Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is a fatal neurodegenerative disease of cattle. BSE has attracted wide attention because it can be transmitted to humans. Pathogenic prions are responsible for transmissible spongiform encephalopathies (TSE), and especially for the occurrence of a new variant of Creutzfeldt-Jakob disease (nvCJD), a human brain-wasting disease. Due to this problem, the specified risk material is burned under high temperatures to avoid any hazards for humans and animals. However, another possibility could be to consider this material as a source for producing biodiesel by transesterification. In fact, production of biodiesel from the risk material could represent a more economic usage than its combustion. Siedel et al. have found that almost every single step of the process leads to a significant reduction in the concen­tration of the pathogenic prion protein (PrPSc) in the main product and by-products. They concluded that biodiesel from materials with a high concentration of pathogenic prions can be considered safe [163]. Animal fats, such as tallow or lard, have been widely investigated as a source of biodiesel [164-169]. Muniyappa et al. have found that transesterifi­cation of beef tallow produced a mixture of esters with a high concen­tration in saturated fatty acids, but with physical properties similar to esters of soybean oil [37]. Ma et al. found that 0.3% NaOH completed methanolysis of beef tallow in 15 min [170]. Some authors have found that absolute ethanol produced higher conversion and less viscosity than absolute methanol at 50°C, after 2 h [171]. Nebel and Mittelbach have found n-hexane was the most suitable solvent for extraction of fat from meat and bone meal. The extracted material was converted into fatty acid methyl esters through a two-step process [172]. Lee et al. have performed a three-step transesterification to produce biodiesel from lard and restaurant grease. They found that a porous substance, such as silica gel, improved the conversion when more than 1 M methanol was used as reaction substrate [173]. Mbaraka et al. also synthesized propylsulfonic acid-functionalized mesoporous silica materials for methanol esterification of the FFA in beef tallow, as a pretreatment step for alkyl ester production [174].

Engine tests also showed a reduction in emission, except oxides of nitrogen that increased up to 11% for the yellow grease methyl ester [157]. Cold-flow properties of the fat-based fuels were found to be less desirable than those of soy-based biodiesel, with comparable lubricity and oxidative stability [175]. To solve this problem, Kazancev et al. blended up to 25% of pork lard methyl esters with other oil methyl esters and fossil diesel fuels. In this case, the CFPP showed a value of — 5°C. In winter, only up to 5% of esters can be added to the fuel. Depressant Viscoplex 10-35 with an optimal dose of 5000 mg/kg was found to be the most effective additive to improve the cold properties [101].

Although the physical and thermodynamic characteristics of alcohols do not make them particularly suitable for compression ignition (CI) engines, with certain modifications, however, they can also be used in CI engines. In heavy vehicles powered by CI engines, ethanol carbure — tion can be employed for bi-fuel operation of the engine with proportional savings in diesel oil. The various methods for using alcohols with diesel are fumigation, dual injection, and alcohol-diesel emulsions.

In a fumigation system the engine is fitted with a suitable carbure­tor and auxiliary ethanol tank. An ethanol-air mixture is carbureted during the induction stroke to provide 50% of the total energy of the cycle and the remaining energy is provided by diesel oil being injected in the conventional manner near the end of the compression stroke. The mate­rials of a fuel tank and fuel system must be compatible with alcohol. The entire system can be used as a retrofit kit, as shown in Fig. 7.6.

Ghosh et al. [4] carried out an investigation on the performance of a tractor diesel engine with ethanol fumigation (see Figs. 7.7 and 7.8). The following observations were recorded:

1. The brake thermal efficiency decreases with an increase in ethanol fumigation rate at a constant engine speed.

2. The BSFC decreases with an increase in ethanol fumigation rate at a constant engine speed

Figure 7.7 Experimental setup of ethanol fumigation.

3. The diesel substitution and the energy replacement increases with an increase in an ethanol fumigation rate at a constant engine speed.

4. The NOx emission level and the exhaust gas temperature decreases with an increase in a ethanol fumigation rate at a constant engine speed.

