal energy of the high-temperature exhaust may be utilized to preheat the incoming air stream, or in internal or external fuel reformer. The high-temperature exhaust may also be used for cogeneration or elec­tricity generation in a downstream gas turbine system.

Quite a few prototype experiments have been done, and a large number of postulations are yet to be worked out, based on the potential differ­ence maintained within and outside the living cell. Two well-known phenomena are the membrane potential and the injury potential.

If the inside and outside walls of a cell membrane are brought to elec­trical continuity, current will flow. Usually the inside is anodic, mainly due to the dominating fixed charges on the membrane protein. When injury is caused, the excess mobile cations from the outer surface infil­trate the inner layer and a local flow of current takes place. A healthy (uninjured) cell maintains an intact membrane, spends some metabolic energy to pump in nutrients and K+, and retains them within the cell against a concentration gradient. Likewise, some of the metabolic prod­ucts, including Na+ are pumped out (exceptions, namely, Halobacterium— are few).

Most of these functions are chemically mediated (by ATPase, ATP — Mg2+, etc.) and amount to mechanical work. Maintenance of the poten­tial difference on the membrane inside out is an indirect electrical mani­festation of the chemical activity. The membrane components, particularly protein, uphold its configuration with desired functional groups pro­jected within. Retention of selective ions with the cell, in addition to offering electrical neutrality, offers colloid osmotic steady state (through Donnan equilibrium).

Another interesting phenomenon associated with chemical activity of cells is the pH specificity of specialized cells. Normally, the mammalian body fluid behaves as an alkaline buffer, pH 7.4, with only about 0.1 M, contributed by metal ions, but has high osmolarity due to colloid osmotic components. In spite of the pH 7.4 of the circulating fluid, the stomach, part of the kidney, and the respiratory system maintain distinct acid pH. This mechanism of upholding higher H+ concentration is by metabolic expenses. In plants, the tissue fluid is usually acidic, say pH 6.5, and certain specialized tissues, namely fruits, exhibit strong acidity. In very rare cases (marine flora), plant tissue fluids show alkaline pH.

These examples are sufficient to indicate that if gastric mucosa is connected to the intravenous system, a potential difference or an EMF will be experienced. Likewise, if the root tissue and the fruit of a tree are short-circuited, current (however feeble) will be experienced. This information is not worth much at this present state of the art because the magnitude of instrumentation will appear prohibitive. But in space research, there was no alternative left but to develop solar cells, and sil­icon cells have found their place despite their cost. Because roughly 4 kcal of energy is available per gram of coal or hydrocarbon, this tech­nique is of limited value at present. However, with enhanced improve­ment, the renewable resources of flora and fauna may be sources of direct energy when we run out of oil and coal and will also appear inex­pensive under those circumstances.

Lignocellulosic materials predominantly contain a mixture of carbohy­drate polymers (cellulose and hemicellulose) and lignin. The carbohydrate polymers are tightly bound to lignin mainly through hydrogen bonding, but also through some covalent bonding. The contents of cellulose, hemi — cellulose, and lignin in common lignocellulosic materials are listed in Table 3.2. Different types of carbohydrates (glucan, xylan, galactan, arabinan, and mannan), lignin, extractive, and ash content of many lig — nocellulosic materials have been analyzed and are available in the lit­erature [2, 11-14] (see Table 3.2).

1.5.1 Cellulose

Cellulose is the main component of most lignocellulosic materials. Cellulose is a linear polymer of up to 27,000 glucosyl residues linked by ^-1,4 bonds. However, each glucose residue is rotated 180° relative to

its neighbors so that the basic repeating unit is in fact cellobiose, a dimer of a two-glucose unit. As glucose units are linked together into polymer chains, a molecule of water is lost, which makes the chemical formula C6H10O5 for each monomer unit of “glucan.” The parallel polyglucan chains form numerous intra — and intermolecular hydrogen bonds, which result in a highly ordered crystalline structure of native cellulose, interspersed with less-ordered amorphous regions [15, 16].

