Crop description. Aleurites fordii (Vernicia fordii) and A. montana— commonly known as the tung tree, Chinese wood, Abrasin, and Mu (see Fig. 4.24)—belong to the family Euphorbiaceae and grow well in cold cli­mates, but will survive in subtropical conditions (A. fordii). A. montana prefers a tropical climate. Major producers are China, Argentina, Paraguay, Brazil, and the United States. The nut of this deciduous tree contains an oil-rich kernel. The oil content of the air-dried fruit lies between 15% and 20% [77]. Major fatty acid composition of oil includes

palmitic acid (5.5%), oleic acid (4.0%), linoleic acid (8.5%), and eleostearic acid (82%) [77].

Main uses. Tung oil is used in paints, varnishes, and so forth. It is also used in the production of linoleum, resins, and chemical coatings. It has been used in motor fuels in China [77]. The seed cake after oil extraction is used as a fertilizer and cannot be used for animal feed as it contains a toxic protein [75]. No references about its use as a raw mate­rial to produce biodiesel have been found to date.

Methanol has high latent heat; therefore, some provision must be pro­vided to heat the intake manifold, because cold starting problems are often caused by A:F vapor mixture being outside the flammability range. Specially, methanol in its pure form is much more inferior to petrol for cold starting. Cold starting more or less becomes impossible with methanol when the ambient temperature falls below system on chip (SOC). Figure 7.12 shows the modification that is provided to avoid the difficulty of cold starting. By preheating, methanol dissociates into CO + 2H2 to obtain gaseous H2, which gives a broad flammability limit. While cranking the engine, a rich gaseous A:F mixture of methanol is collected near the spark plug, which enables good starting of the engine.

The major applications for fuel cells are as stationary electric power plants (including cogeneration units), as a transportation power source

for vehicles, and as portable power sources, besides an electric power source for space vehicles or other closed environments.

Stationary power applications are very favorable for fuel cell systems. Stationary applications mostly require continuous operation, so start­up time is not a very important constraint. Thus, high-temperature fuel cells such as the MCFC and SOFC systems are also suitable for this application in addition to the PAFC and PEMFC systems. The fuel source for stationary applications is most likely to be natural gas, which is relatively easy to reform in the internal reformer of high-temperature fuel cells or in the external reformer for low-temperature fuel cells. An advantage of using natural gas is that the distribution infrastructure for natural gas already exists. Promising applications for stationary fuel cell systems include premium power systems (high-quality uninterruptible/ back-up power supply systems); high-efficiency cogeneration (heat and electricity) systems for residences, commercial buildings, hospitals, and

industrial facilities; and distributed power generation systems for util­ities. Although some demonstration and commercial stationary fuel cell power plants in sizes from a few kilowatts to 11 MW are in operation, widespread commercialization can be expected only if their installation cost drops down from the present cost of $4000/kW to about $400-700/kW (or about $1000/kW for some premium applications).

The recent surge of interest in fuel cell technology is because of its potential use in transportation applications, including personal vehicles. This development is being sponsored by various governments in North America, Europe, and Japan, as well as by major automobile manufac­turers worldwide, who have invested several billion dollars with the goal of producing a high-efficiency and low-emission fuel cell power plant at a cost that is competitive with the existing internal combustion engines. With hydrogen as the onboard fuel, such vehicles would be zero-emission vehicles. With fuels other than hydrogen, an appropriate fuel processor to convert the fuel to hydrogen will be needed. Fuel cell — powered vehicles offer the advantages of electric drive and low mainte­nance, because of the few critical moving parts. The major activity in transportation fuel cell development has focused on the polymer elec­trolyte fuel cell (PEFC), and many of the technical objectives related to the fuel cell stack have been met or are close to being met. The current development efforts are focused on decreasing cost and resolving issues related to fuel supply and system integration.

Besides exotic areas of applications such as space vehicles or sub­marines, another very promising area of application for fuel cells is portable power systems. Portable power systems are small, lightweight systems that power portable devices (e. g., computers, laptops, cellular phones, and entertainment electronic devices), camping and recreational vehicles, military applications in the field, and so forth. These devices need power in the range of a few watts to a few hundred watts. Fuel cell systems based on DMFC or PEMFC technology are well suited for many of these applications. The convenience of transporting and storing liquid methanol makes DMFC systems very attractive for this application. A small container of methanol or a cylinder of compressed hydrogen can be used as a fuel supply. When the fuel is depleted, a new fuel container may be installed in its place after removing the old one.

