Crop description. Cuphea spp., C. carthagenensis, C. painter, C. ignea, and C. viscosissima—commonly known as cuphea—belong to the family Lythraceae and grow in temperate and subtropical climates (see Fig. 4.5). They can be found in Central and South America, and have been grown in trials in Germany and the United States. The seeds of Cuphea con­tain about 30-36% oil [69]. Major fatty acid composition of the oil includes caprylic acid (73% in C. painter, 3% in C. ignea), capric acid (18% in C. carthagenensis, 24% in C. painteri, 87% in C. ignea, and 83-86% in C. llavea), and lauric acid (57% in C. carthagenensis) [70].

Main uses. It contains high levels of short-chain fatty acids, very inter­esting for industrial applications. Previous studies have suggested that oil composition and chemical properties of C. viscosissima VS-320 are not appropriate for use as a substitute for diesel fuel without chemical

conversion of triglycerides to methyl esters. Further genetic modifica­tions must be made [71, 72]. Later studies have revealed that genetically modified oils present relatively low viscosity that is predicted to enhance their performance as alternative diesel fuels [73]. Also, atomization prop­erties suggest better fuel performance, because this oil has short-chain triglycerides, while traditional vegetable oils contain predominantly long-chain triglycerides [74].

Ahindra Nag

5.2 Introduction

Processing of vegetable oils as biodiesel [1, 2] and its engine perform­ance is very challenging. From an environmental point of view, diesel engines are a major source of air pollution. Exhaust gases from diesel engines contain oxides of nitrogen, carbon monoxide, organic compounds consisting of unburned or partially burned hydrocarbons and particu­late matter (consisting primarily of soot).

Interest in clean burning fuels is growing worldwide, and reduction in exhaust emissions from diesel engines is of utmost importance. It is widely recognized that alternative diesel fuels produced from vegetable oils and animal fats can reduce exhaust emissions from compression ignition (CI) engines, without significantly affecting engine perform­ance. But reducing pollutant emissions from diesel engines requires a detailed knowledge of the combustion process. However, the complex nature of the combustion process in an engine makes it difficult to understand the events occurring in the combustion chamber that deter­mine the emission of exhaust gases.

Dr. Rudolf Diesel [3], the inventor of the CI engine, used peanut oil in one of his engines for a demonstration at the Paris exhibition in 1900. Then there was considerable interest in the use of vegetable oils as fuel in diesel engines.

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Several studies have reported the effects of fuel and engine parame­ters on diesel exhaust emissions. Chowdhury [4] claims to have suc­cessfully used raw vegetable oils in diesel engines. He observed that no major changes were necessary in the engine, but the engine could not be run for more than 4 h. The performance and economic aspects of vegetable oil were also discussed.

Barve and Amurthe [5] cite an example of using groundnut oil as fuel in a diesel engine generator set (103 kW) of a local water pump house. They claimed that the power output and fuel consumption were very much comparable with certified diesel fuel. Weibe and Nowakowska [6] have reported the use of palm oil as a motor fuel. The performance was found satisfactory with higher fuel consumption. Fang [7] has reported that soybean and castor oil blended with diesel fuel burns adequately in a small diesel engine. Engelman et al. [8] has presented data on the performance of soybean diesel oil blends compared with diesel fuel. Results from a short-duration test showed that the use of blends was feasible in the diesel engine; but in fact, in the long-term, test problems associated with lubrication, sticking piston rings, and injector atomization patterns contributed to mechanical difficulties in the engine. Cruze et al. [9] have found that atomization of the fuel by the injector, in some cases, has caused delayed ignition characteristics and reduced efficiency of mechanical power production, compared to diesel fuel. Pryde [10] has stated that raw vegetable oil has had no great promise for engine tests and that modified oil esters were required for further engine tests. Bruwer [11] has reported that even without mod­ification, nine diesel engines started and operated almost normally on sunflower oil and delivered power equal to that of diesel fuel. Brake thermal efficiency and maximum engine power were 3% lower, while the specific fuel consumption was 10% higher than that of diesel fuel. The bench test, however, showed that atomization of 100% sunflower oil was much poorer than diesel but could be improved by reducing the viscosity of oil. Energy wise, sunflower oil was favorable for run­ning diesel engines for a shorter duration.

Baranescu et al. [12] have conducted tests on a turbocharged engine, using mixtures of sunflower oil in 25%, 50%, and 75% with diesel fuel. They have concluded that the use of sunflower oil blended with diesel brought modification in the fuel injection process that mainly included an increase in injection pressure and a longer ignition duration. These effects led to longer combustion duration. Cold-temperature operation was very critical due to high viscosity that caused fuel system problems such as starting failure, unacceptable emission levels, and injection pump failure. Engine shutdown for a long duration accelerated gum formation, where the fuel contacted the bare metal. This might further impair the engine or injection system.

