Age-old phenomena of spontaneous combustion of natural gas, continuously or intermittently, were called “will-o-wisp” or “fool’s fire.” Later, these phe­nomena were assigned to “marsh gas” and mainly methane by H. Tappeiner (1882) [7]. Almost a century passed, through which different postulates had to be verified in order to unveil the mechanism behind this natural methanogenesis or biogas formation. First, one-step microbial degradation of cellulose to methane was proposed. This was replaced by a two-step con­cept, where lower-molecular-weight organic acids are produced as interme­diates, which further undergo conversion to methane. Finally, the three-step concept has been prevailing (the entire process is anoxic):

Hydrolytic Acetogenic stage Methane, organic fermentive stage S (Mesophilic) S (Thermophilic)

Organic matter (35°C, pH 5-6) Acetic acid

Organic matter ^ ——— >

Alcohols, H2, CO2 H2,CO2

Methane _ (45°C, pH 4-6)

CO2 *

An oversimplified mass balance may be written as C6H12O6 ^ 3CH4 + 3CO2

The technical values of yield coefficient, biological efficiency, chemical/ biological oxygen demand (COD/BOD), biological efficiency in productivity/ ecologic efficiency rate (BEP/EER) ratios, and so forth are yet to be estab­lished for each setup or system. Mostly obligate anaerobes and a few fac­ultative microbes contributing to these conversions belong to different genera. A few may be mentioned: Actinomyces, Aerobacter, Aeromonas, Arthrobacter, Bacillus, Bacteroides, Cellulomonas, Citrobacter, Clostridium, Corynebacterium, Enterobacter, Escherichia, Klebsiella, Lactobacillus, Laptospira, Micrococcus, Nocardia, Peptococeus, Proteus, Pseudomonas, Ruminococcus, Sarcina, Staphylococcus, Streptococcus, Streptomyces, and many others. A few methanogenic species are also known: Methano- bacterium bryantii, Methanococcus vanniellii, Methano-genum aggre — gans, Methanomicro-bium mobile, Methanosarcina barkeri, Methano — thrix concillii, usually eukaryotic organisms, and blue-green algae are incapable of performing such bioconversions [8].

Morphologically, the organisms belong to wide groups: coccus, sarcina (flower-like), rod, filamentous, and other shapes. G + C (guanine-cytosine) values of DNAof these organisms also suggest that they all have varied origin and hence are likely to have different metabolic patterns. Khan (1980) found that Acetivibrio cellulolyticus producing acetic acid and hydrogen from cellulose are readily utilized by M. Barkeri to produce methane and carbon dioxide. It has been established beyond doubt that the process is chemolithotrophic metabolism, favored by strict anaero­bic condition, and facilitated by the absence of sulfates, abundance of mois­ture, approximate temperature range of 25-40oC (37°C), and pH 6.2-8.0 (pH 6.8). The organic materials on which these organisms survive and grow are usually cellulose in nature. Crop residues, agricultural residues, animal excreta, municipal sewage, and other organic materials derived from terrestrial and aquatic origin are also considered as good sub­strates. Plant materials with high lignin content are an inferior type of feed for such reactions.

A pretreatment or partial putrefaction or degradation makes the process easy. In this respect, animal excreta appear to be a ready-made substrate. The art of producing gaseous fuel out of cattle excreta is well known in the Indian Subcontinent as the gobargas plant, and will be dis­cussed subsequently.

Sargassum tenerrimum, an abundant variety of marine algae found on the Indian coast of the Arabian Sea, shows promising results in lab­oratory experiments by anaerobic digestion. A mixed culture of marine bacteria and methanogens happens to be a better choice. In a prototype experiment, the partially treated marine algal biomass mixed with cattle dung could be the initial feed for a digester. In a mixed culture, the entire process is a complex one. The organisms which are very effi­cient in cellulolytic activities degrade higher-carbohydrate materials into simpler products as lower organic acids, including CO2 and less fre­quently H2, along with other products, but very seldom show a significant amount of reduction reactions. In absence of methanogens, they usually produce H2, CO2 (even CO), formate, acetate, and less favorably other fatty acids and alcohols. It has been established that many methanogens utilize NH4 as their nitrogen source, either H2S or cysteine for their sulfur requirement, and other growth-stimulating amino acids, vita­mins, and some trace minerals.

