1.10.1 Biomass

Imitating the coal-based process, biomass conversion has also been tried and looks promising. Main sources of biomass are agricultural, horti­cultural, and forest wastes. Municipal organic solid wastes (which are also plenty) are potential resources as well. Considering biomass as a renewable resource, the bioconversion may be pyrolytic, where biogas and bio-oil are the main products and yet the residue contains some calo­rie value which can be further utilized (as adsorbents, filter beds, chars, etc.). Supercritical conversion and superheated steam reformation of bio­mass are recent techniques. During 1990—1997, quite a few reports appeared in the literature showing success and promise of catalytic or uncatalytic reformation of biomass to hydrogen (almost to 18% v/v) without any char or residues.

Temperature ranges of 340-650oC, with pressures of 22-35 MPa, are cited with as low as 30-s residence time, through supercritical flow reac­tors. The raw materials are widely varying: water hyacinth, algae, bagasse, whole biomass, sewage sludge, sawdust, and other effluents rich in organic matters. In some efficient carbon bed-catalyzed reactors, other products (i. e., carbon mono — and dioxides and methane) were also detected.

Historically, yeasts have been the most commonly used microorganisms for ethanol production. Yeast strains are generally chosen among S. cere — visiae, S. ellypsoideuse, S. fragilis, S. carlsbergensis, Schizosaccharomyces pombe, Torula cremoris, and Candida pseudotropicalis. Yeast species which can produce ethanol as the main fermentation product are reviewed, e. g., by Lin and Tanaka [8].

Among the ethanol-producing yeasts, the “industrial working horse” S. cerevisiae is by far the most well-known and most widely used yeast in industry and research for ethanol fermentation. This yeast can grow both on simple hexose sugars, such as glucose, and on the disaccharide sucrose. S. cerevisiae is also generally recognized to be safe as a food additive for human consumption and is therefore ideal for producing alcoholic beverages and for leavening bread. However, it cannot fer­ment pentoses such as xylose and arabinose to ethanol [14, 31]. There have been several research efforts to genetically modify S. cerevisiae to be able to consume xylose [33, 48-50]. Several attempts have been made to clone and express various bacterial genes, which is necessary for fer­mentation of xylose in S. cerevisiae [51, 52]. It resulted in great success, but probably not enough yet to efficiently ferment xylose with high yield and productivity [32].

Alternatively, xylose is converted to ethanol by some other naturally occurring recombinant. Among the wild-type xylose-fermenting yeast strains for ethanol production, Pichia stipitis and C. shehatae have reportedly shown promising results for industrial applications in terms of complete sugar utilization, minimal by-product formation, low sensitivity to temperature, and substrate concentration. Furthermore, P. stipitis is able to ferment a wide variety of sugars to ethanol and has no vitamin requirement for xylose fermentation [2].

Olsson and Hahn-Hagerdal [20] have presented a list of bacteria, yeasts, and filamentous fungi that produce ethanol from xylose. Certain species of the yeasts Candida, Pichia, Kluyveromyces, Schizosaccharomyces, and Pachysolen are among the naturally occurring organisms. Jeffries and Kurtzman [53] have reviewed the strain selection, taxonomy, and genetics of xylose-fermenting yeasts.

Utilization of cellobiose is important in ethanol production from lig — nocellulosic materials by SSF. However, a few ethanol-producing microorganisms are cellobiose-utilizing organisms. The requirement for addition of ^-glucosidase has been eliminated by cellobiose utilization during fermentation, since presentation of cellobiose reduces the activity of cellulase. Cellobiose utilization eliminates the need for one class of cellulase enzymes [2]. Brettanomyces custersii is one of the yeasts iden­tified as a promising glucose — and cellobiose-fermenting microorganism for SSF of cellulose for ethanol production [54].