In recent years, there has been a lot of interest in electric power gen­eration using renewable energy sources such as wind energy, solar energy, and tidal energy. A major problem with these energy sources is that all are intermittent in nature. Combining the renewable energy — based power generation system with a fuel cell system would solve this problem to a great extent. A hybrid wind/solar energy-fuel cell system can use wind/solar power for generating hydrogen using the electroly­sis of water, and store it in cylinders at high pressure. This hydrogen can then be used as the fuel for the fuel cell stack. The stored hydrogen can also be used to fuel the fuel cell vehicles and so forth. In a grid — connected wind/solar energy—hydrogen system, wind/solar power whenever available provides electricity for hydrogen production. The grid power is used during off-peak periods for low-cost electricity and hydrogen production; whereas during peak-demand periods or no/low wind/solar energy periods, the fuel cell can generate electricity using the stored hydrogen. These hybrid systems could be configured in several ways.

9.4 Conclusion

Fuel cell systems are one of the most promising technologies to meet our future power generation requirements. Fuel cell systems provide a very clean and efficient technology for electrical and automotive power sys­tems. With cogeneration efficiencies higher than 80%, fuel cells prom­ise to reduce primary energy use and environmental impact. Fuel cells are a very good alternative for rural energy needs, especially in remote places where there are no existing power grids or power supply is unre­liable. The application of fuel cells into the transportation sector will reduce greenhouse emissions considerably; if fuels from renewable energy sources are used, it would nearly eliminate greenhouse gas emis­sions. Utility companies are beginning to locate small, energy-saving power generators closer to loads to overcome right of way problems and transmission line costs. The modular design of fuel cells suits this dis­tributed generation strategy very nicely as new modular units can be added when the demand increases. This reduces the financial risk for utility planners. Biofuel cells are very attractive for implant devices as they can use glucose in blood to power these devices, eliminating the need for surgery for maintenance and battery replacement. Use of digester gas as a fuel in biofuel cells makes them very attractive for power generation from garbage and other organic waste. This will also help in waste disposal, a big problem in the agriculture and food industry.

All fuel cell technologies (PEMFC, DMFC, AFC, PAFC, MCFC, SOFC, and MFC) discussed in this chapter are in a very advanced stage of development and are very near to commercialization. Although a number of demonstration units of different types of fuel cells are oper­ating all over the world and many PAFC and AFC units have been com­mercially sold and are successfully operating, fuel cells are still awaiting widespread commercialization due to their high cost and limitation in the choice of the fuel used. These barriers will be overcome in the next few years, and fuel cells will become a preferred power source with widespread applications.

Small vesicles, called chromatophores, can be isolated from the membranes of photosynthetic bacteria, which exhibit two types of electron transfer chains resembling mitochondria and chloroplasts. Chroma-tophores supported on artificial membranes permit the generation of 200 mV on illumination. The salt-bacteria (Halobacterium halobium) contain a simple protein-vitamin A aldehyde, known as bacteriorhodopsin, when supported on artificial membranes that generate 250 mV on illumina­tion. This system is simpler than its counterpart. There is a probabil­ity that the entire system may be successfully synthesized or assembled. Solar photocells made of bacteriorhodopsin show great promise.

There are several possible ways to hydrolyze lignocellulose (see Fig. 3.5). The most commonly applied methods can be classified into two groups: chemical hydrolysis and enzymatic hydrolysis. In addition, there are

some other hydrolysis methods in which no chemicals or enzymes are applied. For instance, lignocellulose may be hydrolyzed by y-ray or electron-beam irradiation, or microwave irradiation. However, those processes are commercially unimportant [15].

Crop description. Cuphea spp., C. carthagenensis, C. painter, C. ignea, and C. viscosissima—commonly known as cuphea—belong to the family Lythraceae and grow in temperate and subtropical climates (see Fig. 4.5). They can be found in Central and South America, and have been grown in trials in Germany and the United States. The seeds of Cuphea con­tain about 30-36% oil [69]. Major fatty acid composition of the oil includes caprylic acid (73% in C. painter, 3% in C. ignea), capric acid (18% in C. carthagenensis, 24% in C. painteri, 87% in C. ignea, and 83-86% in C. llavea), and lauric acid (57% in C. carthagenensis) [70].