Uncommon in many other anaerobic organisms, methanogens have shown presence of a cofactor (coenzyme) named CoM, identified as HSCH2CH2SO3 (2-mercapto-ethanesulfonic acid), and also another low — molecular-weight factor called F420, as of yet unidentified. This F420 in an oxidized state fluoresces at 420 nm but loses all optical activity when reduced. This compound is neither a ferredoxin nor can it be substituted by ferredoxin. Another interesting part is its dependence on Co II (NADP) and it cannot be substituted by Co I (NAD system). Occurrence of oxidative or substrate-level phosphorylation in methanogens could not be established, and the presence of quinines or cytochrome b/c systems could not be observed. The involvement of methylcobalamin also could not be substantiated. So, a large part of the information is yet to be derived by the next-generation scientists. It will be useful to summarize some of the metabolic steps, so far understood (see Fig. 1.10).

The ecologic role of biogas is manifold. Chemical anoxic transforma­tion reduces the BOD value of the organic residues, which in turn are enriched, proportionately in its C, N, P, and mineral ratios. In lignocel — lulosics, after the anoxic process, enrichment of lignin occurs and may lead to peat formation. This may be the origin of coal; natural gas and coal deposits are likely to be found within a reasonable stretch. This is a built-in machinery of nature for BOD and pollution control.

A simplified central metabolic pathway for ethanol production in yeast and bacteria under anaerobic conditions is presented in Fig. 3.7 [15, 35-37].

Three major interrelated pathways that control catabolism of carbo­hydrate in most ethanol-producing organisms are

■ Embden-Meyerhof pathway (EMP) or glycolysis

■ Pentose phosphate pathway (PPP)

■ Krebs or tricarboxylic acid cycle (TCA)

In glycolysis, glucose is anaerobically converted to pyruvic acid and then to ethanol through acetaldehyde. This pathway provides energy in the form of ATP to the cells. The net yield in glycolysis is 2 moles of pyru­vate (or ethanol) and 2 moles of ATP from each mole of glucose. This pathway is also the entrance of other hexoses such as fructose, mannose, and galactose to metabolic pathways. With only 2 moles of ATP formed per glucose catabolized, large amounts of ethanol (at least 3.7 g of ethanol per gram of biomass) must be formed [15, 38].

The PPP handles pentoses and is important for nucleotide (ribose — 5-phosphate) and fatty acid biosynthesis. The PPP is mainly used to
reduce NADP+. In Saccharomyces cerevisiae, 6-8% of glucose passes through the PPP under anaerobic conditions [8, 15].

The TCA cycle functions to convert pyruvic and lactic acids and ethanol aerobically to the end products CO2 and H2O. It is also a common channel for the ultimate oxidation of fatty acids and the carbon skele­tons of many amino acids. In cells containing the additional aerobic pathways, the NADH that forms during glycolysis results in ATP gen­eration in the TCA cycle [8].

Ethanol production from hexoses is redox-neutral, i. e., no net forma­tion of NADH or NADPH occurs. However, biosynthesis of the cells results in net formation of NADH and consumption of NADPH. The PPP is mainly used to reduce NADP+ to NADPH. Oxidation of surplus NADH under anaerobic conditions in S. cerevisiae is carried out through the glycerol pathway. Furthermore, there are other by-products—mainly carboxylic acids: acetic acid, pyruvic acid, and succinic acid—that add to the surplus NADH. Consequently, glycerol is also formed to com­pensate the NADH formation coupled with these carboxylic acids. Thus, formation of glycerol is coupled with biomass and carboxylic acid for­mation in anaerobic growth of S. cerevisiae [15, 39].

We should keep in mind that growth of the cells and increasing their biomass is the ultimate goal of the cells. They produce ethanol under anaerobic conditions in order to provide energy through catabolic reac­tions. Glycerol is formed to keep the redox balance of the cells, and car­boxylic acids may leak from the cells to the medium. Therefore, the ethanol-producing microorganisms produce ethanol as the major product under anaerobic conditions, while biomass, glycerol, and some carboxylic acids are the by-products.

Crop description. Azadirachta indica—commonly known as the neem tree, nim, margosa, veppam, cho do, or nilayati nimb—belongs to the family Meliaceae and can be found in dry tropical forests (see Fig. 4.11). The major producing countries are India, Sri Lanka, Burma, Pakistan, tropical Australia, and Africa. The evergreen neem tree grows up to 18 m high. The fat content of the kernels ranges from 33 to 45% [77]. The fatty acid content includes 42% oleic acid, 20% palmitic acid, 20% stearic acid, 15% linoleic acid, and 1.4% arachidic acid. Good quality ker­nels yield 40-50% oil. The cakes, which contain 7-12% oil are sold for solvent extraction. Neem oil is unusual in that it contains nonlipid asso­ciates often loosely termed as bitters and organic sulfur compounds that impart a pungent, disagreeable odor [88].

