Crop description. Cyperus esculentus L.—commonly known as tigernut, chufa sedge, yellow nutsedge, and earth almond—belongs to the family Cyperaceae and grows in warm temperate to subtropical regions of the Northern Hemisphere (see Figs. 4.17 and 4.18). It can be found in Africa, South America, Europe, and Asia. It is a perennial herb, growing up to
90 cm high [138]. Tubers contain 20-36% oil. The oil from the tuber con­tains 18% saturated (palmitic acid and stearic acid) and 82% unsatu­rated (oleic acid and linoleic acid) fatty acids [138].

Main uses. The tubers are edible and have high nutritive value. They contain 3-15% protein, 15-20% sugar, 20-25% starch, 4-14% cellulose, and trace amounts of natural resin. They are used in Spain to make a beverage named horchata, and also consumed fresh after soaking. In other countries, the tubers are used in sweetmeats or uncooked as a side dish. New products obtained can enhance the interest in this crop

Figure 4.18 Cyperus esculentus L. (Photo courtesy of Peter Chen [www. cod. edu/people/faculty/ch enpe/PRAIRIE/2005_09_20/ Cyperus_esculentus. jpg].)

as a source of dietary fiber in food technology, as a high-quality cooking/ salad oil, as a source of starch, as an antioxidant-containing food, and so forth [139]. The oil extracted from yellow nutsedge can be used as food oil as well as for industrial purposes. Since the tubers contain 20—36% oil, the crop has been suggested as a potential oil crop for the produc­tion of biodiesel [138]. Preliminary tests using pure nutsedge oil as fuel in a diesel engine have indicated that the engine operated near its rated power [140]. Currently, it is being studied as an oil source for fuel pro­duction in Africa [53].

Alcohol can be used as a blend with gasoline as this has the advantage that the existing engines need not be modified and tetra-ethyl lead (TEL) can be eliminated from gasoline, due to the octane-enhancing quality of alcohol. If the engine is to be operated using only pure alco­hol, then some major modifications are required in the engine and fuel system, as listed below:

1. Both alcohols and blends with gasoline are corrosive to many of the engine materials. These materials have to be changed.

2. Adjustment of the carburetor and fuel injection need to be made to compensate for the leaning effect.

3. Change in the fuel pump and circulation system need to be made to avoid vapor lock, as the methanol vaporization rate is very high.

4. Introduction of high energy ignition system with lean mixture.

5. Increase in compression ratio to make better antiknock properties of the fuel.

6. Addition of detergent and volatile primers to reduce engine deposits and assist in cold starting.

7. Use of cooler-running spark plugs to avoid preignition.

General properties of the blends are listed in Table 7.4. The volatil­ity shown by the American Standard Testing Method (ASTM) distillation characteristics of petrol is a compromise between opposing factors to ensure good performance in petrol engines. This requires petrol to have a sufficiently lighter reaction and a 10% distillation temperature in order to start the engine as well as warm up, but the temperature should

not be so low that vapor-locking takes place and stops the engine due to the nonsupply of fuel. As far as volatility is concerned, ethanol-petrol blends are as good as petrol, if not better. Also gum resistance is greater than that of petrol. Aniline points for blends are lower, which indicates more aromatic content than petrol, due to the adding of ethanol to petrol, which helps to improve the octane number marginally. If a small quan­tity of water is introduced into a gasoline-alcohol blend, phase separation takes place, with gasoline-content in the upper phase and alcohol in the lower. This separation produces some undesirable effects. The alco­hol-water mixture tends to pick up sediment and stall the engine on reaching the carburetor [4]. To improve the water tolerance of the blend, benzene and heptanes are added.

