In fed-batch processes (or semi-continuous processes), the substrate and required nutrients are added continuously or intermittently to the initial medium after the start of cultivation or from the point halfway through the batch process. Fed-batch processes have been utilized to avoid utilizing substrates that inhibit growth rate if present at high con­centration, to overcome catabolic repression, to demand less initial bio­mass, to overcome the problem of contamination, and to avoid mutation and plasmid instability found in continuous culture. Furthermore, fed — batch processes do not face the problem of washout, which can occur in continuous fermentation. A major disadvantage of a fed-batch process is the need for additional control instruments that require a substan­tial amount of operator skill. In addition, for systems without feedback control, where the feed is added on a predetermined fixed schedule, there can be difficulty in dealing with any deviation (i. e., time courses may not always follow the expected profiles) [70]. The fed-batch processes without feedback control can be classified as intermittent fed-batch, constant-rate fed-batch, exponential fed-batch, and optimized fed-batch.

The fed-batch processes with feedback control have been classified as indirect-control and direct-control fed-batch processes [70, 71].

The fed-batch technique is one of the promising methods for fermen­tation of dilute-acid hydrolyzates of lignocellulosic materials. The basic concept behind the success of this technique is the capability of in situ detoxification of hydrolyzates by the fermenting microorganisms. Since the yeast has a limited capacity for conversion of the inhibitors, the achievement of a successful fermentation strongly depends on the feed rate of the hydrolyzate. By adding the substrate at a low rate in fed — batch fermentation, the concentrations of bioconvertible inhibitors such as furfural and HMF in the fermentor remain low, and the inhibiting effect therefore decreases. At a very high feed rate, using an inhibiting hydrolyzate, both ethanol production and cell growth can stop, whereas at a very low feed rate, the hydrolyzate may still be converted, but at a very low productivity rate, which has been experimentally confirmed. Consequently, there should exist an optimum feed rate [15, 18, 21].

Similar to batch operations, higher optimum dilution rate in fed — batch cultivation can be obtained by (a) high initial cell concentration, (b) increasing the tolerance of microorganisms against the inhibitors, and (c) choosing optimal reactor conditions to minimize the effects of inhibitors. Productivity in fed-batch fermentation is generally limited by the feed rate which, in turn, is limited by the cell-mass concentra­tion [21].

Currently, world oil crop production is about 139,000,000 ton [141]. In particular, developing countries (97,370,185 ton) and developed coun­tries (41,193,308 ton) are the largest producers, while least developed countries contribute 4,141,535 ton. Most of this oil is used for deep — frying processes, after which it becomes a disposal problem. Disposal methods often contaminate environmental water and contribute to world pollution. Due to high oxidative thermal stress, such waste frying fats should not be used for human food [142]. Also, since 2002, the EU has enforced a ban on feeding these mixtures to animals, because during frying many harmful compounds are formed, which could result in the return of harmful compounds back into the food chain through the animal meat [143].

Used oils can be recycled through conversion into soap by saponifi­cation and reused as lubricating oil or hydraulic fluid. Nevertheless, bio­fuel production seems to be the most attractive alternative for waste oil treatment. Certainly, it will not solve the energy problem, because only a small percentage of diesel demand can be supplied by this source [20], but it will decrease the dependence on fossil oil while reducing an envi­ronmental problem.

For economic reasons, used frying oil is an interesting feedstock for biodiesel production. In this sense, Nye et al. were the first to describe the transesterification of used frying oil using excess of alcohol under both acidic and basic conditions. The best result was obtained using methanol with catalysis by KOH [144]. The tests were carried out using frying margarine and partially hydrogenated soybean oil. The reaction was carried out at 50°C for 24 h, using methanol in a methanol-triglyceride molar ratio of 3.6:1 and 0.4% KOH. At the same time, Mittelbach et al. investigated the use of waste oils to produce biodiesel and found that the increase in the amount of polymers during heating of the oil is a good indicator for the suitability for biodiesel production [42]. They proposed a low-temperature process (40°C) under alkaline catalysis and excess of methanol [145]. Considering used olive oil, better results were also obtained using KOH and methanol instead of NaOH and ethanol, which decreases transesterification rates. The reaction was optimized at an ambient temperature, using 1.26% KOH and 12% methanol, and stirring for 1 min [40]. Some authors have optimized the reaction by using methanol (alcohol-waste oils molar ratios between 3.6 and 5.4) and 0.2-1% NaOH [146], or methanol (molar ratios in the range of 1:74 to 1:245) and acid catalyst (sulfuric acid) [147]. Al-Widyan and Al-Shyoukh have performed waste palm oil transesterification under various conditions. The best process combination was 2.25 M H2SO4 with 100% excess ethanol in about 3 h of reaction time [148].

