One attractive suggestion is based on harvesting the potential produced in different steps of metabolism in living systems [2]. Basic principles remain the same in all such models. One of them is to tap the oxidative phosphorylation path, and the other one is to use the photosynthetic mechanism. There are a few more novel systems suggested by other schools: (a) calcium pumps in biological systems by Ernesto Carafoli of Swiss Federal Institute of Technology, Zurich, (b) constructing cells from bacteriorhodopsin of the purple membranes of certain bacteria by Lester Packer of the University of California at Berkeley, United States, and

Figure 1.4 Gaseous battery (hydrogen fuel cell).

(c) isolated energy-rich compounds, i. e., iron-sulfur proteins, ATP, and so forth, suggested by many other authors [3].

Acid hydrolysis is an old process still applied in some ethanol industries. Sulfuric acid is the most commonly applied acid in this process, where starch is converted to low-molecular-weight dextrins and glucose [8]. Main advantages of this process are rapid hydrolysis and less cost for catalyst, compared to the enzymatic hydrolysis. However, the acid processes possess drawbacks including (a) high capital cost for an acid — resistant hydrolysis reactor, (b) destruction of sensitive nutrients such as vitamins present in raw materials, and (c) further degradation of sugar to hydroxymethylfurfural (HMF), levulinic acid, and formic acid, which lowers the ethanol yield and inhibits the fermentation process [9].

The acid hydrolysis process can be performed either in batch or in con­tinuous systems. Dilute-acid hydrolysis can also be used as a pretreat­ment for enzymatic hydrolysis. It is common to soak the starch or starchy materials in the dilute acid prior to enzymatic hydrolysis, then to con­tinuously pass it through a steam-jet heater into a cooking tube (called a jet cooker or mash cooker) with a plug flow residence time for a couple of minutes, and then subject it to enzymatic hydrolysis.

Among nonedible feedstock, there are many crops and tree-borne oilseed plants, such as karanja, neem, and jatropha, which have been under­utilized due to the presence of toxic components in their oils. Most of them grow in underdeveloped and developing countries, where a biodiesel program would give multiple benefits in terms of generation of employment for rural people (farmers), leverage of starting many types of industries using by-products from biofuels, and so forth [47]. However, nonedible crops are very much ignored in most cases. They grow on, regardless, waiting for their energetic potential to be discov­ered. The key is to find crops or trees that need very little care, have high oil content, and are resistant to plagues and drainage. The foliage could be used as manure, giving an added value to the crop. In fact, most of the trees and crops mentioned in the following (karanja, neem, etc.) grow well on wasteland and can tolerate long periods of drought and dry conditions.

The iodine value (IV) has been included in the European biodiesel stan­dards to purportedly address the issue of oxidative stability and the propensity of the oil or fat to polymerize and form engine deposits. The IV is a measure of the total unsaturation of a fatty material measured in grams of iodine per 100 g of sample when formally adding iodine to the double bonds. An IV of 120 has been specified in EN 14214 and 130 in EN 14213, which would largely exclude vegetable oils such as soybean and sunflower oils as biodiesel feedstock. Thus the IV has not been included in biodiesel standards in the United States and Australia, and is limited to 140 in the South African standard (which would permit sun­flower and soybean oils); the provisional Brazilian standard requires that it only be noted.

The IV of a vegetable oil or animal fat is almost identical to that of the corresponding methyl esters; however, the IV of alkyl esters decreases with higher alcohols used in their production since the IV is molecular weight dependent. For example, the IV of methyl, ethyl, propyl, and butyl linoleate is 172.4, 164.5, 157.4, and 150.8, respectively [64].

The use of the IV of a mixture for such purposes does not take into consideration that an infinite number of fatty acid profiles can yield the same IV and that different fatty acid structures can give the same IV, although the propensity for oxidation can differ significantly [64]. Other, new structure indices termed allylic and bis-allylic position equivalents (APE and BAPE), which are based on the number of such positions in a fatty acid chain and are independent of molecular weight, are likely more suitable than the IV [64]. The BAPE index distinguishes mixtures having nearly identical IV correctly by their OSI times. Note that the BAPE index is the decisive index compared to the APE because it relates to the more reactive bis-allylic positions. Engine performance tests with a mixture of vegetable oils of different IVs did not yield results that would have justified a low IV [65, 66]. No relationship between the IV and oxidative stability has been observed in another investigation on biodiesel with a wide range of IV [52].