Since 1979, gasohol has been sold at 500 filling stations in the mid­western United States, where the corn from which alcohol is commonly made is abundant. This blend yields about the same mileage as unleaded gasoline and even offers an ever renewable source of energy. Moreover, if this blend were to replace gasoline, it could cut as much as 10% of the nation’s oil imports, which totalled $40 billion in 1979. This fuel has a good future in wealthy countries. The blends have some important advantages over pure ethanol, as listed below:

1. The starting difficulty can be removed.

2. There is no abnormal corrosion compared with pure ethanol.

3. Lubrication in a petrol-alcohol blend is more or less the same.

4. Some benzene is added to prevent separation of the layers of petrol and alcohol.

If blends are used, some minor modifications in the engine are required, as listed below:

1. The carburetor jet should be increased to increase the flow 1.56 times that of petrol.

2. The float has to be weighted down to correct levels due to higher spe­cific gravity.

3. The air inlet should be modified to get less air as blends require less air for complete combustion than petrol.

4. Specific arrangement of heating the carburetor and intake manifold should be provided as lower vapor pressure of alcohol makes the starting difficult below 70°C.

The MCFC has evolved from work in the 1960s, aimed at producing a fuel cell that would operate directly on coal [23, 24]. Although direct oper­ation on coal is no longer a goal, a remarkable feature of the MCFC is that it can directly operate on coal-derived fuel gases or natural gas and is therefore also called a direct fuel cell (DFC). MCFCs operate at high temperatures (600-650°C) compared to phosphoric acid (180-220°C) or PEM fuel cells (60-85°C). Operation at high temperatures eliminates the need for external fuel processors that the lower temperature fuel cells require to extract hydrogen from naturally available fuel. When natu­ral gas is used as fuel, methane (the main ingredient of natural gas) and water (steam) are converted into a hydrogen-rich gas inside the MCFC stack (“internal reforming”) (see Fig. 9.9). High operating temperatures also result in high-temperature exhaust gas, which can be utilized for heat recovery for secondary power generation or cogeneration. MCFCs can therefore achieve a higher fuel-to-electricity and an overall energy use efficiency (>75%) than the low-temperature fuel cells. The MCFC

Fuel + steam

CO,

Internal

reforming

Anode

is a well-developed fuel cell and is a commercially viable technology for a stationary power plant, compared to other fuel cell types. A number of MCFC prototype units in the power range of 200 kW to 1 MW and higher are operating around the world. The cost and useful life issues are the major challenges to overcome before the MCFC can compete with the existing (thermal or other) electric power generation systems for widespread use.

Electrochemistry of MCFC. The electrochemical reactions occurring in the cell are:

Anode half reaction. At the anode, hydrogen reacts with carbonate ions to produce water, carbon dioxide, and electrons. The electrons travel through an external circuit—creating electricity—and return to the cathode.

H2 + CO32~ ^ H2O + CO2 + 2e~

Cathode half reaction. At the cathode, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form car­bonate ions that replenish the electrolyte and transfer the current through the fuel cell, completing the circuit.

2 O2 + CO2 + 2e S CO3

The overall cell reaction is

H2 + 2 O2 + CO2 (cathode) s H2O + CO2 (anode)

If a fuel such as natural gas is used, it has to be reformed either exter­nally or within the cell (internally) in the presence of a suitable cata­lyst to form H2 and CO by the reaction:

CH4 + H2O ^ 3H2 + CO

Although, CO is not directly used by the electrochemical oxidation, but produces additional H2 by the water gas shift reaction:

CO + H2O ^ H2 + CO2

Typically, the CO2 generated at the anode is recycled to the cathode, where it is consumed. This requires additional equipment to either trans­fer CO2 from the anode exit gas to the cathode inlet gas or produce CO2 by combustion of anode exhaust gas and mix with the cathode inlet gas.