Wagner et al. [13] have conducted tests on a number of diesel engines with different blends of winter rape and safflower oil with diesel fuel. The following specific conclusions were drawn from the results obtained:

■ High viscosity and tendency to polymerize within the cylinder were major physical and chemical problems.

■ Attempt to reduce the viscosity of the oil by preheating the fuel by increasing the temperature of the fuel at the injector to the required value was not successful.

■ Short-term engine performance showed power output and fuel con­sumption equivalent to diesel fuel.

■ Severe engine damage occurred within a very short duration when the test was conducted for maximum power with varying engine rpm (revolutions per minute).

■ A blend of 70% winter rape with 30% diesel was successfully used for 50 h. No adverse effect was noted.

■ A diesel injector pump when run for 154 h with safflower oil had no abnormal wear, gumming, or corrosion.

Borgelt et al. [14] have conducted tests on three diesel engines con­taining 25-75% and 50-50% soybean oil and diesel. The engines were operated under 50% load for 1000 horsepower (HP); the output ranged from 2.55 to 2.8 kW. Thermal efficiency ranged from 19.3 to 20%. Engine performances were not significantly different. Carbon deposit increased with increased percentage of soybean oil. Thus, Borgelt et al. concluded that use of 25% or less soybean oil caused negligible changes in engine performance.

Barsic and Humke [15] performed a study in which blends of unre­fined peanut and sunflower with diesel fuel (50-50%) were used in a single-cylinder engine. The engine produced equivalent power or a minor increase (6%) with vegetable oils and blends, with a 20% increase in spe­cific fuel consumption. Performance tests at equal energy showed that the power level remained constant or decreased slightly, thermal effi­ciency decreased slightly, and the exhaust temperature increased with an increase in the percentage of vegetable oil in the fuel. Exhaust emis­sion at equal energy input showed slightly higher NOx for vegetable oils and their blends. Unburned hydrocarbon emission was about 50% higher than pure diesel fuel because the injection system was not optimized for more viscous fuels. Ziejewski et al. [16] reported the results of an endurance test using a 25-75% blend of alkali-refined sunflower oil with diesel and 25-75% blend of safflower oil with diesel on a volume basis. The major problems experienced were premature injection, determination of nozzle performance, and heavier carbon deposits in the piston ring grooves.

There was no significant problem with engine operation when the blend of safflower oil was used. That investigation revealed that chemical dif­ferences between vegetable oil and diesel had a very important influence on long-term engine performance. Bhattacharya et al. [17] have reported that a blend of 50% rice bran oil with diesel could be a supplementary fuel for their 10-bhp CI engine. No significant difference in the brake thermal efficiency was reported.

Samson et al. [18] have reported the use of tallow and stillingia oil in 25-75% and 50-50% blends by mass with diesel. The fuel properties of the blends were found to be within the limits proposed for diesel. The heat of combustion appeared to decrease. Specific gravity and kinematic viscos­ity increased with the increase in concentration of oil. Dunn et al. [19] con­ducted the test on rubber seed oil blended with diesel in 25%, 50%, 75%, and 100% in an air-cooled engine with 4.9 kW at 3600 rpm. Higher spe­cific fuel consumption and slightly higher thermal efficiency were observed. But, carbon deposits were heavier than those for pure diesel fuel.

Samga [20] conducted a test on a water-cooled single-cylinder diesel engine, using hone oil (ken seed oil). He concluded that hone oil gave acceptable performance, smooth running, and ease in starting without preheating. The exhaust temperature and specific fuel consumption were higher than those for diesel. The partial-load efficiency was lower, but full-load efficiency was better than with diesel fuel.

Auld et al. [21] evaluated the potential yield and fuel qualities of winter rape, safflower, and sunflower as sources of fuel for diesel engines. Vegetable oils contained 94-95% heat value of diesel fuel, but were 11.1-17.6 times more viscous and also 7-9% heavier than diesel fuel. Viscosities of vegetable oils were closely related to fatty acid chain length and number of unsaturated bonds. During short-term engine tests, all vegetable oils produced power comparable to that of diesel, and the thermal efficiency was 1.8-2.8% higher than that of diesel. Based on the results, they concluded that vegetable oil as fuel should be selected on identification of the crop species that produced the most optimum yield of fuel quality vegetable oils.