5. The CO emission level increases with an increase in an ethanol fumi­gation rate at a constant engine speed.

6. The smoke level decreases with an increase in an ethanol fumigation rate at a constant engine speed.

7.

The fumigation rate of 1.06 kg/h (40% diesel substitution) is optimal for good engine performance.

Ethanol fumigation in diesel engines can play a major role in envi­ronmental air pollution control, and ethanol is a viable alternative fuel for diesel engines.

Ethanol is a very good SI engine fuel and a rather poor CI engine fuel. Ethanol has a high octane rating of 90 and a low cetane rating of 8, and will not self-ignite reasonably in most CI engines. Dehydrated ethanol is fumigated into the air stream in the intake manifold of a 42-hp trac­tor diesel engine to improve its self-ignition quality. The performance of the engine under dual-fuel (diesel and fumigated ethanol) operation is compared with diesel fuel operation at various speeds (800, 900, 1000, 1100 rpm), loads (0, 4, 8, 12, 16 kgf), and fumigation rates (0.00, 1.06, 1.45, 2.06 kg/h). Analysis of the results shows that ethanol fumigation has the advantages of reduction in BSFC, NOx emission, and smoke level and the disadvantage of slight reduction in brake thermal efficiency. The fumigation rate of 1.06 kg/h (40% diesel substitution) is optimal for good engine performance.

It has been concluded that ethanol is a viable alternative fuel for diesel engines. A dramatic reduction in the NOx and the smoke level sug­gests that fumigation, as an emission control technique in diesel engines, can play a vital role in environmental air pollution control on a farm.

In the dual-injection method, two injection systems are used, one for diesel and the other for alcohol. This method can replace a large per­centage of diesel fuel. In this method, air is sucked and compressed, and then methanol is injected through a primary injector. To ignite this, a small amount of diesel is injected through a pilot injector. The relative injection timing of alcohol and diesel is an important aspect of the system.

As two injection systems are required, two injectors are required on the cylinder head, which limits the application of this method to large-bore engines. An additional pump, fuel tank, and fuel line are also required, making the system more complicated. But this method replaces 60% of diesel at a partial load and 90% at a full load, and provides higher ther­mal efficiency.

A biofuel cell operation is very similar to a conventional fuel cell, except that it uses biocatalysts such as enzymes, or even whole organisms instead of inorganic catalysts like platinum, to catalyze the conversion of chemical energy into electricity. They can use available substrates from renewable sources and convert them into benign by-products with the generation of electricity. As mentioned earlier, in recent years, med­ical science is increasingly relying on implantable electronic devices for treating a number of conditions. These devices demand a very reliable and maintenance-free (any maintenance that might require surgery) power source. Biofuel cells can provide solutions to most of these prob­lems. A biofuel cell can use fuel that is readily available in the body, for example, glucose in the bloodstream, and it would ideally draw on this power for as long as the patient lives. Since they use concentrated sources of chemical energy, they can be small and light.

A biofuel cell can operate in two ways: It can utilize the chemical pathways of living cells (microbial fuel cells), or, alternatively, it can use isolated enzymes [7, 28]. Microbial fuel cells have high efficiency in terms of conversion of chemical energy into electrical energy; however, they suffer from the low volumetric catalytic activity of the whole organ­ism and low power densities due to slow mass transport of the fuel across the cell wall. Isolated enzymes extracted from biological systems can be used as catalysts to oxidize fuel molecules at the anode and to enhance oxygen reduction at the cathode of the biofuel cell. Isolated enzymes are attractive catalysts for biofuel cells due to their high cat­alytic activity and selectivity. The theoretical value of the current that can be generated by an enzymatic catalyst with an activity of 103 U/mg is 1.6 A, a catalytic rate greater than platinum! However, practical observed currents are much lower due to the loss of catalytic activity from immobilization of the enzymes at the electrode surface and energy losses of the overall system. A major challenge in the biofuel cell design
is the electrical coupling of the biological components of the system with the fuel cell electrodes. Molecules known as electron-transfer mediators are needed to provide efficient transport of electrons between the bio­logical components (enzymes or microbial cells) and the electrodes of the biofuel cell. Integrated biocatalytic systems that include biocatalysts, electron-transfer mediators, and electrodes are under research and development. Biofuel cells have much wider fuel options; enzymatic bio­fuel cells can operate on a wide variety of available fuels such as ethanol, sugars, or even waste materials.