Main uses. The products of the neem tree are known to be antibac­terial, antifungal, and antiparasitic. The main uses are in soaps, teas, medicinal preparations, cosmetics, skin care, insecticides, and repel­lents. Neem twigs are used as tooth brushes and ward against gum disease. Neem oil, which is extracted from the seed kernel, has excel­lent healing properties and is used in creams, lotions, and soaps. It is also an effective fungicide. The bitter cake after the extraction of oil

Figure 4.11 Azadirachta indica. (Photo courtesy of Food and Agricultural Organization of the United Nations [www. fao. org].)

has no value for animal feeding but is recognized as both a fertilizer and nematicide [88]. Besides medical use, esters of neem oils have some important fuel properties that can be exploited for alternative fuels for diesel engines [78]. Nabi et al. have produced biodiesel from neem oil by using 20% methyl alcohol and 0.6% anhydrous lye catalyst (NaOH). The temperature of the materials was maintained at 55-60oC. Compared with conventional diesel fuel, exhaust emissions including smoke and CO were reduced, while NOx emission was increased with diesel-biodiesel blends. However, NOx emission with diesel-biodiesel blends was slightly reduced when exhaust gas recirculation (EGR) was applied. According to the results, Nabi et al. have recommended the use of the ester of this oil as an environment-friendly alternative fuel for diesel engines [117].

B. B. Ghosh and Ahindra Nag

7.1 Introduction

The increasing industrialization and motorization of the world has led to a steep rise in the demand of petroleum products. Petroleum-based fuels are stored fuels in the earth. There are limited reserves of these stored fuels, and they are irreplaceable. Figure 7.1 shows the difference in demand and supply of petroleum products, and how this depletion will create a problem before the world within a decade or two.

Geologists throughout the world have been searching for further deposits. Although the present reserves seem vast, the accelerating con­sumption is challenging the world to create new types of fuels to replace the conventional ones. New oil reserves appear to grow arithmetically while consumption is growing geometrically. Under this situation, when consumption overtakes discovery, the world will be heading toward an industrial disaster.

Apart from the problems of fast-vanishing reserves and the irre­placeable nature of petroleum fuels, another important aspect of their use is the extent and nature of environmental pollution caused by com­bustion in vehicular engines. Petroleum-fueled vehicles discharge sig­nificant amounts of pollutants like CO, HC, NOx, soot, lead compounds, and aldehydes.

A light-vehicular engine (car engine) discharges 1-2 kg of pollutants a day, and a heavy automobile discharges 660 kg of CO a year. CO is highly toxic, and exposure for a couple of hours to concentrations of 30 ppm can cause measurable impairments to physiological functions.

Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.

Figure 7.1 Difference in demand and supply of petroleum products.

Oxides of nitrogen and unburned hydrocarbons from exhausts cause environmental fouling by forming photochemical smog. Their interac­tion involves the formation of certain formaldehydes, peroxides, and peroxyacylnitrate, which cause eye and skin irritation, plant damage, and reduced visibility. Present day leaded gasoline contains lead com­pounds. Lead coming out with the exhaust finds its way into the human body, and causes brain damage in infants and children.

Vehicular exhaust fouling of the environment has already become a serious problem in Western countries and is a growing menace in devel­oping countries like India [1]. They exhaust huge quantities of harmful pollutants in urban areas. Everyday, vehicles running in Delhi dis­charge about 240 tons of CO, 30 tons of HC, 20 tons of NOx, and 2 tons of SO2. The disastrous effect of these pollutants on human health, animal and plant life, and property are well known.

In view of these problems, attempts must be made to develop tech­nology to produce alternative, clean-burning synthetic fuels. These fuels should be renewable, should perform well in the engine, and their poten­tial for environmental pollution should be quite low.

Various fuels have been considered as substitutes for petroleum fuels used in automobiles. The most prominent of these include ethanol, methanol, NH3, H2, and natural gases [2]. The suitability of each of these fuels for internal combustion (IC) engines used in automobiles has been under investigation throughout the world. A few of them are already in use in different countries. This chapter introduces different types of uncon­ventional fuel such as ethanol and methanol, their burning properties when used in IC engines, their performance characteristics compared
with conventional engines, the modifications required in the engine if used in practice, and their environmental pollution characteristics.