Several parameters (e. g., heating conditions, FFA composition, and water content) can influence conversion from waste oils into biodiesel. Mittelbach et al. have found that heating over a long period led to a sig­nificantly higher FFA content, which can reach values up to 10% and have detrimental effects during the transesterification process. Nevertheless, in most cases, simple heating and filtering of solid impu­rities is sufficient for further transesterification [20]. The methyl and ethyl esters of fatty acids obtained by alcoholysis of triglycerides seem to be excellent fuels [5]. Anggraini found that it was also important to keep the water content of used cooking oils as low as possible [149]. Dorado et al. have compared biofuels from waste vegetable oils from sev­eral countries (different FFA composition) including Brazil, Spain, and Germany. The transesterification process was carried out in two steps, using a stoichiometric amount of methanol and the necessary amount of KOH, supplemented with the exact amount of KOH to neutralized acidity. Both reactions were completed in 30 min [41]. Results revealed that to carry the reaction to completion, an FFA value lower than 3% is needed. The two-step transesterification process (without any costly purification step) was found to be an economic method for biofuel pro­duction using waste vegetable oils. To reduce FFA content, a two-step transesterification using 0.2% ferric sulfate and 1% KOH with methanol (mole ratio 10:1) was also developed [150]. Acid-catalyzed pretreatment to esterifiy the FFA before transesterification with an alkaline catalyst was also proposed [151]. This procedure can reduce the acid levels to less than 1%. Some authors have proposed a three-step process in a fixed — bed bioreactor with immobilized Candida antarctica lipase [152]. Brenneis et al. also developed a process involving C. antarctica through alcoholysis of waste fats, with excess of water. The optimum amount of water was found to be 80-10% of the amount of fat [153]. Chen et al. preferred the use of immobilized lipase Novozym-435 in transesterifi­cation of both waste oil and methyl acetate. However, they found that the reaction rate decreased with increasing water content [154].

Engine tests have been performed with biodiesel from different kinds of waste oils. Al-Widyan et al. tested several ester-diesel blends in a direct-injection diesel engine. Results indicated that the biodiesel burned more efficiently with less specific fuel consumption. Furthermore, 50% of the blends produced less CO and fewer unburned hydrocarbons than diesel [155]. Also, Mittelbach and Junek stated that it improves exhaust gas emissions, as compared to esters made from fresh oil [156]. However, despite the exhaust emission reduction, there are some dis­crepancies in terms of NOx emission related to the process and raw material [1, 105, 157]. In general terms, most studies show a slight decrease in brake power output, besides an increase in specific fuel consumption [158, 159]. To solve this problem, Kegl and Hribernik have proposed to modify injection characteristics at different fuel temperatures [160].

Several authors have worked on related topics. Kato et al. have used ozone treatment to reduce the flash point of biodiesel from fish waste oil, resulting in easy combustibility [161]. The immiscibility of canola oil in methanol provides a mass-transfer challenge in the early stages of transesterification. To exploit this situation, Dube et al. developed a two-phase membrane reactor. The reactor was particularly useful in removing unreacted oil [162].

The effect of speed on power output, brake specific fuel consumption (BSFC), and thermal efficiency of an engine using ethanol is compared with gaso­line engine, is shown in Figs. 7.2 through 7.5.

The observations are listed below:

1. The power output of the ethanol engine is higher, compared to a gaso­line engine at all speeds.

2. The BSFC is improved with an ethanol engine, compared to a petrol engine.

3. The maximum thermal efficiency of an ethanol engine is higher than that of a petrol engine. The efficiency curve of an ethanol engine is flat for a wide range of speeds, which indicates that the partial-load efficiency is much better, compared with a petrol engine.

4. The engine torque is considerably higher for ethanol as compared to a petrol engine.

Figure 7.2 Effect of speed on power at different compression ratios.

Figure 7.3 Effect of speed on BSFC (brake specific fuel con­sumption).

Figure 7.4 Effect of speed on thermal efficiencies.

Figure 7.5 Effect of speed on the torque.

