The fuel properties of biodiesel are strongly influenced by the proper­ties of the individual fatty esters as well as those of some minor com­ponents. Both moieties, the fatty acid and alcohol, have considerable influence on fuel properties such as CN, with relation to combustion and exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity. It therefore appears reasonable to enrich (a) certain fatty ester(s) with desirable properties in the fuel, in order to improve the properties of the whole fuel. For example, from the presently available data, it appears that isopropyl esters have better fuel properties than methyl esters. The major disadvantage is the higher price of isopropanol in compari­son to methanol, besides modifications needed for the transesterifica­tion reaction. Similar observations likely hold for the fatty acid moiety. It may be possible in the future to improve the properties of biodiesel by means of genetic engineering of the parent oils, which could even­tually lead to a fuel enriched with (a) certain fatty acid(s), possibly oleic acid, that exhibits a combination of improved fuel properties.

At first glance vegetable oil offers a favorable CO2 balance. However, when the extra N2O emission from biofuel production is calculated in “CO2-equivalent” global warming terms, and compared with the quasi­cooling effect of “saving” emissions of fossil fuel derived CO2, the out­come is that production of commonly used biofuels can contribute as much or more to global warming by N2O emissions than cooling by fossil fuel savings [33]. In addition, widespread use of vegetable oil fuels is lim­ited by high viscosity, low volatility, poor cold flow behavior, and lack of oxidation stability during storage [6, 7]. Partial conversion of vegetable oil to hydrocarbons offers the possibility to preserve the favorable envi­ronmental characteristics of vegetable oil-based fuels while improving viscosity and cold flow behavior [34, 35]. Figure 8.2 depicts thermo­gravimetry of vegetable oil without pure oil (dashed line) and in the pres­ence of a Y-zeolite (Koestrolith). The dotted line represents the first derivative from the catalyzed conversion reaction.

О

H

Q

The efficiency of the decarboxylation effect of Y-zeolite activity on pure vegetable oil at T = 450°C may be seen by comparing the IR spec­trum of pure vegetable oil fuel in Fig. 8.3 with the corresponding spec­trum of the conversion product in Fig. 8.4. The carbonyl band at around 1700 cm 1 is an indicator for conversion efficiency.

Table 8.3 summarizes physical and chemical parameters of vegetable oil fuel and conversion products at different temperatures. The change

in viscosity is quite remarkable. In accordance with Fig. 8.2, a reaction temperature of T = 450°C is preferred.

A typical natural phenomenon, probably a unique mating signal by the “firefly,” also exists in other living species, namely, bacteria, protozoa, fungi, and worms, in the forms that emit visible light. In most cases, the
nature of the luminescent light varies in color and intensity; but chem­ical pathways are, to a great extent, common. The chemical products responsible for giving out different colors are different and are not yet fully known.

A heat-labile simple protein enzyme luciferase (MW 105) makes a complex (luciferyl adenylate E) with reduced luciferin, in the presence of ATP (Mg2+), which subsequently breaks down into different products in the presence of molecular oxygen. This results in the excitation of luciferin to a high-energy state. On return of the same to the ground state, emission of visible light produces bioluminescence (see Fig. 1.13).

LH2 + ATP (Mg2+) + E ^ LH2 — AMP — E + PPi
LH2 — AMP — E + O2 ^ Products + Light

The phenomenon appears to be insignificant but a substantial supply of luciferin, ATP (Mg2+), and a little enzyme can deliver an appreciable luminescence of practical use. Whether luciferin, luciferase, and ATP may also be harvested from animal resources, or the chemical compo­nents may be synthesized economically and the enzyme can be pro­cured from flies, remains a matter of investigation and development. Like bee-keeping, culture of “fireflies” is very likely to become a prof­itable art. The dream of producing high voltage by animal tissues, imi­tating the electric eel, may come true in the near future; the fundamentals are known, but economic viability is not assured, hence not discussed here.

OH

I

C—O — P—O—Ribose-Adenine

II II

OO

LH2-Adenylate

Figure 1.13 Firefly bioluminescence.