Although fuel cells have been around for more than a century (William Grove in 1839 first discovered the principle of the fuel cell), it was not until the National Aeoronautics and Space Administration (NASA) demonstrated its potential applications in providing power during space flights in the 1960s that fuel cells became widely known and the indus­try began to recognize the commercial potential of fuel cells. Initially, fuel cells were not economically competitive with existing energy tech­nologies; but with advancements in fuel cell technology, it is now becom­ing competitive for some niche applications [6].

The main components of a fuel cell are anode, anodic catalyst layer, electrolyte, cathodic catalyst layer, and cathode, as shown in Fig. 9.1. The anode and cathode consist of porous gas diffusion layers, usually made of high-electron-conductivity materials such as thin layers of porous graphite. The most common catalyst is platinum for low — temperature fuel cells. Nickel is preferred for high-temperature fuel
cells. Some other materials (Pt-Pt/Ru, Perovskites, etc.) are also used, depending on the fuel cell type [3].

The electrolyte is made up of materials that provide high proton con­ductivity and zero or very low electron conductivity. The charge carriers (from the anode to the cathode or vice versa) are different, depending on the type of fuel cell. A fuel cell stack is obtained by connecting such fuel cells in series/parallel to yield the desired voltage and current out­puts (see Fig. 9.2). The bipolar plates (or interconnects) collect the elec­trical current and also distribute and separate reactive gases in the fuel cell stack. Sometimes, gaskets for sealing/preventing leakage of gases between anode and cathode are also used.

The anode reaction in hydrogen fuel cells is direct oxidation of hydro­gen. For fuel cells using hydrocarbon fuels, the anodic half reaction con­sists of indirect oxidation through a reforming step.

In most fuel cells, the cathode reaction is oxygen (air) reduction. The overall reaction for hydrogen fuel cells is

H2 + 1 O2 s H2O with AG = —237.2 kJ/mol

where, AG is the change in Gibbs free energy of formation. The reaction product is water released at the cathode or anode, depending on the type of fuel cell.

For an ideal fuel cell, the theoretical voltage E0 under standard con­ditions of 25°C and 1 atm pressure is 1.23 V, whereas typical operating voltage for high-performance fuel cells is ~0.7 V. Stack voltage depends on the number of cells in a series in a stack. Cell current depends on the cross-sectional area (the size) of a cell.

Fuel cell systems are not limited by Carnot cycle efficiency. Therefore, a fuel cell system with a combined cycle and/or cogeneration has very high efficiency (55-85%) as compared to the efficiency of about 30-40% of cur­rent power generation systems. In a distributed generation system, fuel cells can reduce costly transmission line installation and transmission losses. There are no moving parts in a fuel cell and very few moving parts (compressors, fans, etc.) in a fuel cell system. Therefore, it has higher reli­ability compared to an internal combustion or gas turbine power plant.

Fuel cell-based power plants have no emissions when pure hydrogen and oxygen are used as fuel. However, if fossil fuels are used for gener­ating hydrogen, fuel cell power plants produce CO2 emissions. Compared to a steam power plant, a fuel cell plant has very low water usage; water/steam is a reaction product in a fuel cell. This clean water/steam does not require any pretreatment and can be used for reactant humid­ification and cogeneration. Another advantage of the fuel cell power plant is that it does not produce any solid waste and its operation is very silent as compared to a steam/gas turbine power plant. The noise gen­erated in a fuel cell power plant is only from the fan/compressor used for pumping/pressurizing the fuel and the air supply to the cathode.

A fuel cell power plant has good load-following capability (it can quickly increase or decrease its output in response to load changes). The modular construction of fuel cell plants provides good planning flexibility (new units can be added to meet the growth in electric demand when needed), and its performance is independent of the power plant size (efficiency does not vary with variation in size from W to MW size).

The major technical challenges in fuel cell commercialization at pres­ent are (1) high cost, (2) durability, and (3) hydrogen availability and infrastructure. For fuel cells to compete with contemporary power gen­eration technology, they have to become competitive in terms of the cost per kilowatt required to purchase and install a power system. A fuel cell system needs to cost ~$30/kW to be competitive for transportation appli­cations and for stationary systems; the acceptable price range is $400-$750/kW for widespread commercial application [9]. Fuel cell tech­nology needs a few breakthroughs in development to become competi­tive with other advanced power generation technologies.