Electrolyte. The MCFC uses a molten carbonate salt mixture as its electrolyte. At operating temperatures of about 650°C, the salt mixture is in a molten (liquid) state and is a good ionic conductor. The composi­tion of salts in the electrolyte may vary but usually consist of lithium/potassium carbonate (Li2CO3/K2CO3, 62-38 mol%) for operation at atmospheric pressure. For operation under pressurized conditions, lithium/sodium carbonate (LiCO3/NaCO3, 52-48 or 60-40 mol%) is used as it provides improved cathode stability and performance. This allows for the use of thicker Li/Na electrolyte for the same performance, resulting in a longer lifetime before a shorting caused by internal precipitation. The composition of the electrolyte has an effect on electrochemical activ­ity, corrosion, and electrolyte loss rate. Li/Na offers better corrosion resistance but has greater temperature sensitivity. Additives are being developed to minimize the temperature sensitivity of the Li/Na elec­trolyte. The electrolyte has a low vapor pressure at the operating tem­perature and may evaporate very slowly; however, this does not have any serious effect on the cell life. The electrolyte is suspended in a porous, insulating, and chemically inert ceramic (LiAlO2) matrix. The ceramic matrix has a significant effect on the ohmic resistance of the electrolyte. It accounts for almost 70% of the ohmic polarization. The electrolyte management in an MCFC ensures that the electrolyte matrix remains completely filled with the molten carbonate, while the porous electrodes are partially filled, depending on their pore size distributions.

Electrode. The anode is made of a porous chromium-doped sintered Ni-Cr/Ni-Al alloy. Because of the high temperatures resulting in a fast anode action, a large surface area is not required on the anode as com­pared to the cathode. Partial flooding of the anode with molten carbon­ate is desirable as it acts as a reservoir that replenishes carbonate in the stack during prolonged use. The cathode is made up of porous lithi — ated nickel oxide. Because of the high operating temperatures, no noble catalysts are needed in the fuel cell. Nickel is used on the anode and nickel oxide on the cathode as catalysts. Bipolar plates or interconnects are made from thin stainless steel sheets with corrugated gas diffusion channels. The anode side of the plate is coated with pure nickel to pro­tect against corrosion.

Performance. At the high operating temperatures of an MCFC, CO is not a poison but acts as a fuel. In the MCFC, CO2 has to be added to oxygen (air) stream at the cathode for generation of carbonate ions. The anode reaction converts these ions back to CO2, resulting in a net transfer of two ions with every molecule of CO2. The need for CO2 in the oxidant stream requires that CO2 from the spent anode gas be separated and mixed with the incoming air stream. Before this can be done, any resid­ual hydrogen in the spent fuel stream must be burned. Systems devel­oped in the future may incorporate membrane separators to remove the hydrogen for recirculation back to the fuel stream to increase efficiency.

Internal reforming of natural gas and partially cracked hydrocarbons is possible in the inlet chamber of the MCFC, eliminating the separate fuel processing of natural gas or other hydrogen-rich fuels. The require­ment for CO2 makes the digester gas (sewage, animal waste, food pro­cessing waste, etc.) an ideal fuel for the MCFC; other fuels such as natural gas, landfill gas, propane, coal gas, and liquid fuels (diesel, methanol, ethanol, LPG, etc.) can also be used in the MCFC system. The elimination of the external fuel reformer also contributes to lower costs, and high-temperature waste heat can be utilized to make additional elec­tricity and cogeneration. MCFCs can reach overall thermal efficiencies as high as 85%.

With the increase in operating temperature, the theoretical operat­ing voltage for a fuel cell decreases, but increases the rate of the elec­trochemical reaction and therefore the current that can be obtained at a given voltage. This results in the MCFC having a higher operating volt­age for the same current density and higher fuel efficiency than a PAFC of the same electrode area. As size and cost scale roughly with the elec­trode area, the MCFC is smaller and less expensive than a PAFC of com­parable output. Another advantage of the MCFC is that the electrodes can be made with cheaper nickel catalysts rather than the more expen­sive platinum used in other low-temperature fuel cells. Endurance of the cell stack is a critical issue in commercialization, and MCFC manufac­turers report an average potential degradation of — 2 mV/1000 h over a cell stack lifetime of 40,000 h. The high temperature limits the use of materials in the MCFC, and safety issues prevent their application for home use. MCFC units require a few minutes of fuel burning at the start up to heat up the cell to its operating temperature and therefore are not very suitable for use in automobiles. However, they are very good for sta­tionary power applications and units with up to 2 MW have been con­structed, and designs for units with up to 100 MW exist [3, 23-25].