Main uses. It contains high levels of short-chain fatty acids, very inter­esting for industrial applications. Previous studies have suggested that oil composition and chemical properties of C. viscosissima VS-320 are not appropriate for use as a substitute for diesel fuel without chemical

conversion of triglycerides to methyl esters. Further genetic modifica­tions must be made [71, 72]. Later studies have revealed that genetically modified oils present relatively low viscosity that is predicted to enhance their performance as alternative diesel fuels [73]. Also, atomization prop­erties suggest better fuel performance, because this oil has short-chain triglycerides, while traditional vegetable oils contain predominantly long-chain triglycerides [74].

Ahindra Nag

5.2 Introduction

Processing of vegetable oils as biodiesel [1, 2] and its engine perform­ance is very challenging. From an environmental point of view, diesel engines are a major source of air pollution. Exhaust gases from diesel engines contain oxides of nitrogen, carbon monoxide, organic compounds consisting of unburned or partially burned hydrocarbons and particu­late matter (consisting primarily of soot).

Interest in clean burning fuels is growing worldwide, and reduction in exhaust emissions from diesel engines is of utmost importance. It is widely recognized that alternative diesel fuels produced from vegetable oils and animal fats can reduce exhaust emissions from compression ignition (CI) engines, without significantly affecting engine perform­ance. But reducing pollutant emissions from diesel engines requires a detailed knowledge of the combustion process. However, the complex nature of the combustion process in an engine makes it difficult to understand the events occurring in the combustion chamber that deter­mine the emission of exhaust gases.

Dr. Rudolf Diesel [3], the inventor of the CI engine, used peanut oil in one of his engines for a demonstration at the Paris exhibition in 1900. Then there was considerable interest in the use of vegetable oils as fuel in diesel engines.

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Several studies have reported the effects of fuel and engine parame­ters on diesel exhaust emissions. Chowdhury [4] claims to have suc­cessfully used raw vegetable oils in diesel engines. He observed that no major changes were necessary in the engine, but the engine could not be run for more than 4 h. The performance and economic aspects of vegetable oil were also discussed.

Barve and Amurthe [5] cite an example of using groundnut oil as fuel in a diesel engine generator set (103 kW) of a local water pump house. They claimed that the power output and fuel consumption were very much comparable with certified diesel fuel. Weibe and Nowakowska [6] have reported the use of palm oil as a motor fuel. The performance was found satisfactory with higher fuel consumption. Fang [7] has reported that soybean and castor oil blended with diesel fuel burns adequately in a small diesel engine. Engelman et al. [8] has presented data on the performance of soybean diesel oil blends compared with diesel fuel. Results from a short-duration test showed that the use of blends was feasible in the diesel engine; but in fact, in the long-term, test problems associated with lubrication, sticking piston rings, and injector atomization patterns contributed to mechanical difficulties in the engine. Cruze et al. [9] have found that atomization of the fuel by the injector, in some cases, has caused delayed ignition characteristics and reduced efficiency of mechanical power production, compared to diesel fuel. Pryde [10] has stated that raw vegetable oil has had no great promise for engine tests and that modified oil esters were required for further engine tests. Bruwer [11] has reported that even without mod­ification, nine diesel engines started and operated almost normally on sunflower oil and delivered power equal to that of diesel fuel. Brake thermal efficiency and maximum engine power were 3% lower, while the specific fuel consumption was 10% higher than that of diesel fuel. The bench test, however, showed that atomization of 100% sunflower oil was much poorer than diesel but could be improved by reducing the viscosity of oil. Energy wise, sunflower oil was favorable for run­ning diesel engines for a shorter duration.

Baranescu et al. [12] have conducted tests on a turbocharged engine, using mixtures of sunflower oil in 25%, 50%, and 75% with diesel fuel. They have concluded that the use of sunflower oil blended with diesel brought modification in the fuel injection process that mainly included an increase in injection pressure and a longer ignition duration. These effects led to longer combustion duration. Cold-temperature operation was very critical due to high viscosity that caused fuel system problems such as starting failure, unacceptable emission levels, and injection pump failure. Engine shutdown for a long duration accelerated gum formation, where the fuel contacted the bare metal. This might further impair the engine or injection system.