Ryan et al. [22] have tested four different types of vegetable oils (soybean, sunflower, cottonseed, and peanut) in at least three different stages of processing. All the oils were characterized according to their physical and chemical properties. The spray characteristics of oils were determined at different fuel temperatures, using a high-pressure, high-temperature injection bomb, and high-speed motion picture camera. The injection study pointed out that vegetable oil behaved differently from diesel fuel. Normally, as the viscosity decreased, the penetration rate decreased and the spray cone angle increased. Using vegetable oils, however, increased the penetration rate, and increasing the temperature of the oil from 45°C to 145°C reduced the cone angle and decreased the viscosity.

Engine test results, based on the specific energy, showed that degummed soybean performed as well as the base fuel, but performance of the deodorized sunflower was the worst of those tested with an energy con­sumption 10% higher than the base fuel. Vegetable oils had a much smaller premixed combustion stage, with the diffused stage of combus­tion being flatter for sunflower and soybean oil than for the diesel fuel. Engine inspection showed that heating of the oil reduced the carbon deposit problem. It was concluded that deposits and overall durability were related to viscosity differences and the chemical structures of the other oil as compared to diesel fuel.

Mathur and Das [23] have conducted tests on diesel engines, using blends of mahua and neem oil with diesel. Results showed that neem oil could be substituted for up to 35% with marginal reduction in effi­ciency and power output. Mahua oil with diesel had exhaust charac­teristics similar to those of diesel. Further, savings in the diesel fuel through the use of both these nonedible oils outweighed the demerits of a marginal drop in efficiency and a slight loss in power output.

Goering et al. [24] conducted tests on a diesel engine using a hybrid fuel formed by micro-emulsion of aqueous ethanol in soybean oil. The test data were compared with the data from a baseline test on diesel fuel. The nonionic emulsion produced the same power as diesel fuel, with 19% lower heating value. Brake specific fuel consumption (BSFC) was 16% higher, and the brake thermal efficiency was 6% higher, with diesel at full power. Diesel knock for the hybrid fuel was not worse than for diesel fuel; thus the low cetane number of the hybrid fuel was not reflected in engine performance. Hybrid fuels were less volatile than ethanol and thus safer. The effect of hybrid fuel on the engine durability was unknown.

Presently, vegetable oil is regarded as a niche application. One liter of rapeseed oil substitutes for approximately 0.96 L of diesel. The annual yield is 1480 L/ha. CO2 reduction in relation to the diesel equivalent is about 80% [36]. However, this is questioned in newer literature [33] in terms of global warming reduction considering the effects of extra N2O entering the atmosphere as a result of using nitrogen-based fer­tilizers to produce crops for biofuels. Before unmodified vegetable oil is used as a fuel, the engine must be refitted for the fuel to correspond to the viscosity and combustion properties of vegetable oil. Refitting concepts include preheating either the fuel and the injection system or the equipment with a two-tank system. The engine is started with diesel and changes to vegetable oil only when the operating tempera­ture has been reached. Blends of pure vegetable oils and a conversion product together with additives (antioxidants) increase oxidation sta­bility, reduce viscosity, and give a better perspective for vegetable oil fuel markets.

The simplest of the elements, containing a single proton and electron each, of mass almost unity, is the first member of the periodic table. Data may vary from different sources; solid at 4.2 K (d 0.089), H has the atomic number (AN) 1, atomic weight (AW) 1.008 g, melting point (mp) —259.14°C, and boiling point (bp) —252.87°C (d 0.071 at 20.4 K). He has a AN 2, AW 4.0026 g, mp —272.2°C (20 atm), and bp —268.93°C (specific gravity 0.124).

Commercial consumption at present is mostly in synthetic fuels, say from coal, mineral oils, petroleum reformation (refineries), and iron and copper ore reductions. Hydrogen is very important because of the ver­satility of its physical, chemical, and biological properties. More impor­tantly for our purposes, is its potential as a source of energy. Hydrogen liquefies at 33.2 K, 12.8 atm, and 0.03 g/mL and occupies a negligible volume (22.4 times less), compared to its gaseous state. Solid hydrogen and helium are academic ideas. When hydrogen combines with oxygen in a volume ratio of 2:1, heat is generated and the product is water in a vapor state. The reaction in a vapor state occurs with a reduction of volume to 1/3 and water vapor to water 1/22, which means the reaction is favored at a higher pressure; alternatively, the change in volume is compensated by utilizing some of the heat that evolves. The calculations are already there. Hydrogen as a combustion fuel or as a material for a fuel cell is less attractive than the fusion reaction such as that which occurs in the sun. Taking it as a model, we may be able to harness huge amounts of thermal and traditional energies, but we should also learn how to manage and handle such enormous outbursts of energy. Two protons fuse to yield a deuterium, a positron, and a neutrino; the last one is the clue to the release of energy that is not yet fully understood by science;

2H1 ^ e+ + v + H2 H2 + H1 ^ H3 + v 2H3 ^ He4 + 2H1

Solar constant = 1.968 cal/(cm2 • min) = 3.86 X 1033 erg/s = 1.373 kW/m2; even at such a long distance, we are unable to use all the energies.