High temperature tolerance could be a good characterization for ethanol production, since it simplifies fermentation cooling. On the other hand, one of the problems associated with SSF is the different optimum temperatures for saccharification and fermentation. Many attempts have been made to find thermotolerant yeasts for SSF. Szczodrak and Targonski [55] tested 58 yeast strains belonging to 12 different genera and capable of growing and fermenting sugars at temperatures of 40-46oC. They selected several strains belonging to the genera Saccharomyces, Kluyveromyces, and Fabospora, in view of their capacity to ferment glucose, galactose, and mannose at 40oC, 43oC, and 46oC, respectively. Kluyveromyces marxianus has been found to be a suitable strain for SSF [56].

Crop description. Dipteryx odorata—commonly known as sarapia, tonka bean, amburana, aumana, yape, charapilla, and cumaru—belongs to the family Leguminacea and grows in tropical areas (see Fig. 4.13). Major producing countries are Guianas and Venezuela. The tonka bean is the seed of a large tree. The kernel contains up to 46% oil on a dry basis. Major fatty acid composition of oil includes palmitic acid (6.1%), stearic acid (5.7%), oleic acid (59.6%), and linoleic acid (51.4%) [77].

Main uses. The oil is used in perfumery and as a flavoring material. Tonka extracts are used in the tobacco industry to impart a particular aroma. Few attempts have been made to use it as a raw material to pro­duce biodiesel. Abreu et al. conducted methanolysis of cumaru oil using different homogeneous metal (Sn, Pb, and Zn) complexes as catalysts. They found that pyrone complexes of different metals are active for cumaru-oil transesterification reaction [122].

Ethanol (ethyl alcohol) as a transport fuel has attracted a lot of atten­tion because it is seen as a relatively cheap nonpetroleum-based fuel. It is produced to a large extent from biomass, which aids agricultural economies by creating a stable market. Ethanol, being a pure compound, has a fixed set of physical as well as chemical properties. This is in con­trast to petrol and diesel, which are mixtures of hydrocarbons [3].

The use of alcohol in spark ignition (SI) engines began in 1954 in countries like the United States, Germany, and France. During World Wars I and II, gasoline shortages occurred in France and Germany, and alcohol was used in all types of vehicles, including military planes. Nowadays, it is used with gasoline (a mixture) in the United States and has become a major fuel in Brazil.

Ethyl alcohol can be produced by fermentation of vegetables and plant materials. But in countries like India, ethanol is a strong candidate since they possess the agricultural resources for the production of ethyl alcohol. It is a more attractive fuel for India because the productive capacity from sugarcane crops is high, of the order 1345 L/ha. Earlier, this fuel was not used in automobiles due to low energy density, high pro­duction cost, and corrosion. The current shortage of gasoline has made it necessary to substitute ethanol as fuel in SI engines.

Any new fuel that is going to be introduced should be evaluated from the aspect of availability, renewability, safety, and cost adaptability to the existing engines’ performance, economy, and finally emission. A mas­sive research effort has been put into the study and analysis of all these aspects for ethanol, which is now an established, viable alternative fuel for IC engines. The comparative properties of ethanol with petrol and diesel are shown in Table 7.1.

The PEMFC uses a solid polymer membrane as an electrolyte. The main components of this fuel cell are an electron-conducting anode consist­ing of a porous gas diffusion layer as an electrode and an anodic cata­lyst layer; a proton-conducting electrolyte, a hydrated solid membrane; an electron-conducting cathode consisting of a cathodic catalyst layer and a porous gas diffusion layer as an electrode; and current collectors with the reactant gas flow fields (see Fig. 9.3).

In the PEMFC, platinum or platinum alloys in nanometer-size par­ticles are used as the electrocatalysts with NafionTM (a DuPont trade­mark) membranes [3, 10—12]. The polymer electrolyte membranes have some unusual properties: In a hydrated membrane, the negative ions are rigidly held within its structure and are not allowed to pass through.

Electric current

,-WV

Only the positive ions contained within the membrane are mobile and free to carry a positive charge through the membrane. The proton exchange membrane (PEM) is a good conductor of hydrogen ions (pro­tons), but it does not allow the flow of electrons through the electrolyte membrane. As the electrons cannot pass through the membrane, elec­trons produced at the anode side of the cell must travel through an external wire to the cathode side of the cell to complete the electrical circuit in the cell.

