Crop description. Allanblackia stuhlmannii and A floribunda—commonly known as allanblackia, mkanyi fat, bouandjo, and kagne butter (see Fig. 4.19)—belong to the family Guttiferae and grow in tropical areas, mainly in East Africa, Congo, and Cameroons. A high content of hard white fat (60-80%) can be extracted from the seed kernels of the trees. Allanblackia fats consist almost entirely of stearic acid (52-58%), oleic acid (39-45%), and palmitic acid (2-3%) [87]. Allanblackia has received considerable attention, based on its fat composition rather than its com­mercial importance [77].

Main uses. The use of the fat in soap manufacture has been suggested [176]. The timber is suitable for use under damp conditions. The pounded bark is used for medicinal purposes [177]. No references about its use as a biodiesel source have been found so far.

This technique replaces 100% diesel. The injection system can be retained as is or replaced by carburetion or port-type fuel injection. A spark plug is introduced in the combustion chamber, and the associated ignition system is added. High compression ratio and positive ignition result in smooth combustion, thereby improving thermal efficiency.

This approach is quite attractive as it uses the high latent heat of the vaporization of alcohols and their octane rating to good advantage. Power output is reduced due to lower heat content of alcohols. Changes in engine operability are not noticeable with alcohol-fired SI engines, rel­ative to the same engines using diesel fuel due to their similar torque. The engines are as efficient as their diesel-fueled counterparts. In fact, huge torque is available at engine speeds below 1400 rpm, which increases engine flexibility and response in use. Converting an existing diesel fleet to an SI technique involves engine modification. Space at the appro­priate place must be available for spark plugs in the cylinder head. Lubricants need to be added to alcohols to increase lubricity and pre­vent wear. Small amounts of cetane improvers may be added, but they are not required. It is not easy to switch between fuels after conversion to the SI technique.

A fuel processor converts a commercially available fuel (gas, liquid, or solid) to a fuel gas reformate suitable for the fuel cell use. Fuel processing involves the following steps:

1. Fuel cleaning—It involves cleaning and removal of harmful species (sulfur, halides, and ammonia) in the fuel. This prevents fuel proces­sor and fuel cell catalyst degradation.

2. Fuel Conversion—In this stage, a naturally available fuel (prima­rily hydrocarbons such as natural gas, petrol, diesel, ethanol, methanol, biofuels [such as produced from biomass, landfill gas, biogas from anaerobic digesters, syngas from gasification of biomass and wastes] etc.) is converted to a hydrogen-rich fuel gas reformat.

3. Downstream processing—It involves reformate gas alteration by converting carbon monoxide (CO) and water (H2O) in the fuel gas reformate to hydrogen (H2) and carbon dioxide (CO2) through the water gas shift reaction, selective oxidation to reduce CO to a few parts per million, or removal of water by condensing to increase the H2 concentration.

A schematic showing the different stages in the fuel-processing system is presented in Fig. 9.14. Major fuel-processing techniques are steam reforming (SR), partial oxidation (POX) (catalytic and noncatalytic), and autothermal reforming (ATR). Some other techniques such as dry reforming, direct hydrocarbon oxidation, and pyrolysis are also used. Most fuel processors use the chemical and heat energy of the fuel cell effluent to provide heat for fuel processing. This enhances system efficiency.

Steam reforming is a popular method of converting light hydrocarbons to hydrogen. In SR, heated and vaporized fuel is injected with super­heated steam (steam-to-carbon molar ratio of about 2.5:1) into a reac­tion vessel. Excess steam ensures complete reaction as well as inhibits soot formation. Although the steam reformer can operate without a cat­alyst, most commercial reformers use a nickel — or cobalt-based catalyst to enhance reaction rates at lower temperatures. Although the water gas shift reaction in the steam reformer reactor is exothermic, the com­bined SR and water gas shift reaction is endothermic. It therefore requires a high-temperature heat source (usually an adjacent high — temperature furnace that burns a small portion of the fuel or the fuel effluent from the fuel cell) to operate the reactor. SR is a slow reaction and requires a large reactor. It is suitable for pipeline gas and light dis­tillates using a fuel cell for stationary power generation but is unsuit­able for systems requiring rapid start and/or fast changes in load.