A great number of molds are able to produce ethanol. The filamentous fungi Fusarium, Mucor, Monilia, Rhizopus, Ryzypose, and Paecilomyces are among the fungi that can ferment pentoses to ethanol [33]. Zygomycetes are saprophytic filamentous fungi, which are able to produce several metabolites including ethanol. Among the three genera Mucor, Rhizopus, and Rhizomucor, Mucor indicus (formerly M. rouxii) and Rhizopus oryzae have shown good performances on ethanol productivity from glucose, xylose, and wood hydrolyzate [60]. M. indicus has several industrial advantages compared to baker’s yeast for ethanol production, such as (a) capability of utilizing xylose, (b) having a valuable biomass, e. g., for production of chitosan, and (c) high optimum temperature of 37°C [61]. Skory et al. [62] examined 19 Aspergilli and 10 Rhizopus strains for their ability to ferment simple sugars (glucose, xylose, and arabinose) as well as complex substrates. An appreciable level of ethanol has been produced by Aspergillus oryzae, R. oryzae, and R. javanicus.

The dimorphic organism M. circinelloides is also used for production of ethanol from pentose and hexose sugars. Large amounts of ethanol have been produced during aerobic growth on glucose under nonoxygen — limiting conditions by this mold. However, ethanol production on galac­tose or xylose has been less significant [63]. Yields as high as 0.48 g/g ethanol from glucose by M. indicus, under anaerobic conditions, have been reported [64]. However, the yield and productivity of ethanol from xylose is lower than that of P stipitis [65].

Although filamentous fungi have been industrially used for a long time for several purposes, a number of process-engineering problems are associated with these organisms due to their filamentous growth. Problems can appear in mixing, mass transfer, and heat transfer. Furthermore, attachment and growth on bioreactor walls, agitators, probes, and baffles cause heterogeneity within the bioreactor and prob­lems in measurement of controlling parameters and cleaning of the bioreactor [66, 67]. Such potential problems might hinder industrial application of M. indicus for ethanol production. However, this fungus is dimorphic, and its morphology can be controlled to be yeast-like or pellet-like through fermentation [65].

Crop description. Brassica carinata, commonly known as Ethiopian mustard, is an adequate oil-bearing crop that is well adapted to mar­ginal regions (see Fig. 4.15). This crop, which is originally from Ethiopia, is drought-resistant and grown in arid regions [127, 128]. Ethiopian mustard presents up to 6% saturated hydrocarbon chains. It is native to the Ethiopian highlands, is widely used as food by the Ethiopians, and pre-sents better agronomic performances in areas such as Spain, California, and Italy. This makes B. carinata a promising oil feedstock for cultivation in coastal Mediterranean areas, which could offer the pos­sibility of exploiting the Mediterranean marginal areas for energetic pur­poses [129]. Its fatty acid composition includes palmitic acid (3.6%), stearic acid (1.3%), oleic acid (14.8%), linoleic acid (12.2%), gadoleic acid (10.3%), and erucic acid (45.4%) [123].

Main uses. It is widely used as food in Ethiopia. Oil from wild species is high in erucic acid, which is toxic, although some cultivars contain little erucic acid and can be used as food. The seed can also be crushed and used as a condiment [127]. There is a genetic relationship among

B. carinata genotypes based on oil content and fatty acid composition. Genet et al. have generated information to plan crosses and maximize the use of genetic diversity and expression of heterosis [130]. Dorado et al. found negative effects of singular fatty acids, such as erucic acid, over alkali-catalyzed transesterification reaction [39]. These researchers described a low-cost transesterification process of B. carinata oil. An oil-methanol molar ratio of 1:4.6, addition of 1.4% of KOH, a reaction tem­perature in the range of 20-45oC, and 30 min of stirring are considered to be the best conditions to develop a low-cost method to produce biodiesel from B. carinata oil [39, 131]. Biodiesel from Ethiopian mustard oil could become of interest if a fuel tax exemption is granted [30]. When com­pared with petroleum diesel fuel, Cardone et al. have found that engine test bench analysis did not show any appreciable variation of output engine torque values, while there was a significant difference in specific fuel consumption data at the lowest loads. Biodiesel produced higher levels of NOx concentrations and lower levels of particulate matter (PM), with respect to diesel fuel. Biodiesel emissions contain less soot [132].

If a mixture of water and alcohol is boiled, the percentage of alcohol to water is greater in vapor than in liquid. Therefore, by repeated distil­lation and condensation, the alcoholic strength of the distillate can be increased until it contains 97.6% alcohol. There are different methods of distillation, but they are not discussed here, as ethanol production is our prime concern.