Hydrogen in absence of air or oxygen, or in vacuum, will not burn, but may have a kind of combustion to produce ammonia in air or nitrogen. Combustion of hydrogen in our atmosphere does not produce simple water vapor, but mixture of others, i. e., ammonia and NOxS (nitrogen and oxygen combine at the vicinity of high temperature generated).

Cryogenic and space research have taught us many more lessons. Liquid hydrogen can be stored in special containers (cylinders), or trans­ported through pipes, and is almost an ideal fuel for rockets and space­ships, perhaps next to azides. But at higher altitudes or in space, in the absence of atmosphere, optimal liquid oxygen is also needed to perform the dynamism or thrust. Water vapor is transformed into ice particles instantly due to the very low temperature in space. Liquid hydrogen for such research or experiment is generated at a very high cost, i. e., elec­trolytic splitting of water. The alternate resource of hydrogen is a by­product in the caustic soda plant. A similar minor and indirect source of hydrogen is water gas (C + H2O ^ CO + H2), almost obsolete for any large-scale production. None of these examples are renewable in nature, continue to be energy and labor intensive, and cannot stand as com­petitors as fuel or energy resources. Other commercial sources of hydro­gen are dependent on the existing limited supply of natural resources,

i. e., coal, naphtha, and natural gas, which are not renewable. The mate­rials are mainly based on fluidization or gasification of coal, and refor­mation by superheated steam or from steam-iron process (3Fe + 4H2O ^ F3O4 + 4H2); these processes can be broadly classified into (a) ther­mochemical or solar gasification and (b) fast pyrolysis or other novel gasification. These processes may be totally or partly catalytic. The basic chemical principles are mostly similar to those of classical water gas: C + H2O ^ CO + H2; CO + H2O ^ CO2 + H2. Major sources of hydro­gen at present are directly or indirectly natural gas; electrolysis; pyrolytic, thermal, and superheated steam; or geothermal, solar, ocean current, ocean thermal gradient, and nuclear reactors. Biomass as a source of hydrogen as well as energy has been discussed in Sec. 1.2.

In this section, we will discuss different fermentation processes appli­cable for ethanol production. Fermentation processes, as well as other biological processes, can be classified into batch, fed-batch, and contin­uous operation. All these methods are applicable in industrial fermen­tation of sugar substances and starch materials. These processes are well established, the fed-batch and continuous modes of operation being dominant in the ethanol market. When configuring the fermentation process, several parameters must be considered, including (a) high ethanol yield and productivity, (b) high conversion of sugars, and (c) low equipment cost. The need for detoxification and choice of the microor­ganism must be evaluated in relation to the fermentation configuration.

Presentation of a variety of inhibitors and their interaction effects, e. g., in lignocellulosic hydrolyzates, makes the fermentation process more complex than with other substrates for ethanol production [17, 21].

In fermentation of this hydrolyzate, the pentoses should be utilized in order to increase the overall yield of the process and to avoid problems in wastewater treatment. Therefore, it is still a challenge to use a hexose-fermenting organism such as S. cerevisiae for fermentation of the hydrolyzate.

When a mixture of hexoses and pentoses is present in the medium, microorganisms usually take up hexoses first and produce ethanol. As the hexose concentration decreases, they start to take up the pentose. Fermentation of hexoses can be successfully performed under anaerobic or microaerobic conditions, with high ethanol yield and productivity. However, fermentation of pentoses is generally a slow and aerobic process. If one adds air to ferment pentoses, the microorganisms will start utilizing the ethanol produced as well. It makes the entire process complicated and demands a well-designed and controlled process.

Crop description. Camelina sativa L. Crantz—commonly known as gold-of-pleasure and camelina—belongs to the family Cruciferae and grows well in temperate climates (see Fig. 4.16). It is an annual oilseed plant and is cultivated in small amounts in France, and to a lesser

Figure 4.16 Camelina sativa L. Crantz. (Photo courtesy of Prof. Arne Anderberg [http://linna. eus. nrm. se/flora/di/brassica/camel/ camemic. html].)

extent in Holland, Belgium, and Russia. The oil content of camelina seeds ranges from 29.9% to 38.3%. However, it is an underexploited oilseed crop at present. Its fatty acid profile includes oleic acid (14-19.5%), linoleic acid (18.8-24%), linolenic acid (27-34.7%), eicosenoic acid (12-15%), and erucic acid (less than 4%) [133]. Budin et al. have concluded that camelina is a low-input crop possessing a potential for food and nonfood exploitation [133].