The dark reaction is independent of light. This reaction is purely enzy­matic and is carried out in the stoma portion of the chloroplast. Ribulose-1, 5-diphosphate (RuDP), a pentose phosphate present in plant cells, acts as the initial acceptor of CO2 and changes thereby into a very unstable C6. The latter is converted into 3-phosphoglyceric acid (3-PGA), which is transferred to 3-phosphoglyceraldehyde. For this reaction, ATP and NADPH2 (produced in the light reaction) are necessary as cofactors. Three molecules of RuDP combine with three molecules of CO2 to give rise to six molecules of PGA. Three molecules of RuDP utilized initially as CO2 acceptors are regenerated by five molecules of phosphoglycer — aldehyde through different intermediates like xylulose-5-phosphate and ribulose-5-phosphate. The only molecule of phosphoglyceraldehyde is converted into fructose-1,6-diphosphate, which may be transformed into sucrose and starch through other reactions.

In SHF, enzymatic hydrolysis for conversion of pretreated cellulose to glucose is the first step. Produced glucose is then converted to ethanol in the second step. Enzymatic hydrolysis can be performed in the opti­mum conditions of the cellulase. The optimum temperature for hydrol­ysis by cellulase is usually between 45°C and 50°C, depending on the microorganism that produces the cellulase. The major disadvantage of SHF is that the released sugars severely inhibit cellulase activity. The activity of cellulose is reduced by 60% at a cellobiose concentration as

low as 6 g/L. Although glucose also decreases the cellulase activity, the inhibitory effect of glucose is lower than that of cellobiose. On the other hand, glucose is a strong inhibitor for ^-glucosidase. At a level of 3 g/L of glucose, the activity of ^-glucosidase reduces by 75% [27, 80]. Another possible problem in SHF is contamination. Hydrolysis is a lengthy process (one or possibly several days), and a dilute solution of sugar always has a risk of contamination, even at rather high temperatures such as 45-50oC.

Crop description. Prunus communis, P americana, and P. amygdalus— commonly known as almond, amandier, mandelbaum, almendro, and mandorlo (see Fig. 4.20)—belong to the family Rosaceae and grow in temperate Mediterranean areas. Major producing countries are Italy, Spain, Morocco, France, Greece, and Iran. The almond tree grows to a height of 3-8 m. Many varieties of almonds are grown, but they can be

Figure 4.19 Allanblackia stuhl­mannii. (Photo courtesy of Josina Kimottho (ICRAF) [www. worldagroforestry. org/Sites/ TreeDBS/aft/imageSearch. asp].)

broadly divided into two types: bitter and sweet. Bitter almonds contain amygdalin and an enzyme that causes its hydrolysis to glucose, ben — zaldehyde, and hydrocyanic acid, making the fruit nonedible. The bitter almond oil yield is around 40-45%, and sometimes as low as 20% [77, 178]. Major fatty acid composition of oil includes palmitic acid (7.5%), stearic acid (1.8%), oleic acid (66.4%), and linoleic acid (23.5%) [178].

Main uses. Bitter almond press cake cannot be used for feed due to its toxic components [179]. They are pressed at low temperatures, gener­ally at about 30oC, to prevent destruction of the hydrolytic enzyme. The press cake is then used for production of bitter almond oil [77]. Despite the oil content and fatty acid composition, no references about the use of bitter almond oil as a raw material to produce biodiesel have been found so far.

Neat alcohols are used in diesel engines by increasing the cetane number sufficiently using ignition improvers. This technique saves the expense and complexity of engine component changes but adds the cost of igni­tion improvers. The cost of 10-20% ignition improvers is quite prohibitive.

The most effective ignition improvers are nitrogen-based compounds which can aggravate exhaust emissions of NOx. Ethylene glycol nitrates have shown promising trends at 5% concentration.

Engines operating on cetane-enhanced alcohol need a few changes, e. g.,

■ Injection volume and timing must be adjusted to obtain optimum performance.

■ A large pump, fuel lines, and injectors are required to satisfy total fuel requirements of the engine for the desired output.

■ A lubricant (generally castor oil used so far) is required to be added to alcohols using improvers.

Besides fuel, a fuel cell also requires an oxidant (usually air). Depending on the application and design, air provided to the fuel cell cathode can be at a low pressure or a high pressure. High pressure of the air improves the reaction kinetics and increases the power density and efficiency of the stack. But increasing the air pressure reduces the water-holding capacity of the air and therefore reduces the humidification require­ments of the membrane (PEMFC). It also increases the power required to compress the air to a high pressure and thereby reduces the net power available. At present, most fuel cell stacks for stationary power applications are designed for operating pressures in the range of 1—8 atm, while automotive fuel cell systems based on the PEMFC technol­ogy are designed to operate at lower pressures of 2—3 atm to increase power density and improve water management.