Alkaline-electrolyte fuel cells (see Fig. 9.7) are one of the most developed fuel cell technologies. They have been in use since the mid-1960s for Apollo and space shuttle programs [3, 6, 18, 19]. The AFCs onboard these spacecraft provide electrical power as well as drinking water. AFCs are among the most efficient electricity-generating fuel cells with an efficiency of nearly 70%. The electrolyte used in the AFC is an alka­line solution in which an OH ion can move freely across the electrolyte.

Electrochemistry of AFCs. The electrolyte used in the AFC is an aqueous (water-based) solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The concentration of KOH can be varied with the fuel cell operating temperature, which ranges from 65 to 220°C.

The charge carrier for an AFC is the hydroxyl ion (OH-) that migrates from the cathode to the anode, where they react with hydrogen to pro­duce water and electrons. Water formed at the anode migrates back to the cathode to regenerate hydroxyl ions.

Anode reaction: 2H2 + 4OH — ^ 4H2O + 4e-

Cathode reaction: O2 + 2H2O + 4e — ^ 4OH-

Hydroxyl ions are the conducting species in the electrolyte.

Overall cell reaction: 2H2 + O2 ^ 2H2O + heat + electricity

In many cell designs, the electrolyte is circulated (mobile electrolyte) so that heat can be removed and water eliminated by evaporation. Since KOH has the highest conductance among the alkaline hydroxides, it is the preferred electrolyte.

Electrolyte. Concentrated KOH (85 wt.%) is used in cells designed for operation at a high temperature (~260°C). For lower temperature (<120oC) operation, less concentrated KOH (35-50 wt.%) is used. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e. g., Ni, Ag, metal oxides, and noble metals). A major advantage of the AFC is the lower activation polar­ization at the cathode, resulting in a higher operating voltage (0.875 V). Another advantage of the AFC is the use of inexpensive electrolyte materials. The electrolyte is replenished through a reservoir on the anode side. The typical performance of this AFC cell is 0.85 V at a cur­rent density of 150 mA/cm2. The AFCs used in the space shuttle orbiter have a rectangular cross-section and weigh 91 kg. They operate at an average power of 7 kW with a peak power rating of 12 kW at 27.5 V. A disadvantage of the AFC is that it is very sensitive to CO2 present in the fuel or air. The alkaline electrolyte reacts with CO2 and severely degrades the fuel cell performance, limiting their application to closed environments, such as space and undersea vehicles, as these cells work well only with pure hydrogen and oxygen as fuel.

Electrodes. A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with non­precious, relatively inexpensive metals. The most important character­istics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). Both metallic (typ­ically hydrophobic) and carbon-based (typically hydrophilic) electrode structures with multilayers and optimized porosity characteristics for the flow of liquid electrolytes and gases (H2 and O2) have been developed. The kinetics of oxygen reduction in alkaline electrolytes is much faster than in acid media; hence AFCs can use low-level Pt catalysts (about 20% Pt, compared with PEMFCs) on a large surface carbon support [20].

Performance. The AFC development has gone through many changes since 1960. To meet the requirements for space applications, the early AFCs were operated at relatively high temperatures and pressures. Now the focus of the technology is to develop low-cost components for AFCs operating at near-ambient temperature and pressure, with air as the oxi­dant for terrestrial applications. This has resulted in lower perform­ance. The reversible cell potential for an H2 and O2 fuel cell decreases by 0.49 mV/°C under standard conditions. An increase in operating tem­perature reduces activation polarization, mass transfer polarization, and ohmic losses, thereby improving cell performance. Alkaline cells operated at low temperatures (~70°C) show reasonable performance.

Pure hydrogen and oxygen are required in order to operate an AFC. Reformed H2 or air containing even trace amounts of CO2 dramatically affects its performance and lifetime. There is a drastic loss in performance when using hydrogen-rich fuels containing even a small amount of CO2 from reformed hydrocarbon fuels and also from the presence of CO2 in the air (~350 ppm CO2 in ambient air). The CO2 reacts with OH (CO2 + 2OH ^ CO32~ + H2O), thereby decreasing their concentration and thus reducing the reaction kinetics. Other ill effects of the presence of CO2 are:

■ Increase in electrolyte viscosity, resulting in lower diffusion rate and

lower limiting currents.