A basic microbial biofuel cell consists of two compartments, an anode compartment and a cathode compartment, separated by a PEM as shown in Fig. 9.11. Usually, Nafion-117 film (an expensive material) is used as the PEM; it allows hydrogen ions generated in the anode compartment to be transferred across the membrane into the cathode compartment [8].

Previously, graphite electrodes were used as the anode and cathode, but they are now replaced by woven graphite felt as it provides a larger surface area than a regular graphite electrode of similar dimensions. This facilitates an increased electron transfer from the microorganisms. A microorganism (e. g., Escherichia coli) is used to breakdown glucose in order to generate adenosine triphosphate (ATP), which is utilized by cells for energy storage. Methylene blue (MB) or neutral red (NR) is used as an electron mediator to efficiently facilitate the transfer of electrons from the microorganism to the electrode. Electron mediators tap into the electron transport chain, chemically reducing nicotinamide adenine din­ucleotide (NAD+) to its protonated form NADH. The exact mechanism by which the transfer of electrons takes place through these electron mediators is not fully known [29]; however, it is known that they insert themselves into the bacterial membrane and essentially “hijack” the electron transport process of glucose metabolism of the bio-electrodes in a biofuel cell. Their activity is very dependent on pH, and a potassium phosphate buffer (pH 7.0) is used to maintain the pH value in the anode compartment. The cathode compartment contains potassium ferricyanide,

a potassium phosphate buffer (pH 7.0), and a woven graphite felt elec­trode. Potassium ferricyanide reaction helps in rapid electron uptake. Hydrogen ions (H+) migrate across the PEM and combine with oxygen from air and the electrons to produce water at the cathode. The cath­ode compartment has to be oxygenated by constant bubbling with air to promote the cathode reactions. It may be worth mentioning that the electron transport chain occurs in the cell membrane of prokaryotes (a unicellular organism having cells lacking membrane-bound nuclei, such as bacteria), while this process occurs in the mitochondrial mem­brane of eukaryotes (animal cells). Therefore, attempts to substitute eukaryotic cells for bacterial cells in a biofuel cell may present a sig­nificant challenge.

Electrochemistry of microbial fuel cells. In a microbial fuel cell, two redox couples are required in order to generate a current: (a) coupling of the reduction of an electron mediator to a bacterial oxidative metabolism and (b) coupling of the oxidation of the electron mediator to the reduction of the electron acceptor on the cathode surface. The electron acceptor is subsequently regenerated by the presence of O2 at the cathode surface. The electrochemical reactions in a biofuel cell using glucose as a fuel are

At the anode:

C6H12O6 + 6H2O S 6CO2 + 24e + 24H+

At the cathode:

4Fe(CN)63~ + 4e2 s 4Fe(CN)642
4Fe(CN)642 + 4H+ + O2 s 4Fe(CN)632 + 2H2O

Complete oxidation of glucose does not always occur. One might often get additional products besides CO2 and water. For example, E. coli forms acetate, being unable to completely breakdown glucose, thereby limit­ing electricity production. Recently, an elegant approach to address this long-standing problem of limited enzyme stability has been reported [30]. It is suggested that the immobilization of enzymes in Nation layers to create a bio-anode results in stable performance over months.

Another way of using a microorganism’s ability to produce electro­chemically active substances for energy generation is to combine a biore­actor with a biofuel cell or a hydrogen fuel cell. The fuel can be produced in a bioreactor at one place and transported to a (H2 or bio-) fuel cell to be used as a fuel. In this case, the biocatalytic microbial reactor produces the fuel, and the biological part of the device is not directly integrated with the electrochemical part (see Fig. 9.12).