In the PEMFC, the positive ions moving through the electrolyte are hydrogen ions, or protons. Therefore, the PEMFC is also called a proton exchange membrane fuel cell. The polymer electrolyte membrane is also an effective gas separator; it keeps the hydrogen fuel separated from the oxidant air. This feature is essential for the efficient operation of a fuel cell.

The heart of a PEMFC is the membrane electrode assembly (MEA), consisting of the anode-electrolyte-cathode assembly that is only a few hundred microns thick [11].

Electrochemistry of PEM fuel cells. All electrochemical reactions consist of two separate reactions: an oxidation half reaction occurring at the anode and a reduction half reaction occurring at the cathode.

Oxidation half reaction: 2H2 ^ 4H+ + 4e~

Reduction half reaction: O2 + 4H+ + 4e~ ^ 2H2O

Overall cell reaction: 2H2 + O2 ^ 2H2O

The H2 half reaction. At the anode, hydrogen (H2) gas molecules diffuse through the porous electrode until they encounter a platinum (Pt) par­ticle. Pt catalyzes the dissociation of the H2 molecule into two hydrogen atoms (H) bonded to two neighboring Pt atoms; here each H atom releases an electron to form a hydrogen ion (H+). These H+ ions move through the hydrated membrane to the cathode while the electrons pass from the anode through the external circuit to the cathode, resulting in a flow of current in the circuit.

The O2 half reaction. The reaction of one oxygen (O2) molecule at the cathode is a four-electron reduction process that occurs in a multistep sequence. The catalysts capable of generating high rates of O2 reduction at relatively low temperatures (~80°C) appear to be the Pt-based expen­sive catalysts. The performance of the PEMFCs is limited primarily by the slow rate of the O2 reduction half reaction, which is many times slower than the H2 oxidation half reaction.

Electrolyte. The polymer electrolyte membrane is a solid organic poly­mer, usually poly-[perfluorosulfonic] acid. Atypical membrane material used in the PEMFC is Nation [11, 12]. It consists of three regions:

1. The teflon-like fluorocarbon backbone, hundreds of repeating —CF2—CF—CF2— units in length

2. The side chains, — O-CF2-CF-O-CF2-CF2-, which connect the molec­ular backbone to the third region

3. The ion clusters consisting of sulfonic acid ions, SO3~ H+

The negative ion SO3~ is permanently attached to the side chain and cannot move. However, when the membrane becomes hydrated by absorbing water, the hydrogen ion becomes mobile. Ion movement occurs by protons (H ) bonded to water molecules, hopping from one SO3 site to another within the membrane. Because of this mechanism, the solid hydrated electrolyte is an excellent conductor of hydrogen ions.

Electrodes. The anode and the cathode are separated from each other by the electrolyte, the PEM. Each electrode consists of porous carbon to which very small Pt particles are bonded. The porous electrodes allow the reactant gases to diffuse through each electrode to reach the catalyst. Both platinum and carbon are good conductors, so electrons are able to move freely through the electrode [13, 14].

Catalyst. The two half reactions occur very slowly under normal con­ditions at the low operating temperature (~80°C) of the PEMFC. Therefore, catalysts are needed on both the anode and cathode to increase the rates of each half reaction. Although platinum is a very expensive metal, it is the best material for a catalyst on each electrode.

The half reactions occurring at each electrode can occur only at a high rate at the surface of the Pt catalyst. A unique feature of Pt is that it is sufficiently reactive in bonding H and O intermediates, as required to facilitate the electrode processes, and is also capable of effectively releas­ing the intermediate to form the final product. The anode process requires Pt sites to bond H atoms when the H2 molecule reacts; next, these Pt sites release the H atoms, as follows:

2H+ + 2e~ ^ H2
H2 + 2Pt ^ 2(Pt-H)