All forms of life are dependent on availability of energy at all levels, the creation, growth, and maintenance (defense, offense, and survival). The requirement and utilization of energy are mainly in two forms; the most important are nutrient and environmental energy in the form of heat and light.

It is easy to observe that extremely cold or hot regions are not favor­able for the growth of living things. Likewise, the absence of light limits the propagation and proliferation of photosynthetic biotic species.

The sun, of course, radiates energy into space of which only an insignif­icant part is shared by this planet of ours called Earth. Because of its spin and its orbital rotation, a seasonal variation occurs in the total insolation on the earth’s surface, which averages approximately 20 kcal/(m2 • yr). The incident radiation comprises 2000-8000 A, 50% of which is in the visible range (3700-7700 A); only a small part of the incident energy is utilized by living systems.

Solar constants are given as 1.968 cal/(cm2 • min) = 3.86 X 1033 erg/s = 1.373 kW/m2. There are variations in the figures, depending on the source of information. However, the energy received on the earth’s sur­face is mostly thermal and wasted. Biological fixation is restricted to pho­tophosphorylation.

Let us look at the components of ecosystems that are capable of uti­lizing incident energy and some interrelationships between them.

Autotrophs (meaning self-surviving), also known as producers, mainly the photosynthetic systems, are the largest users of sunlight. Theoretically, anywhere there is light they should grow, provided other inputs are favorable. In arid land, the lack of nutrients; in deserts, the lack of water; and at higher-altitude, low temperatures, low CO2 tension and other adverse conditions will prevent the proliferation of autotrophs, leaving otherwise sufficient insolation unutilized (energy fixation by photosynthetic pathway is treated elsewhere). Producers growing on detritus (dead organic materials) are not well described in the literature, but these could be autotrophs.

Heterotrophs (mixed surviving or unlike surviving), on the other hand, survive partly depending on the nutrient sources made avail­able by other living systems. Most animals are heterotrophic. Therefore, animals are also called consumers.

If animals survive mainly on autotrophic materials, they are called primary consumers, commonly known as herbivores. If animals largely survive on other animals as their source of food, they are called secondary consumers, popularly known as carnivores. Predators are animals that hunt their animate food, known as prey. The prey-predator relationship plays an important role in nature and con­tributes to the ecologic balance.

Mohammad J. Taherzadeh and Keikhosro Karimi

3.1 Introduction

Ethanol (C2H5OH) is a clear, colorless, flammable chemical. It has been produced and used as an alcoholic beverage for several thousand years. Ethanol also has several industrial applications (e. g., in detergents, toi­letries, coatings, and pharmaceuticals) and has been used as trans­portation fuel for more than a century. Nicholas Otto used ethanol in the internal combustion engine invented in 1897 [1]. However, ethanol did not have a major impact in the fuel market until the 1970s, when two oil crises occurred in 1973 and 1979. Since the 1980s, ethanol has been a major actor in the fuel market as an alternative fuel as well as an oxy­genated compound for gasoline. Ethanol can be produced synthetically from oil and natural gas, or biologically from sugar, starch, and ligno — cellulosic materials. The biologically produced ethanol is sometimes called fermentative ethanol or bioethanol. Application of bioethanol as fuel has no or very limited net emission of CO2 [2] and is able to fulfill the Kyoto Climate Change Protocol (1997) to decrease the net emission of CO2 [3]. In this chapter, the global market and the production of bioethanol are briefly reviewed.

In order to allow blending of alcohol with gasoline, the water content of ethanol must be reduced to less than 1% by volume, which is not pos­sible by distillation. Higher water levels can result in phase separation of an alcohol-water mixture from the gasoline phase, which may cause engine malfunction. Removal of water beyond the last 5% is called dehy­dration or drying of ethanol. Azeotropic distillation was previously employed to produce higher-purity ethanol by adding a third component, such as benzene, cyclohexane, or ether, to break the azeotrope and pro­duce dry ethanol [82]. To avoid illegal transfer of ethanol from the indus­trial market into the potable alcohol market, where it is highly regulated and taxed, dry alcohol usually requires the addition of denaturing agents that render it toxic for human consumption; the azeotropic reagents conveniently meet this requirement [82]. Except in the high-purity reagent-grade ethanol market, azeotropic drying has been supplanted by molecular sieve drying technology.