In POX, a substoichiometric amount of air or oxygen is used to par­tially combust the fuel. POX is highly exothermic, and the resulting high-temperature reaction products are quenched using superheated steam. This promotes the combined water gas shift and steam-reforming

reactions, which cools the gas. In a well-designed POX reformer with controlled preheating of the reactants, the overall reaction is exother­mic and self-sustaining. Both catalytic (870-925oC) and noncatalytic (1175-1400oC) POX reformers have been developed for hydrocarbon fuels. The advantage of POX reforming is that it does not need indi­rect heat transfer, resulting in a compact and lightweight reformer. Also, it is capable of higher reforming efficiencies than steam reformers [3, 6].

Autothermal reforming combines SR with POX reforming in the pres­ence of a catalyst that controls the reaction pathways and thereby deter­mines the relative extents of the POX and SR reactions. The SR reaction absorbs part of the heat generated by the POX reaction, limiting the maximum temperature in the reactor. This results in a slightly exother­mic process, which is self-sustaining, and high H2 concentration. The ATR fuel processor operates at a lower operating cost and lower tem­perature than the POX reformer, and is smaller, quicker starting, and quicker responding than the SR.

Most of the natural hydrocarbon fuels, such as natural gas and gasoline, contain some amount of sulfur, or sulfur-containing odorants are added to them for leak detection. As the fuel cells or reformer cat­alysts do not tolerate sulfur, it must be removed. Sulfur removal is usually achieved with the help of zinc oxide sulfur polisher, which removes the mercaptans and disulfides. A zinc oxide reactor is oper­ated at 350—400OC to minimize bed volume. However, removing sulfur — containing odorants such as thiophane requires the addition of a hydrodesulfurizer stage before the zinc oxide polisher. Hydrogen (sup­plied by recycling a small amount of the natural gas-reformed prod­uct) converts thiophane into H2S in the hydrodesulfurizer. The zinc oxide polisher easily removes H2S.

To reduce the level of CO in the reformat gas, it must be water gas shifted. The shift conversion is often performed in two or more stages when CO levels are high. A first high-temperature stage allows high reaction rates, while a low-temperature converter allows a higher con­version. Excess steam is used to enhance the CO conversion. In a PEMFC, the reformate is passed through a preferential CO catalytic oxi­dizer after being shifted in a shift reactor, as a PEMFC can tolerate a CO level of only about 50 ppm.

A fuel processor is an integrated unit consisting of one or more of the above stages, as per the requirements of a particular type of fuel cell. High-temperature fuel cells such as the SOFC and MCFC are equipped with internal fuel reforming and hence do not require a high- temperature shift, or low-temperature shift stage. The CO removal stage is not required for the SOFC, MCFC, PAFC, and circulating AFC. For the PEMFC, all the stages are required.

Two different metals in contact with a polar or ionic fluid generate the flow of electrons. When touched simultaneously by two different metal­lic rods, muscles contract, a pioneering observation that gave birth to the study of galvanic, voltaic, and Daniel cells.

The potential generated depends on the energy of sublimation, the ion­ization potential, the electronic work function, and the energy of solva­tion of ions. The nature of the solvent influences the last factor. The electronic work function also includes several other conditions of ionic activity. As a result, a potential difference will arise out of a simple con­centration gradient, provided that anionic and cationic stoichiometry is maintained. A review of the existing knowledge is worthwhile here.

If two small baths, each having either Zn or Cu metal and correspond­ing dilute solutions of Zn2+ and Cu2+salts, are in electrical continuity— say through a capillary of a U tube or a Pt wire—then current will flow in the two metals when connected outside, with Cu behaving as a cathode and Zn as an anode (see Fig. 1.3). The setup can also be designed by sep­arating the two systems by a semipermeable membrane.

A similar experience is the cylindrical design of the commonly avail­able dry cells, where a graphite rod at the center serves as a reference

cathode surrounded by a paste of chemicals, usually NH4Cl, totally housed in a small cylindrical cup of metallic Zn as an anode.

In each case, Zn gets oxidized and changes to Zn2+, and Cu2+ is reduced and is deposited as Cu; in the graphite (carbon) electrode, the chemical change is not noticeable. (Theoretically, CH4 should be formed, but slow escape of NH3 takes place.)

The field of electrochemistry has progressed considerably. Standard electrode potentials and electrochemical charts with a fair degree of accuracy and reliability are available. Taking Pt (inert) electrodes, hydrogen gas at 1-atm pressure, immersed in a solution of hydrogen ion of unit activity is usually a reference or standard hydrogen electrode (usually referred as zero or standard scale). If an element goes into a solution, producing cation (Zn ^ Zn2+ = +0.761 V), the half cell will give an oxidation potential with a sign opposite to the potential when the cation of the same species is deposited as the element, giving rise to a reduction (Zn2+ ^ Zn = —0.761 V); the numerical values are expected to remain in the same order.