Substitution of alcohol fuels for conventional fuels will increase the number of jobs in fuel production, distribution, and handling industries. Alcohol fuels are expected to cost more than gasoline over the next 10 years.

As a result, vehicle-operating costs will be somewhat higher if alco­hol blends are used. The price of alcohol blends varies significantly, depending upon the type of alcohol and feedstock used. Blends con­taining methanol derived from coal are the least expensive. The most expensive are alcohol blends containing ethanol produced from corn.

7.10.4 Transportation and infrastructure impacts

The existing fuel distribution system must be modified and expanded to accommodate the increasing use of alcohol fuels in the long run. The changes required will include construction of new pipelines, storage facilities, and retrofitting of existing facilities with alcohol-compatible pumps, hoses, valves, and other components.

The vehicle support services such as refueling, maintenance, repairs, and vehicle sales will be unaffected by the use of alcohol fuels. The use of alcohol fuels is not expected to have a significant impact on the exist­ing transportation system infrastructure.

As already mentioned in the preceeding section of biogas, gobargas is an extended version of the biogas. Usually, when cattle excreta (gobar)

H2, Mg2+, ATP

CH3 — S — CoM———- CH4+ HS — CoM

Methyl reductase

+2H/2e — +2H/2e-

CO2 + MH > MCOOH ——- > MCHO r-> MCH2OH

-H2O I

+2H — H2O

CH3COOH + MH

OH H H

I 2e — I 2e — I 2e-

CO + MH^*O = C — M O = C — M HO-C-M CH3-M—- >CH4+ MH

H2O I H2O 3 4

H

Gunsalus pathway

MH (reduced metabolite/reduced coenzyme/reduced enzyme complex)

Figure 1.10 Methanation.

is the starting material for anoxic fermentation to flammable gas, it is called gobargas. Before a scientific and technical approach was given to this promising field, the technique was developed in the southern part of India in a very crude way. Partly dehydrated animal excreta, when ignited, produces fumes and burn for a short duration with a partially sooty flame a little above the solid fuel. Slurried excreta, when stored in closed earthen vessels for a while, produced flammable gas. Based on these observations, villagers developed techniques of producing gas sim­ilar to illicit brewing.

Perhaps the greatest benefits of gobargas projects are secondary in nature. It takes out the pollution and ecologic problems and yields better biomass as compost and manure. The primary product, the biogas, has of course become very important in the present energy perspective. The fuel value of the gas, though not very high, is relatively safe and pollu­tion free. Out of the many reports available so far, the positive and encouraging points leading to successful implementation of gobargas projects are very restricted. The negative points or factors which make the progress slow down are many, and a few are difficult to overcome. It may be useful to mention a few of them. These points are by no means
insurmountable, but may help us to orient our future course of action, research, and development.

1. Dehydrated cow dung is a popular fuel and does not need special or expensive containers for keeping throughout the year.

2. Untended herds make the collection of dung laborious and cost intensive.

3. Installation of community biogas plants is not easy. Due to the frag­mentized small households, individual plants are also difficult to erect. Most families cannot provide the minimum 50-kg average dung input to the plant. About 50 L of water should also go with it. Fifty percent of the settlements are located in drought-prone areas. The remaining 50% face water shortage during the 5 months of dry season.

4. Temperature fluctuations throughout the year are significant and affect the rate of biogas production.

Disfunctioning and malfunctioning of some of the plants, due to the lack of proper maintenance and servicing, create poor examples to neigh­bors. This reduces the fresh installation potentialities and leads to an unwillingness to invest funds. The increasing cost of installation is another reason for the negative attitude.

The Chinese use mostly underground designs, and their outlays have been more successful because they have already undergone a genera­tion of restructured social order. As per Neelakantan’s (1974-1975) report, the wet-dung yield of a cow is on an average 11.3 kg ( 3.6 to 18.6 kg) and of a buffalo is 11.6 kg ( 5.0 to 19.4 kg). The daily output of dung from an average of five cattle (a minimum of four) may suffice for a house­hold with a miniature gobargas plant. When underground ambient con­ditions (30oC), are favorable, at least 2.7 m3 of gas (50 m3/ton of wet dung) per day is expected out of the plant. This gas has a minimum of 9500 kcal (3500 kcal/m3) of heat value (equivalent to 1.5 L of kerosene), which may serve the daily need of a five-member family. It is estimated that the average daily requirements of the gas per adult per day are 0.3 m3 for cooking and 0.2 m3 for lighting purposes.