The advantage of this scheme is that it allows the electrochemical part to operate under conditions that are not compatible with the biological part of the device. The two parts can even be separated in time, oper­ating completely independently. The most widely used fuel in this scheme is hydrogen gas, allowing well-developed and highly efficient H2/O2 fuel cells to be conjugated with a bioreactor.

In recent years, ethanol has been developed as an alternative to the traditional methanol-powered biofuel cell due to the widespread avail­ability of ethanol for consumer use, its nontoxicity, and increased selec­tivity by alcohol. Ethanol fuel cells with immobilized enzymes have provided higher power densities than the latest state-of-the-art methanol biofuel cells. Open-circuit potentials ranging from 0.61 to 0.82 V and power densities of 1.00-2.04 mW/cm2 have been produced.

Mediatorless microbial fuel cells. Most biofuel cells need a mediator mol­ecule to speed up the electron transfer from the enzyme to the electrode. Recently, mediatorless microbial fuel cells have been developed. These use metal-reducing bacteria, such as members of the families Geobacteraceae or Shewanellaceae, which exhibit special cytochromes bound to their membranes. These are capable of transferring electrons to the elec­trodes directly. Rhodoferax ferrireducens, an iron-reducing microor­ganism, has the ability to directly transfer electrons to the surface of electrodes and does not require the addition of toxic electron-shuttling mediator compounds employed in other microbial fuel cells. Also, this metal-reducing bacterium is able to oxidize glucose at 80% electron effi­ciency (other organisms, such as Clostridium strains, oxidize glucose at only 0.04% efficiency). In other fuel cells that use immobilized enzymes, glucose is oxidized to gluconic acid and generates only two electrons, whereas in microbial fuel cells (MFCs) using R. ferrireducens, glucose is completely oxidized to CO2 releasing 24 electrons. These MFCs have a remarkable long-term stability, providing a steady electron flow over
extended periods. Current density of 31 mA/m2 over a period of more than 600 h has been reported [31]. MFCs using R. ferrireducens have the abil­ity to be recharged, and have a reasonable cycle life and low capacity loss under open-circuit conditions. They allow the harvest of electricity from many types of organic waste matter or renewable biomass. This is an advantage over other microorganisms in the family Geobacteraceae, which cannot metabolize sugars.

Another recent development has been the use of microfibers rather than flat electrodes and the enzyme-based electroactive coatings. The anode coating used is glucose oxidase, which is covalently bound to a reducing-potential copolymer and has osmium complexes attached to its backbone. The cathode coating contains the enzyme laccase and an oxidizing-potential copolymer. The osmium redox centers in the coatings electrically “wire” the reaction centers of the enzymes to the carbon fibers. This electrode design avoids glucose oxidation at the cathode and O2 reduction at the anode, eliminating the need for an electrode — separating membrane. This has led to miniature “one-compartment bio­fuel cells” for implantable devices within humans, such as pacemakers, insulin pumps, sensors, and prosthetic units. Biofuel cells with two 7-^m-diameter, electrocatalyst-coated carbon fiber electrodes placed in 1-mm grooves machined into a polycarbonate support with a power output of 600 nW at 37°C (enough to power small silicon-based micro­electronics) have been reported [32].

Microbial fuel cells have a long way to go before they compete with more established hydrogen fuel cells or electrical batteries. However, a number of factors provide motivation for research into microbial fuel cells for electricity production.

1. Bacteria are adapted to feeding on virtually all available carbon sources (carbohydrates or more complex organic matter present in sewage, sludge, or even marine sediments). This makes them poten­tial catalysts for electricity generation from organic waste.

2. Bacteria are omnipresent in the environment and are self — reproducing, self-renewing catalysts; thus a simple initial inoculation of a suitable strain could be cultured continuously in an MFC for long­term operation.

3. The catalytic core of conventional fuel cells uses very expensive pre­cious metals such as platinum, and biocatalysts like bacteria may become a serious cost-reducing alternative.