The SOFC has the most desirable properties for generating electricity from hydrocarbon fuels. The SOFC uses a solid electrolyte and is very effi­cient. It can internally reform hydrocarbon fuels and is tolerant to impu­rities. The SOFC operates at a very high temperature (700-1000°C) and so does not require any cooling system for maintaining a fuel cell oper­ating temperature. For small systems, insulation has to be provided to maintain the cell temperature. In large SOFC systems, the operating temperature is maintained internally by the reforming action of the fuel and by the cool outside air (oxidant) that is drawn into the fuel cell. At high operating temperatures, chemical reaction rates in the SOFC are high and air compression is not required. This results in a simpler
system, quiet operation, and high efficiencies. Westinghouse has worked at developing a tubular style of the SOFC that operates at 1000°C (see Fig. 9.10) for many years [1-3, 26, 27]. These long tubes have high elec­trical resistance but are simple to seal. Many other manufacturers are now working on a planar SOFC composed of thin ceramic sheets which operate at 800°C or even less. Thin sheets offer low electrical resistance, and cheaper materials such as stainless steel can be used at these lower temperatures [3, 6, 26]. One big advantage of the SOFC over the MCFC is that the electrolyte is a solid. Therefore, no pumps are required to cir­culate a hot electrolyte, and very compact, small planar SOFC systems of a few kW range could be constructed using very thin sheets.

A major advantage of the SOFC is that both hydrogen and carbon monoxide are used in the cell. Therefore, in the SOFC, many common hydrocarbon fuels such as natural gas, diesel, gasoline, alcohol, and coal gas can be safely used. The SOFC can reform these fuels into hydrogen

and carbon monoxide inside the cell, and the high-temperature waste thermal energy can be recycled back for fuel reforming. During oper­ation, the SOFC is at the same time a generator and a user of heat. Heat is generated through exothermic chemical reactions and ohmic losses, while it is absorbed by the reforming reaction. It is possible to design the SOFC to be thermally balanced, thereby eliminating the requirement for external insulation and heating. Small SOFC systems are not thermally self-sustaining and may require an external heat source to start and maintain operation. In large systems, the heat gen­erated is not fully absorbed by fuel reforming, and the excess heat can be used in gas turbines for generating electricity or for cogeneration. Another advantage of the SOFC is that expensive catalysts are not required. However, a few minutes of fuel burning is required to reach the operating temperature of the SOFC at the start. This time delay is a disadvantage for an automotive application, but for stationary electric power plants, this is not a problem as they run continuously for long periods of time.

Electrochemistry of SOFCs. Hydrogen or carbon monoxide in the fuel stream reacts with oxide ions (O2 ) from the electrolyte to produce water or CO2 and to deposit electrons into the anode. The electrons pass out­side the fuel cell, through the load, and back to the cathode, where oxygen from the air receives the electrons and is converted into oxide ions, which are injected into the electrolyte. In the SOFC, oxygen ions are formed at the cathode. The reaction at the cathode is

O2 + 4e~ ^ 2O2~

At the operating temperature, the electrolyte offers high ionic con­ductivity and low electrical conductivity; therefore, oxygen ions migrate through the electrolyte to the anode. The overall reaction occurring at the anode is as follows:

The hydrogen in the fuel reacts with the oxygen ions to produce water and releases two electrons.

H2 + O2 ^ H2O + 2e

Carbon monoxide present in the fuel causes a shift reaction to produce additional fuel (H2).

CO + H2O ^ H2+ CO2

The following internal reforming reaction for the hydrocarbon fuel takes place on the anode side:

CxHy + xH2O ^ xCO + (x + t^)H2

For methane-rich fuels, this reforming reaction is CH4 + H2O — CO + 3H2

This reaction is generally not in chemical equilibrium, and the CO shift reaction takes place to provide more hydrogen. The overall cell reaction is

