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

While there are several factors that affect photosynthetic rate, the three main factors are light intensity, carbon dioxide level, and temperature. The net efficiency of photosynthesis is estimated by the net growth of biosynthesis and the amount used for respiration. The requirements for achieving high energy conversion are optimal temperature, light, nutri­tion, leaf canopy, absence of photorespiration, and so forth. Many plant species can be distinguished by the type of photosynthetic pathway they utilize. Most plants utilize the C3 photosynthesis route. C3 determines the mass of carbon present in the plant material. Poplar, willow, wheat, and most cereals are C3 plant species. Plants such as perennial grass, Miscanthus, sweet sorghum, maize, and artichoke all use the C4 route of photosynthesis and accumulate significantly greater dry mass of carbon than the C3 plants. Advances in crop production, agricultural techniques, and so forth have led to potential applications in low-cost bio­mass production with high conversion efficiencies. Further, introduction of alternative nonfood crops on surplus land and the use of biomass as a sustainable and environmentally safe alternative make biomass an attractive renewable energy resource. The potential of biomass energy derived from forest and agricultural residues worldwide is estimated at about 30 EJ/yr. For the adoption of biomass as a renewable energy source, the cultivation of energy crops using fallow and marginal land and efficient processing methods are vital [3].

C3 metabolism in plants and the pentose phosphate pathway. In C3 plants, the pathway for reduction of carbon dioxide to sugar involves the reduc­tive pentose phosphate cycle. This involves addition of CO2 to the pentose bisphosphate, ribulose-1,5-bisphosphate (RuBP). The enzyme-bound carboxylation product is hydrolytically split, through an internal oxidation — reduction process, into two identical molecules of 3-PGA. An acyl phos­phate of this acid is formed by reaction with ATP. This is further reduced with NADPH. Five molecules of the resulting triose phosphate are con­verted into three molecules of the pentose phosphate, ribulose 5- phosphate. Three molecules of ribulose 5-phosphate are converted with ATP to give the carbon dioxide acceptor, RuBP, thereby completing the cycle. When these three RuBP molecules are carboxylated and split into six PGA molecules and these are reduced to triose phosphate, there is a net gain of one triose phosphate molecule over the five needed to regenerate the carbon dioxide acceptor. Triose phosphate is formed in this cycle and can either be converted into starch for storage of energy inside the chloroplast, or it can serve its primary function by being transported out of the chloroplast for subsequent biosynthetic reactions. In a mature leaf, sucrose is synthesized and exported to the rest of the plant, thus providing energy and reduced carbon for growth [4]. Wheat, potato, rice, and barley are examples of C3 plants. A representative C3 cycle is shown in Fig. 2.3.

C4 metabolism in plants. In air that contains low carbon dioxide in rela­tion to oxygen, oxygen competes for the carbon dioxide binding site of the ribulose bisphosphate carboxylase. This is known to set off a process of photorespiration in plants, and it is believed that the C4 plants have evolved from such a mechanism. Such plants possess a specialized leaf morphology called “Krantz anatomy” and a special additional CO2 trans­port mechanism. This typically overcomes the problem of photorespi­ration. Such avoidance of photorespiration is known to result in higher growth rates. The Krantz anatomy is characterized by the fact that the vascular system of the leaves is surrounded by a vascular bundle, or bundle-sheath cells, which contain enzymes of the reductive pentose phosphate cycle. The reduction of CO2 is similar to that of C3 plants, except that the CO2 for carboxylation of CO2 is derived not from the stomata but is released in bundle-sheath cells by decarboxylation of a four-carbon acid (C4 acid). This C4 acid is supplied by the mesophyll cells that surround the bundle sheath cells. The C4 pathway for the transport of CO2 starts in a mesophyll cell with the condensation of CO2 and phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by phosphoenolpyruvate carboxylase (PEPCase), and the reduction of oxaloacetate to malate [5]. Figure 2.4 shows the C4 cycle of CO2 fixation in photosynthesis.

Due to the elimination of the photorespiration process, C4 plants are proposed to be ideal for increased biomass production especially in mar­ginal conditions. Grasses are suitable for this purpose as they can be

grown on a repetitive cropping mode for continuous and maximum production of biomass. Grasses such as Bermuda grass, Sudan grass, sugarcane, and sorghum are good candidates for energy generation from biomass. A comparison of the characteristics of C3 and C4 plants, in terms of leaf anatomy, is shown in Table 2.1.

SSF combines enzymatic hydrolysis of cellulose and fermentation in one step. As cellulose converts to glucose, a fermenting microorganism is presented in the medium and it immediately consumes the glucose produced. As mentioned, cellobiose and glucose significantly decrease the activity of cellulase. SSF gives higher reported ethanol yields and requires lower amounts of enzyme, because end-product inhibition from cellobiose and glucose formed during enzymatic hydrolysis is relieved by the yeast fermentation. SSF has the following advantages compared to SHF:

■ Fewer vessels are required for SSF, in comparison to SHF, resulting in capital cost savings.

■ Less contamination during enzymatic hydrolysis, since the presence of ethanol reduces the possibility of contamination.

■ Higher yield of ethanol.

■ Lower enzyme-loading requirement.

On the other hand, SSF has the following drawbacks compared to SHF:

■ SSF requires that enzyme and culture conditions be compatible with respect to pH and temperature. In particular, the difference between optimum temperatures of the hydrolyzing enzymes and fermenting microorganisms is usually problematic. Trichoderma reesei cellulases, which constitute the most active preparations, have optimal activity between 450C and 50OC, whereas S. cerevisiae has an optimum tem­perature between 30OC and 350C. The optimal temperature for SSF is around 38oC, which is a compromise between the optimal temper­atures for hydrolysis and fermentation. Hydrolysis is usually the rate-limiting process in SSF [27]. Several thermotolerant yeasts (e. g., C. acidothermophilum and K. marxianus) and bacteria have been used in SSF to raise the temperature close to the optimal hydrolysis temperature.

■ Cellulase is inhibited by ethanol. For instance, at 30 g/L ethanol, the enzyme activity was reduced by 25% [2]. Ethanol inhibition may be a limiting factor in production of high ethanol concentration. However, there has been less attention to ethanol inhibition of cellulase, since practically it is not possible to work with very high substrate con­centration in SSF, because of the problem with mechanical mixing.

■ Another problem arises from the fact that most microorganisms used for converting cellulosic feedstock cannot utilize xylose, a hemicellu- lose hydrolysis product [8].