Although biofuel cells are still in an early stage of development and work toward optimizing the performance of a biofuel cell system is needed, the utilization of white blood cells as a source of electrons for a biofuel cell could mark an important step in developing a perpetual power source for implantable devices. There is still a lot of work to be done as there are many unanswered questions; however, the feasibility of con­structing commercially viable biofuel cell power supplies for a number of applications is very promising.

Energy can be derived from living systems in restricted forms only. Lignocelluloses are burned to get heat, and vegetable oils are often used for illumination. These may also serve as nutrients for different biotic species in various forms, i. e., cellulose, starch, and sugars. In other words, chemically stored energy may be reused in the form of fuel (fire­wood) or nutrients (food, feed, fodder, etc.). Animals can be employed to do different mechanical work. Animals directly (fish, meat) or indirectly (egg, milk) may provide nutrients for others. Use of dried cow dung as

cooking fuel in rural areas is also a well-known example of animal prod­ucts indirectly contributing to this field. But examples of direct energy flow from living systems are still in the conceptual state. Scientists dream that, one day, light emitted by fireflies or high voltage generated by electric eels may be of great use in the near future.

The production of alcohol or methane by microbial fermentation of common plant wastes are well-known phenomena. Recently however, scientists have started looking into these phenomena with greater inter­est, so that, in either gas or liquid form, their production and use can be optimized and made efficient. Plant bodies have been used as anten­nas, and plant leaves have been demonstrated to work as batteries. The survival of all biotic species depends directly or indirectly on solar energy. Studying the energy-based ecosystem raises awareness of this fact. Obviously, the most common question becomes: If the sun happens to be the source of all energy, why then is the solar energy not har­nessed by different devices? There are inherent limitations of most of the physical devices by (a) way of efficiency, (b) critical cost, (c) mainte­nance, (d) reliability, and (e) other factors.

In photosynthetic systems operating in green vegetations of the above points, (b), (c), and (d) are enormously better. Its characteristic limita­tions are [for point (a)] the incident insolation, the ability to use only a narrow spectrum [for point (e)], and requiring the proportionate amount of soil surface area for insolation, optimal nutrients, temperature, and moisture in the microenvironment. Here nature provides several mutants from which we can take, pick, screen, or select the most toler­ant variety. We may resort to genetic engineering for tissue cultures or selective hybridization.

What is our objective? Along with the effort to harness the solar energy by different physical methods, parallel efforts of optimal use of solar energy through biotic fixation should be attempted. This involves under­standing the following:

1. The living world in its entirety, i. e., ecology.

2. The photosynthetic systems in different species: terrestrial, aquatic, or mixed.

3. Application of the above to develop science and technology for:

a. Better management of the biotic systems useful for our purpose

b. Conversion of biological raw materials into energy rich products

4. Coordination for quality of life, pollution abatement, and sparing of nonrenewable resources for future generations.

A few examples that may not be out of place include potato, tomato, eucalyptus, and so forth. Though of wild origin, they have been appreci­ated and have been cultivated for this use after studying and admiring

their productivity and receptivity. Later by scientific manipulation, new strains have been developed for cultivation.

It is justified to discuss certain established facts for making suffi­cient conceptual clarity for special topics. Some aspects of energy rela­tions in living systems will be discussed in detail. Some other aspects will not be discussed in detail because existing “know-how” is rather limited.

The process of ethanol production depends on the raw materials used. A general simplified representation of these processes is shown in Fig. 3.3, and a brief description of different units of the process is presented in the rest of this chapter. It should be noted that if sugar substances, such as molasses and sugarcane juice, are used as raw materials, then milling, pretreatment, hydrolysis, and detoxification are not necessary.

Milling, liquefaction, and saccharification processes are usually nec­essary for production of fermentable sugar from starchy materials, while milling, pretreatment, and hydrolysis are typically used for ethanol pro­duction from lignocellulosic materials. Furthermore, a detoxification unit is not always considered, unless a toxic substrate is fed to the biore­actors

Membranes can also be used for ethanol purification. Reverse osmosis (RO), which employs membranes impermeable to ethanol and perme­able to water, can be used for purification of ethanol from water. Using a membrane permeable to ethanol but not to water is another approach [9]. Pervaporation, a promising membrane technique for separation of organic liquid mixtures such as azeotropic mixtures or near-boiling-point mixtures, can also be used for separation of these azeotropes [81, 83]. It involves the separation of ethanol-water azeotrope or near-azeotropic ethanol-water composition (from about 95 to 99.5 wt% ethanol) through water-permeable (or water-selective) membranes to remove the rest of the water from the concentrated ethanol solution [84].