Installation of a 3 m3 digester (gobargas plant), partly embedded in the earth, or preferably constructed underground, as per improved versions of several designs, suffices for one standard household (see Fig. 1.11). At the present cost, it comes to about Rs. 10,000 (approximately US $200), depending on the remoteness of the house or the community. Attractive cost figures have been developed by competent engineers and social workers who have estimated an annual savings to the tune of Rs. 1000 (approximately US $20) per family, and the initial investment is likely to be paid off within 3 years. The estimated average lifetime of a gobargas

plant is supposed to be 20 years. It is perhaps very important that a semi­skilled person or a trained “know-how” person tend to the plant.

Once installed, a 3 m3 digester plant will require about 50-60 kg (4 buckets) of raw wet cattle dung and an equal amount of water. If the dung is slurried prior to feeding the digester plant, stirring may not be needed. Initially, a 15-day incubation is necessary and combustible gas starts coming out after about 3 weeks, when stabilized, and will continue to produce a gas mixture which is satisfactorily flammable. The average retention time of the materials in the digester is 3-7 weeks (average 5 weeks). The optimal temperature, of course, is 40oC (15-65°C) with a pH 6.8 (pH 6.5-7.5). In a small digester (family unit), control of temperature and pH remains out of bounds for ordi­nary villagers.

The omnipresent microbial flora in the ruminants will start the reac­tion, initially at a slow rate. No additional microbial culture is usually required. The gas is composed mainly of CO2 and methane, and traces of other gases. Objectionable or harmful gases are very rare. Since a mix­ture of carbon dioxide is present, the gas is less flammable and haz­ardous than LPG, but needs sufficient precaution to be handled in the household. Most of the precautions to be observed in handling and using bottled gas will also apply in this case. The pipeline from the plant to the burner needs to be checked occasionally for leaks.

Human excreta and other animal excreta are equally useful for the same purpose. In fact, all such domestic excreta and pulped organic refuge may be mixed together to enrich the feed to the gobargas plant. Social practices and inhibitions prevent people from combining the feed­stock materials. The common septic tank system can also be modified in design and be made to deliver biogas. The quantity of human excreta per family is relatively small, and hence, the gas evolved will hardly meet even the partial requirement of the family, if the biogas plant is fed exclusively with night soil.

The disappearing forests and forage have a cyclic relation in the ecosystem. Rising cost of animal feed of all kinds adds to the crisis. Keeping of cattle in small village households may not be an attractive proposal very soon. A major part of the animal dung is not collected by the owner of the cattle while animals graze. The space required to keep cattle and have a biogas plant will be considered a poor investment, due to soaring price of land, even in remote villages. Considering these and a few more unforeseen factors, better prospects of gobargas plants in a distant future may not be a correct speculation.

In general, yeast and filamentous fungi metabolize xylose through a two — step reaction before they enter the central metabolism (glycolysis) through the PPP. The first step is conversion of xylose to xylitol using xylose reductase (XR), and the second step is conversion of xylitol to xylulose using another enzyme, xylitol dehydrogenase (XDH) [40-42].

Wild strains of S. cerevisiae possess the enzymes XR and XDH, but their activities are too low to allow growth on xylose. Although S. cere­visiae cannot utilize xylose, it can utilize its isomer, xylulose. Thus, if

S. cerevisiae is to be used for xylose fermentation, it requires a genetic modification to encode XR/XDH or XI [40, 43].

Bacteria have a slightly different metabolic pathway for xylose uti­lization. They convert xylose to xylulose in one reaction using XI [10, 44-46].