Main uses. This crop has recently been rediscovered as an oil crop. At the moment, the feasibility of utilizing oil from this plant is being investi­gated [53, 134]. Oil is used as a luminant and emollient for softening the skin. Fiber is obtained from the stems. Frohlich and Rice have investi­gated production of methyl ester from camelina oil. Biodiesel was pre­pared by means of a single-stage esterification using methanol and KOH [135]. Steinke et al. have developed both alkali-catalyzed and lipase-catalyzed alcoholyses of camelina oil [136, 137].

Both ethanol and methanol, as listed in Table 7.3, have high knock resistance (as the octane numbers are 89 and 92, against 85 for gaso­line), wide ignition limit, high latent heat of vaporization, and nearly

the same specific gravity. All those properties are of great advantage if used in SI engines. Some important advantages of alcohol-fueled engines compared with gasoline engines are listed below:

1. The alcohols (both) have higher heat of vaporization. As the liquid fuel evaporates into the air stream being charged to the engine, a higher heat of vaporization cools the air, allowing more mass to be drawn into the cylinder. This increases the power produced from the given engine size. High latent heat of vaporization leads to higher volumetric efficiency and provides good internal cooling.

2. The high octane number of alcohols compared to petrol means higher compression ratios can be used, which results in higher engine effi­ciency and higher power from the engine.

3. Ethanol burns faster than petrol, allowing more uniform and efficient torque development. Both alcohols have wider flamma­bility limits, which results into a rich air—fuel (A:F) ratio being used when needed to maximize power by injecting more fuel per cycle.

4. Alcohols also have lower exhaust emissions than gasoline engines except for aldehydes. Both alcohols have lower carbon-hydrogen ratio than petrol and diesel, and produce less CO2. For the same power output, CO2 produced by an ethanol-fired engine is about 80% of the petrol engine. Because of high heat of vaporization, the fuels burn at lower flame temperatures than petrol, forming less NOx. The CO percentage in both cases (alcohol and petrol) remains more or less the same.

5. Contamination of water in alcohols is less dangerous than petrol or diesel because alcohols are less toxic to humans and have a recog­nizable taste.

6. The alcohols can also be blended with gasoline to form the so-called gasohol (80% petrol and 20% alcohol), which is widely used in the United States.

7. Ethyl alcohol as a fuel offers great safety due to its low degree of volatility and higher flash point (17°C).

8. The heating value of alcohol is 60% of that of petrol (60% only), and it shows equally good thermal efficiency and lower fuel consumption, because the air required for petrol and alcohol is in the ratio of 15:9 by weight, which is the same as their calorific value, i. e., the same heat is developed per cylinder charge in petrol and alcohol engines. The power per unit volume of cylinder for petrol, ethanol, and methanol are closely similar.

9. In many hot-climate countries, more precautions are often taken for the use of more volatile spirit-based fuels, while alcohol is perfectly safe in the hottest climate.

10. The major problem faced with ethanol is corrosion; special metals should be used for the engine parts to avoid corrosion.

Alcohols are clean-burning, renewable alternative fuels that can come to our rescue to meet the duel challenge of vehicular fuel oil scarcity and fouling of the environment by exhaust emissions.

Alcohols inherently make very poor diesel engine fuels as their cetane number is considerably lower. They can be used in dual-fuel engines or with assisted ignition in diesel engine. In the dual-fuel mode, alcohol is inducted along with air, compressed, and then ignited by a pilot spray of diesel oil.

Phosphoric acid fuel cells (see Fig. 9.8) operate at intermediate tem­peratures (~200°C) and are very well developed and commercially avail­able today. Hundreds of PAFC systems are working around the world in hospitals, hotels, offices, schools, utility power plants, landfills and wastewater treatment plants, and so forth. Most of the PAFC plants are in the 50- to 200-kW capacity ranges, but large plants of 1- and 5-MW capacity have also been built; a demonstration unit has achieved 11 MW of grid-quality ac power [3]. PAFCs generate electricity at more than 40% efficiency and if the steam produced is used for cogeneration, efficien­cies of nearly 85% can be achieved. PAFCs use liquid phosphoric acid as the electrolyte. One of the main advantages to this type of fuel cell, besides high efficiency, is that it does not require pure hydrogen as fuel and can tolerate up to 1.5% CO concentration in fuel, which broadens the choice of fuels that can be used. However, any sulfur compounds present in the fuel have to be removed to a concentration of <0.1 ppmV. Temperatures of about 200°C and acid concentrations of 100% H3PO4 are commonly used, while operating pressure in excess of 8 atm has been used in an 11-MW electric utility demonstration plant [3, 22, 23].