The cetane number (CN), which is related to the ignition properties, is a prime indicator of fuel quality in the realm of diesel engines. It is con­ceptually similar to the octane number used for gasoline. Generally, a compound that has a high octane number tends to have a low CN and vice versa. The CN of a DF is related to the ignition delay (ID) time, i. e., the time between injection of the fuel into the cylinder and onset of ignition. The shorter the ID time, the higher the CN, and vice versa.

Standards have been established worldwide for CN determination, e. g., ASTM D613 in the United States, and internationally the International Organization for Standardization (ISO) standard ISO 5165. A long straight-chain hydrocarbon, hexadecane (C16H34; trivial name cetane, giving the cetane scale its name) is the high-quality stan­dard on the cetane scale with an assigned CN of 100. A highly branched isomer of hexadecane, 2,2,4,4,6,8,8-heptamethylnonane (HMN), a com­pound with poor ignition quality, is the low-quality standard with an assigned CN of 15. The two reference compounds on the cetane scale show that CN decreases with decreasing chain length and increasing branching. Aromatic compounds that are present in significant amounts in petrodiesel have low CNs but their CNs increase with increasing size of n-alkyl side chains [12, 13]. The cetane scale is arbitrary, and com­pounds with CN > 100 or CN < 15 have been identified. The American standard for petrodiesel (ASTM D975) prescribes a minimum CN of 40, while the standards for biodiesel prescribe a minimum of 47 (ASTM D6751) or 51 (European standard EN 14214). Due to the high CNs of many fatty compounds, which can exceed the cetane scale, the lipid combustion quality number for these compounds has been suggested [14].

The use of biodiesel reduces most regulated exhaust emissions from a diesel engine. The species reduced include carbon monoxide, hydro­carbons, and particulate matter (PM). Nitrogen oxide (NOx) emissions are slightly increased, however. When blending biodiesel with petrodiesel, the effect of biodiesel is approximately linear to the blend level. A report summarizing exhaust emissions tests with biodiesel is available [15], and other summaries are given in Refs. [16, 17].

The structure of the fatty esters in biodiesel affects the levels of exhaust emissions. When using a 1991-model, 6-cylinder, 345-bhp (257-kW), direct-injection, turbocharged, and intercooled diesel engine, NOx exhaust emission increased with increasing number of double bonds and decreas­ing chain length for saturated chains [18]. Although often a trade-off is observed between NOx and PM exhaust emissions, no trade-off has been observed in this work when varying the chain length [18]. The CN and density were correlated with emission levels [18]. However, emissions are likely affected by the technology level of the engine. When conducting tests on a 2003-model, 6-cylinder, 14-L, direct-injection, turbocharged, intercooled diesel engine with exhaust gas recirculation (EGR), no chain length effect has been observed for NOx exhaust emissions, although the level of saturation still played a significant role [19]. PM exhaust emissions were reduced to levels close to the US 2007 regulations required for ultra-low-sulfur petrodiesel fuel. Also, PM levels were lower than those for neat hydrocarbons which would be enriched in “clean” petrodiesel fuel [19]. In both studies [18, 19], NOx emissions of the sat­urated esters were slightly below those of the reference petrodiesel fuel.

For petrodiesel fuel, higher CNs have been correlated with reduced NOx exhaust emissions [20]. This correlation has led to efforts to improve the CN of biodiesel fuels by using additives known as cetane improvers [8]. Despite the inherent relatively high CNs of fatty compounds, NOx exhaust emissions usually increase slightly when operating a diesel engine on biodiesel, as mentioned above. The relationship between the CN and engine emissions is complicated by many factors, including the technology level of the engine. Older, lower-injection pressure engines are generally very sensitive to CN, with increased CN causing signifi­cant reductions in NOx emissions, due to shorter ID times and the result­ing lower average combustion temperatures. More modern engines that are equipped with injection systems that control the rate of injection are not very sensitive to CN [21-23].