3.7 Microorganisms Related to Ethanol Fermentation

The criteria for an ideal ethanol-producing microorganism are to have (a) high growth and fermentation rate, (b) high ethanol yield, (c) high ethanol and glucose tolerance, (d) osmotolerance, (e) low optimum fer­mentation pH, (f) high optimum temperature, (g) general hardiness under physiological stress, and (h) tolerance to potential inhibitors pres­ent in the substrate [31, 47]. Ethanol and sugar tolerance allows the con­version of concentrated feeds to concentrated products, reducing energy requirements for distillation and stillage handling. Osmotolerance allows handling of relatively dirty raw materials with their high salt con­tent. Low-pH fermentation combats contamination by competing organ­isms. High temperature tolerance simplifies fermentation cooling. General hardiness allows microorganisms to survive stress such as that of handling (e. g., centrifugation) [47]. The microorganisms should also tolerate the inhibitors present in the medium.

Crop description. Hevea brasiliensis—commonly known as Para rubber tree, rubber tree, jebe, cauchotero de para, seringueira, or siringa— belongs to the family Euphorbiaceae (see Fig. 4.12). The rubber tree originates from the Amazon rain forest (Brazil). Today, most rubber tree plantations are located in Southeast Asia and some are also in tropical Africa. The tree can reach up to 30 m high. Oil can be extracted from the seeds. Although there are variations in the oil content of the seed from different countries, the average oil yield has been 40% [118].

Its fatty acid composition includes palmitic acid (10.2%), stearic acid (8.7%), oleic acid (24.6%), linoleic acid (39.6%), and linolenic acid (16.3%) [118].

Main uses. The crop is of major economical importance because it pro­duces latex. The wood from this tree is used in the manufacture of high — end furniture. In Cambodia and other rubber-manufacturing countries, rubber seeds are used to feed livestock. Rubber seed contains cyanogenic glycosides that will release prussic acid in the presence of enzymes or in slightly acidic conditions. Press cake or extracted meal can be cau­tiously used as fertilizer or feed for stock [119].

Several studies to check its feasibility as a source of biodiesel have been undertaken. Ikwuagwu et al. have prepared methyl esters of rubber seed oil using excess of methanol (6 M) containing 1% NaOH as a catalyst. Petroleum ether was added to the reaction to produce two phases. Analysis of the properties have shown a good potential for use as an alternative diesel fuel, with the exception of the oxidative stability [120]. Ramadhas et al. have performed a previously acid — catalyzed esterification to reduce the high FFA content, followed by an alkaline esterification. Sulfuric acid 0.5% by volume and a methanol — oil molar ratio of 6:1 was used in the pretreatment. A molar ratio of 9:1 and 0.5% by weight of sodium hydroxide was used during the second step. The authors found a reduction in exhaust gas emissions. The lower blends of biodiesel increased brake thermal efficiency and reduced fuel consumption [121].

Due to the global energy crisis and continuous increase in petroleum prices, scientists have been in search of new fuels to replace conventional fuels that are used in IC engines. Among all the fuels, alcohols, which can be produced from sugarcane waste and many other agricultural products, are considered the most promising fuels for the future. There are two types of alcohols: ethanol (C2H5OH) and methanol (CH3OH). Many other agri­cultural products (renewable sources) also have a vast potential for alco­hol production, and it is necessary to tap this source to the maximum level in national interest. The use of alcohol as a motor fuel is itself not a new idea. Nicolas Otto, the pioneering German engine designer, suggested it as early as 1895. But, as long as crude oil was plentiful and inexpensive, petroleum gasoline was the most economical fuel for the IC engine.

Due to the global energy crisis, many countries that used to export molasses to be used as cattle feed are now setting up distilleries to man­ufacture ethanol.

Fuel cells are classified primarily on the basis of the electrolyte they use. The electrolyte is the heart of the fuel cell as it decides the important operating parameters such as the electrochemical reactions that take place in the cell, the type of catalysts required, the temperature range of cell operation, and the fuel (reactants) to be used, and therefore the applications for which these cells are most suitable. There are several types of fuel cells currently under development; a few of the most prom­ising types include

■ Polymer electrolyte membrane fuel cells (PEMFCs)

■ Direct methanol fuel cells (DMFCs)

■ Alkaline electrolyte fuel cells (AFCs)

■ Phosphoric acid fuel cells (PAFCs)

■ Molten carbonate fuel cells (MCFCs)

■ Solid oxide fuel cells (SOFCs)

■ Biofuel cells