Electrochemistry of PAFCs. The electrochemical reactions occurring in a PAFC are

At the anode:

H2 ^ 2H+ + 2e~

At the cathode:

—O2 + 2H+ + 2e S H2O The overall cell reaction:

2 O2 + H2 s H2O

The fuel cell operates on H2; CO is a poison when present in a concen­tration greater than 0.5%. If a hydrocarbon such as natural gas is used as a fuel, reforming of the fuel by the reaction

CH4 + H2O ^ 3H2 + CO

and shifting of the reformat by the reaction

CO + H2O ^ H2 + CO2

is required to generate the required fuel for the cell.

Electrolyte. The PAFC uses 100% concentrated phosphoric acid (H3PO4) as an electrolyte. The electrolyte assembly is a 0.1- to 0.2-mm-thick matrix made of silicon carbide particles held together with a small amount of PTFE. The pores of the matrix retain the electrolyte (phosphoric acid) by capillary action. At lower temperatures, H3PO4 is a poor ionic con­ductor and CO poisoning of the Pt electrocatalyst in the anode can become severe. There will be some loss of H3PO4 over long periods, depending upon the operating conditions. Hence, as a general rule, suf­ficient acid reserve is kept in the matrix at the beginning.

Electrode. The PAFC (similar to a PEMFC) uses gas diffusion electrodes. Platinum or platinum alloys are used as the catalyst at both electrodes. In the mid-1960s, the conventional porous electrodes were PTFE-bonded Pt black, and the loadings of Pt were about 9 mg/cm2. In recent years, Pt supported on carbon black has replaced Pt black in porous PTFE — bonded electrode structures. Pt loading has also dramatically reduced to about 0.25 mg Pt/cm2 in the anode and about 0.50 mg Pt/cm2 in the cathode. The porous electrodes used in a PAFC consist of a mixture of the electrocatalyst supported on carbon black and a polymeric binder to bind the carbon black particles together to form an integral structure. A porous carbon paper substrate provides structural support for the electrocatalyst layer and also acts as the current collector. The com­posite structure consisting of a carbon black/binder layer onto the carbon paper substrate forms a three-phase interface, with the electrolyte on one side and the reactant gases on the other side of the carbon paper. The stack consists of a repeating arrangement of a bipolar plate, the anode, electrolyte matrix, and cathode.

Hardware. A bipolar plate separates the individual cells and electri­cally connects them in a series in a fuel cell stack. A bipolar plate has a multifunction design; it has to separate the reactant gases in the adja­cent cells in the stack, so it must be impermeable to reactant gases; it must transmit electrons to the next cell (series connection), so it has to be electrically conducting; and it must be heat conducting for proper heat transfer and thermal management of the fuel cell stack. In some designs, gas channels are also provided on the bipolar plates to feed reactant gases to the porous electrodes and to remove the reaction products. Bipolar plates should have very low porosity so as to minimize phosphoric acid absorption. These plates must be stable and corrosion-resistant in the PAFC environment. Bipolar plates are usually made of graphite — resin mixtures that are carbonized and heat treated to 2700°C to increase corrosion resistance. For 100-kW and larger power generation systems, water cooling has to be used and cooling channels are provided in the bipolar plates to cool the stack.

Temperature and humidity management. Temperature and humidity management are essential for proper operation of a PAFC. The PAFC system has to be heated up to 130°C before the cell can start working. At lower temperatures, concentrated phosphoric acid does not get dis­sociated, resulting in a low availability of protons. Also, due to lower vapor pressure of the concentrated acid, the water generated will not come out with the reactant stream and the moisture retention dilutes the acid. This causes an increase in acid volume, which results in acid oozing out through the electrode. With the start of normal cell operation, its tem­perature increases and acid concentration gets back to its normal value that causes acid volume to shrink, resulting in drying of the electrolyte matrix pores if the acid is not replenished. Controlled stack heating at start-up is achieved by using an insertable heater system. During oper­ation, the temperature of the stack is maintained by controlling the air flow in the oxidant channel. At high loading conditions, insertable coolers may be used to remove excess heat from the stack. Large-power PAFC systems use a water-cooling system.

Moisture generated at the cathode dilutes the acid on the cathode side of the electrolyte matrix, causing higher vapor pressure. This results in more moisture out with the oxidant stream. With the movement of pro­tons from anode to cathode, moisture migration takes place at the cath­ode side also. This water evaporation results in an acid concentration gradient from anode to cathode, causing low availability of protons and a lower potential of the cell. Therefore, water management is needed to maintain humidity of the anode stream gas at a sufficient level so that the vapor pressure matches the acid concentration level at the operat­ing temperature.