One may observe, on the other hand, that alkali metals have a ten­dency to become hydrated oxides in water, so they exhibit a tendency to offer oxidation potential with a + sign. When the element approaches nobility, then converts to the halogen (2X— ^ X2 + 2e—), the situation is reversed. A representative partial list of the standard electrode poten­tials is reproduced (see Table 1.3). So one may expect that in a chemi­cal cell with Zn/ZnCl2-CuCl2/Cu, the EMF will be +0.761 —(—0.340) = 1.101 V.

If the electrode pair is made of the same material in a system, and the concentration difference of electrolyte is maintained between the two electrodes, a standard potential difference is expected, at the rate of 0.054 V per each tenfold rise in ionic concentration (referred to as con­centration cells).

TABLE 1.3 Standard Electrode Potentials at 25°C

If Zn is used as a common electrode, or better inert-metal electrodes are used (e. g., Pt) and immersed into NH4Cl or HCl solutions, say 0.1 and 1.0 N, a potential difference of 0.054 V will be experienced. The effect of temperature and other factors which affect ionic activity will definitely alter the values of EMF. The strength of the current will depend, expect­edly, on the total surface area or participation of the total number of ions and their charge-carrying capacities.

Electrochemical behavior of certain elements, e. g., carbon and silicon, must be determined indirectly. Only graphite exhibits direct application in a chemical cell, but other forms of carbon or silicon do not play any significant role at this state of knowledge (see Fig. 1.4).

There are various raw materials that contain starch and are suitable for ethanol production. Corn is the most widely used on an industrial scale for this purpose. However, there are several other cereals, such as wheat, rye, barley, and sorghum, and crop roots such as potato and cas­sava, which are used as raw materials for ethanol production. The cereals contain about 60-70% starch, 8-12% proteins, 10-15% water, and small

amounts of fats and fibers. The compositions of the crop roots are almost identical to those of the cereals on a dry basis, but the water content of the roots is usually 70-80%. The exact composition of each raw mate­rial depends on the type and variety of materials used and can be found in literature (e. g., [7]). Starch from these materials is used as a carbon and energy source, and part of the proteins as a nitrogen source, by the microorganisms.

Starch contains two fractions: amylose and amylopectin. Amylose, which typically constitutes about 20% of starch, is a straight-chain polymer of a-glucose subunits with a molecular weight that may vary from several thousands to half a million. Amylose is a water-insoluble polymer. The bulk of starch is amylopectin, which is also a polymer of glucose. Amylopectin contains a substantial number of branches in the molecular chains. Branches occur from the ends of amylose seg­ments, averaging 25 glucose units in length. Amylopectin molecules are typically larger than amylose, with molecular weights ranging up to 1-2 Mg. Amylopectin is soluble in water and can form a gel by absorb­ing water.

For ethanol production, hydrolysis is necessary for converting starch into fermentable sugar available to microorganisms. Traditional con­version of starch into sugar monomers requires a two-stage hydrolysis process: liquefaction of large starch molecules to oligomers, and sac­charification of the oligomers to sugar monomers. This hydrolysis may be catalyzed by acid or amylolytic enzymes.

M. P. Dorado

4.1 Introduction

The present energy scenario is undergoing a period of transition, as more and more energy consumers understand the inevitability of exhaustion of fossil fuel. The era of fossil fuel of nonrenewable resources is gradually coming to an end, where oil and natural gas will be depleted first, followed eventually by depletion of coal. In developing countries, the energy problem is rather critical. The price paid for petrol, diesel, and petroleum products now dominates over all other expenditures and forms a major part of a country’s import bill. In view of the prob­lems stated, there is a need for developing alternative energy sources. Alternative fuel options are mainly biogas, producer gas, methanol, ethanol, and vegetable oils. But biogas and producer gas have low energy contents per unit mass and can substitute for diesel fuel only up to 80%. Moreover, there are problems of storage because of their gaseous nature. Methanol and ethanol have very poor calorific value per unit mass, apart from having a low Cetane number. Therefore, these are rather unsuitable as substitutes for high-compression diesel engines. Experimental evidence indicates that methanol and ethanol can be substituted up to only 20-40%. There exists a number of plant species that produce oils and hydrocarbon substances as a part of their metabolism. These products can be used with other fuels in diesel engines with various degrees of processing. The development of veg­etable oils as liquid fuels have several advantages over other alterna­tive fuel options, such as

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1. The technologies for extraction and processing are very easy and simple, as conventional equipment with low energy inputs are needed.