■ Deposition of carbonate salts in the pores of the porous electrode.

■ Reduction in oxygen solubility.

■ Reduction in electrolyte conductivity.

A higher concentration of KOH decreases the life of O2 electrodes when operating with air containing CO2. However, operation at higher tem­peratures is beneficial because it increases the solubility of CO2 in the elec­trolyte. The operational life of air electrodes polytetrafluoroethylene [PTFE] bonded carbon electrodes on porous nickel substrates) at a current density of 65 mA/cm2 in 9-N KOH at 65°C ranges from 4000 to 5500 h with CO2-free air, and their life decreases to 1600-3400 h when air (350-ppm CO2) is used. For large-scale utility applications, operating times >40,000 h are required, which is a very significant hurdle to com­mercialization of AFC devices for stationary electric power generation.

Another problem with the AFC is that the electrodes and catalysts degrade more on no-load or light-load operation than on a loaded condition, because the high open-circuit voltage causes faster carbon oxidation processes and catalyst changes. The AFC with immobilized KOH electrolyte suffers much more from this as the electrolyte has to stay in the cells caus­ing residual carbonate accumulation, separator deterioration, and gas cross leakage during storage or unloaded periods if careful maintenance is not carried out. In circulating an electrolyte-type AFC, the electrolyte is emp­tied from the cell during nonoperating periods. Shutting off the H2 elec­trodes from air establishes an inert atmosphere. This shutdown also eliminates all parasitic currents and increases life expectancy. The exchangeability of the KOH in a circulating electrolyte-type AFC offers the possibility to operate on air without complete removal of the CO2 [20, 21].

In Chap. 1, gasification (pyrolysis) of biomass, biogas, gobargas, hydro­gen, and biohydrogen were discussed in detail.

2.6.1 Liquid products

An important renewable energy resource for transportation purposes is liquid fuel based on plant oils. However, pure plant oils are generally not suitable for use in modern diesel engines. This can be overcome by the process of transesterification. The resultant fatty-acid methyl esters have properties similar to those of diesel and are commonly called biodiesel. Biodiesel presents several advantages, such as better CO2 balance than diesel, low soot content, reduced hydrocarbon emissions, and low carcinogenic potential [20]. The specification standards for the European Union (EU) and the United States are EN14214 and ASTM D6751, respectively. The EU directive established a minimum content of 2% and 5.75% biodiesel for all petrol and diesel used in transport by December 31, 2005, and December 31, 2010, respectively. Biodiesel refers to the pure oil before blending with diesel fuel. Biodiesel blends are represented as “BXX,” with “XX” representing the percentage of biodiesel component in the blend (National Biodiesel Board, 2005) [21]. In the biomass-to-liquid conversion processes, biomass is broken down into a gaseous constituent and a solid constituent by low-temperature gasification. The next step involves production of synthetic gas, which is converted into fuel (termed SunFuel) by the Fischer-Tropsch synthe­sis process, with downstream fuel optimization by hydrogen after treat­ment [22]. Ethanol has already been introduced in countries such as Brazil, the United States, and some European countries. In Brazil, it is currently produced from sugar and, in the United States, from starch at competitive prices. Ethanol is currently produced from sugarcane and starch-containing materials, where the conversion of starch to ethanol includes a liquefaction step (to make the starch soluble) and a hydrolysis step (to produce glucose). There are generally two types of processes for production of bioethanol: the lignocellulosic process and the starch process. Unlike the starch-based process, the lignocellulosic process has not been as widely adopted due to techno-economic reasons.

High ethanol yield requires complete hydrolysis of both cellulosic and hemicellulose with a minimum of sugar dehydration, followed by effi­cient fermentation of all sugars in the biomass. Certain advantages of using lignocellulose-based liquid biofuels are that they are evenly dis­tributed across the globe and hence are readily available, less expensive compared to agricultural feedstock, produced at a lower cost, and have low net greenhouse gas emissions. Enzymatic processes (essentially using bacteria, yeasts, or filamentous fungi) have been considered for lignocellulosic processes. The enzymatic process when coupled with the fermentation process is known as simultaneous saccharification and fermentation. This has proved to be efficient in the fermentation of hexose and pentose sugars [23]. Genencor International (www. genen- cor. com/) and Novozymes, Inc., (www. novozymes. com) have been awarded $17 million each by the U. S. Department of Energy with a goal to reduce the enzyme cost tenfold (www. eere. energy. gov/). The Iogen Corp. (www. iogen. ca/) demo-plant is the only one that produces bioethanol from lignocellulose, using the enzymatic hydrolysis process. This plant is known to handle about 40 ton/day of wheat, oat, barley, and straw and is designed to produce up to 3 ML/yr of cellulose ethanol. Refer to Chap. 3 for bioethanol preparation, Chap. 6 for boidiesel processing, and Chap. 7 for ethanol and methanol used in engines.