Performance. For good performance, the normal operating tempera­ture range of a PAFC is 180°C < T < 250°C; below 200°C, the decrease in cell potential is significant. Although an increased temperature increases performance, higher temperatures also result in increased catalyst sin­tering, component corrosion, electrolyte degradation, and evaporation. PAFCs operate in the current density range of 100-400 mA/cm2 at 600-800 mV/cell. Voltage and power limitations result from increased corrosion of platinum and carbon components at cell potentials above approximately 800 mV. Since the freezing point of phosphoric acid is 42°C, the PAFC must be kept above this temperature once commis­sioned to avoid the thermal stresses due to freezing and thawing. Various factors affect the PAFC life. Acid concentration management by proper humidity control is very important to prevent acid loss and performance degradation. A PAFC has a life of 10,000-50,000 h, commercially avail­able (UTC Fuel Cells) PAFC systems operating at 207°C have shown a
life of 40,000 h with reasonable performance (degradation rate AVlifetime (mV) = —2 mV/1000 h) [3, 23].

A body can do work, or work can be done upon a body; a body of water can turn a turbine, or one may pedal a bike to move it. If work is done on a body, it will possess energy. When energy is possessed by a body, the body can do work.

An agent may do work when it possesses energy, i. e., the amount of work that an agent can do is the amount of energy it possesses. So a body may gain kinetic and potential energy or lose the gained energy by pro­ducing heat or converting it to other forms of work.

Kinetic energy is due to the motion of a body.

Potential energy is due to the position or status of a body.

Frictional or colligative motion energy is produced in a water­fall; heat evolves to overcome a frictional resistance or checks the motion of a body but sets useless motion to others (e. g., rolling of peb­bles in a stream or dust behind a vehicle). Mechanical friction causes a matchstick to ignite.

Units of energy are the same as those of work and are assigned equiv­alent quantities. Some important definitions and units are given in the appendix. Energy content of some common substances are provided in Table 1.1.

1.1.1 Thermodynamics

All three principles of thermodynamics are very much applicable in the area of biological energy and chemical changes related to it. It is worth­while to review a few fundamental points. Chemical reaction can take

place only if the energy status changes, i. e., A will be converted to B only if B has a free energy content less than that of a change in free energy AF that is easy and spontaneous; reactions may be written as

A = B + (-AF) or A = B — AF

or

-AF = Fa

The reaction is called exergonic, or energy is evolved or given out. If AF has a positive expression, the reaction is driven by the input of energy and called endergonic; such reactions are difficult to complete. At equi­librium, AF = 0 (±), a point which may be arrived at by the end of the reaction, or a reaction may be typically of that type (practically sluggish, the progress of the reaction will depend on the change in concentration of reactants, the change of temperature or pressure, etc.).

AF = AF0 + RT ln B/A, where B/Ais the ratio at equilibrium or equilib­rium constant, i. e., Ksq. Then, 0 = AF0 + RT ln B/A or AF0 = — RT ln B/A =

-1363 logio Keq at 25°C. Here, R = 1.987 cal/mol/K, T = (273 + 25) K = 298 K, and ln B/A = 2.303 log10 Keq. This expression can be very useful:

When A and B exist equimolar, then the expression AF = AF0 + RT ln 1 means AF = AF0, and the state is called a standard state.

Chemical conversions and change of state need some other consider­ation in the light of the third law of chemical thermodynamics:

AF = AH — ATS

AH is the change in heat content, T is the absolute temperature at which the reaction occurs, and AS is the change in entropy (change, GR), or degree of disorder in the system, understood as the heat gained isothermally and reversibly per unit rise of temperature at which it happens (unit being calories per kelvin). The absolute value of H and S of a system cannot be directly determined. “Heat content” is also known as “heat content at constant pressure” or “enthalpy.” The third law sug­gests chemical pathway of finding entropy values in absolute terms. The first law of thermodynamics deals with conservation of energy and the second law with the relation between heat and work.

1. Energy cannot be destroyed or created, i. e., the sum of all energies in an isolated system remains constant.

2. All systems tend to approach a state of equilibrium. This means that the entropy change of a system depends only on the initial and final stages of the system, expressed by R. Clausius.

a. The total amount of energy in nature is constant.

b. The total amount of entropy in nature is increasing.