Crop description. Ricinus communis L., commonly known as the castor — oil plant, belongs to the family Euphorbiaceae (see Fig. 4.3). This peren­nial tree or shrub can reach up to 12 m high in tropical or subtropical climates, but it remains 3 m tall in temperate places. Native to Central Africa, it is being cultivated in many hot climates. The oil contains up to 90% ricinoleic acid, which is not suitable for nutritional purposes due to its laxative effect [52]. This hydroxycarboxylic acid is responsi­ble for the extremely high viscosity of castor oil, amounting to almost a hundred times the value observed for other fatty materials [53].

Main uses. Castor bean is cultivated for its seeds, which yield a fast­drying oil used mainly in industry and medicine. Coating fabrics, high — grade lubricants, printing inks, and production of a polyamide nylon-type fiber are among its uses. Dehydrated oil is an excellent drying agent and is used in paints and varnishes. Hydrogenated oil is utilized in the man­ufacture of waxes, polishes, carbon paper, candles, and crayons. The pomace or residue after crushing is used as a nitrogen-rich fertilizer. Although it is highly toxic due to the ricin, a method of detoxicating the meal has been developed, so that it can safely be fed to livestock [54]. Several authors have found that castor-oil biodiesel can be considered as a promising alternative to diesel fuel. Transesterification reactions have been carried out mainly by using both ethanol and NaOH, and through enzymatic methanolysis [55-57]. Several authors have studied the influ­ence of the nature of the catalyst on the yields of biodiesel from castor oil. They found that the most efficient transesterification of castor oil could be achieved in the presence of methoxide and acid catalysts [58]. The influence of alcohol has also been studied. Comparing the use of ethanol versus methanol, Meneghetti et al. have found that similar yields of fatty acid esters may be obtained; however, the reaction with methanolysis is much more rapid [59]. Cvengros et al. produced both ethyl and methyl esters, using NaOH in the presence of ethanol and methanol, respectively. Despite the high viscosity and density values, they concluded that both methyl and ethyl esters can be successfully used as fuel. A positive solution to meet the standard values for both viscosity and density parameters can be a dilution with esters based
on oils/fats without an OH group, or a blending with conventional diesel fuel [60].

With the advent of low-sulfur petroleum-based DFs, the issue of DF lubric­ity is becoming increasingly important. Desulfurization of petrodiesel reduces or eliminates the inherent lubricity of this fuel, which is essen­tial for proper functioning of vital engine components such as fuel pumps and injectors. Several studies [10, 11, 67-82] on the lubricity of biodiesel or fatty compounds have shown a beneficial effect of these materials on the lubricity of petrodiesel, particularly low-sulfur petrodiesel fuel. Adding biodiesel at low levels (1-2%) restores the lubricity to low-sulfur petroleum-derived DFs. However, the lubricity-enhancing effect of biodiesel at low blend levels is mainly caused by minor components of biodiesel such as free fatty acids and monoacylglycerols [83], which have free COOH and OH groups. Other studies [84, 85] also point out the beneficial effect of minor components on biodiesel lubricity, but these studies do not fully agree on the responsible species [83-85]. Thus, biodiesel is required at 1-2% levels in low-lubricity petrodiesel, in order for the minor components to be effective lubricity enhancers [83]. At higher blend levels, such as 5%, the esters are sufficiently effective without the presence of minor components.

While the length of a fatty acid chain does not significantly affect lubricity, unsaturation enhances lubricity slightly; thus an ester such as methyl linoleate or methyl linolenate improves lubricity more than methyl stearate [80, 83]. In accordance with the above observation on the effect of free OH groups on lubricity, castor oil displayed better lubricity than other vegetable oil esters [75, 80, 81]. Ethyl esters have improved lubricity compared to methyl esters [75].

Standards for testing DF lubricity use the scuffing load ball-on-cylinder lubricity evaluator (SLBOCLE) (ASTM D6078) or the high-frequency reciprocating rig (HFRR) (ASTM D6079; ISO 12156). Lubricity has not been included in biodiesel standards despite the definite advantage of biodiesel over petrodiesel with respect to this fuel property. However, the HFRR method has been included in the petrodiesel standards ASTM D975 and EN 590.