H2+ |o2 — H2O

Electrolyte. The use of a solid electrolyte in the SOFC eliminates the electrolyte management problems associated with the liquid electrolyte fuel cells and also reduces corrosion considerations to a great extent. In the SOFC, it is the migration of oxygen ions (O2) through the elec­trolyte that establishes the voltage difference between the anode and cathode. Therefore, the electrolyte must be a good conductor of O2~ ions and a bad conductor of electrons; it must also be stable at the high oper­ating temperature. Some ceramics possess these properties and there­fore are good candidates for this application. With the help of modern ceramic technology and solid-state science, many ceramics can be tai­lored for electrical properties unattainable in metallic or polymer mate­rials. These tailored ceramic materials are termed electroceramics, and one group is known as fast ion conductors or superionic conductors. These superionic conductors when used as a solid electrolyte allow easy passage of ions from the cathode to the anode in an SOFC. The material generally used as an electrolyte in the SOFC is dense yttria-stabilized zirconia. It is an excellent conductor of negatively charged oxygen ions at high temperatures (1000°C), but its conductivity reduces drastically with the drop in temperature. Other materials such as scandia-stabilized zirconia (ScSZ), which shows good ionic conductivity at a lower temper­ature (800°C), are also being investigated, but the electrolyte developed with ScSZ-based materials is very expensive and they degrade very fast.

Electrode. The anode is made of metallic Ni and Y2O3-stabilized ZrO2 (YSZ). Ytrria-stabilized zirconia is added in Ni to inhibit sintering of the metal particles and to provide a thermal expansion coefficient close to those of the other cell materials [26]. Nickel structure is normally obtained from NiO powders; therefore, before starting the operation for the first time, the cell is run with hydrogen in an open-circuit condition to reduce the NiO to nickel. The anode structure is fabricated with a porosity of 20-40% to facilitate mass transport of the reactant and prod­uct gases. The Sr-doped lanthanum manganite (La1_x Srx MnO3, x = 0.10-0.15; known as LSM) is most commonly used for the cathode mate­rial. LSM is a p-type semiconductor. Similar to the anode, the cathode is also a porous structure that permits rapid mass transport of the reac­tant and product gases.

Hardware. In the SOFC, both CO and hydrogen are used as direct fuel. Therefore, it is important that the fuel and air streams are kept sepa­rate, and a thermal balance should be maintained to ensure that oper­ating temperatures remain within an acceptable range. Several designs of the SOFC (tubular and planer) have been developed to accommodate these requirements. The SOFC is a solid-state device and shares certain properties and fabrication techniques with semiconductor devices.

Individual cells in the stack are connected by interconnects, which carry an electrical current between cells and can also act as a separa­tor between the fuel and oxidant supplies. In high-temperature SOFCs, the interconnects that are used are ceramic such as lanthanum chromite, or if the temperature is limited to less than 1000°C, a refractory alloy based on Y/Cr may be used. The interconnects constitute a major pro­portion of the stack cost. Stack and other plant construction materials that are used also need to be refractory to withstand the high-temperature gas streams. Volatility of chromium-containing ceramics and alloys can result in contamination of the stack components, and the presence of a toxic material such as Cr6+ requires special disposal procedures.

The high operating temperature (1000°C) of the SOFC requires a sig­nificant start-up time. The cell performance is very sensitive to operat­ing temperatures. A 10% drop in temperature results in an ~12% drop in cell performance due to the increase in internal resistance to the flow of oxygen ions. The high temperature also demands that the system include significant thermal shielding to protect personnel and to retain heat. Also, the materials required for such high-temperature operation, particularly for interconnect and construction materials, are very expen­sive. Operating the SOFC at temperatures lower than 700°C would be very beneficial as low-cost metallic materials, such as ferritic stainless steels, that can be used as interconnect and construction materials. This will make both the stack and balance of a plant cheaper and more robust. Using ferritic materials also significantly reduces the problems associated with chromium. The other advantages of low/intermediate — temperature operation are rapid start-up and shutdown and signifi­cantly reduced corrosion rates.

However, to operate at reduced temperatures, several changes are required in stack design, cell materials, reformer design and operation, and operating conditions. With the reduction in operating temperature, the ionic conductivity of the electrolyte decreases and the parasitic losses due to the conductivity of the electrodes and interconnects increase. This results in a rapid deterioration of the performance of the SOFC. This can be overcome to some extent by reducing the thickness of the electrolyte to compensate for its reduced ionic conductivity. The thick­ness reduction that is required to accommodate, say a 200°C reduction in the operating temperature, leads to impracticably thin membranes.

Some designs in which a thin, dense layer of the electrolyte is physically supported on one of the electrodes (electrode-supported design) are sug­gested. This structure of a very porous support is difficult to manufac­ture, and an expensive thin-film deposition technique such as chemical vapor deposition (CVD) is needed to manufacture these systems. Even then, the mechanical strength of the structure (defined by the porous electrode) is often poor, and the handling of the structure through sub­sequent processing and assembly is difficult. Another approach to improve SOFC performance at low operating temperatures is to use different materials for the electrolyte and the electrode. Several mate­rials options are being investigated [2, 6, 26, 27].