Refer to Sec. 1.14, Chap. 1, for more details on biomass. Solid products fall under the following categories:

1. Direct outcome of photosynthesis: Products from forest, shrubs, agri­cultures, and aquacultures.

2. Nonphotosynthesis: Mushrooms, animal biomass, indirect from photofixation.

3. Wastes: Forests and agricultural products.

4. Municipal solid wastes: Not all solid biomass may be suitable for dif­ferent end uses, i. e., energy production or energy recovery. For exam­ple, mushrooms are notably useful as food, feed, or fodder, not otherwise. Biomass properties are guidelines to further and more fruitful end uses. The properties depend on the following:

a. Water or moisture content (aqueous/dry)

b. Calorific or combustion value

c. Dry residues/ash content/silicates, and so forth

d. Alkali metal/oxides in the ash

e. Ratio of cellulose/liquid/oils/fats/of other carbonaceous matters

f. Ratio of solid/liquid/volatiles

Direct combustion of biomass for heat generation is the most inefficient technique in energy economy, heat being the most inefficient of all forms of energy. The best way to utilize biomass is to recycle biomass for pro­duction of other or further biomass, namely, agriculture, horticulture, aquaculture, poultry, animal farming, and so forth. Randomness is reduced (low entropy change), and environmental chaos is lessened. Properties (a), (c), and (d) are significant for farming; (b) and (f) are important for hydrolytic processes; and (e) is important for biofuels and biodiesel. All the points are important for fermentations and in biore­fineries. Biorefinery has become a new science and technology harmony for a promising future, which takes care of different aspects of biosafety, minimizes waste, and maximizes energy efficiency. It is a field of engi­neering and technology for the future. Biorefinery is a system similar to that of petroleum in its requirements for producing fuels and chemicals from biomass. A biorefinery is a capital-intensive project and is based on a conversion technology process of biomass. Hence, several technologies— thermochemical, chemical, biochemical, and so forth—are combined to reduce the overall cost. Fernando et al. suggest an integrated biorefinery process from bio-oil produced from pyrolysis of biomas (see Fig. 2.12),

which will not only produce sugar but also different by-products and electricity [24]. The process can produce its own power.

Fermentation is equally important. Anaerobic and restricted aerobic digestion with selected algae species allow us to harvest hydrogen and clean fuels, without much loss of biomass and with the least amount of waste products. In an aerobic process, the process is carried out by oxi­dizing the volatile matter into biodegradable organic fractions of solid waste. Air acts as a source of oxygen, and aerobic bacteria act as a cata­lyst. The change occurring during the process may be represented as

Biomass + O2 (Aerobic bacteria) s CO2 + H2O + Organic manure

Anaerobic digestion is carried out by segregating the nonbiodegrad- ables and the biodegradables at the same time. This may be done man­ually or mechanically. The smaller pieces of inorganic materials like clay and sand may be removed by washing the biomass with water. The washed material is then shredded into a size that will not interfere with mixing and may be more amenable to bacterial action. The shred­ded biomass is then mixed with sufficient quantity of water, and slurry is fed into a digester system. If necessary, nutrients like nitrogen, phos­phorus, and potassium have to be added to the digester. The process involves four groups of bacteria in the digested slurry as follows:

1. Hydrolytic bacteria catabolize carbohydrates, proteins, lipids, and so forth contained in the biomass to fatty acids, H2, and CO2.

2. Hydrogen-producing acetogenic bacteria catabolize certain fatty acids and some neutral end products to acetate, CO2, and H2.

3. Homoacetogenic bacteria synthesize acetate, using H2, CO2, and formate.

4. In the final phase, called the methanogenic phase, methanogenic bacteria cleave acetate to methane and CO2.

Water acts as a catalytic agent in methane formation. Thus water is acted upon by enzymes, itself breaking down to hydrogen and oxygen. Hydrogen is used by microorganisms to reduce CO2 to CH4, while oxygen oxidizes carbon dioxide, i. e., makes it acidic (H2CO3). In simple terms, acetate (in presence of CoI) is simultaneously oxidized to CO2 and reduced to CH4. For details, refer to Chap. 1, methanation, and Baker’s and Ganzalus pathway. Thus, methane-forming bacteria play an impor­tant role in the circulation of substances and energy turnover in nature. They absorb CO, CO2, and H2 to give hydrocarbon and methane and help synthesis of their own cell substances. During anaerobic digestion, gas containing mainly CH4 and CO2 is produced. The gas is known as biogas, which is used for the generation of electricity or fuel. The residual biomass comes out of the digester in the form of a slurry, which is separated into a sludge, which is used as fertilizer and a stream of waste water. Research is ongoing to produce renewable energies from different plant sources, which will necessarily dominate the world’s energy supply in the long-term. Using renewable-energy system technologies will create employment at much higher rates than any other technologies would [1]. There are economic opportunities for industries and craft jobs through production, installation, and maintenance of renewable energy systems.