My sincere thanks to the following people and organizations for their generosity in letting me use their photos: Dr. Kazuo Yamasaki (Teikyo Heisei University, Japan), Abdulrahman Alsirhan (www. alsirhan. com), Eric Winder (Biological Sciences, Michigan Technological University), Jack Bacheler (Department of Entomology, North Carolina State University), Dr. Alvin R. Diamond (Department of Biological and Environmental Sciences, Troy University), Piet Van Wyk and EcoPort, Food and Agricultural Organization of the United Nations, Antoine van den Bos (Botanypictures), Forest and Kim Starr (USGS), Dr. Davison Shillingford (Dominica Academy of Arts and Sciences), Prof. Arne Anderberg (Swedish Museum of Natural History), Rolv Hjelmstad (Urtekilden), Peter Chen (College of DuPage), Josina Kimottho (ICRAF), Gernot Katzer (University of Graz), Prof. Gerald D. Carr (University of Hawaii, Botany Department), Barbara Simonsohn, Dr. Mike Kuhns (Utah State University), and Eugenio Arantes de Melo (Arvores do Brasil).

Methanol is more toxic as compared to petrol, which creates difficulty in its handling. The toxicity of methanol is reduced by adding chemical emetics.

The heterocyst, regularly spread among more numerous vegetative cells (ratio 1:15), receives carbon compounds fixed by the neighboring vege­tative cells in exchange of the nitrogenous compounds fixed by them. Nitrogenase, like hydrogenase, needs an anaerobic environment to func­tion and can produce hydrogen only under certain conditions (absence of molecular nitrogen). The ratio of evolution of hydrogen and oxygen roughly corresponds to the ratio of the heterocysts and vegetative cells and also with the ratio of nitrogen and carbon for nutritive requirements.

If the algal culture is exposed to argon for about 24 hours, due to nitro­gen starvation, differentiation of the heterocysts increases from 6% up to 20%. In addition, a yellowish color appears due to the loss of the light­trapping pigment phytocyanin, resulting in less carbon dioxide fixation, i. e., oxygen evolution and an increase in light conversion efficiency by almost 0.5%. Induction of reversible hydrogenase in the heterocysts, as its theoretically higher turnover principle, is less affected by N2 and O2, and independent of ATP, it becomes more desirable and needs heterocysts to be genetically improved.

Native (indigenous) cellulose fractions of cellulosic materials are recal­citrant to enzymatic breakdown, so a pretreatment step is required to render them amenable to enzymatic hydrolysis to glucose. A number of pretreatment processes have been developed in laboratories, including:

■ Physical pretreatment—mechanical comminution, irradiation, and

pyrolysis

■ Physicochemical pretreatment—steam explosion or autohydrolysis, ammonia fiber explosion (AFEX), SO2 explosion, and CO2 explosion

■ Chemical pretreatment—ozonolysis, dilute-acid hydrolysis, alkaline hydrolysis, organosolvent process, and oxidative delignification

■ Biological pretreatment

However, not all of these methods may be technically or economically feasible for large-scale processes. In some cases, a method is used to increase the efficiency of another method. For instance, milling could be applied to achieve better steam explosion by reducing the chip size. Furthermore, it should be noticed that the selection of pretreatment method should be compatible with the selection of hydrolysis. For exam­ple, if acid hydrolysis is to be applied, a pretreatment with alkali may not be beneficial [18]. Pretreatment methods have been reviewed by Wyman [2] and Sun and Cheng [12].

Among the different types of pretreatment methods, dilute-acid, SO2, and steam explosion methods have been successfully developed for pre­treatment of lignocellulosic materials. The methods show promising results for industrial application. Dilute-sulfuric acid hydrolysis is a favorable method for either pretreatment before enzymatic hydrolysis or conversion of lignocellulose to sugars.