2(Pt-H) ^ 2Pt + 2H+ + 2e~

This optimized bonding to H atoms (neither very weak nor very strong) is a unique property of the Pt catalyst. To increase the reaction rate, the catalyst layer is constructed with the highest possible surface area. This is achieved by using very small Pt particles, about 2 nm in diameter, resulting in an enormously large total surface area of Pt that is accessible to gas molecules. The original MEAs for the Gemini space program used 4 mg of platinum per square centimeter of membrane area (4 mg/cm2). Although the technology varies with the manufacturer, the total platinum loading has decreased from the original 4 mg/cm2 to about 0.5 mg/cm2. Laboratory research now uses platinum loadings of 0.15 mg/cm2. For catalyst layers containing Pt of about 0.15 mg/cm2, the thickness of the catalyst layer is ~10 ^m; the MEA with a total thickness ~200 ^m can generate more than half an ampere of current for every square centimeter of the MEA at a voltage of 0.7 V between the cath­ode and the anode [2, 3, 10-12]. Recently, scientists at Los Alamos National Laboratory, USA have developed a new class of hydrogen fuel cell catalysts that exhibit promising activity and stability. The cata­lysts, cobalt-polypyrrole-carbon (Co-PPY-XC72) composite, are made of low-cost metals entrapped in a heteroatomic-polymer structure.

The cell hardware. The hardware of the fuel cell consists of backing layers, flow fields, and current collectors. These are designed to maxi­mize the current that can be obtained from an MEA. The backing layers placed next to the electrodes are made of a porous carbon paper or carbon cloth, typically 100-300 ^m thick. The porous nature of the back­ing material ensures effective diffusion of the reactant gases to the cat­alyst. The backing layers also assist in water management during the operation of the fuel cell; too little or too much water can halt the cell operation. The correct backing material allows the right amount of water vapor to reach the MEA and keep the membrane humidified.

Carbon is used for backing layers because it can conduct the electrons leaving the anode and entering the cathode. Apiece of hardware, called a plate, is pressed against the outer surface of each backing layer. The plate serves the dual role of a flow field and current collector. The side of the plate next to the backing layer contains channels machined into the plate. The plates are made of a lightweight, strong, gas-impermeable, electron-conducting material; graphite or metals are commonly used, although composite material plates are now being developed. Electrons produced by the oxidation of hydrogen move through the anode, through the backing layer, and through the plate before they can exit the cell, travel through an external circuit, and reenter the cell at the cathode plate. In a single fuel cell, these two plates are the last of the compo­nents making up the cell.

In a fuel cell stack, current collectors are the bipolar plates; they make up over 90% of the volume and 80% of the mass of a fuel cell stack [11, 15, 16].

Water and air management. Although water is a product of the fuel cell reaction and is carried out of the cell during its operation, it is nec­essary that both the fuel and air entering the fuel cell be humidified. This additional water keeps the polymer electrolyte membrane hydrated. The humidity of the gases has to be carefully controlled, as too little water dries up the membrane and prevents it from conducting the H+ ions and the cell current drops. If the air flow past the cathode is too slow, the air cannot carry all the water produced at the cathode out of the fuel cell, and the cathode “floods.” Cell performance deteri­orates because not enough oxygen is able to penetrate the excess liquid water to reach the cathode catalyst sites. Cooling is required to maintain the temperature of a fuel cell stack at about 80°C, and the product water produced at the cathode at this temperature is both liquid and vapor.

Performance of the PEM fuel cell [3,11,16]. Energy conversion in a fuel cell is given by the relation:

Chemical energy of the fuel = electric energy + heat energy

Power is the rate at which energy (E) is made available (P = dE/dt, or AE = PAt). The power delivered by a cell is the product of the current (I) drawn and the terminal voltage (V) at that current (P = IV watts). In order to compute power delivered by a fuel cell, we have to know the cell voltage and load current. The ideal (maximum) cell voltage (E) for the hydrogen/air fuel cell reaction (H2 + 1/2O2 ^ H2O) at a specific tem­perature and pressure is calculated from the maximum electrical energy

AG

Wel = — AG = nFE or E = —

TP

where AG is the change in Gibbs free energy for the reaction, n is the number of moles of electrons involved in the reaction per mole of H2, and F (Faraday’s constant) = 96,487 C (coulombs = joules/volt). At a constant pressure of 1 atm, the change in Gibbs free energy in the fuel cell process (per mole of H2) is calculated from the reaction temperature (T) and from changes in the reaction enthalpy (H) and entropy (S).