Assuming that the wavelength of light remains constant, the intensity influences the rate of photosynthesis, which is why the earlier part of the

forenoon is the most productive, and higher intensity of light energy and higher temperature slow down the photosynthetic rate. Likewise, a cloudy day does not slow down the normal photosynthetic rate of par­ticular species to any observable extent.

Metabolically speaking, reports are insufficient to conclude anything based on this observation, even though the above information itself is very useful and valuable. At the onset of daybreak, the photosynthetic machinery gets into action after a dark rest period and the rate is at its peak; the carbon dioxide tension (partial pressure) at the immediate microenvironment is also higher (it is yet to be established that higher carbon dioxide tension facilitates photosynthesis, though the reverse is true). As the reaction proceeds with time, all other conditions remain­ing the same, the anabolic machineries including the enzymes and coen­zymes (particularly NADP/Co II system) are fully occupied and ATP systems are also fully utilized. ATP production is, in turn, dependent on respiration (oxidative process), which to some extent is competitive with carbon fixation. Geological and geographical factors contribute greatly to ATP productivity.

Let us turn again to the consideration of biogeological and biogeo­graphical distribution on energy. For an energy-based ecosystem, the biosphere may be classified into two major types: terrestrial, and aquatic. These can also be subdivided into eight intraterrestrial types: terrestrial, subterrestrial, epilimnon, mesolimnon, hypolimnon, estuarine, epima — rine, and submarine. What do these have to do with our objective? Natural distribution of flora and fauna largely depend upon the types of microenvironments mentioned above.

At this point, it need not be assumed that the arctic belt, being very cold, is biologically unproductive. The author was surprised to see the existence of almost a minitropical pocket, 66° north latitude and 20° east longitude (Jockmock, Sweden) due to uninterrupted insolation for almost 90 days and prolonged daylight for 60 more days. The flora and fauna have adapted to survival techniques for the cruelty of adverse nature during the long, dark winter months.

Ethanol is produced from a variety of feedstocks. Fermentative ethanol is produced from grains, molasses, sugarcane juice, fruits, surplus wine, [8]

whey, and some other similar sources, which contain simple sugars and their polymers. On the other hand, synthetic ethanol is produced from oil, e. g., through hydration of ethylene:

Oil ^ CH2 = CH2 (ethylene) —CH3CH2OH (ethanol) (3.1)

Several companies, such as Sasol, SADAF, British Petroleum, and Equistar, produce synthetic ethanol, with capacities of 100-400 kilotons/yr. However, the share of synthetic ethanol in world ethanol production was less than 4% in 2006, down from 7% in the 1990s [4]. Furthermore, increasing oil price or declining ethanol price can harm the economic competition of synthetic ethanol production, compared to the fermen­tative one. Ethylene prices in 2005 rose to US $1000 per ton, while ethanol values were around US $500 per ton. If we consider the theo­retical yield of ethanol from ethylene based on Eq. (3.1) as 1.64 kg/kg, the price of raw materials was higher than that of the product. In this case, it is economically feasible to produce biobased plastics through “bioethylene”:

Fermentation H2O

Biomass/crops————— ► CH3CH2OH—- > CH2 = CH2 ^ Plastics (3.2)

The global demand for ethylene is around 120 megatons [4]. It can be considered a new market for ethanol in the future.

The total world ethanol production in 2006 was 49.8 GL (gigaliter) (39 megatons), where 77% of this production was used as fuel, 8% as beverage, and 15% in industrial applications [4]. Since 1975, potable ethanol production has not experienced a major growth, while industrial ethanol production has experienced growth by about 75%. However, fuel ethanol production has increased aggressively from less than 1 GL in 1975 to more than 38 GL in 2006 (see Fig. 3.1).

There is competition between Brazil and the United States to be the dominant ethanol producer in the world. So far, Brazil has been the largest ethanol producer, but the statistics from 2006 imply that the United States is the largest ethanol producer with 19.1 GL, followed by Brazil with 16.7 GL. Both countries produced almost identical amounts of ethanol in 2005 (16.2 and 16.0 GL, respectively). The American conti­nents produced 72% of the world ethanol production (see Fig. 3.2), fol­lowed by Asia, Europe, Oceania, and the African continents.