Wagner et al. [13] have conducted tests on a number of diesel engines with different blends of winter rape and safflower oil with diesel fuel. The following specific conclusions were drawn from the results obtained:

■ High viscosity and tendency to polymerize within the cylinder were major physical and chemical problems.

■ Attempt to reduce the viscosity of the oil by preheating the fuel by increasing the temperature of the fuel at the injector to the required value was not successful.

■ Short-term engine performance showed power output and fuel con­sumption equivalent to diesel fuel.

■ Severe engine damage occurred within a very short duration when the test was conducted for maximum power with varying engine rpm (revolutions per minute).

■ A blend of 70% winter rape with 30% diesel was successfully used for 50 h. No adverse effect was noted.

■ A diesel injector pump when run for 154 h with safflower oil had no abnormal wear, gumming, or corrosion.

Borgelt et al. [14] have conducted tests on three diesel engines con­taining 25-75% and 50-50% soybean oil and diesel. The engines were operated under 50% load for 1000 horsepower (HP); the output ranged from 2.55 to 2.8 kW. Thermal efficiency ranged from 19.3 to 20%. Engine performances were not significantly different. Carbon deposit increased with increased percentage of soybean oil. Thus, Borgelt et al. concluded that use of 25% or less soybean oil caused negligible changes in engine performance.

Barsic and Humke [15] performed a study in which blends of unre­fined peanut and sunflower with diesel fuel (50-50%) were used in a single-cylinder engine. The engine produced equivalent power or a minor increase (6%) with vegetable oils and blends, with a 20% increase in spe­cific fuel consumption. Performance tests at equal energy showed that the power level remained constant or decreased slightly, thermal effi­ciency decreased slightly, and the exhaust temperature increased with an increase in the percentage of vegetable oil in the fuel. Exhaust emis­sion at equal energy input showed slightly higher NOx for vegetable oils and their blends. Unburned hydrocarbon emission was about 50% higher than pure diesel fuel because the injection system was not optimized for more viscous fuels. Ziejewski et al. [16] reported the results of an endurance test using a 25-75% blend of alkali-refined sunflower oil with diesel and 25-75% blend of safflower oil with diesel on a volume basis. The major problems experienced were premature injection, determination of nozzle performance, and heavier carbon deposits in the piston ring grooves.

There was no significant problem with engine operation when the blend of safflower oil was used. That investigation revealed that chemical dif­ferences between vegetable oil and diesel had a very important influence on long-term engine performance. Bhattacharya et al. [17] have reported that a blend of 50% rice bran oil with diesel could be a supplementary fuel for their 10-bhp CI engine. No significant difference in the brake thermal efficiency was reported.

Samson et al. [18] have reported the use of tallow and stillingia oil in 25-75% and 50-50% blends by mass with diesel. The fuel properties of the blends were found to be within the limits proposed for diesel. The heat of combustion appeared to decrease. Specific gravity and kinematic viscos­ity increased with the increase in concentration of oil. Dunn et al. [19] con­ducted the test on rubber seed oil blended with diesel in 25%, 50%, 75%, and 100% in an air-cooled engine with 4.9 kW at 3600 rpm. Higher spe­cific fuel consumption and slightly higher thermal efficiency were observed. But, carbon deposits were heavier than those for pure diesel fuel.

Samga [20] conducted a test on a water-cooled single-cylinder diesel engine, using hone oil (ken seed oil). He concluded that hone oil gave acceptable performance, smooth running, and ease in starting without preheating. The exhaust temperature and specific fuel consumption were higher than those for diesel. The partial-load efficiency was lower, but full-load efficiency was better than with diesel fuel.

Auld et al. [21] evaluated the potential yield and fuel qualities of winter rape, safflower, and sunflower as sources of fuel for diesel engines. Vegetable oils contained 94-95% heat value of diesel fuel, but were 11.1-17.6 times more viscous and also 7-9% heavier than diesel fuel. Viscosities of vegetable oils were closely related to fatty acid chain length and number of unsaturated bonds. During short-term engine tests, all vegetable oils produced power comparable to that of diesel, and the thermal efficiency was 1.8-2.8% higher than that of diesel. Based on the results, they concluded that vegetable oil as fuel should be selected on identification of the crop species that produced the most optimum yield of fuel quality vegetable oils.