Historically, the first CN tests were carried out on palm oil ethyl esters [24, 25], which have a high CN, a result confirmed by later stud­ies on many other vegetable oil-based DFs and individual fatty com­pounds. The influence of the compound structure on CNs of fatty compounds has been discussed in more recent literature [26], with the predictions made in that paper being confirmed by practical cetane tests [7-9, 13]. CNs of neat fatty compounds are given in Table 5.1. In sum­mary, the results show that CNs decrease with increasing unsaturation and increase with increasing chain length, i. e., uninterrupted CH2 moi­eties. However, branched esters derived from alcohols such as iso­propanol have CNs competitive with methyl or other straight-chain alkyl esters [9, 27]. Thus, one long, straight chain suffices to impart a high CN, even if the other moiety is branched. Branched esters are of interest because they exhibit improved low-temperature properties.

Recently, cetane studies on fatty compounds have been conducted using the Ignition Quality Tester™ (IQT) [9]. The IQT is a further, automated development of a constant volume combustion apparatus (CVCA) [28, 29]. The CVCA was originally developed for determining CNs more rapidly with greater experimental ease, better reproducibil­ity, reduced use of fuel, and therefore less cost than the ASTM D613 method utilizing a cetane engine. The IQT method, which is the basis of ASTM D6890, was shown to be reproducible and the results compet­itive with those derived from ASTM D613. Some results from the IQT are included in Table 5.1. For the IQT, ID and CN are related by the fol­lowing equation [9]:

CNiqt = 83.99 (ID — 1.512)-0658 + 3.54 7 (5.1)

In the recently approved method ASTM D6890, which is based on this technology, only ID times of 3.6-5.5 ms [corresponding to 55.3-40.5 DCN (derived CN)] are covered as the precision may be affected outside that range. However, for fatty compounds, the results obtained by using the IQT are comparable to those obtained by other methods [9]. Generally, the results of cetane testing for compounds with lower CNs, such as more unsaturated fatty compounds, show better agreement over various related literature references than the results for compounds with higher CNs, because of the nonlinear relationship [see Eq. (5.1)] between the ID time and the CN, which was observed previously [30]. Thus, small changes at shorter ID times result in greater changes in CN than at longer ID times. This would indicate a leveling-off effect on emissions such as NOx, as discussed above, once a certain ID time with corresponding CN has been reached as the formation of certain species depend on the ID time. However, for newer engines, this aspect must be modified as discussed above.

Most of the properties are similar, with differences of only 5-10%. Ethanol is superior to methanol as it has a wider ignition limit (3.5-17) than methanol (2.15-12.8). Its calorific value (CV) (26,880 kJ/kg) is con­siderably higher than methanol (19,740 kJ/kg).

Ethanol is a much more superior fuel for diesel engines as its cetane number is 8 compared to the cetane number of 3 for methanol. There are wide resources for manufacturing ethanol compared with methanol. Therefore, ethanol is widely used as SI engine fuel in many countries. Methanol is superior to ethanol in one respect: Its vaporization rate is much higher than ethanol. Therefore, mixing with air rapidly forms a uniformly vaporized mixture and also burns uniformly. One major drawback of methanol is that it creates vapor locks because of the higher vaporiza­tion rate. Properties of ethanol and methanol as compared with petrol are listed in Table 7.3.

The biology of an “oxidation pond” is not well understood. The algae-versus — bacterial growth needs to be controlled, and anaerobicity and temperature need to be maintained properly. The carbonaceous matter tends to ferment, and methane is produced instead of carbon dioxide. The end product, methane, can be used either as a direct fuel or through a suitably designed fuel cell. Microbial methane and hydrogen production are discussed later.

The general reaction for ethanol production during fermentation is

In this reaction, the microorganisms work as a catalyst.