There is tough competition between sugar crops (particularly sugar­cane juice and molasses) and starch crops (particularly maize) as feed­stock for fuel ethanol production. While sugar crops were the feedstock for more than 60% of fuel ethanol production at the beginning of the 2000s, its share decreased to 47% in 2006 and starch crops were used for 53% of fuel ethanol production in the same year.

The world fuel ethanol production is predicted to keep the latest trend, at least until 2015. In comparison to 2006, ethanol production by Brazil and the United States is expected to increase by 102% and 93%, respectively. However, total production of the rest of the world is expected to increase by 585% [4]. Therefore, the world fuel ethanol pro­duction is expected to increase to around 100 GL. The main reasons for this sharp increase in ethanol production and demand in the future might be [2, 5, 6]:

■ Possible increase in oil prices

■ Higher demand for liquid fuels in the future

■ Decline of the crude oil supply in the future

■ Environmental legislation in different countries to encourage using biofuels

■

Production of bioplastic materials from ethanol

The molecular sieve is a more energy-efficient method than azeotropic distillation. Furthermore, this method avoids the occupational haz­ards associated with azeotropic chemical admixtures. In molecular sieve drying, 95% ethanol is passed through a bed of synthetic zeolite with uniform pore sizes that preferentially adsorb water molecules. Approximately three-fourths of adsorbed material is water and one — fourth is ethanol. The bed becomes saturated after a few minutes and must be regenerated by heating or evacuation to drive out the adsorbed water. During the regeneration phase, a side stream of ethanol/water (often around 50%) is produced, which must be redistilled before return­ing to the drying process [82].

Gerhard Knothe

5.1 Introduction

Biodiesel is an alternative diesel fuel (DF) derived from vegetable oils or animal fats [1, 2]. Transesterification of an oil or fat with a monohy — dric alcohol, in most cases methanol, yields the corresponding mono­alkyl esters, which are defined as biodiesel. The successful introduction and commercialization of biodiesel in many countries around the world has been accompanied by the development of standards to ensure high product quality and user confidence. Some biodiesel standards are ASTM D6751 (ASTM stands for American Society for Testing and Materials) and the European standard EN 14214, which was developed from pre­viously existing standards in individual European countries.

The suitability of any material as fuel, including biodiesel, is influenced by the nature of its major as well as minor components arising from pro­duction or other sources. The nature of these components ultimately deter­mines the fuel and physical properties. Some of the properties included in standards can be traced to the structure of the fatty esters in the biodiesel. Since biodiesel consists of fatty acid esters, not only the structure of the fatty acids but also that of the ester moiety can influence the fuel proper­ties of biodiesel. The transesterification reaction of an oil or fat leads to a biodiesel fuel corresponding in its fatty acid profile with that of the parent oil or fat. Therefore, biodiesel is largely a mixture of fatty esters with each ester component contributing to the properties of the fuel.

Properties of biodiesel that are determined by the structure of its component fatty esters and the nature of its minor components include [9]

ignition quality, cold flow, oxidative stability, viscosity, and lubricity. This chapter discusses the influence of the structure of fatty esters on these properties. Not all of these properties have been included in biodiesel standards, although all of them are essential to proper func­tioning of the fuel in a diesel engine.

Generally, as the least expensive alcohol, methanol has been used to produce biodiesel. Biodiesel, in most cases, can therefore be termed the fatty acid methyl esters (FAME) of a vegetable oil or animal fat. However, as mentioned above, both the fatty acid chain and the alcohol functionality contribute to the overall properties of a fatty ester. It is worthwhile to consider the properties imparted by other alcohols yield­ing fatty acid alkyl esters (FAAE) that could be used for producing biodiesel. Therefore, both structural moieties will be discussed in this chapter. Table 5.1 lists fuel properties of neat alkyl esters of fatty acids. Besides the fuel properties discussed here, the heat of combustion (HG) of some fatty compounds [3] is included in Table 5. 1, for the sake of under­scoring the suitability of fatty esters as fuel with regard to this property.

The major problem with methanol is high levels of formaldehyde emis­sion, which is negligible with conventional fuels. Formaldehyde emis­sion levels with and without an electric heater are shown in Fig. 7.13. The level with an electric heater is considerably lower compared with its absence.