2. Fuel properties are close to diesel fuel.

3. Vegetable oils are renewable in nature.

4. Being liquid, these oils offer ease of portability and also possess sta­bility and no handling hazards.

5. The by-product leftover after extraction of oil is rich in protein and can be used as animal feed or solid fuel.

6. Cultivation of these oilseeds is adaptable to a wide range of geo­graphic locations and climatic conditions.

7. Biodiesel can be used directly in compression ignition engines with no substantial modifications to the engine.

8. Biodiesel contains no sulfur, and there is no production of oxides of sulfur.

Hence, biodiesel is considered an alternative fuel for internal combus­tion engines derived from oils and fats from renewable biological sources; it emits far less regulated pollutants than the standard diesel fuel [1—10]. It entails minimal reduced engine performance as a result of a slight power loss and increase in specific fuel consumption [8, 11—29]. However, one main concern in further usage of biodiesel is the economic viability of producing biodiesel.

The main economic criteria are manufacturing cost and the price of raw feedstock. Manufacturing costs include direct costs for oil extrac­tion, reagents, and operating supplies, as well as indirect costs related to insurance and storage. Fixed capital costs involved in the construc­tion of processing plants and auxiliary facilities, distribution, and retail­ing must also be taken into consideration [30].

Several authors have found that biodiesel is currently not economi­cally feasible unless tax credits are applied [23, 31]. Peterson [23] has found that diesel fuel costs less than biodiesel, and an emergency or diesel shortage would be required to provide a practical reason for using biodiesel. Some authors have stated that biodiesel could compete with diesel fuel if produced in cooperatives [31, 32].

To promote biodiesel consumption, several countries have exempted biodiesel from their fuel excise tax. Among them, the European Union (EU) approved the biodiesel tax exemption program in May 2002. The financial law funded biofuels through excise exemption over a period of 3 years (Art. 21, Finance Law 2001). The U. S. Senate Finance Committee also approved an excise tax exemption for biodiesel in 2003. Moreover, the legislation provides a 1% reduction in the diesel fuel excise tax for each percentage of biodiesel blended with petroleum diesel up to 20%. Also, among some other countries, the Australian Senate approved an excise exemption on biofuels in 2004. However, the tax exemption will one day come to an end; in order to continue to promote the social inclusion and economic attraction of biodiesel, other steps will be needed. This could be facilitated by the selection of low-cost raw materials, such as nonedible oils, used frying oil, or animal fat, and the use of a lower-cost transes­terification process.

A lower-cost biodiesel production can be achieved by the optimization of the process. Because the chemical properties of the esters determine their feasibility as a fuel, the intent of the optimization is to investigate and optimize the involved parameters maximizing the yield of ester, to develop a low-cost chemical process, and to ensure appropriate oil chemical properties for both the transesterification and the engine performance.

Although it is a well-known process since, in 1864, Rochleder described glycerol preparation through the ethanolysis of castor oil [33], the pro­portion of reagents affects the process, in terms of conversion efficiency [34]; this factor differs according to the vegetable oil. Several researchers have identified the most important variables that influence the trans­esterification reaction, namely, reaction temperature, type and amount of catalyst, ratio of alcohol to vegetable oil, stirring rate, and reaction time [20, 35-42]. In this sense, it is important to characterize the oil (i. e., fatty acid composition, water content, and peroxide value) to determine the correlation between them and the feasibility to convert the oil into biodiesel [39, 43].

However, several studies have identified that the price of feedstock oils is by far one of the most significant factors affecting the economic viability of biodiesel manufacture [30, 44-46]. Approximately 70-95% of the total biodiesel production cost arises from the cost of the raw material [44, 45]. To produce a competitive biodiesel, the feedstock price is a factor that needs to be taken into consideration. Edible oils are too valuable for human feeding to run automobiles. So, the accent must be on nonedible oils and used frying oils.