Mash is usually centrifuged or settled in order to separate the micro­bial biomass from the liquid and then sent to the ethanol recovery system. Distillation is typically used for the separation of ethanol, alde­hydes, fusel oil, and stillage [9]. Ethanol is readily concentrated from mash by distillation, since the volatility of ethanol in a dilute solution is much higher than the volatility of water. Therefore, ethanol is sepa­rated from the rest of the materials and water by distillation. However, ethanol and water form an azeotrope at 95.57 wt% ethanol (89 mol% ethanol) with a minimum boiling point of 78.15°C. This mixture behaves as a single component in a simple distillation, and no further enrichment than 95.57 wt% of ethanol can be achieved by simple distil­lation [9, 47, 81]. Various industrial distillation systems for ethanol purifi­cation are (a) simple two-column systems, (b) three- or four-column barbet systems, (c) three-column Othmer system, (d) vacuum rectification, (e) vapor recompression, (f) multieffect distillation, and (g) six-column reagent alcohol system [9, 47]. These methods are reviewed by Kosaric [9]. The following parameters should be considered for selection of the industrial distillation systems:

■ Energy consumption (e. g., steam consumption or cooling water con­sumption per kilogram of ethanol produced).

■ Quality of ethanol (complete separation of fusel oil and light compo­nents).

■ How to deal with the problem associated with clogging of the first dis­tillation column and its reboiler because of precipitation or formation of solids. Special design and use of a vacuum may be applied for over­coming the problem in the column. Using open steam instead of appli­cation of a reboiler can prevent clogging of the reboiler, in spite of the increase in amount of wastewater.

■ Simplicity in controlling the system.

■ Simplicity in opening column parts and cleaning the columns.

Of course, lower capital investment is also one of the main parameters in the selection of distillation systems.

Ethanol is present in the market with different degrees of purity. The majority of ethanol is 190 proof (95% or 92.4%, minimum) used for sol­vent, pharmaceutical, cosmetic, and chemical applications. Technical — grade ethanol, containing up to 5% volatile organic aldehyde, ester, and sometimes methanol, is used for industrial solvents and chemical syn­theses. High-purity 200 proof (99.85%) anhydrous ethanol is produced for special chemical applications. For fuel use in mixture with gasoline (gasohol), nearly anhydrous (99.2%) ethanol, but with higher available levels of organic impurities, is used [47].

A simple two-column system is described here, while other systems are presented in the literature (e. g., [9, 47]). Simple one — or two — column systems with only a stripping and rectification section are usually used to produce lower-quality industrial alcohol and azeotrope alcohol for further dehydration to fuel grade. The simplest continu­ous ethanol distillation system consists of stripping and rectification sections, either together in one column or separated into two columns (see Fig. 3.10).

The mash produced is pumped into a continuous distillation process, where steam is used to heat the mash to its boiling point in the stripper column. The ethanol-enriched vapors pass through a rectifying column and are condensed and removed from the top of the rectifier at around 95% ethanol. The ethanol-stripped stillage falls to the bottom of the stripper column and is pumped to a stillage tank. Aldehydes are drawn

Stillage <

Figure 3.10 Two-column system for distillation of ethanol.

from the head vapor, condensed, and partly used as reflux. Fusel oil is taken out from several plates of the rectifying section [9, 47, 82].

With efficient distillation, the stillage should contain less than 0.1% ethanol since the presence of ethanol significantly increases the chem­ical oxygen demand (COD) of wastewater. For each 1% ethanol left in the stillage, the COD of the stillage is incremented by more than 20 g/L. Due to the potential impact of residual ethanol content, therefore, proper control over distillation can greatly affect the COD of stillage [82].

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.