The performance characteristics compared with petrol engine are considered as brake thermal efficiency versus air fuel (A:F) ratio, the effect of speed power output and specific heat consumption. In addi­tion, the performance characteristics also include the effect of A:F ratio on exhaust emission. The effects of A:F ratio and speed on brake power are shown in Figs. 7.14a and 14b. Another important charac­teristic is the effect of speed on volumetric efficiency, which is shown in Figs. 7.15 and 7.16.

Both alcohols, as well as their blends, are studied as alternative fuels for IC engines. The power can be increased from 6 to 10% with alcohols or their blends. The use of a leaner mixture provides more O2, which reduces the emission. Because of the high heat of vaporization of these fuels compared to petrol, greater cooling of the inlet mixture occurs, which gives higher thermal efficiency, less specific heat consumption, and smooth operation. At higher speeds, the specific heat consumption is lower than that of petrol. Methanol dissociates in the engine cylinder forming H2. This H2 gas helps the mixture to burn quickly and increases the burning velocity, which brings about complete combustion and makes a leaner mixture more combustible. In a petrol engine, misfiring

Formaldehyde emission
with electric heater

Figure 7.16 Effect of speed on volumetric efficiency.

0 1000 2000 3000 4000 5000 6000 —— ► Speed

occurs while operating at a lean A:F ratio, whereas in an engine using alcohol, the engine can manage to handle leaner mixtures without any misfire. Important objectionable emissions are CO, HC, NOx, and alde­hydes. The effect of equivalence ratio on all these emissions for petrol and methanol are shown in Figs. 7.17 through 7.20.

For all the above graphs, the engine details and compression ratio are as follows:

1. Full throttle rpm = 2500

2. Compression ratio

Methanol Petrol

Rc = 9 Rc = 9

Rc = 12.6

Regarding emission, ethanol and methanol are considered as clean fuels, as emissions of CO, HC, and NOx are reduced by nearly 10-15% compared with a petrol engine. The flame speed of alcohol mixtures is higher than a petrol A:F mixture, and this helps in making the com­bustion more complete without misfiring.

Regarding the production of formaldehyde, its percentage in exhaust is much higher, which is a great problem to extract methanol in pure form

as a replaceable fuel. To avoid this, blends (15-25%) of both alcohols are preferred over pure ethanol or methanol. The properties of blends and their effect lie in between pure alcohol and petrol. As we know, methanol blends have lower stoichiometric air requirements compared to petrol. Therefore, if we use a methanol-petrol blend without any modification in the carburetor, we get more air for combustion, which will reduce the emission of CO and HC as well as NOx as the engine works cooler with the blend compared with a petrol engine.

Oxidizing catalytic devices can control aldehyde emissions. Platinum-rhodium and platinum-palladium catalysts are considered the most effective in tackling aldehyde emissions of methanol fueling. Concerning the alcohol fuels, the following conclusions can be drawn:

1. Alcohol is potentially a better fuel than gasoline for SI engines.

2. Its use improves the thermal efficiency as a higher compression ratio (12:16) can be used.

3. It can avoid knocking even at a higher compression ratio because of the high octane number.

4. It provides better fuel economy and less exhaust emissions.

5. High latent heat of alcohol reduces the working temperature of the engine.

6. It gives more power, specially when used as a blend.

7. Easy availability of raw materials.

8. Cost of production is low because of the price hike in crude petroleum.

In agricultural countries like India, we can get ethyl alcohol easily from vegetables, agricultural material, and sugarcane waste at a much lower cost compared with the cost of petrol today. Therefore, replacing petrol with alcohol in a SI engine has a good future.

Hydrogen production by photoheterotrophic bacteria is principally sim­ilar to that of blue-green algae, capable of fixing nitrogen and produc­ing hydrogen. The microbes are capable of converting large varieties of organic compounds to carbon dioxide and hydrogen up to 50 kg/(m2 • yr). Practical applications of these bacteria are more of an engineering problem than one of scientific “know-how.” The scope of newer research exists on the noncyclic hydrogen production by these microbes unin­hibited by nitrogen. Dilute wastes can be utilized by the photosynthetic bacteria, which is an added advantage over those of the methane fer — mentors. The conventional fermentation of organic substrates to methane or hydrogen is theoretically limited to 80% and 20%, and prac­tically to 65% and 15%. The difference is accounted for by the synthesis of ATP and cell biomass. ATP is produced in presence of light and reac­tions are driven at its expense, if hydrogen is produced by nitrogenase.