Ryan et al. [22] have tested four different types of vegetable oils (soybean, sunflower, cottonseed, and peanut) in at least three different stages of processing. All the oils were characterized according to their physical and chemical properties. The spray characteristics of oils were determined at different fuel temperatures, using a high-pressure, high-temperature injection bomb, and high-speed motion picture camera. The injection study pointed out that vegetable oil behaved differently from diesel fuel. Normally, as the viscosity decreased, the penetration rate decreased and the spray cone angle increased. Using vegetable oils, however, increased the penetration rate, and increasing the temperature of the oil from 45°C to 145°C reduced the cone angle and decreased the viscosity.

Engine test results, based on the specific energy, showed that degummed soybean performed as well as the base fuel, but performance of the deodorized sunflower was the worst of those tested with an energy con­sumption 10% higher than the base fuel. Vegetable oils had a much smaller premixed combustion stage, with the diffused stage of combus­tion being flatter for sunflower and soybean oil than for the diesel fuel. Engine inspection showed that heating of the oil reduced the carbon deposit problem. It was concluded that deposits and overall durability were related to viscosity differences and the chemical structures of the other oil as compared to diesel fuel.

Mathur and Das [23] have conducted tests on diesel engines, using blends of mahua and neem oil with diesel. Results showed that neem oil could be substituted for up to 35% with marginal reduction in effi­ciency and power output. Mahua oil with diesel had exhaust charac­teristics similar to those of diesel. Further, savings in the diesel fuel through the use of both these nonedible oils outweighed the demerits of a marginal drop in efficiency and a slight loss in power output.

Goering et al. [24] conducted tests on a diesel engine using a hybrid fuel formed by micro-emulsion of aqueous ethanol in soybean oil. The test data were compared with the data from a baseline test on diesel fuel. The nonionic emulsion produced the same power as diesel fuel, with 19% lower heating value. Brake specific fuel consumption (BSFC) was 16% higher, and the brake thermal efficiency was 6% higher, with diesel at full power. Diesel knock for the hybrid fuel was not worse than for diesel fuel; thus the low cetane number of the hybrid fuel was not reflected in engine performance. Hybrid fuels were less volatile than ethanol and thus safer. The effect of hybrid fuel on the engine durability was unknown.

Presently, vegetable oil is regarded as a niche application. One liter of rapeseed oil substitutes for approximately 0.96 L of diesel. The annual yield is 1480 L/ha. CO2 reduction in relation to the diesel equivalent is about 80% [36]. However, this is questioned in newer literature [33] in terms of global warming reduction considering the effects of extra N2O entering the atmosphere as a result of using nitrogen-based fer­tilizers to produce crops for biofuels. Before unmodified vegetable oil is used as a fuel, the engine must be refitted for the fuel to correspond to the viscosity and combustion properties of vegetable oil. Refitting concepts include preheating either the fuel and the injection system or the equipment with a two-tank system. The engine is started with diesel and changes to vegetable oil only when the operating tempera­ture has been reached. Blends of pure vegetable oils and a conversion product together with additives (antioxidants) increase oxidation sta­bility, reduce viscosity, and give a better perspective for vegetable oil fuel markets.

The simplest of the elements, containing a single proton and electron each, of mass almost unity, is the first member of the periodic table. Data may vary from different sources; solid at 4.2 K (d 0.089), H has the atomic number (AN) 1, atomic weight (AW) 1.008 g, melting point (mp) —259.14°C, and boiling point (bp) —252.87°C (d 0.071 at 20.4 K). He has a AN 2, AW 4.0026 g, mp —272.2°C (20 atm), and bp —268.93°C (specific gravity 0.124).

Commercial consumption at present is mostly in synthetic fuels, say from coal, mineral oils, petroleum reformation (refineries), and iron and copper ore reductions. Hydrogen is very important because of the ver­satility of its physical, chemical, and biological properties. More impor­tantly for our purposes, is its potential as a source of energy. Hydrogen liquefies at 33.2 K, 12.8 atm, and 0.03 g/mL and occupies a negligible volume (22.4 times less), compared to its gaseous state. Solid hydrogen and helium are academic ideas. When hydrogen combines with oxygen in a volume ratio of 2:1, heat is generated and the product is water in a vapor state. The reaction in a vapor state occurs with a reduction of volume to 1/3 and water vapor to water 1/22, which means the reaction is favored at a higher pressure; alternatively, the change in volume is compensated by utilizing some of the heat that evolves. The calculations are already there. Hydrogen as a combustion fuel or as a material for a fuel cell is less attractive than the fusion reaction such as that which occurs in the sun. Taking it as a model, we may be able to harness huge amounts of thermal and traditional energies, but we should also learn how to manage and handle such enormous outbursts of energy. Two protons fuse to yield a deuterium, a positron, and a neutrino; the last one is the clue to the release of energy that is not yet fully understood by science;