AG = AH — T AS

= -285,800 J — (298 K)(-163.2 J/K) = -237,200 J

For the hydrogen-air fuel cell at 1 atm pressure and 25°C (298 K), the cell voltage is

AG

nF

As temperature rises from room temperature to the PEM fuel cell operating temperature (80°C or 353 K), the change in values of H and S is very small, but T changes by 55°C. Thus the absolute value of AG decreases. Assuming negligible change in the values of H and S,

AG = -285,800 J/mol — (353 K)(163.2 J/mol • K)

= -228,200 J/mol

Therefore,

Thus, for standard pressure of 1 atm, the maximum cell voltage decreases from 1.23 V at 25°C to 1.18 V at 80°C. An additional correc­tion is needed for using air instead of pure oxygen, and also for using humidified air and hydrogen instead of dry gases. This further reduces the maximum voltage from the hydrogen-air fuel cell to 1.16 V at 80°C and 1 atm pressure. With an increase in load current, the actual cell potential is decreased from its no-load potential because of irreversible losses, which are often called polarization or overvoltage (h). These origi­nate primarily from three sources:

■ Activation polarization (hact)

■ Ohmic polarization (hohm)

■ Concentration polarization (hconc)

The polarization losses result in a further decrease in actual cell volt­age (V) from its ideal potential E (V = E — potential drop due to losses). The activation polarization loss is dominant at low current density. This is because electronic barriers have to be overcome prior to current and ion flows. Activation polarization is present when the rate of an electro­chemical reaction at an electrode surface is controlled by sluggish elec­trode kinetics. Therefore, activation polarization is directly related to the rates of electrochemical reactions. In an electrochemical reaction with hact > 50 — 100 mV, activation polarization is described by a semi­empirical equation known as the Tafel equation:

ha“ = ІЗ) ln (t)

where a is the electron transfer coefficient of the reaction at the elec­trode (anode or cathode), and i0 is the exchange current density. The Tafel slope for the PEMFC electrochemical reaction is about 100 mV/decade at room temperature. Thus there is an incentive to develop electrocat­alysts that yield a lower Tafel slope [2, 3, 11, 12].

Ohmic losses occur because of the resistance to the flow of ions in the electrolyte and resistance to the flow of electrons through the electrode materials. Decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte can reduce the ohmic losses. Both the electrolyte and fuel cell electrodes obey Ohm’s law; the ohmic losses can be expressed by the equation: hohm = iR, where i is the current flowing through the cell and R is the total cell resistance, which includes ionic, electronic, and contact resistance. See Figure 9.4.

Due to the consumption of reactants at the electrode by an electro­chemical reaction, the surrounding material is unable to maintain the

initial concentration of the bulk fluid and a concentration gradient is formed, resulting in a loss of electrode potential. Although several processes contribute to concentration polarization, at practical current densities, slow transport of reactants and products to and from the elec­trochemical reaction site is a major contributor to concentration polar­ization. The effect of polarization is to shift the potential of the electrode: For the anode,

and for the cathode,

kathode Ecathode I hcathode I

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential. This reduces the cell voltage. The cell voltage includes the contribution of the anode and cathode potentials and ohmic polarization. See Figure 9.5.

Fcell kathode Fanode iR; or

Fcell Ecathode ^cathode 1 (Eanode ^ |^anode 1) iR; or

Fcell Ecell |Ecathode 1 |^anode 1

where Ecell Ecathode Eanode

The goal of fuel cell developers is to minimize the polarization losses so that the Fcell approaches the Ecell by modifications to the fuel cell

Figure 9.5 PEM fuel cell voltage versus current density curve [3].

design by improvement in the electrode structures, better electrocata­lysts, more conductive electrolytes, thinner cell components, and so forth. It is possible to improve the cell performance by modifying the operat­ing conditions such as higher gas pressure, higher temperature, and a change in gas composition to lower the gas impurity concentration [3].

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.