Oxidative stability of biodiesel has been the subject of considerable research [41-62]. This issue affects biodiesel primarily during extended storage. The influence of parameters such as presence of air, heat, traces of metal, antioxidants, and peroxides as well as nature of the storage container was investigated in the aforementioned studies. Generally, fac­tors such as the presence of air, elevated temperatures, or the presence of metals facilitate oxidation. Studies performed with the automated oil stability index (OSI) method have confirmed the catalyzing effect of metals on oxidation; however, the influence of the compound structure of the fatty esters, especially unsaturation as discussed below, was even greater [52]. Numerous other methods, including not only wet-chemical ones such as the acid value and peroxide value, but also pressurized dif­ferential scanning calorimetry, nuclear magnetic resonance (NMR), and so forth, have been applied in oxidation studies of biodiesel.

Two simple methods for assessing the quality of stored biodiesel are the acid value and viscosity since both increase continuously with increasing fuel degradation, i. e., deteriorating fuel quality. The peroxide value is less suitable because it reaches a maximum and then can decrease again due to the formation of secondary oxidation products [48].

Autoxidation occurs due to the presence of double bonds in the chains of many fatty compounds. Autoxidation of unsaturated fatty compounds proceeds with different rates, depending on the number and position of double bonds [63]. Especially the positions allylic to double bonds are susceptible to oxidation. The bis-allylic positions in common polyun­saturated fatty acids, such as linoleic acid (double bonds at. C-9 and. C-12, giving one bis-allylic position at C-11) and linolenic acid (double bonds at. C-9, .C-12, and C-15, giving two bis-allylic positions at C-11 and C-14), are even more prone to autoxidation than the allylic positions. The relative rates of oxidation given in the literature [63] are 1 for oleates (methyl, ethyl esters), 41 for linoleates, and 98 for linolenates. This is essential because most biodiesel fuels contain significant amounts of esters of oleic, linoleic, or linolenic acids, which influence the oxida­tive stability of the fuels. The species formed during the oxidation process cause the fuel to eventually deteriorate.

A European standard (EN 14112; Rancimat method) for oxidative stability has been included in the American and European biodiesel standards (ASTM D6751 and EN 14214). Both biodiesel standards call for determining oxidative stability at 110°C; however, EN 14214 pre­scribes a minimum induction time of 6 h by the Rancimat method while ASTM D6751 prescribes 3 h. The Rancimat method is nearly identical to the OSI method, which is an AOCS (American Oil Chemists’ Society) method.

Besides preventing exposure of the fatty material to air, adding antiox­idants is a common method to address the issue of oxidative stability. Common antioxidants are synthetic materials such as ieri-butylhydro — quinone (TBHQ), butylated hydroxytoluene (BHT), butylated hydrox — yanisole (BHA), and propyl gallate (PG) as well as natural materials such as tocopherols. Antioxidants delay oxidation but do not prevent it, as oxidation will commence once the antioxidant in a material has been consumed.

The direct exposure of soils to methanol spills results in immediate damage of surface vegetation. The miscibility, volatility, and degrad­ability of alcohols reduce the alcohol residence time in soil and mini­mizes the environmental impact. Fungal and bacterial populations, which are important agents of nutrient cycling, exhibit 80—90% recovery with 3 weeks of exposure. Total recovery of the site occurs within a period of weeks or months. In comparison, recovery of biodegradation by crude oil and petroleum products takes months or years.

7.10.2 Occupational health impacts

Occupational heath risks associated with using alcohol fuels are lower than those associated with conventional fuels. The relative toxicity of alcohol fuels depends on the means of exposure, inhalation, and inges­tion. Gasoline poses a greater occupational health risk than either methanol or ethanol as carcinogens in gasoline can be readily absorbed by the skin or inhaled.

7.10.3 Occupational safety impacts

Two major safety hazards of all fuels are fire and explosion, which can occur because of improper fuel storage, spills, or vehicle accidents. The properties of alcohols and gasoline that pertain to fire and explosion risks include the flash point, auto-ignition temperature, flammability limits, and saturated vapor concentrations. While ethanol and methanol have broader flammability limits than gasoline, gasoline poses a greater risk of fire in open air. Because of the low flash point and auto-ignition temperature of gasoline, gasoline is more likely to ignite and burn rap­idly; therefore, the fire hazard is greater for gasoline.

Alcohol-fueled fire can be more readily contained than a gasoline-fueled fire of equivalent volume because alcohols have a lower heat of combustion than gasoline and less of the energy released is converted to radiant heat. Therefore, energy release and potential damage from an explosion caused by alcohol would be less than that of an explosion caused by gasoline.

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.