2H1 ^ e+ + v + H2 H2 + H1 ^ H3 + v 2H3 ^ He4 + 2H1

Solar constant = 1.968 cal/(cm2 • min) = 3.86 X 1033 erg/s = 1.373 kW/m2; even at such a long distance, we are unable to use all the energies.

Hydrogen in absence of air or oxygen, or in vacuum, will not burn, but may have a kind of combustion to produce ammonia in air or nitrogen. Combustion of hydrogen in our atmosphere does not produce simple water vapor, but mixture of others, i. e., ammonia and NOxS (nitrogen and oxygen combine at the vicinity of high temperature generated).

Cryogenic and space research have taught us many more lessons. Liquid hydrogen can be stored in special containers (cylinders), or trans­ported through pipes, and is almost an ideal fuel for rockets and space­ships, perhaps next to azides. But at higher altitudes or in space, in the absence of atmosphere, optimal liquid oxygen is also needed to perform the dynamism or thrust. Water vapor is transformed into ice particles instantly due to the very low temperature in space. Liquid hydrogen for such research or experiment is generated at a very high cost, i. e., elec­trolytic splitting of water. The alternate resource of hydrogen is a by­product in the caustic soda plant. A similar minor and indirect source of hydrogen is water gas (C + H2O ^ CO + H2), almost obsolete for any large-scale production. None of these examples are renewable in nature, continue to be energy and labor intensive, and cannot stand as com­petitors as fuel or energy resources. Other commercial sources of hydro­gen are dependent on the existing limited supply of natural resources,

i. e., coal, naphtha, and natural gas, which are not renewable. The mate­rials are mainly based on fluidization or gasification of coal, and refor­mation by superheated steam or from steam-iron process (3Fe + 4H2O ^ F3O4 + 4H2); these processes can be broadly classified into (a) ther­mochemical or solar gasification and (b) fast pyrolysis or other novel gasification. These processes may be totally or partly catalytic. The basic chemical principles are mostly similar to those of classical water gas: C + H2O ^ CO + H2; CO + H2O ^ CO2 + H2. Major sources of hydro­gen at present are directly or indirectly natural gas; electrolysis; pyrolytic, thermal, and superheated steam; or geothermal, solar, ocean current, ocean thermal gradient, and nuclear reactors. Biomass as a source of hydrogen as well as energy has been discussed in Sec. 1.2.

In this section, we will discuss different fermentation processes appli­cable for ethanol production. Fermentation processes, as well as other biological processes, can be classified into batch, fed-batch, and contin­uous operation. All these methods are applicable in industrial fermen­tation of sugar substances and starch materials. These processes are well established, the fed-batch and continuous modes of operation being dominant in the ethanol market. When configuring the fermentation process, several parameters must be considered, including (a) high ethanol yield and productivity, (b) high conversion of sugars, and (c) low equipment cost. The need for detoxification and choice of the microor­ganism must be evaluated in relation to the fermentation configuration.

Presentation of a variety of inhibitors and their interaction effects, e. g., in lignocellulosic hydrolyzates, makes the fermentation process more complex than with other substrates for ethanol production [17, 21].

In fermentation of this hydrolyzate, the pentoses should be utilized in order to increase the overall yield of the process and to avoid problems in wastewater treatment. Therefore, it is still a challenge to use a hexose-fermenting organism such as S. cerevisiae for fermentation of the hydrolyzate.

When a mixture of hexoses and pentoses is present in the medium, microorganisms usually take up hexoses first and produce ethanol. As the hexose concentration decreases, they start to take up the pentose. Fermentation of hexoses can be successfully performed under anaerobic or microaerobic conditions, with high ethanol yield and productivity. However, fermentation of pentoses is generally a slow and aerobic process. If one adds air to ferment pentoses, the microorganisms will start utilizing the ethanol produced as well. It makes the entire process complicated and demands a well-designed and controlled process.